Behaviors of Ca ii K line in A-type stars

Abstract

A synthetic spectrum-fitting analysis was applied to the resonance Ca ii line at 3933.68 Å for a large sample of 122 A-type main-sequence stars (7000 ≲ Teff ≲ 10000 K) in a wide range of projected rotational velocity (10 ≲ vsini ≲ 300 km s−1), with an aim of investigating the behaviors of Ca abundances ([Ca/H]39) determined from this strong Ca ii 3934 line, especially in context of (i) how they are related with the Am phenomenon (often seen in slow rotators) and (ii) whether they are consistent with the Ca abundances ([Ca/H]61) previously derived by the author from the weaker Ca i 6162 line. It was confirmed that Ca line strengths in Am stars tend to be weaker and associated abundances are lower compared to non-Am stars at the same Teff, indicating a deficiency of Ca in the photosphere of Am stars. However, an appreciable fraction of cool Am stars (Teff ≲ 8000 K) were found to show extraordinarily anomalous Ca ii 3934 line feature (i.e., unusually broad for its considerable weakness) which is hard to explain. Regarding the comparison between [Ca/H]39 and [Ca/H]61, while both are roughly consistent with each other for hotter stars (Teff ≳ 8000 K), the former tends to be lower (by up to −1 dex or even more) than the latter for cooler A stars (Teff ≲ 8000 K) including those “weak broad K line” objects, This fact suggests that some special mechanism reducing the strength of Ca ii 3934 line is involved at Teff ≲ 8000 K where [Ca/H]39 would be no more reliable. Whereas atomic diffusion causing the deficit of Ca in the photosphere as a result of element segregation in the deeper radiative envelope may be regarded as a promising explanation because it seems to fit in the qualitative trend of [Ca/H]61 in A-type stars, the well-known feature of considerably weak Ca ii K line in classical Am stars should not necessarily be attributed to only this element diffusion scenario, for which some unknown weakening mechanism specific to this resonance line may independently be operative.