Interstellar H2 toward HD 147888

Abstract

The ultraviolet and far-ultraviolet spectra of HD 147888 allows the H 2 vibrational level ν = 0 to be accessed along with higher vibrational levels of the ground H2 electronic level. The large number of H2 absorption lines in the HST spectra allows column densities to be determined even from a noisy spectra. We have determined column densities of the H2 molecule on vibrational levels ν = 0-5 and rotational levels J = 0-6 using the profile fitting method. No variations in the column densities of H2 on vibrationally excited levels were observed from 2000 through 2009. The ortho to para H2 ratio (O/P)* for the excited vibrational states ν = 1-4 equals to 1.13. For the lowest vibrational state ν = 0 and rotational level J = 1 the ortho to para H 2 ratio is only 0.15. The temperature of ortho-para thermodynamical equilibrium isTOP= 42 ± 3 K. The measurements of H2 column densities on excited vibrational levels (from the HST spectra) leads to constraints on the radiation field in photon-dominated region (PDR) models of the interstellar cloud towards HD 147888. The Meudon PDR model locates the cloud 0.62 pc from the star. The modeled hydrogen cloud density (89-336 cm -3) agrees with independent density estimations based on the C 2 molecule and the chemical model. The observed (O/P)J = 1 and (O/P)* H2 ratios cannot be explained by a simple model. © ESO, 2012.