H₂ Fluorescence in HH7 and DR21

A. J. L. Fernandes, P. W. J. L. Brand, M. G. Burton

Abstract
We present evidence for Lyα pumping of the Lyman band system of molecular hydrogen in Herbig-Haro 7 and the bipolar outflow DR 21. For this study we have measured several vibrational-rotational emission lines of H₂ whose energy levels are widely spaced and ranging from 6000 (v = 1) to 25000 Kelvin (v = 4). We show that the near-infrared H₂ emission from the shocked gas in HH 7 can be well described by a bow C-type shock. The enhanced emission observed from the higher energy levels (v > 3) can be well modelled by employing the Lyα pumping mechanism.
In the DR 21 outflow the multi-line study showed that different physical conditions exist in the eastern and western emission lobes. The higher H₂ line ratios measured in the eastern lobe suggests a higher Lyα pump rate which may be locally produced in the fast bowshocks. The FUV radiation field emanating from the central HII regions may in addition be exciting the Lyman and Werner bands of H₂ in the molecular lobes.
We show that the observed H₂ emission can be interpreted in terms of a simple model consisting of a C-type bowshock, which produces the low excitation H₂ emission, and a FUV radiation field with enough Lyα line radiation to produce the high excitation H₂ emission through fluorescence.

Astrophysics and Space Science
Volume 233, Page 45
November 1995

ADS Bibliographic code: 1995Ap&SS.233...45F