Astronomy and Astrophysics – Astronomy
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.1606s&link_type=abstract
American Astronomical Society Meeting 210, #16.06; Bulletin of the American Astronomical Society, Vol. 39, p.116
Astronomy and Astrophysics
Astronomy
Scientific paper
We have obtained deep photometric observations of the globular cluster NGC 6397 using HST's Advanced Camera for Surveys Wide Field Camera (ACS WFC) in B435, R625, and Hα. The white dwarf sequence is well defined, with approximately 150 objects extending from R625=22 to R625=25. Using this sequence and assuming the cluster WDs have masses of 0.53 ± 0.02 M_sun, we find a distance to the cluster of 2.35 ± 0.15 kpc.In addition, we find a second broader sequence of approximately 20 stars that extends as much as two magnitudes brighter than the first at fixed color. This sequence includes the six helium-core white dwarf (HeWD) candidates previously reported by Cool et al. (1998) and Taylor et al. (2001).
In an R625 vs. Hα--R625 color-magnitude diagram, standard carbon-oxygen white dwarfs are 0.2--0.5 magnitudes Hα-faint with respect to the main sequence. The HeWD sequence appears similarly Hα-faint, in contrast to cataclysmic variables which can mimic white dwarfs in a R625 vs. B435--R625 CMD, but which show Hα emission. We find that the objects in the second sequence are strongly centrally concentrated. This implies that the HeWDs have a massive companions cool enough that the HeWD provides the dominant light.
This work was supported in part by NASA grant HST-GO-10257.
Anderson Jeffrey
Cohn Henry
Cool Adrienne
Lugger Phyllis
Strickler Rachel
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