Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.1706c&link_type=abstract
American Astronomical Society, AAS Meeting #212, #17.06; Bulletin of the American Astronomical Society, Vol. 40, p.211
Astronomy and Astrophysics
Astronomy
Scientific paper
The profiles of strong absorption lines in CP stars cannot be fit with classical atmospheric structures. We follow recent work in assuming stratified abundances to explain the profiles of the resonance lines of Ca II and Sr II. The sharp spectral features of 10 Aql emphasize the need for stratification of Sr, even though its resonance lines are significantly weaker than those of Ca II.
In the case of Ca II, the infrared triplet (IRT) lines show large isotope shifts. In some stars, it appears that the dominant isotope may be the rare Ca-48. We investigate the bold hypothesis of Ryabchikova, Kochukhov, and Bagnulo (2008arXiv0801.2245R; RKB) that for some stars, the relative amount of Ca-48 is small, but that its prominence is due to its being concentrated in a high cloud layer. Thus far, we have been unable to confirm this for either 10 Aql, the case best illustrated by RKB, or Gamma Equ for which we have data for all three lines. Usually, an (isotope) unstratified, near 50-50 mix gives a good fit to the profiles for these two stars. The stratification parameters do differ from one IRT line to the next, but it is not yet clear how meaningful these differences are.
We argue that the diversity of isotopic compositions of mercury in HgMn stars means there is some mechanism, --diffusion or light-induced drift--capable of isotopic separation. In HgMn stars, the most likely separation is vertical, leading to isotopic stratification. Similar processes could operate to separate the calcium isotopes. The RKB hypothesis is plausible, though our study does not support it at this time. The stars with the largest (Ca-48) isotope shifts tend to have relatively weak lines, so that separate shifts for the core and wings are difficult to delineate. Our work is thus a progress report.
Cowley Charles
Hubrig Svetlana
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