Luminous blue variable

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Luminous blue variable AG Carinae as seen by the Hubble Space Telescope

Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Large Magellanic Cloud. They are considered to be rare.

Discovery and history

P Cygni profile
of a spectral line

The LBV stars P Cygni and η Carinae have been known as unusual variables since the 17th century, but their true nature was not fully understood until late in the 20th century.

In 1922

M31. Of these, Var A, Var B, Var C, and Var 2 in M33 and Var 19 in M31 were followed up with a detailed study by Hubble and Allan Sandage in 1953. Var 1 in M33 was excluded as being too faint and Var 3 had already been classified as a Cepheid variable. At the time they were simply described as irregular variables, although remarkable for being the brightest stars in those galaxies.[1]
The original Hubble Sandage paper contains a footnote that S Doradus might be the same type of star, but expressed strong reservations, so the link would have to wait several decades to be confirmed.

Later papers referred to these five stars as Hubble–Sandage variables. In the 1970s, Var 83 in M33 and

P Cygni profiles and were compared to η Carinae.[2] In 1978, Roberta M. Humphreys published a study of eight variables in M31 and M33 (excluding Var A) and referred to them as luminous blue variables, as well as making the link to the S Doradus class of variable stars.[3] In 1984 in a presentation at the IAU symposium, Peter Conti formally grouped the S Doradus variables, Hubble–Sandage variables, η Carinae, P Cygni, and other similar stars together under the term "luminous blue variables" and shortened it to LBV. He also clearly separated them from those other luminous blue stars, the Wolf–Rayet stars.[4]

Variable star types are usually named after the first member discovered to be variable, for example

P Cygni profiles, which also occur in other types of stars, and chose the acronym SDOR for "variables of the S Doradus type".[5] The term "S Doradus variable" was used to describe P Cygni, S Doradus, η Carinae, and the Hubble-Sandage variables as a group in 1974.[6]

Physical properties

Upper portion of H-R Diagram showing the location of the S Doradus instability strip and the location of LBV outbursts. Main sequence is the thin sloping line on the lower left.

LBVs are massive unstable supergiant (or hypergiant) stars that show a variety of spectroscopic and photometric variation, most obviously periodic outbursts and occasional much larger eruptions.

In their "quiescent" state they are typically B-type stars, occasionally slightly hotter, with unusual emission lines. They are found in a region of the Hertzsprung–Russell diagram known as the S Doradus instability strip, where the least luminous have a temperature around 10,000 K and a luminosity about 250,000 times that of the Sun, whereas the most luminous have a temperature around 25,000 K and a luminosity over a million times that of the Sun, making them some of the most luminous of all stars.

During a normal outburst the temperature decreases to around 8,500 K for all stars, slightly hotter than the yellow hypergiants. The bolometric luminosity usually remains constant, which means that visual brightness increases somewhat by a magnitude or two. A few examples have been found where luminosity appears to change during an outburst, but the properties of these unusual stars are difficult to determine accurately. For example, AG Carinae may decrease in luminosity by around 30% during outbursts; and AFGL 2298 has been observed to dramatically increase its luminosity during an outburst although it is not clear if that should be classified as a modest giant eruption.[7] S Doradus typifies this behaviour, which has been referred to as strong-active cycle, and it is regarded as a key criterion for identifying luminous blue variables. Two distinct periodicities are seen, either variations taking longer than 20 years, or less than 10 years. In some cases, the variations are much smaller, less than half a magnitude, with only small temperature reductions. These are referred to as weak-active cycles and always occur on timescales of less than 10 years.[8]

Some LBVs have been observed to undergo giant eruptions with dramatically increased mass loss and luminosity, so violent that several were initially catalogued as supernovae. The outbursts mean there are usually nebulae around such stars; η Carinae is the best-studied and most luminous known example, but may not be typical.[9] It is generally assumed that all luminous blue variables undergo one or more of these large eruptions, but they have only been observed in two or three well-studied stars and possibly a handful of supernova imposters. The two clear examples in the Milky Way galaxy, P Cygni and η Carinae, and the possible example in the Small Magellanic Cloud, HD 5980A, have not shown strong-cycle variations. It is still possible that the two types of variability occur in different groups of stars.[10] 3-D simulations have shown that these outbursts may be caused by variations in helium opacity.[11]

Many luminous blue variables also show small amplitude variability with periods less than a year, which appears typical of

Alpha Cygni variables,[7] and stochastic (i.e. totally random) variations.[8]

Luminous blue variables are by definition more luminous than most stars and also more massive, but within a very wide range. The most luminous are more than a million L☉ (Eta Carinae reaches 4.6 million) and have masses approaching, possibly exceeding, 100 M☉. The least luminous have luminosities around a quarter of a million L☉ and masses as low as 10 M☉, although they would have been considerably more massive as main-sequence stars, due to their rapid mass loss. Their high mass loss rates could be due to outbursts and very high luminosity and show some enhancement of helium and nitrogen.[7]

Evolution

The Homunculus Nebula, produced by the Great Outburst of η Carinae

Because of these stars' large mass and high luminosity, their

helium burning phase (LBV with high surface temperature again) before transitioning to the Wolf–Rayet phase,[14] thus being analogous to the red giant and red supergiant
phases of less massive stars.

There appear to be two groups of LBVs, one with luminosities above 630,000 times the Sun and the other with luminosities below 400,000 times the Sun, although this is disputed in more-recent research.

large outbursts that can be mistaken for a faint supernova and these may shed the necessary mass. Recent models all agree that the LBV stage occurs after the main-sequence stage and before the hydrogen-depleted Wolf–Rayet stage, and that essentially all LBV stars will eventually explode as supernovae. LBVs apparently can explode directly as a supernova, but probably only a small fraction do. If the star does not lose enough mass before the end of the LBV stage, it may undergo a particularly powerful supernova created by pair-instability. The latest models of stellar evolution suggest that some single stars with initial masses around 20 times that of the Sun will explode as LBVs as type II-P, type IIb, or type Ib supernovae,[13] whereas binary stars undergo much-more-complex evolution through envelope stripping leading to less predictable outcomes.[18]

Supernova-like outbursts

Stars similar to η Carinae in nearby galaxies

Luminous blue variable stars can undergo "giant outbursts" with dramatically increased mass loss and luminosity. η Carinae is the prototypical example,

supernova imposter
events have declined to normal brightness within months. Well-studied examples are:

Early models of stellar evolution had predicted that although the high-mass stars that produce LBVs would often or always end their lives as supernovae, the supernova explosion would not occur at the LBV stage. Prompted by the progenitor of SN 1987A being a blue supergiant, and most likely an LBV, several subsequent supernovae have been associated with LBV progenitors. The progenitor of SN 2005gl has been shown to be an LBV apparently in outburst only a few years earlier.[23] Progenitors of several other type IIn supernovae have been detected and were likely to have been LBVs:[24]

Modelling suggests that at near-solar metallicity, stars with an initial mass around 20–25 M☉ will explode as a supernova while in the LBV stage of their lives. They will be post-red-supergiants with luminosities a few hundred thousand times that of the Sun. The supernova is expected to be of type II, most likely type IIb, although possibly type IIn due to episodes of enhanced mass loss that occur as an LBV and in the yellow-hypergiant stage.[25]

List of LBVs

See also: List of most luminous stars

The identification of LBVs requires confirmation of the characteristic spectral and photometric variations, but these stars can be "quiescent" for decades or centuries at which time they are indistinguishable from many other hot luminous stars. A candidate luminous blue variable (cLBV) can be identified relatively quickly on the basis of its spectrum or luminosity, and dozens have been catalogued in the Milky Way during recent surveys.[26]

Recent studies of dense clusters and mass spectrographic analysis of luminous stars have identified dozens of probable LBVs in the Milky Way out of a likely total population of just a few hundred, although few have been observed in enough detail to confirm the characteristic types of variability. In addition the majority of the LBVs in the Magellanic Clouds have been identified, several dozen in M31 and M33, plus a handful in other local group galaxies.[27]

η Carinae, a luminous blue variable as seen from the Chandra X-ray Observatory
HD 168607 is the right star of the pair below the Omega Nebula. The other is the hypergiant HD 168625.
A selection of LBVs and suspected LBVs with nebula, observed with the Spitzer Space Telescope.

Milky Way

Suspected:

Several more LBV's have been found near or in the Galactic Center:

Large Magellanic Cloud

Small Magellanic Cloud

Andromeda Galaxy

Triangulum Galaxy

NGC 2403:

NGC 1156

  • J025941.21+251412.2[42]
  • J025941.54+251421.8[42]

NGC 2366 (NGC 2363)

NGC 4449

  • J122809.72+440514.8[43]
  • J122817.83+440630.8[43]

NGC 4736 (Messier 94
)

PHL 293B

  • Unnamed star that underwent an outburst from 1998 to 2008 in an unusual supernova-like event, and has now disappeared[45]

Sunburst galaxy

Other

A number of cLBVs in the Milky Way (and in the case of

Sanduleak -69° 202
, in the LMC) are well known because of their extreme luminosity or unusual characteristics, including:

Further well-known stars have been LBVs relatively recently, are LBVs in a stable phase or are not currently classified as LBVs but may be transitioning into LBVs:[citation needed]

  • Zeta-1 Scorpii
    (naked-eye hypergiant)
  • IRC+10420
    (yellow hypergiant that has increased its temperature into the LBV range)
  • V509 Cassiopeiae (= HR 8752, an unusual yellow hypergiant evolving bluewards)
  • Rho Cassiopeiae (unstable yellow hypergiant suffering periodic outbursts)

See also

References

  1. doi:10.1086/145764
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  2. Bibcode:1975A&A....42..289B
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  3. doi:10.1086/155797
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  4. ISBN 978-90-277-1775-7
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  6. doi:10.1093/mnras/168.1.221
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  7. ^
    S2CID 17983157
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  8. ^
    Bibcode:2003ASPC..292..437S
    .
  9. arXiv:1402.0257v1 [SR astro-ph. SR
    ].
  10. ISSN 0004-6361
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  11. S2CID 205570660
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  12. S2CID 118870118
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  13. ^
    S2CID 119227339
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  14. S2CID 118870118
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  15. S2CID 84177572
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  16. ^
    doi:10.1086/177740
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  17. S2CID 15424181
    .
  18. S2CID 53596517
    .
  19. S2CID 118614725
    .
  20. doi:10.1086/316420
    .
  21. S2CID 85440811
    .
  22. S2CID 118623783
    .
  23. S2CID 4392537
    .
  24. S2CID 22996021
    .
  25. S2CID 119227339
    .
  26. S2CID 43688343
    .
  27. S2CID 119509358
    .
  28. S2CID 39425211
    .
  29. S2CID 41026239
    .
  30. S2CID 7905464
    .
  31. S2CID 119198192
    .
  32. S2CID 119233401
    .
  33. S2CID 118616519
    .
  34. ^ "HD 269700". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 16 July 2017.
  35. doi:10.3847/1538-3881/aa6195
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  36. ^
    S2CID 119177378
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  37. ^
    S2CID 118374186
    .
  38. S2CID 202558925
    .
  39. S2CID 118947657
    .
  40. S2CID 118409541
    .
  41. ^
    S2CID 119379139
    .
  42. ^
    doi:10.1093/mnras/stac3408
    .
  43. ^
    doi:10.1093/mnras/stab2036
    .
  44. doi:10.1093/mnrasl/sly241
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  45. S2CID 211572824
    .

External links
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