A&A 385, 131-142 (2002)
DOI: 10.1051/0004-6361:20020099

Physical parameters for subdwarf B stars with composite spectra

R. Aznar Cuadrado and C. S. Jeffery

Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland

(Received 5 November 2001 / Accepted 16 January 2002 )

Abstract
New intermediate-resolution spectra have been obtained for a number of subdwarf B stars having both single and composite spectra. Physical parameters have been determined for the sdB stars and, in composite-spectrum systems, their cool companions. For these binaries, we have developed a method which uses the blue-optical spectrum to determine the effective temperatures of both stars, the surface gravity of the hot stars and the radius ratio of the system. The surface gravity of the cool star is measured using the infrared calcium triplet. The surface gravities of these cool companions identify them as main-sequence stars with masses in the range 0.8-1.2, confirming a previous energy distribution analysis. There is also evidence that the composite-spectrum sdBs are more helium-poor than single-spectrum sdBs.

Key words: stars: formation -- stars: early-type -- stars: subdwarfs -- stars: fundamental parameters -- stars: binaries: spectroscopic

Offprint request: C. S. Jeffery, csj@star.arm.ac.uk

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