An MK-like system of spectral classification for hot subdwarfs^⋆

¹ Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
e-mail: johndrilling@comcast.net
² Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
e-mail: csj@arm.ac.uk
³ Dr. Remeis Sternwarte, Universität Erlangen-Nürnberg, 96049 Bamberg, Germany
e-mail: Ulrich.Heber@sternwarte.uni-erlangen.de
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: smoehler@eso.org
⁵ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
e-mail: r.napiwotzki@gmail.com

Received: 17 April 2012
Accepted: 27 November 2012

Abstract

An MK (Morgan-Keenan)-like system of spectral classification for hot subdwarfs is presented. We find that a three-dimensional spectral type, consisting of a “spectral” class, a “luminosity” class, and a “helium” class, is necessary to classify the sdO (subdwarf O) and sdB (subdwarf B) stars. In addition, the helium-strong stars appear to form two parallel spectral sequences: one showing strong lines of CII, CIII, or CIV, and the other with these same lines weak or absent. We also give a preliminary calibration of the new spectral types in terms of effective temperature, surface gravity, and surface helium-to-hydrogen abundance ratio, and show the relation between the new spectral types and the natural groups defined by the Palomar-Green (PG) survey.