What causes a star to be over 100 times brighter? Astronomers have been pondering that question since 1936, when a star in the constellation Orion brightened from 16th to 8th magnitude in a single year.
The star is named FU Ori. Astronomers have come up with different explanations for the star, but they don't give a complete explanation.
We might have one.
A new study presents an explanation for FU Ori. The Monthly Notices of the Royal Astronomical Society will publish it. Elizabeth Borchert is a PhD candidate at the school.
Borchert said in a press release that there were no completely explainable reasons for the phenomenon.
Astronomers know that FU Ori is the only one of its kind. The stars are named after it. They haven't entered the main sequence yet, because they're T-Tauri stars. They formed out of gas and dust and will form planets. The fact helps explain the change.
One of the explanations for FU Ori involves two stars. The astronomer who discovered FU Ori S thinks it is a pre-Main Sequence star.
This is an ALMA image of two animals. Perez et al 2020.
The original star that brightened in 1936 is not the primary star in the pair. The primary star is FU Ori S. Fu Ori N is a small amount of solar mass.
The ALMA images show a complex environment. The stars are marked with crosses. The top is FU Ori N. Perez et al 2020.
Since 2004, follow-up studies have shown that the disk has experienced problems due to interactions with other objects. The team behind this work wanted to find out if a flyby of one star into the other's disc could account for the historical brightening of the other. They wrote in their paper that their main aim was to test whether a stellar flyby that penetrated the disc could produce a fast-rising but long- lasting outburst.
The team performed simulations.
The images are from simulations. The top and bottom rows have side views. During a flyby of the secondary star, the material from the primary is grabbed. The stars are marked by red dots. Borchert et al 2021.
The secondary star experiences a spike in accretion when it passes through the primary's disc. There is no disc surrounding the secondary until it passes through the primary. The authors write that they observe a rapid rise of the mass accretion rate onto the secondary in all simulations.
The primary star also experiences a weaker spike in accretion. Borchert et al 2021.
In 1936, it did so in a year. It is a very small amount of time. How can the rapid rise be explained?
They explain that the primary receives its accretion outburst from inward evolution of disc disturbances, while the secondary goes into outburst when penetrating the disc of the primary. The accretion burst can be sustained for more than 100 years.
A star crashing into a disc of gas and dust results in a 250 times change in brightness within a couple of years, according to the co-author.
The small star is the one that becomes bright, which is nice, as in FU Ori the low-mass star is the bright one of the pair, Borchert said.
We find chondrules in our own Solar System. They are round droplets of once-molten material. Chondrules are melted in space before they reach their parent asteroids. Is it possible that the disc interactions could account for chondrules?
The team saw the temperature rise. The existence of chondrules in our solar system could be explained by rapid heating of the disc with dust melting. Evidence for similar heating in the solar system doesn't necessarily mean that our Sun was the culprit.
The study shows the temperatures during flyby and accretion. The highest temperature is hot enough to cause chondrules to melt. The top and bottom rows are side views. Borchert et al 2021.
A lot of the dust found in meteorites seems to have been melted rapidly, which could explain the mystery of our solar system.
Does this study show that the stars are caused by the interactions of two discs? The team was able to reconstruct the 1936 brightening, but they weren't able to completely reconstruct it.
In our simulations, we have not tried a detailed reconstruction of the close encounter. They write that the reason we are not able to maintain the outburst is likely due to this.
Disc interactions could be the cause. The simulations showed the fast rise in luminosity, the higher accretion rate experienced by the secondary star, and the lower mass star having an outburst.
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