A non-LTE stellar atmosphere study of nitrogen-rich early-type stars

Schonberner, D.; Herrero, A.; Becker, S.; Eber, F.; Butler, K.; Kudritzki, R. P.; Simon, K. P.
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Astronomy and Astrophysics (ISSN 0004-6361), vol. 197, no. 1-2, May 1988, p. 209-222.

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Spectrograms of a sample of nitrogen-rich early type stars (OBN stars) are analyzed differentially to normal stars of similar spectral types by means of NLTE methods. Effective temperatures, gravities and atmospheric helium contents are derived by fitting theoretical profiles of hydrogen and helium lines to the observations. The abundance analysis yields the following striking result: All analyzed OBN stars have helium-enriched atmospheres, the (number) fraction of helium being between 0.15 and 0.26. The normal stars have solar helim contents. The elemental distribution of carbon, nitrogen and oxygen is also essentially solar in the normal stars, but it is completely different in the atmospheres of the OBN stars. Their nitrogen is enriched, carbon depleted, and oxygen about normal.