HST/WFPC-2 Observations of the Ionizing Star Cluster of the Giant HII region NGC 595 in The Local Group Galaxy M33
Session 76 -- Spirals I
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

## [76.04] HST/WFPC-2 Observations of the Ionizing Star Cluster of the Giant HII region NGC 595 in The Local Group Galaxy M33

E.M. Malumuth (CSC/GSFC), W. Waller, J.W. Parker (HSTX/GSFC)

As part of a program to study the resolved and composite properties of the giant HII regions in the Local Group galaxy M33, we have obtained, U, B, V, and UV images of the star cluster NGC 595 using the PC chip of the WFPC-2 camera aboard the HST. All of the images were reduced using the standard pipeline'' processing by the STScI. Cosmic-ray hits were cleaned using the CR-split option.

Photometric reductions were done using the DAOPHOT-2 package (Stetson, Davis, \& Crabtree 1990). We find 561 stars on the V-band image (the deepest of our 4 images), 345 stars on the B-band image, 272 stars on the U-band image, and 100 stars on the UV image. A total of 267 stars are common to the U, B, and V images while 85 are detected on all 4 images.

We present U-B vs. B-V and UV-U vs. U-B color-color diagrams, from which the individual reddening of each star is determined. The average reddening derived from these diagrams is E(B-V)=0.29 which is in good agreement with that derived from fitting the blue-plume in the color magnitude diagram to the main sequence. Using the individual reddenings of the stars with U, B, and V magnitudes, and the average reddening for all other stars we derive a M$_{\rm V}$ vs (B-V)$_{\rm o}$ color magnitude diagram. With a distance modulus of 24.64 the brightest stars in the cluster have M$_{\rm V} \sim -9$. This is consistent with stars which are 2.8 Myr old and have a zero age main sequence mass of $\sim$ 85 M$_{\odot}$. Using a 2.8 Myr isochrone we derive the luminosity and initial mass functions for NGC595. The IMF has a slope of ${\Gamma}=-1.52$, where the standard Salpeter IMF has a slope of ${\Gamma}=-1.35$. This value is considerably steeper then that derived by Drissen et al. (1993) based on a F439W image obtained with the PC camera of the WFPC. The difference is largely attributable to the ability for the WFPC-2 to go deeper and to resolve fainter stars near bright neighbors than the WFPC.