Accretion outburst from a massive protostar: a sequence of extraordinary observational results

Date: 
Thursday, June 24, 2021 - 12:30pm PDT
Speaker: 
Todd Hunter
Affiliation: 
NRAO
Location: 
Online
Event Type: 
Summer Series

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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.

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