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Astronomers observe the cooling process of a young stellar object following an accretion burst

Using the NOrthern Extended Millimeter Array (NOEMA) and the Atacama Large Millimeter/submillimeter Array (ALMA), Chinese astronomers have observed a young stellar object known as G24.33+0.14 in the process of cooling after an accretion burst. Results of the observations are on the arXiv preprint server.

In general, young stellar objects (YSOs) are stars in early stages of evolution; in particular, protostars and pre-main sequence (PMS) stars. They are usually detected within dense molecular clusters, which are abundant in molecular gas and interstellar particles.

Observations show that episodic accretion processes occur in YSOs. Therefore, these objects may experience accretion-driven outbursts. Astronomers usually divide such events into EX Lup (also known as EXors) and FU Ori outbursts (or FUors). EXors are a few magnitudes in amplitude, and last from a few months to one or two years; FUors are more extreme and rare, can be up to 5 to 6 magnitudes in amplitude, and last from decades to even centuries.

G24.33+0.14 (or G24 for short) is a high-mass young stellar object (HMYSO) at a distance of some 23,500 light years away from the Earth. Previous observations have found that G24 is a recurrent accretion burst source, exhibiting a cycle of 8.5 years and a burst duration of about two years.

Recently, a team of astronomers led by Xiaoyun Xu of the Guangzhou University in China performed interferometric observations of G24 during its post-burst phase, in order to better understand its evolution during this period. Their study was complemented by data from ALMA.

The observations found that after the accretion burst, the continuum emission in the inner core region of G24 decreased by some 20%, while the emission in the outer region increased by about 30%. These findings suggest that the burst caused a heat wave, which radiated outward from the core's interior to its periphery over a period of about six months.

Furthermore, it was found that the methanol emission intensity decreased notably in G24. It turned out that as the energy of the upper state of G24 increases, the decrease in the methanol emission intensity becomes more pronounced.

Based on the analysis of the methanol rotational temperature, the astronomers identified a modest decrease in both the gas temperature and column density of methanol in the innermost layer of the methanol emission region. This indicates that both the temperature ratio and column density ratio between the two observation periods generally increased with increasing distance from the core of G24.

According to the authors of the paper, the results confirm that while the core region of G24 cooled down after the burst, the outer region experienced persistent heating. The observations also proved that variations in the physical environment are a direct consequence of the bursts that occur during the high-mass star formation process.