Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...279..188j&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 279, no. 1, p. 188-196
Astronomy and Astrophysics
Astrophysics
18
Abundance, B Stars, Emission Spectra, Line Spectra, Photosphere, Spectrum Analysis, Stellar Temperature, Visible Spectrum, Carbon, Hydrogen, Iron, Nitrogen, Oxygen, Stellar Atmospheres, Stellar Spectrophotometry
Scientific paper
LSE 78 is a hydrogen-deficient early-B supergiant, spectroscopically similar to the pulsating extreme helium star BD - 9 deg 4395 and the hot R CrB star DY Cen. The photospheric parameters of LSE 78 have been derived from optical high-resolution spectra by the method of fine analysis. The principal results are Teff = 18000 K, log g = 2.00, nH/nHe is less than 0.0001, n(suc C)/nHe = 0.01; hydrogen is extremely deficient. The effective temperature is consistent with broadband visual and ultraviolet spectrophotometry and an extinction EB - V approximately 0.25, subject to peculiar extinction in the UV (lambda less than 2000A). Assuming a He-shell burning model, a mass approximately 0.8 solar mass may be estimated for LSE 78. Its previous evolution is reflected in the chemistry of the atmosphere, which contains enriched nitrogen from CNO-cycle hydrogen burning, carbon from 3 alpha helium burning, and oxygen from alpha-capture reactions. The latter makes LSE 78 the most oxygen-rich extreme helium star analysed so far, with nC/nO approximately 3. This low ratio indicates that the atmospheric carbon and oxygen are the products of near complete helium burning. The iron abundance is sim 0.2 times solar. Weak and variable emission lines due to He I and C II are detected, but are never as strong as those detected in BD-9 deg 4395 or DY Cen. By analogy, they provide evidence for a recombination shell, or extended atmosphere, with a temperature slightly below Teff, around LSE 78.
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