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H.B. Richer (UBC), J. Anderson (Rice), J.P. Brewer, S. Davis (UBC), G.G. Fahlman (HIA/NRC), B.M.S. Hansen (UCLA), J.R. Hurley (Monash), J. Kalirai (UCSC), I.R. King (UWash.), D. Reitzel, R.M. Rich (UCLA), A. Ruberg (UBC), M.M. Shara (AMNH), P.B. Stetson (HIA/NRC), D.R. Zurek (AMNH)
Using HST/ACS imaging, we report the deepest globular cluster photometry ever obtained, reaching an F606W magnitude fainter than 30 in NGC 6397. With proper motion cleaning and photometric classification criteria to reject non-member and non-stellar objects, we reveal a color-mangitude diagram extending from above the turnoff down to main sequence stars with masses within a few hundredths of a solar mass of that of the hydrogen-burning limit. In addition, the cluster white dwarf sequence extends over approximately 6.5 magnitudes and includes a stubby ``blue hook" which is expected as a consequence of collisionally induced molecular hydrogen opacity in these very cool objects. The turn-over in the white dwarf luminosity function is seen and this facilitates an accurate white dwarf cooling age for the cluster. This luminosity function is compared with that of the intermediate metallicity cluster Messier 4 which we also observed recently with HST and is used to establish any differential age between the two.
The data were secured during HST Cycle 13 and 126 orbits were used to image a single field in this nearby metal-poor globular cluster. One third of the time was spent exposing through the F606W filter and two thirds through the F814W filter. The field observed was located at 5 arcmin from the cluster center. It overlaps images taken in 1994 and 1997 so that proper motion selection was possible in order to isolate pure cluster populations of stars. Simultaneous observations of the cluster core were carried out with WFPC2.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.