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

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 mu m and C ii at 157.7 mu m 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) x 1021 cm-2, while that of the hot component at 685 +/- 68 K is (1.2 +/- 0.4) x 1019 cm-2. Fine-structure 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) x 103 cm-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 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.