Primeval very low-mass stars and brown dwarfs – III. The halo transitional brown dwarfs

Z. H. Zhang, D. J. Pinfield, M. C. Galvez-Ortiz, D. Homeier, A. J. Burgasser, N. Lodieu, E. L. Martin, M. R. Zapatero Osorio, F. Allard, H. R. A. Jones, R. L. Smart, B. Lopez Marti, B. Burningham, R. Rebolo. 2018. Primeval very low-mass stars and brown dwarfs – III. The halo transitional brown dwarfs. Monthly Notices of the Royal Astronomical Society 479, 1, 1383-1391, DOI: 10.1093/mnras/sty1352

We report the discovery of an esdL3 subdwarf, ULAS J020858,62+020657,0, and a usdL4.5 subdwarf, ULAS J230711.01+014447.1. They were identified as L subdwarfs by optical spectra obtained with the Gran Telescopio Canarias, and followed up by optical-to-near-infrared spectroscopy with the Very Large Telescope. We also obtained an optical-to-near infrared spectrum of a previously known L subdwarf, ULAS J135058.85+081506.8, and reclassified it as a usdL3 subdwarf. These three objects all have typical halo kinematics. They have T-eff around 2050-2250 K, -1.8 <= [Fe/H] <= -1.5, and mass around 0.0822-0.0833 M-circle dot, according to model spectral fitting and evolutionary models. These sources are likely halo transitional brown dwarfs with unsteady hydrogen fusion, as their masses are just below the hydrogen-burning minimum mass, which is similar to 0.0845 M-circle dot at [Fe/H] = -1.6 and similar to 0.0855 M-circle dot at [Fe/H] = -1.8. Including these, there are now nine objects in the ‘halo brown dwarf transition zone’, which is a ‘substellar subdwarf gap’ that spans a wide temperature range within a narrow mass range of the substellar population.

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