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Thorén, Patrik. „Dwarf and Subgiant Stars as Probes of Galactic Chemical and Dynamical Evolution“. Doctoral thesis, Uppsala universitet, Uppsala Astronomiska Observatorium, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-636.
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Stellar chemical abundances provide astronomers with vital information about the production of chemical elements. Some stars preserve the composition of the environment in which they were born on their surfaces. By analysing the light from a star, the abundances of elements, its age and its path in space can be derived, and translated into the language of galactic history. The spallative history of boron in the early Galaxy was reinvestigated by observations of an ultraviolet spectral line in the old star HD 140283 with the Hubble Space Telescope. The line was barely detected and the upper limit abundance derived was lower than expected, which calls for further observations of this line in halo stars. Stars evolved into subgiants were observed with the ESO CAT, La Silla, and NOT, La Palma, to deduce their usefulness for galactic evolution studies. The high resolution spectroscopy study of the 26 objects showed that these stars are indeed useful for such studies. They are more luminous than dwarf stars and their ages can be accurately derived. They do not seem to have changed their surface abundances due to their evolution into giants. Subgiants can successfully be used to observationally reach regions further from the Earth, which can remove local biases that may appear when only observing nearby dwarf stars. A NLTE investigation of neutral Ca showed that cool metal rich dwarf stars did not deviate significantly from LTE, as had earlier been suggested. By an LTE analysis of a sample of 17 such dwarfs, using recent MARCS atmospheres, synthetic spectroscopy and modern atomic line data, cool metal rich dwarfs were shown not to deviate significantly from the expected abundance patterns in a number of elements. This increases the number of potential targets for studies of galactic chemical evolution in the metal rich regime since most stars are cool.
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Thorén, Patrik. „Dwarf and subgiant stars as probes of galactic chemical and dynamical evolution /“. Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2001. http://publications.uu.se/theses/91-554-4991-3/.
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3
Ortiz, Mauricio, Sabine Reffert, Trifon Trifonov, Andreas Quirrenbach, David S. Mitchell, Grzegorz Nowak, Esther Buenzli et al. „Precise radial velocities of giant stars“. EDP SCIENCES S A, 2016. http://hdl.handle.net/10150/622444.
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Context. For over 12 yr, we have carried out a precise radial velocity (RV) survey of a sample of 373 G- and K-giant stars using the Hamilton Echelle Spectrograph at the Lick Observatory. There are, among others, a number of multiple planetary systems in our sample as well as several planetary candidates in stellar binaries. Aims. We aim at detecting and characterizing substellar and stellar companions to the giant star HD 59686 A (HR 2877, HIP 36616). Methods. We obtained high-precision RV measurements of the star HD 59686 A. By fitting a Keplerian model to the periodic changes in the RVs, we can assess the nature of companions in the system. To distinguish between RV variations that are due to non-radial pulsation or stellar spots, we used infrared RVs taken with the CRIRES spectrograph at the Very Large Telescope. Additionally, to characterize the system in more detail, we obtained high-resolution images with LMIRCam at the Large Binocular Telescope. Results. We report the probable discovery of a giant planet with a mass of m(p) sin i = 6.92(-0.24)(+0.18) M-Jup orbiting at a(p) = 1.0860(-0.0007)(+0.0006) aufrom the giant star HD 59686 A. In addition to the planetary signal, we discovered an eccentric (e(B) = 0.729(-0.003)(+0.004)) binary companionwith a mass of m(B) sin i = 0.5296(-0.0008)(+0.0011) M-circle dot orbiting at a close separation from the giant primary with a semi-major axis of a(B) = 13.56(-0.14)(+0.18) au. Conclusions. The existence of the planet HD 59686 Ab in a tight eccentric binary system severely challenges standard giant planet formation theories and requires substantial improvements to such theories in tight binaries. Otherwise, alternative planet formation scenarios such as second-generation planets or dynamical interactions in an early phase of the system's lifetime need to be seriously considered to better understand the origin of this enigmatic planet.
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Papakonstantinou, Nikolaos. „Examining LUMBA UVES pipeline spectroscopy on giant and sub-giant stars of M67“. Thesis, Uppsala universitet, Observationell astrofysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-448637.
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In this work, the efficiency of the LUMBA UVES pipeline for processing of spectroscopic observations is tested by use on 23 high-resolution spectra of the open star cluster M67. An abundance trend discovered by Gavel et al. (2019) concerning iron abundances of giant and sub-giant stars of that cluster is examined. An initial run for a set of ”Gaia FGK benchmark stars”, as described in Blanco-Cuaresma et al. (2014) and Heiter et al. (2015) helps inspect the structure, method and output of the pipeline. Through Python language programming, processes are greatly automatized and the pipeline is run for a total of 460 weak and strong iron lines of our 23-star sample. The line fitting and efficiency of the pipeline is appreciated by statistically analyzing the results and looking into individual discrepant ones. The abundance trend is reproduced while using FeI lines, unlike runs using FeII lines. Trends in abundance over line strength plots also hint at bias through the Gaia-Eso Survey (GES) microturbulence relation. Using internal Data Release 6 (iDR6) and LUMBA-derived starting parameters, log(g) - Teff plots of our sample stars agree with a previously established 4.3 Gyr cluster age. An alternate run is performed for those stars, using LUMBA-derived starting parameters. The choice of starting parameters does impact abundance derivation, but is not the primary source of persistent systematic discrepancies.
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Willems, Frits Jan. „Infrared studies of asymptotic giant branch stars“. [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 1987. http://dare.uva.nl/document/113520.
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Norgren, Ofelia. „Pulsation Properties in Asymptotic Giant Branch Stars“. Thesis, Uppsala universitet, Teoretisk astrofysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-388388.
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Asymptotic Giant Branch (AGB) stars are stars with low- to intermediate mass in a late stage in their stellar evolution. An important feature of stellar evolution is the ongoing nucleosynthesis, the creation of heavier elements. Unlike main sequence stars, the AGB stars have a thick convective envelope which makes it possible to dredge-up the heavier fused elements from the stellar core to its surface. AGB stars are also pulsating variable stars, meaning the interior expands and contracts, causing the brightness to fluctuate. These pulsations will also play a major role in the mass loss observed in these stars. The mass loss is caused by stellar winds that accelerate gas and dust from the surface of these stars and thereby chemical enrich the interstellar medium. It is important to understand the properties of these pulsations since they play a key role in how stellar winds are produced and then enrich the galaxy with heavier synthesized elements. These pulsation periods can be observed with their corresponding Light-Curves, where the periodic motion of the brightness can be clearly seen. The main goal with this project is to calculate these pulsation periods for different AGB stars and compare these values with the periods listed in the General Catalogue of Variable Stars (GCVS). The comparison between these values gives a better understanding of methods of determining these periods and the uncertainties that follow.Asymptotiska jättegrenen är en del av slutstadiet för låg- till medelmassiva stjärnor (AGB stjärnor). Ett viktigt kännetecken hos stjärnutvecklingen är den pågående nukleosyntesen, sammanslagningen av tyngre ämnen i stjärnans inre. Till skillnad mot stjärnor på huvudserien har AGB stjärnor ett tjockt konvektivt lager som gör det möjligt att dra upp dessa nybildade ämnen till stjärnans yta. AGB stjärnor är pulserande variabla stjärnor där variationer i stjärnans radie gör att ljusstyrkan varierar. Dessa pulsationer kommer även att spela en viktig roll för den massförlust som observeras hos dessa stjärnor. Massförlusten orsakas av stjärnvindar som accelererar gas och stoft från stjärnans yta och därmed kemiskt berikar det interstellära mediet. Det är viktigt att förstå dessa pulsationer eftersom de är en viktig komponent för hur stjärnvindar uppstår och sedan berikar galaxer med tyngre ämnen. Dessa pulsationsperioder kan studeras genom att observera stjärnornas ljuskurvor, där man tydligt ser det periodiska beteendet hos ljusstyrkan. Det huvudsakliga målet med detta projekt är att beräkna dessa perioder för olika AGB stjärnor och att sedan jämföra dem med värden från General Catalogue of Variable Stars (GCVS). Jämförelsen mellan dessa värden ger en bättre förståelse för metoderna som används för att bestämma dessa perioder och hur osäkra dessa värden är.
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Van, Eck Sophie. „Binary intruders among peculiar red giant stars“. Doctoral thesis, Universite Libre de Bruxelles, 1999. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211875.
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COOK, KEM HOLLAND. „ASYMPTOTIC GIANT BRANCH POPULATIONS IN COMPOSITE STELLAR SYSTEMS“. Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184208.
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This dissertation presents a technique for the identification and classification of late-type stars and for the estimation of M star metallicities. The technique uses broad-band, V and I, CCD images to identify red stars and two intermediate-band CCD images to classify these as carbon or M types. One of the intermediate passbands is centered on a TiO absorption band at 7750Å and the other is centered on a CN absorption band at 8100Å. Color-color plots of V-I versus the intermediate-band index, 77-81, clearly distinguishes carbon from M stars. Observations of both early- and late-type stars were used to define the 77-81 system based upon the intermediate-band filters. The TiO bandstrength deduced from the 77-81 color as a function of V-I color was investigated for field giants and giants in 12 globular clusters. A linear correlation between [Fe/H] and the V-I color at a given TiO bandstrength was found. This correlation can be used to estimate the metallicity of M giants. The stellar population of a field in Baade's Window was examined using this technique. Many late-M stars and no carbon stars were found. The color-color diagram for Baade's Window suggests a range of metallicities for the M giants of [Fe/H] ≈ -0.4 to > +0.2. The stellar population of the Sagittarius Dwarf Irregular galaxy (Sagdig) was examined using the 77-81 system. A method for estimating reddening based upon the color mode of foreground stars was developed for the analysis of the Sagdig data. Sagdig is estimated to be ~ 1.3 megaparsecs distant. Bright blue and red stars in Sagdig are evidence for recent star formation. Carbon stars were identified in Sagdig. They display a bimodal luminosity and color distribution which suggests distinct epochs of star forming activity between 1 and 10 Gyr ago. The spatial distribution of carbon stars and bright red stars in Sagdig shows this galaxy to be much larger than previously thought.
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Weldrake, David Thomas Fredrick, und weldrake@mpia-hd mpg de. „Giant Planets and Variable Stars in Globular Clusters“. The Australian National University. Research School of Astronomy and Astrophysics, 2005. http://thesis.anu.edu.au./public/adt-ANU20050616.191315.
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Over the last decade, 135 extrasolar planets have been discovered, the vast majority found by ongoing radial velocity searches. Of the stars sampled in these searches, 1% have `Hot Jupiter' planets associated with them. Having masses equivalent to Jupiter yet orbital periods of only a few days, this new class of planet is clearly unlike anything in our Solar System.¶
Hot Jupiters present us with an intriguing prospect. If the orientation of the planetary orbit is close to edge-on, the planet will periodically transit across the face of its star, resulting in a small drop in brightness. This transit phenomenon has been successfully used for planet detection over the last couple of years, allowing determination of the planetary radius and accurate mass estimates when coupled with radial velocity observations.¶
To aid understanding of the effect stellar environment plays on Hot Jupiter formation and survivability, this thesis presents the results of a wide-field search for transiting Hot Jupiters in the globular cluster 47 Tucanae. This cluster presents many thousands of stars in a moderate field of view and provides the perfect target for a search of this nature. One previous transit search has been made in the central core of 47 Tuc; using the HST for 8.3 continuous days, Gilliland et.al (2000) expected 17 transits yet found none. This null result suggests that either system metallicity or stellar density may be inhibiting Hot Jupiter formation or survivability in the cluster.¶
This thesis presents a search for transits with a field of view 250 times larger than the HST search and samples the uncrowded outer halo of the cluster (previously unsampled for transits), providing important constraints on the effect of environment on Hot Jupiter formation. If planets are found, then stellar density would seem responsible for the Gilliland et.al (2000) core null result. If no planets are found to a significant level, the survey would provide strong evidence that system metallicity is the dominant factor. Using the ANU 40'' (1m) telescope at Siding Spring Observatory, a 30.4 night observing run was executed and photometry was derived via differential imaging. The dataset numbers 109,000 cluster (and field) stars for photometric analysis, of which 22,000 are suitable for the transit search. With a custom-written transit detection algorithm and extensive Monte Carlo simulations to model the dataset, seven planets should be detectable if the occurrence rate of Hot Jupiters is the same in the cluster as in the Solar Neighbourhood.¶
Despite a detailed search, no transit signatures were identified. This result strongly indicates that the low metallicity of the cluster is the dominant factor inhibiting planet formation in 47 Tuc. Current results in the Solar Neighbourhood show that planet frequency is strongly biased towards stars of high metallicity. This thesis shows that the metallicity trend is likely a universal phenomenon, not only limited to the immediate Solar Neighbourhood and raises questions of whether planets were much rarer in the earlier Universe.¶
As a side result of the search, 100 variable stars were identified in the field, 69 of which are new discoveries. Subsequent analysis reveals a strong period segregation among the cluster eclipsing binaries, indicating previously unobserved dynamical effects in the cluster. Distance estimates for both 47 Tuc and the SMC are in agreement with previously published values and an independent identification of the binary period-colour relation was observed. Two binaries seem to have low-luminosity companions worthy of followup and one variable is likely a star in the early phases of planetary nebula formation. All of the results presented in this thesis have been published in three separately refereed research papers.
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Weldrake, David Thomas Frederick. „Giant planets and variable stars in globular clusters /“. View thesis entry in Australian Digital Theses Program, 2005. http://thesis.anu.edu.au/public/adt-ANU20050616.191315/index.html.
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Tarrant, Neil James. „Asteroseismology from the main sequence to giant stars“. Thesis, University of Birmingham, 2010. http://etheses.bham.ac.uk//id/eprint/736/.
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The thesis focuses on the use of asteroseismology to probe the structure of stellar sources. By observing pulsational variability, a detailed description of the interior of stars, and the determination of stellar parameters can be obtained. An extensive sample of bright K and M class giant stars were surveyed to detect the presence of any noticeable variability. Three giant stars have been studied in detail. In Arcturus (alpha Boo), no oscillations at a significant amplitude were detected. In beta UMi (Kochab) multiperiodic oscillations have been observed for the first time, allowing an asteroseismic estimate for the mass. In gamma Crucis (Gacrux) previously observed frequencies have been confirmed, and the modes shown to be sinusoidal in nature. Two main-sequence stars have been studied in detail. In the first, gamma Dor, prototype star of the class of gamma Doradus variable stars, three oscillations further to those previously known were unambiguously detected in the star. In the second, nu Eri, a beta Cephei class variable star, modes detected by an extensive previous campaign were confirmed, with determined frequencies in excellent agreement between the two studies. All results gathered should provide a valuable input to future models of these stars, and provide an interesting starting point for further, detailed studies.
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Wylie, Elizabeth Claire. „Nucleosynthesis and s-process element formation in giant stars“. Thesis, University of Canterbury. Physics and Astronomy, 2006. http://hdl.handle.net/10092/1346.
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A thorough understanding of nucleosynthesis and element formation in stars of all evolutionary phases is of vital importance in stellar astrophysics. It provides information about internal structure, conditions and nuclear processes occurring in the stellar interior. The heavy elements formed in a star throughout its life are returned to the interstellar medium through mass loss processes. New populations of stars are then formed from this previously enriched material. This continues the cycle of element recycling in the Universe and has great consequence for galactic chemical evolution. As both modelling and observing techniques advance, more surveys are required to ensure there is agreement between the two. It is hoped that when a thorough understanding of the internal processes in giant stars is reached, the evolutionary models will reproduce the observed elemental yields. This work provides an internally self-consistent analysis of the element abundances produced via nucleosynthesis and s-process element formation occurring in giant stars in different stellar environments. High resolution spectroscopic observations have been taken of Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) stars in three different stellar environments. Spectrum synthesis has been used to determine s-process element abundances for RGB stars in the Hyades open cluster, RGB and AGB stars in the globular cluster, 47 Tucanae, and AGB stars in the galactic field. It was found that the two Hyades giant studied showed solar, or near-solar, abundances of s-process elements. Enhancements in the light s-process elements, Y and Zr, of +0.02 to +0.11 were observed, while enhancements in the heavy s-process elements, La, Pr and Nd, ranged from +0.06 to +0.16. These results are consistent with previous findings of enhancements in Y of ~+0.12, and of ~+0.15 for the heavy s-process elements. The results from 47 Tucanae suggest a genuine star-to-star scatter in the s-process element abundances in the giant stars of this globular cluster. This is unexpected due to the fact that stars in a globular cluster are thought to have the same formation and chemical history. However, spreads in s-process element abundances of as much as +-0.7 dex are observed between this study and three other studies of similar stars in the same cluster. A range of field stars along the AGB phase, ranging from M to MS to S to SC, have been analysed for s-process enrichment. The observed element abundances are compared with those predicted by recent modelling of the AGB phase of evolution. Enhancements in s-process element abundances range from [s/Fe]~0.00 for M stars, to ~+0.50 for MS stars, through to ~+0.95 for S stars. The comparison of these enhancements with those predicted by modelling provides an indication of the success of these models and will enable theoreticians to further refine their understanding of the internal nucleosynthetic processes present in giant stars.
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Judge, P. G. „Ultraviolet spectroscopy of late-type giant and supergiant stars“. Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.354829.
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Sahin, Timur. „An observational study of post-asymptotic giant branch stars“. Thesis, Queen's University Belfast, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492494.
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In this thesis, we present an LTE model atmosphere analyses of a group of early B-type postasymptotic giant branch (pAGB) stars. With initial masses::; 9Mo, post-AGB stars form an important group of evolved stars and provide a unique opportunity to study stellar evolution almost on a human time-scale. Post-AGB stars have spectral types ranging from K to Band luminosities between 103 and 104Lo. These objects ended their asymptotic giant branch (AGB) evolution phase with a peliod of strong mass loss (10-7 - 1O-4Moyr-l ) and have been evolving from cooler to hotter temperatures at almost constant luminosity on a timescale of ~ 104yr. B-type pAGB stars span a wide range in effective temperature (10 000 - 30000K). Their expected surface gravities (log g) and effective temperatures (Terr ) coincide with those of B stars evolving from the main sequence. Therefore systematic observational analyses are required to to distinguish these two groups. Furthermore, post-AGB stars may be divided into four distinct groups bqsed on their chemical composition. In this thesis, groups I and II represent post-AGB stars which are very metal deficient with CjO~l and metal poor with CjO
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Bennett, Philip Desmond. „Spherically symmetric model atmospheres for late-type giant stars“. Thesis, University of British Columbia, 1991. http://hdl.handle.net/2429/30908.
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The ATHENA computer code has been developed to model the extended atmospheres of late-type giant and supergiant stars. The atmospheres are assumed to be static, spherically
symmetric and in radiative and hydrostatic equilibrium. Molecular line blanketing (for now) is handled using the simplifying assumption of mean opacities. The complete linearization method of Auer and Mihalas [7], adapted to spherical geometry, is used to solve the model system. The radiative transfer is solved by using variable Eddington factors to close the system of moment transfer equations, and the entire system of transfer
equations plus constraints is solved efficiently by arrangement into the Rybicki [83] block matrix form. The variable Eddington factors are calculated from the full angle-dependent formal solution of the radiative transfer problem using the impact parameter method of Hummer, Kunasz and Kunasz [47]. We were guided by the work of Mihalas and Hummer [72] in their development of extended models of O stars, but our method differs in the choice of the independent variable. The radius depth scale used by Mihalas and Hummer was found to fail because of the strongly temperature-dependent opacities of late-type atmospheres. Instead, we were able to achieve an exact linearization of the radius. This permitted the use of the numerically well-behaved column mass or optical
depth scales. The resulting formulation is analogous to the plane-parallel complete linearization method and reduces to this method in the compact atmosphere limit.
Models of M giants were calculated for T[formula omitted] = 3000K and 3500K with opacities of the CN, TiO and H₂0 molecules included, and the results were in general agreement with other published spherical models. These models were calculated assuming radiative equilibrium. The importance of convective energy transport was estimated by calculating the convective flux that would result from the temperature structure of the models. The
standard local mixing length theory was used for this purpose. Convection was found to be important only at depths with Ƭ[formula omitted] > 15 for the low gravity models with log g = 0, but significant out to Ƭ[formula omitted] ~ 1 at the most transparent frequencies for the higher gravity models with log g = 2. Thus, the temperature structure of the surface layers and the emergent flux for the log g = 0 models should be accurately modelled but the emergent flux for the log g = 2 models may be in error by up to 5% at the most transparent frequencies.Science, Faculty of
Physics and Astronomy, Department of
Graduate
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Jorissen, Alain. „Asymptotic giant branch stars in binaries and their progeny“. Doctoral thesis, Universite Libre de Bruxelles, 2002. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211437.
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Szigeti, László, Szabolcs Mészáros, Verne V. Smith, Katia Cunha, Nadège Lagarde, Corinne Charbonnel, D. A. García-Hernández et al. „12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791“. OXFORD UNIV PRESS, 2018. http://hdl.handle.net/10150/627135.
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Carbon isotope ratios, along with carbon and nitrogen abundances, are derived in a sample of 11 red-giant members of one of the most metal-rich clusters in the Milky Way, NGC 6791. The selected red-giants have a mean metallicity and standard deviation of [Fe/H] = +0.39 +/- 0.06 (Cunha et al. 2015). We used high-resolution H-band spectra obtained by the SDSS-IV Apache Point Observatory Galactic Evolution Experiment. The advantage of using high-resolution spectra in the H band is that lines of CO are well represented and their line profiles are sensitive to the variation of C-12/C-13. Values of the C-12/C-13 ratio were obtained from a spectrum synthesis analysis. The derived C-12/C-13 ratios varied between 6.3 and 10.6 in NGC 6791, in agreement with the final isotopic ratios from thermohaline-induced mixing models. The ratios derived here are combined with those obtained for more metal poor red-giants from the literature to examine the correlation between C-12/C-13, mass, metallicity, and evolutionary status.
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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.
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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.
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Skelton, Brooke P. „Giant H II regions in M33 /“. Thesis, Connect to this title online; UW restricted, 1999. http://hdl.handle.net/1773/5417.
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Famaey, Benoît. „Kinematics and dynamics pf giant stars in the solar neighbourhood“. Doctoral thesis, Universite Libre de Bruxelles, 2004. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211090.
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We study the motion of giant stars in the Solar neighbourhood and what they tell us about the dynamics of the Galaxy: we thus contribute to the huge project of understanding the structure and evolution of the Galaxy as a whole. We present a kinematic analysis of 5952 K and 739 M giant stars which includes for the first time radial velocity data from an important survey performed with the CORAVEL spectrovelocimeter at the Observatoire de Haute Provence. Parallaxes from the Hipparcos catalogue and proper motions from the Tycho-2 catalogue are also used.
A maximum-likelihood method, based on a bayesian approach, is applied to the data, in order to make full use of all the available stars, and to derive the kinematic properties of the subgroups forming a rich small-scale structure in velocity space. Isochrones in the Hertzsprung-Russell diagram reveal a very wide range of ages for stars belonging to these subgroups, which are thus most probably related to the dynamical perturbation by transient spiral waves rather than to cluster remnants. A possible explanation for the presence of young group/clusters in the same area of velocity space is that they have been put there by the spiral wave associated with their formation, while the kinematics of the older stars of our sample has also been disturbed by the same wave. The emerging picture is thus one of "dynamical streams" pervading the Solar neighbourhood and travelling in the Galaxy with a similar spatial velocity. The term "dynamical stream" is more appropriate than the traditional term "supercluster" since it involves stars of different ages, not born at the same place nor at the same time. We then discuss, in the light of our results, the validity of older evaluations of the Solar motion in the Galaxy.
We finally argue that dynamical modeling is essential for a better understanding of the physics hiding behind the observed kinematics. An accurate axisymmetric model of the Galaxy is a necessary starting point in order to understand the true effects of non-axisymmetric perturbations such as spiral waves. To establish such a model, we develop new galactic potentials that fit some fundamental parameters of the Milky Way. We also develop new component distribution functions that depend on three analytic integrals of the motion and that can represent realistic stellar disks.
Doctorat en sciences, Spécialisation physique
info:eu-repo/semantics/nonPublished
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Grunhut, J., G. Wade, J. Sundqvist, A. ud-Doula, C. Neiner, Richard Ignace, W. Marcolino et al. „Investigating the Spectroscopic, Magnetic and Circumstellar Variability of the O9 Subgiant Star HD 57682“. Digital Commons @ East Tennessee State University, 2013. https://dc.etsu.edu/etsu-works/6236.
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The O9IV star HD 57682, discovered to be magnetic within the context of the Magnetism in Massive Stars (MiMeS) survey in 2009, is one of only eight convincingly detected magnetic O-type stars. Among this select group, it stands out due to its sharp-lined photospheric spectrum. Since its discovery, the MiMeS Collaboration has continued to obtain spectroscopic and magnetic observations in order to refine our knowledge of its magnetic field strength and geometry, rotational period and spectral properties and variability. In this paper we report new Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) spectropolarimetric observations of HD 57682, which are combined with previously published ESPaDOnS data and archival Hα spectroscopy. This data set is used to determine the rotational period (63.5708 ± 0.0057 d), refine the longitudinal magnetic field variation and magnetic geometry (dipole surface field strength of 880 ± 50 G and magnetic obliquity of 79° ± 4° as measured from the magnetic longitudinal field variations, assuming an inclination of 60°) and examine the phase variation of various lines. In particular, we demonstrate that the Hα equivalent width undergoes a double-wave variation during a single rotation of the star, consistent with the derived magnetic geometry. We group the variable lines into two classes: those that, like Hα, exhibit non-sinusoidal variability, often with multiple maxima during the rotation cycle, and those that vary essentially sinusoidally. Based on our modelling of the Hα emission, we show that the variability is consistent with emission being generated from an optically thick, flattened distribution of magnetically confined plasma that is roughly distributed about the magnetic equator. Finally, we discuss our findings in the magnetospheric framework proposed in our earlier study.
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Allsopp, James. „Triggered and spontaneous star formation in the W3 giant molecular cloud“. Thesis, Liverpool John Moores University, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.571899.
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The thesis goes on to extend the work of Bretherton (2003) and Moore et al. (2007) on the W3 Giant Molecular Cloud, by performing NH3 follow up of a sample of the cores discovered in the 850um SCUBA map and observing the whole cloud in 13CO(J=1-0) and C18O(J=1-0). - The NH3(1,1) and NH3(2,2) observations of the SCUBA cores used the fact that NH3 only traces the densest regions of the cloud, and hence can be used to find the temperature and kinematics of the cores themselves. This was used to test if the individual cores were virially bound, and from this find if cores in the more densely star-forming region of the cloud (High-Density Layer, HDL) were more likely to be bound than those in the Low-Density Layer (LDL). There are a mixture of virially bound and unbound cores in both the HDL and the LDL but no statistical difference in ratio of these between the two regions. This has an important bearing on models of environmentally-dependent star-formation, which divide into two categories; those, such as Collect and Collapse (Dale et al., 2007), which state that external pressures create dense structure, and those such as Radiatively- Driven Instability (RD I) (Bertoldi, 1989), which state that those external pressures cause dense structure to collapse. The evidence from this thesis favours models in which dense structure is created according to the Collect and Collapse scenario.
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Karakas, Amanda I. (Amanda Irene) 1974. „Asymptotic giant branch stars : their influence on binary systems and the interstellar medium“. Monash University, School of Mathematical Sciences, 2003. http://arrow.monash.edu.au/hdl/1959.1/5570.
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Pooley, Daniel. „Spectroscopic and photometric monitoring of southern post asymptotic giant branch stars“. Thesis, University of Canterbury. Physics and Astronomy, 2003. http://hdl.handle.net/10092/5564.
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A programme of photometric and spectroscopic monitoring has been undertaken in order to investigate the characteristics of suspected post asymptotic giant branch (post-AGB) stars (AI CMi, HD 179821, SAO 209008, HD 70379, HD 95767, HD 172481, HD 168625 and CPD-59 6723).
Long time baseline BV RI photometry was obtained at the Mount John University Observatory (MJUO) over intervals of 1032 and 1463 days. This was complemented with contemporaneous high-resolution spectroscopy also acquired at MJUO.
As a group, post-AGB stars are predicted to have certain broad properties. Analyses of the light curves show that none of the observed stars have stable light and radial velocity curves. Most showed the small amplitude (∆V ~ 0.2 mag), semi-regular light variations expected for objects with high luminosity to mass ratios. Semi-regular radial velocity variations of between 10-20 km s-1 were seen in most of the stars.
Examination of the sodium D lines at high resolution showed many components, between 4 and 7 per star. These are due to a combination of interstellar, circumstellar and photospheric lines. The interstellar lines were used to place lower limits on the distances to the stars. None of the stars showed a clearly identifiable photospheric component. To account for the profiles a range of models were developed, with a combination of photospheric and circum stellar absorptions, in conjunction with chromospheric emission fitting.
Radial velocity measurements revealed generally large mean radial velocities consistent with the stars belonging to the Galactic thick disk population. HD 172481 has a mean radial velocity of
~ -83 km s-1, indicating likely Galactic halo membership.
Hα profiles all showed varying degrees of emission. The hotter stars exhibited P Cygni profiles. For cooler stars the profiles were a combination of large photospheric absorption with a smaller self-absorbed emission superimposed. This emission is caused by mass loss on the order of 10-7 Mסּ yr-1 and arises in a region within a few stellar radii of the star. The profiles are also variable due to relative motion between the three underlying components.
All but one of the stars observed showed signs of cool, 100-400 K, circumstellar material. The amount of extinction provided by this material is expected to be small, on the order of a few tenths of a magnitude. Only HD 172481 showed clear indications of circumstellar extinction. The masses of these shells were estimated from the 60 µm fluxes acquired by the IRAS satellite.
With the loose classification criteria for post-AGB stars, we have also observed some quite individual behaviour in a number of our objects. AI CMi showed light and spectrum variations similar to the RV Tauri class of variables. HD 70379 showed a variable amplitude due to a combination of two simultaneous radial oscillations. HD 172481 has a cool Mira type companion which causes the I-band light curve to vary with a consistent large amplitude (∆V ~ 0.55 mag) but with a variable amplitude in the BVR-bands.
HD 95767 proved to be a binary star with a cool companion. During the time of our observations, initial low amplitude variations due to pulsations of the primary, were disrupted by the periastron passage of the companion. This also caused emission lines to be seen in the spectrum.
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Bossini, Diego. „Asteroseismology of red giant stars : a tool for constraining stellar models“. Thesis, University of Birmingham, 2016. http://etheses.bham.ac.uk//id/eprint/7090/.
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The aim of this thesis is to study stellar evolution and asteroseimology of red-giant stars mainly from a modelling point of view, in particular the impact on core-convective-burning stars of adopting different mixing schemes. Thanks to NASA space telescope Kepler, asteroseismology of thousands of giants provided us new information related to their internal structure, that can be used for finding constraints on their cores. I used several stellar evolution codes (MESA, BaSTI, and PARSEC) to investigate the effect of different mixing schemes in the helium-core-burning stars. Comparing them with observed stars, I concluded that standard stellar models, largely used in literature, cannot describe the combined observed distribution of luminosity and period spacing. I then proposed as solution a penetrative convection model with moderate overshooting parameter. Additional tests on Kepler's open clusters (NGC6791 and NGC6819) and secondary clump stars, allowed me to revised to my mixing model.
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Stancliffe, Richard James. „The evolution and nucleosynthesis of thermally pulsing asymptotic giant branch stars“. Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615303.
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27
Herzberg, Wiebke [Verfasser], und Markus [Akademischer Betreuer] Roth. „Effects of large-scale poloidal flows on oscillation modes of subgiant stars and the Sun / Wiebke Herzberg ; Betreuer: Markus Roth“. Freiburg : Albert-Ludwigs-Universität Freiburg, 2016. http://d-nb.info/1126920746/34.
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Zinn, Joel Coyle. „Accurate red giant distances and radii with asteroseismology“. The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555582282659221.
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Bagnulo, Stefano. „Modelling of circumstellar environments around carbon and oxygen rich stars“. Thesis, Queen's University Belfast, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337050.
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30
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.
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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.
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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.
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32
Weston, Simon. „Post asymptotic giant branch and central stars of planetary nebulae in the Galactic halo“. Thesis, University of Hertfordshire, 2012. http://hdl.handle.net/2299/9042.
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Post asymptotic giant branch (post-AGB) stars, central stars of planetary nebulae (CSPNe) and planetary nebulae (PNe) are important phases of stellar evolution as the material they feedback is the seed of subsequent star formation in a galaxy. The majority of low and intermediate mass stars are expected to evolve through these channels, however, it is uncertain how many actually do, and at what rate. The Galactic halo, with its older population, provides a direct test of evolutionary models for low mass stars. Birthrate estimates of PNe are uncertain and worse still, are in contradiction with accepted white dwarf (WD) birthrate estimates. Much of the uncertainty stems from the lack of complete samples and poorly determined distance estimates. New surveys such as the Sloan Digital Sky Survey (SDSS), Galaxy Evolutionary Ex- plorer (GALEX) and the INT Photometric H® Survey (IPHAS) have discovered many new PNe and have observed the far edges of the Galaxy. Improved methods of determining distances to CSPNe are presented here, using model atmospheres, evolutionary tracks and high resolution reddening maps utilising these revolution- ary surveys. Locating the CSPN is non-trivial particularly for evolved PNe, as they are ex- tended with their central star often displaced from the centre of the nebula. There- fore, photometric criteria are required to locate the CSPN in the nebula’s field. Synthetic photometry of the CSPNe is derived from spectral energy distributions (SEDs) computed from a grid of model atmospheres covering the parameter range of CSPNe. The SEDs are convolved with filter transmission curves to compute synthetic magnitudes for a given photometric system which are then calibrated with standard stars and WDs. A further project borne out of a search for luminous central stars of faint PNe, resulted in a systematic search for post-AGB stars in the Galactic halo. In this work, new candidate halo post-AGB stars are discovered from a search through the SDSS spectroscopic database. Combined with previously identified halo post- AGB stars, including the results of a sub-sample from the Palomar-Green (PG) survey, the number of observed and predicted populations are compared. The number of observed post-AGB candidates shows a remarkable deficit to expecta- tions. A survey within a subset of the photometric database of SDSS supports the findings of the PG and SDSS spectroscopic surveys. These findings provide strong evidence for a lack of post-AGB stars in the Galac- tic halo and thick disc. A plausible explanation is that a large fraction of stars in these old, metal-poor populations are evolving via alternative channels. The implications of such a result are far reaching with knock on effects for stellar evolutionary theory, galactic evolution and extragalactic redshift estimates.
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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.
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34
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.
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Forde, Kieran Patrick. „Shock-excited molecular hydrogen in the outflows of post-asymptotic giant branch stars“. Thesis, University of Hertfordshire, 2014. http://hdl.handle.net/2299/14448.
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Since the identi cation of proto-planetary nebulae (PPNe) as transition objects between the asymptotic giant branch stars and planetary nebulae more than two decades ago, astronomers have attempted to characterise these exciting objects. Today many questions still elude a conclusive answer, partly due to the sheer diversity observed within this small subset of stellar objects, and partly due to the low numbers detected. Fortunately, many of these objects display a rich spectrum of emission/absorption lines that can be used as diagnostics for these nebulae. This dissertation presents a study of six PPNe using the relatively new (at NIR wavelengths) integral eld spectroscopy technique. This method has allowed the investigation of distinct regions of these nebulae, and in certain cases the application of magneto-hydrodynamic shock models to the data. The goal of this research has been to investigate the evolution of PPNe by detailed examination of a small sample of objects consisting of a full range of evolutionary types. Near-IR ro-vibrational lines were employed as the primary tool to tackle this problem. In all six sources the 1!0S(1) line is used to map the spatial extent of the H2. In three of these objects the maps represent the rst images of their H2 emission nebulae. In the case of the earliest-type object (IRAS 14331-6435) in this sample, the line map gives the rst image of its nebula at any wavelength. In the only M-type object (OH 231.8+4.2) in the sample, high-velocity H2 is detected in discrete clumps along the edges of the bipolar out ow, while a possible ring of slower moving H2 is found around the equatorial region. This is the rst detection of H2 in such a late-type object but due its peculiarities, it is possibly not representative of what is expected of M-type objects. In IRAS 19500-1709, an intermediate-type object, the line map shows the H2 emission to originate in clumpy structures along the edges of a bipolar shell/out ow. The remaining three objects have all been the subject of previous studies but in each case new H2 lines are detected in this work along with other emission lines (Mg ii, Na i & CO). In the case of IRAS 16594-4656, MHD shock models have been used to determine the gas density and shock velocity. Two new python modules/classes have been written. The rst one to deal with the data cubes, extract ux measurements, rebin regions of interest, and produce line maps. The second class allows the easy calculation of many important parameters, for example, excitation temperatures, column density ratio values, extinction estimates from several line-pairs, column density values, and total mass of the H2. The class also allows the production of input les for the shock tting procedure, and simulated shocks for testing this tting process. A new framework to t NIR shock models to data has been developed, employing Monte Carlo techniques and the extensive computing cluster at the University of Hertfordshire (UH). This method builds on the approach used by many other authors, with the added advantages that this framework provides a method of correctly sampling the shock model parameter space, and providing error estimates on the model t. Using this approach, data from IRAS 16594-4656 have been successfully modelled using the shock models. A full description of this class of stellar objects from such a small sample is not possible due to their diverse nature. Although H2 was detected across the full spectral vi range of post-AGB objects, the phase at which H2 emission begins is still not clear. The only M-type object in this work is a peculiar object and may not be representative of a typical post-AGB star. The H2 PPNe appear to be located at lower Galactic latitudes (b 20 ) than the total PPNe population, possibly pointing to an above average mass and hence younger age of these objects.
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DePew, Kyle David. „On the astromineralogy of the 13 [mu]m feature in the spectra of oxygen-rich AGB stars“. Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/4562.
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Thesis (M.S.)--University of Missouri-Columbia, 2006.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (February 6, 2007) Includes bibliographical references.
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Maciel, Saulo Carneiro. „Uma an?lise te?rica da evolu??o da rota??o estelar“. Universidade Federal do Rio Grande do Norte, 2007. http://repositorio.ufrn.br:8080/jspui/handle/123456789/16525.
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Made available in DSpace on 2014-12-17T15:13:41Z (GMT). No. of bitstreams: 1
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Previous issue date: 2007-10-22Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
In the past 50 years, large efforts have been made toward the understanding of the stellar evolution. In the observational context, large sets of precise measurements of projected rotational velocity were produced, in particular by the Natal and Geneva groups. From these data, it is now possible to establish the behavior of stellar rotation from the turnoff to the red giant branch. In addition, these data have shown the role of tidal effects on stellar rotation in close binary systems. Nevertheless, relatively little attention has been paid to theoretical studies on the evolution of rotation along the HR Diagram, a topic itself directly associated to the evolution of the stars. Basically, there are two reasons for such a fact, (i) spherical symmetry is not assumed, what leads to a substantial increase in the numerical complexity of equations and (ii) non rotating models have been very successful in explaining relevant observational data, including the mass-luminosity relation and chemical abundances. In spite of these facts, it is clear that considerable work remains to be done on the role of rotation in the later stages of the evolution, where clear disagreements arise from confrontations between theoretical predictions and observations. In the present work we study the evolutionary behavior of stellar rotation along the HR Diagram, taking into account constraint conditions issued from recent observational survey of rotational velocity carried out with high precision procedures and new evolutionary codes
Nos ?ltimos cinq?enta anos, grandes esfor?os foram feitos no intuito de se entender a evolu??o estelar. No contexto observacional, medidas precisas da velocidade rotacional projetada foram produzidas, em particular, pelos grupos de Natal e Genebra. Destes dados ? poss?vel no presente momento estabelecer o comportamento da rota??o estelar do turnoff at? o ramo das gigantes vermelhas. Em adicional, estes dados t?m mostrado o papel dos efeitos de mar?s na rota??o estelar em sistemas bin?rios pr?ximos. Mesmo tendo sido feitos bons avan?os c no ambito observacional, pouca aten?? tem sido dada a estudos te?ricos da evolu??o da rota??o ao longo do diagrama HR, um t?pico que est? intimamente ligado a evolu??o das estrelas. Basicamente, existem dois motivos para tal fato,(i) a n?o suposi??o de simetria esf?rica leva a um aumento substancial da complexidade num?rica das equa??es, e (ii) os modelos sem rota??o t?m tido bastante sucesso na explica??o de relevantes dados observacionais, incluindo as rela??es entre massa e luminosidade e abund?ncias qu?micas. Diante do avan?o do estudo da rota??o, ainda restam trabalhos a serem feitos sobre o seu papel em est?gios mais tardios da evolu??o, uma vez que existem discord?ncias entre predi??es te?ricas e observacionais. No presente trabalho, n?s estudamos o comportamento evolucion?rio da rota??o ao longo do diagrama HR, levando em conta condi??es de contorno baseadas em recentes medidas de velocidade rotacional obtidas a partir de procedimentos de alta precis?o e novos c?digos evolutivos
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Saffer, R. A., und J. Liebert. „Search for Close Binary Evolved Stars“. Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623897.
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We report on a search for short -period binary systems composed of pairs of
evolved stars. The search is being carried out concurrently with a program to characterize
the kinematical properties of two different samples of stars. Each sample has produced one
close binary candidate for which further spectroscopic observations are planned. We also
recapitulate the discovery of a close detached binary system composed of two cool DA
white dwarfs, and we discuss the null results of Ha observations of the suspected white
dwarf /brown dwarf system G 29-38.
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Hjort, Adam. „Future directions in the study of Asymptotic Giant Branch Stars with the James Webb Space Telescope“. Thesis, Uppsala universitet, Observationell astrofysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-303344.
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In this study we present photometric predictions for C-type Asymptotic Giant Branch Stars (AGB) stars from Eriksson et al. (2014) for the James Webb Space Telescope (JWST) and the Wide-field Infrared Survey Explorer (WISE) instruments. The photometric predictions we have done are for JWST’s general purpose wide-band filters on NIRCam and MIRI covering wavelengths of 0.7 — 21 microns. AGB stars contribute substantially to the integrated light of intermediate-age stellar popula- tions and is a substantial source of the metals (especially carbon) in galaxies. Studies of AGB stars are (among other reasons) important for the understanding of the chemical evolution and dust cycle of galaxies. Since the JWST is scheduled for launch in 2018 it should be a high priority to prepare observing strategies. With these predictions we hope it will be possible to optimize observing strategies of AGB stars and maximize the science return of JWST. By testing our method on Whitelock et al. (2006) objects from the WISE catalog and comparing them with our photometric results based on Eriksson et al. (2014) we have been able to fit 20 objects with models. The photometric data set can be accessed at: http://www.astro.uu.se/AGBmodels/I den här studien har jag gjort fotometriska förutsägelser för asymptotis- ka jättegrensstjärnor (AGB-stjärnor) av C typ från Eriksson et al. (2014) modifierade för instrument ombord på James Webb Space Telescope (JWST) och Wide-field Infrared Survey Explorer (WISE). AGB-stjärnor bidrar kraftigt till det totala ljuset av stjärnor av intermediär ålder och är också en stor källa till metaller (speciellt kol) i galaxer. Studier av AGB stjärnor är viktiga av flera anledningar, däribland för att förstå den kemiska evolutionen och stoftcykler i galaxer. JWST är planerad att skjutas upp 2018 och fram till dess bör det vara en hög prioritet att förbereda observeringsstrategier. Med den fotometriska datan i den här studien hoppas vi att användare av JWST kommer kunna optimera sina observeringsstrategier av AGB-stjärnor och få ut så mycket som möjligt av sin obseravtionstid med teleskopet. Vi har testat metoden genom att titta på objekt från Whitelock et al. (2006) i WISE-katalogen och jämföra dem med de fotometriska resultaten baserade på modellerna från Eriksson et al. (2014). På detta sett har vi lyckats matcha 20 objekt med modeller. Den fotometriska datan går att ladda ner ifrån: http://www.astro.uu.se/AGBmodels/
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Simpson, Jeffrey. „Spectral Matching for Elemental Abundances of Evolved Stars of Globular Clusters“. Thesis, University of Canterbury. Physics and Astronomy, 2013. http://hdl.handle.net/10092/8460.
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In order to understand the origin of globular clusters, large samples of their stars need to be observed and analyzed for their chemical composition. This is especially true for the complex, multimetallic cluster ω Centauri, with its large range of iron, carbon, nitrogen, oxygen, sodium and barium abundances. In order to accomplish this, an automated spectral matching pipeline was developed to determine these abundances. This thesis made use of photometry and low resolution spectroscopy to analyze the chemical composition of evolved stars in three clusters: ω Cen, 47 Tuc and NGC 6752. The latter two clusters are monometallic and selected due to their similar metallicities to the metal-rich and metal-poor stars in ω Cen. This allowed them to be used as test-cases for the spectral matching pipeline.
For ω Cen, two analyses were performed. In the first, 221 giant branch stars were selected that had known [O/Fe]. These stars showed the expected anticorrelation in [C/Fe]to [N/Fe]. In the second, spectral indices were used to estimate the oxygen abundance of the stars, leading to a determination of whether a particular star was oxygen-rich or oxygen-poor. From this a catalogue of abundances of iron, carbon and barium of 848 giant branch stars were determined, of which 557 also had well-defined nitrogen abundances. k-means clustering analysis was used to group the stars in ω Cen into four homogeneous groups based upon these abundances. These groups suggest that there were at least four main periods of star formation in the cluster. The exact order of these star formation events is not yet understood, with some models predicting the groups formed from iron-poorest to iron-richest, while others suggest the potential for iron-poorer groups to form after iron-rich groups.
These results compare well with those found from higher resolution studies and show the value of more extensive lower resolution spectral surveys. They also highlight the need for large samples of stars when working with a complex object like ω Cen.
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Wagstaff, Graham [Verfasser], und Achim [Akademischer Betreuer] Weiss. „Convective and atmospheric boundaries of asymptotic giant branch stars / Graham Wagstaff ; Betreuer: Achim Weiss“. München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2018. http://d-nb.info/1156533694/34.
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Chadney, Joshua. „Modelling of the upper atmosphere of gas-giant exoplanets irradiated by low-mass stars“. Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/28075.
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Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmosphere of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. The work descried in this thesis details the development of a new 1D ionospheric model to describe the upper atmospheres of Extrasolar Giant Plants (EGPs). The model is time-dependent and includes photo-chemistry and diffusive transport. Electron-impact ionisation processes are taken into account through coupling with a suprathermal electron transport code. Neutral composition and temperature profiles are obtained by using a thermospheric model that incorporates atmospheric escape. Atmospheres composed of H, H2, He, and their associated ions are considered. Efforts have been made to obtain accurate X-ray and Extreme Ultraviolet (EUV) spectral irradiance of the stars studied. To this effect, synthetic spectra are used originating from a detailed coronal model for three different low-mass stars of different activity levels: epsilon Eridani, AD Leonis and AU Microscopii. This work is the first study of the ionosphere of EGPs that takes into account the different spectral energy distribution of low-mass stars. In planets subjected to radiation from active stars, the transition from slow, Jeans escape to a regime of rapid hydrodynamic escape at the top of the atmosphere is found to occur at larger orbital distances than for planets around low activity stars (such as the Sun). To correctly estimate the critical orbital distance of this transition, the spectral shape of stellar XUV radiation is important. A novel method to scale the EUV region of the solar spectrum based upon stellar X-ray emission is developed in this work. This new method produces an outcome in terms of the planet's upper atmosphere and escape regime that is very similar to that obtained using a detailed coronal model of the host star. EGP ionospheres at all orbital distances and around all stars studied are dominated by the long-lived H+ ion. In addition, planets in the Jeans escape regime also have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, H3+ densities undergo significant diurnal variations, their peak value being determined by the stellar X-ray flux. In contrast, H+ densities show very little day/night variability and their value is determined by the level of stellar EUV flux. The H3+ peak in EGPs in the rapid hydrodynamic escape regime under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (C, O, etc.).
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Harries, Timothy James. „Spectropolarimetry as a probe of stellar winds“. Thesis, University College London (University of London), 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367038.
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Brogan, Róisín O'Rourke. „Testing the multi-epoch luminosity function of asymptotic giant branch stars in the Small Magellanic Cloud with VISTA“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-419074.
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The physics pertaining to the asymptotic giant branch (AGB) phase of stellar evolution has been studied for many years. However, the mechanics behind many characteristics displayed at this stage are still not fully understood. As a member of the Long Period Variable class of stars, AGB stars are invaluable in creating three-dimensional maps of the Milky Way, the Magellanic System and other galaxies with resolved stellar populations. Variable stars can be used to determine radial distances from Earth using their periodic luminosity variations. As this type of star has unknown qualities, models of AGB populations need to be calibrated with observed data. Previous research has derived a best-fitting model using the TRILEGAL code (a TRIdimensional modeL of thE GALaxy). This model was calibrated against single-epoch luminosity functions (LFs) calculated from resolved stellar populations in the Small Magellanic Cloud (SMC). With multi-epoch data now available from the VISTA survey of the Magellanic Clouds (VMC), this best-fitting model can now be compared with the LFs as they vary with time. Firstly, statistical tests are completed to measure the extent of the LF variation between epochs and from the mean LF for both the full VMC AGB catalogue and for the oxygen-rich, carbon-rich and extreme AGB classes. Statistical tests are then performed to measure the similarity between the LFs from different epochs and the simulated LFs, again for the entire sample and the three classes. This investigation shows that, while the current best-fitting model is a good approximation of many individual epochs’ AGB LFs in the SMC to within 3σ, inclusion of multi-epoch data would make for a more robust analysis. In order to do this, it would be desirable to have more epochs with deeper and regular observations that could cover full lightcurves of some of the sources. There also seems to be a statistical difference between the inner and outer areas of the SMC, perhaps due to tidal disruptions. It would be interesting to see the results of a similar study using the LMC, which is less affected by the gravitational influence of its smaller companion.This thesis was written under the supervision of Maria-Rosa Cioni at the Leibniz Institute for Astrophysics in Potsdam. The presentation was held online due to the COVID-19 pandemic.
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Call, Scott. „Long-Period Variable Stars in the Globular Cluster NGC 6553“. Bowling Green State University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1626973295690519.
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Themeẞl, Nathalie [Verfasser], Saskia [Akademischer Betreuer] Hekker, Saskia [Gutachter] Hekker und Stefan [Gutachter] Dreizler. „Asteroseismic inferences from red-giant stars / Nathalie Themeẞl ; Gutachter: Saskia Hekker, Stefan Dreizler ; Betreuer: Saskia Hekker“. Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2019. http://d-nb.info/1182677932/34.
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Themeßl, Nathalie [Verfasser], Saskia [Akademischer Betreuer] Hekker, Saskia [Gutachter] Hekker und Stefan [Gutachter] Dreizler. „Asteroseismic inferences from red-giant stars / Nathalie Themeẞl ; Gutachter: Saskia Hekker, Stefan Dreizler ; Betreuer: Saskia Hekker“. Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2019. http://d-nb.info/1182677932/34.
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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/.
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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.
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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.
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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.
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Massa, Derek, Lidi Oskinova, Raman Prinja und Richard Ignace. „Coordinated UV and X-Ray Spectroscopic Observations of the O-type Giant ξ Per: The Connection between X-Rays and Large-scale Wind Structure“. Digital Commons @ East Tennessee State University, 2019. https://dc.etsu.edu/etsu-works/5501.
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We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7 III(n)((f)) star ξ Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of ξ Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N iv λ1718 and Si iv λ1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si iv λ1402, N iv λ1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180°, on the surface of the star. We note that the presence and persistence of two spots separated by 180° suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in ξ Per.