AAS 197, January 2001
Session 41. Star Clusters and Associations
Display, Tuesday, January 9, 2001, 9:30am-7:00pm, Exhibit Hall

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[41.15] HST Color-Magnitude Data for Globular Clusters: Empirical Transformations Between STIS LP Magnitudes and WFPC2 F606W and F814W Magnitudes and Comparisons with Synthetic Photometry

G. Gilmore (Inst. of Astronomy, Cambridge Univ.), M. L. Houdashelt, R. F. G. Wyse (Johns Hopkins Univ.)

We present deep Hubble Space Telescope optical imaging observations of two Galactic globular clusters, 47 Tuc and M15, using the Space Telescope Imaging Spectrograph (STIS) longpass (LP) filter and the Wide Field and Planetary Camera 2 (WFPC2) F606W and F814W filters. These globular clusters have very different metallicities ([Fe/H] ~ -0.7 dex for 47 Tuc, and [Fe/H] ~ -2.2 dex for M15), essentially spanning the metallicity distribution of the entire Milky Way globular cluster system. They were chosen to investigate the relationship between magnitudes in the non-standard STIS LP system and the better characterized WFPC2 magnitudes. We determine this relationship by combining our new STIS data with archival WFPC2 data, taking care to provide a robust and reliable transformation. Examining the 47 Tuc and M15 data separately, there is no evidence for a metallicity dependence in the transformation relations. This allows us to combine the data for both clusters, and also include the stars in the Small Magellanic Cloud that lie within the field of view of the 47 Tuc images, to determine final transformations that are valid for main sequence stars with colors in the range 0.3 < V606-I814 < 1.8. These final relations predict STIS magnitudes within 0.05 mag (1 sigma) of those measured for stars in both clusters. However, our empirical transformation relations differ from those predicted by synthetic photometry. We ascribe these differences to a known property of STIS -- increased scattering of red light within the instrument at progressively longer wavelengths -- which introduces a color dependence in the aperture corrections to the STIS LP magnitudes.

Support for this work was provided by NASA grant number GO-7419 from STScI, operated by AURA Inc, under NASA contract NAS5-26555.

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The author(s) of this abstract have provided an email address for comments about the abstract: mlh@pha.jhu.edu

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