Primeval very low-mass stars and brown dwarfs – I. Six new L subdwarfs, classification and atmospheric properties

Abstract

We have conducted a search for L subdwarf candidates within the photometric catalogues of the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Six of our candidates are confirmed as L subdwarfs spectroscopically at optical and/or nearinfrared wavelengths. We also present new optical spectra of three previously known L subdwarfs (WISEA J001450.17-083823.4, 2MASS J00412179+3547133 and ULAS J124425.75+102439.3). We examined the spectral type and metallicity classification of subclasses of known L subdwarfs. We summarized the spectroscopic properties of L subdwarfs with different spectral types and subclasses. We classify these new L subdwarfs by comparing their spectra to known L subdwarfs and L dwarf standards. We estimate temperatures and metallicities of 22 late-type M and L subdwarfs by comparing their spectra to BT-Settl models. We find that L subdwarfs have temperatures between 1500 and 2700 K, which are higher than similar-typed L dwarfs by around 100-400 K depending on different subclasses and subtypes. We constrained the metallicity ranges of subclasses of M, L, and T subdwarfs. We also discussed the spectral-type and absolute magnitude relationships for L and T subdwarfs.

Understanding Fundamental Properties and Atmospheric Features of Subdwarfs via a Case Study of SDSS J125637.13–022452.4∗

“…Our values for the bolometric luminosity and radius differ from those of Lodieu et al (2015), because they have used an incorrect parallax value in their calculations. Our derived values for T eff and log g are consistent with the model based values from Zhang et al (2017b).…”

“…Allers & Liu (2013) classified the near-infrared spectra of J1256−0224 using their method as an M6 and with field-gravity, noting the stark contrast to the optical spectral type of sdL3.5. Most recently, Zhang et al (2017b) reclassified J1256−0224 as an usdL3 using the 0.73 − 0.88 µm region of the optical spectrum, which was based on their classification scheme anchored to the work of Lépine et al (2007).…”

“…We note that Lodieu et al (2015) also determined T eff , L bol , and radius values for J1256−0224, however these values used the parallax and proper motion of J1256−1408 instead, therefore they are not valid. Lastly, Zhang et al (2017b) derived T eff = 2250 ± 120 K, [Fe/H] = −1.8 ± 0.2 dex, and log g = 5.50 ± 0.2 dex for J1256−0224 from BT-Settl model fits. Scholz et al (2009) measured the TiO5, VO-a, and PC3 spectral indices in order to classify J1256−0224 as an sdL4 ± 1 using a spectral type versus spectral index relationship.…”

Discovery of wide low and very low-mass binary systems using Virtual Observatory tools

“…They retain the original chemistry of the early Galaxy and, therefore, are good tracers of the Galactic halo formation and evolution. We have marked the three targets in Table A1 with proper motion over than 150 mas yr −1 whose (J − K) and (z − J) colours follow the criteria of Lodieu et al (2010) for the selection of this type of objects and also lay in the appropriate area of the (i − J) vs (J − K) diagram of Zhang et al (2017). Moreover, although the reduced proper motion diagram (Figure 3) seems to not discriminate between dwarfs and subdwarfs in the L spectral type region, the three targets lay close to other L subwarfs identified in the literature.…”

Discovery of a Possible Early-T Thick-disk Subdwarf from the AllWISE2 Motion Survey*

“…Recently, dedicated searches for the old, low-mass population of objects in our galaxy have been carried out using the Wide-field Infrared Survey Explorer (WISE; Kirkpatrick et al 2014Kirkpatrick et al , 2016Pinfield et al 2014), UKIDSS Large-Area Survey (ULAS; Zhang et al 2017b), Two-Micron All Sky Survey (2MASS; Kirkpatrick et al 2010), and Sloan Digital Sky Survey (SDSS; Sivarani et al 2009). The totality of the results of these surveys reveals that the lowest-mass stars and brown dwarfs with low metallicity are relatively rare.…”