Very Low-Mass Stars and Brown Dwarfs

Contact

Suzanne Hawley
University of Washington
slh@astro.washington.edu

Summary

A search for M stars and L dwarfs in the BOSS survey area (9,376 square degrees)

Finding Targets

An object whose ANCILLARY_TARGET1 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
BRIGHTERL 44 Bright likely L dwarf 424 391
BRIGHTERM 45 Bright likely M dwarf 2,165 1,854
FAINTERL 46 Faint likely L dwarf 1,283 1,158
FAINTERM 47 Faint likely M dwarf 3,912 3,400

Description

Very-low-mass stars and brown dwarfs (spectral types M8, M9 and L) are ideal tracers of the kinematic properties of the Milky Way thin disk. While the program The Transient Universe Through Stripe 82 uses variability information over Stripe 82 to identify rare classes of stars, this ancillary target program uses photometric selection over the much larger BOSS survey area to identify new low-mass stars and brown dwarfs.

SDSS-I and SDSS-II yielded a wealth of spectroscopic data of these ultracool dwarfs (Schmidt, West, Hawley, & Pineda 2010; West et al. 2011), but were limited by the small number of spectra observed for stars with spectral types later than M7. Additional observations of these objects are essential to understand the properties of magnetic activity in these ultracool dwarfs (extending the results of West et al. 2008). Data from this ancillary target program also enable studies using kinematics to understand the distribution of stellar ages, especially at the stellar/sub-stellar boundary. Finally, the sample contains a class of L dwarfs which are peculiarly blue in the near-infrared, but have typical L dwarf colors in SDSS i – z (Schmidt et al. 2010).

Target Selection

The ancillary target program was divided into two survey areas: the stripe 82 footprint (with target density 5 deg-2) and the overall BOSS footprint (with target density 1 deg-2). Within the Stripe 82 footprint, targets were sought at five per square degree; in the rest of the BOSS footprint, targets were sought at one per square degree. To select the cleanest possible sample, the photometric selection criteria included 2MASS magnitudes.

The target selection criteria in the Stripe 82 footprint are:

• iPSF – zPSF > 1.14
• iPSF < 21
• iPSF – J > 3.7
• 1.9 < zPSF – J < 4

Criteria for the rest of the BOSS footprint are:

• iPSF – zPSF > 1.44
• iPSF < 20.5
• iPSF – J > 3.7
• 1.9 < zPSF – J < 4

Here, the J-band photometry, unlike the SDSS photometry, is Vega-based.

In both survey areas, targets are divided into subsamples with different priorities. L dwarfs are both less common and fainter, so our priority was to detect L dwarfs over late-M dwarfs. Targets likely to be bright L dwarfs are indicated by the BRIGHTERL sub-program, and are assigned first priority. Fainter L dwarfs (FAINTERL) are assigned second priority. Lower priorities are assigned to likely M dwarfs, both brighter (BRIGHTERM) and fainter (FAINTERM).

The selection criteria used to assign targets to each of the four sub-programs is:

Sub-program Magnitude cut Color cut
BRIGHTERL iPSF < 19.5 iPSFzPSF > 1.14
FAINTERL iPSF > 19.5 iPSFzPSF > 1.14
BRIGHTERM iPSF < 19.5 iPSFzPSF < 1.14
FAINTERM iPSF > 19.5 iPSFzPSF < 1.14