Metallicity, temperature, and gravity scales of M subdwarfs^,

Abstract

Aims. The aim of the project is to define a metallicity/gravity/temperature scale vs spectral types for metal-poor M dwarfs. Methods. We obtained intermediate-resolution ultraviolet (R∼3300), optical (R∼5400), and near-infrared (R∼3900) spectra of 43 M subdwarfs (sdM), extreme subdwarfs (esdM), and ultra-subdwarfs (usdM) with the X-shooter spectrograph on the European Southern Observatory Very Large Telescope. We compared our atlas of spectra to the latest BT-Settl synthetic spectral energy distribution over a wide range of metallicities, gravities, and effective temperatures to infer the physical properties for the whole M dwarf sequence (M0-M9.5) at sub-solar metallicities and constrain the latest state-of-the-art atmospheric models. Results. The BT-Settl models reproduce well the observed spectra across the 450-2500 nm wavelength range except for a few regions. We find that the best fits are obtained for gravities of log(g) = 5.0-5.5 for the three metal classes. We infer metallicities of [Fe/H] = −0.5, −1.5, and −2.0±0.5 dex and effective temperatures of 3700-2600 K, 3800-2900 K, and 3700-2900 K for subdwarfs, extreme subdwarfs, and ultra-subdwarfs, respectively. Metal-poor M dwarfs tend to be warmer by about 200±100 K and exhibit higher gravity than their solar-metallicity counterparts. We derive abundances of several elements (Fe, Na, K, Ca, Ti) for our sample but cannot describe their atmospheres with a single metallicity parameter. Our metallicity scale expands the current scales available for midly metal-poor planet-host low-mass stars. Our compendium of moderate-resolution spectra covering the 0.45-2.5 micron range represents an important legacy value for large-scale surveys and space missions to come.

Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

“…While refinements of metallicity classes for M dwarfs are ongoing (Lodieu et al 2019;Zhang et al 2019), there has also been a concerted effort to identify metal-sensitive features in the near-infrared spectra of M-type stars.…”

WISEA J041451.67–585456.7 and WISEA J181006.18–101000.5: The First Extreme T-type Subdwarfs?

“…The lowmass stellar (M-type) subdwarf population has been extensively characterized (e.g., Gizis 1997;Lépine et al 2007). There are three recognized subclasses of low-mass M-type subdwarfs: subdwarfs (sd), extreme subdwarfs (esd), and ultra-subdwarfs (usd), which roughly correspond to metallicity values ([Fe/H]) of −0.5, −1.0, and −1.5 dex, respectively (Pavlenko et al 2015), though these classes are still undergoing refinements (Lodieu et al 2019;. The intermediate subdwarf class (d/sd; [Fe/H] > −0.5) has also been suggested (Mould & McElroy 1978;Burgasser et al 2007).…”

Wide companions to M and L subdwarfs withGaiaand the Virtual Observatory

“…The effective temperatures provided by VOSA for the subdwarf and the companion candidate are 2 200±100 K (consistent with the spectral type) and 3 000±100 K, respectively. As we did for other subdwarfs, we estimated the spectral type of the companion to sdM7.0±0.5 from its temperature (Lodieu et al 2019a). Their masses and distances provide us a value for the binding energy of 88% of the minimum; with a low RUWE of 1.17 for the companion candidate, we conclude that this pair has a low chance of being a real system.…”

“…In the estimation of the spectral type, for the case of lowmetallicity dwarfs, we check Table 2 from Lodieu et al (2019a), and Figure 4 from Zhang et al (2018b). For solar metallicities, we can estimate the spectral type following Reid & Gizis (2005).…”

“…We obtained their SEDs with VOSA, from which we derived effective temperatures of 2 600±100 K and 2 900±100 K for Id 11 and Id 11-17, respectively. Using Lodieu et al (2019a) and Zhang et al (2018b), we estimated its spectral type as sdM8.0±0.5. We also calculated its binding energy, which is very low (about ten times lower than the accepted minimum energy to be considered a bound pair).…”