C II Abundances in Early-Type Stars: Solution to a Notorious Non-LTE Problem

We address a long-standing discrepancy between non-LTE analyses of the prominent C II 4267 and 6578/82 Å multiplets in early-type stars. A comprehensive non-LTE model atom of C II is constructed based on critically selected atomic data. This model atom is used for an abundance study of six apparently slow-rotating main-sequence and giant early B-type stars. High-resolution and high S/N spectra allow us to derive highly consistent abundances not only from the classical features but also from up to 18 additional C II lines in the visual—including two so far unreported emission features equally well reproduced in non-LTE. These results require the stellar atmospheric parameters to be determined with care. A homogeneous (slightly) subsolar present-day carbon abundance from young stars in the solar vicinity (in associations and in the field) of log(C/H) + 12 = 8.29 ± 0.03 is indicated.