HEAPOW: The Next Red Nova? (2017 Jan 09)

[bystander]

HEAPOW: The Next Red Nova? (2017 Jan 09)

Binary stars, two stars bound by gravity which orbit around their center of mass, are quite common. As known since the time of Kepler, the orbital periods of binaries are dependent on the separation of the stars. Some binary stars, called contact binaries, are so close that (as the name implies) they touch each other (as shown in the illustration above). Contact binaries are peculiar and complex objects - both stars may be surrounded and submerged within a common envelope of gas, and the stars may exchange mass from one to the other through this envelope. Changes in the stellar masses of the component stars, and interactions between the stellar magnetic fields, can help drive the stars closer and closer, making them revolve faster and faster until they violently merge into a single object, producing an enormous outpouring of energy. Astronomers have seen a number of cosmic explosions which they have called red novae, which are believed to be produced by such stellar mergers. These events, as the name implies, are characterized by a reddish color, and their explosions are brighter than normal novae (where outbursts are driven by the accumulation of matter from a companion star onto the surface of a white dwarf) but fainter than supernovae (where the outburst is produced by the complete destruction of a star). Astronomers have recently identified an otherwise innocuous star called KIC 9832227 as a contact binary system whose orbital period is decreasing with time (shown in the inset above). The shrinking period indicates that the stars are getting closer to each other, and analysis of the orbital period change (shown by the solid line in the graph) indicates that the orbit will shrink so much that the two stars will merge in only about 5 years. Keep an eye on this binary to watch the fireworks.

Science: Colliding stars will light up the night sky in 2022
http://asterisk.apod.com/viewtopic.php?t=36727

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Author: Dr. Michael F. Corcoran

[Ann]

Me being me, I was of course absolutely fascinated when I saw the illustration of a contact binary consisting of two brilliantly blue stars. Are the stars of KIC 9832227 really hot and blue?

I tried to read the paper, and I think the answer is no. The mass of the primary star of this contact binary, compared with the mass of the Sun, is 1.395, if I understood things correctly. That's pretty similar to F5-type star Procyon, whose mass is 1.42 solar, according to Jim Kaler. On the other hand, Procyon is about 7 times as luminous as the Sun, and the primary of KIC 9832227 is only 2.609 times brighter than our own star. Interestingly, too, the primary is decidedly cooler than Procyon, 5800 Kelvin versus 6530 Kelvin for Procyon. Nevertheless, the primary is a little bit hotter than the Sun, whose temperature is 5 777 K, and I would think that the primary component of KIC 9832227 is a late F-type star.

The secondary star is even weirder. It is quite low in mass, 0.318 solar, but it is extraordinarily bright for its puny mass: 0.789 solar. And it is hot, too, 5920 Kelvin, which is more than 100 K hotter than the Sun. Presumably the primary component of KIC 9832227 has "stolen" a lot of mass from the secondary and exposed more and more of the secondary's hot interior.

Well, fascinating! It would of course be spectacular if these two stars really produced a red nova within the next few years. And forgive me if I'm wrong, but wouldn't the more long term outcome of such a merger be a blue straggler star?

Ann