2002ApJ...575..234L

We analyze H₂ and CO absorption along the line of sight toward HD 37903 over the 1045-1060 Å and 1086-1102 Å 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. HD 37903 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 H₂ lines from the J''=0-5 rotational levels. In addition, we identify the CO absorption band at 1088 Å and calculate the abundance ratio [CO]/[H₂] directly, which is found to be 1.3x10^–7. The higher rotational levels of H₂ are excited in the outer boundary of a molecular cloud where UV pumping dominates, while the lower J levels of H₂ 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 H₂ column density of the foreground gas. By measuring the CO and ¹³CO integrated intensities via radio observations, we find the conversion factor N_tot(H₂)/W(CO) to be 1.5x10²⁰ cm^–2.(K.km.s^–1)^–1, which is in accord with canonical conversion factors.