The Space Density and Kinematics of Metal-Poor Blue Main Sequence Stars Near the Solar Circle

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Session 78 -- Galactic Structure and Star Counts
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[78.06] The Space Density and Kinematics of Metal-Poor Blue Main Sequence Stars Near the Solar Circle

G.W. Preston (OCIW), T.C. Beers (Michigan State University), S.A. Shectman (OCIW)

We have used $UBV$ photometry to identify 172 blue ($0.15 \le (B-V)_o \le 0.35$), metal-poor ([Fe/H] $\le -1.0$) main sequence stars (hereafter BMPs) among candidates from the HK objective-prism/interference-filter survey. The BMP fractions in the principal spectral groups of the survey have been used to estimate the space density of this species within 2 kpc of the sun, $\nu = 600$ kpc$^{-3}$. Accordingly, the specific frequency of the field BMPs, reckoned as the number of BMPs per field horizontal-branch star, is 12, which is 12 times greater than the largest value of specific frequency found among the seven globular clusters that have been searched for blue stragglers to their centers. From radial velocities which are available for 104 of these stars we obtain a Galactic rotation of $V_{rot} \approx 130$ kms$^{-1}$, and a velocity ellipsoid of ($\sigma_r \approx 85, \sigma_{\phi} \approx 85, \sigma_{\theta} \approx 85$) kms$^{-1}$, a quartet of values unlike those of either the halo or thick-disk populations.

These results indicate that the great majority of BMPs cannot be field analogues of the blue stragglers found in halo globular clusters. Rather, we suggest that BMPs are comparatively young main sequence stars formed in one or more Milky Way satellites, similar perhaps to Fornax or Carina, that have been captured by the Galaxy during the past 3 to 10 Gyrs. Our results reinforce earlier arguments by Rodgers, Harding, \& Sadler (1981, ApJ 244, 912) and by Lance (1988, ApJ 344, 927) on the basis of more limited data for less metal-deficient early-type stars. Such satellite captures may be viewed as an extension to relatively recent times of the Searle-Zinn fragment dissipation process. At earliest times satellites were both numerous and gassy, and collisions among them produced a nearly non-rotating halo of stars. As the number of satellites decreased, mergers with the disk became the dominant interaction, and the tendency for dynamical friction to select satellites with prograde orbits led to the now-observed kinematic properties of the BMPs, which are intermediate between those of the halo and the thick-disk.

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