A homogeneous sample of 34 000 M7-M9.5 dwarfs brighter than $J$=17.5 with accurate spectral types

Abstract

The space density of late M dwarfs, subtypes M7-M9.5, is not well determined. We applied the photo-type method to $iz$ photometry from the Sloan Digital Sky Survey and $YJHK$ photometry from the UKIRT Infrared Deep Sky Survey, over an effective area of 3070 $de{g}^2$, to produce a new, bright $J$(Vega) < 17.5, homogeneous sample of 33 665 M7-M9.5 dwarfs. The typical S/N of each source summed over the six bands is > 100. Classifications are provided to the nearest half spectral subtype. Through a comparison with the classifications in the BOSS Ultracool Dwarfs (BUD) spectroscopic sample, the typing is shown to be accurately calibrated to the BUD classifications and the precision is better than 0.5 subtypes rms; i.e. the photo-type classifcations are as precise as good spectroscopic classifications. Sources with large ${\chi }^2$ > 20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example, L subdwarfs are misclassified towards earlier types by approximately two spectral subtypes. We estimate that this bias only affects ~1% of the course. Therefore the sample is well suited to measure the luminsoity function and investigate the softening towards the Galactic plane of the exponential variation of density with height.

Accepted for publication in A&A. 8 pages, 7 figures, and 5 tables. Catalogue of M dwarfs available on CDS (link in paper)

References in corpus (10)

• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• Constraining the Age-Activity Relation for Cool Stars: The SDSS DR5 Low-Mass Star Spectroscopic Sample
• The WFCAM Science Archive
• The UKIRT Wide Field Camera ZYJHK Photometric System: Calibration from 2MASS
• Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field
• BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs
• The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk: I. The Calibration Region
• Primeval very low-mass stars and brown dwarfs. I. Six new L subdwarfs, classification and atmospheric properties
• Photometric brown-dwarf classification. I. A method to identify and accurately classify large samples of brown dwarfs without spectroscopy
• Milky Way Tomography with K and M Dwarf Stars: the Vertical Structure of the Galactic Disk

Cited by in corpus (6)

• Predicting the Yield of Small Transiting Exoplanets around Mid-M and Ultra-Cool Dwarfs in the Nancy Grace Roman Space Telescope Galactic Bulge Time Domain Survey
• NGTS clusters survey -- III: A low-mass eclipsing binary in the Blanco 1 open cluster spanning the fully convective boundary
• A $\Delta R\sim 9.5$ mag Super Flare of An Ultracool Star Detected by $\text{SVOM/GWAC}$ System
• Ultracool dwarfs identified using spectra in LAMOST DR7
• J-PLUS: Discovery and characterisation of ultracool dwarfs using Virtual Observatory tools II. Second data release and machine learning methodology
• The absolute magnitudes $M_J$, the binary fraction, and the binary mass ratios of M7 to M9.5 dwarfs