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Luminous Blue Variables

Abstract

The defining characteristics of luminous blue variables are presented and interpreted in an evolutionary context. Some occurences of these extremely rare objects are listed with their known physical properties.

Keywords: luminous blue variable, LBV, P Cygni, S Doradus, Hubble-Sandage

Introduction

LBVs are massive, intrinsically bright stars which display different scales of light and colour variability, ranging from rapid microvariations to rare outbreaks of catastrophic mass loss. They represent a very short-lived (perhaps as little as 40,000 years) strongly mass-losing phase in the evolution of massive stars, during which they undergo deep erosion of the outer layers before they enter the Wolf-Rayet phase.

They include P Cygni stars, S Doradus stars, and Hubble-Sandage variables.

The LBV class was first defined by Conti (1984).
 
 

Related Topics

Further Reading

  • K. Davidson et al., eds. (1989): Physics of Luminous Blue Variables
  • Nota, A.; Lamers, H.J.G.L.M., eds. (1997): Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120.

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Characteristics

Spectra
LBV spectra typically exhibit prominent emission lines of H, HeI, FeII and [FeII], often with P Cygni profiles [® sidebar]. These emission lines are much stronger than normal for the temperature and luminosity of the stars.

The spectra, luminosity and temperature are highly variable. During quiet periods of minimum brightness (so-called ‘quiescence’) they appear as blue B-type supergiants with temperatures greater than around 15 - 20,000 K showing H and HeI emission. During periods of maximum brightness they appear as later A- or F-type supergiants with temperatures near 8000 K and FeII and [FeII] emission reaches maximum. It is widely believed that the bolometric magnitudes, i.e. total radiation, remain roughly constant throughout these cycles and that the apparent cooling is due to absorption by surrounding shells of ejected matter which re-radiate the stars’ visible and blueward radiation at longer wavelengths (Parker et al. 1993).
P Cygni spectra are characterised by emission lines flanked on their violet edges by absorptions and are produced by an expanding envelope of gas or stellar wind.

Other than that which is "blowing" straight towards or away from us, the gas streaming away from the star is travelling more or less perpendicular to our line of sight, producing emission lines which are not Doppler shifted. The gas which is travelling straight towards us, however, lies directly in front of the star, so producing an absorption feature in the stellar continuum, which is Doppler shifted to shorter wavelengths and appears on the violet edge of the emission lines. The gas streaming directly away from us on the far side of the star is not seen at all.

(After Kahler 1992, pp.193-194. See also Crowther 1997, pp. 51-57.)

Mass and Luminosity

Luminosity is typically in the order of 106 L¤ (Mbol ~ -10), which is near the observed upper luminosity limit of all stars, and their episodic outbursts may be a clue to the mechanism which brings about the limit in the first place.

From their luminosity and positions on the H-R diagram, we can categorise LBVs into three broad luminosity types:

  • The most luminous star, h Carinae, occupies a class of its own.
  • The second group contains stars which range from -11 > Mbol > -9.9 and includes most of the well-known LBVs: R127, S Dor, P Cyg and AG Car.
  • The lowest luminosity LBVs are found below the Humphreys-Davidson limit and are typified by R71.

The underlying basis for these apparently natural groupings may lie in the available range for radius variation before the star runs into whatever physical process drives the Humphreys-Davidson limit.

(After Bohannan 1989, p. 36.)

Variability/Mass Loss

Most characteristically, LBVs exhibit variability ranging from day to day microvariations of around 0.1 to 0.2 magnitudes, through ‘normal’ variations of 1 to 2 magnitudes on time scales of years to a few decades, to rare eruptions which can increase their brightness by more than 3 magnitudes occurring less frequently, perhaps once in a few centuries.

  1. LBVs exhibit microvariations of around 0.1 to 0.2 magnitudes on a time scale of days or a few tens of days. Wolf (1989, p. 93) notes recorded time scales for R71 of ~23.5 days between 1983 and 1985, shortening to ~14.3 days between 1986 and 1987. "[It] is easily seen that even small photometric variations are paralleled by significant spectroscopic variations in the sense that the spectrum becomes later when the star becomes visually brighter." Van Genderen et al. 1989, p. 273, reports similar findings for AG Car (fig. 1) (P ~ 10 days) and HR Car (P ~ 20 days).
     
  2. During ‘normal’ (S Doradus type) eruptions, the stars may brighten by up to two magnitudes within a few months.

    At these times the stars’ atmospheres form a slowly (100 to 200 km s-1) expanding "optically thick pseudo-photosphere" (Humphreys 1989, p. 4). Mass loss increases 10 to 100 times from quiescent levels, which are near those of normal supergiants of the same temperature and luminosity, up to 10-5 to 10-4 M¤ yr-1. Both the minima and the maxima may last several years (Wolf 1989, p. 92). Superimposed over these variations are smaller oscillations of around half a magnitude, occurring on a time scale of months to a few years.

    Stothers and Chin have recently (1995) found that the periodicity of the normal variations seems to be related to absolute bolometric magnitude (table 1).

  

 

Variable Galaxy Mbol Period
h Car MW -11.6 3 – 5 yrs
AF And M31 -11.4 3 – 7
AG Car MW -10.8 4 – 14
Var. B M33 -10.2 5 – 8
P Cyg MW -9.9 ~18
S Dor LMC -9.8 10 – 30
Var. C M33 -9.8 19 – 20
Table 1: Table of some luminous blue variable stars illustrating the possible relation between Mbol and periodicity of S Dor-type eruptions.

Mbol = Bolometric magnitude, MW = Milky Way, LMC = Large Magellanic Cloud (after Stothers and Chin 1995).
  1. LBVs are perhaps most famous for their sporadic giant (‘plinian’ or ‘Eta Carinae type’) eruptions which may represent the causal mechanism behind the Humphreys-Davidson limit. During these eruptions, large amounts of mass are suddenly ejected. The most famous example is certainly the brightening of h Carinae between 1837 and 1860, when it outshone Canopus. Less well-known, P Cygni brightened from below naked-eye visibility to around third magnitude in the 1600s.

    The interval between major eruptions, during which the stars remain quiescent, is estimated in the order of a few hundreds to a few thousands of years.

    The sporadic ejections of mass and high rates of steady mass loss at other times surrounds the LBV with a circumstellar shell of dust which may even be clearly visible as in the case of the Homunculus Nebula surrounding h Car or the ring nebula surrounding AG Carinae (fig. 1). Over the life-time of the LBV, usually estimated as ~105 years, they may shed around 1 M¤ or more through steady mass loss and more during major eruptions. Stahl 1987 estimates the average mass loss rate for a number of LBVs, including AG Car and R127, to be >10-4 M¤ yr-1.

    Analyses have indicated that the ejecta are rich in nitrogen and helium, presumably arising from fusion in the core and brought to the surface by mixing or convective overshooting.

    The lifetime of LBVs can be estimated from the relative numbers of LBVs versus other stars. For example, in the LMC there are known six LBVs and 115 Wolf-Rayet stars. “If one assumes that there are ten current LBVs to account for a few unknown, the phase between core-hydrogen and core-helium burning would then be about one-twelfth of the entire W-R lifetime (5 x 105y), ~ 40,000 years” (Bohannan 1997, p.4).
(A) AG Carinae field (19146 bytes)     (B) AG Carinae jet (26597 bytes)

Fig 1: AG Carinae.

(A) AG Car (blue star), at a distance of 6 to 7 kpc (19,500 to 22,800 light years), has over 1 million times the luminosity of the Sun, and changes in spectral type from early A to late O; the star has varied greatly in luminosity over the last two decades.  More importantly, it shows a dusty circumstellar nebula, visible in the full size 2MASS image as the blue ring around the star. The ring nebula surrounding AG Carinae is roughly elliptical, about 30" ´ 40", and expanding at an estimated 50 km s-1. Smith et al. 1997 concludes from a compositional study of the nebula, that AG Car probably experienced an earlier, brief red supergiant phase, when it ejected its hydrogen-rich outer layers to form the ~1 pc-diameter ring nebula; the ring matter has subsequently been swept up by the present hot supergiant wind. (The purplish "star" to the north of AG Car's diffraction spike is a known persistence artifact; a similar red artifact can be seen to the north of the very red star, which is to the southwest of AG Car.)

[Atlas text and image courtesy of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.]

(B) V-band image in which light from the star (located at the cross) is almost completely suppressed by a coronograph.  The occulting wedge has a width at the centre of ~2 arcsecs. A jet-like structure can be seen projecting SW from a distance of approximately 5 arcsecs from the star towards the outer portion of the circumstellar ring at ~15 arcsecs from the centre. "This feature is extremely well defined in the B, V, I images and seems to consist of two thick helical filaments spiralling towards the external portion of the nebula" (Nota and Paresce 1989, p. 161, fig. 1a).

Interpretation

Progenitors

It is widely believed that most stars with an initial mass greater than about 40 M¤ go through a short (up to about 105 years) LBV phase in the late stages of their evolution, after leaving the main sequence (e.g. Drissen et al. 1997).  Moffat et al. 1989 (p. 229) goes even further, suggesting that "the LBV mechanism is an essential step to ‘force’ massive stars (MZAMS / 40 M¤ ) to finally enter the Wolf-Rayet (W-R) domain".

Stars somewhat below this mass may become variable red supergiants showing some LBV characteristics (occasionally referred to as "red LBVs," e.g. by Moffat et al. 1989, p.234).  This class includes the highly variable OH/IR stars. Note, however, that there are some supposed LBVs which have relatively low luminosity and interpreted low mass: perhaps as low as 20 to 30 M¤.   This group includes R66, AE Andromedae, and ‘var. 15’ from M31. Their evolutionary future is unclear (Moffat et al. 1989, p.234).

The obvious place to seek candidate progenitor stars is in adjacent areas of the HR Diagram, between the H-ZAMS and the Humphreys-Davidson Limit. This area includes early type luminous supergiants "that have not yet shown any obvious LBV activity beyond quiescent microvariability.... They evolve essentially horizontally in the H-R Diagram, both before and after encountering the H-D Limit" (Moffat et al. 1989, p.229).

As noted above, detailed photometric studies reveal microvariability in "non-variable" supergiants, most pronounced in early (O and early B) type stars of high bolometric magnitude, the amplitudes of which "resemble the microvariations observed in LBVs during quiescence" (Moffat et al. 1989, p.230).

Evolution

During the so-called "normal" (S Doradus type) outbursts, the objects evolve red-wards to cooler temperatures, bringing them into contact with the Humphreys-Davidson limit and stimulating accelerated mass loss. "After an LBV has lost enough mass in an eruption, the instability is temporarily relieved and the star returns to its quiescent [blue] state, presumably until the instability develops again" (Humphreys 1989 p. 7).

Stars may run into several LBV episodes. "As they careen across the H-R diagram in repeated blue-red-blue excursions, they lose mass through constant winds and occasional outbursts. This scenario may lead an LBV to a Wolf-Rayet phase or even a supernova, which perhaps could occur either during the outburst phase as a red supergiant or the quiescent phase as a blue supergiant such as the progenitor of SN 1987A. These temperature variations should not be confused with the ‘blue loops’ that are thought to occur in the post-main-sequence evolution of massive stars due to nuclear evolution; in LBVs the variability is due to photospheric instabilities and changes" (Parker et al. 1993, p. 770; also see Bohannan 1997).

Quiescent States
"Ofpe/WN9 stars appear also to be related to the LBV phenomenon. The best known examples of this connection are R127 and AG Carinae. R127 had been originally classified as an Ofpe/WN9 star till the '80s when Stahl et al. (1983) observed it undergoing a typical LBV outburst. AG Carinae is a proven LBV star which assumes an Ofpe/WN9-like spectral morphology during its quiescent periods (Stahl 1986). As a consequence of these observations Ofpe/WN9 stars are now suspected to be dormant LBVs" (Pasquali et al. 1996).

"This relationship has been further explored by Smith, Crowther, & Prinja (1994) who propose the some LBVs are the extension of the WN sequence towards later spectral types, hence unifying the classes of WNL, LBV, and Ofpe/WN9 stars" (Nota et al. 1996, p. 385).

"The above properties make Ofpe/WN9 stars ideal to verify stellar evolution models and to study the mass loss phenomenon. In particular, it is interesting to compare the mass loss parameters (such as mass loss rate, stellar radiation field and wind momentum) found for Ofpe/WN9 with the ones derived for O and WR stars. In this way, it becomes possible to compare the mass loss properties during the evolution from the main sequence to the early Wolf-Rayet stage" (Pasquali et al. 1996).

Higher mass-loss rate and enhanced mixing between core and envelope are required in order to yield models compatible with the observed properties of Ofpe/WN9 stars. The emerging picture may be consistent with earlier evidence of Ofpe/WN9 stars being quiescent LBVs. This idea is further strengthened by the highly reduced surface H mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5 to 0.3, which still excludes Ofpe/WN9 stars from being core He burning objects, but is almost identical to the Xs values recently measured in LBVs.

A number of Ofpe/WN9 star spectra show the presence of nebular emission lines, indicating a surrounding nebulosity though only in some cases (S119, BE 381) is there a clear case for an expanding shell associated with the star. "This would strengthen the connection between Ofpe/WN9 stars and luminous blue variables, which also are often surrounded by circumstellar nebulae generated during one or more 'violent outbursts' in their recent evolution history (Nota et al. 1996, p. 383).
"In or about 1980, the Ofpe/WN9 prototype HDE 269858 = Radcliffe 127 unexpectedly entered an outburst phase (Stahl et al. 1983, Walborn 1984), during which its spectrum has evolved through an intermediate B-type to a peculiar supergiant A-type identical to that of S Doradus, while the bolometric luminosity has remained constant (Stahl and Wolf 1986a, Wolf et al. 1988). Thus HDE 269858 has firmly established its own LBV nature, as well as a connection between the Ofpe/WN9 and LBV classes. A connection in the opposite direction has been made by the recent discovery of Ofpe/WN9 characteristics during a light minimum of the galactic LBV AG Carinae (Stahl 1986), which at other times displays a B-type spectrum identical to that of P Cygni (Hutsemékers and Kohoutek 1988), and an A-type similar to S Doradus (Wolf and Stahl 1982).

"These remarkable observations indicate previously unsuspected relationships among the apparently disparate objects in question. Moreover, strong hypotheses are suggested that all Ofpe/WN9 objects may be quiescent LBVs, and conversely that many or perhaps all LBVs are of Ofpe/WN9 type at minimum. A corollary of the latter would be that the true minimum states of P Cygni and S Doradus have not yet been observed, and that they are in extended intermediate or maximum states" (Walborn 1989, p. 28).

Successors

H-depletion and N-enrichment have been noted in some LBVs (see Langer 1990 for references) so "it does seem likely that after several major LBV eruptions, one should look among luminous stars with enhanced CNO-cycle abundances. The W-R stars are the most obvious candidates, in particular the most luminous ones, of type WNL (late-type W-R stars of subtypes WN6-9)" (Moffat et al. 1989, p.229). An example supporting this conjecture is R127 in the LMC, originally recorded as an Of+WN binary until its recent eruption.

Occurence/Examples

LBVs are among the rarest of stars; only about 32 stars in the local group of galaxies are known to be LBVs and six of these are in the LMC (Parker et al. 1993, p. 770; Bohannon 1997).
Star Galaxy Temp (at min L) Temp (at max L) Mbol Mass Loss Rate
h Car Milky Way 27,000   -11.3 10-3 to 10-1
R127/HDE 269850 LMC 30,000 8,500 -10.5 6 ´ 10-5
AG Car Milky Way 25,000 9,000 -10.1 3 ´ 10-5
P Cyg Milky Way 19,000   -9.9 2 ´ 10-5
S Dor LMC 20-25,000 8,000 -9.8 5 ´ 10-5
Var C M33 20-25,000 7,500-8,000 -9.8 4 ´ 10-5
Var A M33 3,500 8,000 -9.5 2 ´ 10-4
R71/HDE 269006 LMC 13,600 9,000 -8.8 5 ´ 10-5
Table 2:  Observed properties of some well-studied LBVs (after Humphreys 1989, Table 2). Some additional information can be found in Humphreys 1989 p. 5 (AE And), p. 6 (R81) and p. 9 (R127, Var C, Var A, R71 and AG Car).
Some other examples are HR Car in the Milky Way, AE And and AF And in M31, and HDE 269582 in the LMC.

Another candidate LBV, R81 in the LMC, is an eclipsing binary.

Additional Note

For a discussion of binary scenarios, see Gallagher 1989.

References

Bohannan, Bruce 1989: The Distribution of Types of Luminous Blue Variables. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 35-43.

—— 1997: The Definition of Luminous Blue Variables. In Nota, A.; Lamers, H.J.G.L.M. (eds) 1997: Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120.

Conti, P.S. 1984: In Maeder, A.; Renzini, A. (eds.) 1984: Observational Tests of Stellar Evolution Theory. IAU Symposium 105: 233.

Crowther 1997: Physical Properties of LBVs and Related Objects. In Nota, A.; Lamers, H.J.G.L.M. (eds) 1997: Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120, pp. 51-57.

Drissen et al. 1997

Gallagher, J.S. 1989: Close Binary Models for Luminous Blue Variable Stars. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 185-194.

Humphreys, Roberta M. 1989: What are LBVs? – Their Characteristics and Role in the Upper H-R Diagram. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 3-12.

Kaler, James B. 1992: Stars. Scientific American Library.

Langer 1990

Moffat, A.F.J.; Drissen, L.; Robert, C. 1989: Observational Connections Between LBVs and Other Stars, with Emphasis on Wolf-Rayet Stars. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 229-240.

Nota, Antonella; Paresce, Francesco 1989:  High Resolution Coronographic Imaging of AG Carinae.  In K. Davidson et al. (eds). 1989: Physics of Luminous Blue Variables.  Kluwer Academic Publishers.

Nota, Antonella; Pasquali, Anna; Drissen, Laurent; Leitherer, Claus; Robert, Carmelle; Moffat, Anthony F. J.; Schmutz, Werner 1996: O Stars in Transition. I. Optical Spectroscopy of Ofpe/WN9 and Related Stars. Astrophysical Journal Supplement v.102, pp. 383-410.

Parker et al. 1993

Pasquali, Anna; Schmutz, Werner; Leitherer, Claus; Nota, Antonella; Hubeny, Ivan; Langer, Norbert; Drissen, Laurent; Robert, Carmelle 1996: Fundamental Properties of Ofpe/WN9 Stars from Ultraviolet HST Spectra. Science with the HST 2: 386-392.

Smith et al. 1997 MNRAS 290:265

Stahl 1987

Stothers and Chin 1995

Van Genderen, A.M.; Thé, P.S.; Heemskerk, M.; Heynderickx, D.; Larsen, I.; Wanders, I.; van Weeren, N. 1989: The Optical Micro Variations of the Two S Dor Type Stars AG Car and HR Car. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 273-274.

Walborn, Nolan R. 1989: Evolutionary Diagnostics of LBV Spectra and Systems in the LMC. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 27-34.

Wolf, B. 1989: "Normal" LBV Eruptions a la S Doradus. In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, pp. 91-99.
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