The Local Population II Main Sequence
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**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.

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