Weak H₂ emission from diffuse and translucent clouds

W.F. Thi ¹, E.F. van Dishoeck ¹, J.H. Black ², N.J. Evans ³, D.T. Jaffe ³ & D.J. Jansen ¹,

¹ Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands

² Onsala Space Observatory, S-43992 Onsala, Sweden

³ Astronomy Dept., University of Texas, Austin, TX 78712, USA

We present the first results of a survey of the molecular hydrogen pure-rotational lines observed with the ISO-SWS in different interstellar environments: diffuse clouds, translucent clouds and photon-dominated regions (PDR). Results for the S(0) at 28.218 m and S(1) at 17.035 m lines will be presented for different positions in the outskirts of the molecular cloud S140, the diffuse cloud in the line of sight toward  Ophiuchi, and the PDR IC 63. The density ranges from n 10² cm^-3 in the diffuse cloud toward  Ophiuchi to n 10⁵ cm^-3 in IC 63, and the incident UV radiation varies from I -1000.

The levels involved in the H₂ lines lie quite high in energy: E 510 K and 1015 K for J=2 and J=3 respectively, and thus appear excellent tracers of the ``warm'' (T 100 K) gas component of these objects.

The observations are interpreted with comprehensive models, which take both collisional excitation and ultraviolet pumping of H₂ into account. In the case of  Ophiuchi, the ISO results are compared with those of H₂ observations at ultraviolet wavelengths. Lines up to S(5) are detected from the IC 63 cloud illuminated by the B0.5 star Cas, which are compared with the detailed available physical models for this cloud. The data form tests of the temperature structure and heating mechanisms of these clouds.