A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics

TL;DR

This paper introduces a large new catalog of low-mass subdwarfs from SDSS, analyzing their properties, kinematics, and activity, revealing their likely Galactic halo membership and distinct characteristics from disk stars.

Contribution

The study provides the largest spectroscopically confirmed subdwarf sample to date, including new classifications, spectra, and detailed kinematic and activity analyses.

Findings

Subdwarfs are separated into three metallicity classes with distinct color-magnitude properties.

Extreme and ultra subdwarfs are likely part of the Galactic halo based on kinematics.

Activity fraction varies with spectral class but remains flat for extreme and ultra subdwarfs.

Abstract

We present a new sample of M subdwarfs compiled from the 7th data release of the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample significantly increases the number of spectroscopically confirmed low-mass subdwarfs. This catalog also includes 905 extreme and 534 ultra sudwarfs. We present the entire catalog including observed and derived quantities, and template spectra created from co-added subdwarf spectra. We show color-color and reduced proper motion diagrams of the three metallicity classes, which are shown to separate from the disk dwarf population. The extreme and ultra subdwarfs are seen at larger values of reduced proper motion as expected for more dynamically heated populations. We determine 3D kinematics for all of the stars with proper motions. The color-magnitude diagrams show a clear separation of the three metallicity classes with the ultra and extreme subdwarfs being significantly closer to the main sequence than the ordinary subdwarfs. All subdwarfs lie below (fainter) and to the left (bluer) of the main sequence. Based on the average $(U,V,W)$ velocities and their dispersions, the extreme and ultra subdwarfs likely belong to the Galactic halo, while the ordinary subdwarfs are likely part of the old Galactic (or thick) disk. An extensive activity analysis of subdwarfs is performed using H$\alpha$ emission and 208 active subdwarfs are found. We show that while the activity fraction of subdwarfs rises with spectral class and levels off at the latest spectral classes, consistent with the behavior of M dwarfs, the extreme and ultra subdwarfs are basically flat.