H. Alwyn Wootten

Water Maser Survey toward Low-Mass Young Stellar Objects in the Northern Sky: Observational Constraints on Maser Excitation Conditions

We present the results from a multiepoch H2O maser survey toward low-mass young stellar objects using the Nobeyama 45 m telescope and the Very Large Array. Our Nobeyama survey is the first complete H2O maser survey toward known Class 0 sources in the northern sky (δ > -35°). During the series of the monitoring observations, we detected the maser emission toward none of the 31 pre-protostellar cores, 15 of 30 Class 0, two of 32 Class I, and zero of nine Class II sources. From this, we conclude that Class 0 sources are favorable sites to harbor the masers: the detection rates are derived to be 39.7% for Class 0, 4.0% for Class I, and 0.0% for Class II sources taking time variation into account. In addition, we found that the H2O maser luminosities in low-mass stars are more closely related to the luminosities of 100 AU scale radio jets rather than the mechanical luminosities of large-scale CO outflows. This fact suggests that the masers are associated with the shocked regions that are impacted by neutral protostellar jets emanating from the central stars. The drastic decrease of the maser detection rate in Class I sources is likely to be caused by the dissipation of dense gas around the central objects. We base this on the fact that the radio jets are found to have similar luminosities in Class 0 and Class I. It seems difficult even for active protostellar jets to excite masers in the remaining tenuous gas around Class I sources.

High Angular Resolution Observations of Maser Kinematics Near Low Mass Young Stellar Objects

With the advent of new correlators and dedicated arrays, spectral line VLBI is entering its ascendancy as a probe of a variety of interesting astrophysical environments. One of the most interesting environments where spectroscopic VLBI techniques are valuable are the regions directly coincident with forming stars. In these sources, water maser emission is observed when the outflowing jets of material interact with the surrounding medium. Observations of these water masers dramatically reveal the innermost regions of the star formation process at or below the 1-AU scale. We have found that the water masers clearly trace the jets at these scales. The masers show space motions on the order of 60 to 100 kms −1 and form within a few AU of the exciting protostar. By observing the distributions and motions of the water masers associated with these objects, we may be able to address in greater detail the collimation mechanism of the jets seen in these protostars. In this brief poster proceeding, we provide a summary image of the water masers associated with SVS13, the driving source for the HH 7-11 objects. We have also mapped the masers associated with IRAS 16293-2422, IRAS 05413-0104, IRAS 4A and IRAS 4B, both in the NGC 1333 star forming region. For further information on these sources, please contact any of the authors directly.

Water Masers toward Low-Luminosity Young Stellar Objects

A review of the importance of water maser observations toward young stellar objects (YSOs) is presented. Also, we present recent, differing types of observations of water masers near YSOs. Single antenna observations, taken regularly, characterize the variability of the masers and allow estimates of time and spatial scales. High resolution (~ 1 mas) multi-epoch observations allow proper motions to be studied. Detailed analysis of such proper motions will allow the placement of the masers in the circumstellar (a disk) or near-stellar environment at the base of the outflow. Radio interferometric techniques are the best method of making estimates of the kinematics of the gas in these regions.