Shocked Molecular Hydrogen in the Orion ``Bullets''

Details Shocked Molecular Hydrogen in the Orion ``Bullets'' Shocked Molecular Hydrogen in the Orion ``Bullets''

Mar 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........38t&link_type=abstract

PhD Thesis, Institute for Astronomy, University of Edinburgh, UK, 1997.

Astronomy and Astrophysics

Astronomy

Scientific paper

The physics of shocked outflows in molecular clouds is one of the fundamental astrophysical processes by which the cycle of star formation in our Galaxy is regulated. I outline the basis of our understanding of the star formation process and the viol ent outflow always associated with it, the physics of shocks in molecular gas, and the consequent excitation of molecular hydrogen (H2). It is demonstrated that molecular hydrogen is the best observational diagnostic of this hot, shocked molecular gas and an introduction is given to the observational techniques of near-infrared spectroscopy required in its measurement. I describe a detailed observational study of the physics of shocked H2 excitation and dynamics in the nearby massive star forming re gion of the Orion giant molecular cloud, the brightest source of its type, using the recently upgraded CGS4 near-IR spectrometer at UKIRT. We have demonstrated that integrated [FeII] 1.644micron line profiles in the Orion ``bullets'' are consistent with theoretical bow-shock predictions for two different ``bullets''. We have identified a uniform, broad background component pervading the region in both Fe+ and H2 which is inconsistent with a fluorescent component due to the ionizing radiation of the Trapezium stars alone. A collisionally broadened background component of unidentified origin is measured to be Gaussian in profile with an average FWHM of 26 +/- 2.5kms-1 in the H2 1-0 S(1) line after deconvolution of the instrumental profile and a peak velocity of 2.5 +/- 0.5kms-1, close to the local ambient rest velocity. Crucially, the extended H2 ``bullet'' wakes have allowed us to dissect individual molecular bow shock structures but the broad (intrinsic FWHM <= 27kms-1), singly-peaked H2 1-0 S(1) profiles observed in the two most clearly resolved, plane-of-sky oriented wakes challenge our present understanding. It is very difficult to reconcile any steady-state molecular bow shock model with these observations in Orion. To fit a single C shock absorber model to individual H2 profiles implies a magnetic field strength far in excess of observed estimates and is not consistent with the bow-shaped wake morphology. Alternatively, we may still not be resolving multiple H2 shock fronts along the line-of-sight. For example, multiple overlapping bullet wakes could give rise to merged sets of doubly-peaked profiles resulting in approximately Gaussian shaped profiles. However, given the appearance of single bow shaped wakes at many observed positions, the accuracy of single Gaussian line-fits, the velocity resolution of our observations (FWHM = 23.1 +/- 0.3kms-1) and that we see this phenomenon in two differe nt wakes, this explanation is expected to be excluded. If we cannot fit the profiles in Orion with steady state molecular shocks it may be necessary to model the effects of instabilities and turbulence. This will have important consequences. Not only will line profiles be broadened but level populations of shocked species will be altered and hence the observed column densities over a range of transitions. Observations of a range of H2 column densities in the K band have confirmed the existence of a near-constant background excitation mechanism pervading the entire Orion ``bullets'' region. The background H2 emission can be modelled by a combination o f fluorescent and shock excited mechanisms, in agreement with the broad H2 line profiles observed. It is thermalized in the v = 1 levels but higher levels are dominated by fluorescence. Measurement of the H2 excitation in the ``bullet'' wakes M42 HH 126-053 and M42 HH120-114 shows a near constant emission spectrum, within each wake, that may be modelled by a combination of shocked and fluorescent excitation, now more strongly dominated by collisional processes but also containing an intrinsic wake-on ly fluorescent component. The column density ratios clearly show a range of gas temperatures as expected for cooling, post-shock gas. Furthermore, the uniformity of these ratios on small-scales (these observations) and also on large scales, contradicts co mbinations of fundamentally different types of shock. However, the near constancy of this excitation with position within each individual wake is inconsistent with bow C shock models previously fitted at OMC-1, in which significantly different line ratios occur depending on the shock velocity which varies in the bow.

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