A luminous red nova (abbr. LRN, pl. luminous red novae, pl.abbr. LRNe) is a stellar explosion thought to be caused by the merger of two stars. They are characterised by a distinct red colour, and a light curve that lingers with resurgent brightness in the infrared. Luminous red novae are not to be confused with standard novae, explosions that occur on the surface of white dwarf stars.
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A small number of objects exhibiting the characteristics of luminous red novae have been observed over the last 30 years or so. The red star M31 RV in the Andromeda Galaxy, flared brightly during 1988 and may have been a luminous red nova. In 1994, V4332 Sgr, a star in the Milky Way galaxy, flared similarly, and in 2002, V838 Mon followed suit and was studied quite closely.
The first confirmed Luminous Red Nova was the object M85 OT2006-1, in the Messier 85 galaxy. It was first observed during the Lick Observatory Supernova Search, and subsequently investigated by a team of astronomers from both U.C. Berkeley and Caltech. The team, led by Shrinivas Kulkarni confirmed its difference from known explosions such as novae and thermal pulses, and announced Luminous Red Novae as a new class of stellar explosion in a press release on May 23 2007 and by publication in the journal Nature.
A luminous red nova has the following observed characteristics:
The team investigating M85 OT2006-1 believe it to have formed when two main sequence stars merged. (See the article on V838 Mon for further information on mergebursts and alternative possibilities.)
At the time the mergeburst occurs, the LRNe appears to expand extremely rapidly, reaching thousands to tens of thousands of solar radii in only a few months. This would cause the object to cool, explaining the intriguing co-existence of a bright flash with a cool post-flash object.
Some astronomers believe it to be premature to declare a new class of stellar explosions based on such a limited number of observations. For instance, Pastorello et al. 2007[1] explained that the event may be due to a type II-p supernova and Todd et al. 2008[2] pointed out that supernovae undergoing a high level of extinction will naturally be both red and of low luminosity.