V838 Monocerotis

Coordinates: Sky map 07h 04m 04.85s, −03° 50′ 50.1″
Source: Wikipedia, the free encyclopedia.
V838 Monocerotis
ESA
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation
Monoceros
Right ascension 07h 04m 04.822s[1]
Declination −03° 50′ 50.53″[1]
Apparent magnitude (V) 6.75 (2002), 15.6[2]
Characteristics
Evolutionary stage
M-type supergiant[3]
Spectral type M7.5I -> M5.5I + B3V[3]
Variable type
LRN[3]
Distance
19,200 ly
(5,900±400[6] pc)
Details
Myr
V838
 Mon, Nova Monocerotis 2002, GSC 04822-00039
Database references
SIMBADdata

V838 Monocerotis (Nova Monocerotis 2002) is a

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.[10] 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.[11]

Outburst

The visual (blue points) and K band infrared (red points) light curves of the 2002 eruption of V838 Monocerotis, adapted from Starrfield et al., 2004[12]
Light echo of V838 Mon as imaged April 30, 2002

On January 6, 2002, an unknown star was seen to brighten in the constellation

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.[2]
In 2009, the star was about 15,000 L☉,[11] which, in the absence of extinction, would correspond to an apparent magnitude of 8.5.[a]

Comparison between the size of V838 Monocerotis and the Inner Solar System.

The star brightened to about a million times solar luminosity[14] and an absolute magnitude of −9.8,[15] 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

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.[18] Since then, it has shrank 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.[15]

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.[19]

Progenitor star

Location of V838 Monocerotis within the Milky Way galaxy.

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.[10]

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☉).[7] 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).[7][b] 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[7]). Munari et al. (2005) suggested that the progenitor star was a very massive supergiant with an initial mass of about 65 M☉,[9] but this has been contested.[7] 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.[7] It is also possible that the progenitor was slightly less massive than the companion and only just entering the main sequence.[10]

Based on the photometric parallax of the companion, Munari et al. calculated a greater distance, 36,000 light years (10 kpc).[9]

Light echo

ESA
.
The evolution of the light echo around V838 Monocerotis.[20]
Animation of 11 images of light echo of V838 Mon

Rapidly brightening objects like novae and supernovae are known to produce a phenomenon known as light echo. The light that travels directly from the object arrives first. If there are clouds of interstellar matter around the star, some light is reflected from the clouds. Because of the longer path, the reflected light arrives later, producing a vision of expanding rings of light around the erupted object. The rings appear to travel faster than the speed of light, but in fact they do not.[2][21]

In the case of V838 Monocerotis, the light echo produced was unprecedented and is well documented in images taken by the Hubble Space Telescope. While the photos appear to depict an expanding spherical shell of debris, they are actually formed by the illumination of an ever-expanding ellipsoid with the progenitor star at one focus and the observer at the other. Hence, despite appearances, the structures in these photos are actually concave toward the viewer.

By March 2003 the size of the light echo in the sky was twice the

arcseconds
.

It is not yet clear if the surrounding nebulosity is associated with the star itself. If that is the case, they may have been produced by the star in earlier eruptions which would rule out several models that are based on single catastrophic events.[2] However, there is strong evidence that the V838 Monocerotis system is very young and still embedded in the nebula from which it formed.[14]

The eruption initially emitted at shorter wavelengths (i.e. was bluer), which can be seen in the light echo: the outer border is bluish in the Hubble images.[2]

Hypotheses

Two pictures taken in November 2005 and September 2006 showing the changes that occurred to the bright echo of V838 Mon.

This and other luminous red nova events are thought to be caused by the merger of two stars. In the case of V838 Monocerotis, it was the merger of two main sequence stars, or an 8 M☉ main sequence star and a 0.3 M☉ pre-main sequence star. The merger model explains the multiple peaks in the light curve observed during the outburst.[14] Based on further observations of stars similar to V838 Monocerotis, such as V1309 Scorpii, astronomers have reached the conclusion that this is the most likely scenario.[1][23]

Other explanations for the eruption of V838 Monocerotis have also been published.[24]

Atypical nova outburst

The outburst of V838 Monocerotis may be a nova eruption after all, albeit a very unusual one. However, this is very unlikely considering that the system includes a B-type star, and stars of this type are young and massive. There has not been enough time for a possible white dwarf to cool and accrete enough material to cause the eruption.[19]

Thermal pulse of a dying star

V838 Monocerotis may be a post-asymptotic giant branch star, on the verge of going supernova. The nebulosity illuminated by the light echo may actually be shells of dust surrounding the star, created by the star during previous similar outbursts. The brightening may have been a so-called helium flash, where the core of a dying low-mass star suddenly ignites helium fusion disrupting, but not destroying, the star. Such an event is known to have occurred in Sakurai's Object. However, several pieces of evidence support the argument that the dust is interstellar rather than centered on V838 Monocerotis. A dying star that has lost its outer envelopes would be appropriately hot, but the evidence points to a young star instead.[7]

Thermonuclear event within a massive supergiant

According to some evidence, V838 Monocerotis may be a very massive

kiloparsecs, respectively.[9]

Planetary capture event

Another possibility is that V838 Monocerotis may have swallowed its giant planets. If one of the planets entered into the atmosphere of the star, the stellar atmosphere would have begun slowing down the planet. As the planet penetrated deeper into the atmosphere, friction would become stronger and kinetic energy would be released into the star more rapidly. The star's envelope would then warm up enough to trigger deuterium fusion, which would lead to rapid expansion. The later peaks may then have occurred when two other planets entered into the expanded envelope. The authors of this model calculate that every year about 0.4 planetary capture events occur in Sun-like stars in the Milky Way galaxy, whereas for massive stars like V838 Monocerotis the rate is approximately 0.5–2.5 events per year.[25]

Common envelope event

See Common envelope

See also

Notes

  1. ^ The sun's absolute magnitude is 4.83, meaning that its apparent magnitude would be 4.83 at 10 parsecs, and V838 Mon was 15,000 times more luminous than the sun, assuming that it is 6,500 parsecs away, so the apparent magnitude of V838 Mon comes to 4.83 − 2.5×log(15000) + 5×log(6500/10) ≈ 8.5.
  2. ^ The reference does not explicitly give the luminosity in the first case.

References

  1. ^
    S2CID 239768384
    .
  2. ^
    S2CID 90973
    .
  3. ^
    S2CID 221839336
    .
  4. ^
    doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR
    .
  5. S2CID 203904869
    .
  6. S2CID 214794885
    .
  7. ^
    S2CID 119363054
    . Retrieved 10 August 2006.
  8. ^
    S2CID 235422695
    .
  9. ^
    S2CID 119330637
    .
  10. ^
    S2CID 3566688
    .
  11. ^
    S2CID 118649108
    .
  12. ^ Starrfield, S.; Wagner, R. M.; Hauschildt, P. H.; Bond, H. E.; Evans, A.; Rushton, M. T.; Rushton, M. T.; Munari, U.; Henden, A.; Levay, Z. G.; Panagia, N.; Sparks, W. B.; Corradi, R. L. M. (July 2004). The 2002 Outburst of V838 Mon: As Cool As It Gets. Retrieved 29 September 2021.
  13. Bibcode:2002IAUC.7785....1B
    .
  14. ^
    Bibcode:2007ASPC..363..280S
    .
  15. ^
    S2CID 13459320
    .
  16. S2CID 119473906
    .
  17. S2CID 11558941
    .
  18. doi:10.1046/j.1365-8711.2003.06755.x
    .
  19. ^
    S2CID 18582562
    . M31-RV - 0402313
  20. ^ The evolution of the light echo around V838 Monocerotis, retrieved 2015-08-27
  21. ^ "Many Epochs of V838 Mon". The Hubble Heritage Project. Retrieved 3 October 2015.
  22. ^ "Hubble Watches Light from Mysterious Erupting Star Reverberate Through Space". Hubblesite. Mar 26, 2003. Archived from the original on December 22, 2017.
  23. S2CID 237194996
    . 183.
  24. ISSN 0362-4331
    . Retrieved 2015-08-30.
  25. doi:10.1111/j.1365-2966.2006.10579.x
    .

External links

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