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

ZengHua Zhang

doi:10.1093/mnras/stz2196

paper

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

arXiv:1908.02390 · doi:10.1093/mnras/stz2196

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 $_{- 0.67}^{+ 0.68}$ pc with a projected separation of 15828 $\pm$ 78 au. I classified Gaia J0452 $-$ 36AB as esdM1 and esdL0 subdwarfs, respectively. Gaia J0452 $-$ 36AB have typical halo kinematics, metallicity of [Fe/H] $\approx - 1.4$ , and temperature of $\sim$ 3550 and 2600 K, respectively. Gaia J0452 $-$ 36AB is a pair of very low-mass stars with masses of 0.151 $_{- 0.019}^{+ 0.029}$ and 0.0855 $_{- 0.0010}^{+ 0.0014}$ 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.

14 pages, 10 figures

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

Abstract

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