Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004apj...602l.125l&link_type=abstract
The Astrophysical Journal, Volume 602, Issue 2, pp. L125-L128.
Astronomy and Astrophysics
Astronomy
15
Galaxy: Halo, Galaxy: Stellar Content, Stars: Late-Type, Stars: Low-Mass, Brown Dwarfs, Stars: Population Ii, Stars: Subdwarfs
Scientific paper
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.
Based on observations made at the MDM Observatory, maintained and operated by the University of Michigan, Dartmouth College, the Ohio State University, and Columbia University.
Lepine Sebastien
Rich Robert Michael
Shara Michael M.
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