- ID:
- ivo://CDS.VizieR/J/A+A/567/A72
- Title:
- Stellar parameters and abundances in NGC 6752
- Short Name:
- J/A+A/567/A72
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6752. These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC 6752. We perform an abundance analysis by combining photometric and spectroscopic data of 194 stars located between the turnoff point and the base of the red giant branch. Stellar parameters are derived from uvby Stromgren photometry. Using the quantitative-spectroscopy package SME, stellar surface abundances for light elements such as Li, Na, Mg, Al, and Si as well as heavier elements such as Ca, Ti, and Fe are derived in an automated way by fitting synthetic spectra to individual lines in the stellar spectra, obtained with the VLT/FLAMES-GIRAFFE spectrograph. Based on uvby Stromgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC 6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC 6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC 6752.
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- ID:
- ivo://CDS.VizieR/J/A+A/627/A46
- Title:
- Sunspot penumbra and umbral flashes models
- Short Name:
- J/A+A/627/A46
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The solar chromosphere and the lower transition region are believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution (CRD) in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. We aim to obtain semi-empirical model atmospheres that reproduce the features of the MgII H & K line profiles that sample the middle chromosphere with focus on a sunspot. We used spectropolarimetric observations of the CaII 8542{AA} spectra obtained with the Swedish 1-m Solar Telescope and used NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These were used to synthesize MgII H & K spectra using the RH1.5D code, which we compared with observations taken with the Interface Region Imaging Spectrograph (IRIS). Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed MgII H & K profiles are very well reproduced. In addition, we find that supersonic hot downflows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS MgII H & K profiles, with the exception of the line core. We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot. These are available in electronic as well as in table formats.
- ID:
- ivo://CDS.VizieR/J/A+A/528/A113
- Title:
- Synthetic lines in the Sun
- Short Name:
- J/A+A/528/A113
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have computed synthetic spectra from a realistic 3D numerical simulation of the solar photosphere. We provide the spatially averaged spectra for selected lines that are commonly used on solar applications. These data can be used to calibrate Doppler velocity measurements in the solar photosphere. The calculations are carried out along the solar disk from heliocentric angle mu=1.0 to mu=0.3.
- ID:
- ivo://CDS.VizieR/J/ApJ/717/257
- Title:
- Synthetic spectra of dark stars
- Short Name:
- J/ApJ/717/257
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The first stars in the history of the Universe are likely to form in the dense central regions of {sim.to}10^5^-10^6^ M_{sun}_ cold dark matter halos at z{approx}10--50. The annihilation of dark matter particles in these environments may lead to the formation of so-called dark stars, which are predicted to be cooler, larger, more massive and potentially more long-lived than conventional population III stars. Here, we investigate the prospects of detecting high-redshift dark stars with the upcoming James Webb Space Telescope (JWST). We find that all dark stars with masses up to 10^3^ M_{sun}_ are intrinsically too faint to be detected by JWST at z above 6. However, by exploiting foreground galaxy clusters as gravitational telescopes, certain varieties of cool (T_eff_<=30000K) dark stars should be within reach at redshifts up to z{approx}10. If the lifetimes of dark stars are sufficiently long, many such objects may also congregate inside the first galaxies. We demonstrate that this could give rise to peculiar features in the integrated spectra of galaxies at high redshifts, provided that dark stars make up at least {sim.to}1% of the total stellar mass in such objects.
- ID:
- ivo://CDS.VizieR/J/MNRAS/412/1070
- Title:
- Synthetic supernova extinction curves
- Short Name:
- J/MNRAS/412/1070
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We apply the supernova (SN) extinction curves to reproduce the observed properties of SST J1604+4304 which is a young infrared (IR) galaxy at z~1. The SN extinction curves used in this work were obtained from models of unmixed ejecta of Type II supernovae for the Salpeter initial mass function with a mass range from 8 to 30M_{sun}_ or 8 to 40M_{sun}_. The effect of dust distributions on the attenuation of starlight is investigated by performing the {chi}^2^ fitting method against various dust distributions. These are the commonly used uniform dust screen, the clumpy dust screen and the internal dust geometry. We add to these geometries three scattering properties, namely, no scattering, isotropic scattering and forward-only scattering. Judging from the {chi}^2^ values, we find that the uniform screen models with any scattering property provide good approximations to the real dust geometry.
- ID:
- ivo://CDS.VizieR/J/A+A/512/A54
- Title:
- Teff and Fbol from Infrared Flux Method
- Short Name:
- J/A+A/512/A54
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Various effective temperature scales have been proposed over the years. Despite much work and the high internal precision usually achieved, systematic differences of order 100K (or more) among various scales are still present. We present an investigation based on the Infrared Flux Method aimed at assessing the source of such discrepancies and pin down their origin. We break the impasse among different scales by using a large set of solar twins, stars which are spectroscopically and photometrically identical to the Sun, to set the absolute zero point of the effective temperature scale to within few degrees. Our newly calibrated, accurate and precise temperature scale applies to dwarfs and subgiants, from super-solar metallicities to the most metal-poor stars currently known. At solar metallicities our results validate spectroscopic effective temperature scales, whereas for [Fe/H]<-2.5 our temperatures are roughly 100 K hotter than those determined from model fits to the Balmer lines and 200 K hotter than those obtained from the excitation equilibrium of Fe lines. Empirical bolometric corrections and useful relations linking photometric indices to effective temperatures and angular diameters have been derived. Our results take full advantage of the high accuracy reached in absolute calibration in recent years and are further validated by interferometric angular diameters and space based spectrophotometry over a wide range of effective temperatures and metallicities.
- ID:
- ivo://CDS.VizieR/J/ApJ/761/166
- Title:
- Terrestrial exoplanet atmospheres. I.
- Short Name:
- J/ApJ/761/166
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a comprehensive photochemistry model for exploration of the chemical composition of terrestrial exoplanet atmospheres. The photochemistry model is designed from the ground up to have the capacity to treat all types of terrestrial planet atmospheres, ranging from oxidizing through reducing, which makes the code suitable for applications for the wide range of anticipated terrestrial exoplanet compositions. The one-dimensional chemical transport model treats up to 800 chemical reactions, photochemical processes, dry and wet deposition, surface emission, and thermal escape of O, H, C, N, and S bearing species, as well as formation and deposition of elemental sulfur and sulfuric acid aerosols. We validate the model by computing the atmospheric composition of current Earth and Mars and find agreement with observations of major trace gases in Earth's and Mars' atmospheres. We simulate several plausible atmospheric scenarios of terrestrial exoplanets and choose three benchmark cases for atmospheres from reducing to oxidizing. The most interesting finding is that atomic hydrogen is always a more abundant reactive radical than the hydroxyl radical in anoxic atmospheres. Whether atomic hydrogen is the most important removal path for a molecule of interest also depends on the relevant reaction rates. We also find that volcanic carbon compounds (i.e., CH_4_ and CO_2_) are chemically long-lived and tend to be well mixed in both reducing and oxidizing atmospheres, and their dry deposition velocities to the surface control the atmospheric oxidation states. Furthermore, we revisit whether photochemically produced oxygen can cause false positives for detecting oxygenic photosynthesis, and find that in 1 bar CO_2_-rich atmospheres oxygen and ozone may build up to levels that have conventionally been accepted as signatures of life, if there is no surface emission of reducing gases. The atmospheric scenarios presented in this paper can serve as the benchmark atmospheres for quickly assessing the lifetime of trace gases in reducing, weakly oxidizing, and highly oxidizing atmospheres on terrestrial exoplanets for the exploration of possible biosignature gases.
- ID:
- ivo://CDS.VizieR/J/A+A/386/1001
- Title:
- Thermal emission from low-field neutron stars
- Short Name:
- J/A+A/386/1001
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a new grid of LTE model atmospheres for weakly magnetic (B<~10^10^G) neutron stars, using X opacity and equation of state data from the OPAL project and employing a fully frequency- and angle-dependent radiation transfer. Model spectra for low-field neutron stars with three different photospheric compositions are presented: (1) pure hydrogen atmospheres, (2) atmospheres with solar abundances, and (3) pure iron atmospheres. Each grid covers 29 effective temperatures log(Teff[K])=5.10-6.50 in steps of 0.05. The model spectra are tabulated as angle-averaged unredshifted (surface) Eddington fluxes. All spectra have been calculated for a canonical neutron star configuration, M_ns_=1.4M_{sun}_, R_ns_=10km, corresponding to a surface gravitational acceleration of log(g)=14.386. The same model grids are also available as XSPEC tables at http://legacy.gsfc.nasa.gov/docs/xanadu/xspec/models/gbr.html
- ID:
- ivo://CDS.VizieR/J/A+A/457/265
- Title:
- The VLT-FLAMES Survey of Massive Stars
- Short Name:
- J/A+A/457/265
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmosphere calculations have been used to estimate effective temperatures (from either the helium spectrum or the silicon ionization equilibrium) and gravities (from the hydrogen spectrum). Projected rotational velocities have been deduced from the helium spectrum (for fast and moderate rotators) or the metal line spectrum (for slow rotators). The origin of the low gravity estimates for apparently near main sequence objects is discussed and is related to the stellar rotational velocity. The atmospheric parameters have been used to estimate cluster distances (which are generally in good agreement with previous determinations) and these have been used to estimate stellar luminosities and evolutionary masses. The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed. Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters. Projected rotational velocities for targets in the older clusters, NGC 3293 and NGC 4755, have been found to be systematically larger than those for the field, confirming recent results in other similar age clusters. The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities. For the relatively unevolved targets in the older clusters, NGC 3293 and NGC 4755, the peak of the velocity distribution would be 250km/s with a full-width-half-maximum of approximately 180km/s. For NGC 6611, the sample size is relatively small but implies a lower mean rotational velocity. This may be evidence for the spin-down effect due to angular momentum loss through stellar winds, although our results are consistent with those found for very young high mass stars. For all three clusters we deduce present day mass functions with Gamma-values in the range of -1.5 to -1.8, which are similar to other young stellar clusters in the Milky Way.
- ID:
- ivo://CDS.VizieR/J/ApJ/751/156
- Title:
- Ti and Fe lines in red supergiants
- Short Name:
- J/ApJ/751/156
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Detailed non-LTE (NLTE) calculations for red supergiant (RSG) stars are presented to investigate the influence of NLTE on the formation of atomic iron and titanium lines in the J band. With their enormous brightness at J band RSG stars are ideal probes of cosmic abundances. Recent LTE studies have found that metallicities accurate to 0.15 dex can be determined from medium-resolution spectroscopy of individual RSGs in galaxies as distant as 10 Mpc. The NLTE results obtained in this investigation support these findings. NLTE abundance corrections for iron are smaller than 0.05 dex for effective temperatures between 3400 K and 4200 K and 0.1 dex at 4400 K. For titanium the NLTE abundance corrections vary smoothly between -0.4 dex and +0.2 dex as a function of effective temperature. For both elements, the corrections also depend on stellar gravity and metallicity. The physical reasons behind the NLTE corrections and the consequences for extragalactic J-band abundance studies are discussed.
