Abundance anomalies in main sequence A stars. II - C, Si, CA and the heavy elements SR and BA

Details Abundance anomalies in main sequence A stars. II - C, Si, CA and the heavy elements SR and BA Abundance anomalies in main sequence A stars. II - C, Si, CA and the heavy elements SR and BA

Dec 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990a%26a...240..331l&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 240, no. 2, Dec. 1990, p. 331-345.

Astronomy and Astrophysics

Astrophysics

56

A Stars, Abundance, Anomalies, Main Sequence Stars, Barium, Calcium, Carbon, Silicon, Strontium, Stars: Abundances, Stars: A Type, Am Type, Vega, Sirius, Nlte, Diffusion

Scientific paper

Based on new high resolution Reticon spectra, abundances of carbon, silicon, calcium, strontium, and barium have been derived for 16 sharp lined, main sequence A stars classified as "normal", i.e. not chemically peculiar. NLTE effects of calcium and barium were taken into account.
Non-solar compositions were found with non-uniform abundance patterns. Carbon turned out to vary by up to one order of magnitude with no detectable correlation to other parameters.
Variations of silicon are closely correlated with those of iron as derived in a previous paper (Lemke, 1989). The Si/Fe ratio is solar except in HR 5959. In this object, the ratio is enhanced by 0.9 dex and barium is depleted by more than one order of magnitude.
Both strontium and barium are enhanced by typically 1.0 dex relative to the sun. Neither the Ba/Fe nor the Sr/Fe ratio is constant. Ba ii shows noticeable deviations from LTE, dependent on the barium abundance. Assuming LTE, calcium is found to be solar in most objects, Sirius is Ca deficient and four objects are probably depleted as well. Non-LTE effects have proven to be very important; they might completely compensate for the underabundances.
The AOV reference Vega also is exceptional. Solely the carbon abundance is normal; all other elements investigated have abundances distinctly lower than most of the program stars. Relating the abundances to iron, only the Si/Fe ratio is normal. Both the Sr/Fe and Ba/Fe ratios are relatively low compared to the other stars, but Ba/Fe is still higher than in the sun. The Ca/Fe ratio, however, is slightly enhanced and the C/Fe ratio is one of the highest values found.
The abundance patterns of the program stars resemble to a different degree the compositions of the chemically peculiar Am stars. Based on abundances it is hard to draw a line between Am and non-Am stars. Rather smooth transitions of metallicity were detected, ranging from abundances typical for Am stars to those of relatively metal-poor objects. This suggests a common physical origin of abundance variations, favouring diffusion models with weak stellar winds of different strengths.

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