Luminous blue variable

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Luminous blue variable AG Carinae as seen by the Hubble Space Telescope AG Carinae (HD 94910).png
Luminous blue variable AG Carinae as seen by the Hubble Space Telescope

Luminous blue variables (LBVs) are rare, massive and 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.

Contents

Discovery and history

P Cygni profile of a spectral line P Cygni Profile-en.svg
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 John Charles Duncan published the first three variable stars ever detected in an external galaxy, variables 1, 2, and 3, in the Triangulum Galaxy (M33). These were followed up by Edwin Hubble with three more in 1926: A, B, and C in M33. Then in 1929 Hubble added a list of variables detected in 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 AE Andromedae, AF Andromedae (=Var 19), Var 15, and Var A-1 in M31 were added to the list and described by several authors as "luminous blue variables", although it was not considered a formal name at the time. The spectra were found to contain lines with 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 δ Sct variables named after the star δ Sct. The first luminous blue variable to be identified as a variable star was P Cygni, and these stars have been referred to as P Cygni type variables. The General Catalogue of Variable Stars decided there was a possibility of confusion with 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. Lbvstar.png
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 e Carinae EtaCarinae-HST-1995-09.jpg
The Homunculus Nebula, produced by the Great Outburst of η Carinae

Because of these stars' large mass and high luminosity, their lifetime is very short—only a few million years in total and much less than a million years in the LBV phase. [12] They are rapidly evolving on observable timescales; examples have been detected where stars with Wolf–Rayet spectra (WNL/Ofpe) have developed to show LBV outbursts and a handful of supernovae have been traced to likely LBV progenitors. Recent theoretical research confirms the latter scenario, where luminous blue variable stars are the final evolutionary stage of some massive stars before they explode as supernovae, for at least stars with initial masses between 20 and 25 solar masses. [13] For more-massive stars, computer simulations of their evolution suggest the luminous blue variable phase takes place during the latest phases of core hydrogen burning (LBV with high surface temperature), the hydrogen shell burning phase (LBV with lower surface temperature), and the earliest part of the core 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. [15] Models have been constructed showing that the lower-luminosity group are post-red-supergiants with initial masses of 30–60 times the Sun, whereas the higher-luminosity group are population-II stars with initial masses 60–90 times the Sun that never develop to red supergiants, although they may become yellow hypergiants. [16] Some models suggest that LBVs are a stage in the evolution of very massive stars required for them to shed excess mass, [17] whereas others require that most of the mass is lost at an earlier cool-supergiant stage. [16] Normal outbursts and the stellar winds in the quiescent state are not sufficient for the required mass loss, but LBVs occasionally produce abnormally 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 e Carinae in nearby galaxies Stars similar to Eta Carinae in nearby galaxies.jpg
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, [19] with P Cygni showing one or more similar outbursts 300–400 years ago, [20] but dozens have now been catalogued in external galaxies. Many of these were initially classified as supernovae but re-examined because of unusual features. [21] The nature of the outbursts and of the progenitor stars seems to be highly variable, [22] with the outbursts most likely having several different causes. The historical η Carinae and P Cygni outbursts, and several seen more recently in external galaxies, have lasted years or decades whereas some of the 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

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]

e Carinae, a luminous blue variable as seen from the Chandra X-ray Observatory EtaCarinaeStarSystem-ChandraXRayObservatory-20140826.jpg
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. The star formation region Messier 17.jpg
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. Luminous blue variable stars with Spitzer.png
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

NGC 2366 (NGC 2363)

NGC 4449

NGC 4559

NGC 4736 (Messier 94)

PHL 293B

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 ]

See also

Related Research Articles

<span class="mw-page-title-main">Supergiant</span> Type of star that is massive and luminous

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.

<span class="mw-page-title-main">Red supergiant</span> Stars with a supergiant luminosity class with a spectral type of K or M

Red supergiants (RSGs) are stars with a supergiant luminosity class and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

<span class="mw-page-title-main">Blue supergiant</span> Hot, luminous star with a spectral type of A9 or earlier

A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They are usually considered to be those with luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blue supergiants.

<span class="mw-page-title-main">S Doradus</span> Star in the Large Magellanic Cloud

S Doradus is one of the brightest stars in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, located roughly 160,000 light-years away. The star is a luminous blue variable, and one of the most luminous stars known, having a luminosity varying widely above and below 1,000,000 times the luminosity of the Sun, although it is too far away to be seen with the naked eye.

<span class="mw-page-title-main">P Cygni</span> Variable star in the constellation Cygnus

P Cygni is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in Uranometria as a nova. Located about 5,300 light-years from Earth, it is a hypergiant luminous blue variable (LBV) star of spectral type B1-2 Ia-0ep that is one of the most luminous stars in the Milky Way.

<span class="mw-page-title-main">Yellow hypergiant</span> Class of massive star with a spectral type of A to K

A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest, with just 20 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.

<span class="mw-page-title-main">AG Carinae</span> Luminous variable star in the constellation Carina

AG Carinae is a star in the constellation Carina. It is classified as a luminous blue variable (LBV) and is one of the most luminous stars in the Milky Way. The great distance and intervening dust mean that the star is not usually visible to the naked eye; its apparent brightness varies erratically between magnitude 5.7 and 9.0.

<span class="mw-page-title-main">HR Carinae</span> Star in the constellation Carina

HR Carinae is a luminous blue variable star located in the constellation Carina. It is surrounded by a vast nebula of ejected nuclear-processed material because this star has a multiple shell expanding atmosphere. This star is among the most luminous stars in the Milky Way. It has very broad emission wings on the Balmer lines, reminiscent from the broad lines observed in the spectra of O and Wolf–Rayet stars. A distance of 5 kpc and a bolometric magnitude of −9.4 put HR Car among the most luminous stars of the galaxy.

<span class="mw-page-title-main">Hypergiant</span> Rare star with tremendous luminosity and high rates of mass loss by stellar winds

A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially star formation, stability, and their expected demise as supernovae. Notable examples of hypergiants include the Pistol Star, a blue hypergiant located close to the Galactic Center and one of the most luminous stars known; Rho Cassiopeiae, a yellow hypergiant that is one of the brightest to the naked eye; and Mu Cephei (Herschel's "Garnet Star"), one of the largest and brightest stars known.

<span class="mw-page-title-main">Supernova impostor</span> Stellar explosions that appear to be supernovae

Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).

<span class="mw-page-title-main">HD 5980</span> Triple star system in the constellation Tucana

HD 5980 is a multiple star system on the outskirts of NGC 346 in the Small Magellanic Cloud (SMC) and is one of the brightest stars in the SMC.

<span class="mw-page-title-main">HD 168607</span> Star in the constellation Sagittarius

HD 168607 is a blue hypergiant and luminous blue variable (LBV) star located in the constellation of Sagittarius, easy to see with amateur telescopes. It forms a pair with HD 168625, also a blue hypergiant and possible luminous blue variable, that can be seen at the south-east of M17, the Omega Nebula.

<span class="mw-page-title-main">NGC 2363-V1</span> Luminous blue variable star in the constellation Camelopardalis

NGC 2363-V1 is a luminous blue variable star in the star-forming region NGC 2363, at the far southwestern part of the irregular galaxy NGC 2366 in the constellation Camelopardalis, near the north celestial pole nearly 11 million light years away from our galaxy. It was discovered in 1996 by Laurent Drissen, Jean-René Roy, and Carmelle Robert while examining images taken by the Hubble Space Telescope Wide Field Planetary Camera 2.

<span class="mw-page-title-main">AE Andromedae</span> Star in the constellation Andromeda

AE Andromedae is a luminous blue variable (LBV), a type of variable star. The star is one of the most luminous variables in M31, the Andromeda Galaxy.

<span class="mw-page-title-main">HD 316285</span> Star in the constellation Sagittarius

HDE 316285 is a blue supergiant star in the constellation Sagittarius. It is a candidate luminous blue variable and lies about 6,000 light years away in the direction of the Galactic Center.

<span class="mw-page-title-main">R71 (star)</span> Star in the Large Magellanic Cloud

R71 is a star in the Large Magellanic Cloud (LMC) in the constellation Mensa. It is classified as a luminous blue variable and is one of the most luminous stars in the LMC. It lies three arc-minutes southwest of the naked-eye star β Mensae.

<span class="mw-page-title-main">AF Andromedae</span> Luminous blue variable star in the constellation Andromeda

AF Andromedae is a luminous blue variable (LBV), a type of variable star. The star is one of the most luminous variables in M31, the Andromeda Galaxy.

<span class="mw-page-title-main">HD 37836</span> Star in Large Magellanic Cloud

HD 37836 is a candidate luminous blue variable located in the Large Magellanic Cloud and one of the brightest stars in its galaxy.

<span class="mw-page-title-main">B324</span> Star in the Triangulum Galaxy

B324 is a yellow hypergiant in the Triangulum Galaxy, located near the giant H II region IC 142 around 2.7 million light years away. It is the brightest star in the Triangulum Galaxy in terms of apparent magnitude.

References

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