Hot subdwarf stars in close-up view

IV. Helium abundances and the ³He isotopic anomaly of subdwarf B stars^⋆

¹ Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: sgeier@eso.org
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Department of Astrophysics, Faculty of Science, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Department of Physics, University of Western Cape, Private Bag X17, 7535 Bellville, South Africa
⁵ South African Astronomical Observatory, PO Box 9, 7935 Observatory Cape Town, South Africa
⁶ Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory Cape Town, South Africa
⁷ Department of Physics, University of Warwick, Conventry CV4 7AL, UK
⁸ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH 41 1 LD, UK

Received: 11 June 2013
Accepted: 22 July 2013

Abstract

Atmospheric parameters and helium abundances of 44 bright subdwarf B stars have been determined. More than half of our sample consists of newly discovered stars from the Edinburgh Cape survey. We showed that effective temperatures and surface gravities can be derived from high resolution echelle spectra with sufficient accuracy. Systematic uncertainties have been determined by comparing the parameters derived from the high resolution data with the ones derived from medium resolution spectra. Helium abundances have been measured with high accuracy. Besides the known correlation of helium abundance with temperature, two distinct sequences in helium abundance have been confirmed. Significant isotopic shifts of helium lines due to an enrichment in ³He have been found in the spectra of 8 subluminous B stars (sdBs). Most of these stars cluster in a small temperature range between 27 000 K and 31 000 K very similar to the known ³He-rich main sequence B stars, which also cluster in such a small strip, but at different temperatures. Both the helium sequences and the isotopic anomaly are discussed.