HST WFPC2 Study of the Density Profile and Stellar Populations in M15

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Session 104 -- Globilar Clusters
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[104.14] HST WFPC2 Study of the Density Profile and Stellar Populations in M15

P.~Guhathakurta (STScI), B.~Yanny (Fermilab), D.~P.~Schneider (Penn State U), J.~N.~Bahcall (IAS, Princeton)

We describe images of the center of the dense globular cluster M15 (NGC~7078) obtained with HST's WFPC2 instrument. Data taken in the F336W, F439W, and F555W filters ($UBV$) yield a color--magnitude diagram of $\sim3\times10^4$~stars in a 5~arcmin$^2$ region within $r<2'$ (6.7$\,$pc) of the cluster center. Realistic simulated images have been used to estimate photometric error and incompleteness in the star counts. The measurement error in color is $1\sigma\la0.05$~mag for stars with $V<18$, increasing to $1\sigma\sim0.2$~mag at $V=20.5$, which is 1.5~mag fainter than the main sequence turnoff. The larger number of stars detected with WFPC2 (compared to WF/PC-1) result in smaller Poisson errors in the determination of the star count profile. The projected radial distribution of stars (after correction for incompleteness and photometric bias/error) is well represented by a power law: $N(r)\sim{r}^\alpha$, with slope $\alpha=-0.7\pm0.15$, into radii less than 0\farcs3 (0.015$\,$pc). Any flat core of radius larger than $1''$ (0.05$\,$pc) in the stellar distribution is ruled out at the 99\% level. Only 30 blue straggler candidates are identified from the $UBV$ data within $r\la30''$, and we use the simulations to correct for contamination by subgiant and turnoff stars. The specific frequency of blue straggler stars (relative to red giant branch [RGB] and subgiant stars) in the central region of M15 is $f_{\rm BSS}=0.05\pm0.02$ which is significantly lower than in other dense clusters. The incompleteness-corrected radial distribution of $19 Thursday program listing