Astron. J., 118, 2940-2961 (1999/December-0)
Parsec-scale CO outflow and H2 jets in Barnard 5.
YU K.C., BILLAWALA Y. and BALLY J.
Abstract (from CDS):
New observations of the Barnard 5 IRS 1 molecular outflow, including maps in the 12CO J=2-1 transition and images and high-resolution spectra in the H2 v=1-0 S(1) line, are presented. In the 12CO J=2-1 maps, the outflow has a projected length of over 30' (3 pc) and is highly collimated with a width smaller than 2' (0.2 pc), with one outflow lobe containing clear evidence of a limb-brightened cavity. Like the associated Herbig-Haro flow, the CO lobes exhibit C-shaped symmetry about IRS 1. Bow- or cone-shaped clumps, which are not associated with visible shocks, are located at the ends of the CO outflow. While the presence of Herbig-Haro objects and associated shock excited H2 emission in the outer parts of the CO flow several arcminutes closer to the source indicate that a relatively recent mass-loss episode is still transferring momentum to CO-bearing gas, these terminal CO structures may provide a fossil record of a much older mass-loss episode. The new observations provide support for bow shock entrainment models for the acceleration of CO-bearing gas. Several 15' long 12CO-emitting filaments lie parallel to but displaced by several arcminutes from the IRS 1 outflow. These features may trace perturbations excited by magnetosonic waves triggered by major mass-loss eruption episodes of IRS 1. The terminal H2 emission closely traces the Hα and [S II] emission produced by Herbig-Haro objects located near the ends of the main CO outflow body and is likely to be powered by shocks. However, the H2 emission is systematically displaced downstream from the Herbig-Haro objects. Since the B5 outflow appears to lie within 13° of the plane of the sky, this displacement is not likely to be a geometric projection effect. The specific excitation mechanism may require heating by a magnetic precursor or fluorescence produced by radiation originating in the shocks associated with the Herbig-Haro objects. A compact chain of H2 knots located within 30" of IRS 1 appears to delineate a bipolar jet originating from this source. This H2 feature and the associated Hα emission bisects the limb-brightened CO cones found within 20" of IRS 1. The presence of both axial knots and a wide-angle cavity implies that the central source may simultaneously power both a jet and a wide-angle wind that are formed within 2000 AU of IRS 1. A new method that accounts for the velocity dependence of the 12CO optical depth is used to estimate the mass in the outflow lobes. The resulting power-law mass spectra have slopes that are much steeper than those obtained by assuming that the 12CO line is optically thin in the outflow lobes, an assumption that has been frequently used in other studies. The flow orientation, the outflow evolution, and the velocity at which the outflow lobes becomes optically thin also affect the mass spectrum. The source luminosity, outflow dynamic timescale, outflow strength, and embedded nature of IRS 1 imply that it is in an intermediate evolutionary stage between a Class I and Class 0 source. We also present a new optical spectrum of HH 367, which originates from IRS 3, confirming its Herbig-Haro nature and showing evidence for different excitation conditions along the flow and variable mass ejection rates from the source.
ISM: Herbig-Haro objects - ISM: individual (Barnard 5) - ISM: Jets and Outflows - Techniques: Spectroscopic - Stars: Formation
Figs 2, 3: [YBB99] FN (Nos F0, F1, F2), [YBB99] CN (Nos C1-C2).
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