Possible Overionization of C II, N II, and O II Ions in the Atmospheres of Early B- and Late O-Type Stars

ISSN： 0571-7256

Source： Astrophysics, Vol.56, Iss.4, 2013-12, pp. : 472-487

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Abstract

The reliability of C, N, and O abundances determined from lines of C II, N II, and O II in the main-sequence (MS) early B- and late O-type stars is examined. This analysis is based on a fact of primary importance for this problem: the C II, N II, and O II ions in the atmospheres of these stars are photoionized by radiation in the far UV. Observations show that the actual flux in this range for hot stars can be considerably higher than theoretical levels; thus, overionization of the C II, N II, and O II ions beyond the ionization calculated with the standard model atmospheres occurs in the actual atmospheres of these stars. Underestimates in the calculated ionization lead to reduced values of the C, N, and O abundances. This is confirmed by the observed dependence on the effective temperature T _eff: the C, N and O abundances tend to be lower with increasing T _eff. It is shown that overionization becomes significant for sufficiently hot stars, specifically when T _eff > 18500 K for the C II lines and when T _eff > 26000 K for the N II and O II lines. The systematic difference in the C, N and O abundances between these relatively hot stars and the cooler B-type MS stars is about 0.2 dex. The relatively cool B-type MS stars (T _eff < 18100 K for C II and T _eff < 25000 K for N II and O II) yield the undistorted C, N and O abundances, and their average values agree very well with modern estimates for the sun. This confirms that the metallicity of B- and O-type MS stars in the Sun’s vicinity is the same as that of the sun. An explanation is given for the slight deficit of carbon found for the early B-type MS stars in previous work. It is noted that when an alternative method based on examining the ionization balance (e.g., He I-II, C II-III, Ne I-II, Si III-IV) is used to determine the parameters T _eff and log g, the overionization problem essentially vanishes; however, this method leads to a systematic enhancement in the values of T _eff and log g. The overionization problem can be solved by shifting to an improved model for the atmospheres of early B- and late O-type stars that takes their sphericity into account and, possibly, stellar winds and magnetic fields.