A search for shock-excited molecular hydrogen knots in Chamaeleon I very low mass YSOs

Abstract

We have obtained narrow-band images of three selected areas of the Chamaeleon I dark cloud which harbor very low mass young stars, centered on the H_2 and Br_gamma lines and neighboring continuum as well as on the broad band K_s. One region is located in the northern part of the cloud, roughly coinciding with the densest area. The other two regions are in the southern section of the cloud. Our aim is to search for H_2 outflows associated with these objects. In the northern region, we found seven new H_2 knots, five of which are aligned in the direction of a previously known 12^CO molecular bipolar outflow. Further evidence that the class I low mass stellar object ISO-ChaI 192 is the driving source of the molecular flow is given by the presence of a 960 AU long elongated structure at 2.2 micron emanating from this star and oriented parallel to the bipolar structure. Another pair of H_2 knots, although lying relatively nearby, is not aligned with the outflow direction. They are located on opposite sides of C1-6, a low mass class II object in the northern part of the Chamaeleon I dark cloud. In contrast, we fail to detect any H_2 emission object brighter than our sensitivity limit (about 6 x 10^-32 W/m^2 Hz arcsec^2) in the two southern areas of the cloud that also harbor several very low mass stars, including two transition stellar/sub-stellar objects. This negative result is probably not surprising in view of the extremely low accretion rates measured for brown dwarfs (M_dot = 10^-12 -- 10^-9 M_sun yr^-1). Deeper H_2 observations are required to better constraint the outflow event in sub-stellar objects.