The SALT survey of helium-rich hot subdwarfs: methods, classification, and coarse analysis

Abstract

ABSTRACT A medium- and high-resolution spectroscopic survey of helium-rich hot subdwarfs is being carried out using the Southern African Large Telescope (SALT). Objectives include the discovery of exotic hot subdwarfs and of sequences connecting chemically peculiar subdwarfs of different types. The first phase consists of medium-resolution spectroscopy of over 100 stars selected from low-resolution surveys. This paper describes the selection criteria, and the observing, classification, and analysis methods. It presents 107 spectral classifications on the MK-like Drilling system and 106 coarse analyses (${T_{\rm eff}}, \log g, \log y$) based on a hybrid grid of zero-metal non-LTE and line-blanketed LTE model atmospheres. For 75 stars, atmospheric parameters have been derived for the first time. The sample may be divided into six distinct groups including the classical ‘helium-rich’ sdO stars with spectral types (Sp) sdO6.5–sdB1 (74) comprising carbon-rich (35) and carbon-weak (39) stars, very hot He-sdO’s with Sp ≲ sdO6 (13), extreme helium stars with luminosity class ≲5 (5), intermediate helium-rich subdwarfs with helium class 25–35 (8), and intermediate helium-rich subdwarfs with helium class 10–25 (6). The last covers a narrow spectral range (sdB0–sdB1) including two known and four candidate heavy-metal subdwarfs. Within other groups are several stars of individual interest, including an extremely metal-poor helium star, candidate double-helium subdwarf binaries, and a candidate low-gravity He-sdO star.