DOI

https://doi.org/10.48717/zt1h-ct57

Project

RB006

Title

e-EVN: rb006a

Abstract

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 expectedcomprise less than 2 percent of the star’s formative years. Only three burst events have been identified to datemassive 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 effortsunderstand the inner workings of these deeply embedded, minute regions. As part of the ongoing M2O project, we propose Target of Opportunity observations to map the 6.7 GHz methanol maser transition to trace the propagation of enhanced radiation field driven by the accretion burst which is happening now in real-time (see the example in the Scientific Justification). We also pursue the shock-driven 22 GHz water maser emission which is also currently bursting in G24. Water masers in such an event have never been observed with VLBI during an accretion burst since previous attempts were hindered by its sudden dissapearence. The maser spectra in G24 exhibit extreme, daily evolution, indicative of fast changing physical conditions, and necessitating quick-response observations. As with the case of G358 we have reached out to as many VLBI arrays as possible to contribute any available time to observe the event. Observational constraint of the G24 event will be a global effort in which the EVN can provide invaluable contribution.