AAS 201st Meeting, January, 2003
Session 50. The Milky Way: From Center to Halo
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[50.12] Stars in the Galactic Inner Halo

J.S. Kalirai (University of British Columbia), J. Brewer (UBC), G.G. Fahlman (CFHT), B.K. Gibson (Swinburne), B.M. Hansen (UCLA), R. Ibata (Strabourg), R.M. Rich (UCLA), H.B. Richer (UBC), I. Saviane (ESO), M.M. Shara (AMNH), P.B. Stetson (DAO)

We report on the field stellar population measured in one of the deepest Hubble Space Telescope (HST) imaging fields obtained to date, the 123 orbit integration on the globular cluster M4, using the Wide Field Planetary Camera 2 (WFPC2). Combining positions and photometry from 1995 images with the deep 2001 data set, we use proper-motions to separate field stars along the line of sight from M4 members down to V = 30. M4 has Galactic coordinates, l, b = (351 deg, 16 deg); the majority of this field population consists of inner halo stars at a projected distance of 2.2 kpc from the Galactic nucleus. Part of the stellar population is consistent with metal-poor main sequence stars lying at the tangent point, 7.6 kpc. We find a redder sequence of field stars, which appear to be nearby metal-rich stars along the same sight line. Based on these assumptions we derive photometric parallaxes for the field stars, and apply theorertical mass-luminosity relations to derive mass functions for each population. We identify an extragalactic population based on image morphology, and show that reduced proper-motion diagrams are consistent with our separation of populations. The absolute proper-motion of the members of M4 with respect to this zero motion frame of reference is found to be in good agreement with other indirect measurements. The galaxies also provide a solar reflex velocity, with respect to the halo, which is in good agreement with known values. By integrating the inner halo mass function slope from the present-day turnoff to the progenitor mass which represents the limit that a white dwarf could have cooled to in our photometry (V = 30), we predict the number of expected inner halo white dwarfs in our field and compare to the number of faint-blue objects with consistent proper-motions and location in the colour-magnitude diagram.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.