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C. R. Cowley, S. Hubrig, I. Kamp (U. Mich., ESO, StSci)
We present spectra of number of cool, magnetic Ap (or CP2) stars showing sharp, deep, ``nibs'' at the cores of the Ca II K-lines. At high resolution, the contrast with normal stars is pronounced. Similar nibs are found for Ca II H-lines, but the profiles are strongly perturbed by H-\epsilon. The Ca K profile of the Am star HR 1353 resembles that of normal stars. All spectra are from the ESO UVES spectrograph. They are of generally higher quality than those used in previous investigations because of resolution, S/N, and the intrinsically narrow lines of the spectra studied. Babel (A&A, 258, 449, 1992), working with specially-defined equivalent widths, found that the absorption within 0.3Å\ of Ca II K-line cores of magnetic CP stars is characteristically less than that of normal stars. His sample, though more numerous than ours, had relatively few objects with truly sharp-lined spectra. In spite of the nibs, our stars still show the absorption within a few tenths of an Angstrom of the line cores is lower than in normal stars. This agrees with Babel's result. The nibs are not apparent in hotter CP2 stars; weaker total Ca K-line absorption reduces the contrast between the line center and near wings. Ryabchikova et al. (A&A, 423, 705, 2004) synthesized a nib-like core in LTE for a Gamma Equ model with a stratified calcium abundance. We find that NLTE calculations modify the K-line cores, though not substantially. We show that other possibilities in addition to chemical stratification may yield nib-like cores.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.