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Model Atmospheres (model + atmosphere)

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Selected Abstracts

Parameters of the components of visually close binary systems: Hip 11352

ASTRONOMISCHE NACHRICHTEN, Issue 4 2009
M.A. Al-Wardat
Abstract Model atmospheres of the components of the visually close binary system Hip 11352 were constructed using a grid of Kurucz's blanketed models. The entire observational spectral energy distribution of the system was used as a reference for the comparison with synthetic ones created from the model atmospheres. The parameters of the components of the system were derived as Teffa = 5650 ± 50 K, Teffb = 5550 ± 50 K, log ga = 4.47 ± 0.13, log gb = 4.49 ± 0.13, Ra = 0.92 ± 0.03 R,, Rb = 0.88 ± 0.02 R,, Ma = 0.93 ± 0.05 M,, Mb = 0.89 ± 0.05 M,, with a spectral type G7 for the primary and G9 for the secondary. The synthetic magnitudes and of both components were calculated using Johnson-Cousins, Strömgren, and Tycho photometrical systems. Finally the formation and evolution of the system was discussed (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

HD 209621: abundances of neutron-capture elements,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2010
Aruna Goswami
ABSTRACT High-resolution spectra obtained from the Subaru Telescope High Dispersion Spectrograph have been used to update the stellar atmospheric parameters and metallicity of the star HD 209621. We have derived a metallicity of [Fe/H]=,1.93 for this star, and have found a large enhancement of carbon and of heavy elements, with respect to iron. Updates on the elemental abundances of four s-process elements (Y, Ce, Pr, Nd) along with the first estimates of abundances for a number of other heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are reported. The stellar atmospheric parameters, the effective temperature, Teff, and the surface gravity, log g (4500 K, 2.0), are determined from local thermodynamic equilibrium analysis using model atmospheres. Estimated [Ba/Eu]=+0.35, places the star in the group of CEMP-(r+s) stars; however, the s-elements abundance pattern seen in HD 209621 is characteristic of CH stars; notably, the second-peak s-process elements are more enhanced than the first-peak s-process elements. HD 209621 is also found to show a large enhancement of the third-peak s-process element lead (Pb) with [Pb/Fe]=+1.88. The relative contributions of the two neutron-capture processes, r and s, to the observed abundances are examined using a parametric model-based analysis, which hints that the neutron-capture elements in HD 209621 primarily originate in s-process. [source]

Testing the accuracy of synthetic stellar libraries

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Lucimara P. Martins
ABSTRACT One of the main ingredients of stellar population synthesis models is a library of stellar spectra. Both empirical and theoretical libraries are used for this purpose, and the question about which one is preferable is still debated in the literature. Empirical and theoretical libraries are being improved significantly over the years, and many libraries have become available lately. However, it is not clear in the literature what are the advantages of using each of these new libraries, and how far behind models are compared to observations. Here we compare in detail some of the major theoretical libraries available in the literature with observations, aiming at detecting weaknesses and strengths from the stellar population modelling point of view. Our test is twofold: we compared model predictions and observations for broad-band colours and for high-resolution spectral features. Concerning the broad-band colours, we measured the stellar colour given by three recent sets of model atmospheres and flux distributions, and compared them with a recent UBVRIJHK calibration which is mostly based on empirical data. We found that the models can reproduce with reasonable accuracy the stellar colours for a fair interval in effective temperatures and gravities. The exceptions are (1) the U,B colour, where the models are typically redder than the observations, and (2) the very cool stars in general (V,K, 3). Castelli & Kurucz is the set of models that best reproduce the bluest colours (U,B, B,V) while Gustafsson et al. and Brott & Hauschildt more accurately predict the visual colours. The three sets of models perform in a similar way for the infrared colours. Concerning the high-resolution spectral features, we measured 35 spectral indices defined in the literature on three high-resolution synthetic libraries, and compared them with the observed measurements given by three empirical libraries. The measured indices cover the wavelength range from ,3500 to ,8700 Å. We found that the direct comparison between models and observations is not a simple task, given the uncertainties in parameter determinations of empirical libraries. Taking that aside, we found that in general the three libraries present similar behaviours and systematic deviations. For stars with Teff, 7000 K, the library by Coelho et al.is the one with best average performance. We detect that lists of atomic and molecular line opacities still need improvement, specially in the blue region of the spectrum, and for the cool stars (Teff, 4500 K). [source]

Accurate fundamental parameters for lower main-sequence stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006
Luca Casagrande
ABSTRACT We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our study is based on 104 stars for which we have excellent BV(RI)CJHKS photometry, excellent parallaxes and good metallicities. Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement. A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology. Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices. We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem. [source]

Abundance analysis of the cool extreme helium star LSS 3378

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Gajendra Pandey
ABSTRACT Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using local thermodynamic equilibrium (LTE) line formation and LTE model atmospheres constructed for EHe stars. The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s -process nucleosynthesis. The derived abundances of iron peak and ,-elements indicate the absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance [log ,(Fe) = 6.1] is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group. [source]

Surface abundances of light elements for a large sample of early B-type stars , III.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
An analysis of helium lines in spectra of 102 stars
ABSTRACT Non-local thermodynamic equilibrium analysis of He i lines in spectra of 102 B stars is implemented in order to derive the helium abundance He/H, the microturbulent parameter Vt and the projected rotation velocity v sin i. A simultaneous determination of He/H and Vt for the stars is effected by analysing equivalent widths of the 4471- and 4922-Å lines primarily as indicators of He/H and the 4713-, 5016-, 5876- and 6678-Å lines primarily as indicators of Vt. The rotation velocities v sin i are found from profiles of the same lines. It is shown that, when Vt > 7 km s,1, the Vt(He i) values determined from He i lines are systematically overestimated as compared with the Vt(O ii, N ii) values derived from O ii and N ii lines. This discrepancy is especially appreciable for hot evolved B giants with Vt(He i) = 16,23 km s,1 and may indicate a failure of classical model atmospheres to represent the strong He i lines for these stars. Two programme stars, HR 1512 and 7651, are found to be helium-weak stars. The remaining 100 stars are divided into three groups according to their masses M. The microturbulent parameter Vt(He i) is low for all stars of group A (M= 4.1,6.9 M,) and for all stars with the relative ages t/tMS < 0.8 of group B (M= 7.0,11.2 M,). Their Vt(He i) values are within the 0 to 5 km s,1 range, as a rule; the mean value is Vt= 1.7 km s,1. Only evolved giants of group B, which are close to the termination of the main-sequence (MS) evolutionary phase (t/tMS > 0.8), show Vt(He i) up to 11 km s,1. The helium abundance He/H is correlated with the relative age t/tMS in both groups; the averaged He/H enhancement during the MS phase is 26 per cent. For group C, containing the most massive stars (M= 12.4,18.8 M,), the Vt(He i) values display a correlation with t/tMS, varying from 4 to 23 km s,1. The He/H determination for hot evolved B giants of the group with Vt(He i) > 15 km s,1 depends on a choice between the Vt(He i) and Vt(O ii, N ii) scales. The mean He/H enrichment by 67 per cent during the MS phase is found, if the abundances He/H are based on the Vt(O ii, N ii) scale; however, two evolved giants with especially high v sin i, HR 7446 and 7993, show the He/H enhancement by about a factor of 2.5. When using the same Vt scale, we found a trend of He/H with projected rotational velocities v sin i; a large dispersion for v sin i > 150 km s,1 can result from differences in masses M. A comparison with the stellar model computations with rotationally induced mixing shows that the observed helium enrichment during the MS phase can be explained by rotation with initial velocities 250,400 km s,1. The He/H distribution on M and v sin i based on the Vt(O ii, N ii) scale seems to be in better agreement with the theory than one based on the Vt(He i) scale. The mean value He/H = 0.10 derived for stars in the zero age main sequence (ZAMS) vicinity can be adopted as the typical initial helium abundance for early B stars in the solar neighbourhood. [source]

A method for the direct determination of the surface gravities of transiting extrasolar planets

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2007
John Southworth
ABSTRACT We show that the surface gravity of a transiting extrasolar planet can be calculated from only the spectroscopic orbit of its parent star and the analysis of its transit light curve. This does not require additional constraints, such as are often inferred from theoretical stellar models or model atmospheres. The surface gravity of the planet can therefore be measured precisely and from only directly observable quantities. We outline the method and apply it to the case of the first known transiting extrasolar planet, HD 209458b. We find a surface gravity of gp= 9.28 ± 0.15 m s,2, which is an order of magnitude more precise than the best available measurements of its mass, radius and density. This confirms that the planet has a much lower surface gravity than that predicted by published theoretical models of gas giant planets. We apply our method to all 14 known transiting extrasolar planets and find a significant correlation between surface gravity and orbital period, which is related to the known correlation between mass and period. This correlation may be the underlying effect as surface gravity is a fundamental parameter in the evaporation of planetary atmospheres. [source]

Non-LTE model atmospheres for supersoft X-ray sources

ASTRONOMISCHE NACHRICHTEN, Issue 2 2010
T. Rauch
Abstract In the last decade, X-ray observations of hot stellar objects became available with unprecedented resolution and S/N ratio. For an adequate interpretation, fully metal-line blanketed Non-LTE model-atmospheres are necessary. The Tübingen Non-LTE Model Atmosphere Package (TMAP) can calculate such model atmospheres at a high level of sophistication. Although TMAP is not especially designed for the calculation of spectral energy distributions (SEDs) at extreme photospheric parameters, it can be employed for the spectral analysis ofburst spectra of novae like V4743 Sgr or line identifications in observations of neutron stars with low magnetic fields in low-mass X-ray binaries (LMXBs) like EXO 0748-676 (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]