- ID:
- ivo://CDS.VizieR/J/A+A/326/249
- Title:
- Chromospheric models of dwarf M stars
- Short Name:
- J/A+A/326/249
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Tables 4 and 5 give the atmospheric parameters for our models of dM stars Gl 588 and Gl 628. The columns give the column mass in g/cm^-2^, the electron temperature in K, the microturbulent velocity in km/s; the continuum optical depth at 5000{AA}; the hydrogen, proton, and electron density in particles per cm^-3^; and the height h (in km) above the level where tau_5000_=1.
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- ID:
- ivo://CDS.VizieR/J/AJ/119/1424
- Title:
- Color-temperature relations of M giants
- Short Name:
- J/AJ/119/1424
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As part of a project to model the integrated spectra and colors of elliptical galaxies through evolutionary synthesis, we have refined our synthetic spectrum calculations of M giants. After critically assessing three effective temperature scales for M giants, we adopted the relation of Dyck et al. for our models. Using empirical spectra of field M giants as a guide, we then calculated MARCS stellar atmosphere models (Gustafsson et al. 1975; Bell et al. 1976) and SSG synthetic spectra (Bell & Gustafsson 1978; Gustafsson & Bell 1979) of these cool stars, adjusting the band absorption oscillator strengths of the TiO bands to better reproduce the observational data. The resulting synthetic spectra are found to be in very good agreement with the K-band spectra of stars of the appropriate spectral type taken from Kleinmann & Hall (1986) as well. Spectral types estimated from the strengths of the TiO bands and the depth of the band head of CO near 2.3 {mu}m quantitatively confirm that the synthetic spectra are good representations of those of field M giants. The broadband colors of the models match the field relations of K and early-M giants very well; for late-M giants, differences between the field star and synthetic colors are probably caused by the omission of spectral lines of VO and H_2_O in the spectrum synthesis calculations. Here, we present four grids of K-band bolometric corrections and colors - Johnson U-V and B-V, Cousins V-R and V-I, Johnson-Glass V-K, J-K, and H-K, and CIT/CTIO V-K, J-K, H-K, and CO - for models having 3000 K {<=} Teff {<=} 4000 K and -0.5 {<=} log(g) {<=} 1.5.
- ID:
- ivo://CDS.VizieR/J/A+A/547/A108
- Title:
- Comparative modelling of cool giants spectra
- Short Name:
- J/A+A/547/A108
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. The corresponding paper aims to compare a wide variety of model codes and strategies for the analysis of stellar spectra that are available today. The online tables list spectral lines used during the analysis together with the basic line parameters.
- ID:
- ivo://CDS.VizieR/J/A+A/454/333
- Title:
- Computed Hbeta indices from ATLAS9 model
- Short Name:
- J/A+A/454/333
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Grids of H{beta} indices based on updated (new-ODF) ATLAS9 model atmospheres were computed for solar and scaled solar metallicities [+0.5], [+0.2], [0.0], [-0.5], [-1.0], [-1.5], [-2.0], [-2.5] and for alpha enhanced compositions [+0.5a], [0.0a], [-0.5a], [-1.0a], [-1.5a], [-2.0a], [-2.5a], and [-4.0a]. Indices for Teff>5000K were computed with the same methods as described by Lester, Gray & Kurucz (1986ApJS...61..509L) (LGK86) except for a different normalization of the computed natural system to the standard system. LGK86 used special ODFs to compute the fluxes. For Teff less or equal to 5000K we computed the fluxes using the synthetic spectrum method. In order to assess the accuracy of the computed indices comparisons were made with the indices computed by Smalley & Dworetsky (1995A&A...293..446S) (MD95) and with the empirical relations Teff-H{beta} given by Alonso et al. (1996A&A...313..873A) for several metallicities. Furthermore, for cool stars, temperatures inferred from the computed indices were compared with those of the fundamental stars listed by MD95. The same kind of comparison was made between gravities for B-type stars. The temperatures from the computed indices are in good agreement, within the error limits, with the literature values for Teff between 4750K and 8000K, while the gravities agree for Teff>9000K. The computed H{beta} indices for the Sun and for Procyon are very close to the observed values. The comparison between the observed and computed H{beta} indices as function of the observed H{beta} has shown a very small trend which almost completely disappears when only stars hotter than 10000K are considered. The trend due to the cool stars is probably related with the low accuracy of the fundamental Teff which are affected by large errors for most of the stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/878/63
- Title:
- Cool WD atmosphere models. IV. Spectral evolution
- Short Name:
- J/ApJ/878/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As a result of competing physical mechanisms, the atmospheric composition of white dwarfs changes throughout their evolution, a process known as spectral evolution. Because of the ambiguity of their atmospheric compositions and the difficulties inherent to the modeling of their dense atmospheres, no consensus exists regarding the spectral evolution of cool white dwarfs (Teff<6000K). In the previous papers of this series, we presented and observationally validated a new generation of cool white dwarf atmosphere models that include all the necessary constitutive physics to accurately model those objects. Using these new models and a homogeneous sample of 501 cool white dwarfs, we revisit the spectral evolution of cool white dwarfs. Our sample includes all spectroscopically identified white dwarfs cooler than 8300K for which a parallax is available in Gaia DR2 and photometric observations are available in Pan-STARRS1 and 2MASS. Except for a few cool carbon-polluted objects, our models allow an excellent fit to the spectroscopic and photometric observations of all objects included in our sample. We identify a decrease of the ratio of hydrogen- to helium-rich objects between 7500 and 6250K, which we interpret as the signature of convective mixing. After this decrease, hydrogen-rich objects become more abundant up to 5000K. This puzzling increase, reminiscent of the non-DA gap, has yet to be explained. At lower temperatures, below 5000K, hydrogen-rich white dwarfs become rarer, which rules out the scenario in which the accretion of hydrogen from the interstellar medium dominates the spectral evolution of cool white dwarfs.
- ID:
- ivo://CDS.VizieR/J/ApJ/882/106
- Title:
- DB white dwarfs with SDSS and Gaia data
- Short Name:
- J/ApJ/882/106
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a comprehensive analysis of DB white dwarfs drawn from the Sloan Digital Sky Survey, based on model fits to ugriz photometry and medium-resolution spectroscopy from the Sloan Digital Sky Survey. We also take advantage of the exquisite trigonometric parallax measurements recently obtained by the Gaia mission. Using the so-called photometric and spectroscopic techniques, we measure the atmospheric and physical parameters of each object in our sample (Teff, logg, H/He, Ca/He, R, M), and compare the values obtained from both techniques in order to assess the precision and accuracy of each method. We then explore in great detail the surface gravity, stellar mass, and hydrogen abundance distributions of DB white dwarfs as a function of effective temperature. We present some clear evidence for a large population of unresolved double-degenerate binaries composed of DB+DB and even DB+DA white dwarfs. In the light of our results, we finally discuss the spectral evolution of DB white dwarfs, in particular the evolution of the DB-to-DA ratio as a function of Teff, and we revisit the question of the origin of hydrogen in DBA white dwarfs.
- ID:
- ivo://CDS.VizieR/J/A+A/611/A68
- Title:
- 3D correction in 5 photometric systems
- Short Name:
- J/A+A/611/A68
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities. We aim to study, in a quantitative way, the impact of granulation on colours. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L=2,3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBVRI, 2MASS, Hipparcos, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a "3D correction" that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200K and overestimation of gravity by up to 0.5dex, when using colours as diagnostics.We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01mag, especially for colours involving blue and UV bands. We have investigated the Gaia and Hipparcos photometric systems and found that the (G-Hp),(BP-RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F,G,K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering.
- ID:
- ivo://CDS.VizieR/J/A+A/615/A139
- Title:
- 3D non-LTE Balmer line formation
- Short Name:
- J/A+A/615/A139
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Hydrogen Balmer lines are commonly used as spectroscopic effective temperature diagnostics of late-type stars. However, reliable inferences require accurate model spectra, and the absolute accuracy of classical methods that are based on one-dimensional (1D) hydrostatic model atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To investigate this, we carry out 3D non-LTE calculations for the Balmer lines, performed, for the first time, over an extensive grid of 3D hydrodynamic STAGGER model atmospheres. For H{alpha}, H{beta}, and H{gamma} we find significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer wings tend to be stronger in 3D models, particularly for H{gamma}, while the inner wings can be weaker in 3D models, particularly for H{alpha}. For H{alpha}, we also find significant 3D LTE versus 3D non-LTE differences (non-LTE effects): in warmer stars (Teff~~6500K) the inner wings tend to be weaker in non-LTE models, while at lower effective temperatures (Teff~~4500K) the inner wings can be stronger in non-LTE models; the non-LTE effects are more severe at lower metallicities. We test our 3D non-LTE models against observations of well-studied benchmark stars. For the Sun, we infer concordant effective temperatures from H{alpha}, H{beta}, and H{gamma}; however the value is too low by around 50K which could signal residual modelling shortcomings. For other benchmark stars, our 3D non-LTE models generally reproduce the effective temperatures to within 1{sigma} uncertainties. For H{alpha}, the absolute 3D effects and non-LTE effects can separately reach around 100K, in terms of inferred effective temperatures. For metal-poor turn-off stars, 1D LTE models of H{alpha} can underestimate effective temperatures by around 150K. Our 3D non-LTE model spectra are publicly available, and can be used for more reliable spectroscopic effective temperature determinations.
- ID:
- ivo://CDS.VizieR/J/A+A/658/A47
- Title:
- Dwarf stars limb-darkening coefficients
- Short Name:
- J/A+A/658/A47
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Stellar models applied to large stellar surveys of the Milky Way need to be properly tested against a sample of stars with highly reliable fundamental stellar parameters. We have established a program aiming to deliver such a sample of stars. Here we present new fundamental stellar parameters of nine dwarf stars that will be used as benchmark stars for large stellar surveys. One of these stars is the solar-twin 18Sco, which is also one of the Gaia-ESO benchmarks. The goal is to reach a precision of 1% in effective temperatures (Teff). This precision is important for accurate determinations of the full set of fundamental parameters and abundances of stars observed by the surveys. We observed HD131156 (xi Boo), HD146233 (18 Sco), HD152391, HD173701, HD185395 (theta Cyg), HD186408 (16 Cyg A), HD186427 (16 Cyg B), HD190360 and HD207978 (15 Peg) using the high angular resolution optical interferometric instrument PAVO at the CHARA Array. We derived limb-darkening corrections from 3D model atmospheres and determined Teff directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE spectrograph and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analyses of unblended lines of neutral and singly ionized iron. For eight of the nine stars we measure the Teff less than 1%, and for one star better than 2%. We determined the median uncertainties in log g and [Fe/H] as 0.015dex and 0.05dex, respectively. This study presents updated fundamental stellar parameters of nine dwarf stars that can be used as new set of benchmarks. All fundamental stellar parameters were based on consistently combining interferometric observations, 3D limb-darkening modelling and spectroscopic analysis. The next paper in this series will extend our sample to giants in the metal-rich range.
- ID:
- ivo://CDS.VizieR/J/A+AS/107/445
- Title:
- Envelopes of oxygen-rich AGB stars
- Short Name:
- J/A+AS/107/445
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- (no description available)
