Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...276..171r&link_type=abstract
Astronomy and Astrophysics, Vol.276, NO. 1/SEPI, P. 171, 1993
Astronomy and Astrophysics
Astrophysics
20
Stars: Abundances, Stars: Atmospheres, Stars: Evolution, Stars: Individual: Hd 128220B, Stars: Post-Agb, Subdwarfs
Scientific paper
We present a precise NLTE analysis of the hot subdwarf in the double-lined spectroscopic-binary system HD128220 (BD+19°2824, SAO 101094), based on 38 high- resolution IUE spectra. Very detailed NLTE models were constructed using the Accelerated Lambda Iteration technique. Model atoms for H I, He I/He II, C III/C IV, N III/N IV and O IV/O V are carefully tailored to match the needs of the observed UV lines. The importance of an accurate treatment of the broadening of the He II and C IV lines is evaluated. The effective temperature is determined from the C III/C IV ionization balance: Teff = 40.6±0.4kK. The surface gravity (log g = 4.5±0.1dex) and helium abundance result from fitting several He II lines (nHe/nH = 0.30±.05). Resulting abundances of C, N and O (nC/nH = 0.0005±0.0002, (nN/nH = 0.0015±0.0003, nO/nH = 0.0003±0.0002) indicate that the surface material is nitrogen-rich and oxygen-poor but that carbon is almost solar.
Comparing the observed atmospheric parameters to evolutionary calculations it is concluded that HD128220B is a post-AGB star with a mass of M = 0.54±0.01 Msun. The observed N and O abundances support the scenario by Howarth & Heber (1990) that the HD128220 binary system has undergone mass transfer when the sdO progenitor was on the AGB. The carbon abundance can probably be explained if mass transfer occurred after the onset of thermal pulses.
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