Water maser variability in a high-mass YSO outburst

VERA and ALMA observations of S255 NIRS 3

¹ Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
e-mail: tomoya.hirota@nao.ac.jp
² Department of Astronomical Sciences, SOKENDAI (The Graduate University for Advanced Studies), Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ National Astronomical Research Institute of Thailand (Public Organization), 260 Moo 4, T. Donkaew, Amphur Maerim, Chiang Mai 50180, Thailand
⁵ Korea Astronomy and Space Science Institute, Hwaam-dong 61-1, Yuseong-gu, Daejeon 305-348, Republic of Korea
⁶ Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, Hoshigaoka 2-12, Mizusawa, Oshu, Iwate 023-0861, Japan
⁷ Department of Astronomical Sciences, SOKENDAI (The Graduate University for Advanced Studies), Hoshigaoka 2-12, Mizusawa, Oshu, Iwate 023-0861, Japan
⁸ Center for Astronomy, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan

Received: 30 October 2020
Accepted: 9 December 2020

Abstract

Context. Clarifying the relationship between mass accretion and ejection history is one of the key issues in understanding high-mass star formation processes.

Aims. We aim to investigate the possible relationship between the mass accretion burst event in mid-June 2015 and the jet ejection in the high-mass protostar S255 NIRS 3.

Methods. The Very Long Baseline Interferometer (VLBI) monitoring observations of the 22 GHz H₂O masers were carried out using VLBI Exploration of Radio Astrometry (VERA) to reveal the 3D velocity and spatial structure of the outflow/jet traced by the H₂O masers in S255 NIRS 3. In addition, we conducted follow-up observations of the submillimeter continuum and the 321 GHz H₂O masers with the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7.

Results. We successfully measured the proper motions of the 22 GHz H₂O masers associated with a bipolar outflow. The structure is almost the same as was observed in 2005 and 2010. The expansion velocity of the blueshifted bow shock traced by the 22 GHz H₂O masers was measured to be 28 km s⁻¹, corresponding to a dynamical timescale of 60 yr. The direction of the maser outflow is slightly tilted compared with the radio jet, which could suggest a more recent ejection episode during the accretion burst event. The total flux density of the 22 GHz H₂O masers gradually increases from the beginning of the VLBI monitoring in early 2017 and becomes almost constant in subsequent single-dish monitoring in 2018. The brightening of the H₂O masers is more prominent in the northeast outflow lobe. For the first time, we revealed extended H₂O maser emission at 22 GHz in a star-forming region, which is partly resolved out by VERA and even by the most extended Very Large Array (VLA) configurations. We find that the flux variation of such an extended component is similar to that of the unresolved maser emission. The ALMA Band 7 continuum emission did not show significant variations compared with the previous observations performed five months before. We mapped the 321 GHz H₂O masers in S255 NIRS 3 providing the fourth example, for this maser, of the spatial distribution in a high-mass star-forming region.

Conclusions. We conclude that the bow shock structure traced by the 22 GHz H₂O maser features is unlikely to originate at the interface between the radio jet powered by the recent accretion outburst and the surrounding medium. The brightening of the 22 GHz H₂O masers could be due to radiative excitation by photons form the (declining) infrared (IR) outburst escaping along the cavity created by the newly ejected material. The lower ratio of the 22 GHz/321 GHz maser luminosity in the blueshifted bow shock suggests a temperature ( >1000 K), higher than for the other maser features in this region.