ISOobservation of molecular hydrogen and fine-structure lines in the photodissociation region IC���63

Abstract

We wish to constrain the main physical properties of the photodissociation region (PDR) IC 63. We present the results of a survey for the lowest pure-rotational lines of H 2 with the Short Wavelength Spectrometer and for the major fine-structure cooling lines of O I at 63 and 145 μm and C II at 157.7 μm with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO) in the high-density PDR IC 63. The observations are compared with available photochemical models based on optical absorption and/or millimetre emission line data with and without enhanced H 2 formation rate on grain surfaces. The cloud density n H is constrained by the fine-structure lines. The models include both collisional excitation and ultraviolet (UV) pumping of the H 2 ro-vibrational levels. Molecular pure-rotational lines up to S(5) are detected. The inferred column density of warm H 2 at 106 ± 11 K is (5.9 ± 1.8) +0.9 −0.7 × 10 21 cm −2 , while that of the hot component at 685 ± 68 K is (1.2 ± 0.4) × 10 19 cm −2 . Finestructure lines are also detected in the far-infrared spectrum of IC 63. The fine-structure lines constrain the density of the PDR to be (1-5) × 10 3 cm −3 . The impinging UV field on the PDR is enhanced by a factor of 10 3 compared to the mean interstellar field and is consistent with direct measurements in the UV. PDR models that include an enhanced H 2 formation at high dust temperature give higher H 2 intensities than models without enhancement. However, the predicted intensities are still lower than the observed intensities.

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

“…Theoretical calculations, confirmed by experimental results studying H 2 formation on graphite (Latimer et al, 2008;Islam et al, 2007Islam et al, , 2010, show that ro-vibrational excitation of H 2 leaving the surface upon formation is concentrated around high values of the vibrational quantum number (3-4) and low values of the rotational quantum number. There have been a few attempts to detect a signature of such H 2 formation, via observations of transitions of H 2 in appropriate regions of the ISM, but with no success to date (Tiné et al, 2003;Thi et al, 2009). The ortho (odd rotational quantum number J) to para (even J) ratio of H 2 can yield information concerning the thermal history of the associated cloud, as well as concerning the conditions associated with H 2 formation on the grains of the cloud (Wilgenbus et al, 2000;Le Bourlot, 2000), although presence of other H 2 molecules co-adsorbed on the grain can hinder the release of excited molecules (Congiu et al, 2009).…”

Extreme cosmic ray dominated regions: a new paradigm for high star formation density events in the Universe

“…This model has already been successfully used several times to model PDRs in the Galaxy (e.g. Thi et al 2009).…”

Competing Mechanisms of Molecular Hydrogen Formation in Conditions Relevant to the Interstellar Medium

“…Searches of rovibrational excitation of H 2 that could be ascribed to "UV formation pumping" in PDR and dense clouds yielded no positive detection (Tinéetal. 2003;Lemaire & Field 2001;Thi et al 2009). …”