Disk and Outflow in Cepheus A–HW2: Interferometric SiO and HCO+Observations

Abstract

This paper reports observations of the HCO+ (1→0) and SiO (2→1) lines and of continuum emission at λ=3.4 mm toward the Cepheus A East star-forming region. The HCO+ line shows emission up to velocities of ~50 km s-1 relative to the ambient cloud velocity. The spatial distribution of the high-velocity gas is bipolar, centered on HW2, and extends for ~1' along P.A.=55°-60°. The orientation of this molecular outflow is very similar to that of the radio jet and CO emission associated with HW2. This confirms that current mass loss from HW2 takes place in the northeast-southwest direction. The momentum rate of the HCO+ outflow is 1.3×10−2 M☉ km s-1 yr-1, 20 times larger than that of the ionized jet from HW2, suggesting that the jet could be largely neutral. Peaks of HCO+ emission coincide with some of the radio-continuum sources in the region, consistent with outflow-ambient gas interaction. Multiple episodes of outflow activity from either one precessing source or a number of powering sources would explain the detection of moderate-velocity HCO+ emission toward the HW7 chain of sources. The continuum emission at 3.4 mm is strongly peaked toward HW2, confirming that this is the most likely powering source for the outflow. The SiO emission is barely elongated along P.A.=-33°, i.e., almost perpendicular to the HCO+ outflow. A velocity trend consistent with a gradient of ~31 km s-1 over 2'' is detected along the major axis of the SiO structure; these motions could be bound by 200 M☉. Published H2O and CS observations support our suggestion that this represents the outer, ~750 AU radius, part of a circumstellar disk, although further observations are needed to confirm this interpretation.