ISO observation of molecular hydrogen and fine-structure lines in the photodissociation region IC 63

Journal article, 2009

We wish to constrain the main physical properties of the photodissociation region (PDR) IC63. We present the results of a survey for the lowest pure-rotational lines of H2 with the Short Wavelength Spectrometer and for the major fine-structure cooling lines of OI at 63 and 145μm and CII at 157.7μm with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO) in the high-density PDR IC63. The observations are compared with available photochemical models based on optical absorption and/or millimetre emission line data with and without enhanced H2 formation rate on grain surfaces. The cloud density nH is constrained by the fine-structure lines. The models include both collisional excitation and ultraviolet (UV) pumping of the H2 ro-vibrational levels. Molecular pure-rotational lines up to S(5) are detected. The inferred column density of warm H2 at 106 +/- 11K is (5.9 +/- 1.8)+0.9-0.7 × 1021cm-2, while that of the hot component at 685 +/- 68K is (1.2 +/- 0.4) × 1019cm-2. Fine-structure lines are also detected in the far-infrared spectrum of IC63. The fine-structure lines constrain the density of the PDR to be (1-5) × 103cm-3. The impinging UV field on the PDR is enhanced by a factor of 103 compared to the mean interstellar field and is consistent with direct measurements in the UV. PDR models that include an enhanced H2 formation at high dust temperature give higher H2 intensities than models without enhancement. However, the predicted intensities are still lower than the observed intensities.