The Gaia ultracool dwarf sample – II. Structure at the end of the main sequence

Abstract

We identify and investigate known late M, L and T dwarfs in the Gaia second data release. This sample is being used as a training set in the Gaia data processing chain of the ultra-cool dwarfs work package. We find 695 objects in the optical spectral range M8 to T6 with accurate Gaia coordinates, proper motions, and parallaxes which we combine with published spectral types and photometry from large area optical and infrared sky surveys. We find that 100 objects are in 47 multiple systems, of which 27 systems are published and 20 are new. These will be useful benchmark systems and we discuss the requirements to produce a complete catalog of multiple systems with an ultra-cool dwarf component. We examine the magnitudes in the Gaia passbands and find that the G BP magnitudes are unreliable and should not be used for these objects. We examine progressively redder colour-magnitude diagrams and see a notable increase in the main sequence scatter and a bi-variate main sequence for old and young objects. We provide an absolute magnitude -spectral sub-type calibration for G and G RP passbands along with linear fits over the range M8-L8 for other passbands.

CARMENES input catalogue of M dwarfs

“…This extra scatter at the reddest colours is more likely due to the intrinsic spectral variations at the M/L boundary (à la Hawley et al 2002; e.g. metallicity) than due to data analysis systematics or Poissonian error at the survey magnitude limits (à la Smart et al 2019;e.g. background).…”

“…(a) For the K dwarfs, we tabulate the SUPERBLINK catalogue identifier (Lépine & Shara 2005;Lépine et al 2013). (b) For the ultracool dwarfs, we tabulate the Gaia UltraCool Dwarf Catalogue identifier (Smart et al 2017(Smart et al , 2019). (c) SpTnum = -2 for K5 V, -1 for K7 V, 0.0 for M0.0 V, 0.5 for M0.5 V. .…”

Gaia Early Data Release 3

“…The full sky coverage and high-precision observations of Gaia offer the means of uncovering nearby UCDs through astrometric rather than purely photometric selection (Reylé 2018;Smart et al 2019;Scholz 2020). Gaia provides a large homogeneous sample.…”

The Gaia Ultra-Cool Dwarf Sample – III: seven new multiple systems containing at least one Gaia DR2 ultracool dwarf.

“…The maximum distance is 373 pc, the distance at which the brightest, hottest UCD (T eff ∼ 2500 K) would be fainter than the Gaia limiting magnitude (G = 20.7 mag). We required the G − G RP colour to be redder than 1.4 mag (since UCDs are typically redder than that; Smart et al 2017Smart et al , 2019. To minimize the number of sources with spurious astrometric measurements, we removed candidates within 5 • of the Galactic plane and inside an ellipse centred at the Galactic centre with semimajor axis along the Galactic longitude axis of 50 • , and 8 • along the Galactic latitude axis.…”

“…We computed posterior probability densities of the distance given the parallax measurements and associated uncertainties using an exponentially decreasing constant volume density prior, and selected sources with a posterior probability to be within 373 pc greater than 0.5. We then fit a principal curve (Hastie & Stuetzle 1989) in the M G versus G − G RP plane to the values of the resulting set, and calibrated the curve in effective temperature using the spectral types of sources in the Gaia Ultra-Cool Dwarfs Sample (GUCDS, Smart et al 2017, 2019 and the Stephens et al (2009) conversion between spectral types and effective temperatures. Finally, we computed the projections of the UCD candidate positions in the M G versus G RP plane along the principal curve and assigned effective temperatures accordingly.…”