Observational properties of rotationally excited molecular hydrogen in translucent lines of sight

Adam G. Jensen, Theodore P. Snow, George Sonneborn, Brian L. Rachford

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


The Far Ultraviolet Spectroscopic Explorer (FUSE) has allowed precise determinations of the column densities of molecular hydrogen (H2) in Galactic lines of sight with a wide range of pathlengths and extinction properties. However, survey studies of lines of sight with greater extinction have been mostly restricted to the low-J states (lower total angular momentum) in which most molecular hydrogen is observed. This paper presents a survey of column densities for the molecular hydrogen in states of greater rotational excitation (J ≥ 2) in Galactic lines of sight with log N(H2) ≳ 20. This study is comprehensive through the highest excited state detectable in each line of sight. J = 5 is observed in every line of sight, and we detect J = 7 in four lines of sight, J = 8 in one line of sight, and vibrationally excited H2 in two lines of sight. We compared the apparent b-values and velocity offsets of the higher-J states relative to the dominant low-J states and we found no evidence of any trends that might provide insight into the formation of higher-J H2, although these results are the most affected by the limits of the FUSE resolution. We also derive excitation temperatures based on the column densities of the different states. We confirm that at least two distinct temperatures are necessary to adequately describe these lines of sight, and that more temperatures are probably necessary. Total H2 column density is known to be correlated with other molecules; we explore if correlations vary as a function of J for several molecules, most importantly CH and CH+. Finally, we briefly discuss interpretations of selected lines of sight by comparing them to models computed using the Meudon PDR code.

Original languageEnglish (US)
Pages (from-to)1236-1256
Number of pages21
JournalAstrophysical Journal
Issue number2
StatePublished - 2010
Externally publishedYes


  • ISM: molecules
  • Ultraviolet: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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