The CepHeus-A Star formation and proper Motions (CHASM) Survey

The “CepHeus-A Star formation and proper Motions” (CHASM) survey is a large project consisting of a combination of astrometric Very Long Baseline Array (VLBA) and Jansky Very Large Array (VLA) observations, to map both the stellar and dense molecular gas components in the star-forming region Cepheus A. With the VLBA, we make use of the CH3OH and H2O maser emission in the vicinity of Cepheus A HW2, in order to measure accurate proper motions and parallax distances to both T Tauri stars and massive young stellar objects (YSOs) belonging to the same star-forming region. With the Jansky VLA, we make use of the interstellar thermometer NH3, in order to image the molecular clump surrounding Cepheus A HW2 and to determine its physical conditions. By combining these informations all together, we can provide, for instance, a direct measurement of the Bondi-Hoyle accretion radius for a massive young star, namely, HW2.

New water maser source near HW3d in the massive star-forming region Cepheus A

Cepheus A is the second nearest high mass star-forming region after Orion. It is characterized by the presence of several phenomena, such as a complex molecular outflow, and multiple radio continuum sources, known as HW sources. The radio continuum and water maser emission have been detected toward HW2, HW3b and HW3d regions, and all of them are considered harboring young stellar objects. In 2014, we performed KaVA observations and detected a new bright maser feature, ~700 mas apart from HW3d, which has not been detected with previous VLBI observations. The relative proper motion of the new maser feature is faster than other regions. It can be a clue for a newly forming star. Alternatively, it may be caused by outflow shock from the star-forming regions such as HW3d or HW3c.

A Highly-collimated Water Maser Bipolar Outflow in the Cepheus A HW3d Massive Protostellar Object

We report the results of multi-epoch very long baseline interferometry (VLBI) water (H2O) maser observations carried out with the VLBI Exploration of Radio Astrometry (VERA) toward the HW3d object within the Cepheus A star-forming region. We measured proper motions of 30 water maser features, tracing a compact bipolar outflow. This outflow is highly collimated, extending through ~400 mas (290 AU), and having a typical proper motion velocity of ~6 mas yr−1 (~21 km s−1). The dynamical timescale of the outflow was estimated to be ~100 years, showing that the outflow is tracing a very early star-formation phase. Our results provide strong support that the HW3d object harbors an internal massive protostar, as previous observations suggested. In addition, we have analyzed Very Large Array (VLA) archive 1.3 cm continuum data of the 1995 and 2006 epochs obtained towards Cepheus A. These results indicate possible different protostars around HW3d and/or strong variability in its radio continuum emission.