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Giant mid-infrared outburst detected from a distant young stellar object

By conducting a blind search for large amplitude mid-infrared variables in the Wide Infrared Sky Explorer (WISE) archive, Chinese astronomers have serendipitously discovered a giant mid-infrared outburst from a distant young stellar object designated J064722.95+031644.6. The finding is reported in a paper published September 20 on the pre-print server arXiv.

Young Stellar Objects (YSOs) are stars in early stages of evolution, in particular, protostars and pre-main sequence stars. They are usually observed embedded in dense molecular clumps, environments containing plenty of molecular gas and interstellar dust.

Given that episodic accretion processes occur in YSOs, 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 five to six magnitudes in amplitude and last from decades to even centuries.

J064722.95+031644.6 (or J0647 for short) was first identified as an infrared source near a star-forming region in the constellation Monoceros. The true nature of J0647 remained unknown until a new study was conducted by a team of astronomers led by Tinggui Wang of University of Science and Technology of China in Hefei, China.

Wang's team has recently examined mid-infrared light curves from the W1 and W2 bands of the AllWISE and NEOWISE single-exposure photometric database in the Infrared Science Archive (IRSA) within 6 arcseconds of the ALLWISE position. They found that J0647 is a YSO that exhibited a giant mid-infrared outburst.

"In this paper, we report the serendipitous discovery of a gigantic eruption in a previously unknown YSO (RA = 06:47:22.95, DEC = +03:16:44.56) in the mid-infrared," the researchers wrote.

According to the study, J0647 is a deeply embedded Class I (possess a disk and envelope) YSO with a quiescent luminosity of about 9.0 solar luminosities. The mass of J0647 is estimated to be between 0.58 and 1.3 solar masses.

The astronomers found that during the mid-infrared outburst, J0647 gradually brightened by a factor of 5 from 2014 to 2016, what was followed by an abrupt rise by a factor of more than 100 in 2017. Therefore, the YSO exhibited a 500-fold increase in mid-infrared brightness over a two-year period, followed by a slow decline. This eruption amplitude is the second largest recorded among all known YSO eruptions in the mid-infrared band.

Based on the light curve alone, the astronomers classified J0647 as an intermediate-type eruption YSO with an exceptional amplitude. They noted that its near-infrared spectrum is different from that of classical FUors, EXors, or many other known intermediate-type outbursts in YSO due to its lack of absorption or emission lines other than diatomic hydrogen (H₂).