ORFEUS observations of the foreground gas toward HD 37903

Abstract

We analyze H-2 and CO absorption along the line of sight toward HD 37903 over the 1045-1060 Angstrom and 1086-1102 Angstrom wavelength regions, which were observed by the Berkeley Extreme and Far-Ultraviolet Spectrometer on the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) telescope. HD37903 is a bright UV-emitting star embedded in the L1630 molecular cloud, creating the reflection nebula NGC 2023. Using the theory of formation and dissociation of molecular hydrogen in the far-UV spectral range, we derive the physical conditions of the foreground gas toward HD 37903, such as the density n, temperature T, and the UV intensity I-UV, by analyzing the H-2 lines from the J" = 0-5 rotational levels. In addition, we identify the CO absorption band at 1088 Angstrom and calculate the abundance ratio [CO]/[H-2] directly, which is found to be 1.3 x 10(-7). The higher rotational levels of H-2 are excited in the outer boundary of a molecular cloud where UV pumping dominates, while the lower J levels of H-2 and other molecules, such as CO, prevail in the neutral core of the cloud. In this regard, our analysis indicates that the observed gas extends from the neutral region to the photodissociation region of NGC 2023 at a distance of 0.2 pc from HD 37903. These results are similar to those found in previous studies of the photodissociation region of NGC 2023 and chemical analysis of the foreground gas toward HD 37903. The total amount of molecular hydrogen, including the gas behind HD 37903, is estimated from the total visual extinction of the dark cloud and our H-2 column density of the foreground gas. By measuring the CO and (CO)-C-13 integrated intensities via radio observations, we find the conversion factor N-tot(H-2) = W(CO) to be 1.5 x 10(20) cm(-2) (K km s(-1))(-1), which is in accord with canonical conversion factors.