The Extraordinary Outburst in the Massive Protostellar System NGC 6334I-MM1: Emergence of Strong 6.7 GHz Methanol Masers

Abstract

We report the first sub-arcsecond VLA imaging of 6 GHz continuum, methanol maser, and excitedstate hydroxyl maser emission toward the massive protostellar cluster NGC6334I following the recent 2015 outburst in (sub)millimeter continuum toward MM1, the strongest (sub)millimeter source in the protocluster. In addition to detections toward the previously known 6.7 GHz Class II methanol maser sites in the hot core MM2 and the UCHII region MM3 (NGC6334F), we find new maser features toward several components of MM1, along with weaker features ∼ 1 ′′ north, west, and southwest of MM1, and toward the non-thermal radio continuum source CM2. None of these areas have heretofore exhibited Class II methanol maser emission in three decades of observations. The strongest MM1 masers trace a dust cavity, while no masers are seen toward the strongest dust sources MM1A, 1B and 1D. The locations of the masers are consistent with a combination of increased radiative pumping due to elevated dust grain temperature following the outburst, the presence of infrared photon propagation cavities, and the presence of high methanol column densities as indicated by ALMA images of thermal transitions. The non-thermal radio emission source CM2 (2 ′′ north of MM1) also exhibits new maser emission from the excited 6.035 and 6.030 GHz OH lines. Using the Zeeman effect, we measure a line-of-sight magnetic field of +0.5 to +3.7 mG toward CM2. In agreement with previous studies, we also detect numerous methanol and excited OH maser spots toward the UCHII region MM3, with predominantly negative line-of-sight magnetic field strengths of −2 to −5 mG and an intriguing south-north field reversal.

The Extraordinary Outburst in the Massive Protostellar System NGC 6334I-MM1: Flaring of the Water Masers in a North–South Bipolar Outflow Driven by MM1B

“…When the water masers within these groups are concentrated into spatially distinct "associations" they are further labeled with the suffix -W1, -W2, etc for ease of referral. The naming of these associations parallels that used for the 6.7 GHz methanol and 6.035 GHz OH masers reported in Hunter et al (2018) toward this source. The assignment of each fitted maser spot to an association is provided in a column in Tables 2, 3, and 4.…”

“…The pumping of the methanol masers to the southeast of CM2-W1 (Fig. 9b) is most likely supported by the increased infrared radiation from MM1B itself, as it travels up the outflow cavity and outward through a few low extinction pathways as postulated by Hunter et al (2018).…”

“…4.1. Comparison between post-outburst water, 6.7 GHz methanol and 6.035 GHz excited OH masers Figure 9a shows the locations of the 2017.8 epoch water masers, as well as the 2016.9 epoch 6.7 GHz methanol and 6.035 GHz excited OH masers reported by Hunter et al (2018) with respect to the 1 mm dust continuum, 5 cm emission, and the N-S outflow traced by CS (6-5). The north-south morphology of the ensemble of maser species is striking.…”

First Results of an ALMA Band 10 Spectral Line Survey of NGC 6334I: Detections of Glycolaldehyde (HC(O)CH2OH) and a New Compact Bipolar Outflow in HDO and CS

“…The ability to monitor both the thermal water in the outflow and pinpoint the locations of the H 2 O maser emission to the impact sites of the outflow into the surrounding media demonstrates the powerful combination of ALMA band 10 data with VLA cm-wave observations. In this case, the detection of a collimated outflow in thermal and maser lines along the same axis as the compact radio jet from MM1B Hunter et al 2018) enables the recognition of these various distinct phenomena as arising from a unified structure. (Figure 3).…”

Radio jets from young stellar objects

“…Image adapted from Osorio et al (2017) mainly carried out at the VLA. After the confirmation in 2010 of synchrotron emission in the HH 80-81 radio jet, the improvement in sensitivity of radio interferometers has facilitated higher sensitivity observations at multiple wavelengths of star forming regions, and new non-thermal protostellar jet candidates are emerging (e.g., Purser et al 2016;Osorio et al 2017;Hunter et al 2018;Tychoniec et al 2018;see Fig. 5).…”