New Kinematic Results for the Blue Metal-Poor Stars

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Session 50 -- The Milky Way
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

[50.02] New Kinematic Results for the Blue Metal-Poor Stars

R. Wilhelm, T.C. Beers (Michigan State), A.C. Layden (CTIO)

Preston, Beers, \& Shectman (1994, AJ 108, 538) have identified a substantial number of blue metal-poor main sequence (BMP) stars within a few kpc of the Galactic plane. These stars, chosen to be blueward of the main-sequence turnoff of an old metal-deficient population, have been referred to in the past as halo blue stragglers. However, the BMPs appear in great numbers (on the order of 400 kpc$^{-3}$ ) relative to evolved members of the halo, and exhibit kinematics which do not seem to indicate membership in either the thick disk or halo population. Thus the identification with halo blue stragglers no longer seems viable. Rather, these authors submit that the BMPs are young metal-deficient stars shorn from collisions between dwarf galaxies and the disk of the Milky Way in the past 10 Gyrs.

We present a kinematic analysis for 221 BMP stars (with [Fe/H] $\le -1.0$) chosen from the ongoing HK survey. The systemic rotation velocity is found to be $V_{rot} = 105\; \pm \; 17$ km/s, somewhat lower than the value reported by Preston et al. , but still consistent within the stated uncertainties. The velocity ellipsoid we derive for the enlarged sample of BMPs is also consistent with that obtained by Preston et al. (90 km/s for all three components). We search for changes in the kinematics of the BMPs with abundance, dividing the sample at a metallicity [Fe/H] $ = -2.2$. We obtain $V_{rot} = 132 \; \pm \; 19$ km/s and $V_{rot} = 8 \pm 31$ km/s for the metal-richer and metal-poorer halves of this sample, respectively, though the velocity ellipsoids remain roughly the same. This behavior suggests that there may be a rapid change in the mixing fractions for the BMP sample at low metallicity, either due to mis-identification of halo FHB stars, or inclusion of true halo blue straggler stars. If the BMPs are confined to metallicities [Fe/H] $> -2.2$, as our results suggest, then stronger constraints can be placed on the nature of the accreted parent populations.

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