Young star caught devouring a planet


Recent fluctuations in the luminosity of a very young star would be caused by the matter released by repeated collisions between rocky planetary bodies. Fragments of these bodies would fall on this star, a phenomenon predicted but never observed for a young sun a few million years old.

Since Descartes, Kant and Laplace, we have made gigantic progress in the theory of the formation of the Solar System. The first great leap forward came in the second half of the twentieth century with analytical and numerical models of planet formation derived from the theory of accretion initially developed by researchers like Viktor Safronov and George Wetherill. The second leap forward, which is also accompanied by the progress of numerical simulations, comes mainly from the progress of observational astronomy for a quarter of a century and which shows us the accretion discs around young stars in stellar nurseries, as well as exoplanets.

We still do not understand everything in the cosmogony of planetary systems and there are still areas of shadows regarding the birth of giant planets in the Solar System. This is why astronomers and astrophysicists who deal with these issues are tirelessly scanning certain cosmic nurseries for additional information to test. One of the most interesting is the one located on the borders of the Taurus and Bouvier constellations (a region called Taurus-Auriga), which includes the Taurus Molecular Cloud (TMC-1) 1), a molecular cloud located approximately 450 light-years from the Sun and which would be the most important star formation region closest to the Solar System.

Also in this region of the celestial vault, scientists have observed since 1937 the intriguing (not as much as those of the star of Tabby) variations of luminosity of the star of solar type RW Aur A, which constitutes a binary system with another star Similar RW Aur B. These stars are a few million years old and RW Aur A is still surrounded by his protoplanetary disk rich in gas and dust in which the formation of gas giants is probably in progress. We know that this type of disc dissipates after about 5 to 10 million years, which means that in the case of the Solar System, Saturn and Jupiter have necessarily formed before the dissipation of an equivalent disk having him. – even existed for a similar duration.

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Significant drops in the brightness of RW Aur A usually occur after a few decades and last about a month. However, in 2011, the star’s light flux dropped again but for about half a year. The decrease in luminosity was then repeated in mid-2014 and it was not until November 2016 that the star became “normal” again. What could be due to this phenomenon?

The astrophysicists are on a track thanks to observations in the X-ray field of the Chandra satellite and they have advanced a hypothesis that they explained in an available publication on arXiv.

In fact RW Aur A saw its brightness lowered again in January 2017 and this time Chandra made it possible to observe it during a total duration of 14 hours. The X emissions came from both the star itself and the disc as the RW Aur A radiation in that same band of the electromagnetic spectrum spread. Two results have been deduced, including a particularly surprising one. The first is that the star is warmer than we thought and the second is that some parts of the disc and especially the atmosphere of the star are much richer in iron than expected because of the dependence between the temperature of the star and the composition of its protoplanetary disk by a factor of about 10. Typically hot and active stars have less iron than others, while RW Aur A seems to have more in his crown for some time compared to previous observations.

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The most appealing idea for researchers is that this would be the result of recent collisions between two planetary celestial bodies of sufficiently large size that they differentiated and formed the equivalent of the iron and nickel-rich nucleus in the center. of the earth. We know that this type of phenomenon can occur because in the Solar System itself, siderite-type meteorites made up in the great majority of an alloy of iron and nickel are supposed to be the remains of similar collisions.

Around RW Aur A, such collisions would then again release the iron and nickel trapped in large planetesimals, probably several hundred kilometers in radius, or even a terrestrial type planet embryo. This would explain the sudden iron enrichment of the protoplanetary disk and the crown of the star. The gases and the released dust blocking also the light of the star would be due to the recent decreases of luminosity with repetition in the visible spectrum.

Several such collisions have occurred in recent years, and it is likely that these are in fact repeated collisions between debris left behind by larger collisions. The iron enrichment of the RW Aur A crown would be due to the arrival on the star of some of these debris.

If this is the case, then it is a great first, the observation of the remains of planets falling on a young star. This type of scenario was previously confined to numerical computer simulations of planetary system formation.

Carl Frantz

Polyglot, humanitarian, Carl was born in Germany but raised in the USA. He writes mostly on tech, science and culture.