Peter D. Edmonds

Chandra Exposes the Core-collapsed Globular Cluster NGC 6397

We report results of the Chandra deep-imaging observation of the closest post-core-collapse globular cluster, NGC 6397. Some 25 sources are detected within 2 of the cluster center, of which ~20 are likely cluster members, with LX 3 × 1029 ergs s-1. The X-ray spectra suggest identifications with one quiescent low-mass X-ray binary (qLMXB) detected by the thermal emission from its neutron star (NS) and nine cataclysmic variables (CVs), eight of which are identified in our deep Hubble Space Telescope (HST) imaging survey (reported separately). Three (of 16) BY Dra main-sequence binary candidates identified in our earlier HST imaging study (Taylor et al.) are detected, of which one is indeed the counterpart of the eclipsing millisecond pulsar (MSP) as recently identified by Ferraro et al. Two other BY Dra candidates are also detected, whereas none of the probable He white dwarf (WD) binaries identified by Taylor et al. are, indicating they do not contain MSP primaries. The ratio of CVs to MSPs appears to be ~10 times greater than in 47 Tuc.

K Giants in 47 Tucanae: Detection of a New Class of Variable Stars*

We report the discovery of variability among K giants in the globular cluster 47 Tucanae, using a time series of U exposures with the Hubble Space Telescope. The variables lie along a narrow band in the color-magnitude diagram, joining the faint end of the asymptotic giant branch to the red giant branch. The variations, if coherent, mostly have periods between ~2 and ~4 days, consistent with low-overtone radial pulsation or nonradial pulsation, and V amplitudes in the range 5-15 mmag, which explains their nondetection so far in clusters. One of the variables may have a period of 1.1 days and a V amplitude of 5.3 mmag. These stars define a new class of variable stars and probably contain variable field K giants such as α Boo as members. An understanding of their variations may have significant ramifications for theories of stellar structure and stellar evolution.

The Variable Star Population in the Center of 47 Tuc as denned by HST Observations

The high stellar densities at the centre of some globular clusters provide prolific breeding grounds for exotic stars, e.g., the 11 msec pulsars in 47 Tuc and numerous blue stragglers. The same high density of stars as viewed on the sky makes ground-based photometry problematic for variable star detection. Even a very few tight binaries can be of fundamental importance for the dynamic evolution of the cluster core. We present results of a continuous 40-hour sequence of U-band CCD exposures acquired with WF/PC on HST. The high spatial resolution coupled with a stable PSF allows nearly Poisson limited time series on some 20,000 stars. The 1000 second sampling and long coverage provide excellent sensitivity to W UMa and 6 Scuti variations. Light curves are shown for selected variables and the relative frequencies of binary and variable stars are discussed.