Characterizing Low-mass M Dwarfs Using Wide Binaries


Photo of Saurav Dhital
Saurav Dhital
Embry-Riddle Aeronautical University


Spectra of candidate binary star pairs in which one star is an M dwarf, obtained to help better characterize the ages and metallicities of M dwarfs

Finding Targets

An object whose ANCILLARY_TARGET2 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 Spectra Number of Unique Primary Objects
SPOKE2 17 Candidate binary pair in which one star is an M dwarf 95 93


M dwarf stars make up about 70% of the stars in the Galaxy by number, and have lifetimes longer than the age of the Universe. They are thus valuable tracers of the chemical and dynamical evolution of the Milky Way, but their complex spectra dominated by molecular bands make it difficult to determine their ages and metallicities.

Binary systems have the potential to improve our calibration of M dwarf properties, because both stars in each system were presumably formed from the same material at the same time – but the companion stars have atmospheres that are easier to interpret. These systems can be used to refine relations between stellar properties and spectral signatures (e.g. Stassun et al. 2008, Dhital et al. 2012).

The newly-observed binaries serve as coeval laboratories to extend metallicity indicators to late-type and low-metallicity M dwarfs. The ages of the newly-observed binary companions will allow studies of the activity-age relationship in M dwarfs.

Target Selection

Fibers denoted by the SPOKE2 target flag were assigned to candidate binary companions of spectroscopically confirmed low-mass stars in the Sloan Low mass Wide Pairs of Kinematically Equivalent Stars (Dhital, West, Stassun, & Bochanski 2010, Dhital et al. 2015).

These binaries complement those from a previous BOSS ancillary target program, Low-Mass Binary Stars (Dawson et al. 2013), which consisted of systems with angular separations 65-180″.

The SPOKE2 ancillary target program described here extends that target sample to late-M spectral types by using a new catalog that identified binaries at small angular separations (3-20″) without the need to impose any proper motion requirement (Dhital et al. 2015).

Targets have magnitudes 17 < iPSF < 21.3 and lie in pairs that are wider than 3″ to avoid contamination by the other component.


Dawson, K.S. et al. 2013, AJ, 145, 10.

Dhital, S., West, A.A., Stassun, K.G., & Bochanski, J.J. 2010, AJ, 139, 2566.

Dhital, S. et al. 2012, AJ, 143, 67.

Dhital, S. et al. 2015, AJ, 150, 57

Stassun, K.G. et al. 2008, Nature, 453(7198), 1079