V838 Monocerotis

V838 Monocerotis

V838 Monocerotis and its light echo as imaged by the Hubble Space Telescope on December 17, 2002. Credit: NASA/ESA
Observation data Epoch 2000.0      Equinox 2000.0
Constellation	Monoceros
Right ascension	07h 04m 04.85s
Declination	−03° 50′ 50.1″
Apparent magnitude (V)	15.74
Details
Radius	1,570 ± 400 R☉
Other designations
Nova Monocerotis 2002, GSC 04822-00039
Database references
SIMBAD	data

V838 Monocerotis (V838 Mon) is a red variable star in the constellation Monoceros about 20,000 light years (6 kpc)^[1] from the Sun, and possibly one of the largest known stars. The previously unknown star was observed in early 2002 experiencing a major outburst. Originally believed to be a typical nova eruption, it was then realized to be something completely different. The reason for the outburst is still uncertain, but several conjectures have been put forward, including an eruption related to stellar death processes and a merger of a binary star or planets.

Outburst

On January 6, 2002, an unknown star was seen to brighten up in Monoceros, the Unicorn.^[2] 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 has accumulated on the surface of a white dwarf from its close binary companion. Therefore it was also designated Nova Monocerotis 2002. V838 Monocerotis reached 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, this time mostly in infrared wavelengths. Yet another brightening in infrared occurred in early April, after which the star returned to near its original brightness before the eruption, magnitude 15.6. The lightcurve produced by the eruption is unlike anything previously seen.^[3]

The star brightened to about a million times solar luminosity^[4] ensuring that at the time of maximum V838 Monocerotis was one of the most luminous stars in the Milky Way galaxy. The brightening was caused by a rapid expansion of the outer layers of the star. The star was observed using the Palomar Testbed Interferometer which provided a radius of 1,570 ± 400 solar radii (comparable to Jupiter's orbital radius), confirming the earlier indirect calculations.^[5] The expansion took only a couple of months, meaning that its speed was abnormal. The laws of thermodynamics dictate that expanding gas cools. Therefore the star became extremely cool and deep red. In fact, some astronomers argue that the spectra of the star resembled that of L-type brown dwarfs. If that is the case, V838 Monocerotis would be the first known L-type supergiant.^[6]

Other possibly similar events

There are a handful of outbursts that resemble the one which occurred on 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 a luminosity of 0.75 million times solar) before dimming beyond detectability. A similar eruption occurred in 1994 in the Milky Way (V4332 Sagittarii).^[7]

Progenitor star

Some details are emerging on the nature of the star that experienced the outburst. 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 not much unlike our Sun which posed a considerable enigma.^[8]

More accurate measurements gave a much larger distance, 20,000 light years (6 kpc). It appears that the star is considerably more massive and luminous than the Sun. The mass of the star is probably from 5 to 10 times solar,^[9] and luminosity from 550 to 5,000 times solar. The original radius may have been about 5 times solar and temperature 4,700–30,000 K.^[1] Needless to say, these values are very approximate. Munari et al. (2005) suggest that the progenitor star is in fact a very massive supergiant with a mass of about 65 times solar. They also conclude that the system may be only about 4 million years old.^[10]

The spectrum of V838 Monocerotis reveals a companion, a hot blue B-type main sequence star probably not much different from the erupted star.^[9] It is also possible that the erupted star was slightly less massive than the companion and only just entering the main sequence.^[8]

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

Light echo

Images showing the expansion of the light echo. Credit: NASA/ESA.

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. In addition, the rings appear to travel faster than the speed of light.^[3]

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.

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.^[3] However, there is strong evidence that the V838 Monocerotis system is very young and still embedded in the nebula from which it formed.^[4]

Interestingly, the first eruption occurred at shorter wavelengths (i.e. was bluer) and can be seen in the light echo: the outer border of the echo is bluish in the Hubble images.^[3]

Models

So far several rather different explanations for the eruption of V838 Monocerotis have been published.

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.^[7]

Thermal pulse of a dying star

V838 Monocerotis may be a post-asymptotic giant branch star, on the verge of its death. 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 carbon fusion disrupting, but not destroying, the star. Such an event is known to have occurred in Sakurai's Object. However, several pieces of evidence supports the argument that the dust is interstellar rather than centered around V838 Monoceros. A dying star that has lost its outer envelopes would be appropriately hot, but the evidence points to a young star instead.^[9]

Thermonuclear event within a massive supergiant

According to some evidence, V838 Monocerotis may be a very massive supergiant. If that is the case, the outburst may have been a so-called helium flash, a thermonuclear event where a shell in the star containing helium suddenly ignites and starts to fuse carbon. Very massive stars survive multiple such events, however they experience heavy mass loss (about half of the original mass is lost while in the main sequence) before settling as extremely hot Wolf-Rayet stars. This theory may also explain the apparent dust shells around the star. V838 Monoceros is located in the approximate direction of the Galactic anticenter and off from the disk of the Milky Way. Stellar birth is less active in outer galactic regions, and it is not clear how such a massive star can form there. However, there are very young clusters like Ruprecht 44 and the 4 million years old NGC 1893 at a distance of ca. 7 kpc and 6 kpc, respectively.^[10]

Mergeburst

The outburst may have been the result of a so-called mergeburst, the merger of two main sequence stars (or an 8 M_☉ main sequence star and a 0.3 M_☉ pre-main sequence star). This model is strengthened by the apparent youth of the system and the fact that multiple stellar systems may be unstable. The less massive component may have been in a very eccentric orbit or deflected towards the massive one. Computer simulations have shown the merger model to be plausible. The simulations also show that the inflated envelope would have come almost entirely from the smaller component. In addition, the merger model explains the multiple peaks in the light curve observed during the outburst.^[4]

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.^[1]

See also

• Stellar evolution

References

1. ^ ^{a b c} Retter, A.; Zhang, B.; Siess, L.; Levinson, A. (May 22, 2006). The planets capture model of V838 Monocerotis: conclusions for the penetration depth of the planet/s. 1. arXiv:astro-ph/0605552. Bibcode 2006MNRAS.370.1565C. doi:10.1111/j.1365-2966.2006.10579.x. 
2. ^ Brown, N. J. (January 10, 2002). "IAU Circular No. 7785". http://cfa-www.harvard.edu/iauc/07700/07785.html#Item1. Retrieved 10 August 2006. 
3. ^ ^{a b c d} Bond, Howard E.; Henden, Arne; Levay, Zoltan G.; Panagia, Nino; Sparks, William B.; Starrfield, Sumner; Wagner, R. Mark; Corradi, R. L. M.; Munari, U. (March 27, 2003). "An energetic stellar outburst accompanied by circumstellar light echoes". Nature 422 (6930): 405–408. arXiv:astro-ph/0303513. Bibcode 2003Natur.422..405B. doi:10.1038/nature01508. PMID 12660776. 
4. ^ ^{a b c} Soker, N.; Tylenda, R. (June 15, 2006). "Modelling V838 Monocerotis as a Mergeburst Object". arXiv:astro-ph/0606371 [astro-ph]. 
5. ^ Lane, B. F.; Retter, A.; Thompson, R. R.; Eisner, J. A. (April, 2005). "Interferometric Observations of V838 Monocerotis". The Astrophysical Journal (The American Astronomical Society) 622 (2): L137–L140. arXiv:astro-ph/0502293. Bibcode 2005ApJ...622L.137L. doi:10.1086/429619. 
6. ^ Evans, A.; Geballe, T. R.; Rushton, M. T.; Smalley, B.; van Loon, J. Th.; Eyres, S. P. S.; Tyne, V. H. (August, 2003). "V838 Mon: an L supergiant?". Monthly Notice of the Royal Astronomical Society (Royal Astronomical Society) 343 (3): 1054. Bibcode 2003MNRAS.343.1054E. doi:10.1046/j.1365-8711.2003.06755.x. 
7. ^ ^{a b} Boschi, F.; Munari, U. (May, 2004). "M 31-RV evolution and its alleged multi-outburst pattern". Astronomy & Astrophysics 418 (3): 869–875. arXiv:astro-ph/0402313. Bibcode 2004A&A...418..869B. doi:10.1051/0004-6361:20035716.  M31-RV - 0402313
8. ^ ^{a b} Tylenda, R. (June 4, 2005). "Evolution of V838 Monocerotis during and after the 2002 eruption". Astronomy and Astrophysics 436 (3): 1009–1020. arXiv:astro-ph/0502060. Bibcode 2005A&A...436.1009T. doi:10.1051/0004-6361:20052800. 
9. ^ ^{a b c} Tylenda, R.; Soker, N.; Szczerba, R. (October 2005). "On the progenitor of V838 Monocerotis". Astronomy and Astrophysics 441 (3): 1099–1109. arXiv:astro-ph/0412183. Bibcode 2005A&A...441.1099T. doi:10.1051/0004-6361:20042485. http://adsabs.harvard.edu/cgi-bin/bib_query?astro-ph/0412183. Retrieved 10 August 2006. 
10. ^ ^{a b c} Munari, U.; Munari, U.; Henden, A.; Vallenari, A.; Bond, H. E.; Corradi, R. L. M.; Crause, L.; Desidera, S.; Giro, E.; Marrese, P. M.; Ragaini, S.; Siviero, A.; Sordo, R.; Starrfield, S.; Tomov, T.; Villanova, S.; Zwitter, T.; Wagner, R. M. (May 2, 2005). "On the distance, reddening and progenitor of V838 Mon". Astronomy and Astrophysics 434 (3): 1107–1116. arXiv:astro-ph/0501604. Bibcode 2005A&A...434.1107M. doi:10.1051/0004-6361:20041751. 

External links

• AAVSO Variable Star of the Month. December, 2002: V838 Mon
• Show 12: A Flash of Brilliance Dr. Frank Summers tells about the Mysterious Erupting Star
• Hubble Watches Light from Mysterious Erupting Star Reverberate Through Space — STScI press release STScI-2003-10 (March 26, 2003; received on 10 August 2006)
• Light Continues to Echo Three Years After Stellar Outburst — STScI press release STScI-2005-02 (February 3, 2005; received on 10 August 2006)
• The Hubble Heritage Project — more information and images (received on 10 August 2006)
• Hubble's Latest Views of Light Echo from Star V838 Monocerotis — STScI press release STScI-2006-50 (Exposure dates: November 17, 2005 and September 9, 2006. Released October 26, 2006.)
• V838 at ESA/Hubble
• Slacker Astronomy video podcast and animation of V838 Mon
• NASA Astronomy Picture of the Day for 2002-10-03. Photos taken by Lisa Crause using the 1 meter telescope at the South African Astronomical Observatory

Coordinates: 07^h 04^m 04.85^s, +03° 50′ 50.1″