ULTRAVIOLET PHOTOMETRY OF STARS IN THE COMPACT CLUSTER R136 IN THE LARGE MAGELLANIC CLOUD

We present ultraviolet photometry of stars in the Large Magellanic Clo ud star cluster R136. The data were obtained with the refurbished Wide Field/Planetary Camera on the Hubble Space Telescope through an ultra violet filter centered at 1750 Angstrom (F170W). These data, combined with optical HST observations presented previously, are used to explor e the hot, luminous stellar population within the cluster. The first q uestion we wanted to address was whether the ultraviolet could place b etter constraints on differential reddening and the degree of coevalit y within the cluster than optical photometry alone. The stellar sequen ce in the color magnitude diagram using the F170W-F555W color is broad er than it was in the optical and is broader than would be expected fr om photometric uncertainties. Although there could be a modest age spr ead among the massive stars, a large part of the intrinsic spread in t he color-magnitude diagram is likely to be due to reddening difference s. There is also some evidence that stars in the outer parts of the cl uster are affected by a larger range in reddening than those in the sm aller region of the cluster core. The second question we addressed is whether ultraviolet photometry in combination with optical photometry can distinguish the most massive stars more readily than is possible w ith the optical colors. The F170W-F555W color is better at separating B supergiants, of which there is only one in R136, from comparably bri ght O and Wolf-Rayet stars. However, spectral classifications of the s tars within R136 will be necessary to properly identify stellar types of the rest of the stars and to disentangle age and reddening effects. (C) 1997 American Astronomical Society.