Sulfur Abundances in Orion B Stars

AbstractSulfur abundances are derived for a sample of ten B MS star members of the Orion association. The analysis is based on LTE model atmospheres and non-LTE line formation theory by means of spectrum synthesis analysis of Sii and Siii lines. The abundance distribution obtained for the Orion targets is homogeneous within the errors in the analysis: A(S)=7.15±0.05. This abundance result is in agreement with the solar value and with results for the Orion nebula. The sulfur abundances for Orion combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of −0.037±0.012 dex Kpc−1.

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Boron Abundances of B Stars of the Orion Association

The Astrophysical Journal ◽

10.1086/303791 ◽

1997 ◽

Vol 478 (1) ◽

pp. 211-224 ◽

Cited By ~ 25

Author(s):

Katia Cunha ◽

David L. Lambert ◽

Michael Lemke ◽

Douglas R. Gies ◽

Lewis C. Roberts

Keyword(s):

B Stars ◽

Orion Association

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Three Dimensional image Reconstruction of an Hα Flare

Symposium - International Astronomical Union ◽

10.1017/s0074180900219372 ◽

2001 ◽

Vol 203 ◽

pp. 303-306 ◽

Cited By ~ 1

Author(s):

Z. Q. Qu ◽

Y. C. Jiang ◽

T. Luan ◽

Z. Xu

Keyword(s):

Image Reconstruction ◽

Line Intensity ◽

Three Dimensional ◽

Model Atmosphere ◽

Line Center ◽

Line Formation ◽

3D Image ◽

Photon Distribution ◽

Formation Theory ◽

Specific Line

The three-dimensional (3D) image of an Hα flare is reconstructed from its line-center and seriary off-center filtergrams. The reconstruction is based on the principle of the line formation theory that the contribution function (CF) of emergent specific line intensity describes the escape photon distribution from stellar atmospheric layers. Thus the reconstruction depends on the model atmosphere accepted which determines the CF. An Hα flare taking place on April 22, 1992 is utilized to illustrate the process.

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Non-local thermodynamic equilibrium line formation for Si i–ii–iii in A–B stars and the origin of Si ii emission lines in ι Her

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa653 ◽

2020 ◽

Vol 493 (4) ◽

pp. 6095-6108 ◽

Cited By ~ 4

Author(s):

Lyudmila Mashonkina

Keyword(s):

Thermodynamic Equilibrium ◽

Local Thermodynamic Equilibrium ◽

Emission Lines ◽

Atomic Data ◽

Line Formation ◽

Atmospheric Parameters ◽

B Stars ◽

Non Local ◽

Effective Temperatures ◽

Model Atom

ABSTRACT A comprehensive model atom was developed for Si i–ii–iii using the most up-to-date atomic data available so far. Based on non-local thermodynamic equilibrium (NLTE) line formation for Si i, Si ii and Si iii and high-resolution observed spectra, we determined the NLTE abundances for a sample of nine unevolved A9–B3 type stars with well-determined atmospheric parameters. For each star, NLTE reduces the line-to-line scatter for Si ii substantially compared with the LTE case and leads to consistent mean abundances from lines of different ionization stages. In the hottest star of our sample, ι Her, Si ii is subject to overionization that drives emission in the lines arising from the high-excitation doublet levels. Our NLTE calculations reproduced 10 emission lines of Si ii observed in ι Her. The same overionization effect leads to greatly weakened Si ii lines, which are observed in absorption in ι Her. Large positive NLTE abundance corrections (up to 0.98 dex for 5055 Å) were useful for achieving consistent mean abundances from lines of the two ionization stages, Si ii and Si iii. It was found that NLTE effects are overestimated for the Si ii 6347, 6371 Å doublet in ι Her, while the new model atom works well for cooler stars. At this stage, we failed to understand this problem. We computed a grid of the NLTE abundance corrections for lines of Si i, Si ii and Si iii in model atmospheres with effective temperatures and surface gravities characteristic of unevolved A–B type stars.

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Puzzling problems of He I line formation in early B stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900215052 ◽

1994 ◽

Vol 162 ◽

pp. 273-274

Author(s):

Myron A. Smith ◽

Ivan Hubeny ◽

Thierry Lanz

Keyword(s):

Equivalent Width ◽

Be Stars ◽

Line Formation ◽

B Stars ◽

Thermal Changes ◽

Red Line

Although NLTE model atmospheres have been shown to resolve most of the equivalent width (EW) discrepancies for blue He I lines (Auer and Mihalas 1972, 1973), Wolff and Heasley (1984, 1985) have demonstrated that discrepancies remain for the leading members of the singlet/triplet 2P – nD series, viz. λ6678 and λ5876. These two lines are the strongest nonresonance He I transitions and are important because they respond to thermal changes in the superficial atmosphere (τ~ 10-3) of early B stars. In order to understand the observed rapid variations of the λ6678 line in mild Be stars, we undertook a survey of EWs of λ6678 and λ4388, namely the first and third member of the same series. These two lines have a log gf ratio of 15 but have similar EWs in B star spectra. Our new observations confirm the red line discrepancy noted by WH85 and point to additional EW differences among various groups of B stars not noted hitherto.

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