Hubble Space Telescope NICMOS and WFPC2 images of the HH 1 jet: A comparative study

We present new Hubble Space Telescope (HST) NICMOS images of the source reg ion of the HH 1/2 flow. The HH 1 jet is traced in the [Fe II] 1.64 mu m and the H-2 2.12 mu m lines to about 2 " 5 of the deeply embedded VLA source. In general, the structure of the jet is similar in H-2 and [Fe II] with all the features having comparable brightness in both species. However, there is a gradient in the H-2/[Fe II] ratio which increases with distance from t he source. We also compare our infrared images with almost contemporary HST WFPC2 images in H alpha and [S II]. The ratio of [Fe II] to [S II], both l ow-excitation tracers of weak shocks, is almost constant along the visible part of the jet but then increases by an order of magnitude where the optic al jet disappears. If this is due to reddening, then the extinction, A(V), increases by at least 4 mag in a space of 0." 9 corresponding to 420 AU. Be yond this obscuring ridge, we detect five more infrared knots. Less than 2 " now separate the infrared jet and the tip of the compact 3.6 cm radio con tinuum jet centered on the VLA source. The heavy extinction which obscures the base of the IR jet is probably due to the compact cloud core recently d etected in HCO+. The jet is observed to steadily increase in width, and we argue that this is either due to sideways ejection of shocked material from internal working surfaces or a sonic expansion of the hot jet beam. Surpri singly, the width of the jet is the same to within measurement errors in H- 2 and [Fe II], contrary to the expectations of some models. Two small, prev iously detected off-axis knots appear to form an independent HH flow, which we call HH 501. An infrared cometary nebula (dubbed the X nebula) is found next to the HH 1 jet, along a line through the HH 501 knots, possibly supp orting earlier speculations that yet one more source exists in the region. However, new proper motions of the HH 501 flow suggest an origin of these k nots near VLA 1, and it is therefore possible that the VLA 1 source itself may be a close binary, thus forming a triple system with the more distant V LA 2.