The Missing Luminous Blue Variables and the Bistability Jump

By N. Smith, J. Vink, A. de Koter. 2004

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We discuss an interesting feature of the distribution of luminous blue variables (LBVs) on the H-R diagram, and we propose a connection with the bistability jump seen in the winds of early-type supergiants. There appears to be a deficiency of quiescent LBVs on the S Doradus instability strip at luminosities between log(L/L☉) ≃ 5.6 and 5.8. The upper boundary, interestingly, is also where the temperature-dependent S Doradus instability strip intersects the bistability jump at about Teff ≃ 21,000 K. Because of increased opacity, winds of early-type supergiants are slower and denser on the cool side of the bistability jump, and we postulate that this may trigger optically thick winds that inhibit quiescent LBVs from residing there. We conduct numerical simulations of radiation-driven winds for a range of temperatures, masses, and velocity laws at log(L/L☉) = 5.7 to see what effect the bistability jump should have. We find that for relatively low stellar masses, the order-of-magnitude increase in the wind density at the bistability jump leads to the formation of a modest to strong pseudophotosphere that might alter a star's apparent position on the H-R diagram. The effect is strongest for LBVs approaching 10 M☉, where the pseudophotospheres are sufficiently extended to make an early B-type star appear as a yellow hypergiant. Thus, the proposed mechanism will be most relevant for LBVs that are post-red supergiants [curiously, the upper boundary at log(L/L☉) ≃ 5.8 coincides with the upper luminosity limit for red supergiants]. Further work is obviously needed, especially with regard to a possible evolutionary connection between the "missing" LBVs and the most luminous red supergiants and yellow hypergiants. Specifically, yellow hypergiants such as IRC +10420 and ρ Cas occupy the same luminosity range as the missing LBVs and show apparent temperature variations at constant luminosity. If these yellow hypergiants do eventually become Wolf-Rayet stars, we speculate that they may skip the normal LBV phase, at least as far as their apparent positions on the H-R diagram are concerned.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/f31f6d85287a1c8332dded5f0806c7412648b319

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Citations in the corpus

A Massive Cluster of Red Supergiants at the Base of the Scutum-Crux Arm (2007, background): “Since evolved then the star has apparently evolved to an A-type supergiant (Klochkova et al. 2002), although this may be due to the dissipation of the pseudophotosphere created by the dense wind (Smith et al. 2004).”
Luminous blue variable eruptions and related transients: diversity of progenitors and outburst properties (2010, background): “It has been well-established that the instability we associate with LBVs occupies a large range of initial mass, from the most massive stars that may exist down to about 20-25 M⊙ (Smith et al. 2004).”
Discovery of the Dust-Enshrouded Progenitor of SN 2008S with Spitzer (2008, background): “Mid-IR Detection of the SN 2008S Progenitor 5 8 The lowest-mass LBVs known have initial masses of 20−25 M⊙ and luminosities > 105 L⊙ (e.g., Smith et al. 2004; Smith 2007).”
Mass loss from hot massive stars (2008, background): “Vink and de Koter (2002) and Smith et al (2004) showed with their Monte Carlo computations that when lowering the mass for constantluminosity, the mass-loss rate increases more rapidly than Eq.”
Luminous blue variables are antisocial: their isolation implies that they are kicked mass gainers in binary evolution (2014, background): “Figure 6 shows the locations of LBVs on the HR diagram, mostly taken from Smith et al. (2004), except as noted in the caption.”
Fundamental properties of core-collapse Supernova and GRB progenitors: predicting the look of massive stars before death (2013, background): “13 that LBVs are distributed over a range of luminosities (see also van Genderen 2001; Smith et al. 2004; Clark et al. 2005, 2009a, 2012b; Vink 2012).”
Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars (2014, methodology): “Their initial masses are uncertain, but comparing their luminosities to single-star evolution tracks suggests initial masses greater than 25 M⊙ (Smith et al. 2004).”
PREPARING FOR AN EXPLOSION: HYDRODYNAMIC INSTABILITIES AND TURBULENCE IN PRESUPERNOVAE (2013, background): “LBVs The LBVs are a rather diverse class, consisting of a wide range of irregular variable phenomena associated with evolved massive stars (Conti 1984; Humphreys & Davidson 1994; van Genderen 2001; Smith et al. 2004, 2011b), excluding those seen in cool supergiants.”
THE INCIDENCE OF STELLAR MERGERS AND MASS GAINERS AMONG MASSIVE STARS (2013, background): “1987; Smith et al. 2004; Nazé et al. 2008, 2010) or a circum-binary disk (as seen for example around the interacting system RY Scuti, Grundstrom et al. 2007).”
DISCOVERY OF PRECURSOR LUMINOUS BLUE VARIABLE OUTBURSTS IN TWO RECENT OPTICAL TRANSIENTS: THE FITFULLY VARIABLE MISSING LINKS UGC 2773-OT AND SN 2009ip (2009, background): “LBVs represent a highly unstable, rapid mass-loss phase in the late evolution of massive stars (see Smith & Owocki 2006; Smith et al. 2004; Humphreys & Davidson 1994), which is still poorly understood.”
RED SUPERGIANTS AS POTENTIAL TYPE IIn SUPERNOVA PROGENITORS: SPATIALLY RESOLVED 4.6 μm CO EMISSION AROUND VY CMa AND BETELGEUSE (2008, background): “Will VY CMa explode in its current extreme RSG state, or will it evolve blueward to become a yellow hypergiant with a dense environment (like IRC+10420), or a blue supergiant with a dense shell nebula in a postRSG LBV phase (Smith 2007; Smith et al. 2004)?”
On the massive stellar population of the super star cluster Westerlund 1 (2005, background): “Moreover, Smith et al. (2004) identify an apparent lack of known LBVs in the luminosity range occupied by the YHGs and suggest a novel explanation for this – that the effect of the bi-stability jump on the mass loss rate of post RSG stars evolving bluewards into the S Doradus instability strip is…”
Observed Fractions of Core-Collapse Supernova Types and Initial Masses of their Single and Binary Progenitor Stars (2010, background): “(1) The WR phase for many lower-mass stars is not observed because it is extremely short lived, perhaps because eruptions in late evolutionary phases remove the remaining H envelope even down to lower masses than we normally associate with LBVs.”
SN 2006gy: Discovery of the Most Luminous Supernova Ever Recorded, Powered by the Death of an Extremely Massive Star like η Carinae (2006, background): “Gal-Yam et al. (2007) have drawn a similar conjecture, considering LBVs as themost likely progenitors of SNe IIn.”
Wind modelling of very massive stars up to 300 solar masses (2011, result): “Still adopting a velocity stratiﬁcation, Vink & de Koter (2002) and Smith et al. (2004) predicted mass-loss rates for Luminous Blue Variables (LBVs), and showed that ˙ M is a strong function of the Eddington factor for these objects.”