Globular Cluster Photometry with HST: Color-Mag Diagrams in the Dense Cores of 47 TUC and M15

Details Globular Cluster Photometry with HST: Color-Mag Diagrams in the Dense Cores of 47 TUC and M15 Globular Cluster Photometry with HST: Color-Mag Diagrams in the Dense Cores of 47 TUC and M15

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.4307g&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #43.07; Bulletin of the American Astronomical Society, Vol. 24, p.1188

Astronomy and Astrophysics

Astronomy

Scientific paper

We present accurate (sigma ~ 0.1 mag), multi-color photometry of stars in the cores of two of the densest known Galactic globular clusters, 47 Tuc and M15, using images obtained with the Hubble Space Telescope's Planetary Camera (HST/PC). Despite the spherical aberration in HST's primary mirror, the high angular resolution of the PC images makes it possible to construct reliable color-magnitude diagrams of post-main sequence stars even in the most crowded portions of these clusters. This task has proven impossible from the ground in the best of seeing conditions (FWHM ~ 0.5''). We have developed a technique that constructs an empirical model of the stellar point spread function (PSF) from a PC image of a globular cluster core. The technique corrects for saturation in the stellar cores (caused by the limited dynamic range of the CCD detector) and attempts to track the variation in the shape of the PSF across the image. Detailed knowledge of the PSF is essential in order to do crowded field stellar photometry; note, the low level wings of the PSF extend beyond a radius of 2.5'' while the average projected separation between stars in these cluster cores is less than 1''. The PSF modeling and stellar photometry technique has been tested and calibrated with the help of realistic simulated images. Color-magnitude diagrams (U-V and V-I versus V) of 47 Tuc and M15 reveal a few dozen blue straggler stars within the central 1 arcmin(2) of each cluster. The blue stragglers are more centrally concentrated than the average post-main sequence star and are consistent with having masses equal to 2m_turnoff. The ratio of the number of bright red giant branch stars to horizontal branch stars does not vary with radius within the inner 1' of 47 Tuc. In M15, that ratio declines near the cluster center, consistent with the hypothesis that the red giant envelopes have been destroyed in this region of extremely high stellar density.

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