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 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 . 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) × 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.