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The Isolation of Luminous Blue Variables Resembles Aging B-Type Supergiants, Not the Most Massive Unevolved Stars

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The Isolation of Luminous Blue Variables Resembles Aging B-Type Supergiants, Not the Most Massive Unevolved Stars The isolation of luminous blue variables resembles aging B-type supergiants, not the most massive unevolved stars Item Type Article Authors Smith, Nathan Citation Nathan Smith, The isolation of luminous blue variables resembles aging B-type supergiants, not the most massive unevolved stars, Monthly Notices of the Royal Astronomical Society, Volume 489, Issue 3, November 2019, Pages 4378–4388, https:// doi.org/10.1093/mnras/stz2277 DOI 10.1093/mnras/stz2277 Publisher OXFORD UNIV PRESS Journal MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY Rights Copyright © 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. Download date 27/09/2021 14:40:50 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version Link to Item http://hdl.handle.net/10150/634943 MNRAS 489, 4378–4388 (2019) doi:10.1093/mnras/stz2277 Advance Access publication 2019 August 16 The isolation of luminous blue variables resembles aging B-type supergiants, not the most massive unevolved stars Downloaded from https://academic.oup.com/mnras/article-abstract/489/3/4378/5550754 by University of Arizona Health Sciences Library user on 31 October 2019 Nathan Smith‹ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA Accepted 2019 August 5. Received 2019 July 25; in original form 2019 May 1 ABSTRACT Luminous blue variables (LBVs) are suprisingly isolated from the massive O-type stars that are their putative progenitors in single-star evolution, implicating LBVs as binary evolution products. Aadland et al. found that LBVs are, however, only marginally more dispersed than a photometrically selected sample of bright blue stars (BBS) in the Large Magellanic Cloud (LMC), leading them to suggest that LBV environments may not exclude a single-star origin. In both comparisons, LBVs have the same median separation, confirming that any incompleteness in the O-star sample does not fabricate LBV isolation. Instead, the relative difference arises because the photometric BBS sample is far more dispersed than known O-type stars. Evidence suggests that the large BBS separation arises because it traces less massive (∼20 M), aging blue supergiants. Although photometric criteria used by A19 aimed to select only the most massive unevolved stars, visual-wavelength colour selection cannot avoid contamination because O and early B stars have almost the same intrinsic colour. Spectral types confirm that the BBS sample contains many B supergiants. Moreover, the observed BBS separation distribution matches that of spectroscopically confirmed early B supergiants, not O-type stars, and matches predictions for a roughly 10 Myr population, not a 3–4 Myr population. A broader implication for ages of stellar populations is that bright blue stars are not a good tracer of the youngest massive O-type stars. Bright blue stars in nearby galaxies (and unresolved blue light in distant galaxies) generally trace evolved blue supergiants akin to SN 1987A’s progenitor. Key words: binaries: general – blue stragglers – stars: evolution – stars: massive – stars: Wolf–Rayet. The standard view of LBVs has been that they correspond to a 1 INTRODUCTION very brief transitional phase of the most massive single stars, when The massive eruptive stars known as luminous blue variables the star moves from core H burning to core He burning. In this (LBVs) are critical for understanding the evolution and fates of view, LBV winds or eruptions are the prime agent that removes the massive stars. This is because LBVs have the highest observed mass- H envelope to produce Wolf–Rayet (WR) stars (Langer et al. 1994; loss rates of any class of stars, and because this mass-loss (which Heger et al. 2003; Meynet & Maeder 2003;Meynetetal.2011). may or may not remove the H envelope) profoundly influences the This transition from single O-type stars to WR through their own fate of the star and the type of eventual supernova (SN) explosion mass-loss is often referred to as the ‘Conti scenario’ (Conti 1976). (see Smith 2014). Understanding the physical mechanism of this The reliance upon the LBV phase for making WR stars from single mass-loss and its metallicity dependence is therefore critical for stars is even more acute because of lowered O-star wind mass- models of stellar evolution, whether it is driven by normal winds, loss rates (Bouret, Lanz & Hillier 2005; Fullerton, Massa & Prinja eruptive events when a massive star exceeds the Eddington limit, 2006; Smith & Owocki 2006; Smith 2014). It is therefore critical or binary interaction episodes (Owocki, Gayley & Shaviv 2004; to have model-independent tests of this single-star evolutionary Smith&Owocki2006; Podsiadlowski 2010; Smith et al. 2011; paradigm. Groh et al. 2013a; Groh, Meynet & Ekstrom¨ 2013b; Justham, Single versus binary scenarios can be addressed by studying Podsiadlowski & Vink 2014; Smith & Arnett 2014; Blagovest, the ages and environments of LBVs. For stars at the same Vink & Grafener¨ 2016;Gotberg,¨ de Mink & Groh 2017). place on the HR diagram, the age of the surrounding envi- ronment can differentiate binary evolution products from single stars, since mass gainers and mergers may have had significantly E-mail: [email protected] lower initial masses and longer lifetimes than effectively sin- C 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society LBVs and BBS 4379 gle supergiant stars of the same current luminosity (Gallagher theoretical studies seeking to understand how LBVs might be SN 1989). A clear prediction is that in the single-star scenario, progenitor stars (Justham et al. 2014). where LBVs occur immediately after core H exhaustion in tran- This new blue straggler view of LBVs is in direct contradiction sition to their He burning phase as WR stars, the spatial lo- to the traditional view for their role in stellar evolution. In addition cations of LBVs should follow those of massive, young, early to giving a different origin for LBVs themselves, it also has the Downloaded from https://academic.oup.com/mnras/article-abstract/489/3/4378/5550754 by University of Arizona Health Sciences Library user on 31 October 2019 O-type stars that are their immediate progenitors. At these high consequence of removing LBVs from the single-star evolutionary initial masses, the lifetimes are very short (3–4 Myr), and there scenario, wherein they play a crucial role in removing the H is not enough time to move far from their birth sites. In a binary envelope to make WR stars. This modification has sparked some scenario, on the other hand, LBVs should be more dispersed than debate. In particular, Humphreys et al. (2016) had a different young O-type stars because they have been rejuvenated after a take on subdividing the data, and preferred the traditional single- delay due to their longer main-sequence lifetime (or they may have star view. Humphreys et al. (2016) noticed that if one excludes received a kick from a companion’s SN), whereas the most massive most of the LBV sample, then the three most luminous LBVs O-type stars have already died. in the LMC do have a median separation similar to that of O- Most stellar age indicators are too imprecise for this task, because type stars, which in their interpretation supported the single-star one is interested in being able to distinguish between ages of scenario after all. Humphreys et al. (2016) also pointed out that around 3–4 Myr (main-sequence lifetimes of MZAMS > 40 M stars, the lower luminosity LBVs have a separation distribution similar for e.g. appropriate to classical LBVs) or a factor of only about to red supergiants (RSGs), taken as support for a single-star view 2–3 older corresponding to ∼20 M stars with longer lifetimes wherein these LBVs are post-RSGs from initially 30–40 M stars. that have been rejuvented though mass accretion or mergers. For For both points, however, Smith (2016) showed that this was a example, in the star formation history study of the LMC by Harris & mischaracterization of the data. The most luminous LBVs should Zaritsky (2009), there is one single age bin for all ages <9Myr; have initial masses of around 50–100 M, but the common O- so whether they are single or binary, almost all the LBVs should type stars with a similar spatial distribution noted by Humphreys be lumped into one bin. Since LBVs are generally not in clusters, et al. (2016) were dominated by late O-type stars with initial age estimates based on turnoffs, RSG luminosity, or luminosity masses around 18–25 M. Similarly, the population of RSGs was functions (Schneider et al. 2014; Eldridge et al. 2017;Beasoretal. dominated by relatively low initial masses of ∼15 M,sotheir 2019) generally can’t be applied to LBVs. The most reliable clock similarity to the low-luminosity LBV distribution (expected to for the highest mass stars turns out to be using a spatial association have single-star initial masses of 30–40 M) contradicts a single- with other stars that must have very short lifetimes: i.e. early O- star scenario. Moreover, Smith (2016) demonstrated that there is type stars. A spectrum of a single O star doesn’t provide an age, no significant difference between LBVs and LBV candidates, so of course, but the relative degree of clustering of those O-type that including ‘candidate’ LBVs would not skew the results as stars does give a relative statistical age, because O stars are born Humphreys et al. (2016) argued. (Note that ‘candidate’ LBVs are in clusters that disperse with time. These are the same stars that stars with similar spectra and luminosities to LBVs, often with should be the single-star progenitors of classical LBVs.
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