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Origin of the H2 emission: excitation mechanisms

There are several mechanisms which could contribute to the population of the ro-vibrational levels of the electronic ground state and thus give rise to emission of near infrared ro-vibrational H2 lines. First it has to be noted that generally the gas in the molecular clouds is much too cold to substantially populate even the lowest ro-vibrational levels (a few times 10 K, compared to level energies corresponding to temperatures of more than 6000 K even for the lowest ro-vibrational levels). A detailed discussion of the various possible H2 excitation mechanisms in HH-objects is given by Wolfire & Königl (1991), along with possible means of distinguishing between them.

The most relevant excitation mechanism for the present work is without doubt collisional excitation of the H2 molecules in the hot post-shock gas in the protostellar flows. There, the ro-vibrational levels are populated by collisions with other H2 molecules, atoms, or electrons. The temperature of the gas is typically of order 2000-3000 K, thus only the lower v levels will be populated (H2 would be dissociated in hotter gas). The signatures of shock-excited H2 emission are the absence of transitions from high-v levels and a high ratio (~10:1) of fluxes in the 2.12 µm 1-0 S(1) and the 2.24 µm 2-1 S(1) lines (see, e.g., Wolfire & Königl 1991; Smith 1995).

As a second excitation mechanism UV fluorescence must be kept in mind. In this case, the H2 molecule is lifted into an electronic excited state through absorption of UV photons in the Lyman and Werner bands. Subsequent decay either leads to dissociation of the molecule (in about 10 % of all transitions) or to decay into bound ro-vibrational levels of the ground state. From there, the H2 molecule decays through a cascade of ro-vibrational transitions (e.g., Black & Dalgarno 1976; Black & van Dishoeck 1987). UV-fluorescence leads to population of both, high- and low-v states. Consequently, transitions from higher v states can be observed; another often used first discriminant against collisional excitation is the usually comparably low ratio (2:1) of fluxes in the 2.12 µm 1-0 S(1) and the 2.24 µm 2-1 S(1) lines (e.g., Black & Dalgarno 1976; Black & van Dishoeck 1987; Wolfire & Königl 1991). Similar to UV continuum pumping, H2 molecules in vibrationally excited levels of the ground state might absorb Lyman- photons from atomic hydrogen and thus be pumped to the first excited electronic state.

Collisional excitation is the dominant H2 excitation mechanism in outflows from young stellar objects (see below). However, the possibility of H2 excitation through UV fluorescence has to be kept in mind, particularly if H2 features near hot stars are to be explained.


next up previous contents
Next:Origin of the H2 emission: (molecular) shocks Up:2.3 Molecular hydrogen jets Previous:Observations of interstellar H2: Milestones

Thomas Stanke

Mon Aug 14 17:51:08 CEST 2000
Last modified Thu Jul 12 15:38 CEST 2001