Interstellar H2 toward HD 147888 ^⋆

Institute of Theoretical Physics and Astrophysics, University of Gdańsk, ul. Wita Stwosza 57, 80-952 Gdańsk, Poland
e-mail: fizpg@univ.gda.pl

Received: 31 July 2012
Accepted: 8 November 2012

Abstract

The ultraviolet and far-ultraviolet spectra of HD 147888 allows the H₂ 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 H₂ ratio is only 0.15. The temperature of ortho-para thermodynamical equilibrium is T_OP = 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 C₂ molecule and the chemical model. The observed (O/P)_J = 1 and (O/P)* H₂ ratios cannot be explained by a simple model.