The JCMT Newsletter Number 10
 Bow shock entrainment in the W75N molecular outflow
With recent CO observations - obtained by Gerald Moriarty-Schieven
through the service/flexible observing programme at the JCMT - we
have searched for extensive outflow activity associated with a number
of luminous young stars. Here we report on some striking new
observations of the massive molecular outflow associated with W75N.
Prior to our observing run at JCMT, wide-field near-IR images
obtained at the Calar Alto observatory revealed extensive bow shocks
equidistant, though on opposite sides of, the well-known, luminous
cluster of massive young stars in W75N. However, it is the CO
J = 3-2 observations which illustrate the full history of the flow, and
our comparison of the two datasets which reveal the true nature
of this source.
In Fig.1 we show contours of the integrated high-velocity
blue-shifted (dashed lines) and red-shifted (full lines) gas in the
W75N outflow. Although the blue lobe appears only as a compact peak
coincident with the central core region (and presumably the outflow
source) the red lobe extends almost 150 arcsecs (1.5 pc) to the southwest
(note the rotated frame coodinates in the figure), streching as far as
the 'bow shock' observed in molecular hydrogen 2.122 micron emission.
Indeed, the CO outflow lobe fits snuggly within the curve of the bow
shock. The flow lobe and bow are seemingly intimately related. In
fact, W75N represents perhaps the best example yet discovered of 'bow
shock entrainment', a model invoked to explain how collimated
optical/Herbig-Haro jets may power massive molecular (CO) outflows.
In this case, it seems likely that much of the observed high-velocity
CO was swept up by the bow shock, the bow itself being driven by an
'unseen' collimated jet. Comparison of these data with numerical
simulations of jet-driven outflows certainly looks promising (the data
and interpretation are discussed in detail in a paper recently
submitted to MNRAS; Davis, Smith & Moriarty-Schieven 1998).
Figure 1: CO 3-2 integrated intensity map of the molecular outflow
associated with W75N, superimposed onto a narrow-band H2 (+ continuum)
image of the region. The luminous source that drives this flow
appears as a dark, nebulous patch in the image at the (0,0) position.
The red lobe of the outflow (full contours) extends to the SW; note
the orientation of N and E at left. The features labelled 'bow
shock' are not evident in our continuum image of the region and are
therefore thought to be the near-IR counterpart of a Herbig-Haro object.
Chris Davis, Joint Astronomy Centre
Last Modification Date 1998/3/2 - Last Modification Author: gdw
Graeme Watt (gdw)
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