ISO observation 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). 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 H2 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.