A luminous red nova (abbr. ''LRN'', pl. ''luminous red novae'', pl.abbr. ''LRNe'') is a stellar explosion thought to be caused by the merging of two stars. They are characterised by a distinct red colour, and a

light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

that fades slowly with resurgent brightness in the

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

. Luminous red novae are not related to standard novae, which are explosions that occur on the surface of

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

stars.

Discovery

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 Sagittarii, a star in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy, flared similarly, and in 2002, V838 Monocerotis followed suit and was studied quite closely. The first confirmed luminous red nova was the object M85 OT2006-1, in the galaxy

Messier 85 Messier 85 (also known as M85 or NGC 4382 or PGC 40515 or ISD 0135852) is a lenticular galaxy, or elliptical galaxy for other authors, in the Coma Berenices constellation. It is 60 million light-years away, and has a diameter of about across. ...

. It was first observed during the

Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton (California), Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The ...

Supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

Search, and subsequently investigated by a team of

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

s from both U.C. Berkeley and

Caltech The California Institute of Technology (branded as Caltech) is a private university, private research university in Pasadena, California, United States. The university is responsible for many modern scientific advancements and is among a small g ...

. They confirmed its difference from known explosions such as novae and thermal pulses, and announced luminous red novae as a new class of stellar explosion. V1309 Scorpii is a luminous red nova that followed the merger of a contact binary in 2008. In January 2015, a luminous red nova was observed in the Andromeda Galaxy. On February 10, 2015, a luminous red nova, known as M101 OT2015-1 was discovered in the Pinwheel Galaxy (M101). ZTF SLRN-2020 is a subluminous red nova that involved the engulfment of a planet. These events are rare and between 0.1 and several should occur in the

per year. In addition to these novae, 2020nqq (type ILRT, mag. 17.8), was discovered on 27 June 2020 in the Centaurus A galaxy.

Characteristics

The

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

of the explosion occurring in luminous red novae is between that of a supernova (which is brighter) and a nova (dimmer). The visible light lasts for weeks or months, and is distinctively red in colour, becoming dimmer and redder over time. As the visible light dims, the infrared light grows and also lasts for an extended period of time, usually dimming and brightening a number of times. Infrared observations of M85 OT2006-1 have shown that temperature of this star is slightly less than 1000 K, a very low temperature (corresponding to the spectral type T, usually only seen in

Brown Dwarfs Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars. Their mass is approximately 13 to 80 times that of Jupiter ()not big enough to sustain nuclear fu ...

). It is not clear if this characteristic is shared by other luminous red novae.

Evolution

The team investigating M85 OT2006-1 believe it to have formed when two

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

stars merged. (See the article on V838 Mon for further information on ''mergebursts'' and alternative possibilities.) At the time the mergeburst occurs, the LRN appears to expand extremely rapidly, reaching thousands to tens of thousands of

solar radii Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicat ...

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.

Other viewpoints

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, they may be due to a type II-p supernova; alternatively, supernovae undergoing a high level of extinction will naturally be both red and of low luminosity.

Prediction

In 2017 KIC 9832227, a binary star system, was predicted to merge and produce a red nova by early 2022. In September 2018, a typo was discovered in data used for the initial prediction, and it was determined that the merger would likely not take place at the predicted time.

References

External links

Caltech Press Release "Caltech and Berkeley Astronomers Identify a New Class of Cosmic Explosions"Smithsonian/NASA ADS Astronomy Abstract Service "Spitzer Observations of the New Luminous Red Nova M85 OT2006-1"

Cosmos Online "Stars merge in new cosmic explosion"
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