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mass loss is inexorably linked to evolution for high - mass stars . in fact , it has a _ deterministic _ influence , which in turn has tremendous impact on other areas of astronomy influenced by stellar feedback ( regulating star formation , galaxy evolution , chemical evolution , reionization , etc . ) . for most of their lives , massive stars above @xmath020 @xmath1 shed mass in fast winds that affect their subsequent evolution , but in post - main sequence ( post - ms ) phases the mass loss becomes critical in determining the type of resulting supernova ( sn ) . the most dramatic mass loss in post - ms evolution is during the luminous blue variable ( lbv ) phase . some of the most pressing issues in massive star research concern resolving problems with mass - loss rates and how they are incorporated into evolutionary models , as well as the treatment ( or neglect ) of close binaries in these models ( see recent reviews by @xcite ) . a majority of massive stars are in binary systems whose separation is small enough that they will exchange mass @xcite . with current uncertainties in mass loss and mass transfer , connecting massive stars to their end fates as various types of sn explosions is still very challenging . lbvs and their eruptive mass loss have emerged as the linchpin in our paradigm for the evolution of single massive stars . in light of recent estimates of lower mass - loss rates for clumpy winds , eruptive mass loss of lbvs would need to be the primary agent responsible for turning most h - rich o stars ( above @xmath030 - 35 @xmath1 , the upper limit for red supergiants ) into h - free wolf - rayet ( wr ) stars before they die as sne  ibc @xcite . however , our understanding of the physics and evolutionary states of lbvs is still poor , and their connection to ( or overlap with ) the evolution of close binary systems leaves many open questions . when ideas about lbvs were taking shape , @xcite discussed mass - transferring binaries and how they may provide suitable explanations for lbvs , but for whatever reason , these ideas did not appear to dominate the interpretation of lbvs and so they were mostly discussed as very massive single - stars . later , @xcite mentioned difficulties in explaining the formation of the nebula around @xmath2 carinae with a single star , although that paper did not discuss evolutionary aspects of lbvs . the role of binarity and initial mass in evolution is a central question we address here from a new perspective , by studying lbv environments . lbvs were recognized early - on as the brightest blue irregular variables in nearby spiral galaxies like m31 , m33 , and ngc  2403 @xcite , originally referred to as the `` hubble - sandage variables '' . famous galactic objects like p  cygni and @xmath2 carinae had spectacular outbursts in the 17th and 19th centuries , respectively , and appeared to share many of the same properties , so they were grouped together and called `` lbvs '' by @xcite . over a dozen true lbvs are now identified in the milky way ( mw ) and the large and small magellanic clouds ( lmc / smc ) , and a similar number resides in other local group galaxies @xcite . stars that spectroscopically resemble lbvs with similar luminosity and color , but which have not ( as yet ) been observed to show the signature eruptive variability of lbvs , are often called `` lbv candidates '' ; these are presumed to be temporarily dormant lbvs . the traditional view of lbvs , which emerged in the 1980s and 1990s , is that they correspond to a very brief _ transitional _ phase of evolution of the most massive single stars , when the star moves from core h burning , through the onset of h shell burning , to the core he burning phase @xcite . the motivation for a very brief phase comes from the fact that lbvs are extremely rare compared to o stars : the duration of the lbv phase is thought to be only a few 10@xmath3 yr @xcite based on this line of reasoning . 10@xmath4 yr @xcite , but it is unclear if this longer lbv lifetime ( and shorter wr lifetime ) agrees with observed statistics . ] a typical and often quoted monotonic evolutionary scheme for a star of 60 - 100 @xmath1 initially is :    o star @xmath5 of / wnh @xmath5 lbv @xmath5 wn @xmath5 wc @xmath5 sn ibc .    in this scenario , the strong mass - loss experienced by lbvs is a crucial component of the central paradigm of massive star evolution , which is that a star s own mass loss determines its evolution . high luminosity powers strong radiatively driven winds , and this wind mass loss propels the evolution that converts the most massive h - rich main - sequence stars to become h - free wr stars . this is the co - called `` conti scenario '' @xcite . lbvs are crucial to this picture because of the recent downward revisions of o - star mass - loss rates , as noted above ( see review by @xcite and many references therein ) . thus , single massive stars of 30 - 80 @xmath1 do not have strong enough winds to make wr stars on their own ; lbv eruptions are needed @xcite . ( especially wnh stars ) , might be strong enough to make wr stars without an intervening lbv phase @xcite . in this case , the remaining he cores would be more massive than any observed h - free wr stars @xcite . such massive he stars might be rare , so this is not yet a solved issue . ] the lbv phase is still generally presumed to arise in the evolution of single stars @xcite , and it is _ required _ in the standard scenario for single massive - star evolution . the origin of the instability is not fully understood , but it is expected that the core luminosity goes up with time as the growing he core contracts , while winds steadily reduce the stellar mass . thus , the ratio @xmath6 goes up until the star confronts the classical eddington limit and somehow erupts with catastrophic mass loss @xcite . as seen in the case of @xmath2 car , these eruptions can eject 10 - 20 @xmath1 in a single event @xcite . while this traditional view of lbvs persisted through the 1990s @xcite , in the past decade or so , several problems have emerged that threaten it . one issue has to do with the interpretation of lbv variability and the behavior of their winds . the proposed interpretation was that during a standard s  dor eruption , the star brightens while staying at constant bolometric luminosity because its mass - loss rate is assumed to increase , which in turn initiates a cooler and brighter `` pseudo photosphere '' @xcite . however , quantitative spectroscopy has since revealed that mass - loss rates in s  dor outbursts do not increase enough to cause true pseudo photospheres @xcite , and the changes do not really occur at constant bolometric luminosity @xcite . instead , the brightening in an s  dor event is more akin to a pulsation or temporary inflation of the envelope , perhaps driven by the sub - surface fe opacity peak @xcite .    similarly , the traditional explanation for lbv giant eruptions was that they experience a substantial _ increase _ in their bolometric radiative luminosity ( for reasons unexplained ) that temporarily pushes them well above the classical eddington limit @xcite ; this necessarily initiates a super - eddington continuum - driven wind @xcite . however , several lines of evidence now point to an explosive mechanism driving lbv giant eruptions , instead of ( or in addition to ) a wind @xcite . moreover , spectra of the light echoes from @xmath2 car s 19th century great eruption seem inconsistent with a wind pseudo - photosphere @xcite . these considerations point to problems with our interpretation of the physics behind lbv eruptions , but even more serious problems have arisen with the traditional role played by lbvs in evolution . the first clear sign of something very fishy came from the recognition that eruptive lbvs are the most likely progenitors of a particular class of luminous narrow - lined sne with type  iin spectra ( sne  iin ) . the dense h - rich circumstellar material ( csm ) of some sne iin requires eruptive pre - sn mass loss akin to lbvs , and in some cases very large csm mass budgets ( see review by @xcite ) . other aspects of the csm around sne  iin point to wind variability reminiscent of lbvs @xcite . moreover , we now have four direct detections of lbv - like progenitors of sne  iin , including sn  1961v , sn  2005gl , sn  2010jl , and sn  2009ip ( @xcite ; see review by @xcite ) . in the traditional paradigm of single - star evolution , massive lbvs are prohibited from exploding as sne  iin , because lbvs represent merely a brief intermediate phase after core - h burning and before core - he burning . they should still have 0.5 - 1 myr to live as a wr star , and should die without their h envelopes . another clue that something was seriously amiss came from `` sn impostors '' , which are thought to be lbv giant eruptions associated with very massive stars in external galaxies @xcite . for some nearby sn impostors , detections of their relatively low - luminosity dust - enshrouded progenitors , as well as the ages of surrounding stars , suggest either progenitors that are 10 - 15 @xmath1 bsgs / rsgs or even @xmath08 @xmath1 super - agb stars ( e.g. , @xcite ) . this is much lower than the initial masses attributed to eruptive lbvs , which are generally above 25 @xmath1 @xcite . the reason this is physically significant is because these lower - mass stars ( @xmath08 @xmath1 ) never approach the eddington limit in their normal evolution , but they appear to suffer lbv - like eruptions anyway . if they can do this , then the physical cause of the outbursts might not be related exclusively to high initial mass , but perhaps some other exotic mechanism ( collisions , mergers , etc . ) . additionally , anderson and collaborators @xcite examined the spatial correlation between various types of extragalactic sne or sn impostors , as compared to presumed indicators of youth such as h@xmath7 emission in their host galaxies . on the one hand , they found the expected result that sne  ii - p ( arising from 8 - 20 @xmath1 stars ; @xcite ) are less correlated with h@xmath7 than stripped - envelope sne . however , they also found the very surprising result that _ sn impostors and sne  iin are apparently even less correlated with star formation than sne ii - p_. this was curious , and motivated us to take another look at lbvs in the mw and lmc / smc . in this paper , we will argue that the likely solution to many of these problems with lbvs resides in binary evolution , and that the resulting sn kicks may be quite important in explaining their isolation . the idea that binary interaction influences the outcomes of stellar evolution is not a new one @xcite , and various aspects including sn kicks have been studied in theoretical work by several groups @xcite . uncertainty persists in the difficult problems of mass transfer and the subsequent evolution of the mass gainer . the way that lbvs may fit into this picture is disussed in more detail later in the paper . we begin with a discussion of the projected locations on the sky of classic galactic and magellanic cloud lbvs as compared to the locations of o - type stars (  2 ) . this includes a qualitative examination of the sites of individual lbvs , as well as a quantitative statistical comparison of the relative isolation of early , mid , and late - type o stars , wr stars , lbvs , b[e ] supergiants , and rsgs . for this statistical comparison , we only consider stars in the lmc and smc , due to large distance and reddening uncertainties in the milky way . in 3 we discuss how these results are fundamentally incompatible with the standard monotonic evolutionary view of lbvs as the descendants of very massive single stars , and in  4 we argue for an alternative view of bifurcated evolutionary paths for massive stars ( where the `` bifurcation '' corresponds to mass donors and mass gainers ) . in  5 we discuss the environments of extragalactic sn impostors and sne . we end (  6 ) with an epilogue about why something so simple as the avoidance of ob - star clusters by lbvs was not emphasized before .    , but for the two confirmed lbvs in the smc.,width=268 ] in figures  [ fig : mw ] , [ fig : lmc ] , and [ fig : smc ] we present maps of o - type stars in the surrounding environments for several individual cases of well - studied lbvs . the positions of o - type stars are taken from the simbad databse . this depends on the spectral type listed in simbad , which in some cases contains errors . as a check , we also made similar plots using the very recently revised galactic o - star catalog @xcite . using this newer catalog ( not shown ) , some spectral types changed slightly ( for example , some b0 stars changed to late o - types and vice versa , and some o stars shifted from our early - type bin to late - type or vice versa ) , but this did not change the overall result described below where lbvs generally avoid clusters of o stars . it did not , for example , produce o - star clusters where there were none previously . for galactic lbvs we plot a projected size on the sky of @xmath83.8 deg , and for stars in the magellanic clouds we plot @xmath80.3 deg . in each case this represents ( very roughly ) the surrounding several 10@xmath9 pc of each lbv . this is the 2d location projected on the sky , which gives the lbvs the `` benefit of the doubt '' that they are actually at the same distance as the o - type stars seen near them on the sky . we did not attempt to correct for distances along the line - of - sight to further cull the sample of o - type stars , because that may introduce a bias . for example , in many cases , such as milky way lbvs whose distances are based on radial velocities , the distance adopted might be substantially wrong if the star has a peculiar velocity . since our main finding is that lbvs are relatively isolated , treating all stars projected on the sky as being at the same distance is conservative . [ cols="<,<,^,^,^,^,^ " , ]     notes : + p1 = ks p - value for separations to the nearest o star . + p2 = ks p - value for separations to the second - nearest o star . +    [ tab : ks ]      the large fraction of lbvs that appear isolated should sound alarm bells for anyone familiar with the traditional view of lbvs as `` massive stars in transition '' , just having finished core - h burning and soon moving on to he burning as wr stars . upon comparing positions of o - type stars to the coordinates of known lbvs , the result is astonishing . basically , _ lbvs systematically avoid clusters of o - type stars , and they are rarely associated with o - type stars of similar ( presumed ) initial mass_. this is the opposite of what is expected in the standard paradigm of massive star evolution where lbvs are the immediate descendants of the most massive o stars , and the predecessors of luminous wr stars . [ fig : cum ] and [ fig : cum2 ] indicate that the vast majority of o - type stars are indeed in highly clustered environments . wr stars are significantly more dispersed , consistent with their more evolved status . lbvs should be intermediate between o stars and wr stars , but in fact they are more isolated than wr stars . this requires that they are older , on average , than wr stars , which in turn implies that they arise either from lower initial masses ( longer core - h burning lifetimes ) , that they are a more advanced evolutionary state than wr stars ( improbable based on chemical composition ) , or that some mechanism has systematically removed them from clusters without disturbing other massive stars . initial masses implied by lbv environments differ considerably from other inferences about their likely initial masses based primarily on turnoff masses and ages of nearby stars . ( actually , there have been very few studies of the ages and masses of lbvs based on their environments , perhaps reflecting the fact that many of them are not in clusters where this is usually done . ) @xmath2 car is located in the tr16 cluster . tr16 is inferred to have an age of @xmath03 myr , which is , however , based largely on the fact that @xmath2 car itself has not yet gone sn . excluding @xmath2 car , tr16 has a turnoff mass of roughly 80 - 100 @xmath1 , which is significantly below the presumed initial mass of @xmath2 car itself  although perhaps consistent enough that it does not immediately raise questions , due to the similar main - sequence lifetimes of very massive stars at @xmath10100 @xmath1 . based on studying ob associations in the lmc / smc and adopting characteristic turnoff masses for the earliest o - type stars in each , @xcite inferred initial masses of lbvs and wr stars . they favored an initial mass of @xmath1185 @xmath1 for the two lbvs in their study ( s  dor and r85 ) , and high initial masses for wr stars as well ( @xmath1170 @xmath1 in the smc , and 30 - 60 @xmath1 in the lmc ) . there are , however , two key effects that compromise this method of determining initial masses : ( 1 ) it requires that the cluster or association is coeval , but many of the regions studied have large age spreads compared to the lifetimes of very massive stars . for example , the association lh41 in which s  dor and r85 reside has a very large age spread ; while its most luminous members do suggest a turnoff mass around 85 @xmath1 by comparison to single - star evolutionary tracks , this same association also contains cool supergiants with initial masses of only 10 - 15 @xmath1 @xcite . ( 2 ) this method ignores important effects of binary evolution that corrupt the inferred turnoff mass ( see , e.g. , @xcite ) . both binary mass transfer and binary mergers will produce luminous stars that repopulate the upper main sequence ( e.g. , @xcite ) . when compared to isochrones of single - star evolutionary models , these massive blue stragglers will imply an age younger than the true age of the cluster , and will overestimate the main - sequence turnoff mass . convolved with an age spread due to star formation that may be comparable to teh lifetimes of massive stars , this makes it difficult to use cluster turnoffs to estimate the initial masses of the most luminous evolved stars . looking at the spatial relationship between lbvs and other massive stars gives an independent and quantitative check on relative ages that does not have the same pitfalls as cluster turnoffs . if we assume that the vast majority of massive stars form in clusters ( indeed they do , as o - type stars are observed to be highly clustered in figure  [ fig : cum ] ) , then as the most massive stars explode early and the cluster spreads out due to random motions , the distribution of distances to a nearest neighboring o - type star must slowly and steadily increase with age . this method has no reliance on uncertain input physics of stellar evolution models or their neglect of binaries . it does not give a precise absolute age , but it does provide a reliable measure of the _ relative _ ages of different populations of massive stars . this method produces a very different result from cluster turnoffs @xcite . the relative isolation of lbvs is apparent in the mw as well as the lmc / smc , although only the lmc / smc produce a meaningful statistical analysis due to distance and reddening uncertainties in the mw . if there were only a few isolated lbvs , it would nt be so troubling because we might expect a few cases where the lbv was initially the most massive star in a small cluster that contains no other o - type stars . indeed , a few percent of o stars ( mostly late o - type stars ) appear to have been born in relative isolation ( e.g. , fig . [ fig : cum ] and @xcite )  but @xmath090% of early o - type stars _ are _ in clusters . lbvs are the opposite : only rare exceptions are found in clusters ( @xmath2 car and r127 ) and most of them are either in loose associations with large age spreads , or they are found 10s or even 100s of pc from other o - type stars . this isolation requires that lbvs are either very massive stars that for some reason were preferentially born well outside clusters ( hard to justify physically ) , or that they have lived longer than we expect based on single - star evolution and have systematically moved away from their birth environments . this can not be reconciled with the traditional view of lbvs in single - star evolution . as noted in the introduction , the current standard paradigm of massive - star evolution is that single massive o - type stars evolve into h - deficient wr stars by virtue of their own radiation - driven mass loss . for all but the most luminous and most massive stars ( i.e. @xmath10100 @xmath1 ) that pass through a strong - winded wnh phase , accounting for clumping effects indicates that line - driven stellar winds are too weak to accomplish this ( see @xcite and references therein ) . thus , if lbvs are presumed to fit into the evolutionary sequence of single stars :    o star @xmath5 of / wnh @xmath5 lbv @xmath5 wn @xmath5 wc @xmath5 sn ibc ,    then enhanced eruptive lbv mass loss would be essential in order for single massive o - type stars to become wr stars @xcite . in this scenario , the lbv phase is a very brief , fleeting transitional phase ( see , e.g. , @xcite ) . in evolutionary models , there is little time ( @xmath010@xmath4 yr ) after the end of core - h burning and before the onset of the wr phase when a star can be an lbv . in this short time interval , stars can not move very far , and so lbvs are expected to have a spatial distribution on the sky that is very similar to the o stars that are supposedly their immediate progenitors . for a typical velocity dispersion in a cluster of a few km s@xmath12 , a single star will move less than @xmath010 pc in 3 myr , and much less than 1 pc in 10@xmath13 yr . therefore , if early o - type stars are mostly in clusters ( they are ; see figs . [ fig : cum ] and [ fig : cum2 ] ) , then their immediate single - star descendants must be as well . in fig . [ fig : cum ] , the cumulative distribution of lbv separations should therefore be in between o - type stars and wn stars , and lbvs should be much more clustered than wc stars . in this paper we have shown that the opposite is true  that lbvs are surprisingly isolated from o stars . most critically , observations indicate that in terms of their separation from o - type stars , _ lbvs are even more isolated than wr stars ( both wn and wc ) . _    the observed population of lbvs therefore can not evolve into the observed population of wr stars , because lbvs would need to turn around and systematically move back toward clusters and associations in order to match the locations of wr stars . this can not be . the standard view of lbvs as single `` massive stars in transition '' is incompatible with observations of their locations , and must no longer be considered as a viable evolutionary scenario for the majority of massive stars . this rules out the monotonic evolutionary paradigm outlined above , making it difficult for single massive stars to become h - poor wr stars and sne  ibc ( because without lbv mass loss , their winds are nt strong enough ) . most often , the lbv phase must play a different role . a straightforward alternative is discussed next . the fact that lbvs are more isolated than wr stars , plus the fact that some stars appear to remain in an lbv phase until death as sne  iin ( see @xcite for a review ) , suggests an entirely different picture . instead of the monotonic evolutionary scheme for single stars discussed above , the spatial distributions discussed in this paper suggest that massive star populations and especially lbvs are dominated by bifurcated evolutionary trajectories :    o star @xmath5 @xmath14 }    \rightarrow { \rm sniin } & { \rm ( gainer ) } \end{cases}$ ] .    in this scenario , o - type stars evolve off the main sequence , and through binary interaction the majority of massive stars either ( 1 ) lose their h envelope through mass transfer to a companion , become wr stars and die as stripped - envelope sne , or ( 2 ) gain mass from a companion , become bsgs , sgb[e]s , and lbvs , and then retain their h envelopes until they die as sne  iin . after the mass transfer phase , the subsequent evolution of the mass gainer may be quite varied , and so not all mass gainers will necessarily be lbvs and sne iin . in an insightful early paper , @xcite discussed the fact that a few of the hubble - sandage variables in m31 appeared relatively isolated , and they suggested a similar binary mass - transfer scenario for these , while still favoring a single - star evolutionary scheme for most lbvs . here , we show that the environments of _ most _ lbvs violate the single - star picture , not just a few exceptions . instead of lbvs being a brief transitional phase for all very massive stars , they become the dominant late evolutionary phase for a subset of massive binaries . lbvs are rare enough that the fraction of stars which do this and for how long they remain in the lbv phase is poorly constrained observationally . ( note that when the oversimplified , monotonic evolutionary scheme is abandoned , the number ratio of lbvs to o - type stars offers no meaningful constraint on the lbv duration without additional information . ) [ fig : hrd ] shows the locations of lbvs on the hr diagram , mostly taken from @xcite , except as noted in the caption . the locations of lbvs are compared to standard evolutionary tracks for single stars , and also to an example of an evolutionary track for an interacting binary system from @xcite . using single - star evolution tracks as a reference , the initial masses of lbvs would appear to range from 30 - 40 @xmath1 for the lower - l lbvs , and from 50 - 250 @xmath1 or more for the classical high - luminosity lbvs . we infer from figure  [ fig : hrd ] are approximate . values of the luminosity depend on the analysis technique used for each star and the distance assumed , and there are multiple values available in the literature for several lbvs ( see , e.g. , @xcite ) . ] a main point of our paper is that the isolated environments of lbvs are incompatible with the short lifetimes experienced by stars with such high initial mass , and they are incompatible with the key idea that most lbvs continue their evolution to wr stars .    instead , ascribing lbvs as the mass gainers in interacting binaries provides an attractive and plausible alternative . as noted earlier , the plausibility of this basic idea was mentioned long ago @xcite , but did not become the dominant view . through the process of rlof , the mass gainer can substantially increase its mass and luminosity to resemble a star that had an initially much higher mass . one example is shown in fig .  [ fig : hrd ] , where a star with an initial mass of 14 @xmath1 accretes mass in a binary and ends up with a luminosity commensurate with a @xmath026 @xmath1 star @xcite . it is easy to see that such an evolutionary path could , in principle , give rise to the low - luminosity group of lbvs . similarly , somewhat more massive stars in binaries ( initial masses of 30 - 40 @xmath1 ) , could accrete mass to appear as 60 - 80 @xmath1 classical lbvs . one can imagine higher mass analogs of this that could correspond to the classical lbvs like ag  car and r127 . a crucial point is that this mass accretion and corresponding increase in luminosity may occur after a long delay , after both stars in the binary system have lived through much or all of their h - core burning main - sequence lifetimes . as initially lower mass stars , their core - h burning lifetimes may be much longer than the main - sequence lifetime expected for the more massive star that they become . as such , a mass gainer may appear younger than the population of stars around it . in cases where the discrepancy between the initial mass and @xmath15 is close to a factor of 2 , the lifetime can be doubled for moderately massive stars ( 15 - 30 @xmath1 ) . for example , the approximate lifetimes of single stars of 15 and 30 @xmath1 initial mass are about 14 and 6.5 myr , respectively @xcite . if lbvs fit this role , then they are essentially evolved massive blue stragglers , not massive stars in transition . lbvs originating this way may help solve a number of issues that have been problematic for a single - star scenario :    \1 . as noted above , the mass gainer will have a longer age than expected for its current mass and luminosity , helping to rectify the isolation of lbvs discussed in this paper and the initial mass discrepancy . in addition to becoming more massive and more luminous , the mass gainer will also gain angular momentum during rlof . this may cause lbvs to appear as rapid rotators late in life . a number of lbvs do exhibit properties consistent with rapid rotation @xcite . moreover , rotation has been inferred to be quite important in some ideas about lbv instability @xcite . in a single - star scenario , it is difficult to understand how they can shed large amounts of mass in steady winds while retaining their angular momentum . single - star models generally predict negligible rotational speeds for the lbv phase @xcite . this scenario also implies that a large fraction of the fast runaway stars that are kicked out of clusters by their companion s sn will be these rapidly rotating mass gainers ( i.e. there must be a fair number of them to account for the observed distribution of lbvs ) . it is not clear yet if this is the case , but further study of this may provide an important test rlof or other binary interaction may produce very asymmetric csm , as seen around many lbvs with bipolar or elliptical nebulae ( @xmath2 car , ag  car , hr  car , hd  168625 , etc . ) . using spectropolarimetry , equatorial distributions of csm around sne  iin have also been inferred @xcite . some of the more extreme observed asymmetries in the csm may be difficult to achieve with single stars . \4 . removing the restriction that lbvs must transition into h - poor wr stars before they die , mass gainers might retain some of their h envelopes until death . this would reconcile the problem of lbvs exploding as sne  iin . the mass gained from a more evolved companion may be significantly enriched in nitrogen . this by itself does not necessarily argue against a single - star scenario if efficient mixing can bring n to the star s surface @xcite , but the n - enriched nebulae around lbvs are consistent with n enrichment in a binary mass - gainer scenario . similarly , the accretion of enriched material from a companion could help explain why lbv relative h / he abundances are similar to those of wn stars with h ( actually between those of very luminous wnh stars and other wn stars with h ; @xcite ) , even though their locations on the sky are very different . when their stripped - envelope companion explodes as a sn  ibc or iib , the mass - gainer star is likely to receive a kick . this provides an attractive explanation for why lbvs would preferentially and systematically seem to avoid star clusters , and adds to their longer lifetimes in explaining their observed isolation . even if they are born in clusters and live in associations for several myr , they may get kicked out of the cluster . because the initially more massive star will usually explode first , the kicked mass gainer may have a few more myr to live after that sn event . traveling at 50 - 100 km s@xmath12 , the star can move about 50 - 100 pc in 1 myr . such motion for only 1 - 2 myr is therefore sufficient to explain the relative isolation of most of the lbvs discussed here . it is important to recognize that the mass gainer may not exhibit the lbv instability immediately upon accreting mass , but is likely to do so in its own time when it finishes core - h burning and evolves to become a supergiant . ( in the mean time , it will be a much hotter o - type star that is fainter in visual light ; thus , we would not expect bright lbvs to be seen at the positions of most sne  ibc , because it may take them another 10@xmath16 yr to evolve into their own supergiant phase . ) the delayed onset of the lbv phase may lead them to preferentially appear outside clusters . depending on how long it takes them to die , this may also lead sn impostor eruptions and the eventual sne  iin to appear isolated . even if they are a product of binary evolution , lbvs may appear as single stars if their companion has already exploded . in some cases , of course , the close companion may not have exploded yet . if they were born in triple systems , they may still be in binaries . \9 . in the mw , lmc / smc , and in other nearby galaxies , there is a large population of massive stars that resemble lbvs but that have not yet exhibited the eruptive lbv instability . these are usually called `` lbv candidates '' as noted earlier . @xcite find an order of magnitude more lbv candidates than bona fide lbvs in nearby galaxies , and massive dusty shells around massive stars in the mw suggest a similarly large number here @xcite . counting only lbvs , their small number compared to o - type stars has been used to justify a very brief transitional phase ( e.g. , @xcite ) before becoming a wr star . however , if lbv candidates are included in the count , the implied lbv lifetime becomes much longer ( see @xcite and references therein ) . in the bifurcated evolutionary scheme discussed above , the post main - sequence lifetime of the h - rich mass gainer is much longer than the lbv phase envisioned in single - star models , allowing lbv candidates to be the same stars as lbvs , if the lbv eruptive instability is only active for part of that time or in a subset of circumstances . in addition to mass accretion through rlof , stellar mergers or more exotic systems such as blue thorne - zytkow - like objects ( tzos ) might also yield some of the same results that are compatible with observed properties of lbvs ( overluminous or young compared to surrounding stars , rapid rotators , enriched and asymmetric csm , etc).10@xmath16 @xmath17 ) , rsgs do not exist . very luminous shell burning stars that would otherwise be luminous rsgs are thought to reside in the blue because of mass loss and instability in their outer envelopes . the same might apply to the envelopes of tzos in this high - luminosity range , if they exist . this is still speculative , but we mention it because blue tzos can not be ruled out for lbvs without more detailed study . ] the role of mergers in producing asymmetric csm , in particular , has been discussed extensively ( see review by @xcite and references therein ) . theoretical predictions for tzos are highly uncertain , and it remains difficult to rule out the possibility that some lbvs are merger products or blue tzos , especially those that may still be in clusters or associations . however , lacking a significant kick from a companion s sn , stellar mergers do not offer a compelling explanation for why the larger population of lbvs seem to systematically avoid clusters . so , what about the fates of single stars or binaries that merge early on the main sequence ? if lbvs are exclusively or mostly a binary phenomenon , then it becomes very unclear if wr stars can arise from single stars . perhaps rsg mass loss becomes critical . uncertainties ( in most cases overestimates ) in mass loss , plus errors in models that have arisen from comparing single - star models to binary populations , prohibit existing evolution models from making unique predictions ( again , see @xcite ) . with all known stellar mass black holes in binaries , there are few observational constraints on the end fates ( back hole or neutron star ; successful exoplosion or quiet collapse ) for truly single high - mass stars . a key conclusion from lbv environments as compared to other types of stars is that while lbvs are indeed very massive stars based on their current high luminosity , they did nt necessarily begin their lives that way . although this paper advocates a different origin for lbvs than is usually assumed , much of the traditionally discussed phenomena associated with lbvs may still apply . they are still stars with extreme mass loss and instability that have eruptive super - eddington winds or explosive mass loss ( see , e.g. , @xcite ) . what is very different in the evolutionary scheme advocated above is the path by which the stars have come to be in their current state . hence , their initial masses and ages are very different than previously assumed . lbvs have been discussed as a normal transitional state for the most massive stars that have evolved to be in close proximity to the classical eddington limit . instead , we argue that in most cases they are stars that have become more luminous , rapidly rotating , and unstable as a consequence of mass accretion in a binary system ( or perhaps in some cases something more exotic , such as a merger or blue tzo ) . although the scenario suggested here does not solve the overarching mystery of the physical trigger of lbv eruptions , the evolutionary path to arrive at this point may be a critical part of the puzzle ( especially where angular momentum and stellar structure are concerned ) . this remains a challenge for future theoretical work . a clear message with broader significance is that cluster turnoffs analyzed with single - star model isochrones will systematically underestimate the age and overestimate the turnoff mass of clusters and associations , because the apparent turnoffs are contaminated by massive blue stragglers . this method is especially unreliable for very massive stars , where age spreads in clusters are comparable to their absolute age . similarly , we must be mindful of sn progenitors and their `` initial mass '' , as discussed next . evidence presented in recent years has begun to shift our central paradigm of massive star evolution . on the one hand , wind mass - loss rates are lower than we used to think , while the close binary fraction among o - type stars is shown to be quite high ( roughly 2/3 are interacting , as noted in the introduction ) . the isolation of lbvs now seems to require a major shift . the interaction process of rlof or mergers has a profound impact on the types of evolved stars that are observed , and binarity must therefore also have a strong ( or dominant ) impact on the distribution of sn subtypes that mark the end fates of these massive stars  perhaps even moreso than metallicity dependent winds and initial mass , as has generally been assumed .    observed statistics of sn subtypes and other considerations already argue that binary rlof and not stellar winds must dominate the removal of the h envelope for most stripped - envelope sn progenitors @xcite , and binary population synthesis studies have suggested this for some time @xcite . there are too many sne  ibc and iib than there are stars initially massive enough to remove their h envelopes via their own winds . including sne  iib , the fraction of core - collapse sne that have stripped - envelope progenitors is @xmath036% , whereas stars that are luminous enough to shed their h envelopes through their own winds comprise only 10 - 15% of a normal sn - producing imf @xcite , or perhaps even less if some massive stars collapse to black holes without producing bright sne . interestingly , from the fraction of o stars that will interact and exchange mass , @xcite estimate that 1/3 is the approximate fraction of massive stars that we might expect to lose their h envelope in rlof . comparing this 33% to the observed value of 36% of ccsne that have stripped - envelope progenitors , there appears to be little wiggle room to allow massive single stars to die as sne  ibc . similar arguments in favor of binary progenitors have been made based on the mass and composition of the ejecta in stripped - envelope sne @xcite . a natural question arises . if binary rlof strips the h envelope to make most sne  iib , ib , and ic , then what happens to the mass - gainer companions of these stripped - envelope stars ? what do those mass - gainer stars look like , and which sne do they yield ? in a theoretical study , @xcite assumed that the spun - up mass gainers would experience enhanced rotational mixing and would undergo quasi - homogeneous chemical evolution , suggesting that they would die as h - free wr stars and possibly grbs . @xcite therefore inferred that there may be a population of relatively isolated wr stars and grbs . in this paper we have argued something different : that lbvs might be the direct products of mass transfer in binaries , and in particular , the mass gainers . ( if this is correct , it may suggest that quasi - homogeneous chemical evolution may not be appropriate for these stars , but a further discussion is beyond the scope of this paper . ) we have discussed above that this binary scenario alleviates the apparent paradox of lbvs exploding as sne  iin , which is prohibited in the standard single - star evolution framework ( although see @xcite for a possible caveat ) . if lbvs do indeed explode as sne  iin , then we should expect the relative isolation of lbvs discussed herein to be reflected in their resulting sne as well . as noted above , if these stars receive a kick when a companion in the binary system explodes , they may travel 100 pc or more from their birthsites by the time they die . they should die at locations that are preferentially outside clusters , causing them to systematically avoid bright h  ii regions or blue star clusters . even without a kick , the extended lifetime of an lbv ( because its true initial mass was much lower than its apparent @xmath15 ) would allow it to live much longer then the h  ii region that it might have been born in , since the massive stars that ionized the region may have long since died away . evidence consistent with this from observations of sn host environments has already been published in the literature , although the results were interpreted somewhat differently . as noted in the introduction , anderson and collaborators @xcite have found that sn  iin show a weaker association with h@xmath7 emission in host galaxies , as compared to sne  ibc , and their h@xmath7 association is more like that of sne  ii - p . those authors interpreted these results in a context where initial mass dominates sn types ; they attributed the correlation of sne  ibc with h@xmath7 to indicate the highest range of initial mass , and they interpreted the lack of correlation between sne  iin and h@xmath7 as indicating lower initial masses . they therefore concluded that lbvs are a less likely progenitor channel for sne  iin .    as we noted earlier , this result motivated us to examine the environments of nearby lbvs . we found that despite their high luminosities and initial masses higher than those of most rsgs , lbvs do actually appear to be very isolated from ob star clusters ( moreso than wr stars , and almost as isolated as rsgs ) . we have argued that this is most likely due to receiving a kick from a companion s sn that preferentially sends the mass gainers flying out of clusters . binary evolution will have an impact not only on the stars that are kicked ( in this case the lbvs and resulting sne  iin ) , but also on the mass donors that explode first . they ( in this case sne  ibc or iib ) will occur preferentially in young clusters as compared to other sn types that are found anywhere , and will be anticorrelated with those that are preferentially kicked out of clusters . thus , assuming that both sne  ibc and sne  iin come from a wide range of overlapping initial masses appears to be consistent with the data . because of the important influence of binary evolution , interpreting sn host invironments only in terms of progenitor initial mass is overly simple . some stars and certain types of sne can selectively move from their birthsites ; a theoretical study of the delay - time distributions and correlations ( or not ) with h  ii regions is needed to quantify this effect .    moreover , there are reasons why a correlation with h@xmath7 as adopted by anderson and collaborators is not necessarily a reliable indicator of extreme youth and the highest intial masses . that interpretation was criticized by @xcite , who pointed out that the brightest ( and therefore the most easily detected h  ii regions in distant galaxies ) are giant h  ii regions that are actually quite long lived ( @xmath020 myr ) , with ages comparable to the ages of stars with initial masses of only @xmath012  @xmath1 . anderson and collaborators have based their metric of association with h@xmath7 , and hence youth , as flux - depenent , so that a sn landing on a bright source of h@xmath7 is interpreted as younger than a sn landing on pixels with fainter h@xmath7 . in some sense this is backwards  the brightest h  ii region complexes are the longest lived ones with median ages @xmath1110 myr , whereas more isolated , smaller , and fainter h  ii regions are fleeting , and have shorter ages of only @xmath03 - 4 myr . therefore , finding a sn in a smaller isolated h  ii region is more likely to indicate a very high initial mass above 60 @xmath1 than if a sn is seen in a very bright and long - lived giant h  ii region complex @xcite . when these fainter and more isolated h  ii regions go preferentially undetected in ground - based imaging of distant galaxies , their associated sne may be mistakenly assumed to come from an older population . note that sn  1987a , with an initial mass of @xmath018 @xmath1 would have a high correlation with star formation as seen from a distant galaxy because of its proximity to 30 dor ( only @xmath01 at a distance of 50 mpc ) , whereas @xmath2 carinae would explode in the fainter carina nebula , which in some cases would not be detected . anderson et al .  have discussed potential biases in detecting various sn types and found that it does not strongly impact the trends in their observations , but there may be other selection effects besides those that govern the discovery of various sn types . for example , a metallicity bias may cause there to be more of some type of sn in the inner regions of a galaxy as compared to outer regions . in a typical spiral galaxy , inner regions are more densely packed with star formation , and they have a larger fraction of the observed surface area covered by very bright h  ii regions than in the much sparser outer regions . indeed , @xcite found that sne  ibc in their sample were systematically found at smaller galactic radii than sne  ii . thus , even if sn types have the same range of progenitor initial mass , a type of sn that has a preference for smaller galactocentric radii because of metallicity may have a higher probably of coincidence with a very bright h  ii region . the potential influence of this bias deserves further attention . the sensitivity and potential bias in the flux - dependent h@xmath7 diagnostic used by anderson and collaborators @xcite may explain why their conclusions differed from other studies of sn host environments . earlier studies of spatial proximity to h  ii regions ( rather than the strength of h@xmath7 flux ) found no statistically significant preference between sne  ii and ibc @xcite . some of these might be chance coincidence with long - lived giant h  ii regions at relatively course ground - based angular resolution . in general , one expects no direct correlation between a sn and its own h  ii region for any but the most massive stars above 75  @xmath1 @xcite . indeed , @xcite found that high resolution _ images for nearby sne with good constraints on the progenitor stars do not support a correlation with h  ii regions , and @xcite argued against the monotonic increasing progenitor mass from type  ii to iib to ibc based on this and a number of other considerations ( see also @xcite ) . when examining the sn host color at the location of sne rather than host h@xmath7 , @xcite found that sne  ibc , sne  ii , and sne  iin show basically the same distribution of @xmath18 colors . they did , however , find bluer colors for broad - lined sne  ic and sne  iib ; it is unlikely that this is due to initial mass , since progenitor detections suggest rather low masses for sne  iib . one may ask , if lbvs are the mass gainers that get a kick from their companion s sn  ibc in a cluster or association , then why do we not detect bright lbvs at the positions of sne  ibc ? the most likely answer is that the mass gainer will appear as a more luminous but hot o - type star that is still quite difficult to detect ( see figure  [ fig : hrd ] ) , and may not become a visually bright lbv until 1 - 2 myr later , when its own core evolution drives it off the main sequence to become a cooler b[e]/lbv supergiant . moreover , in the binary scenario , the majority of sne  ibc will be from relatively low initial masses , compared to the high - mass wr progenitors envisioned in the single - star scenario . overall , we find it likely that the sequential order of explosion in a binary system and subsequent kicks out of clusters ( and to a somewhat lesser extent , metallicity and star cluster density ) , plus the extended main sequence lifetime before mass accretion , will have a more important influence on the observed environments of core - collapse sne than monotonic differences in the range of initial mass . this may be especially true for more extreme or rare explosions that may require special evolutionary paths in binaries , such as long grbs @xcite . of course , the observed isolation of sne  iin might also be influenced to some degree by sn  iin progenitor contamination from non - lbv objects . indeed , there is strong evidence for this already , although the fraction of contamination is poorly constrained . in principle , any type of explosion can yield a type  iin event , since the iin designation depends on csm interaction and not the explosion mechanism . while lbv mass loss fits the bill @xcite , the case for lbvs is strongest for the most luminous sne iin where the csm mass budget requires extreme parameters @xcite . b[e ] supergiants are also suitable for providing moderate - luminosity sne  iin , and their spatial distribution is even more isolated than lbvs , comparable to rsgs ( figures [ fig : cum ] and [ fig : cum2 ] ) . moreover , the concentration of eruptive mass loss associated with the final nuclear burning sequences in the last few years before core collapse @xcite means that the progenitor need not have been in a dense - wind phase for very long . besides massive lbvs exploding as sne  iin , there are also examples where sne  iin may result from : ( 1 ) extreme rsgs with strong clumpy winds @xcite , ( 2 ) electron - capture sne ( ecsne ) from 8 - 10 @xmath1 super - agb stars ( such as the class of sne  iin - p and possibly the crab nebula ; @xcite ) , and ( 3 ) some hybrid type ia / iin events that are apparently thermonuclear sne  ia exploding in a dense h - rich csm , like sn  2002ic and several similar objects @xcite . when these other objects having much lower mass progenitors are included with sne  iin , it would make the mixed population appear statistically even more isolated . this is the likely explanation for why @xcite found that sne  iin had a spatial distribution similar to sne  ii - p , even though we find lbvs have isolation intermediate between wr stars and rsgs . it is worth reiterating the point that not all mass gainers will become lbvs , since this likely depends on the mass accreted and final luminosity ( many may become b[e ] supergiants instead , or they may evolve to become rsgs , etc . ) , and not all mass gainers will necessarily become sne  iin , since these sne require special conditions ( pre - sn mass ejections ) . comparing the observed fraction of sne iin ( 89% of core collapse sne ; @xcite ) to theoretical expectations requires continued modeling of the evolution of mass gainers , which still depends on uncertain assumptions . in addition to core - collapse sne , caution about inferring an initial mass also applies to sn impostors and related transients . some eruptive transients have been discussed where the apparent age of surrounding stellar population indicates a surprisingly low initial mass . the most representative cases are sn  2008s and the 2008 transient in ngc  300 @xcite . if lbvs or lower - mass analogs preferentially receive a kick from an exploded companion , then this needs to be included in the uncertainty when inferring an age and initial mass from their surrounding stellar populations . one last important consequence to mention is that stars with initial masses below 8 @xmath1 may , if in a binary system , accrete mass and explode as a ccsn or an electron - capture sn , even if their initial mass would have precluded that fate in a single - star scenario . due to the slope of the imf , these initially lower - mass stars may potentially contribute a substantial number of observed sne . we encourage a detailed study of this effect , including the delay time distribution and its contribution to sn statistics . this paper suggests a major reversal in our interpretation of the nature of lbvs . instead of being the result of instability in very massive single stars that have evolved to become unstable through their own core evolution and mass loss , they must preferentially or exclusively be the products of binary evolution . they are most likely the mass gainers in binary rlof or mergers because their ages and initial masses ( inferred from single - star evolution tracks ) systematically disgree with the stars around them . while counter arguments can point to one or two possible exceptions , this must apply to most of the lbvs . why was this seemingly obvious result missed ? the answer may be partly sociological : lbvs are very few in number , and @xmath2 car garners disproportionate attention , so its location in the carina nebula did not arouse suspicion . also , interest in lbvs has concentrated more on driving their eruptive mass loss , as opposed to the evolutionary history that led them to be unstable in the first place . last , lbvs seem to fit nicely into the picture of single - star evolution ( and indeed they are needed for it ) , so the question of whether or not their environments contradict models was not pursued with adequate vigor .    actually , a suggestive result along similar lines was found more than a decade ago by n.l . king and collaborators ( 2000 ; see also @xcite , as well as @xcite ) . based on environments of some candidate lbvs in m31 , it was found that they seem to reside outside ob associations and clusters , implying stellar ages of several myr . that work did not instigate a major rethinking of our standard paradigm of massive star evolution because the paper was not accepted for publication in the refereed literature ( the first author has since left the field ) . in hindsight , the main result of that work appears to have pointed in the correct direction after all . ns thanks j.s . gallagher for discussions regarding binary evolution and lbvs , and in particular for drawing our attention to previous work by n.  king . we benefitted from discussion about sn host environments with p.  crowther , j.  anderson , and p.  kelly , and from further discussions concerning binary evolution with s.  de mink . we also thank the referee ( j. eldridge ) for a careful reading and thoughtful comments that improved the paper . partial support was provided by nsf award ast-1312221 . this research has made use of the simbad database , operated at cds , strasbourg , france . smith n , li w , silverman jm , ganeshalingam m , filippenko av . 2011a , mnras , 415 , 773 smith n , li w , filippenko av , chornock r. 2011b , mnras , 412 , 1522 smith n , li w , miller aa , silverman jm , filippenko av , cuillandre jc , cooper mc , matheson t , van dyk sd . 2011c , apj , 732 , 63
based on their relatively isolated environments , we argue that luminous blue variables ( lbvs ) must be primarily the product of binary evolution , challenging the traditional single - star view wherein lbvs mark a brief transition between massive o - type stars and wolf - rayet ( wr ) stars . 
 if the latter were true , then lbvs should be concentrated in young massive clusters like early o - type stars . 
 this is decidedly not the case . 
 examining locations of lbvs in our galaxy and the magellanic clouds reveals that , with only a few exceptions , lbvs systematically avoid clusters of o - type stars . in the large magellanic cloud , 
 lbvs are statistically much more isolated than o - type stars , and ( perhaps most surprisingly ) even more isolated than wr stars . 
 this makes it impossible for lbvs to be single `` massive stars in transition '' to wr stars . 
 instead , we propose that massive stars and supernova ( sn ) subtypes are dominated by bifurcated evolutionary paths in interacting binaries , wherein most wr stars and sne  ibc correspond to the mass donors , while lbvs ( and their lower - mass analogs like b[e ] supergiants , which are even more isolated ) are the mass gainers . in this view , lbvs are evolved massive blue stragglers . through binary mass transfer , 
 rejuvinated mass gainers get enriched , spun up , and sometimes kicked far from their clustered birthsites by their companion s sn . 
 this scenario agrees better with lbvs exploding as type  iin sne in isolation , and it predicts that many massive runaway stars may be rapid rotators . 
 mergers or blue thorne - zykow - like objects might also give rise to lbvs , but these scenarios may have a harder time explaining why lbvs avoid clusters . 
 [ firstpage ]    binaries : general  stars : evolution  stars : winds , outflows