Bachelor defense by Martine Lützen

Title: Observing thermal water emission toward L1448-MM with ALMA

Stars form deep inside molecular clouds when the gas becomes dense enough for gravity to pull it together. This accretion process is always accompanied by the ejection of material in bipolar outflows driven by stellar jets and winds. However, the details of the driving mechanisms are still poorly understood and as such, observations of a good outflow tracer is needed. Previously, the Herschel Space Observatory showed that water is an excellent tracer of protostellar outflows, and high-velocity water emission was found to be a universal feature for the observed Class 0 sources across transition lines. However, the origin of this emission is still unknown, essentially due to the large Herschel beam (20′′ or ∼ 5000 AU).

There are two main hypotheses for the origin of high-velocity water emission; a disk wind model and a cavity shock model - both of which predict a small emitting region on arcsec scales. To distinguish between the two hypotheses, high resolution observations are needed to spatially resolve the emission. An ideal target is the protostellar source L1448-MM, which is one of the brightest H2O emitting regions.

My aim is to investigate the spatial distribution of the H2O maser transition at 183 GHz toward this source. This will allow us to determine an appropriate outflow model and compare new measurements to previous Herschel data. Initially, we found that ALMA can detect water and the H2O transition at 183 GHz is identified toward the source. Mapping out the spatial distribution revealed the emission to be compactly located around the protostar, agreeing with previous estimates based on Herschel measurements. Therefore, it was a surprise when an average of the 183 GHz spectrum showed a clear discrepancy when compared to the 988 GHz Herschel data. The two spectra showed to fit well at high velocities, however, the ALMA measurements did not exhibit the broad, central component as otherwise expected. Possible explanations were presented, but in order to explore this further, observations at lower angular resolution might be needed.

The morphology of the total H2O emission showed to be well-collimated and for that reason was it no surprise when the isolated thermal emission aligned with the disk wind solution. However, because of the apparent missing central component, we can not draw any conclusions regarding which model fits the emission at low velocities close to v_LSR. As such, the presence of a cavity shock component is still a possibility. In conclusion, the spatial distribution of the thermal H2O emission on small angular scales align with the expectations of the disk wind solution, thus bring us one step closer to uncovering the driving mechanisms behind protostellar outflows.