Primeval very low-mass stars and brown dwarfs – VII. The discovery of the first wide M + L extreme subdwarf binary

Abstract

ABSTRACT 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$\odot$, 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 ζCaH/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 ζCaH/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.