V 838 Monocerotis: the prototypical “red nova”?

V 838 Mon is a unique object whose precise nature is still uncertain. Once thought to be a normal nova – originating in a close binary, with a white dwarf companion – it is known to be a different kind of binary which may have merged:

For the first time, astronomers have watched the spiralling dance performed by two stars merging into a single star. The observations, taken between 2001 and 2008, suggest a solution to the vexed problem of how rare ‘red novae’ form. Most novae are blue and occur when material on a white dwarf star explodes. But what causes red novae has been a mystery. The best-known red nova was spotted in January 2002 toward the edge of our Galaxy’s disk. Named V838 Monocerotis, it was more luminous than normal novae — at peak brightness, it briefly rivalled the most powerful stars in the Galaxy.

The so-called red novae originate in contact binary stars. An example is the case of V1309 Scorpii :

A contact binary consists of two stars that circle each other so closely that they touch. If viewed from an orbiting planet, the stuck-together suns would resemble a glowing peanut-shaped object. Exotic though they seem, contact binaries are common: the nearest, named 44 Boötis B, is just 41 light years (13 parsecs) from Earth.

Because they are so close together, the two stars continually eclipse each other, causing the brightness we see to vary. This allowed Tylenda and his team to deduce the nature of V1309 Scorpii, which is roughly 10,000 light years (3,000 parsecs) from Earth.

Before the explosion, the two stars danced around each other every 1.4 days. As they spiralled together, this period shortened until the stars merged and exploded, upping their brightness by 10,000 times. Tylenda and his colleagues estimate that the larger star had about as much mass as the Sun. Current observations indicate that the system is now single.

Source: “Dancing stars turn on the red light”

Update: Ivanova et al. (2013) new paper in Science with theoretical treatment of common-envelope events.

“The natural range of time scales and energies from this model, as well as the expected colors, light-curve shapes, ejection velocities, and event rate, match those of a recently recognized class of red transient outbursts…”


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