Far‐Ultraviolet Observations of the Ophiuchus Region with SPEAR

Abstract

We present the first far-ultraviolet (FUV; 1370-1670 Å) image of the Ophiuchus molecular cloud region, observed with the SPEAR imaging spectrograph. The flux levels of the diffuse FUV continuum a re in reasonable agreement with those of the Voyager observations in the shorter FUV wavelengths (912-1216 Å), provided that the diffuse FUV emission is dominated by the spectra from late O- and early B-type stars. The observed region of the present study was divided into five subregions according to their FUV intensities, and the spectrum was obtained for each subregion with prominent H 2 fluorescent emission lines. A synthetic model of the H 2 fluorescent emission indicates that the molecular cloud has more or less uniform physical parameters over the Ophiuchus region, with a hydrogen density n H of 500 cm -3 and a H 2 column density N(H 2 ) of 2 × 10 20 cm -2 . It is notable that the observed diffuse FUV continuum is well reproduced by a single-scattering model with scattered starlight from the dust cloud located at ∼120-130 pc, except at a couple of regions with high optical depth. The model also gives reasonable properties of the dust grains of the cloud with an albedo a of 0:36 ± 0:20 and a phase function asymmetry factor g of 0.52 ± 0:22. © 2008. The American Astronomical Society. All rights reserved.