Analysis of the MG II resonance lines in the spectrum of Sirius.

Abstract

The atmospheric layers above the photosphere of Sirius A (Alpha CMa, AlV) are analyzed using high-resolution spectra of the Mg II resonance lines obtained with the Copernicus and IUE satellites. The line cores are formed at optical depths so small that an extension of the existing photospheric models for Sirius is necessary. Two tentative sets of atmospheric models based on the Bell and Dreiling (1981 and 1982) photospheric model are investigated: (1) the photospheric model prolonged toward the exterior, with different temperature plateaux (6000, 7000, and 7500 K); and (2) a chromospherelike temperature rise operating about m = 0.0001 g/sq cm. Both kinds of atmospheric models may be invoked at present as alternative models for the external atmospheric layers in Sirius. The observations are compared with computed NLTE line profiles with complete or partial redistribution. An abundance ratio of Mg/H = 0.000035 is deduced. The contribution of the Mg II interstellar absorption is also discussed.