2009MNRAS.400..622T

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 n_H is constrained by the fine-structure lines. The models include both collisional excitation and ultraviolet (UV) pumping of the H₂ro-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_–0.7x10²¹/cm2, while that of the hot component at 685±68K is (1.2±0.4)x10¹⁹/cm2. 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)x10³/cm3. 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.