2019MNRAS.489.1423Z

I present the discovery of the first wide M + L extreme subdwarf binary system Gaia J0452-36AB. The binary is located at a distance of 137.27 ^+0.68_–0.67 pc with a projected separation of 15828 ± 78 au. I classified Gaia J0452-36AB as esdM1 and esdL0 subdwarfs, respectively. Gaia J0452-36AB have typical halo kinematics, metallicity of [Fe/H] ≃-1.4, and temperature of ∼ 3550 and 2600 K, respectively. Gaia J0452-36AB is a pair of very low-mass stars with masses of 0.151 ^+0.029_–0.019 and 0.0855^+0.0014_–0.0010 M_☉, and is a gravitationally bound system. I tested the metallicity consistency of existing M subdwarf classification schemes with Gaia J0452-36AB and a sample of M and L subdwarfs with known metallicity. I found that the metallicity of each M subclass defined by the the metallicity index ζ_CaH/TiO is not consistent from mid-to-late M subtypes. Because late-type M and L subdwarfs have dusty atmospheres and high surface gravity which have significant impacts on CaH and TiO indices that used in the classification. The metallicity scale of late-type M subdwarfs would be overestimated by the ζ_CaH/TiO index. I discussed the mass range of M subdwarfs, and explained the lack of late-type M extreme and ultra subdwarfs, and decreasing binary fraction from sdM, to esdM, and usdM subclasses. The four M subclasses have different mass ranges. The comparison between M subclasses is between populations in different mass ranges. I also present the discovery of Ruiz 440-469B, an M8 dwarf wide companion to a cool DA white dwarf, Ruiz 440-469.