The detection of binary companions to subdwarf B stars

C. Simon Jeffery¹ and Don L. Pollacco²

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
² Isaac Newton Group, La Palma

MNRAS 298, 179-184

Abstract. Intermediate dispersion spectroscopy of a sample of 40 hot subdwarf B stars between 5500 and 9000A has been obtained. The sample includes a large fraction of targets which have been studied photometrically. Seven targets show strong lines due to the infrared Ca II triplet, the unequivocal signature of a cool companion. The positive Ca II identifications include known photometric binaries and new targets; all are associated with a photometric red excess. Assuming a canonical value for the subdwarf luminosity, all of the detected companions are overluminous compared with the main-sequence. The detection procedure indicates an improved and more secure method for the measurement of the binary frequency of hot subdwarfs.

Keywords. Stars: early-type, subdwarfs, binaries: spectroscopic.

Calcium triplet detections. Spectra of seven sdB stars in the region of the Ca II infrared triplet. The spectra have been normalized and offset, but have not had any velocity corrections applied. Pixels badly affected by a cosmic ray (cr) have been removed.

Two-colour diagram for sdBs. The two-colour diagram for subdwarf B stars, as described in the paper. Triangles represent sdB stars which show the Ca II triplet (CaT). Squares represent sdB stars which do not show CaT. Open symbols represent reported composite objects. Filled symbols have not previously been reported as composite. The reddening vector is shown. Labelled stars are discussed in the paper.