V838 Monocerotis

V838 Monocerotis (Nova Monocerotis 2002) is a cataclysmic binary star in the constellation Monoceros about 19,000 light years (6 kpc) from the Sun. The previously unremarked star was observed in early 2002 experiencing a major outburst, and was one of the largest known stars for a short period following the outburst. Originally believed to be a typical nova eruption, it was then identified as the first of a new class of eruptive variables known as luminous red novae. The reason for the outburst is still uncertain, but is thought to have been a merger of two stars within a triple system. The eruption occurred on one of two B3 main sequence stars in a close binary orbit. The erupting star appeared as an unusually cool supergiant and for a while engulfed its companion. By 2009 the temperature of the supergiant had increased (since 2005) to 3,270 K and its luminosity was 15,000 times solar (L☉), but its radius had decreased to 380 times that of the Sun (R☉), although the ejecta continues to expand.

Outburst

On January 6, 2002, an unknown star was seen to brighten in the constellation Monoceros, the Unicorn. Being a new variable star, it was designated V838 Monocerotis, the 838th variable star of Monoceros. The initial light curve resembled that of a nova, an eruption that occurs when enough hydrogen gas accumulates on the surface of a white dwarf from its close binary companion. Therefore, the object was also designated Nova Monocerotis 2002. V838 Monocerotis reached a maximum visual magnitude of 6.75 on February 6, 2002, after which it started to dim rapidly, as expected. However, in early March, the star started to brighten again, particularly in infrared wavelengths. Yet another brightening in infrared occurred in early April. In 2003, the star returned to near its original brightness before the eruption (magnitude 15.6), but as a red supergiant rather than a blue main-sequence star. The light curve produced by the eruption was unlike anything previously seen. In 2009, the star was about 15,000 L☉, which, in the absence of extinction, would correspond to an apparent magnitude of 8.5.

The star brightened to about a million times solar luminosity and an absolute magnitude of −9.8, ensuring that at the time of maximum, it was one of the most luminous stars in the Milky Way galaxy. Its brightening was caused by a rapid expansion of its outer layers. V838 Monocerotis was observed by use of the Palomar Testbed Interferometer, which indicated a radius of 1,570±400 R☉ (comparable to Jupiter's orbital radius), confirming the earlier indirect calculations. At the currently accepted distance of 6,100 pc, the measured angular diameter in late 2004 (1.83 mas) corresponded to a radius of 1,200±150 R☉, but by 2014, it shrunk to 750±200 R☉, around 70 solar radii larger than Antares. The expansion took only a couple of months, meaning that its speed was abnormal. Because the laws of thermodynamics dictate that expanding gases cool, the star became extremely cool and deep red. In fact, some astronomers argue that the spectrum of the star initially resembled that of L-type brown dwarfs. If that is the case, V838 Monocerotis was the first known L-type supergiant. Since then, it has shrunk and heated up and is now considered an M-type supergiant. However, current estimates of the distance, and hence of the radius, are about 25% lower than assumed in those papers.

Other possibly similar events There are a handful of outbursts resembling that of V838 Monocerotis. In 1988, a red star was detected erupting in the Andromeda Galaxy. The star, designated M31-RV, reached the absolute bolometric magnitude of −9.95 at maximum (corresponding to a luminosity of 0.75 million L☉) before dimming beyond detectability. A similar eruption, V4332 Sagittarii, occurred in 1994 in the Milky Way.

Progenitor star

Based on an incorrect interpretation of the light echo the eruption generated, the distance of the star was first estimated to be 1,900 to 2,900 light years. Combined with the apparent magnitude measured from pre-eruption photographs, it was thought to be an underluminous F-type dwarf, which posed a considerable enigma. More accurate measurements gave a much larger distance, 20,000 light years (6 kpc). It appears that the star was considerably more massive and luminous than the Sun. The star probably has a mass of from 5 to 10 times the mass of the Sun (M☉). It was apparently either a B1.5V star with a B3V companion, or an A0.5V with a B4V companion. In the latter case it would have had a luminosity around 550 L☉ (being 0.43 times as luminous as its companion), and in the former case it would have been more luminous (about 1.9 times as luminous as its companion). The star may have originally had a radius roughly 5 R☉ and its temperature would have been that of a B-type star (more than 10,000K but less than 30,000K). Munari et al. (2005) suggested that the progenitor star was a very massive supergiant with an initial mass of about 65 M☉, but this has been contested. There seems to be agreement that the star system is relatively young. Munari et al. conclude that the system may be only about 4 million years old. The spectrum of V838 Monocerotis reveals a companion, a hot blue B-type main sequence star probably not very different from the progenitor star. It is also possible that the progenitor was slightly less massive than the companion and only just entering the main sequence. Based on the photometric parallax of the companion, Munari et al. calculated a greater distance, 36,000 light years (10 kpc).

Light echo

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