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A.A. Goodman (CfA), H.G. Arce (CalTech)
Three independent lines of evidence imply that the young star PV Ceph is moving at roughly 20 km s-1 through the interstellar medium. The first, and strongest, suggestion of motion comes from the geometry of the HH knots in the ``giant" Herbig-Haro flow associated with PV Ceph. Bisectors of lines drawn between pairs of knots at nearly equal distances from PV Ceph imply an E-W motion of the source, and a plasmon model fit to the knot positions gives a good fit of 22 km s-1 motion for the star. The second bit of damning evidence comes from a redshifted ``trail" of molecular gas, pointing in the same E-W direction implied by the HH knot geometry. The third exhibit we offer in accusing PV Ceph of speeding involves the tilt apparent in the high-velocity molecular jet now emanating from the star. This tilt is best explained if the true, current, jet direction is N-S, as it is in HST WFPC images, and the star is moving--again at roughly 20 km s-1.
Tracing the motion of PV Ceph backward in time, to the nearest cluster from which it might have been ejected, we find that it is very likely to have been thrown out of the massive star-forming cluster NGC7023--more than 10 pc away. PV Ceph and NGC7023 are at similar distances, and the backward-trace of PV Ceph's motion is astonishingly well-aligned with a dark, previously unexplained, rift in NGC7023. We propose that PV Ceph was ejected, at a speed large enough to escape NGC7023, at least 100,000 years ago, but that it did not enter the molecular cloud in which it now finds itself until more like 10,000 years ago. Our calculations show that currently-observable molecular outflow associated with PV Ceph is about 10,000 years old, so that the flow has had plenty of time to form while in its current molecular cloud. But, the question of what PV Ceph was doing, and what gas/disk it took along with it in the time it was traveling through the low-density region between NGC7023 and its current home is open to question.
Recent numerical simulations have suggested that condensed objects should be ejected at high velocity before they have ``finished" forming in a cluster. Prior to this work, a handful of pre-main-sequence stars have been shown to be moving at speeds > 10 km s-1. To the best of our knowledge, though, the analysis of PV Ceph and NGC7023 described here is the first observational work associating a speeding young star with a distant ancestral cluster. These high-speed ejections from clusters will create a class of rapidly-moving young stars in molecluar clouds. If these ejections are at all common, their existence confounds both calculations of clouds' star-forming efficiency and theories of star formation that do not allow for stars to move rapidly through a reservoir of star-forming material while they form.
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The author(s) of this abstract have provided an email address for comments about the abstract: agoodman@cfa.harvard.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.