engleski

Fitting of silicon lines in HD 198478 UV and optical spectra

Atmospheric parameters of the Galactic early B-supergiant HD 198478 (55 Cyg) were determined from the UV and optical silicon lines taken from the high-resolution 1150-1980 Å IUE and 4100-4900 Å optical spectra. The TLUSTY numerical code was used to model the stellar atmosphere and to determine the temperature and surface gravity assuming a non-LTE plane parallel hydrostatic stellar atmosphere with microturbulence. The synthesized spectra were broadened by the appropriate instrumental profile, rotational and macroturbulent velocity with ROTIN numerical code. The silicon lines of different stages of ionization (Si II and Si III) in optical and UV were modeled, leading to the temperature, surface gravity, rotational and macroturbulent velocity values. Balmer lines were used to further constrain surface gravity value. Our results have shown that the line broadening cannot be explained by rotational velocity only, but an additional macroturbulent velocity component should be taken into account. HD 198478 shows a significant degeneracy in velocity, which means that the individual contributions of the macroturbulence and rotation in the total velocity broadening cannot be distinguished. The adequate fit of TLUSTY models to the observed non-resonant silicon lines and comparison between the atmospheric parameters obtained from UV and optical spectra suggests that the non-LTE plane-parallel hydrostatic stellar model without wind contribution can be used to explain such lines both in optical and ultraviolet. We have obtained similar results using the HST STIS spectra in the same procedure, showing that the IUE spectra, despite their lacking quality compared to the STIS spectra, are reliable enough to determine the B supergiants' photospheric parameters.

line: profiles; stars: fundamental parameters; stars:

nije evidentirano