Spectroscopic Properties of Cool Stars in the Sloan Digital Sky Survey: An Analysis of Magnetic Activity and a Search for Subdwarfs

Abstract

We present a spectroscopic analysis of nearly 8000 late-type dwarfs in the Sloan Digital Sky Survey. Using the Hα emission line as an activity indicator, we investigate the fraction of active stars as a function of spectral type and find a peak near type M8, confirming previous results. In contrast to past findings, we find that not all M7-M8 stars are active. We show that this may be a selection effect of the distance distributions of previous samples, as the active stars appear to be concentrated near the Galactic Plane. We also examine the activity strength (ratio of the luminosity emitted in Hα to the bolometric luminosity) for each star, and find that the mean activity strength is constant over the range M0-M5 and declines at later types. The decline begins at a slightly earlier spectral type than previously found. We explore the effect that activity has on the broadband photometric colors and find no significant differences between active and inactive stars. We also carry out a search for subdwarfs using spectroscopic metallicity indicators, and find 60 subdwarf candidates. Several of these candidates are near the extreme subdwarf boundary. The spectroscopic subdwarf candidates are redder by ∼ 0.2 magnitudes in g − r compared to disk dwarfs at the same r − i color.

A Catalog of Galex Ultraviolet Emission From Spectroscopically Confirmed M Dwarfs*

“…Although Kendall's τ is negative, the median trend appears robust as it is less affected by high-luminosity DR7 dwarfs. Figure 8 is noisy, but both data sets show that the fractions of emitting M dwarfs appear to reach a maximum at mid or later spectral types, similar to the trend in Hα activity, which is maximized at a spectral type of M8 (West et al 2004). The emitting fractions show a significant bump between spectral types M3 and M4, where M dwarfs begin to become fully convective (West et al 2008), mirroring the increase in Hα activity at these types.…”

“…At later spectral types, the emitting fractions may decrease, but this is a low-significance trend that requires a much larger data set. The increase to later spectral types could be similar to the Hα active fraction, which is a maximum at a spectral type of M8 (West et al 2004). The maximum NUV-active fraction is ∼0.3 for both DR7 and PMSU M dwarfs.…”

“…For low-mass stars, the SDSS has produced photometric and spectroscopic samples of more than 30 million and 70,000 M dwarfs respectively (Bochanski et al 2010;West et al 2011). Low-mass stars in SDSS have been used as tracers of Galactic dynamics (Bochanski et al 2007Fuchs et al 2009) and Galactic extinction (Jones et al 2011) and to investigate several aspects of the stars themselves, including but not limited to, the stellar mass and luminosity functions (Covey et al 2008b;Bochanski et al 2010), properties of magnetic activity (West et al 2004(West et al , 2008, and the low-metallicity population of the Galactic disk (Savcheva et al 2014).…”

“…Magnetic activity is best understood at optical wavelengths, where large spectroscopic studies such as the Sloan Digital Sky Survey (York et al 2000;Abazajian et al 2009, SDSS) have enabled a statistical treatment of M dwarf activity, particularly by using Hα emission as a diagnostic (L L ; H bol a West et al 2004West et al , 2008West et al , 2011. West et al (2008) showed that activity in the optical varies as a function of both spectral type and stellar age, with a peak in the active fraction occurring near a spectral type of M8 (Reid et al 1995;Hawley et al 1996;Gizis et al 2000;West et al 2004).…”

M dwarfs and the fraction of high carbon-to-oxygen stars in the solar neighbourhood

“…A third important sample is the SDSS stars with spectroscopy, although some subdwarfs were deliberately targeted and the sample includes color selections that may be biased for or against subdwarfs. West et al (2004) noted 60 SDSS M subdwarfs of all types compared to ∼ 8000 M dwarfs. West et al (2011) identified ∼ 70, 000 spectroscopically confirmed M dwarfs in SDSS data, of which only ∼ 2000 are M subdwarfs even including more extreme ones (Bochanski et al 2013).…”