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1
Shaviv, Nir J. "Extremely Luminous Atmospheres". International Astronomical Union Colloquium 169 (1999): 155–58. http://dx.doi.org/10.1017/s025292110007192x.
Testo completoAbstract (sommario):
AbstractWe present the effects that inhomogeneities have on radiating atmospheres. It is shown that nonuniformities in a medium induce a reduction of the effective opacity which subsequently increases the Eddington Luminosity. The most striking effect however that arises from the dependence of the opacity on the inhomogeneities, is the possibility of a phase transition, where the atmosphere energetically favors exciting horizontally propagating waves due to large fluxes.
2
de Jager, Cornells, Joost Carpay, Alex de Koter, Hans Nieuwenhuijzen e Erik Schellekens. "Atmospheric dynamics of luminous stars". International Astronomical Union Colloquium 113 (1989): 211–20. http://dx.doi.org/10.1017/s0252921100004474.
Testo completoAbstract (sommario):
AbstractA review is given of data and theories on the motion fields in super and hypergiants with special reference to LBV’s. We show that the radiative momentum flux is incapable of driving the episodical bursts of mass loss of these stars, and that there are several indications that the LBV-phenomenon is hydrodynamically driven. The sum of turbulent and radiative accelerations in the atmospheres of the most luminous stars compensates the gravitational acceleration for stars near the Humphreys-Davidson limit. This explains their atmospheric near-instability. The motion field in the atmosphere of a typical LBV consists mainly of low-order gravity waves, while acoustic waves are rapidly damped. These gravitation waves may be stochastic rather than coherently ordered. These stochastic pulsations are assumed to be responsible for the LBV phenomenon.
3
Iping, Rosina C., George Sonneborn e Derck L. Massa. "FUSE observations of Luminous Blue Variables". Symposium - International Astronomical Union 212 (2003): 208–9. http://dx.doi.org/10.1017/s0074180900212059.
Testo completoAbstract (sommario):
P Cyg, AG Car, HD 5980 and η Car were observed with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. FUSE covers the spectral range from 980 Å to 1187 Å at a resolution of 0.05 Å. In this paper we discuss the far-UV properties of these LBVs and explore their similarities and differences. The FUSE observations of P Cyg and AG Car, both spectral type B2pe, are very similar. The atmospheres of both η Car and HD 5980 appear to be somewhat hotter and have much higher ionization stages (Si iv, S iv, and P v) in the FUSE spectrum than P Cyg and AG Car. There is a very good agreement between the FUSE spectrum of P Cygni and the model atmosphere computed by John Hillier with his code cmfgen. The FUSE spectrum of η Car, however, does not agree very well with existing model spectra.
4
Forman, W., C. Jones, A. Bogdan, R. Kraft, E. Churazov, S. Randall, M. Sun, E. O’Sullivan, J. Vrtilek e P. Nulsen. "Supermassive Black Hole feedback in early type galaxies". Proceedings of the International Astronomical Union 15, S359 (marzo 2020): 119–25. http://dx.doi.org/10.1017/s1743921320004081.
Testo completoAbstract (sommario):
AbstractOptically luminous early type galaxies host X-ray luminous, hot atmospheres. These hot atmospheres, which we refer to as coronae, undergo the same cooling and feedback processes as are commonly found in their more massive cousins, the gas rich atmospheres of galaxy groups and galaxy clusters. In particular, the hot coronae around galaxies radiatively cool and show cavities in X-ray images that are filled with relativistic plasma originating from jets powered by supermassive black holes (SMBH) at the galaxy centers. We discuss the SMBH feedback using an X-ray survey of early type galaxies carried out using Chandra X-ray Observatory observations. Early type galaxies with coronae very commonly have weak X-ray active nuclei and have associated radio sources. Based on the enthalpy of observed cavities in the coronae, there is sufficient energy to “balance” the observed radiative cooling. There are a very few remarkable examples of optically faint galaxies that are 1) unusually X-ray luminous, 2) have large dark matter halo masses, and 3) have large SMBHs (e.g., NGC4342 and NGC4291). These properties suggest that, in some galaxies, star formation may have been truncated at early times, breaking the simple scaling relations.
5
Malik, Matej, Daniel Kitzmann, João M. Mendonça, Simon L. Grimm, Gabriel-Dominique Marleau, Esther F. Linder, Shang-Min Tsai e Kevin Heng. "Self-luminous and Irradiated Exoplanetary Atmospheres Explored withHELIOS". Astronomical Journal 157, n. 5 (11 aprile 2019): 170. http://dx.doi.org/10.3847/1538-3881/ab1084.
Testo completo
6
Höfner, S., S. Bladh, B. Aringer e K. Eriksson. "Dynamic atmospheres and winds of cool luminous giants". Astronomy & Astrophysics 657 (gennaio 2022): A109. http://dx.doi.org/10.1051/0004-6361/202141224.
Testo completoAbstract (sommario):
Context. The winds observed around asymptotic giant branch (AGB) stars are generally attributed to radiation pressure on dust formed in the extended dynamical atmospheres of these long-period variables. The composition of wind-driving grains is affected by a feedback between their optical properties and the resulting heating due to stellar radiation. Aims. We explore the gradual Fe enrichment of wind-driving silicate grains in M-type AGB stars to derive typical values for Fe/Mg and to test the effects on wind properties and synthetic spectra. Methods. We present new radiation-hydrodynamical DARWIN models that allow for the growth of silicate grains with a variable Fe/Mg ratio and predict mass-loss rates, wind velocities, and grain properties. Synthetic spectra and other observables are computed a posteriori with the COMA code. Results. The self-regulating feedback between grain composition and radiative heating, in combination with quickly falling densities in the stellar wind, leads to low values of Fe/Mg, typically a few percent. Nevertheless, the new models show distinct silicate features around 10 and 18 microns. Fe enrichment affects visual and near-IR photometry moderately, and the new DARWIN models agree well with observations in (J − K) versus (V − K) and Spitzer color–color diagrams. The enrichment of the silicate dust with Fe is a secondary process, taking place in the stellar wind on the surface of large Fe-free grains that have initiated the outflow. Therefore, the mass-loss rates are basically unaffected, while the wind velocities tend to be slightly higher than in corresponding models with Fe-free silicate dust. Conclusions. The gradual Fe enrichment of silicate grains in the inner wind region should produce signatures observable in mid-IR spectro-interferometrical measurements. Mass-loss rates derived from existing DARWIN models, based on Fe-free silicates, can be applied to stellar evolution models since the mass-loss rates are not significantly affected by the inclusion of Fe in the silicate grains.
7
Höfner, S., S. Bladh, B. Aringer e R. Ahuja. "Dynamic atmospheres and winds of cool luminous giants". Astronomy & Astrophysics 594 (ottobre 2016): A108. http://dx.doi.org/10.1051/0004-6361/201628424.
Testo completo
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Chakrabarty, Aritra, Sujan Sengupta e Mark S. Marley. "Polarization of Rotationally Oblate Self-luminous Exoplanets with Anisotropic Atmospheres". Astrophysical Journal 927, n. 1 (1 marzo 2022): 51. http://dx.doi.org/10.3847/1538-4357/ac4d33.
Testo completoAbstract (sommario):
Abstract The young self-luminous giant exoplanets are expected to be oblate in shape, owing to the high rotational speeds observed for some objects. Similar to the case of brown dwarfs, the thermal emission from these planets should be polarized by scatterings of molecules and condensate cloud particles, and the rotation-induced asymmetry of the planet’s disk would yield to net nonzero detectable polarization. Considering an anisotropic atmosphere, we present here a three-dimensional approach to estimating the disk-averaged polarization that arises due to the oblateness of the planets. We solve the multiple-scattering vector radiative transfer equations at each location on the planet’s disk and calculate the local Stokes vectors, and then calculate the disk-integrated flux and linear polarization. For a cloud-free atmosphere, the polarization signal is observable only in the visible wavelength region. However, the presence of clouds in the planetary atmospheres leads to a detectable amount of polarization in the infrared wavelength region where the planetary thermal emission peaks. Considering the different broadband filters of the SPHERE-IRDIS instrument of the Very Large Telescope, we present generic models for the polarization at different wavelength bands as a function of their rotation period. We also present polarization models for the exoplanets β Pic b and ROXs 42B b, as two representative cases that can guide future observations. Our insights into the polarization of young giant planets presented here would be useful for the upcoming polarimetric observations of the directly imaged planets.
9
Dorfl, Ernst A., Michael U. Feuchtinger e Alfred Gautschy. "Nonlinear Pulsations of Luminous Blue Variables". International Astronomical Union Colloquium 176 (2000): 109–12. http://dx.doi.org/10.1017/s0252921100057262.
Testo completoAbstract (sommario):
AbstractWe present the results of various nonlinear radial pulsation calculations of LBVs with different luminosity to mass ratios. Depending on the stellar parameters, most of these objects undergo strongly non-adiabatic pulsations which can also modify their stellar atmospheres. In some cases part of the kinetic energy of the motions is transferred by shock waves into the atmospheric layers leading to an overall decrease or increase of the stellar radius. This results in different mean density accompanied by a change of the pulsational behavior which can be compared to the results of a linear stability analysis. We also find that regular pulsations can occur around a different equilibrium than the initial configuration which has been determined by stellar evolution calculations. In cases of even higher luminosity to mass ratios the linear stability analysis predicts several unstable modes and the nonlinear computations can exhibit rather irregular light curves. Due to the large non-adiabaticity of the pulsation the corresponding velocity curves still remain more regular, but show increasing amplitudes until the energy stored is released. The pulsations are far from equilibrium conditions and may also lead to a pulsationally driven mass loss for certain stellar parameters.
10
Linsky, Jeffrey L. "What is the essential physics of mass loss from late-type stars?" Symposium - International Astronomical Union 122 (1987): 271–87. http://dx.doi.org/10.1017/s007418090015658x.
Testo completoAbstract (sommario):
In this review I consider what clues the data are providing us concerning the mass loss from late-type stars. I consider in turn the major classes of mass-loss mechanisms (thermally-driven winds, radiatively-driven winds, and wave-driven winds), and consider whether the empirical mass loss rates and other data are consistent with any of these mechanisms acting alone. It is likely that several mechanisms act together to produce the large mass loss rates in the Mira and non-pulsating M supergiants. Studies of the solar atmosphere suggest that thermal bifurcation driven by molecular condensation instabilities may play a critical role in cooling the atmospheres of luminous cool stars and forming silicate dust. It is possible that several metastable modes of atmospheric structure may exist for a given set of stellar parameters.
11
Boer, B., J. Carpay, A. de Koter, C. de Jager, H. Nieuwenhuijzen, A. Piters e F. Spaan. "Turbulence-Driven Atmospheric Instability and Large-Scale Motions in Super- and Hypergiants". International Astronomical Union Colloquium 108 (1988): 131–32. http://dx.doi.org/10.1017/s025292110009360x.
Testo completoAbstract (sommario):
AbstractSpectral studies of super- and hypergiants show that the (outward directed) turbulent acceleration approaches the value of the gravitational acceleration for the most luminous stars, which makes their atmospheres unstable.
12
Nieuwenhuijzen, Hans, e Cornells de Jager. "Atmospheric parameters and accelerations in the outer parts of luminous hot stars". International Astronomical Union Colloquium 113 (1989): 287–88. http://dx.doi.org/10.1017/s0252921100004668.
Testo completoAbstract (sommario):
In the atmospheres of the most extreme luminous stars, close to the Humphreys-Davidson limit, the inward gravitational acceleration is for a great part compensated by outward accelerations due to radiation pressure, turbulent pressure and dynamic pressure of the stellar winds. As a result the effective acceleration is very small, resulting in blown-up atmospheres that can no longer be considered plane-parallel or in hydrostatic equilibrium.
13
Kudritzki, R. P., A. Gabler, R. Gabler, H. G. Groth, A. W. A. Pauldrach e J. Puls. "Model atmospheres and quantitative spectroscopy of luminous blue stars". International Astronomical Union Colloquium 113 (1989): 67–82. http://dx.doi.org/10.1017/s0252921100004309.
Testo completoAbstract (sommario):
AbstractThe basic physics of model atmospheres for Luminous Blue Stars (LBS) are discussed. The discussion comprises the model description of both subsonic photospheres and supersonic winds and the application of model calculations for the purpose of quantitative spectroscopy.
14
Dermer, Charles D., e Steven J. Sturner. "Existence of scattering atmospheres near luminous, magnetized compact objects". Astrophysical Journal 382 (novembre 1991): L23. http://dx.doi.org/10.1086/186205.
Testo completo
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Gerhard, Ortwin. "Dark matter in massive galaxies". Proceedings of the International Astronomical Union 8, S295 (agosto 2012): 211–20. http://dx.doi.org/10.1017/s174392131300481x.
Testo completoAbstract (sommario):
AbstractThe spatial distributions of luminous and dark matter in massive early-type galaxies (ETGs) reflect the formation processes which shaped these systems. This article reviews the predictions of cosmological simulations for the dark and baryonic components of ETGs, and the observational constraints from lensing, hydrostatic X-ray gas atmospheres, and outer halo stellar dynamics.
16
Brunton, Ian R., Connor O’Mahoney, Brian D. Fields, Adrian L. Melott e Brian C. Thomas. "X-Ray-luminous Supernovae: Threats to Terrestrial Biospheres". Astrophysical Journal 947, n. 2 (1 aprile 2023): 42. http://dx.doi.org/10.3847/1538-4357/acc728.
Testo completoAbstract (sommario):
Abstract The spectacular outbursts of energy associated with supernovae (SNe) have long motivated research into their potentially hazardous effects on Earth and analogous environments. Much of this research has focused primarily on the atmospheric damage associated with the prompt arrival of ionizing photons within days or months of the initial outburst, and the high-energy cosmic rays that arrive thousands of years after the explosion. In this study, we turn the focus to persistent X-ray emission, arising in certain SNe that have interactions with a dense circumstellar medium and observed months and/or years after the initial outburst. The sustained high X-ray luminosity leads to large doses of ionizing radiation out to formidable distances. We assess the threat posed by these X-ray-luminous SNe for Earth-like planetary atmospheres; our results are rooted in the X-ray SN observations from Chandra, Swift-XRT, XMM-Newton, NuSTAR, and others. We find that this threat is particularly acute for SNe showing evidence of strong circumstellar interaction, such as Type IIn explosions, which have significantly larger ranges of influence than previously expected and lethal consequences up to ∼50 pc away. Furthermore, X-ray-bright SNe could pose a substantial and distinct threat to terrestrial biospheres and tighten the Galactic habitable zone. We urge follow-up X-ray observations of interacting SNe for months and years after the explosion to shed light on the physical nature and full-time evolution of the emission and to clarify the danger that these events pose for life in our galaxy and other star-forming regions.
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Gräfener, Götz, e Wolf-Rainer Hamann. "Wolf-Rayet Wind Models from Hydrodynamic Model Atmospheres". Proceedings of the International Astronomical Union 3, S250 (dicembre 2007): 63–70. http://dx.doi.org/10.1017/s1743921308020346.
Testo completoAbstract (sommario):
AbstractWe present a parameter study of WR-type mass loss, based on the PoWR hydrodynamic model atmospheres. These new models imply that optically thick WR-type winds are generally formed close to the Eddington limit. This is demonstrated for the case of hydrogen rich WNL stars, which turn out to be extremely massive, luminous stars with progenitor masses above ≈ 80 M⊙. We investigate the dependence of WR-type mass loss on various stellar parameters, including the metallicity Z. The results depend strongly on the L/M ratio, the stellar temperature T*, and the assumed wind clumping. For high L/M ratios, strong WR-type winds can be maintained down to very low Z. Even for primordial massive stars we predict considerable mass loss if their surfaces are self-enriched by primary elements.
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Bandiera, Rino, Paola Focardi, Aldo Altamore, Corinne Rossi e Otmar Stahl. "High-resolution emission line profiles in blue luminous stars". International Astronomical Union Colloquium 113 (1989): 279–80. http://dx.doi.org/10.1017/s0252921100004620.
Testo completoAbstract (sommario):
Emission lines are often observed in high luminosity stars and provide evidence of the presence of extended stellar envelopes. Ha is the most frequently observed across the H-R diagram, but lines of Hel or Fell are also found in emission in these stars. They could be used as diagnostics of the structure of their outer atmospheres and winds. High resolution (1/dl ~ 105) high S/N profiles of Ha and Hel 5876 in the galactic LBVs η Car, AG and HR Car, and in the LMC star S22 have been obtained with the ESO CAT-CES during 1984-87, and are described in Figs.1-5. We find that these stars show a large variety of profiles with narrow and broad emissions, wide or multiple blue-shifted absorptions. The profiles are largely variable. Once, a kind of inverse P Cyg profile was observed in HR Car (Fig.4). These results indicate the presence of large scale phenomena and high velocity fields which are dramatically variable in time. Continuous HIRES monitoring of these stars is urgently needed.
19
Aufdenberg, Jason P. "Line‐blanketed Spherically Extended Model Atmospheres of Hot Luminous Stars with and without Winds". Publications of the Astronomical Society of the Pacific 113, n. 779 (gennaio 2001): 119–20. http://dx.doi.org/10.1086/317970.
Testo completo
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Phillips, M. W., P. Tremblin, I. Baraffe, G. Chabrier, N. F. Allard, F. Spiegelman, J. M. Goyal, B. Drummond e E. Hébrard. "A new set of atmosphere and evolution models for cool T–Y brown dwarfs and giant exoplanets". Astronomy & Astrophysics 637 (maggio 2020): A38. http://dx.doi.org/10.1051/0004-6361/201937381.
Testo completoAbstract (sommario):
We present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which we term ATMO 2020. Atmosphere models are generated with our state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of 0.001–0.075 M⊙ objects. Our models include several key improvements to the input physics used in previous models available in the literature. Most notably, the use of a new H–He equation of state including ab initio quantum molecular dynamics calculations has raised the mass by ~1−2% at the stellar–substellar boundary and has altered the cooling tracks around the hydrogen and deuterium burning minimum masses. A second key improvement concerns updated molecular opacities in our atmosphere model ATMO, which now contains significantly more line transitions required to accurately capture the opacity in these hot atmospheres. This leads to warmer atmospheric temperature structures, further changing the cooling curves and predicted emission spectra of substellar objects. We present significant improvement for the treatment of the collisionally broadened potassium resonance doublet, and highlight the importance of these lines in shaping the red-optical and near-infrared spectrum of brown dwarfs. We generate three different grids of model simulations, one using equilibrium chemistry and two using non-equilibrium chemistry due to vertical mixing, all three computed self-consistently with the pressure-temperature structure of the atmosphere. We show the impact of vertical mixing on emission spectra and in colour-magnitude diagrams, highlighting how the 3.5−5.5 μm flux window can be used to calibrate vertical mixing in cool T–Y spectral type objects.
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Durbin, M. J., R. L. Beaton, A. J. Monson, B. Swidler e J. J. Dalcanton. "Empirical 2MASS–WFC3/IR Filter Transformations Across the H-R Diagram from Synthetic Photometry". Astronomical Journal 166, n. 6 (9 novembre 2023): 236. http://dx.doi.org/10.3847/1538-3881/acfaa1.
Testo completoAbstract (sommario):
Abstract Near-infrared bandpasses on spaceborne observatories diverge from their ground-based counterparts as they are free of atmospheric telluric absorption. Available transformations between respective filter systems in the literature rely on theoretical stellar atmospheres, which are known to have difficulties reproducing the observed spectral energy distributions of cool giants. We present new transformations between the Two Micron All Sky Survey JHK S and Hubble Space Telescope WFC3/IR F110W, F125W, and F160W photometric systems based on synthetic photometry of empirical stellar spectra from four spectral libraries. This sample comprises over 1000 individual stars, which together span nearly the full H-R diagram and sample stellar populations from the solar neighborhood out to the Magellanic Clouds, covering a broad range of ages, metallicities, and other relevant stellar properties. In addition to global color-dependent transformations, we examine band-to-band differences for cool, luminous giant stars in particular, including multiple types of primary distance indicators.
22
Gräfener, Götz, e Wolf-Rainer Hamann. "The masses of late-type WN stars". Proceedings of the International Astronomical Union 2, n. 14 (agosto 2006): 199. http://dx.doi.org/10.1017/s1743921307010125.
Testo completoAbstract (sommario):
AbstractWe present recent results for galactic WNL stars, obtained with the new Potsdam Wolf-Rayet (PoWR) hydrodynamic model atmospheres. Based on a combination of stellar wind modeling and spectral analysis we identify the galactic WNL subtypes as a group of extremely luminous stars close to the Eddington limit. Their luminosities imply progenitor masses around 120 M⊙ or even above, making them the direct descendants of the most massive stars in the galaxy. Because of the proximity to the Eddington limit our models are very sensitive to the L/M ratio, thus allowing for a direct estimate of the present masses of these objects.
23
Humphreys, Roberta M. "Luminous Blue Variables, cool hypergiants and some impostors in the H-R diagram". Symposium - International Astronomical Union 212 (2003): 38–46. http://dx.doi.org/10.1017/s0074180900211625.
Testo completoAbstract (sommario):
Current observations of the S Dor/LBVs and candidates and the implications for their important role in massive star evolution are reviewed. Recent observations of the cool hypergiants are altering our ideas about their evolutionary state, their atmospheres and winds, and the possible mechanisms for their asymmetric high mass loss episodes which may involve surface activity and magnetic fields. Recent results for IRC+10420, ρ Cas and VY CMa are highlighted. S Dor/LBVs in eruption, and the cool hypergiants in their high mass loss phases with their optically thick winds are not what their apparent spectra and temperatures imply; they are then ‘impostors’ on the H-R diagram. The importance of the very most massive stars, like η Carinae and the ‘supernovae impostors’ are also discussed.
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Liu, T., e K. A. Joens. "The Luminosities of 13 Field RR Lyrae Stars: The Correlation with Metallicities". International Astronomical Union Colloquium 111 (1989): 272. http://dx.doi.org/10.1017/s0252921100011805.
Testo completoAbstract (sommario):
AbstractWe have obtained UBVRI and JK photometry and radial velocities for 13 field RR Lyrae stars with a large range of metallicity. Our sample includes eleven ab-type and two c-type variables. The surface brightness method is applied to these RR Lyrae stars to determine their mean absolute magnitudes <MV> as well as their distances and radii. We have used the Kurucz’s model atmospheres to provide metallicity- and gravity-dependent color-temperature transformations and bolometric corrections. Our results indicate a clear correlation between the luminosities of the RR Lyrae stars and their metallicities such that metal-poor RR Lyrae stars are more luminous.
25
Zhang 张, Zhoujian 周健, Paul Mollière, Keith Hawkins, Catherine Manea, Jonathan J. Fortney, Caroline V. Morley, Andrew Skemer et al. "ELemental abundances of Planets and brown dwarfs Imaged around Stars (ELPIS). I. Potential Metal Enrichment of the Exoplanet AF Lep b and a Novel Retrieval Approach for Cloudy Self-luminous Atmospheres". Astronomical Journal 166, n. 5 (17 ottobre 2023): 198. http://dx.doi.org/10.3847/1538-3881/acf768.
Testo completoAbstract (sommario):
Abstract AF Lep A+b is a remarkable planetary system hosting a gas-giant planet that has the lowest dynamical mass among directly imaged exoplanets. We present an in-depth analysis of the atmospheric composition of the star and planet to probe the planet’s formation pathway. Based on new high-resolution spectroscopy of AF Lep A, we measure a uniform set of stellar parameters and elemental abundances (e.g., [Fe/H] = −0.27 ± 0.31 dex). The planet’s dynamical mass ( 2.8 − 0.5 + 0.6 M Jup) and orbit are also refined using published radial velocities, relative astrometry, and absolute astrometry. We use petitRADTRANS to perform chemically consistent atmospheric retrievals for AF Lep b. The radiative–convective equilibrium temperature profiles are incorporated as parameterized priors on the planet’s thermal structure, leading to a robust characterization for cloudy self-luminous atmospheres. This novel approach is enabled by constraining the temperature–pressure profiles via the temperature gradient ( d ln T / d ln P ) , a departure from previous studies that solely modeled the temperature. Through multiple retrievals performed on different portions of the 0.9–4.2 μm spectrophotometry, along with different priors on the planet’s mass and radius, we infer that AF Lep b likely possesses a metal-enriched atmosphere ([Fe/H] > 1.0 dex). AF Lep b’s potential metal enrichment may be due to planetesimal accretion, giant impacts, and/or core erosion. The first process coincides with the debris disk in the system, which could be dynamically excited by AF Lep b and lead to planetesimal bombardment. Our analysis also determines T eff ≈ 800 K, log ( g ) ≈ 3.7 dex, and the presence of silicate clouds and disequilibrium chemistry in the atmosphere. Straddling the L/T transition, AF Lep b is thus far the coldest exoplanet with suggested evidence of silicate clouds.
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Kravchenko, K., A. Chiavassa, S. Van Eck, A. Jorissen, T. Merle, B. Freytag e B. Plez. "Tomography of cool giant and supergiant star atmospheres". Astronomy & Astrophysics 632 (25 novembre 2019): A28. http://dx.doi.org/10.1051/0004-6361/201935809.
Testo completoAbstract (sommario):
Context. Red supergiants are cool massive stars and are the largest and the most luminous stars in the Universe. They are characterized by irregular or semi-regular photometric variations, the physics of which is not clearly understood. Aims. The paper aims to derive the velocity field in the red supergiant star μ Cep and to relate it to the photometric variability with the help of the tomographic method. Methods. The tomographic method allows one to recover the line-of-sight velocity distribution over the stellar disk and within different optical-depth slices. This method was applied to a series of high-resolution spectra of μ Cep, and these results are compared to those obtained from 3D radiative-hydrodynamics CO5BOLD simulations of red supergiants. Fluctuations in the velocity field are compared with photometric and spectroscopic variations, the latter were derived from the TiO band strength and serve, at least partly, as a proxy of the variations in effective temperature. Results. The tomographic method reveals a phase shift between the velocity and spectroscopic and photometric variations. This phase shift results in a hysteresis loop in the temperature – velocity plane with a timescale of a few hundred days, which is similar to the photometric one. The similarity between the hysteresis loop timescale measured in μ Cep and the timescale of acoustic waves disturbing the convective pattern suggests that such waves play an important role in triggering the hysteresis loops.
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Zinn, Joel C., Marc H. Pinsonneault, Lars Bildsten e Dennis Stello. "The effect of the adiabatic assumption on asteroseismic scaling relations for luminous red giants". Monthly Notices of the Royal Astronomical Society 525, n. 4 (11 settembre 2023): 5540–53. http://dx.doi.org/10.1093/mnras/stad2560.
Testo completoAbstract (sommario):
ABSTRACT Although stellar radii from asteroseismic scaling relations agree at the per cent level with independent estimates for main sequence and most first-ascent red giant branch (RGB) stars, the scaling relations over-predict radii at the tens of per cent level for the most luminous stars ($R \gtrsim 30 \, \mathrm{R}_{\odot }$). These evolved stars have significantly superadiabatic envelopes, and the extent of these regions increase with increasing radius. However, adiabaticity is assumed in the theoretical derivation of the scaling relations as well as in corrections to the large frequency separation. Here, we show that a part of the scaling relation radius inflation may arise from this assumption of adiabaticity. With a new reduction of Kepler asteroseismic data, we find that scaling relation radii and Gaia radii agree to within at least 2 per cent for stars with $R \lesssim 30\, \mathrm{R}_{\odot }$, when treated under the adiabatic assumption. The accuracy of scaling relation radii for stars with $50\, \mathrm{R}_{\odot }\lesssim R \lesssim 100\, \mathrm{R}_{\odot }$, however, is not better than $10~{{\ \rm per \, cent}}-15~{{\ \rm per \, cent}}$ using adiabatic large frequency separation corrections. We find that up to one third of this disagreement for stars with $R \approx 100\, \mathrm{R}_{\odot }$ could be caused by the adiabatic assumption, and that this adiabatic error increases with radius to reach 10 per cent at the tip of the RGB. We demonstrate that, unlike the solar case, the superadiabatic gradient remains large very deep in luminous stars. A large fraction of the acoustic cavity is also in the optically thin atmosphere. The observed discrepancies may therefore reflect the simplified treatment of convection and atmospheres.
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Parthasarathy, Mudumba. "Analysis of Gaia DR3 Data of Selected R Stars (Warm Carbon Stars)". Research Notes of the AAS 8, n. 12 (23 dicembre 2024): 324. https://doi.org/10.3847/2515-5172/ada1c3.
Testo completoAbstract (sommario):
Abstract An analysis of Gaia DR3 data of 67 R stars is presented and 57 of them are at high galactic latitudes. Three new variable stars are found. The long period luminous variable R stars and R8 stars may be variable carbon stars on the Asymptotic Giant Branch. The R stars seem to have luminosities throughout the Red Giant Branch range. Several R stars are found to be high velocity stars. The determination of s-process elements abundances and C12/C13 ratio of the stars listed in Table 1 will enable us to further understand the R stars phenomenon. The process by which the atmospheres of these stars are enriched with carbon is still a puzzle.
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Linsky, Jeffrey L. "Radio and X-Ray Emissions from Chemically Peculiar B- and A-Type Stars: Observations and a Model". International Astronomical Union Colloquium 138 (1993): 507–16. http://dx.doi.org/10.1017/s0252921100020972.
Testo completoAbstract (sommario):
AbstractConventional wisdom holds that early-type and late-type stars have very different outer atmospheres, because the early-type stars lack deep convective zones. I argue that the magnetic chemically peculiar (CP) stars hotter than about spectral type A2 display many of the activity phenomena seen in the most active late-type stars. In particular, many CP stars are luminous nonthermal radio and coronal x-ray sources like the RS CVn systems. A wind-fed magnetosphere model has been proposed to explain both the nonthermal radio and the x-ray emission. In this model the stellar wind plays the role of a mechanical energy source analogous to the role played by convection in the active late-type stars.
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Rossi, Corinne, Gerard Muratorio, Michael Friedjung e Roberto Viotti. "Analysis of LBV circumstellar envelopes using FeII lines". International Astronomical Union Colloquium 113 (1989): 281–82. http://dx.doi.org/10.1017/s0252921100004632.
Testo completoAbstract (sommario):
Emission and absorption lines of Fell frequently dominate the optical and UV spectra of LBVs. Many luminous blue stars possess a rich optical Fell emission line spectrum, which can be used as diagnostic of their extended atmospheres (Viotti 1976a). Since the launch ofIUEseveral LBVs were observed in UV, and in many cases (n Car, AG Car, P Cyg, Hubble Sandage variables, etc.) their spectra were found dominated by a large amount of Fell absorptions, presumably formed in cooler envelopes. Often these absorptions are so strong as to seriously affect the UV energy distribution. In order to study line formation outside LTE of such a complex ion, the Fell line intensities in luminous stars were analyzed using the Self Absorption Curve (SAC) method by Friedjung and Muratorio (1987). The fitting of the emission line fluxes in different multiplets to theoretical SACs obtained with different wind models, leads to the determination of the relative upper and also relative lower level populations. The overlapping of the individual multiplet curves gives the observational SAC which provides an estimate of the wind parameters (column density, acceleration parametera, mass loss rate). This method of analysis was applied to the optical and UV high resolution spectra of several luminous stars. For instance we found that in AG Car the relative population of the Fell levels from 1.6 to 8.6 eV can be fitted to the same Boltzmann distribution with an excitation temperature of Tex«=7200+/-350K, whereas for the W Cep star KQ Pup the linearity extends from 0 to about 9 eV with Tex=6700 K (Viotti et al. 1988).
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Güdel, Manuel, e Edward F. Guinan. "The Sun in Time: Evolution of Coronae of Solar-Type Stars". International Astronomical Union Colloquium 152 (1996): 519–24. http://dx.doi.org/10.1017/s0252921100036484.
Testo completoAbstract (sommario):
We report on the results of a multi-frequency program to study coronal X-ray, EUV, and microwave activity of solar-type (G0-5 V) stars of greatly different ages. These stars are of interest as proxies for the Sun for ages from 50 Myr to 10 Gyr. Coronal temperatures decrease with X-ray/EUV luminosity and with increasing age and rotation period according to power-law relations. The young Sun had an extremely luminous corona with LEUV,X = 100 − 600LEUV,X (present Sun) and temperatures up to 10 MK; this information is pivotal for the study of the young planetary atmospheres. The findings further suggest an intimate connection between the presence of very hot coronal plasma and non-thermal radio-emitting particles.
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Höfner, Susanne, e Bernd Freytag. "Exploring the origin of clumpy dust clouds around cool giants". Astronomy & Astrophysics 623 (marzo 2019): A158. http://dx.doi.org/10.1051/0004-6361/201834799.
Testo completoAbstract (sommario):
Context. Dust grains forming in the extended atmospheres of AGB stars are critical for the heavy mass loss of these cool luminous giants, as they provide radiative acceleration for the stellar winds. Characteristic mid-IR spectral features indicate that the grains consist mainly of silicates and corundum. The latter species seems to form in a narrow zone within about 2 stellar radii, preceding the condensation of silicate dust, which triggers the outflow. Recent high-angular-resolution observations show clumpy, variable dust clouds at these distances. Aims. We explore possible causes for the formation of inhomogeneous dust layers, using 3D dynamical simulations. Methods. We modeled the outer convective envelope and the dust-forming atmosphere of an M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The simulations account for frequency-dependent gas opacities, and include a time-dependent description of grain growth and evaporation for corundum (Al2O3) and olivine-type silicates (Mg2SiO4). Results. In the inner, gravitationally bound, and corundum-dominated layers of the circumstellar envelope, a patchy distribution of the dust emerges naturally, due to atmospheric shock waves that are generated by large-scale convective flows and pulsations. The formation of silicate dust at somewhat larger distances probably indicates the outer limit of the gravitationally bound layers. The current models do not describe wind acceleration, but the cloud formation mechanism should also work for stars with outflows. Timescales of atmospheric dynamics and grain growth are similar to observed values. In spherical averages of dust densities, more easily comparable to unresolved observations and 1D models, the variable 3D morphology manifests itself as cycle-to-cycle variations. Conclusions. Grain growth in the wake of large-scale non-spherical shock waves, generated by convection and pulsations, is a likely mechanism for producing the observed clumpy dust clouds, and for explaining their physical and dynamical properties.
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Mollière, P., e I. A. G. Snellen. "Detecting isotopologues in exoplanet atmospheres using ground-based high-dispersion spectroscopy". Astronomy & Astrophysics 622 (febbraio 2019): A139. http://dx.doi.org/10.1051/0004-6361/201834169.
Testo completoAbstract (sommario):
Context. The cross-correlation technique is a well-tested method for exoplanet characterization, having lead to the detection of various molecules, to constraints on atmospheric temperature profiles, wind speeds, and planetary spin rates. A new, potentially powerful application of this technique is the measurement of atmospheric isotope ratios. In particular D/H can give unique insights into the formation and evolution of planets, and their atmospheres. Aims. In this paper we aim to study the detectability of molecular isotopologues in the high-dispersion spectra of exoplanet atmospheres, to identify the optimal wavelength ranges to conduct such studies, and to predict the required observational efforts – both with current and future ground-based instrumentation. Methods. High-dispersion (R = 100 000) thermal emission spectra, and in some cases reflection spectra, were simulated by self-consistent modeling of the atmospheric structures and abundances of exoplanets over a wide range of effective temperatures. These were synthetically observed with a telescope equivalent to the VLT and/or ELT, and analyzed using the cross-correlation technique, resulting in signal-to-noise ratio predictions for the 13CO, HDO, and CH3D isotopologues. Results. We find that for the best observable exoplanets, 13CO is well in range of current telescopes. We predict it will be most favorably detectable at 2.4 μm, just longward of the wavelength regions probed by several high-dispersion spectroscopic observations presented in the literature. CH3D can be best targeted at 4.7 μm, and may be detectable using 40 m-class telescopes for planets below 600 K in equilibrium temperature. In this case, the sky background becomes the dominating noise source for self-luminous planets. HDO is best targeted at 3.7 μm, and is less affected by sky background noise. 40 m-class telescopes may lead to its detection for planets with Tequ below 900 K. It could already be in the range of current 8 m-class telescopes in the case of quenched methane abundances. Finally, if Proxima Cen b is water-rich, the HDO isotopologue could be detected with the ELT in ~1 night of observing time in its reflected-light spectrum. Conclusions. Isotopologues will soon be a part of the exoplanet characterisation tools. Measuring D/H in exoplanets, and ratios of other isotopes, could become a prime science case for the first-light instrument METIS on the European ELT, especially for nearby temperate rocky and ice giant planets. This can provide unique insights in their history of icy-body enrichment and atmospheric evaporation processes.
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Bohannan, B., S. A. Voels, D. C. Abbott e D. G. Hummer. "Effective Temperatures and Gravities for O-Type Stars Determined from High Precision Line Profiles and Wind-Blanketed Model Atmospheres". Symposium - International Astronomical Union 132 (1988): 127–30. http://dx.doi.org/10.1017/s0074180900034896.
Testo completoAbstract (sommario):
Analysis of line profiles obtained with astronomical instrumentation capable of high signal-to-noise ratio spectroscopy have contributed significant new precision to the determination of the basic stellar parameters of hot, luminous, mass-loosing stars. Accurate measurement of such stellar properties as effective temperature and helium abundance for stars of spectral type O and early B is important not only to the physics of these stars but also to the environment in which they are located. The overall goals of the work we summarize here are to refine the spectral-type vs. temperature calibration for the most massive stars and to determine helium abundances for stars that are loosing mass at a rate sufficiently high to affect their evolution. Details of our procedures are described in the analysis of ξ Puppis by Bohannan et al. (1986).
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Stolker, T., S. P. Quanz, K. O. Todorov, J. Kühn, P. Mollière, M. R. Meyer, T. Currie, S. Daemgen e B. Lavie. "MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm". Astronomy & Astrophysics 635 (marzo 2020): A182. http://dx.doi.org/10.1051/0004-6361/201937159.
Testo completoAbstract (sommario):
Context. Directly imaged planets and substellar companions are key targets for the characterization of self-luminous atmospheres. Their photometric appearance at 4–5 μm is sensitive to the chemical composition and cloud content of their atmosphere. Aims. We aim to systematically characterize the atmospheres of directly imaged low-mass companions at 4–5 μm. We want to homogeneously process the data, provide robust flux measurements, and compile a photometric library at thermal wavelengths of these mostly young, low-gravity objects. In this way, we want to find trends related to their spectral type and surface gravity by comparing with isolated brown dwarfs and predictions from atmospheric models. Methods. We used the high-resolution, high-contrast capabilities of NACO at the Very Large Telescope (VLT) to directly image the companions of HIP 65426, PZ Tel, and HD 206893 in the NB4.05 and/or M′ filters. For the same targets, and additionally β Pic, we also analyzed six archival VLT/NACO datasets which were taken with the NB3.74, L′, NB4.05, and M′ filters. The data processing and photometric extraction of the companions was done with PynPoint while the species toolkit was used to further analyze and interpret the fluxes and colors. Results. We detect for the first time HIP 65426 b, PZ Tel B, and HD 206893 B in the NB4.05 filter, PZ Tel B and HD 206893 B in the M′ filter, and β Pic b in the NB3.74 filter. We provide calibrated magnitudes and fluxes with a careful analysis of the error budget, both for the new and archival datasets. The L′–NB4.05 and L′–M′ colors of the studied sample are all red while the NB4.05–M′ color is blue for β Pic b, gray for PZ Tel B, and red for HIP 65426 b and HD 206893 B (although typically with low significance). The absolute NB4.05 and M′ fluxes of our sample are all larger than those of field dwarfs with similar spectral types. Finally, the surface gravity of β Pic b has been constrained to log g = 4.17−0.13+0.10 dex from its photometry and dynamical mass. Conclusions. A red color at 3–4 μm and a blue color at 4–5 μm might be (partially) caused by H2O and CO absorption, respectively, which are expected to be the most dominant gaseous opacities in hot (Teff ≳ 1300 K) atmospheres. The red characteristics of β Pic b, HIP 65426 b, and HD 206893 B at 3–5μm, as well as their higher fluxes in NB4.05 and M′ compared to field dwarfs, indicate that cloud densities are enhanced close to the photosphere as a result of their low surface gravity.
36
Chentsov, Eugene. "On the Variable Winds of BA Supergiants". International Astronomical Union Colloquium 169 (1999): 206–13. http://dx.doi.org/10.1017/s0252921100072006.
Testo completoAbstract (sommario):
AbstractThe manifestation of instability and nonhomogeneity of the atmospheres and winds of β Ori, HD 168607, 6 Cas and other highly luminous B7–A3 stars are considered on the basis of visible and near-infrared spectra obtained with CCD–echelle spectrometers of the 1–m and 6–m telescopes SAO RAS. Changes in the profile shapes, radial velocities and differential shifts of lines create an impression that unstable disk– or ring–shaped structures may exist at the wind bases of super-giants such as β Ori. In the process of destruction they occasionally produce more or less radial flows of escaping and infalling gas. It looks that, as the luminosity increased, the geometry and kinematics of the wind simplify. The wind becomes more symmetrical, the sings of compression diappear and even in the lower layers steady (at least over an interval of several years), although unstable expansion is observable.
37
Russell, S. C., M. S. Bessell e M. A. Dopita. "Strömgren photometry of supergiants in the Magellanic Clouds". Publications of the Astronomical Society of Australia 7, n. 1 (1987): 65–68. http://dx.doi.org/10.1017/s132335800002186x.
Testo completoAbstract (sommario):
AbstractThis is a preliminary report on the first Strömgren–Hβ photometric survey of yellow supergiants in the Magellanic Clouds. Less luminous supergiants have been chosen in preference for this program in order to facilitate calculation of their physical parameters. This will not only provide a basis for future detailed spectral analysis of these stars, but will allow a better calibration of the colours derived from model atmospheres for this type of star. The colours of these stars are more affected by changes in metallicity than the hotter stars more frequently studied, and since they are also young, they reflect the metallicity of their local InterStellar Medium (ISM). If indeed the Strömgren abundance index m1 can be calibrated successfully for these stars, then their observation in any galaxy will provide the metallicity of that particular ISM. Our results show that it is important for future colours to be calculated over the range of turbulent velocities important for supergiants.
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Moehler, S., W. B. Landsman, T. Lanz e M. M. Miller Bertolami. "Hot UV-bright stars of galactic globular clusters". Astronomy & Astrophysics 627 (27 giugno 2019): A34. http://dx.doi.org/10.1051/0004-6361/201935694.
Testo completoAbstract (sommario):
Context. We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. Aims. We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory. Methods. We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of −2.3 < [Fe/H] < −1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. Results. We find that metal-rich model spectra are required to analyse stars hotter than 40 000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, while two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low. Conclusions. Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. While the observed numbers of post-AGB and post-early AGB stars roughly agree with the predictions, our current comparison is affected by low number statistics.
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Wambsganss, Joachim. "Strong gravitational lensing: relativity in action". Proceedings of the International Astronomical Union 5, S261 (aprile 2009): 249–59. http://dx.doi.org/10.1017/s1743921309990470.
Testo completoAbstract (sommario):
AbstractDeflection of light by gravity was predicted by Einstein's Theory of General Relativity and observationally confirmed in 1919. In the following decades, various aspects of the gravitational lens effect were explored theoretically, among them measuring the Hubble constant from multiple images of a background source, making use of the magnifying effect as a gravitational telescope, or the possibility of a “relativistic eclipse” as a perfect test of GR. Only in 1979, gravitational lensing became an observational science when the first doubly imaged quasar was discovered. Today lensing is a booming part of astrophysics and cosmology. A whole suite of strong lensing phenomena have been investigated: multiple quasars, giant luminous arcs, Einstein rings, quasar microlensing, and galactic microlensing. The most recent lensing application is the detection of extrasolar planets. Lensing has contributed significant new results in areas as different as the cosmological distance scale, mass determination of galaxy clusters, physics of quasars, searches for dark matter in galaxy halos, structure of the Milky Way, stellar atmospheres and exoplanets. A guided tour through some of these applications will illustrate how gravitational lensing has established itself as a very useful universal astrophysical tool.
40
Eckart, A., R. Genzel, R. Hofmann, B. J. Sams e L. E. Tacconi-Garman. "Diffraction Limited Near Infrared Imaging of the Central Parsec of the Galaxy". Symposium - International Astronomical Union 158 (1994): 379–81. http://dx.doi.org/10.1017/s0074180900107983.
Testo completoAbstract (sommario):
We present deep 1.6 and 2.2 μm images of the central parsec of the Galaxy at a resolution of 0.15″. Most of the flux in earlier seeing limited images comes from about 340 unresolved stellar sources with K≤14. The IRS 16 and 13 complexes are resolved into about two dozen and half a dozen sources, a number of which are probably luminous hot stars. We confirm the presence of a blue near infrared object (K≈13) at the position of the compact radio source Sgr A∗. The spatial centroid of the source number distribution is consistent with the position of Sgr A∗ but not with a position in the IRS 16 complex. The stellar surface density in the central 10″ is very well fitted by an isothermal cluster model with a well defined core radius. The derived core radius of all 340 sources is 0.15±0.05 pc. The central stellar density is a few times 107 M⊙ pc−3. Buildup of massive stars by merging of lower mass stars and collisional disruption of giant atmospheres are very probable processes in the central 0.2 pc.
41
Mirson, Juan Carlos. "Indirect lighting in Contemplative Spaces – an Endless Depth". IOP Conference Series: Earth and Environmental Science 1320, n. 1 (1 marzo 2024): 012010. http://dx.doi.org/10.1088/1755-1315/1320/1/012010.
Testo completoAbstract (sommario):
Abstract This paper explores the relationship between indirect lighting and contemplative spaces. Through an examination of notable architectural works, the study identifies a consistent preference for hidden light sources in contemplative spaces across various religions and eras. The research begins by defining the fundamental aspects of contemplation and its correlation with architecture. This is followed by a bibliographic survey of indirectly illuminated contemplative spaces. Within this investigation, three approaches to understanding these spaces are considered. Firstly, the conceptual ideas that architects sought to infuse into their creations; secondly, sensory descriptions provided by experts aimed at elucidating the luminous atmospheres of these locations. Lastly, the study delves into these diverse themes through the lens of phenomenology, highlighting perceptual peculiarities such as the predominance of sight, the significance of peripheral stimuli, and hapticity. This offers insights into the relationship between specific attributes of indirect lighting and transcendental architectural experiences. The consistent suggestion of perceiving endless depth in contemplative spaces prompts a brief discussion on the essence of our perception of depth. This study underscores light’s significance as a tool to connect with architectural spaces, transforming them into realms of imagination and awe.
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Tsuji, T. "CO molecule in transition region between chromosphere and cool stellar wind: A new probe on the outer atmospheres of cool luminous stars". Symposium - International Astronomical Union 122 (1987): 377–78. http://dx.doi.org/10.1017/s0074180900156773.
Testo completoAbstract (sommario):
Presence of CO layer well separated from photosphere is confirmed and this revealed a presence of quasi-static turbulent transition layer in normal red (super)giant stars. This layer may be related to an outer part of the extended chromosphere and/or a cool part of the chromospheric inhomogeneity, and will play major role in stellar mass-loss.
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Bond, Howard E., e R. Earle Luck. "S-Process Deficiencies in Low-Mass Supergiant Variables". International Astronomical Union Colloquium 106 (1989): 154. http://dx.doi.org/10.1017/s0252921100062849.
Testo completoAbstract (sommario):
We have carried out abundance analyses of four low-mass supergiant variable stars (the RV Tauri or RV Tau-like variables AI Cmi, RU Cen, and U Mon, and the Type II Cepheid Kappa Pav) and two Population I Cepheids (CO Aur and V378 Cen). We used model atmospheres in which hydrostatic equilibrium, plane-parallel geometry, and local thermodynamic equilibrium (LTE) were assumed. Discussion of the results, and of published analyses of additional low-mass variables, leads to the following conclusions. (1) The Population I Cepheids show normal, solar elemental abundance ratios (except for the CNO elements, which have been altered by hydrogen burning), lending some support to the validity of the above assumptions for analyses of luminous variable stars. (2) The low-mass variables show metallicities ranging from solar down to [Fe/H] values typical of thick-disk and, in a few cases, of halo stars. (3) Most low-mass variables show a systematic underabundance of the heavy s- and r-process elements. In a few cases this may indicate that the stars were initially of extremely low metal content, and are now hydrogen deficient. However, most of the variables do not appear to belong to the halo population, nor do they show other abundance patterns seen in halo stars. The origin of these underabundances, and their apparent confinement to luminous variables, are difficult to understand in the context of nuclear processing. (4) The heavy-element underabundances correlate with second ionization potential in a manner suggesting that they are non-LTE phenomenan arising from overionization by Lyman-continuum photons. Why a similar effect is not seen in Population I Cepheids is unclear, but may be related to their generally weaker hydrogen emission. (5) Several low-mass variables, including RU Cen and V553 Cen, show carbon enhancements and solar s-process abundances. Relative to the majority of the Type II variables, these stars are s-process enhanced, and we argue that they are related to the Ba II and CH stars.
44
Mathiasen, Nanet, Turid Borgestrand Øien e Carlo Volf. "Empathic Lighting Design for Healthcare Environments". IOP Conference Series: Earth and Environmental Science 1320, n. 1 (1 marzo 2024): 012031. http://dx.doi.org/10.1088/1755-1315/1320/1/012031.
Testo completoAbstract (sommario):
Abstract Light has the powerful capacity to generate a certain quality and atmosphere within a room. However, lighting design specification is often only characterized via quantitative metrics. In healthcare institutions, such as care homes, hospitals and rehabilitation centres, the use of lighting design may support a positive atmosphere, and potentially also support the health and wellbeing of patients. When people are hospitalised, they are in a difficult and often stressful situation, in which they meet nurses, physicians and relatives within new, unfamiliar surroundings. These situations call for supportive architectural spaces with atmospheres that help people relax and feel safe. Nevertheless, these spaces often lack a lighting design that supports this intent. The objective of this study is to implement two different lighting scenarios and investigate how they affect patients in a psychiatric affective healthcare unit at Copenhagen Psychiatric Centre. Two different lighting scenarios were implemented and tested in a SW-facing group therapy room, during one winter period (November 2022–March 2023). In all, the study included 12 patients. Through questionnaires, the patients were asked how they perceived the luminous environments of two lighting scenarios, one in the existing healthcare lighting vs. a new lighting setup. The patients indicated their preferences and how the lighting design affected and supported heir therapeutic session. The two lighting scenarios are: a) Four high-positioned LED-luminaires (H=2,65 m) in the ceiling; and b) Two low-positioned LED-luminaires suspended above the meeting table (H=1,50 m). The results of the survey showed that patients overall preferred the low-positioned, non-uniform lighting, which scored a total of 60 points in the semantic analysis, compared to 35 points for the high-positioned, uniform lighting. In addition, from a sustainability viewpoint, energy consumption for the low-positioned lighting scenario was only 21 W, compared to 140 W for the high-positioned lighting scenario, providing an 85% reduction in total energy consumption.
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Pauldrach, A. W. A., D. Vanbeveren e T. L. Hoffmann. "Radiation-driven winds of hot luminous stars XVI. Expanding atmospheres of massive and very massive stars and the evolution of dense stellar clusters". Astronomy & Astrophysics 538 (febbraio 2012): A75. http://dx.doi.org/10.1051/0004-6361/201117621.
Testo completo
46
Fanelli, C., L. Origlia, A. Mucciarelli, N. Sanna, E. Oliva e E. Dalessandro. "Lithium Detection in Red Supergiant Stars of the Perseus Complex". Astrophysical Journal 931, n. 1 (1 maggio 2022): 61. http://dx.doi.org/10.3847/1538-4357/ac69e7.
Testo completoAbstract (sommario):
Abstract We present the first systematic study of lithium abundance in a chemically homogeneous sample of 27 red supergiants (RSGs) in the young Perseus complex. For these stars, accurate stellar parameters and detailed chemical abundances of iron and iron peak, CNO, alpha, light, and neutron capture elements have already been obtained by means of high-resolution optical and near-infrared spectroscopy. The observed RSGs have half-solar metallicity, 10–30 Myr ages, bolometric luminosities in the 104–105 L ⊙ range, and likely mass progenitors in the 9–14 M ⊙ range. We detected the optical Li i doublet in eight out of the 27 observed K- and M-type RSGs, finding relatively low A(Li) < 1.0 dex abundances, while for the remaining 19 RSGs upper limits of A(Li) < –0.2 dex have been set. Warmer and less luminous (i.e., likely less massive) as well as less mixed (i.e., with lower [C/N] and 12C/13C depletion) RSGs with Li detection show somewhat higher Li abundances. In order to explain the Li detection in ∼30% of the observed RSGs, we speculate that some stochasticity should be at work, in a scenario where the Li was not completely destroyed in the convective atmospheres and/or a secondary production took place during the post-main-sequence evolution.
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Negueruela, Ignacio, Javier Alonso-Santiago, Ricardo Dorda e Lee R. Patrick. "Strong lithium lines in red supergiants at different metallicities". Proceedings of the International Astronomical Union 18, S361 (maggio 2022): 410–15. http://dx.doi.org/10.1017/s1743921322003040.
Testo completoAbstract (sommario):
AbstractCurrent models of stellar evolution predict that stars more massive than ∼6 M⊙ should have completely depleted all lithium (Li) in their atmospheres by the time when they reach the He core burning phase. Against this, a non-negligible number of red giants with masses ≳6 M⊙ presenting strong Li lines have recently been reported. Motivated by this finding, we have carried out a spectroscopic survey of red supergiants (RSGs) in the Perseus Arm and a selection of young open clusters in the Magellanic Clouds to assess the presence of the Li <sc>i</sc> 6708Å doublet line. Based on a sample of > 70 objects, close to one third of RSGs in the Perseus Arm display noticeable Li lines, with perhaps a trend towards a lower fraction among more luminous stars. The samples in the Magellanic Clouds are not as large, but hint at a metallicity dependence. Twenty one RSGs in 5 LMC clusters show a very high fraction of Li detection, around 40%. Conversely, 17 RSGs in 5 SMC clusters give only one secure detection. The interpretation of these observational results is not straightforward, but a mechanism for Li production seems most likely. Further characterisation work is ongoing, while theoretical studies into this matter are urgently needed.
48
Smith, Linda J. "Properties of hot massive stars from studies of their ring nebulae". Symposium - International Astronomical Union 163 (1995): 24–33. http://dx.doi.org/10.1017/s0074180900201526.
Testo completoAbstract (sommario):
Wolf-Rayet (WR) stars and their possible precursors, the Luminous Blue Variables (LBVs), are often surrounded by ring nebulae. It is believed that these nebulae are formed by the action of the stellar wind, matter being ejected from the star in the past, or a combination of these two processes. The various research applications of LBV and WR nebulae are reviewed with regard to the information they provide on the properties of the central stars. They are very useful probes of stellar evolution since the details of the previous evolutionary phases of the central stars are contained in the nebulae. In particular, abundance studies can provide insights into the chemical processes operating during the precursor phases. The nebulae can also be used as probes of the far-UV ionizing flux distribution of the central WR stars by comparing the observed levels of nebular ionization with those produced with non-LTE WR model flux distributions. Comparisons of stellar effective temperatures and luminosities derived using this technique with those determined by modelling stellar emission lines can identify deficiencies such as the lack of line-blanketing in WR model atmospheres. Very hot WR stars can also be identified by searching for nebular HeII emission. Studies of bipolar structures in LBV and WR nebulae provide valuable clues on wind asymmetries in the central stars.
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Ratajczak, Renata, Elzbieta Guziewicz, Slawomir Prucnal, Cyprian Mieszczynski, Przemysław Jozwik, Marek Barlak, Svitlana Romaniuk et al. "Enhanced Luminescence of Yb3+ Ions Implanted to ZnO through the Selection of Optimal Implantation and Annealing Conditions". Materials 16, n. 5 (21 febbraio 2023): 1756. http://dx.doi.org/10.3390/ma16051756.
Testo completoAbstract (sommario):
Rare earth-doped zinc oxide (ZnO:RE) systems are attractive for future optoelectronic devices such as phosphors, displays, and LEDs with emission in the visible spectral range, working even in a radiation-intense environment. The technology of these systems is currently under development, opening up new fields of application due to the low-cost production. Ion implantation is a very promising technique to incorporate rare-earth dopants into ZnO. However, the ballistic nature of this process makes the use of annealing essential. The selection of implantation parameters, as well as post-implantation annealing, turns out to be non-trivial because they determine the luminous efficiency of the ZnO:RE system. This paper presents a comprehensive study of the optimal implantation and annealing conditions, ensuring the most efficient luminescence of RE3+ ions in the ZnO matrix. Deep and shallow implantations, implantations performed at high and room temperature with various fluencies, as well as a range of post-RT implantation annealing processes are tested: rapid thermal annealing (minute duration) under different temperatures, times, and atmospheres (O2, N2, and Ar), flash lamp annealing (millisecond duration) and pulse plasma annealing (microsecond duration). It is shown that the highest luminescence efficiency of RE3+ is obtained for the shallow implantation at RT with the optimal fluence of 1.0 × 1015 RE ions/cm2 followed by a 10 min annealing in oxygen at 800 °C, and the light emission from such a ZnO:RE system is so bright that can be observed with the naked eye.
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Buiten, Victorine A., Paul P. van der Werf, Serena Viti, Lee Armus, Andrew G. Barr, Loreto Barcos-Muñoz, Aaron S. Evans et al. "GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114". Astrophysical Journal 966, n. 2 (1 maggio 2024): 166. http://dx.doi.org/10.3847/1538-4357/ad344b.
Testo completoAbstract (sommario):
Abstract The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.