The Local Population II Main Sequence
Session 51 -- Brown Dwarfs and Very Low Mass Stars
Oral presentation, Wednesday, 1, 1994, 8:30-12:30

## [51.06] The Local Population II Main Sequence

C. Dahn (U.S. Naval Observatory Flagstaff Station)

Ground-based CCD trigonometric parallax determinations with formal errors in the range 0.5$\leq\epsilon_{\pi} \leq$1.5 mas can now readily be obtained for stars as faint as R$\sim$20 from only 3--4 years of observations. As a result, some 45 lower-luminosity (10.0$<$M$_{V}<$15.7) stars with Pop.II characteristics (i.e., either marked subluminosity and/or V$_{tan} \geq$180 km/sec) now have absolute magnitudes determined to $\pm$0.22 mag ($\pi/\epsilon_{\pi}$=10). An additional 25 Pop.II stars have determinations good to $\pm$0.4 mag ($\pi/\epsilon_{\pi}$=5). These 70 stars form a rather loose sequence in the M$_{V}$ versus V$-$I diagram, falling as much as 2.5 mag below the locus of the young disk stars. MMT spectrophotometry ($\lambda\lambda4700-8700$\AA; $\sim18$\AA\ resolution) has been obtained for 19 of the more extreme stars and qualitatively confirms their very metal-poor abundances. Detailed abundance estimates await improved model atmosphere calculations, at which point a quantitative connection can be established to the more luminous sdF$-$G stars. An estimate of the local Pop.II luminosity function will be presented based on a 1/V$_{max}$ analysis of the LHS stars with $\delta\geq-20\deg$ and $\mu\geq$0.8 arcsec/yr.