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Accretion outburst from a massive protostar: a sequence of extraordinary observational results
Thursday, June 24, 2021 - 12:30pm PDT
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In 2015, a brief but prescient news release from Hartebeesthoek Radio Astronomy Observatory (HartRAO) announced "A Star is Born", reporting that many maser lines had suddenly brightened in unprecedented unison toward the protocluster NGC6334-I (Macleod+2018). Unaware of this news in North America, the first ALMA observations of this region had only recently been obtained for our Cycle 2 project. Comparison of these images with our previous SMA images from 2008 revealed an extraordinary increase in dust continuum emission from the deeply-embedded hot core MM1 (Hunter+2017). VLA DDT observations in late 2016 then pinpointed the new 6.7 GHz methanol maser emission as coming from MM1 and its immediate neighborhood, which had never been detected in this maser line (Hunter+2018). ALMA images of CS and HDO in Bands 7 and 10 revealed a compact bipolar outflow cavity centered on the hypercompact HII region MM1B and aligned with a faint centimeter jet and water maser bowshock (Brogan+2018, McGuire+2018). While MM1 was undetected at 18 microns in pre-outburst ground-based observations, SOFIA FORCAST and HAWC+ images reveal that it is now the brightest object in the region at 25, 37 and 53 microns, outshining the ultracompact HII region NGC6334F. The combination of SOFIA and ALMA measurements has enabled the first SED of the outburst, yielding a luminosity of 48000 Lsun, which is approximately 16 times greater than the pre-outburst era, equivalent to 3 magnitudes of luminosity increase (Hunter+2021). The outburst has persisted in maser and dust emission over 6 years. Such sizes and durations have been predicted by hydrodynamic simluations of accretion outbursts in the context of disk fragmentation (Meyer+2021). The impact of these outbursts on the structure of the protostar is only beginning to be assessed, and I will preview our work on MM1B in this area. Meanwhile, the search for similar events in other massive protostars is ongoing by the international Maser Monitoring Organization using HartRAO and other single dish facilities. With Target-of-Opportunity proposals submitted to infrared through centimeter facilities, SOFIA is most crucial for measuring the magnitude of the outburst, while ALMA is essential to understand the protocluster and the kinematics of the thermal gas surrounding the accreting protostar.
The video of this talk is available here.