An MK-like system of spectral classification for hot subdwarfs

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.

The population of hot subdwarf stars studied withGaia

“…It is almost impossible to homogenize the classifications attributed by different people at different times based on very different types of data. Furthermore, quite diverse classification schemes have emerged in the last few decades, from the quite detailed classifications in the PG catalog (Green et al 1986) and the more general classes introduced (and widely used today) by Moehler et al (1990) to the sophisticated MK-like classification system developed by Drilling et al (2013).…”

A search for new hot subdwarf stars by means of virtual observatory tools II

“…In Drilling et al (2013) an evolved MK-like system is developed. In this system hot subdwarfs are divided in four sequences: He-weak, He-normal, He-strong and He-strong C, with carbon or other metallic lines.…”

“…Stars former classified within the somehow ad-hoc sdOB subclass (Moehler et al 1990) are placed naturally in this scheme, in the transition between late sdO and early sdB subclasses. In the present work we will appply the Drilling et al (2013) system to classify the candidates with spectrum.…”

“…To classify the subdwarfs within the Drilling et al (2013) system we cut and normalized the spectra between 4000 and 5000 Å and then compared them with the standard stars defining the system. The complete classification of our candidates can be found in the electronic tables.…”

“…Additionally, a variety of He i lines may appear in both classes, and some sdOs show metallic C or N lines. More complex classification schemes have been proposed in Green, Schmidt & Liebert (1986) or more recently in Drilling et al (2013).…”