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Using ESA's XMM-Newton satellite and NASA's Chandra X-ray observatory, astronomers have observed a young stellar object known as HL Tauri. Results of the observation campaign, Feb. 11 on the arXiv pre-print server, yield important insights into long-term X-ray variability and properties of this object.

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.

Based on their evolutionary stage and spectral energy distribution, YSOs are divided into several categories—from Class 0 to Class III. Class I YSOs have begun to clear their thick surrounding envelope and are developing a circumstellar disk.

Located some 450 light years away in the Taurus molecular cloud, HL Tauri (or HL Tau for short) is one of the most well-studied Class I YSOs. The star is less than 100,000 years old, has an estimated mass of about 0.7–1.2 solar masses, and hosts one of the most thoroughly studied protoplanetary disks.

Now, a team of astronomers led by Steven M. Silverberg of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, has employed XMM-Newton and Chandra to investigate HL Tau in X-rays.

"Here we present analysis of recent X-ray observations of HL Tau with the X-ray Multimirror Mission (XMM-Newton) and Chandra X-ray observatory, in conjunction with re-analysis of archival X-ray data," the researchers write.

The study found that the X-ray spectrum of HL Tau is consistently hot and heavily absorbed at all 31 observational epochs spanning 20 years. This is common for typical heavily-absorbed Class I YSOs.

Data collected with XMM-Newton show a recurrent variability on a timescale of about 21 days. According to the astronomers, the variability seen in 2020 can primarily be explained by the X-ray emission changing over time. However, they assume that this is not necessarily the case at earlier epochs.

The authors of the paper underlined that the simplest explanation for HL Tau's X-ray variability is stochastic, which means that there is no pattern, but the variability manifests as periodicity in the collected data.

Furthermore, Chandra X-ray spectrum of HL Tau suggests the presence of both cool and hot plasma, with the pseudo-continuum of cool plasma hidden behind the high absorption. Analysis of this spectrum also allowed the team to detect faint emission from the cold iron fluorescence line at 6.4 keV—indicative of hot emission from the star stimulating fluorescence in cold neutral iron in the disk.

Summing up the results, the researchers note that additional observations are necessary to comprehensively characterize the X-ray variability of HL Tau and, in particular, to further explore the potential for long-term recurrent variability.

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