Observations of interstellar molecular hydrogen emission

Citation

Beckwith, Steven Van Walter (1978) Observations of interstellar molecular hydrogen emission. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/p3xb-x280. https://resolver.caltech.edu/CaltechETD:etd-04292005-093316

Abstract

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A search for interstellar molecular hydrogen emission, as seen in the v = 1 -> 0 S(l) line, is presented. Thirty-six objects were observed as part of this search; seven objects show emission from vibrationally excited H2. Of the seven new sources, six are associated with planetary nebulae and one is associated with T Tauri. By considering the current theories of H2 excitation and comparing the emission line intensities from the new sources with the upper limits to emission from nine HII regions, it is concluded that excitation of the molecules originates in the boundary layers of strong shock waves. The new sources provide a basis for futher study of interstellar H2 emission, planetary nebulae, and T Tauri stars.

In addition to the search, an in-depth study of the molecular hydrogen emission from the Orion nebula is presented. The data consist of maps at both 13" and 5" resolution in the v = 1 -> 0 S(l) emission line. Intensity measurements of the v = 1 -> 0 S(0), S(1), S(2), and Q(3) transitions and the v = 2 -> 1 S(1) transition are used to derive a vibrational temperature of 2000 K ± 300 K and the associated column densities at the two emission peaks. The data presented here improve upon previous data of the Orion H2 emission.

Measurements of the v = 1 -> 0 Q(3)/S(1) intensity ratio are given. Comparison of this ratio with that observed in NGC 7027 leads to a 2.1 [...] extinction of 4.5 magnitudes to the emission region in Orion. A measurement of the Q(4)/S(2) intensity ratio gives a similar value. The extinction implies that the emission region is inside the molecular cloud and probably near the BN/KL infrared cluster. The extinction measurement is used with previous line intensity measurements to correct the derived excitation temperatures. The resultant rotational and vibrational temperatures of H2 are found to be equal within the uncertainties.

The measurements of Orion strongly imply that the vibrationally excited molecular hydrogen exists behind the discontinuity of a strong shock wave. A rough estimate of the energy involved in the shock indicates that no observed object in Orion could drive the shock. The results lend support to the suggestion that a supernova explosion occurred within the molecular cloud several thousand years ago.

Thesis Files