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An international team of astronomers has analyzed multi-color photometric and spectroscopic observations of the star Gaia20bdk. As a result, they found that Gaia20bdk is a FUor-type young stellar object. The finding is presented in a paper Dec. 10 on the arXiv preprint server.

Young stellar objects (YSOs) are stars in the 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. They can be up to 5 to 6 magnitudes in amplitude and last from decades to even centuries.

Gaia20bdk was identified in March 2020 by the Gaia Photometric Science Alert system as a 1.2 mag brightening of a red star known as 2MASS 07101491–1827010. Further inspection of this source found that it may be a YSO in the star-forming region Sharpless 2-301, which is located some 11,500 light years away. The mass of Gaia20bdk was determined to be about 2.67 solar masses, while its age was estimated to be 1.6 million years. Archival data indicate that the outburst of this source started in 2018.

Now, a group of astronomers led by Michal Siwak of the Konkoly Observatory in Budapest, Hungary, has performed a comprehensive study of Gaia20bdk, hoping to shed more light on its nature.

"In order to study the star during the quiescent phase, we collected archival public-domain visual and infrared photometry.... We use our [own] and public-domain optical and infrared photometry and spectroscopy to study the outburst," the researchers wrote in the paper.

The study found that Gaia20bdk is G7-type Class I YSO with an effective temperature of approximately 5,300 K and bolometric luminosity of 11 solar luminosities. In general, Class I YSOs are the ones with a thick envelope and a spectral index above 0.3.

The near-infrared spectra of Gaia20bdk investigated by Siwak's team showcase all the features of typical FUors. In particular, the astronomers noted that these spectra show carbon monoxide-bandhead in absorption, a triangular shape of the H-band spectrum, and mostly hydrogen and heavier elements in absorption.

Furthermore, the light curve plateau of Gaia20bdk shows small-scale photometric variability, which is usually associated with disk flickering observed in other FUors. The disk of Gaia20bdk was found to have an inner radius of about four solar radii, bolometric luminosity of around 133-165 solar luminosities, and mass accretion rate at a level of 0.000015 solar masses per year. These values are also typical for YSOs of the FUor type.

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