Luminosity Outburst Energized by the Collision between the Infalling Streamer and Disk in W51 North

Abstract

We report the detection of the disk/torus, outflow, and inflow structures traced by H2O masers toward a high-mass young stellar object W51 North during its H2O maser outburst stage using the Karl G. Jansky Very Large Array (VLA). It is found that the disk has a radius of ∼4000 au and an inclination angle with respect to the sky plane of ∼60° by combining the VLA and the Atacama Large Millimeter/submillimeter Array data. Additionally, a peculiar flow perpendicular to the SiO bipolar outflow is detected in the H2O maser, SiO, and HC3N lines, which is newly-identified as an infalling streamer rather than an old outflow from this source, as reported in previous studies. Combining the VLA map and the Tianma radio telescope monitoring of the H2O masers suggests that the origin of the luminosity outburst of H2O masers during 2020 January–April is likely related to the energy release from the collision between the infalling streamer and the disk. This may provide an additional mechanism to account for the luminosity outburst or episodic accretion beyond disk fragmentation caused by gravitational instability.

Masers and Star Formation Activities in W51A

“…We recently had applied a VLA observation of W51-IRS2 to resolve the spatial distributions of the H 2 O, CH 3 OH and NH 3 masers (Zhang et al 2023), following the detection of a luminous outburst in the target using the TMRT (Zhang et al 2022). In this paper, combining more masers detected in our VLA data may provide additional evidence to support our proposed scenario that a collision between a clump in the infalling streamer and the disk can be one cause of maser flares.…”

“…Figure 5 illustrates the blue-and redshifted emissions of these two transitions in blue and red, respectively, while the 1.3 mm continuum is shown in green. Examining this figure, it is found that while both the SiO J = 5-4 and HC 3 N J = 24-23 emissions trace the infalling streamer in W51-North (see Zhang et al 2023 for details), the HC 3 N J = 24-23 transition emission poorly indicates the presence of the bipolar outflow in W51-North, appearing much fainter and weaker compared to the SiO J = 5-4 transition emission. Apart from W51-North, their emissions in two additional continuum sources also exhibit great differences.…”

“…To help understand the structure of W51 and the results of the VLA observation, the SiO J = 5-4, HC 3 N J = 24-23 transition, and the 1.3 mm continuum data in the public ALMA data set from the Cycle 3 program 2015.1.01596.S were analyzed. As shown in previous involved works (Goddi et al 2020;Tang et al 2022;Zhang et al 2023), this observation was carried out from 2015 October 27 to 31, with a high angular resolution being ∼0 02 at Band 6 (216-237 GHz), and two fields centering on W51-North (J2000 position: R.A. = 19 h 23 m 40 05, decl. =  ¢  14 31 05.…”

“…In Zhang et al (2023), the details of our VLA observation and data reduction were introduced. Here, we give a short summary about it and append a bit information about the data reduction for this article: the K-band (18-26.5 GHz) VLA observation toward W51-IRS2 (J2000 position: R.A. =19 h 23 m 40 047, decl.…”

“…Therefore, one possible origin of this flare is a collision between the infalling streamer and the disk surrounding the protostar. On or even before 2020 January, a ∼0.25 M e clump within the infalling streamer collided with the outer disk around W51-North, releasing its gravitational potential into the energy of the heatwave that was then propagated outward spherically from the collision region (Zhang et al 2023). To check the scenario about the origin of this maser flare and the association between the SiO J = 5-4 and HC 3 N J = 24-23 emissions, we further analyzed the distributions of all the three maser species (H 2 O, NH 3 , and CH 3 OH) obtained from our VLA observation and the SiO J = 5-4 and HC 3 N J = 24-23 emissions from the ALMA archived data toward the entire region of W51A, including W51-IRS2 and W51e2/e8 regions in this work.…”