Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.7502r&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #75.02; Bulletin of the American Astronomical Society, Vol. 24, p.1241
Other
Scientific paper
We present preliminary results of a program to study the cores of nearby clusters using repeated observations with the CTIO 0.9m telescope. We have obtained a total of 296 frames in Johnson B & V of NGC 6397 between June 1991 and August 1992. The 6.7' Tek 1024 x 1024 CCD fields centered on the cluster center are shifted and averaged to produce a template field from which accurate stellar positions are obtained. These fiducial coordinates are utilized to provide an additional constraint for the standard DAOPHOT (Stetson 1987, PASP, 99, 191.) reduction of each individual (nonaveraged) frame. The results of the analysis of each frame is assembled into a time series light curve for each star found in multiple exposures. Based on the analysis of a subset of the data, we find 18 blue straggler stars (BSSs), ten of which have been previously identified (Auriere et al. 1990, Nature, 344, 638; Lauzeral et al. 1992, preprint). The 1.5" seeing of the combined frames was insufficient to resolve the remaining 8 known BSSs. These new BSSs bring to 26 the total number of BSSs identified in NGC6397. The BSSs are more centrally concentrated than sub-giants of the same magnitude, reinforcing the interpretation that they are more massive than main sequence turn-off stars. Other interesting features of the CMD include the complete lack of blue subdwarfs below the blue end of the horizontal branch, which would certainly have been detected. This indicates that, unlike blue stragglers, blue subdwarfs are not ubiquitous features of globular cluster cores. One of the BSSs is a likely variable star with an apparent period 75 minutes and amplitude 0.2 magnitude. This variable star candidate's light curve is consistent with the time variability displayed by SX Phoenicis type pulsators. Several other blue stragglers and other stars show signs of variability. Further reductions of the complete data set are in progress.
Bailyn Charles D.
Rubenstein Eric Paul
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