A tiny star and its companion, a 鈥渇ailed star鈥� known as a brown dwarf, are locked in an incredibly tight orbit, rotating in a volume smaller than the sun.
Unless a star has collapsed into a white dwarf or a neutron star, it can only shrink its orbit around another object through a process called magnetic braking, where stellar winds strip away material and carry away momentum. However, it was unclear whether this worked for brown dwarfs or low mass stars or just how close it could pull two orbiting objects together.
Now, at the California Institute of Technology and his colleagues have found a brown dwarf, called ZTF J2020+5033, that orbits a star roughly every 2 hours, the tightest known orbit for two uncollapsed stars.
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鈥淚f you imagine looking at them billions of years ago, when they formed, both of the stars would have been like 5 to 10 times larger than they currently are,鈥� says El-Badry. 鈥淭hey wouldn鈥檛 fit inside their current orbit, so that means they must have started out further apart.鈥�
The only way they could have developed such a tight orbit as they shrank is if magnetic braking pulled them together, says El-Badry, which also suggests that this phenomenon can occur in other tightly orbiting systems.
Eventually, in a few tens of millions of years, ZTF J2020+5033 will become close enough to its partner star that it will begin to lose material to it, due to its low density. This could lead to a massive explosion, or the two could merge to form a new star. Either way, there would no longer be a brown dwarf. Detailed simulations will have to be run to see exactly which scenario might happen, says El-Badry.
鈥淚t鈥檚 the shortest period by a factor of seven,鈥� says at the University of Warwick, UK. 鈥淓very time I think we鈥檝e reached the short period limit, we鈥檝e managed to find something that proves me wrong, which is fascinating.鈥�
arXiv
