PV Ceph: Young Star Caught Speeding?

Abstract

Three independent lines of evidence imply that the young star PV Ceph is moving at roughly 20 km/s through the interstellar medium. The first, and strongest, suggestion of motion comes from the geometry of the HH knots in the "giant" Herbig-Haro (HH) 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 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 images, and the star is moving at roughly 20 km/s. 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 NGC 7023 (more than 10 pc away). We propose that PV Ceph was ejected, at a speed large enough to escape NGC 7023, at least 100,000 years ago, but that it did not enter the molecular cloud in which it now finds itself until more like 35,000 years ago. Our calculations show that the 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 NGC 7023 and its current home is an open question.