WIDE-FIELD NEAR-INFRARED IMAGING OF THE L1551 DARK CLOUD

We present wide-field near-infrared images of the densest part of the L1551 dark cloud taken with the narrow-band filters for [Fe II] lambda 1.644 mu m and H-2 v = 1-0 S(1) lines, together with the broadband H and K-s filters. Numerous [Fe II] and H-2 emission features were detected from the regions around HL/XZ Tau, HH 30, HH 262, L1551 NE, and L1551 IRS 5. Most of the [Fe II] features are compact or jet-like, suggesting that the emission arises from fast shocks occurring in the ejecta of jets. The H-2 features are more diffuse and widely distributed in outflow lobes, with none of the H-2 features showing the well-collimated emission associated with jets. This implies that the H-2 emission originates from slower shocks where the ejecta interacts with ambient material. The outflow structure in the vicinity of the deeply embedded object L1551 NE is revealed, featuring a well-collimated, spatially continuous [Fe II] jet penetrating a fan-shaped infrared reflection nebula. The tangential velocities of knots in the L1551 NE jet are estimated to be 140-190 km s(-1) from their proper motions, implying an inclination of 45 degrees-60 degrees. for the jet axis. A counter-jet from L1551 IRS 5 is detected for the first time in [Fe II]; this probably corresponds to the northern-most of the two jets on the blueshifted side. The relative brightness of the counter-jet suggests a visual extinction of 20-30 mag. The [Fe II] emissions from collimated jets are relatively strong toward L1551 NE and L1551 IRS 5 compared to those toward HL Tau and HH 30. This implies that the jets from the former objects, which are more embedded, have a higher shock velocity and/or a larger gas density than the latter, more revealed objects. The results presented here show that the near-infrared [Fe II] emission is a useful probe of well-collimated jets from deeply embedded sources, in much the same way that optical [S II] emission is used for relatively revealed objects.