Shocked molecular hydrogen in the bipolar outflow NGC 2071

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Abstract
Emission from the |$\upsilon = 1-0$| S(1) line of molecular hydrogen has been mapped in the bipolar outflow NGC 2071. The line emission peaks at six positions distributed irregularly along the two lobes, which extend over a distance of ∼ 1 pc. These lobes are parallel to, but offset ∼ 20 arcsec from, the lobes of high-velocity CO line emission. Spectra from 2.1 to 2.45 µm of the H2 emission lines are typical of shock-excited emission. The total H2 line luminosity is estimated to be |$\sim4.5 \enspace L_\odot$|⁠. Profiles of the 1—0 S(1) line are relatively narrow (< 30 km s−1 FWHM) for shocked gas. The peak velocity varies systematically across the source, in a manner consistent with the observed bipolarity of the millimetre-wave CO line emission. The H2 lines probably originate from dense clumps of molecular gas, lying along one edge of the outflow cavity, which are being shocked by a high-velocity wind. There is evidence for partial dissociation of the molecular gas into atomic gas by the shock, but the lack of detectable Br γ line emission puts a low upper limit on the degree of ionization of the gas. The energetics and composition of the high-velocity gas seem consistent with a model in which the driving agent is a bipolar atomic wind, arising from the vicinity of the central IR sources, sweeping up and shocking the surrounding molecular cloud and evacuating a cavity within it.

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