Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...309..879v&link_type=abstract
Astronomy and Astrophysics, v.309, p.879-891
Astronomy and Astrophysics
Astrophysics
64
Stars: Atmospheres, Stars: Chemically Peculiar, Stars: Fundamental Parameters, Stars: Individual 63 Tau, Stars: Individual {Tau} Uma
Scientific paper
With a view of improving the effective temperature determinations, two independent methods have been used, Balmer line profile fitting and the Infrared Flux Method. The effective temperature is the first fundamental parameter to be established before doing abundance analyses of Am stars. For this purpose we used new CCD observations, and the latest versions of Kurucz codes for stellar atmospheres and Balmer line profile calculations. We used the Balmer line profile property of being gravity independent for effective temperatures less than 8500K. However examination of ATLAS9 and BALMER9 code results show that Balmer line profiles are not only dependent on the effective temperature of the atmosphere, but also on its metallicity, and are moreover strongly sensitive to its structure. In a first step we adjusted the convection parameter of atmospheres of the Sun, whose T_eff_, gravity and metallicity are well and independently established, and of Procyon used as a standard for hotter stars. The results showed that in order to fit the observed profiles, unless we increase the temperature of the Sun and Procyon by 400K, we had to lower the mixing length parameter l/H_p_ to 0.5, and to remove the overshooting option in ATLAS9 code, showing in this way a large effect of this process on computed profiles. We recomputed the models with these new options, and adopted for the two Am stars 63 Tauri and τ UMa the parameters [M/H]=+0.5 and logg=4.0 as the most reasonable working hypotheses. Concerning the Infrared Flux Method, it had to be used in its modified form, to take into account the presence of a companion, since both Am stars are single lined spectroscopic binaries. It turned out that the agreement between the effective temperatures derived independently by the two methods was very good. We combined the effective temperatures obtained from the two methods, which yielded T_eff_=7190K for 63 Tauri, and T_eff_=7045K for τ UMa, with the remarkably low internal uncertainty of 45K, due to their good consistency. We discuss the discrepancies with previous deteminations, and the part played by the metal abundances included in the opacities. Moreover we would stress the physical questions raised by the classical treatment of convective energy transport, as well as the necessity to be cautious in the use of atmosphere models for the interpretation of observations. In particular our work raises questions about the physical meaning of the mixing-length theory usually included in the convection treatment.
Megessier C.
van't Veer-Menneret C.
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