Herbig-Haro (HH) jets are well collimated, parsec scale, supersonic outflows from very young stars. These jets interact with the surrounding medium to produce shock fronts, entrainment of material, and knots that may be discrete structures or wave phenomena. While the precise mechanisms of jet formation are unclear, a key to locate their source. It may well be optically invisible, still buried in a natal dust cloud. The flow direction usually points to where to look at longer wavelength--almost all well collimated flows have their sources located along their axes.
A multi-wavelength, multi-observatory study of the prominent HH jet HH110 by B. Reipurth (ESO), A.C. Raga (UNAM), and S. Heathcote (CTIO) shows it to be an outstanding exception to this rule. In narrowband CCD images obtained with the 1.5-m Danish telescope on La Silla, HH110 is a spectacular, sinuous jet emanating from the dark cloud L1617. With a length of half a parsec (200") it is among the largest jets known in star forming regions. Numerous knots are visible, embedded in faint gas outlining the flow. The jet begins narrow and well collimated, strongly suggesting that the energy source is located very near its apex. However, neither an IRAS catalog search, nor a 2.2m search with the ESO 2.2-m, nor mapping at 1300m with the Swedish ESO Submillimeter Telescope SEST detected anything near the apex. While several other IRAS sources and a faint nebulous star were noted in the field, these were thought to be unrelated, and the driving source to be intrinsically faint or highly obscured (Reipurth and Olberg, 1991).
Recent work has dramatically revised this understanding. New deep large-field interference filter images have been obtained with the ESO 3.5-m New Technology Telescope. An [SII] image is shown in the figure. While HH110 is by far the most prominent object in the field, the "faint nebulous star" is now identified as the westernmost knot in a new, faint HH flow that can be traced 0.2 parsecs northeast, designated HH270. A faint infrared source and an IRAS source (marked with a cross in the figure) are located near the position of flow origin suggested by the morphology. There is evidence of a deeply buried counter lobe extending to the east. The axis of this newly discovered flow points almost directly at the HH110 apex, and the "nebulous star" is a tiny bow shock facing this apex. Proper motions on a six year baseline give an average tangential velocity of about 80 km/s for HH110, but a very high speed of 300 km/s directly towards the HH110 apex for the HH270 bow shock.
Both the morphology and the proper motions strongly suggest a connection between HH110 and HH270. A long slit echelle spectrum of HH110 was obtained with a 2" slit and 2.5 hours exposure time on the CTIO 4-m. The figure includes a position-velocity contour plot of the sum of the [SII] 6717/6731 lines. The HH flow is blueshifted of order 30 km/s relative to its surroundings. This very low radial velocity indicates a space motion for the flow effectively in the plane of the sky. The apparent flow apex, at the top of the spectrum, is kinematically distinct from the other knots in being divided into a higher velocity northern part and a southern part at the cloud rest velocity. This double structure is also seen in the morphology and proper motion of this knot. While spectroscopy is not yet available for HH270, the appearance and extent of a counter lobe indicate that its motion, too, is in the plane of the sky.
Velocity integrated line ratios in [SII], [NII], and H were also obtained from the 4-m spectrum. These suggest a stronger shock at the HH110 apex than elsewhere along its length, and clearly indicate a gradient in electron density, decreasing along the flow axis away from the apex.
The morphology, kinematics, and physics all demonstrate that the prominent HH110 flow has its origin in the HH270 energy source, with the HH270 flow being deflected by an obstruction. The result of this collision is a strong shock at the impact point together with a change of direction. To test this idea further, SEST was used to map the molecular cloud around HH270. The resulting CO map, which traces high density cold gas, is overlaid as contours on the [SII] image. A dense clump is seen west of the HH270-HH110 impact point, with the flow direction of HH110 tangential to its surface. This is exactly what theory predicts for such a glancing collision. The incident jet divides into a reflected tangential component (the HH110 jet) and a second shock that propagates into the clump, eventually dispersing it. Previous analytical studies provide predictions of the geometry and velocity characteristics of such collisions. The radial velocity and proper motion information permit determination of the true orientation and kinematics of the incident and reflected beams, and these are found to agree closely with the theory.
Reipurth and collaborators believe this to be the first well proven case of a grazing collision between a jet and a cloud core. Besides its intrinsic physical interest, this investigation illustrates the advantage to be had by combining unique and powerful observational facilities, in close proximity in Chile, in an international collaboration.