Z. H. Zhang, M. C. Galvez-Ortiz, D. J. Pinfield, A. J. Burgasser, N. Lodieu, H. R. A. Jones, E. L. Martin, B. Burningham, D. Homeier, F. Allard, M. R. Zapatero Osorio, L. C. Smith, R. L. Smart, B. Lopez Marti, F. Marocco, R. Rebolo. 2018. Primeval very low-mass stars and brown dwarfs – IV. New L subdwarfs, Gaia astrometry, population properties, and a blue brown dwarf binary. Monthly Notices of the Royal Astronomical Society 480, 4, 5447-5474 DOI: 10.1093/mnras/sty2054
We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf candidates were selected with the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Spectroscopic follow-up was carried out primarily with the OSIRIS spectrograph on the Gran Telescopio Canarias. Some of these new objects were followed up with the X-shooter instrument on the Very Large Telescope. We studied the photometric properties of the population of known L subdwarfs using colour-spectral type diagrams and colour-colour diagrams, by comparison with L dwarfs and main sequence stars, and identified new colour spaces for L subdwarf selection/study in current and future surveys. We further discussed the brown dwarf transition-zone and the observational stellar/substellar boundary. We found that about one-third of 66 known L subdwarfs are substellar objects, with two-thirds being very low-mass stars. We also present the Hertzsprung-Russell diagrams, spectral type-absolute magnitude corrections, and tangential velocities of 20 known L subdwarfs observed by the Gaia astrometry satellite. One of our L subdwarf candidates, ULAS J233227.03+123452.0, is a mildly metal-poor spectroscopic binary brown dwarf: a similar to L6p dwarf and a similar to T4p dwarf. This binary is likely a thick disc member according to its kinematics.