The esdM6.5 Star LSR J0822+1700: A New Ultracool Extreme Subdwarf

Spectroscopic observations reveal that the recently discovered high proper motion star LSR J0822+1700 is an ultracool extreme subdwarf, with spectral type esdM6.5. This is only the second known extreme subdwarf with a spectral type later than esdM6.0, the other one being the star APMPM J0559-2903 (esdM7.0). Our new spectra of LSR J0822+1700, of APMPM J0559-2903, and also of the very metal-poor esdM6.0 star LHS 1826 all show the defining signature of cool esdM objects: very deep absorption bands due to molecular CaH flanked by very weak absorption due to TiO. Our new spectrum reveals than the molecular bands in LHS 1826 are a little weaker than previously reported, and we reclassify the star as esdM5.5. We show that all three stars have low metallicities ([ m /H] ≤ -1.5) and kinematics consistent with halo membership. These objects represent the low-mass end of Population II stars in the vicinity of the Sun. The new ultracool extreme subdwarf LSR J0822+1700 appears to be significantly more metal-poor than APMPM J0559-2903, as evidenced by its much weaker TiO absorption. An interesting feature in the spectrum of LSR J0822+1700 is the existence of detectable absorption lines of Rb I and Cs I, usually associated with much cooler subtypes in dwarf stars (L dwarfs). We suggest that Rb I and Cs I are relatively strong in LSR J0822+1700 because of a critical combination of very low temperature and very low metallicity.