Archive abstract

Abstract of the proposal for the experiment RB007


The theory of massive star formation is approaching resolution of some of its
major hurdles. An episodic disk- mediated accretion mechanism is now emerging
as the most likely process by which massive stars acheive their final masses.
However observational evidence is scarce since the proposed ‘bursts’ phases of
accretion are expected to comprise less than 2 percent of the star’s formative
years. Only three burst events have been identified to date in massive stars:
S255IR, NGC6334I and G358.

G24.33+0.14 has been identified as the fourth candidate in this class by maser
monitoring campaigns (via the Maser Monitoring Organisation) and is now under
intense observational scrutiny to at various frequencies and facilities from
radio to infrared. Low opacity and high angular resolution grants radio VLBI
a clear advantage in efforts to understand the inner workings of these deeply
embedded, minute regions.

Maser activity in G24 has been continuously monitored by the M2O and
preliminary results from a 5cm ToO conducted recently indicated changes in the
methanol maser distribution. Now a flare has been identified in the 1667 MHz
hydroxyl maser line, which is also a radiatively pumped maser transition and
is thus expected to react to changes in the radiation field produced by
an accretion burst. The 18 cm OH masers trace different physical conditions
to 5 cm methanol masers and thus could present a novel tool in investigating
accretion bursts. However, to date there have been no OH maser imaging
observations conducted during an accretion burst in a high-mass protostar.

In order to establish the use(s) of OH maser imaging studies in the
investigation of high mass star accretion bursts we propose EVN+MERLIN
observations to compare with earlier observations, in addition to making use
of the more well established role of the paramagnetic OH molecule; as a probe
of the magnetic field.
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