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1
Young, Kenneth Harbour Phillips Thomas G. "Submillimeter and infrared studies of mass lost by asymptotic giant branch stars /". Diss., Pasadena, Calif. : California Institute of Technology, 1994. http://resolver.caltech.edu/CaltechETD:etd-09172008-085430.
Testo completo
2
Tout, Christopher Adam. "Binary stars and mass loss". Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316766.
Testo completo
3
Prieto, Jose L. "Massive Stars: Life and Death". The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1248987393.
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4
Burnley, Adam Warwick. "Mass loss from hot, luminous stars". Thesis, University College London (University of London), 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.408417.
Testo completo
5
Viviers, Etienne. "Lost in the stars Kurt Weill and Maxwell Anderson's musical adaptation of Alan Paton's novel Cry the beloved country /". Diss., Pretoria : [s.n.], 2008. http://upetd.up.ac.za/thesis/available/etd-11252008-142122/.
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6
Oskinova, Lidia M., Richard Ignace e D. P. Huenemoerder. "X-ray Diagnostics of Massive Star Winds". Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etsu-works/2703.
Testo completoAbstract (sommario):
Observations with powerful X-ray telescopes, such as XMM-Newton and Chandra, significantly advance our understanding of massive stars. Nearly all early-type stars are X-ray sources. Studies of their X-ray emission provide important diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are well explained when stellar wind clumping is taking into account, providing further support to a modern picture of stellar winds as non-stationary, inhomogeneous outflows. X-ray variability is detected from such winds, on time scales likely associated with stellar rotation. High-resolution X-ray spectroscopy indicates that the winds of late O-type stars are predominantly in a hot phase. Consequently, X-rays provide the best observational window to study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows to probe their powerful metal enhanced winds, while the mechanisms responsible for the X-ray emission of these stars are not yet understood.
7
Coté, Jacqueline. "Formation and mass loss of be stars". Amsterdam : Amsterdam : Sterrenkundig Instituut " Anton Pannekoek" ; Universiteit van Amsterdam [Host], 1993. http://dare.uva.nl/document/91638.
Testo completo
8
Smith, Nathan, Jose H. Groh, Kevin France e Richard McCray. "Ultraviolet spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analogue". OXFORD UNIV PRESS, 2017. http://hdl.handle.net/10150/624061.
Testo completoAbstract (sommario):
The Galactic blue supergiant SBW1 with its circumstellar ring nebula represents the best known analogue of the progenitor of SN 1987A. High-resolution imaging has shown H alpha and infrared structures arising in an ionized flow that partly fills the ring's interior. To constrain the influence of the stellar wind on this structure, we obtained an ultraviolet (UV) spectrum of the central star of SBW1 with the Hubble Space Telescope Cosmic Origins Spectrograph. The UV spectrum shows none of the typical wind signatures, indicating a very low mass-loss rate. Radiative transfer models suggest an extremely low rate below 10(-10) M-circle dot yr(-1), although we find that cooling time-scales probably become comparable to (or longer than) the flow time below 10(-8) M-circle dot yr(-1). We therefore adopt this latter value as a conservative upper limit. For the central star, the model yields T-eff = 21 000 +/- 1000 K, log(g(eff)) = 3.0, L similar or equal to 5 x 10(4) L-circle dot, and roughly Solar composition except for enhanced N abundance. SBW1' s very low mass-loss rate may hinder the wind's ability to shape its nebula and to shed angular momentum. The spin-down time-scale for magnetic breaking is more than 500 times longer than the age of the ring. This, combined with the star's slow rotation rate, constrains merger scenarios to form ring nebulae. The mass-loss rate is at least 10 times lower than expected from mass-loss recipes, without any account of clumping. The physical explanation for why SBW1' s wind is so weak presents an interesting mystery.
9
Young, Patrick Allen. "Hydrodynamics, nucleosynthesis, and mass loss in massive stars". Diss., The University of Arizona, 2004. http://hdl.handle.net/10150/280579.
Testo completoAbstract (sommario):
I test the predictive power of the stellar evolution code TYCHO. Systematic errors are present in the predictions for double-lined eclipsing binary stars when only standard physics common to the majority of stellar evolution codes is included. A mechanism for driving slow circulation and mixing in the radiative regions of stars is identified in numerical simulations of convection and a physical theory developed. Mixing is caused by dissipation of inertial waves driven by the interaction of convective fluid motions with the boundary of the convection zone. Evolutionary calculations incorporating this physics are tested in several observational regimes. Light element depletion in young clusters, turnoff ages of young clusters with brown dwarf Li depletion ages, and evolution of carbon stars on the asymptotic giant branch are all predicted satisfactorily. Tests of solar models yield good agreement with surface observables, chemical abundances, helioseismological data, and neutrino fluxes. The predictive accuracy of a non-calibrated, state-of-the-art stellar evolution code is ∼7% for surface observables. The main sequence sun is relatively easy to model, so this gives an estimate of our minimum predictive error. The solar models also highlight problems with uniqueness of evolutionary tracks converging on a given point and the potential for avoiding the effects of missing physics by calibration. A reanalysis of the binary sample with the more complete physics shows a dramatic improvement in the accuracy of the models. The potential for avoiding the effects of missing physics by calibration is explored. A TYCHO model for a late AGB star is used for the boundary conditions on a hydrodynamic simulation of proto-planetary nebula evolution as an illustration of the unified technique. NaCl and NaCn are observed at large radii in the Egg Nebula. These molecules require high densities to form, which are difficult to explain at large distances from the star. The 2-D simulation of a fast wind interacting with earlier mass loss produces clumps of material through a thermal instability with the necessary conditions for formation of the molecules. In conclusion, the effects of the more complete physics on the core size and abundance profiles of a massive star during core Si burning are examined as an example of future developments.
10
Ignace, Richard. "Long-Wavelength, Free–Free Spectral Energy Distributions from Porous Stellar Winds". Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etsu-works/2685.
Testo completoAbstract (sommario):
The influence of macroclumps for free–free spectral energy distributions (SEDs) of ionized winds is considered. The goal is to emphasize distinctions between microclumping and macroclumping effects. Microclumping can alter SED slopes and flux levels if the volume filling factor of the clumps varies with radius; however, the modifications are independent of the clump geometry. To what extent does macroclumping alter SED slopes and flux levels? In addressing the question, two specific types of macroclump geometries are explored: shell fragments (pancake-shaped) and spherical clumps. Analytic and semi-analytic results are derived in the limiting case that clumps never obscure one another. Numerical calculations based on a porosity formalism is used when clumps do overlap. Under the assumptions of a constant expansion, isothermal, and fixed ionization wind, the fragment model leads to results that are essentially identical to the microclumping result. Mass-loss rate determinations are not affected by porosity effects for shell fragments. By contrast, spherical clumps can lead to a reduction in long-wavelength fluxes, but the reductions are only significant for extreme volume filling factors.
11
Kiminki, Megan M., Megan Reiter e Nathan Smith. "Ancient eruptions of η Carinae: a tale written in proper motions". OXFORD UNIV PRESS, 2016. http://hdl.handle.net/10150/622109.
Testo completoAbstract (sommario):
We analyse eight epochs of Hubble Space Telescope H alpha+[N ii] imaging of eta Carinae's outer ejecta. Proper motions of nearly 800 knots reveal that the detected ejecta are divided into three apparent age groups, dating to around 1250 A.D., to around 1550 A.D., and to during or shortly before the Great Eruption of the 1840s. Ejecta from these groups reside in different locations and provide a firm constraint that eta Car experienced multiple major eruptions prior to the nineteenth century. The 1250 and 1550 events did not share the same axisymmetry as the Homunculus; the 1250 event was particularly asymmetric, even one-sided. In addition, the ejecta in the S ridge, which have been associated with the Great Eruption, appear to predate the ejection of the Homunculus by several decades. We detect essentially ballistic expansion across multiple epochs. We find no evidence for large-scale deceleration of the observed knots that could power the soft X-ray shell by ploughing into surrounding material, suggesting that the observed X-rays arise instead from fast, rarefied ejecta from the 1840s overtaking the older dense knots. Early deceleration and subsequent coasting cannot explain the origin of the older outer ejecta - significant episodic mass loss prior to the nineteenth century is required. The time-scale and geometry of the past eruptions provide important constraints for any theoretical physical mechanisms driving eta Car's behaviour. Non-repeating mechanisms such as the merger of a close binary in a triple system would require additional complexities to explain the observations.
12
Neilson, Hilding R., e Richard Ignace. "Period Change and Stellar Evolution of β Cephei Stars". Digital Commons @ East Tennessee State University, 2015. https://dc.etsu.edu/etsu-works/2689.
Testo completoAbstract (sommario):
The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change.
13
Ignace, Richard, Z. Damrau e K. T. Hole. "Variability in X-ray Line Ratios in Helium-Like Ions of Massive Stars: The Wind-Driven Case". Digital Commons @ East Tennessee State University, 2019. https://dc.etsu.edu/etsu-works/5500.
Testo completoAbstract (sommario):
Context. High spectral resolution and long exposure times are providing unprecedented levels of data quality of massive stars at X-ray wavelengths.
Aims. A key diagnostic of the X-ray emitting plasma are the fir lines for He-like triplets. In particular, owing to radiative pumping effects, the forbidden-to-intercombination line luminosity ratio, R = f∕i, can be used to determine the proximity of the hot plasma to the UV-bright photospheres of massive stars. Moreover, the era of large observing programs additionally allows for investigation of line variability.
Methods. This contribution is the second to explore how variability in the line ratio can provide new diagnostic information about distributed X-rays in a massive star wind. We focus on wind integration for total line luminosities, taking account of radiative pumping and stellar occultation. While the case of a variable stellar radiation field was explored in the first paper, the effects of wind variability are emphasized in this work.
Results. We formulate an expression for the ratio of line luminosities f∕i that closely resembles the classic expression for the on-the-spot result. While there are many ways to drive variability in the line ratio, we use variable mass loss as an illustrative example for wind integration, particularly since this produces no variability for the on-the-spot case. The f∕i ratio can be significantly modulated owing to evolving wind properties. The extent of the variation depends on how the timescale for the wind flow compares to the timescale over which the line emissivities change.
Conclusions. While a variety of factors can ellicit variable line ratios, a time-varying mass-loss rate serves to demonstrate the range of amplitude and phased-dependent behavior in f∕i line ratios. Importantly, we evaluate how variable mass loss might bias measures of f∕i. For observational exposures that are less than the timescale of variable mass loss, biased measures (relative to the time-averaged wind) can result; if exposures are long, the f∕i ratio is reflective of the time-averaged spherical wind.
14
Fossati, L., T. Koskinen, K. France, P. E. Cubillos, C. A. Haswell, A. F. Lanza e I. Pillitteri. "Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System". IOP PUBLISHING LTD, 2018. http://hdl.handle.net/10150/627089.
Testo completoAbstract (sommario):
WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (< 1 Gyr), the star presents an anomalously low stellar activity level: the measured log R'(HK) activity parameter lies slightly below the basal level; there is no significant time-variability in the log R'(HK) value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of Hubble Space Telescope aimed at explaining this anomaly. From the star's spectral energy distribution, we infer the extinction (E(B-V) approximate to 0.01 mag) and then the interstellar medium (ISM) column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (> 5 Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg s(-1) cm(-2). We employ the rescaled XUV solar fluxes to models of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10(-20) M-J Gyr(-1). For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.
15
Oskinova, Lidia M., David P. Huenemoerder, Wolf-Rainer Hamann, Tomer Shenar, A. A. C. Sander, Richard Ignace, H. Todt e R. Hainich. "On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary - IOPscience". Digital Commons @ East Tennessee State University, 2017. https://dc.etsu.edu/etsu-works/2688.
Testo completoAbstract (sommario):
The blue hypergiant Cyg OB2 12 (B3Ia+) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si xivand Mg xii. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.
16
Drabek-Maunder, Emily Rae. "A submillimetre study of nearby star formation using molecular line data". Thesis, University of Exeter, 2013. http://hdl.handle.net/10871/14587.
Testo completoAbstract (sommario):
This thesis primarily uses submillimetre molecular line data from HARP, a heterodyne array on the James Clerk Maxwell Telescope (JCMT), to further investigate star formation in the Ophiuchus L1688 cloud. HARP was used to observe CO J = 3-2 isotopologues: 12CO, 13CO and C18O; and the dense gas tracer HCO+ J = 4-3. A method for calculating molecular line contamination in the SCUBA-2 450 and 850 μm dust continuum data was developed, which can be used to convert 12CO J =6-5and J =3-2 maps of integrated intensity (K km s−1) to molecular line flux (mJy beam−1) contaminating the continuum emission. Using HARP maps of 12CO J = 3-2, I quantified the amount of molecular line contamination found in the SCUBA-2 850 μm maps of three different regions, including NGC 1333 of Perseus and NGC 2071 and NGC 2024 of Orion B. Regions with ‘significant’ (i.e. > 20%) molecular line contamination correspond to molecular outflows. This method is now being used to remove molecular line contamination from regions with both SCUBA-2 dust continuum and HARP 12CO map coverage in the Gould Belt Legacy Survey (GBS). The Ophiuchus L1688 cloud was observed in all three CO J = 3-2 isotopologues. I carried out a molecular outflow analysis in the region on a list of 30 sources from the Spitzer ‘c2d’ survey [Evans et al., 2009]. Out of the 30 sources, 8 had confirmed bipolar outflows, 20 sources had ‘confused’ outflow detections and 2 sources did not have outflow detections. The Ophiuchus cloud was found to be gravitationally bound with the turbulent kinetic energy a factor of 7 lower than the gravitational binding energy. The high-velocity outflowing gas was found to be only 21% of the turbulence in the cloud, suggesting outflows are significant but not the dominant source of turbulence in the region. Other factors were found to influence the global high-velocity outflowing gas in addition to molecular outflows, including hot dust from nearby B-type stars, outflow remnants from less embedded sources and stellar winds from the Upper Scorpius OB association. To trace high density gas in the Ophiuchus L1688 cloud, HCO+ J = 4-3 was observed to further investigate the relationship between high column density and high density in the molecular cloud. Non-LTE codes RADEX and TORUS were used to develop density models corresponding to the HCO+ emission. The models involved both constant density and peaked density profiles. RADEX [van der Tak et al., 2007] models used a constant density model along the line-of-sight and indicated the HCO+ traced densities that were predominantly subthermally excited with den- sities ranging from 10^3–10^5 cm^−3. Line-of-sight estimates ranged from several parsecs to 90 pc, which was unrealistic for the Ophiuchus cloud. This lead to the implementation of peaked density profiles using the TORUS non-LTE radiative transfer code. Initial models used a ‘triangle’ density profile and a more complicated log-normal density probability density function (PDF) profile was subsequently implemented. Peaked density models were relatively successful at fitting the HCO+ data. Triangle models had density fits ranging from 0.2–2.0×10^6 cm^−3 and 0.1–0.3×10^6 cm^−3 for the 0.2 and 0.3 pc cloud length models re- spectively. Log-normal density models with constant-σ had peak density ranges from 0.2–1.0 ×10^5 cm^−3 and 0.6–2.0×10^5 cm^−3 for 0.2 and 0.3 pc models respectively. Similarly, log-normal models with varying-σ had lower and upper density limits corresponding to the range of FWHM velocities. Densities (lower and upper limits) ranged from 0.1–1.0 ×10^6 and 0.5–3.0 ×10^5 cm^-3 for the 0.2 and 0.3 pc models respectively. The result of the HCO+ density modelling indicated the distributions of starless, prestellar and protostellar cores do not have a preference for higher densities with respect to the rest of the cloud. This is contrary to past research suggesting the probability of finding a submillimetre core steeply rises as a function of column density (i.e. density; Belloche et al. 2011; Hatchell et al. 2005). Since the majority of sources are less embedded (i.e Class II/III), it is possible the evolutionary state of Ophiuchus is the main reason the small sample of Class 0/I protostars do not appear to have a preference for higher densities in the cloud.
17
Diago, Nebot Pascual David. "Asteroseismology and mass loss in Be stars. Study with CoRoT". Doctoral thesis, Universitat de València, 2010. http://hdl.handle.net/10803/52185.
Testo completoAbstract (sommario):
The interiors of the stars are among the most difficult parts of the Universe to
observe. Essentially, asteroseismology tries to make use of the oscillations to
prove the stellar interiors, which are not directly observable. The basic
principles of asteroseismology are, to a certain extent, similar to those
developed and employed by Earth seismologists. Asteroseismology relies on
advanced mathematical descriptions of oscillations in a three-dimensional body
and numerical modeling. It is therefore a prominent example of interdisciplinary
science.
The general aim of this work is the study of Be stars with the CoRoT space
mission. Classical Be stars are B-type stars that exhibit line emission over the
pho- tospheric spectrum. The excess is attributed to a circumstellar gaseous
component that is commonly accepted to be in the form of an equatorial disk.
The mechanisms responsible for the production and dynamics of the
circumstellar gas are still not constrained. Observations of non-radial pulsation
beating phenomena connected to outbursts point toward a relevance of
pulsation, but this mechanism cannot be gene- ralized. In this regard, the
observation of classical Be stars with the high-precision CoRoT satellite is
providing important keys to understand the physics of these objects and the
nature of the Be phenomenon.
In order to study the light variations of the selected stars we use photometric
and spectroscopic observations. These observations allow us to extract
frequencies, amplitudes and phases of these variations. As we will show, these
light variations can be connected with pulsations on the stellar surface. For
carrying out the frequency analysis we have developed a new code based on
standard Fourier analysis. The point is that this code, called pasper, allows the
frequency analysis of large sets of light curves in an automatic mode.
This Ph.D. thesis is arranged as follows: In the first three Chapters we describe
the scientific framework of this project, giving a brief description on Asteroseismology,
presenting the current status of Be stars, and describing the basics of
the Fourier analysis and the rudiments of the time series analysis.
At the early begin of this Ph.D. thesis, the CoRoT satellite was still on ground
getting ready for the launch. In this context, we perform a search for shortperiod
B and Be star variables in the low metallicity environment of the
Magellanic Clouds. This study constitutes the Part I of this Ph.D. thesis. This
Part has a double goal: i) to test the frequency analysis codes; and ii) to detect
observationally β Cephei and SPB-like B-type pulsators in low metallicity
environments, actually not predicted by the pulsational theory and models. This
constitutes the Part I.
Part II is devoted to the study of Be stars with the CoRoT space mission. Here
we depict a complete review on the CoRoT mission. We also describe the
results on the analysis of three Be stars from the CoRoT exoplanet field. Finally,
we present the results on the frequency analysis of the late Be star HD 50 209,
observed in the seismology field of the CoRoT satellite. The analysis of this Be
star has revealed up to sixty frequencies, grouped in six different and separated
sets, attributed to g-mode pulsations.
Finally, we resume the main conclusions of the whole project, including prospects
and future work to be done. An addendum with all the published results
derived from this project has been added at the end of this Part II. Part III encloses
the Appendixes, providing a brief summary of this work in Spanish, a
complete description on basic equations of non-radial oscillation, the user guide
of the PASPER code and the user guide of the KURTZ_BOS code.El objetivo de la asterosismología es describir el interior de las estrellas a partir de las oscilaciones. El interior de las estrellas es, posiblemente, el lugar más difícil de estudiar de todo el Universo, ya que las condiciones especiales que allí se dan no pueden reproducirse en ningún laboratorio terrestre. Así, la importancia de esta ciencia en el estudio global de la física estelar es crucial. Los principios básicos de la asterosismología son, en esencia, similares a los que los geólogos usan en el estudio de terremotos terrestres. La asterosismología hace uso de descripciones matemáticas de las oscilaciones en cuerpos tridimensionales, creando para ello sofisticadas simulaciones numéricas. Es, por tanto, un claro ejemplo de ciencia multidisciplinar. El objetivo general de esta Tesis Doctoral es el estudio de las estrellas Be con las misión espacial CoRoT. Las estrellas Be son objetos de secuencia principal, con alta velocidad de rotación, que presentan un exceso infrarrojo y emisión en las líneas de Balmer, debido a la presencia de una envoltura circumestelar concentrada en el ecuador y generada por eyecciones discretas de materia originado por mecanismos que todavía no son bien conocidos. Los mecanismos propuestos para explicar la eyección de materia son hasta ahora, las pulsaciones no radiales combinadas con la alta velocidad de rotación característica de este tipo de estrellas. La observación de estrellas Be con el satélite de alta precisión CoRoT está aportando claves importantísimas para comprender la física de estos objetos y la naturaleza del fenómeno Be. Para la detección de señales periódicas en las estrellas seleccionadas utilizamos tanto observaciones fotométricas como espectroscópicas. A partir de las observaciones obtenemos las frecuencias, amplitudes y fases de las variaciones. Como se muestra en la tesis, las variaciones en las curvas de luz estudiadas podrían estar estrechamente relacionadas con los mecanismos de pulsación estelar. Para realizar el análisis de frecuencias hemos desarrollado un código basado en técnicas estándar de análisis de Fourier. Este código, llamado Pasper, nos permite analizar grandes conjuntos de datos de forma casi automática.
18
Andronov, Nikolay I. "Evolution of close binary stars with application to cataclysmic variables and Blue Stragglers". Connect to resource, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1126025974.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Ohio State University, 2005.Title from first page of PDF file. Document formatted into pages; contains xii, 190 p.; also includes graphics (some col.). Includes bibliographical references (p. 181-190). Available online via OhioLINK's ETD Center.
19
Bladh, Sara. "Dynamical atmospheres and winds of M-type AGB stars". Doctoral thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-230645.
Testo completoAbstract (sommario):
Mass loss, in the form of slow stellar winds, is a decisive factor for the evolution of cool luminous giants, eventually turning them into white dwarfs. These dense outflows are also a key factor in the enrichment of the interstellar medium with newly produced elements from the interior of these stars. There are strong indications that these winds are accelerated by radiation pressure on dust grains, but the actual grain species responsible for driving the outflows in M-type Asymptotic Giant Branch stars are still a matter of debate. Observations of dust features in the circumstellar environment of these stars suggest that magnesium-iron silicates are possible wind-drivers. However, the optical properties of these silicate grains are strongly influenced by the Fe-content. Fe-bearing condensates heat up strongly when interacting with the radiation field and therefore cannot form close enough to the star to trigger outflows. Fe-free condensates, on the other hand, have a low absorption cross-section at near-IR wavelengths where AGB stars emit most of their flux. To solve this conundrum, it has been suggested that winds of M-type AGB stars may be driven by photon scattering on Fe-free silicate grains with sizes comparable to the wavelength of the flux maximum, rather than by true absorption. In this thesis we investigate dynamical models of M-type AGB stars, using Fe-free silicates as the wind-driving dust species. According to our findings these models produce both dynamic and photometric properties consistent with observations. Especially noteworthy are the large photometric variations in the visual band during a pulsation cycle, seen both in the observed and synthetic fluxes. A closer examination of the models reveals that these variations are caused by changes in the molecular layers, and not by changes in the dust. This is a strong indication that stellar winds of M-type AGB stars are driven by dust materials that are very transparent in the visual and near-infrared wavelength regions, otherwise these molecular effects would not be visible.
20
Sandin, Christer. "A Study of Grain Drift in C Stars : Theoretical Modeling of Dust-Driven Winds in Carbon-Rich Pulsating Giant Stars". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3397.
Testo completo
21
Jennerholm, Hammar Filip. "Dust driven winds of cool giant stars : dependency on grain size". Thesis, Uppsala universitet, Teoretisk astrofysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-156814.
Testo completoAbstract (sommario):
Aim. In this project, theoretical models of dust driven winds of asymptotic giant branch (AGB) stars with effective temperatures within a range of 2400 − 3200 [K] and relative carbon-to-oxygen abundance C/O > 1 are studied. The aim is to understand if and how a detailed description of the grain size in winds of carbon rich AGB stars affects the wind formation and wind driving processes. Method. The computations were performed with a well tested FORTRAN code by calculating a grid of 60 models with different stellar parameters using grain size-dependent opacities. The results were then compared with models where the small particle approximation (SPA) had been used. Conclusions. The results indicate a certain dependency on grain size of the wind properties. The results from the computations of the majority of the models show no significant diferences however, especially not for the mass loss rates. Thus earlier computations performed using the SPA need not necessarily to be rejected.
22
Toalá, Jesus A., Lidia M. Oskinova e Richard Ignace. "On the Absence of Non-thermal X-Ray Emission around Runaway O Stars". Digital Commons @ East Tennessee State University, 2017. https://dc.etsu.edu/etsu-works/2687.
Testo completoAbstract (sommario):
Theoretical models predict that the compressed interstellar medium around runaway O stars can produce high-energy non-thermal diffuse emission, in particular, non-thermal X-ray and γ-ray emission. So far, detection of non-thermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six well-determined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward ζ Oph and BD+43°3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.
23
Smith, Nathan. "Independent signs of lower mass-loss rates for O-type stars". Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1765/.
Testo completoAbstract (sommario):
I discuss observational evidence – independent of the direct spectral diagnostics of stellar winds themselves – suggesting that mass-loss rates for O stars need to be revised downward by roughly a factor of three or more, in line with recent observed mass-loss rates for clumped winds. These independent constraints include the large observed mass-loss rates in LBV eruptions, the large masses of evolved massive stars like LBVs and WNH stars, WR stars in lower metallicity environments, observed rotation rates of massive stars at different metallicity, supernovae that seem to defy expectations of high mass-loss rates in stellar evolution, and other clues. I pay particular attention to the role of feedback that would result from higher mass-loss rates, driving the star to the Eddington limit too soon, and therefore making higher rates appear highly implausible. Some of these arguments by themselves may have more than one interpretation, but together they paint a consistent picture that steady line-driven winds of O-type stars have lower mass-loss rates and are significantly clumped.
24
Bestenlehner, Joachim Michael. "Mass-loss properties of the most massive stars in 30 Doradus". Thesis, Queen's University Belfast, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.602438.
Testo completoAbstract (sommario):
Our understanding of high-mass star evolution is tightly connected to our understanding of their mass-loss properties. The initial and final mass are significantly different as a result of mass loss. Massive stars have strong stellar winds and high ionising fluxes and are the main sources for mechanical and radiative feedback in giant H II regions. However, the wind and mass-loss properties and how they change during their evolution are not understood. To provide new observational evidences and to test current theoretical predictions, this thesis explores the mass-loss rate range from optically thin O-star winds to optically WR-star winds. In the context of the VLT-Flames Tarantula Survey we study the transition region from O-stars to very massive WNh-stars. For this study, we selected 62 0, Of, Of/WN, and WNh-stars in the Tarantula Nebula. For the analysis, we used the non-LTE radiative transfer code CMFGEN to obtain accurate stellar parameters, and to investigate the mass-loss propelties and evolution of these massive stars. We show that Of/WN-stars are transition objects from optically thin O-star winds to optically thick WR-star winds. Our sample covers an age range from 1 to 3 Myr. Of/WN and WNh-stars show hydrogen depletion at the surface on much shorter timescales (< 1 Myr) than the less luminous 0- stars. The predictions of the wind momenta for the' most luminous O-stars are overestimated and for the WNh-stars are underestimated. This is an indication that new theoretical mass-loss predictions are required for the most luminous stars. In our analysis yve could confirm the theoretical predictions which suggest a notable change of the mass-loss behaviour at a certain Eddington factor. The Eddington factor is well defined in evolutionary models. Using this mass loss-Eddington factor relation it is possible to provide more accurate mass-loss rates for new evolutionary models.
25
Fullerton, A. W., D. L. Massa e R. K. Prinja. "Revised mass-loss rates for O stars from the Pv resonance line". Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1764/.
Testo completoAbstract (sommario):
The P v λλ1118, 1128 resonance doublet is an extraordinarily useful diagnostic of O-star winds, because it bypasses the traditional problems associated with determining mass-loss rates from UV resonance lines. We discuss critically the assumptions and uncertainties involved with using P v to diagnose mass-loss rates, and conclude that the large discrepancies between massloss rates determined from P v and the rates determined from “density squared” emission processes pose a significant challenge to the “standard model” of hot-star winds. The disparate measurements can be reconciled if the winds of O-type stars are strongly clumped on small spatial scales, which in turn implies that mass-loss rates based on Hα or radio emission are too large by up to an order of magnitude.
26
Lam, Doan Duc. "The circumstellar envelope of the S-type AGB star π1 Gruis". Licentiate thesis, Uppsala universitet, Teoretisk astrofysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-327397.
Testo completo
27
Decin, L., A. M. S. Richards, T. J. Millar, A. Baudry, Beck E. De, W. Homan, N. Smith, de Sande M. Van e C. Walsh. "ALMA-resolved salt emission traces the chemical footprint and inner wind morphology of VY Canis Majoris". EDP SCIENCES S A, 2016. http://hdl.handle.net/10150/621992.
Testo completoAbstract (sommario):
Context. At the end of their lives, most stars lose a significant amount of mass through a stellar wind. The specific physical and chemical circumstances that lead to the onset of the stellar wind for cool luminous stars are not yet understood. Complex geometrical morphologies in the circumstellar envelopes prove that various dynamical and chemical processes are interlocked and that their relative contributions are not easy to disentangle. Aims. We aim to study the inner-wind structure (R < 250 R-star) of the well-known red supergiant VY CMa, the archetype for the class of luminous red supergiant stars experiencing high mass loss. Specifically, the objective is to unravel the density structure in the inner envelope and to examine the chemical interaction between gas and dust species. Methods. We analyse high spatial resolution (similar to 0 ''.024 x 0 ''.13) ALMA science verification (SV) data in band 7, in which four thermal emission lines of gaseous sodium chloride (NaCl) are present at high signal-to-noise ratio. Results. For the first time, the NaCl emission in the inner wind region of VY CMa is spatially resolved. The ALMA observations reveal the contribution of up to four different spatial regions. The NaCl emission pattern is different compared to the dust continuum and TiO2 emission already analysed from the ALMA SV data. The emission can be reconciled with an axisymmetric geometry, where the lower density polar/rotation axis has a position angle of similar to 50 degrees measured from north to east. However, this picture cannot capture the full morphological diversity, and discrete mass ejection events need to be invoked to explain localized higher-density regions. The velocity traced by the gaseous NaCl line profiles is significantly lower than the average wind terminal velocity, and much slower than some of the fastest mass ejections, signalling a wide range of characteristic speeds for the mass loss. Gaseous NaCl is detected far beyond the main dust condensation region. Realising the refractory nature of this metal halide, this hints at a chemical process that prevents all NaCl from condensing onto dust grains. We show that in the case of the ratio of the surface binding temperature to the grain temperature being similar to 50, only some 10% of NaCl remains in gaseous form while, for lower values of this ratio, thermal desorption efficiently evaporates NaCl. Photodesorption by stellar photons does not seem to be a viable explanation for the detection of gaseous NaCl at 220 R-star from the central star, so instead, we propose shock-induced sputtering driven by localized mass ejection events as an alternative. Conclusions. The analysis of the NaCl lines demonstrates the capabilities of ALMA to decode the geometric morphologies and chemical pathways prevailing in the winds of evolved stars. These early ALMA results prove that the envelopes surrounding evolved stars are far from homogeneous, and that a variety of dynamical and chemical processes dictate the wind structure.
28
Clayton, Matthew. "Dynamical mass loss from unstable giants". Thesis, University of Oxford, 2018. http://ora.ox.ac.uk/objects/uuid:3aa54f27-e13e-4b16-bafd-28b3e7059e8d.
Testo completoAbstract (sommario):
Giant stars are believed to lose significant fractions of their total mass over their lifetimes, but the mechanisms responsible for this are ill-understood. One possible mechanism is dynamical mass loss - a hydrodynamical process in which matter is ejected from the stellar surface in ballistic outflows. In this thesis, dynamical mass loss is studied in three stellar regimes: common-envelope objects, asymptotic giant branch stars, and red supergiants. Using hydrodynamical simulations performed with the stellar evolution code MESA, we examine the dynamical behaviour and stability of stars in each of these regimes. We examine the dynamical properties of common-envelope objects during the slow spiral-in phase using a parameterised 1-dimensional model of orbital dissipatory heating. We find that the envelope becomes unstable to high-amplitude dynamical pulsations that can lead to repeated mass-ejection events capable of removing the entire envelope and terminating the common-envelope phase. We estimate this process's α efficiency value and suggest how these results might be employed in parameterised common-envelope models. We employ coupled evolutionary and hydrodynamical simulations of AGB stars to study their dynamical properties as they traverse the TP-AGB and examine their dependence on basic stellar properties and on the thermal pulse cycle. We find that these models experience large amounts of dynamical mass loss, and we construct a parameterised model to estimate its strength. We find that this model is successful at locating the termination of the AGB. We apply a similar approach to a study of RSGs, and find that dynamical mass loss also emerges in this regime. We estimate the conditions under which this occurs and discuss how this mechanism may resolve theoretical problems relating to the Humphreys-Davidson limit and the progenitors of SNe IIn. We conclude that dynamical mass loss is likely to form a vital part of the mass-loss histories of cool giant stars.
29
Shivvers, Isaac, WeiKang Zheng, Dyk Schuyler D. Van, Jon Mauerhan, Alexei V. Filippenko, Nathan Smith, Ryan J. Foley et al. "The nearby Type Ibn supernova 2015G: signatures of asymmetry and progenitor constraints". OXFORD UNIV PRESS, 2017. http://hdl.handle.net/10150/625796.
Testo completoAbstract (sommario):
We present the results of an extensive observational campaign on the nearby Type Ibn SN 2015G, including data from radio through ultravioletwavelengths. SN2015Gwas asymmetric, showing late-time nebular lines redshifted by similar to 1000 km s(-1). It shared many features with the prototypical SN Ibn 2006jc, including extremely strong He I emission lines and a late-time blue pseudo-continuum. The young SN 2015G showed narrow P-Cygni profiles of He I, but never in its evolution did it showany signature of hydrogen -arguing for a dense, ionized and hydrogenfree circumstellar medium moving outward with a velocity of similar to 1000 km s(-1) and created by relatively recent mass-loss from the progenitor star. Ultraviolet through infrared observations show that the fading SN 2015G (which was probably discovered some 20 d post-peak) had a spectral energy distribution that was well described by a simple, single-component blackbody. Archival HST images provide upper limits on the luminosity of SN 2015G's progenitor, while non-detections of any luminous radio afterglow and optical non-detections of outbursts over the past two decades provide constraints upon its mass-loss history.
30
Smith, Nathan, Charles D. Kilpatrick, Jon C. Mauerhan, Jennifer E. Andrews, Raffaella Margutti, Wen-Fai Fong, Melissa L. Graham et al. "Endurance of SN 2005ip after a decade: X-rays, radio and Hα like SN 1988Z require long-lived pre-supernova mass-loss". OXFORD UNIV PRESS, 2017. http://hdl.handle.net/10150/623937.
Testo completoAbstract (sommario):
Supernova (SN) 2005ip was a Type IIn event notable for its sustained strong interaction with circumstellar material (CSM), coronal emission lines and infrared (IR) excess, interpreted as shock interaction with the very dense and clumpy wind of an extreme red supergiant. We present a series of late- time spectra of SN 2005ip and a first radio detection of this SN, plus late-time X-rays, all of which indicate that its CSM interaction is still strong a decade post- explosion. We also present and discuss new spectra of geriatric SNe with continued CSM interaction: SN 1988Z, SN 1993J and SN 1998S. From 3 to 10 yr post- explosion, SN 2005ip's Ha luminosity and other observed characteristics were nearly identical to those of the radio- luminous SN 1988Z, and much more luminous than SNe 1993J and 1998S. At 10 yr after explosion, SN 2005ip showed a drop in Ha luminosity, followed by a quick resurgence over several months. We interpret this Ha variability as ejecta crashing into a dense shell located less than or similar to 0.05 pc from the star, which may be the same shell that caused the IR echo at earlier epochs. The extreme Ha luminosities in SN 2005ip and SN 1988Z are still dominated by the forward shock at 10 yr post- explosion, whereas SN 1993J and SN 1998S are dominated by the reverse shock at a similar age. Continuous strong CSM interaction in SNe 2005ip and 1988Z is indicative of enhanced mass- loss for similar to 10(3) yr before core collapse, longer than Ne, O or Si burning phases. Instead, the episodic mass- loss must extend back through C burning and perhaps even part of He burning.
31
Neilson, Hilding, Richard Ignace, Beverly Smith, Gary Henson e Alyssa Adams. "Evidence of a Mira-like tail and bow shock about the semi-regular variable V CVn from four decades of polarization measurements". Digital Commons @ East Tennessee State University, 2014. https://dc.etsu.edu/etsu-works/6233.
Testo completoAbstract (sommario):
Polarization is a powerful tool for understanding stellar atmospheres and circumstellar environments. Mira and semi-regular variable stars have been observed for decades and some are known to be polarimetrically variable, however, the semi-regular variable V Canes Venatici displays an unusually large, unexplained amount of polarization. We present ten years of optical polarization observations obtained with the HPOL instrument, supplemented by published observations spanning a total interval of about forty years for V CVn. We find that V CVn shows large polarization variations ranging from 1 - 6%. We also find that for the past forty years the position angle measured for V CVn has been virtually constant suggesting a long-term, stable, asymmetric structure about the star. We suggest that this asymmetry is caused by the presence of a stellar wind bow shock and tail, consistent with the star's large space velocity.
32
Hirschi, Raphael. "The impact of reduced mass loss rates on the evolution of massive stars". Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1761/.
Testo completoAbstract (sommario):
Mass loss is a very important aspect of the life of massive stars. After briefly reviewing its importance, we discuss the impact of the recently proposed downward revision of mass loss rates due to clumping (difficulty to form Wolf-Rayet stars and production of critically rotating stars). Although a small reduction might be allowed, large reduction factors around ten are disfavoured.
We then discuss the possibility of significant mass loss at very low metallicity due to stars reaching break-up velocities and especially due to the metal enrichment of the surface of the star via rotational and convective mixing. This significant mass loss may help the first very massive stars avoid the fate of pair-creation supernova, the chemical signature of which is not observed in extremely metal poor stars. The chemical composition of the very low metallicity winds is very similar to that of the most metal poor star known to date, HE1327-2326 and offer an interesting explanation for the origin of the metals in this star.
We also discuss the importance of mass loss in the context of long and soft gamma-ray bursts and pair-creation supernovae. Finally, we would like to stress that mass loss in cooler parts of the HR-diagram (luminous blue variable and yellow and red supergiant stages) are much more uncertain than in the hot part. More work needs to be done in these areas to better constrain the evolution of the most massive stars.
33
Loon, Jacobus Theodorus van. "Mass loss and evolution of asymptotic giant branch stars in the Magellanic Clouds". [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 1999. http://dare.uva.nl/document/91817.
Testo completo
34
Kasliwal, Mansi M., John Bally, Frank Masci, Ann Marie Cody, Howard E. Bond, Jacob E. Jencson, Samaporn Tinyanont et al. "SPIRITS: Uncovering Unusual Infrared Transients with Spitzer". IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624045.
Testo completoAbstract (sommario):
We present an ongoing, five-year systematic search for extragalactic infrared transients, dubbed SPIRITS-SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag yr(-1) to > 7 mag yr(-1). SPRITEs occur in star-forming galaxies. We present an indepth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a protostellar merger.
35
Lau, Ryan M., Mansi M. Kasliwal, Howard E. Bond, Nathan Smith, Ori D. Fox, Robert Carlon, Ann Marie Cody et al. "RISING FROM THE ASHES: MID-INFRARED RE-BRIGHTENING OF THE IMPOSTOR SN 2010da IN NGC 300". IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/624070.
Testo completoAbstract (sommario):
We present multi-epoch mid-infrared (IR) photometry and the optical discovery observations of the "impostor" supernova (SN) 2010da in NGC. 300 using new and archival Spitzer Space Telescope images and ground-based observatories. The mid-infrared counterpart of SN. 2010da was detected as Spitzer Infrared Intensive Transient Survey (SPIRITS). 14bme in the SPIRITS, an ongoing systematic search for IR transients. Before erupting on 2010 May 24, the SN. 2010da progenitor exhibited a constant mid-IR flux at 3.6 and only a slight similar to 10% decrease at 4.5 mu m between 2003 November and 2007 December. A sharp increase in the 3.6 mu m flux followed by a rapid decrease measured similar to 150 days before and similar to 80 days after the initial outburst, respectively, reveal a mid-IR counterpart to the coincident optical and high luminosity X-ray outbursts. At late times, after the outburst (similar to 2000 days), the 3.6 and 4.5 mu m emission increased to over a factor of two. times the progenitor flux and is currently observed (as of 2016 Feb) to be fading, but still above the progenitor flux. We attribute the re-brightening mid-IR emission to continued dust production and increasing luminosity of the surviving system associated with SN. 2010da. We analyze the evolution of the dust temperature (T-d similar to 700-1000 K), mass (Md similar to 0.5-3.8 x. 10(-7) M circle dot), luminosity (L-IR similar to 1.3-3.5 x 10(4) L circle dot), and the equilibrium temperature radius (R-eq similar to 6.4-12.2 au) in order to resolve the nature of SN. 2010da. We address the leading interpretation of SN. 2010da as an eruption from a luminous blue variable high-mass X-ray binary (HMXB) system. We propose that SN. 2010da is instead a supergiant (sg)B[e]-HMXB based on similar luminosities and dust masses exhibited by two other known sgB[e]-HMXB systems. Additionally, the SN. 2010da progenitor occupies a similar region on a mid-IR color-magnitude diagram (CMD) with known sgB[e] stars in the Large Magellanic Cloud. The lower limit estimated for the orbital eccentricity of the sgB[e]-HMXB (e > 0.82) from X-ray luminosity measurements is high compared to known sgHMXBs and supports the claim that SN. 2010da may be associated with a newly formed HMXB system.
36
Lowe, Krispian Tom Edward. "Infrared polarimetry and integral field spectroscopy of post-asymptotic giant branch stars". Thesis, University of Hertfordshire, 2008. http://hdl.handle.net/2299/2449.
Testo completoAbstract (sommario):
In this thesis, I present the properties of IRAS 19306+1407 central source and its sur- rounding circumstellar envelope (CSE), from the analysis of near-infrared (near-IR) polarimetry and integral field spectroscopy (IFS), with supporting archived HST im- ages and sub-millimetre (sub-mm) photometry. This is supported by axi-symmetric light scattering (ALS), axi-symmetric radiative transfer (DART) and molecular hy-drogen (H2) shock models. The polarimetric images show that IRAS 19306+1407 has a dusty torus, which deviates from axisymmetry and exhibits a ‘twist’ feature. The DART and ALS modelling shows that the CSE consists of Oxygen-rich sub-micrometre dust grains, with a range in temperature from 130±30 to 40±20 K at the inner and outer radius, respectively, with inner and outer radii of 1.9±0.1×1014 and 2.7±0.1×1015 m. The CSE detached 400±10 years ago and the mass loss lasted 5700±160 years, assuming a constant asymptotic giant branch (AGB) outflow speed of 15 km s−1. The dust mass and total mass of the CSE is 8.9±5×10−4 and 1.8±1.0×10 −1 M⊙, assuming a gas-to-dust ratio of 200. The mass loss rate was 3.4±2.1×10−5 M⊙ year−1. The central source is consistent with a B1I-type star with a radius of 3.8 ± 0.6 R⊙, luminosity of 4500 ± 340 L⊙ at a distance of 2.7 ± 0.1 kpc. A purpose built idl package (fus) was developed and used in the SINFONI IFS data critical final reduction steps. It also produced emission line, kinematic and line ratio images. The IFS observations show that H2 is detected throughout the CSE, located in bright arcs and in the bipolar lobes. The velocity of the H2 is greatest at the end of the lobes. Brγ emission originates from, or close, to the central source – produced by a fast jump (J) shock or photo-ionised atomic gas. The 1-0 S(1)/2- 1 S(1) and 1-0 S(1)/3-2 S(3) ratios were used as a diagnostic and determined that H2 was excited by bow shaped shocks; however, these shock models could not wholly explain the observed rotational and vibrational temperatures. The CDR values were fitted by combining continuous (C) or J-bow shock and fluorescence models, with a contribution from the latter, observed throughout the CSE (5–77 per cent). The majority of shock can be described by a C-bow shock model with B = 0.02 to 1.28 mG. Shocks are predominately seen in the equatorial regions. Polarimetry and IFS highlight a ‘twist’ feature, which could be due to an episodic jet undergoing a recent change in the outflow direction. The sub-arcsecond IFS observations reveal a flocculent structure in the south- east bright arc, consisting of several clumps interpreted as a fast-wind eroding an equatorial torus, possibly forming H2 knots seen in (some) evolved planetary nebulae (PNe). My analysis has effectively constrained the following: spectral type, stellar radius, luminosity and distance, chemistry, dust grain properties, geometry, age, mass loss, excitation mechanism and evolutionary state of the post-AGB star and its surrounding CSE. I conclude that IRAS 19306+1407 is a post-AGB object on the verge becoming a PN.
37
Groh, J. H., D. J. Hillier e A. Damineli. "Mass-loss rate and clumping in LBV stars : the impact of time-dependent effects". Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1777/.
Testo completoAbstract (sommario):
This paper outlines a newly-developed method to include the effects of time variability in the radiative transfer code CMFGEN. It is shown that the flow timescale is often large compared to the variability timescale of LBVs. Thus, time-dependent effects significantly change the velocity law and density structure of the wind, affecting the derivation of the mass-loss rate, volume filling factor, wind terminal velocity, and luminosity. The results of this work are directly applicable to all active LBVs in the Galaxy and in the LMC, such as AG Car, HR Car, S Dor and R 127, and could result in a revision of stellar and wind parameters. The massloss rate evolution of AG Car during the last 20 years is presented, highlighting the need for time-dependent models to correctly interpret the evolution of LBVs.
38
Maercker, Matthias. "Asymptotic Giant Branch stars viewed up-close and far-off the physics, chemistry, and evolution of their circumstellar envelopes /". Doctoral thesis, Stockholm : Department of Astronomy, Stockholm University, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-31425.
Testo completoAbstract (sommario):
Diss. (sammanfattning) Stockholm : Stockholms universitet, 2010.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2:submitted. Paper 4: in progress. Härtill 4 uppsatser.
39
Mattsson, Lars. "On the Winds of Carbon Stars and the Origin of Carbon : A Theoretical Study". Doctoral thesis, Uppsala universitet, Astronomi och rymdfysik, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-99593.
Testo completoAbstract (sommario):
Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy. In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold. It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses. The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction. Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.
40
de, Koter A., J. S. Vink e L. Muijres. "Constraints on wind clumping from the empirical mass-loss vs. metallicity relation for early-type stars". Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1773/.
Testo completoAbstract (sommario):
We present the latest results on the observational dependence of the mass-loss rate in stellar winds of O and early-B stars on the metal content of their atmospheres, and compare these with predictions. Absolute empirical rates for the mass loss of stars brighter than 10$^{5.2} L_{odot}$, based on H$alpha$ and ultraviolet (UV) wind lines, are found to be about a factor of two higher than predictions. If this difference is attributed to inhomogeneities in the wind this would imply that luminous O and early-B stars have clumping factors in their H$alpha$ and UV line forming regime of about a factor of 3--5. The investigated stars cover a metallicity range $Z$ from 0.2 to 1 $Z_{odot}$. We find a hint towards smaller clumping factors for lower $Z$. The derived clumping factors, however, presuppose that clumping does not impact the predictions of the mass-loss rate. We discuss this assumption and explain how we intend to investigate its validity in more detail.
41
Felli, Derek Sean. "The Morphology and Uniformity of Circumstellar OH/H2O Masers around OH/IR Stars". BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6633.
Testo completoAbstract (sommario):
Even though low mass stars (< 8 solar masses) vastly outnumber high mass stars (< 8 solar masses), the more massive stars drive the chemical evolution of galaxies from which the next generation of stars and planets can form. Understanding mass loss of asymptotic giant branch stars contributes to our understanding of the chemical evolution of the galaxy, stellar populations, and star formation history. Stars with mass < 8 solar masses form planetary nebulae, while those with mass < 8 solar masses go supernova. In both cases, these stars enrich their environments with elements heavier than simple hydrogen and helium molecules. While some general info about how stars die and form planetary nebulae are known, specific details are missing due to a lack of high-resolution observations and analysis of the intermediate stages. For example, we know that mass loss in stars creates morphologically diverse planetary nebulae, but we do not know the uniformity of these processes, and therefore lack detailed models to better predict how spherically symmetric stars form asymmetric nebulae. We have selected a specific group of late-stage stars and observed them at different scales to reveal the uniformity of mass loss through different layers close to the star. This includes observing nearby masers that trace the molecular shell structure around these stars. This study revealed detailed structure that was analyzed for uniformity to place constraints on how the mass loss processes behave in models. These results will feed into our ability to create more detailed models to better predict the chemical evolution of the next generation of stars and planets.
42
Liermann, A., e Wolf-Rainer Hamann. "Clumping in Galactic WN stars : a comparison of mass loss rates from UV/optical & radio diagnostics". Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1781/.
Testo completoAbstract (sommario):
The mass loss rates and other parameters for a large sample of Galactic WN stars have been revised by Hamann et al. (2006), using the most up-to date Potsdam Wolf-Rayet (PoWR) model atmospheres. For a sub-sample of these stars exist measurements of their radio free-free emission. After harmonizing the adopted distance and terminal wind velocities, we compare the mass loss rates obtained from the two diagnostics. The differences are discussed as a possible consequence of different clumping contrast in the line-forming and radio-emitting regions.
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Goldman, Steve. "The metallicity dependence of maser emission and mass loss from red supergiants and asymptotic giant branch stars". Thesis, Keele University, 2017. http://eprints.keele.ac.uk/4258/.
Testo completoAbstract (sommario):
Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) are some of the largest contributors of dust and material back to the Universe. While a substantial amount of RSG dust will be destroyed by the subsequent supernova, the dust from AGB stars will be injected into the Interstellar Medium. Understanding the contribution of these stars and how it is affected by changes in stellar parameters is critically important to determining stellar lifetimes, constraining stellar models, and understanding the evolutionary paths of core-collapse progenitors and their subsequent supernova. This study has discovered four new circumstellar masers in the Large Magellanic Cloud (LMC), and increased the number of reliable wind speeds found outside of our galaxy from 5 to 13. Results have been used to develop a relation for the wind speed as a function of luminosity and metallicity. A further analysis of the spectral energy distributions of these and Galactic OH/IR stars has led to the development of a robust empirical mass-loss prescription that uses luminosity, pulsation period and gas-to-dust ratio, a measure of metallicity. These results suggest that mass loss in this phase is (nearly) independent of metallicity between a half and twice solar metallicity. A radio survey of evolved stars in the Small Magellanic Cloud (SMC) has found that none of the most luminous sources expected to show OH maser emission, do so. While sources may lie just below both the detection threshold and/or the required OH column density, a number of sources should still exhibit OH maser emission. It is possible that the OH masing phase of lower metallicity massive AGB stars and RSGs is cut short. New observations in the radio, optical and infra-red have been used to constrain the location and mass-loss rate of the prominent dust producing LMC RSGs, IRAS 05280−6910 and IRAS 05346−6949. These results suggest that these sources may exhibit a dusty torus geometry, but confirmation will require further high angular resolution studies. The results of this work have made progress in understanding the wind driving and mass loss mechanism, and have set the stage for much larger upcoming surveys with the SKA and its pathfinders ASKAP and MeerKAT.
44
Crause, Lisa. "Investigation of the mass loss, circumstellar environment and evolution of the hydrogen-deficient stars and V838 Mon". Doctoral thesis, University of Cape Town, 2006. http://hdl.handle.net/11427/4417.
Testo completo
45
Marshall, Jonathan. "Mass loss from dust-enshrouded Asymptotic Giant Branch stars and red supergiants in the Large Magellanic Cloud". Thesis, Keele University, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.423432.
Testo completoAbstract (sommario):
The process of mass-loss from evolved stars is the single largest contributor of matter back into the ISM. Intense mass-loss during the AGB phase of low-intermediate mass stars via a radiatively-driven wind can lead the stars to become enshrouded in an optically-thick layer of dust which condenses out of an extended molecular atmosphere. This thesis attempts to gain further insights into the mass-loss process that is presently poorly understood. We used the Parkes radio telescope to observe dust-enshrouded AGB stars and supergiants in the LMC and SMC, deriving the speed of the superwind from the doublepeaked OH maser profiles. Out of 8 targets in the LMC we detected 5, of which 3 are new detections. Our results confirm the simple theory for radiatively driven winds, this verifies the scaling relations we use in determining mass-loss rates and allows us to speculate on the chemical enrichment at different metallicities. From investigating mass-loss from clusters in the Magellanic Clouds we find that the mass-loss rate increases with larger progenitor mass, possibly due to a dependence on the initial metallicity or the stellar luminosity. We investigate the dust-enshrouded carbon star LI-LMC 1813 in more depth and derive an accurate mass-loss rate and the stellar parameters, mass and metallicity. It is now one of the few AGB stars currently undergoing the superwind phase for which values for the fundamental astrophysical parameters are known. With the ESO Very Large Telescope we obtained 3-4/-Lm spectra of IR stars in the LMC. 28 of 30 targets are identified as carbon stars, significantly adding to the known population of optically invisible carbon stars in the LMC. We find evidence for a high abundance of C2H2, suggestive of high carbon-to-oxygen abundance ratios at the low metallicity which would explain the large population of carbon stars.
46
Hubrig, S., M. Schöller, A. Kholtygin, H. Tsumura, A. Hoshino, S. Kitamoto, L. Oskinova, Richard Ignace, H. Todt e I. Ilyin. "New Multiwavelength Observations of the Of?p Star CPD -28◦ 2561". Digital Commons @ East Tennessee State University, 2015. https://dc.etsu.edu/etsu-works/6241.
Testo completoAbstract (sommario):
A rather strong mean longitudinal magnetic field of the order of a few hundred gauss was detected a few years ago in the Of?p star CPD −28° 2561 using FORS2 (FOcal Reducer low dispersion Spectrograph 2) low-resolution spectropolarimetric observations. In this work, we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS 2 mounted at the 8-m Antu telescope of the Very Large Telescope (VLT). These observations cover a little less than half of the stellar rotation period of 73.41 d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole Bd is larger than 1.15 kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD −28° 2561 with that of another Of?p star, HD 148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log LX/Lbol ≈ −5.7. The low-resolution X-ray spectra reveal the presence of a plasma heated up to 24 MK. We associate the 24 MK plasma in CPD −28° 2561 with the presence of a kG strong magnetic field capable to confine stellar wind.
47
Crager, Kelly Eugene. "Lone Star under the Rising Sun: Texas's "Lost Battalion," 2nd Battalion, 131st Field Artillery Regiment, During World War II". Thesis, University of North Texas, 2005. https://digital.library.unt.edu/ark:/67531/metadc4737/.
Testo completoAbstract (sommario):
In March 1942, the 2nd Battalion, 131st Field Artillery Regiment, 36th Division, surrendered to the Japanese Imperial Army on Java in the Dutch East Indies. Shortly after the surrender, the men of the 2nd Battalion were joined as prisoners-of-war by the sailors and Marines who survived the sinking of the heavy cruiser USS Houston. From March 1942 until the end of World War II, these men lived in various Japanese prison camps throughout the Dutch East Indies, Southeast Asia, and in the Japanese home islands. Forced to labor for their captors for the duration of the conflict, they performed extremely difficult tasks, including working in industrial plants and mining coal in Japan, and most notably, constructing the infamous Burma-Thailand Death Railway. During their three-and-one-half years of captivity, these prisoners experienced brutality at the hands of the Japanese. Enduring prolonged malnutrition and extreme overwork, they suffered from numerous tropical and dietary diseases while receiving almost no medical care. Each day, these men lived in fear of being beaten and tortured, and for months at a time they witnessed the agonizing deaths of their friends and countrymen. In spite of the conditions they faced, most survived to return to the United States at war's end. This study examines the experiences of these former prisoners from 1940 to 1945 and attempts to explain how they survived.
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Kelly, Gillian Patricia. "A Taylor made star : male beauty, changes in masculinity and the 'lost' stardom of Robert Taylor, Hollywood 1934-1969". Thesis, University of Glasgow, 2015. http://theses.gla.ac.uk/6396/.
Testo completoAbstract (sommario):
Despite being a central figure of Hollywood’s Classical era, Robert Taylor can now be regarded as a ‘lost’ or forgotten screen star, an interesting paradox when considering his continued success and longevity during his career. This thesis presents the first substantial study of Taylor’s star persona, examining its initial construction and subsequent developments from 1934 to 1969. Considering ideas surrounding gender, ageing and film genre, the thesis draws on existing literature on stardom and masculinity, and examines Taylor’s persona within the histories of both Hollywood’s Classical era and mid-20th century America. This helps to place Taylor within the wider industrial, cultural and social contexts in which he worked. Taylor, I argue, presents the model for the ‘perfect’ star because of his ability to consistently fit the film industry across time. While star studies continues to concentrate on more unusual stars, I feel that it is important to discuss what makes a typical star, the kind which Taylor embodied. This typicality, however, did not prevent him enjoying a long and successful career and he remained a popular leading man for 35 years until his death. The complex paradox of Taylor’s persona remaining consistently recognisable while also developing over time allowed him to seemingly seamlessly fit changes within America and the film industry. Furthermore, as he matured he developed a sense of nostalgia strongly connected with Hollywood’s ‘Golden Age’. The findings in this thesis are drawn from extensive viewing of most of Taylor’s films and television appearances and the examination of ephemeral material, including magazine covers and press articles, in order to assess Taylor’s on- and off-screen personas and their development over time. The thesis takes a mostly chronological approach, allowing Taylor’s persona to be placed within specific historical moments. Decade overviews are presented alongside detailed case studies covering key genres Taylor worked in. Through doing this, I not only trace Taylor’s star persona over the entirety of his career, but am able to compare his career trajectory and constructed image to other (similar) male stars working at this time. Contextualising my analysis of Taylor’s star persona further allows me to develop ideas around the concepts of male beauty, men as object of the erotic gaze, white American masculinity, and the (somewhat unusual) longevity of a career initially based on a star’s good looks. Most notably, I identify Taylor’s importance to Hollywood cinema by demonstrating how a star persona like his can ‘fit’ so well, and for so long, that it almost becomes invisible, resulting in the star becoming ‘lost’ or forgotten after their career has ended.
49
Igance, Richard. "Modeling X-ray Emission Line Profiles from Massive Star Winds - A Review". Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etsu-works/2686.
Testo completoAbstract (sommario):
The Chandra and XMM-Newton X-ray telescopes have led to numerous advances in the study and understanding of astrophysical X-ray sources. Particularly important has been the much increased spectral resolution of modern X-ray instrumentation. Wind-broadened emission lines have been spectroscopically resolved for many massive stars. This contribution reviews approaches to the modeling of X-ray emission line profile shapes from single stars, including smooth winds, winds with clumping, optically thin versus thick lines, and the effect of a radius-dependent photoabsorption coefficient.
50
Pablo, Herbert, Noel Richardson, Anthony Moffat, Michael Corcoran, Tomer Shenar, Omar Benvenuto, Jim Fuller et al. "A Coordinated X-ray and Optical Campaign of the Nearest Massive Eclipsing Binary, δ Orionis Aa. III. Analysis of Optical Photometric (MOST) and Spectroscopic (Ground Based) Variations". Digital Commons @ East Tennessee State University, 2015. https://dc.etsu.edu/etsu-works/6234.
Testo completoAbstract (sommario):
We report on both high-precision photometry from the Microvariability and Oscillations of Stars (MOST) space telescope and ground-based spectroscopy of the triple system δ Ori A, consisting of a binary O9.5II+early-B(Aa1 and Aa2) with P=5.7 days, and a more distant tertiary(O9 IV P 400 years). This data was collected in concert with X-ray spectroscopy from the Chandra X-ray Observatory. Thanks to continuous coverage for three weeks, the MOST light curve reveals clear eclipses between Aa1 and Aa2 for the first time in non-phased data. From the spectroscopy, we have a well-constrained radial velocity (RV)curve of Aa1. While we are unable to recover RV variations of the secondary star, we are able to constrain several fundamental parameters of this system and determine an approximate mass of the primary using apsidal motion. We also detected second order modulations at 12 separate frequencies with spacings indicative of tidally influenced oscillations. These spacings have never been seen in a massive binary, making this system one of only a handful of such binaries that show evidence for tidally induced pulsations.