RADIATIVELY AND SHOCK EXCITED H2 IN MAGELLANIC H-II REGIONS

Near-IR spectroscopy of 14 small Magellanic Cloud H II regions has led to the detection of the 1-0 S (1) transition of molecular hydrogen (H-2) in 8 objects. In the latter, relatively strong emission due to the He I 4(3) S-3(3) P0 transition was also found. As a consequence, low-resolution CVF spectrophotometry of (extragalactic) H II regions carries the danger of significantly overestimating the 1-0 S (1) H-2 transition strength, and thereby the relative importance of shock excitation. In 5 objects (SMC: N81, N88; LMC: N83B, N159A5 and N160A2) various H-2 transitions were observed in order to identify the H-2 excitation mechanism. Observed line ratios indicate that N83B and N81 are radiatively excited. The remaining objects either contain a shock-excited H-2 component or have densities sufficiently high that collisional de-excitation influences observed line ratios. However, in both cases radiative excitation is in all objects the energetically dominant mechanism. For the observed H-2 Clouds, we derive volume densities n(H) = 2000-10 000 cm-3, with a mean of 5000 cm-3. If significant subparsec clumping occurs, densities may be higher. Limits on shock contributions to the observed H-2 emission suggest shock velocities V(s) < 10 km s-1 (J-type shocks) or V(s) < 15-23 km s-1 (C-type shocks). In turn, this suggests that early-type stars in the observed objects have stellar winds weaker than Galactic O-type stars. The strength of the observed near-IR He I transitions is briefly discussed. Further investigation of He I lines in Magellanic Cloud H II regions may be of interest.