scholarly journals Model atmospheres and spectral analyses of Wolf-Rayet stars

1995 ◽  
Vol 163 ◽  
pp. 105-115
Author(s):  
W.-R. Hamann
Keyword(s):  
Chemical Compositions ◽  
Vertical Strip ◽  
Hydrogen Burning ◽  
Spectral Analyses ◽  
Fundamental Parameters ◽  
Processed Material ◽  
Cno Cycle ◽  
Interesting Correlation ◽  
Effective Temperatures ◽  
Expanding Atmospheres

Stratified non-LTE models for expanding atmospheres have become available in the recent years. They are based on the idealized assumptions of spherical symmetry, stationarity and radiative equilibrium. The satisfactory agreement between calculated and observed Wolf-Rayet spectra suggests that this “standard model” is basically adequate for describing real WR atmospheres and hence can be applied for their quantitative spectral analyses. By the application of these models, the fundamental parameters have been determined meanwhile for the majority of the known Galactic WR stars. Most WN stars populate a vertical strip in the Hertzspung-Russell diagram at effective temperatures of ≈35 kK, the luminosities ranging from 104.5 to 105.9L⊙. Only three WN stars of earliest subtype, other early-type WN stars if they have strong lines, and the WC stars are hotter. The chemical compositions of the WR atmospheres correspond to nuclear-processed material (WN: hydrogen burning in the CNO cycle; WC: helium burning). Hydrogen is depleted but still detectable in the cooler members of the WN subclass. Quantitatively, the hydrogen abundances show an interesting correlation with the luminosity which can be compared with the predictions from evolutionary calculations.

scholarly journals Fundamental Parameters and Models of Stellar Atmospheres

1985 ◽  
Vol 111 ◽  
pp. 303-329
Author(s):  
Bengt Gustafsson ◽  
Uffe Graae-Jørgensen
Keyword(s):  
Model Atmosphere ◽  
Stellar Atmospheres ◽  
Chemical Compositions ◽  
Fundamental Parameters ◽  
Different Types ◽  
Effective Temperatures

The use of photometric and spectroscopic criteria, calibrated by model-atmosphere calculations, for determining effective temperatures, surface gravities and chemical compositions of stars is illustrated and commented on. The accuracy that can be obtained today in such calibrations is discussed, as well as possible ways of improving this accuracy further for different types of stars.

scholarly journals Search For Cool White Dwarf Pulsators

2000 ◽  
Vol 176 ◽  
pp. 525-526
Author(s):  
Atsuko Nitta ◽  
A. Mukadam ◽  
D. E. Winget ◽  
A. Kanaan ◽  
S. J. Kleinman ◽  
...  
Keyword(s):  
Lower Limit ◽  
White Dwarf ◽  
White Dwarfs ◽  
Galactic Disk ◽  
Chemical Compositions ◽  
Physical Processes ◽  
Effective Temperatures

AbstractWe are searching for pulsations in cool (< 6000 K) white dwarfs (WDs), hoping to apply asteroseismological techniques to improve our understanding of their structure and the physical processes inside them. This information is important as we use cool WDs to estimate the lower limit of the age of the Galactic disk. Within a spectroscopic and photometric survey of 110 cool WDs by Bergeron, Ruiz, & Legget, we find 28 candidates with appropriate effective temperatures, masses, and chemical compositions for possible pulsations in nonradial g modes with periods similar to those we observe in DAVs. So far, we have observed 4 candidates, but have found no evidence of large variation.

scholarly journals Hydrogen-Poor Binary Stars

1985 ◽  
Vol 87 ◽  
pp. 230-243 ◽  
Author(s):  
Mirek J. Plavec
Keyword(s):  
Mass Transfer ◽  
Mass Loss ◽  
Binary Stars ◽  
Large Scale ◽  
Hydrogen Burning ◽  
Complex Process ◽  
Processed Material ◽  
Helium Stars ◽  
Two Stages

AbstractHydrogen-poor and helium-rich stars are easy to produce in interacting binaries. Thus they should be found among Population I binaries, in which a large-scale mass transfer has occurred between the components (possibly associated with mass loss from the system). For in such cases, those layers are now on the surface of the “loser” (and, most likely, also on the surface of the “gainer”) that were subject to hydrogen burning and the associated mixing of processed material. Helium overabundance in these objects will be accompanied by an overabundance of nitrogen and underabundance of carbon, as a result of the CNO process. All the Algol-like semidetached binaries should be mild helium stars; so far this has been demonstrated only in β Lyrae, for the He/H ratio is not extreme in such cases. Extreme helium stars require a more complex process, with two stages of mass transfer and/or loss (“case BB”); υ Sagittarii and KS Persei seem to be good examples of this process. The optically invisible components of these two stars seem to have been detected with the IUE. Good model atmospheres do not exist yet, so caution must be exercised in interpreting the UV data.

Hydrogen burning of 17O in the CNO cycle

1975 ◽  
Vol 250 (2) ◽  
pp. 295-308 ◽  
Author(s):  
C. Rolfs ◽  
W.S. Rodney
Keyword(s):  
Hydrogen Burning ◽  
Cno Cycle

scholarly journals News of Effective Temperatures of L Dwarfs

2003 ◽  
Vol 211 ◽  
pp. 403-404
Author(s):  
Andreas Schweitzer ◽  
John E. Gizis ◽  
France Allard ◽  
Peter H. Hauschildt
Keyword(s):  
Ir Spectra ◽  
Spectral Analyses ◽  
Effective Temperatures ◽  
L Dwarfs

In this work we report recent spectral analyses of L dwarfs and our success in measuring Teff and log(g). Using dust filled atmospheres for early L dwarfs and rained out atmospheres for late L dwarfs we could derive Teff of 1400 to 2000 K for L8 to M9.5 dwarfs respectively. We also give an outlook what we can achieve with future models that are improving the fits to intermediate L dwarfs and IR spectra.

scholarly journals Chemical fingerprints of He-sdO stars

2018 ◽  
Vol 27 (1) ◽  
pp. 27-34 ◽  
Author(s):  
Markus Schindewolf ◽  
Peter Németh ◽  
Ulrich Heber ◽  
Tiara Battich ◽  
Marcelo M. Miller Bertolami ◽  
...  
Keyword(s):  
Spectroscopic Analysis ◽  
Atmospheric Parameters ◽  
Intermediate Mass ◽  
Solar Abundance ◽  
Hydrogen Burning ◽  
Cd 31 ◽  
Chemical Fingerprints ◽  
Synthetic Spectra ◽  
Cno Cycle ◽  
Formation History

Abstract The chemical composition of helium-rich hot subluminous O stars plays an important role to understand and model their formation history. We present a spectroscopic analysis of four He-sdO stars,CD-31° 4800, [CW83] 0904- 02, LSS 1274 and LS IV +10° 9. The analysis is based on archival optical and UV high-resolution spectra. We used Tlusty200/Synspec48 to compute line blanketed non-LTE model atmospheres and their corresponding synthetic spectra and derive the atmospheric parameters as well as the abundances of the most prominent elements. All stars have helium-dominated atmospheres with hardly any hydrogen and temperatures between 42000 K and 47000 K while their surface gravity spans between log g = 5.4 and 5.7. CD-31° 4800 shows an enrichment of nitrogen and the characteristic pattern of hydrogen burning via the CNO-cycle, while the rest of the elements have about the solar abundance. This points to the slow merger of two helium white dwarfs as the most likely origin for this system. The other three stars are enriched in carbon, nitrogen and neon while their intermediate mass element’s abundance scatters around the solar value. They were possibly formed in the deep mixing late hot flasher scenario.

scholarly journals Fundamental parameters of B type stars

2014 ◽  
Vol 9 (S307) ◽  
pp. 129-130
Author(s):  
María-Fernanda Nieva
Keyword(s):  
Mass Range ◽  
Eclipsing Binaries ◽  
Precision Data ◽  
Hydrogen Burning ◽  
Fundamental Parameters ◽  
Accuracy And Precision ◽  
High Precision Data ◽  
Ob Associations ◽  
Better Than

AbstractFundamental parameters of 26 well-studied sharp-lined single early B-type stars in OB associations and in the field within a distance of ≤400 pc from the Sun are compared to high-precision data from detached eclipsing binaries (DEBs). Fundamental parameters are derived from accurate and precise atmospheric parameters determined earlier by us from non-LTE analyses of high-quality spectra, utilising the new Geneva stellar evolution models in the mass-range ~6 to 18 M⊙ at metallicity Z = 0.014. Evolutionary masses, radii and luminosities are determined to better than typically 5%, 10%, and 20% uncertainty, respectively, facilitating the mass-radius and mass-luminosity relationships to be recovered for single core hydrogen-burning objects with a similar precision as derived from DEBs. Good agreement between evolutionary and spectroscopic masses is found. Absolute visual and bolometric magnitudes are derived to typically ~0.15-0.20 mag uncertainty. Metallicities are constrained to better than 15-20% uncertainty and tight constraints on evolutionary ages of the stars are provided. The spectroscopic distances and ages of individual sample stars agree with independently derived values for the host OB associations. The accuracy and precision achieved in the determination of fundamental stellar parameters from the quantitative spectroscopy of single early B-type stars comes close (within a factor 2-4) to data derived from DEBs.

scholarly journals Vibrational instability of Population III very massive main-sequence stars due to the -mechanism

2012 ◽  
Vol 421 (1) ◽  
pp. L34-L38 ◽  
Author(s):  
Takafumi Sonoi ◽  
Hideyuki Umeda
Keyword(s):  
Nuclear Energy ◽  
Main Sequence ◽  
Early Stage ◽  
Cooling Process ◽  
Main Sequence Stars ◽  
Hydrogen Burning ◽  
Cno Cycle ◽  
The Universe ◽  
Vibrational Instability ◽  
Population Iii

ABSTRACT Very massive stars are thought to be formed in the early Universe because of a lack of cooling process by heavy elements, and might have been responsible for the later evolution of the Universe. We had an interest in the vibrational stability of their evolution and have carried out a linear non-adiabatic analysis of radial and non-radial oscillations for Population III very massive main-sequence stars with . We found that only the radial fundamental mode becomes unstable due to the -mechanism for these stars. The instability appears just after the CNO cycle is activated and the nuclear energy generation rate becomes large enough to stop the pre-main-sequence contraction, and continues during the early stage of core hydrogen burning. Also, we have roughly estimated the amount of mass loss due to the instability to evaluate its significance.

scholarly journals Fundamental stellar parameters of benchmark stars from CHARA interferometry

2020 ◽  
Vol 640 ◽  
pp. A25
Author(s):  
I. Karovicova ◽  
T. R. White ◽  
T. Nordlander ◽  
L. Casagrande ◽  
M. Ireland ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Local Thermodynamic Equilibrium ◽  
High Angular Resolution ◽  
Fundamental Parameters ◽  
Spectroscopic Observations ◽  
Non Local ◽  
Fundamental Stellar Parameters ◽  
Effective Temperatures ◽  
Limb Darkening ◽  
Better Than

Context. Benchmark stars are crucial as validating standards for current as well as future large stellar surveys of the Milky Way. However, the number of suitable metal-poor benchmark stars is currently limited, owing to the difficulty in determining reliable effective temperatures (Teff) in this regime. Aims. We aim to construct a new set of metal-poor benchmark stars based on reliable interferometric effective temperature determinations and a homogeneous analysis. The aim is to reach a precision of 1% in Teff, as is crucial for sufficiently accurate determinations of the full set of fundamental parameters and abundances for the survey sources. Methods. We observed ten late-type metal-poor dwarfs and giants: HD 2665, HD 6755, HD 6833, HD 103095, HD 122563, HD 127243, HD 140283, HD 175305, HD 221170, and HD 224930. Only three of them (HD 103095, HD 122563, and HD 140283) have previously been used as benchmark stars. For the observations, we used the high-angular-resolution optical interferometric instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb-darkening models and determined effective temperatures directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities (log(g)) were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE and FIES spectrographs and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutral and singly ionised iron. Results. We inferred Teff to better than 1% for five of the stars (HD 103095, HD 122563, HD 127243, HD 140283, and HD 224930). The effective temperatures of the other five stars are reliable to between 2 and 3%; the higher uncertainty on the Teff for those stars is mainly due to their having a larger uncertainty in the bolometric fluxes. We also determined log(g) and [Fe/H] with median uncertainties of 0.03 dex and 0.09 dex, respectively. Conclusions. This study presents reliable and homogeneous fundamental stellar parameters for ten metal-poor stars that can be adopted as a new set of benchmarks. The parameters are based on our consistent approach of combining interferometric observations, 3D limb-darkening-modelling and spectroscopic observations. The next paper in this series will extend this approach to dwarfs and giants in the metal-rich regime.

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