A NEW LOOK AT THE DUST TOWARD OMICRON PERSEI AND SIGMA SCORPII WITH THE INTERNATIONAL ULTRAVIOLET EXPLORER

The first International Ultraviolet Explorer (IUE) low dispersion spectra for o Per and sigma Sco have recently been obtained and extinction curves from 1150 to 3250 angstrom are presented. Our IUE data are compared with earlier studies using OAO, Copernicus, and ANS satellite data. The extinction toward o Per has not been previously measured from 1850 to 3200 angstrom. The interstellar medium along these lines of sight has been extensively studied so it is of interest to consider the sigma Seo and o Per dust in the context of the average extinction law of Cardelli et al. [ApJ, 345, 245 (1989), hereafter referred to as CCM] which depends on one parameter, chosen to be R(v), the ratio of total-to-selective extinction. Apparently o Per lies behind the edge of a dark cloud, while sigma Sco lies within a H II region near a dark cloud. The o Per dust shows a broad bump shifted to shorter wavelengths similar to other dark cloud lines of sight but shows no deviation from the CCM average curve in the far ultraviolet. The sigma Sco dust does show a marked deviation from the CCM average law in the far ultraviolet. Although large, these deviations are more typical of dark cloud or diffuse sight lines rather than bright nebula sight lines. The sigma Sco and o Per UV extinction curves are not anomalous, nor are they prototypes for other lines of sight. The observed characteristics for the sigma Sco and o Per dust are examples of a continuum of characteristics observed along various sight lines depending on the local dust environments.