Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars

“Since massive stars have short lifetimes, their ultraviolet (UV) radiation (reprocessed in various forms) is the most observable tracer of star formation and is used to calculate the star formation rates (Kennicutt 1998, Kennicutt & Evans 2012).”

“Perhaps new open-source codes like MESA (Paxton et al. 2011) will facilitate this.”

“This single-star paradigm then permitted one to extend even further, to compute models for the collective radiative output of entire stellar populations as a function of age (e.g., codes like starburst99; Leitherer et al. 1999).”

“Later, estimates for the solar wind (Parker 1958) powered by hot gas pressure had such low mass-loss rates that winds would not matter much in stellar evolution.”

“Since massive stars have short lifetimes, their ultraviolet (UV) radiation (reprocessed in various forms) is the most observable tracer of star formation and is used to calculate the star formation rates (Kennicutt 1998, Kennicutt & Evans 2012).”

“They leave behind exotic corpses such as black holes, neutron stars, pulsars, magnetars, and all the bizarre high-energy phenomena that occur when compact objects remain bound in a binary system (e.g., Remillard & McClintock 2006).”

“The fact that GRB progenitors have shed most of their envelopes while still retaining a great deal of angular momentum (MacFadyen & Woosley 1999) would seem to strongly favor their origin in binary evolution with mass transfer to spin up the star, or even a merger.”

“…very massive, and they made the first SNe, the first stellar mass black holes, the first dust, ejected the first metals back into the ISM, enhanced or dominated reionization, and probably triggered the formation of the earliest generations of low-mass stars (Bromm & Larson 2004, Heger et al. 2003).”

“…from several young clusters suggest that, when corrected for obervational bias, the fraction of massive stars in binary systems whose orbital period is so short that the stars must exhange mass or merge is something like 3/4 (Kiminki & Kobulnicky 2012, Kobulnicky & Fryer 2007, Sana et al. 2012).”

“Through their violent deaths as SNe, massive stars provide brilliant displays that permit us to dissect individual stellar interiors at distances of many Mpc while the star’s inner layers are peeled away for us to see (Filippenko 1997).”

“This is for physical parameters from model ZAMS stars from Ekström et al. (2012); note that the Vink et al. recipe depends on L, eff , M , and v∞, whereas the de Jager prescription uses only L and Teff .”

“Gamma ray bursts (GRBs; Gehrels et al. 2009, Woosley & Bloom 2006) and SNe also provide a probe of very distant populations and stellar evolution in early environments, but this requires us to understand the connection between stars of various types, their mass loss, and the eventual type of SN…”

“Inferences about the initial mass function (IMF; Bastian et al. 2010) and variation of the star-formation rate through cosmic time (Hopkins & Beacom 2006, Madau et al. 1998) hinge upon converting reprocessed massive star UV luminosity to a collective star-formation rate.”

“In the wind of a hot star, momentum is transfered from the outwardly propagating radiation to the gas through absorption and scattering by UV metal lines (Castor et al. 1975, Lucy & Solomon 1970).”

“Inferences about the initial mass function (IMF; Bastian et al. 2010) and variation of the star-formation rate through cosmic time (Hopkins & Beacom 2006, Madau et al. 1998) hinge upon converting reprocessed massive star UV luminosity to a collective star-formation rate.”

“In fact, there are currently 3 direct progenitor detections for SNe IIb (SN 1993J, 2011dh, and 2013df) that are thought to be YSGs in binary systems with inferred initial masses of 13- 17 M⊙ (Maund & Smartt 2009, Van Dyk et al. 2013a,b).”

“The first stars in Z-poor environments are expected to be very massive (Bromm & Larson 2004), and hence, the scaling of mass loss with Z affects abundances of the lowest Z stars (Beers & Christlieb 2005, Bromm & Yoshida 2011) polluted by a small number of early SNe.”

“Therefore, the dominant paradigm (e.g., Paczynski 1966, 1967, 1971) was that, as in low-mass stars, binary Roche-Lobe Overflow (RLOF) played a major role in making massive stripped-envelope stars like the He-rich WR stars.”

“Inferences about the initial mass function (IMF; Bastian et al. 2010) and variation of the star-formation rate through cosmic time (Hopkins & Beacom 2006, Madau et al. 1998) hinge upon converting reprocessed massive star UV luminosity to a collective star-formation rate.”

“Feedback in the form of UV radiation, stellar winds, and supernovae (SNe) stirs interstellar gas, driving turbulence and perhaps triggering new generations of stars by sweeping gas into dense filaments (Elmegreen & Lada 1977).”

“The distance must be known, but the Magellanic Clouds and clusters with known distances can help, and GAIA will soon lower distance uncertainties for the nearest massive stars in the Milky Way.”

“In current generations of stellar evolution models (see the recent review by Langer 2012), the most commonly used prescription for the mass-loss rates of hot stars with line-driven winds is from Vink et al. (2001).”

“…Winds Traditionally, LBV giant eruptions have been discussed as super-Eddington (SE) winds driven by a sudden unexplained increase in the star’s bolometric luminosity (Humphreys & Davidson 1994; Humphreys, Davidson, & Smith 1999; Owocki et al. 2004; Shaviv 2000; Smith & Owocki 2006).”

“However, these SNe may also be explained by magnetar birth (Kasen & Bildsten 2010, Woosley 2010), so they are not necessarily the product of eruptive pre-SN mass loss.”

“Volume-limited rates of the various core-collapse SN sub-types (excluding SNe Ia) measured in a controlled sample are now available (Smith et al. 2011b); these come from the Lick Observatory SN Search, which targeted mainly large galaxies representative of ∼Z⊙ (Li et al. 2011).”

“RLOF on the MS is referred to as Case A, whereas Case B is for RLOF during H shell burning and Case C for core He burning (Langer 2012, Petrovic et al. 2005, Podsiadlowski et al. 1992).”

“A previous review by Kudritzki & Puls (2000) (KP00 hereafter) dealt with line-driven winds of hot stars, focussing on the theory of the driving mechanism as well as common scaling relations used to convert observables to rates.”

“Significant wind clumping is expected on theoretical grounds (Dessart & Owocki 2005, Feldmeier 1995, Owocki & Rybicki 1984, Owocki & Puls 1999, Owocki et al. 1988, Sundqvist & Owocki 2013), due mostly to the line-driven instability (this arises because the force of line driving is velocity…”

“…first observed event that instigated these ideas of massive CSM shell collisions (Ofek et al. 2007, Smith & McCray 2007, Smith et al. 2007, 2010b, Woosley et al. 2007), but a number of very luminous SNe IIn have been studied in detail since then, including objects like SN 2006tf (Smith et al.…”

“There were some important early indications and expectations of mass loss from hot stars (Sobolev 1960), but the mass-loss rates were very uncertain.”

“There are several diagnostics of the wind density, the most common being the strength of wind free-free emission in the IR or radio (Wright & Barlow 1975), Hα or other recombination emission lines, and the strength of blueshifted P Cygni absorption features in unsaturated UV resonance lines (see…”

“Again, see KP0 and Puls et al. (2008) for reviews concerning steady, line-driven winds from hot stars.”

“In the wind of a hot star, momentum is transfered from the outwardly propagating radiation to the gas through absorption and scattering by UV metal lines (Castor et al. 1975, Lucy & Solomon 1970).”

“This, in turn, suggests that influence of rotation and rotational mixing in current single-star models may be overstimated (de Mink et al. 2013, Vanbeveren 2009).”

“…spectroscopic binary fraction among massive stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012,…”

“There have been a number of physical mechanisms discussed in connection with LBVs, recently reviewed in detail by Smith et al. (2011a).”

“These SLSNe Ic (Gal-Yam 2012, Quimby et al. 2011) and II-L (Gezari et al. 2009, Miller et al. 2009) might be explained by CSM interaction with a dense shell in special cases where the shell has relatively sharp outer boundary.”

“A SN blast wave expands outward into the CSM and the ensuing collision, referred to as “CSM interaction”, is commonly observed in the form of X-ray or radio emission (Chevalier & Fransson 1994) for normal winds (Figure 3).”

“However, these SNe may also be explained by magnetar birth (Kasen & Bildsten 2010, Woosley 2010), so they are not necessarily the product of eruptive pre-SN mass loss.”

“…of the observed statistical distributions of stellar types, as well as the relative rates of various types of SNe (Dessart et al. 2011, Eldridge & Stanway 2009, Eldridge et al. 2008, Petrovic et al. 2005, Podsiadlowski et al. 1992, Sana et al. 2012, Vanbeveren et al. 1998, 2007, Yoon et al. 2010).”

“Massive star feedback may also terminate star formation locally, blowing giant bubbles (Cox 2005) and leaking hot processed gas into the Galactic Halo (Putman et al. 2012).”

“…estimates of mass-loss rates and their scaling with metallicity can profoundly impact topics as remote as reionization of the universe and the interpretation of spectra from galaxies at the highest redshifts currently being detected (Fan et al. 2006, Loeb & Barkana 2001, Morales & Wyithe 2010).”

“For extreme mass-loss rates, one can also use masers (Habing 1996), and spatially resolved circumstellar material using a variety of techniques like IR interferometry (Monnier et al. 2004).”

“…discrete absorption components (Cranmer & Owocki 1996, Fullerton et al. 1997, Howarth et al. 1995, Massa et al. 1995), or axisymmetric winds due to rapid rotation (Owocki et al. 1996), and a wide array of magnetically induced inhomogeneities (Townsend et al. 2005, ud-Doula & Owocki 2002).”

“…of the observed statistical distributions of stellar types, as well as the relative rates of various types of SNe (Dessart et al. 2011, Eldridge & Stanway 2009, Eldridge et al. 2008, Petrovic et al. 2005, Podsiadlowski et al. 1992, Sana et al. 2012, Vanbeveren et al. 1998, 2007, Yoon et al. 2010).”

“Diverse SLSN light curve shapes are possible, depending on the distribution of the mass and explosion properties (Chatzopoulos & Wheeler 2013, Chevalier & Irwin 2011, Moriya et al. 2013, van Marle et al. 2010).”

“This makes it hard to explain the nebula around η Car, with most of the mass moving at speeds of 500-600 km s−1.”

“Massive star feedback may also terminate star formation locally, blowing giant bubbles (Cox 2005) and leaking hot processed gas into the Galactic Halo (Putman et al. 2012).”

“Some models adopt the significantly higher empirical RSG mass-loss prescription of van Loon et al. (2005), which is intended for dust-enshrouded RSGs.”

“Shock fronts in their beautifully complex SN remnants echo through the ISM for thousands of years after the bright SN display has faded (Reynolds 2008).”

“Masses of LBV nebulae occupy a very large range from ∼20 M⊙ at the upper end down to 0.1 M⊙, although even smaller masses become difficult to detect Mass Loss 17 Masses of circumstellar shells around LBVs and LBV-like stars, as a function of luminosity, from Smith & Owocki (2006).”

“With the assumption that mass loss in steady winds dominates mass loss, theorists could adopt simple prescriptions for that mass loss and calculate single-star evolutionary tracks on the HR Diagram (see reviews by Chiosi & Maeder 1986, Maeder & Meynet 2000).”

“Using UV resonance absorption lines in O-type stars, Fullerton et al. (2006) have proposed a reduction by a factor of 10 or more from traditional mass-loss rates (fcl values of 100 or more), while Bouret et al. (2005) require reductions by factors of ∼3.”

“Based on unifying Hα meaurements with the theoretical mind momentum 8 Smith relation, both Repolust et al. (2004) and Markova et al. (2004) found fcl=5, or a mass-loss raduction by 2.3.1 Puls et al. (2006, 2008) and others have discussed that the larger mass-loss rate reduction of 10 found by…”

“…(Woosley et al. 2002, 2007), other explosive shell burning instabilities (Dessart et al. 2010, Smith & Arnett 2014), wave-driven 20 Smith mass loss (Meakin & Arnett 2007, Quataert & Shiode 2012, Shiode & Quataert 2013), and stellar collisions or mergers in a binary system (Podsiadlowski et al.…”

“…estimates of mass-loss rates and their scaling with metallicity can profoundly impact topics as remote as reionization of the universe and the interpretation of spectra from galaxies at the highest redshifts currently being detected (Fan et al. 2006, Loeb & Barkana 2001, Morales & Wyithe 2010).”

“…from several young clusters suggest that, when corrected for obervational bias, the fraction of massive stars in binary systems whose orbital period is so short that the stars must exhange mass or merge is something like 3/4 (Kiminki & Kobulnicky 2012, Kobulnicky & Fryer 2007, Sana et al. 2012).”

“For Ṁ corresponding to normal winds of LBVs, values were compiled from a number of studies (Groh et al. 2009, Hillier et al. 2001, Smith et al. 2004, Vink & de Koter 2002).”

“In the end we may find that observed trends with Z hinge upon the formation of massive binaries (Kratter et al. 2010) and its dependence on fragmentation and cooling as a function of Z. 5.”

“It is suspicious that most H-poor WR stars have luminosities of log(L/L⊙)=5.5 to 5.8 (Crowther 2007, Hamann et al. 2006), which also corresponds to the strongest RSG mass loss (Fig.”

“…for many of the observed statistical distributions of stellar types, as well as the relative rates of various types of SNe (Dessart et al. 2011, Eldridge & Stanway 2009, Eldridge et al. 2008, Petrovic et al. 2005, Podsiadlowski et al. 1992, Sana et al. 2012, Vanbeveren et al. 1998, 2007, Yoon…”

“This was a normal SN IIn with pre-explosion HST images that showed a source at the SN position, which then faded below detection limits after the SN had faded (Gal-Yam et al. 2007, Gal-Yam & Leonard 2009).”

“Thus, the rate at which mass is lifted from the star by this 6 Smith mechanism depends on the UV luminosity of the star, the temperature (and ionization), and the metallicity, Z (Mokiem et al. 2007, Puls et al. 2008).”

“…the effects of relatively rapid rotation (Georgy et al. 2012).6 At first glance, models and observations would seem to be reasonably well aligned (Massey 2003): MWR is inferred to be about 25 M⊙ at Z⊙ (Crowther 2007), increasing to about 30 and 70 M⊙ at ZLMC and ZSMC , respectively (Massey et…”

“No spectra were obtained, but the coincident SN explosion 2 years later was a Type Ibn with strong narrow He i emission lines (Foley et al. 2007, Pastorello et al. 2007).”

“For the Milky Way, LMC, and SMC, various studies using modern non-LTE analysis find fcl ≃ 10 (Crowther et al. 2002, Evans et al. 2004, Figer et al. 2002, Hillier et al. 2003, Massa et al. 2003, Puls et al. 2006), corresponding to Ṁ reductions by a factor of ∼3 with typical uncertainties of ∼30%.”

“…for this study and if they may overestimate the reductions in Ṁ that are applied, this nevertheless forced an important discussion in massive star research by highlighting the potential influence of clumping. because of sub-surface convection driven by the Fe opacity bump (Cantiello et al. 2009).”

“The mass-loss rates indicated by the CSM suggest that viable progenitors could be LBVs (Gal-Yam et al. 2007, Taddia et al. 2013) as well as extreme RSGs or YHGs (Smith et al. 2009) (see Fig 3).”

“Resulting mass transfer rates can be extremely high; during fast Case A or Case B and C phases, mass-loss rates can be of order 10−3 M⊙ yr −1 or higher (Langer 2012, Taam & Sandquist 2000), although there is a wide range (only order of magnitude values are represented in Figure 3).”

“…& Robert 1994) and inhomogeneous on larger scales; large scales include such complexities as the time-variable discrete absorption components (Cranmer & Owocki 1996, Fullerton et al. 1997, Howarth et al. 1995, Massa et al. 1995), or axisymmetric winds due to rapid rotation (Owocki et al.…”

“…although there has been some controversy about whether the 2012 event was a core-collapse SN (Mauerhan et al. 2013a, Ofek et al. 2013a,a, Prieto et al. 2013, Smith et al. 2014) or some type of extremely bright non-terminal event (Fraser et al. 2013a, Margutti et al. 2013, Pastorello et al. 2013).”

“RSG mass loss is time dependent, and there appears to be a wide dispersion even for a given luminosity and temperature (Willson 2000).”

“Standard single-star models predict MWR ≃ 35M⊙ at Z⊙ (Georgy et al. 2012, Heger et al. 2003), in- 26 Smith creasing to about 45 and 70M⊙ at ZLMC and ZSMC , respectively (Heger et al. 2003).”

“There have been a number of physical mechanisms discussed in connection with LBVs, recently reviewed in detail by Smith et al. (2011a).”

“Typical expansion speeds observed in outburst spectra are 100-1000 km s−1 (Smith et al. 2011a), although lower speeds can be seen along the line of sight if the ejection speed is latitude dependent (Smith 2006).”

“Some models predict that this very strong mass-loss phase is short-lived, and enhanced before core collapse caused by increasingly violent pulsations during C burning due to a high L/M ratio (Arnett & Meakin 2011, Heger et al. 1997, Yoon & Cantiello 2010).”

“Using UV resonance absorption lines in O-type stars, Fullerton et al. (2006) have proposed a reduction by a factor of 10 or more from traditional mass-loss rates (fcl values of 100 or more), while Bouret et al. (2005) require reductions by factors of ∼3.”

“…the theoretical mind momentum 8 Smith relation, both Repolust et al. (2004) and Markova et al. (2004) found fcl=5, or a mass-loss raduction by 2.3.1 Puls et al. (2006, 2008) and others have discussed that the larger mass-loss rate reduction of 10 found by Fullerton et al. (2006) may be an…”

“The best studied example is the Galactic object η Carinae, providing us with its historically observed light curve (Smith & Frew 2011), as well as its complex ejecta that contain 10-20 M⊙ and ∼1050 ergs of kinetic energy (Smith 2006, Smith et al. 2003).”

“Significant wind clumping is expected on theoretical grounds (Dessart & Owocki 2005, Feldmeier 1995, Owocki & Rybicki 1984, Owocki & Puls 1999, Owocki et al. 1988, Sundqvist & Owocki 2013), due mostly to the line-driven instability (this arises because the force of line driving is velocity…”

“The increased instability at very high initial masses is extreme in cases where pre-SN eruptions result from the pulsational pair instability (Woosley et al. 2002, 2007), but eruptions may extend to other nuclear burning instabilities as well (Arnett & Meakin 2011, Smith & Arnett 2014).”

“…although there has been some controversy about whether the 2012 event was a core-collapse SN (Mauerhan et al. 2013a, Ofek et al. 2013a,a, Prieto et al. 2013, Smith et al. 2014) or some type of extremely bright non-terminal event (Fraser et al. 2013a, Margutti et al. 2013, Pastorello et al. 2013).”

“…recently, pre-SN eruptive mass loss and connections to LBVs were mostly hypothetical, limited to (reasonable) conjectures supported by the circumstantial evidence that something must deposit a large mass of outflowing H-rich CSM so close to the star (Chugai et al. 2004, Smith et al. 2008, 2010b).”

“The LBVs are a rather diverse class, consisting of a wide range of irregular variable phenomena (Clark et al. 2005, Humphreys & Davidson 1994, Smith et al. 2004, 2011a, Van Dyk & Matheson 2012, van Genderen 2001).”

“This may echo the observation that some SNe IIn appear to be caused by an underlying thermonculaer Type Ia interacting with dense CSM (Silverman et al. 2013).”

“A number of extragalactic LBV-like eruptions show spectra that closely resemble shock-powered Type IIn SNe and also show evidence for extremely fast ejecta that may signify a shock-powered event, such as the precursor outbursts of SN2009ip (Foley et al. 2011, Smith et al. 2010a).”

“…stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012, Mahy et al. 2009, Sana et al. 2008, 2009,…”

“The main uncertainties are in the degree to which mass transfer is conservative5 (Cantiello et al. 2007, de Mink et al. 2007), the related question of how the mass gainer responds to the added angular momentum that may lead to critical rotation (de Mink et al. 2013), and the consequent orbital…”

“A realization in the past decade is that there is wide diversity among the SN impostors and their progenitors; some events that resemble LBV eruptions may actually arise in lower-mass progenitor stars (Prieto et al. 2008a, Thompson et al. 2009).”

“SN 2010mc. Ofek et al. (2013b) reported the detection of a precursor event ∼40 d before the peak of the Type IIn SN 2010mc.”

“These rates are too high to be driven by traditional stellar winds because the material is opaque (Owocki et al. 2004, Smith & Owocki 2006).”

“…recently, pre-SN eruptive mass loss and connections to LBVs were mostly hypothetical, limited to (reasonable) conjectures supported by the circumstantial evidence that something must deposit a large mass of outflowing H-rich CSM so close to the star (Chugai et al. 2004, Smith et al. 2008, 2010b).”

“…with Type IIn spectra represent the most extreme cases of eruptive pre-SN mass loss, with luminosities ∼10 times higher than a normal bright SN Ia. Smith & McCray (2007) proposed that these extremely high luminosities could be achieved wth normal energy core-collapse SNe (a few 1051 ergs) if…”

“…channel agrees with relatively low ejecta masses and H/He mass fractions inferred from detailed radiative transfer models of stripped-envelope SNe (Dessart et al. 2011, Hachinger et al. 2012, Yoon et al. 2010), which seem to rule out the idea that SNe IIb and Ib can come from progenitors much…”

“The extremely short timescale of only a few years probably hints at severe instability in the final nuclear burning sequences, especially Ne and O burning (Arnett & Meakin 2011, Quataert & Shiode 2012, Shiode & Quataert 2013, Smith & Arnett 2014), each of which lasts about 1 yr.”

“For the Milky Way, LMC, and SMC, various studies using modern non-LTE analysis find fcl ≃ 10 (Crowther et al. 2002, Evans et al. 2004, Figer et al. 2002, Hillier et al. 2003, Massa et al. 2003, Puls et al. 2006), corresponding to Ṁ reductions by a factor of ∼3 with typical uncertainties of ∼30%.”

“…discrete absorption components (Cranmer & Owocki 1996, Fullerton et al. 1997, Howarth et al. 1995, Massa et al. 1995), or axisymmetric winds due to rapid rotation (Owocki et al. 1996), and a wide array of magnetically induced inhomogeneities (Townsend et al. 2005, ud-Doula & Owocki 2002).”

“A realization in the past decade is that there is wide diversity among the SN impostors and their progenitors; some events that resemble LBV eruptions may actually arise in lower-mass progenitor stars (Prieto et al. 2008a, Thompson et al. 2009).”

“With thermal timescales of order 104 yr, binary RLOF is therefore capable of quickly removing almost the entire H envelope of a massive star and leaving behind a WR star (Petrovic et al. 2005).”

“For the Milky Way, LMC, and SMC, various studies using modern non-LTE analysis find fcl ≃ 10 (Crowther et al. 2002, Evans et al. 2004, Figer et al. 2002, Hillier et al. 2003, Massa et al. 2003, Puls et al. 2006), corresponding to Ṁ reductions by a factor of ∼3 with typical uncertainties of ∼30%.”

“8 The recent study by Arcavi et al. (2010) finds that compared to giant galaxies at high Z, there is a much larger fraction of SNe IIb and a lower fraction of normal SNe Ibc in lower-Z dwarf galaxies.”

“The mass-loss rates indicated by the CSM suggest that viable progenitors could be LBVs (Gal-Yam et al. 2007, Taddia et al. 2013) as well as extreme RSGs or YHGs (Smith et al. 2009) (see Fig 3).”

“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).”

“…recently, pre-SN eruptive mass loss and connections to LBVs were mostly hypothetical, limited to (reasonable) conjectures supported by the circumstantial evidence that something must deposit a large mass of outflowing H-rich CSM so close to the star (Chugai et al. 2004, Smith et al. 2008, 2010b).”

“High Γ values in hot stars can substantially affect the winds and increase mass-loss rates (Gräfener et al. 2011, Vink et al. 2011).”

“For YHGs, see de Jager (1998).”

“A number of extragalactic LBV-like eruptions show spectra that closely resemble shock-powered Type IIn SNe and also show evidence for extremely fast ejecta that may signify a shock-powered event, such as the precursor outbursts of SN2009ip (Foley et al. 2011, Smith et al. 2010a).”

“This is known as the “weak-wind problem” for O dwarfs (Muijres et al. 2012, Puls et al. 2008), and may indicate inefficient line driving in hot dwarfs.”

“The main uncertainties are in the degree to which mass transfer is conservative5 (Cantiello et al. 2007, de Mink et al. 2007), the related question of how the mass gainer responds to the added angular momentum that may lead to critical rotation (de Mink et al. 2013), and the consequent orbital…”

“A possible cause of this inflation of the star’s outer layers may be near-Eddington luminosities in the sub-surface Fe opacity bump (Gräfener et al. 2012, Guzik & Lovekin 2012).”

“However, Smith et al. (2011b) pointed out that the relevant ratio really is (IIb + Ibc)/(II-P + II-L + IIn), since SNe IIb are almost identical to SNe Ib but for <0.01 M⊙ of H in their outermost layer (Dessart et al. 2011, Hachinger et al. 2012).”

“These were recognized early as the brightest blue irregular variables in nearby galaxies (Hubble & Sandage 1953, Tammann & Sandage 1968), and these classic examples were referred to as the “Hubble-Sandage variables”.”

“…of relatively rapid rotation (Georgy et al. 2012).6 At first glance, models and observations would seem to be reasonably well aligned (Massey 2003): MWR is inferred to be about 25 M⊙ at Z⊙ (Crowther 2007), increasing to about 30 and 70 M⊙ at ZLMC and ZSMC , respectively (Massey et al. 2000).”

“High Γ values in hot stars can substantially affect the winds and increase mass-loss rates (Gräfener et al. 2011, Vink et al. 2011).”

“Examining populations in nearby galaxies, Massey et al. (2007) find that there are more than an order of magnitude more spectroscopically similar LBV candidates than there are LBVs confirmed by their variability.”

“The best studied Galactic object in this category is VY Canis Majoris, which has an extended dust-scattering nebula seen in HST images (Smith et al. 2001).”

“As with UV observations, the development of IR detectors led to a similar recognition of the importance of dust-driven winds in red supergiants (Gehrz & Woolf 1971).”

“SLSNe Ic do, of course, require a stripped H envelope like other SNe Ic, and they are interestingly similar to SNe Ic-BL and GRBs in that they seem to prefer low-Z host galaxies (Neill et al. 2011).”

“These SLSNe Ic (Gal-Yam 2012, Quimby et al. 2011) and II-L (Gezari et al. 2009, Miller et al. 2009) might be explained by CSM interaction with a dense shell in special cases where the shell has relatively sharp outer boundary.”

“SN 2009ip is so far unique among SN progenitor detections: not only did it have a detection of the quiescent progenitor and multiple pre-SN eruptions, but unlike any other object we also have detailed high-quality spectra of the pre-SN eruptions (Foley et al. 2011, Smith et al. 2010a).”

“…stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012, Mahy et al. 2009, Sana et al. 2008, 2009, 2012).”

“…stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012, Mahy et al. 2009, Sana et al. 2008, 2009, 2012).”

“In the case of VY CMa, the density of its nebula drops off sharply at ∼8000 AU from the star, indicating that its high average mass-loss rate of ∼10−3 M⊙ yr−1 has been limited to the previous 103 yr or so (Decin et al. 2006, Smith et al. 2001, 2009).”

“Smith et al. (2014) showed that the double-peaked light curve of SN 2010mc was nearly identical to SN 2009ip, for which it has been suggested that the −40 d precursor was actually the faint SN explosion of a blue supergiant, and the later rise to peak was caused by CSM interaction.”

“S Dor events were originally proposed to occur at constant bolometric luminosity (Humphreys & Davidson 1994), but quantitative studies do reveal varitions in LBol (Groh et al. 2009).”

“The birth of UV astronomy triggered a revolution in our understanding of massive stars by providing the decisive evidence that all luminous hot stars have strong winds, indicated by deep P Cyg absorption in their UV resonance lines (Morton 1967).”

“There have been a number of physical mechanisms discussed in connection with LBVs, recently reviewed in detail by Smith et al. (2011a).”

“In a binary evolution paradigm, on the other hand, He stars stripped of their H envelope can occur over a wider range of initial mass (Claeys et al. 2011, Vanbeveren et al. 2007).”

“8Many studies group SNe IIb and II-L together into a transitional class between SNe II-P and Ib in a single-star framework (e.g., Figure 1).”

“The LBVs are a rather diverse class, consisting of a wide range of irregular variable phenomena (Clark et al. 2005, Humphreys & Davidson 1994, Smith et al. 2004, 2011a, Van Dyk & Matheson 2012, van Genderen 2001).”

“…discrete absorption components (Cranmer & Owocki 1996, Fullerton et al. 1997, Howarth et al. 1995, Massa et al. 1995), or axisymmetric winds due to rapid rotation (Owocki et al. 1996), and a wide array of magnetically induced inhomogeneities (Townsend et al. 2005, ud-Doula & Owocki 2002).”

“Based on unifying Hα meaurements with the theoretical mind momentum 8 Smith relation, both Repolust et al. (2004) and Markova et al. (2004) found fcl=5, or a mass-loss raduction by 2.3.1 Puls et al. (2006, 2008) and others have discussed that the larger mass-loss rate reduction of 10 found by…”

“…although there has been some controversy about whether the 2012 event was a core-collapse SN (Mauerhan et al. 2013a, Ofek et al. 2013a,a, Prieto et al. 2013, Smith et al. 2014) or some type of extremely bright non-terminal event (Fraser et al. 2013a, Margutti et al. 2013, Pastorello et al. 2013).”

“This is further evidence that SNe Ibc may arise from a large range of initial mass, including relatively low-luminosity stars with weak winds 7The upper bound of this range is uncertain, and higher than found by Smartt (2009) if one accounts for progenitor reddening (Walmswell & Eldridge 2012).”

“Smith (2013a) showed that a shock-powered event with CSM interaction could account for the 1845-1860 light curve of η Car, using a SN IIn-type model but with lower explosion energy.”

“…WC/WN ratio that increases with Z (Massey 2003), as well as the fact that even early-type WR stars in the SMC tend to have some small amount of H present in their atmospheres (Foellmi et al. 2003); both of these are generally atributed to the important role of Z-dependent, line-driven winds.”

“The traditional explanation for the apparent temperature change was that the star dramatically increases its mass-loss rate, driving the wind to very high optical depth and causing a pseudo photosphere (Davidson 1987, Humphreys & Davidson 1994).”

“In fact, there are currently 3 direct progenitor detections for SNe IIb (SN 1993J, 2011dh, and 2013df) that are thought to be YSGs in binary systems with inferred initial masses of 13- 17 M⊙ (Maund & Smartt 2009, Van Dyk et al. 2013a,b).”

“For extreme mass-loss rates, one can also use masers (Habing 1996), and spatially resolved circumstellar material using a variety of techniques like IR interferometry (Monnier et al. 2004).”

“SE winds are expected to be very dense but porous, allowing the star’s atmosphere to remain in steady state while exceeding the classical Eddington limit (Owocki et al. 2004, Shaviv 2000).”

“Significant wind clumping is expected on theoretical grounds (Dessart & Owocki 2005, Feldmeier 1995, Owocki & Rybicki 1984, Owocki & Puls 1999, Owocki et al. 1988, Sundqvist & Owocki 2013), due mostly to the line-driven instability (this arises because the force of line driving is velocity…”

“These were recognized early as the brightest blue irregular variables in nearby galaxies (Hubble & Sandage 1953, Tammann & Sandage 1968), and these classic examples were referred to as the “Hubble-Sandage variables”.”

“The large number of shell nebulae recently discovered by IR surveys (Gvaramadze et al. 2010, Wachter et al. 2010) may be very helpful in improving statistics and associations with the central stars.”

“Adapted from Davies et al. (2008) with permission. tions are shown in Figure 3.”

“…stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012, Mahy et al. 2009, Sana et al. 2008, 2009, 2012).”

“Many SNe IIn and SLSNe occur in low-Z dwarf galaxies (Neill et al. 2011, Stoll et al. 2011), demonstrating that low Z does not inhibit strong eruptive mass loss.”

“A new subclass of SNe IIn with plateau-shaped light curves, which show a pronounced (3-6 mag) drop in flux at times around 120 days (Mauerhan et al. 2013b).”

“…calculations are still using mass-loss rates 1Note that long ago, small scale clumping with fcl values of 4-20 (a reduction in mass-loss rates by factors of 2-4) was required to fit both the emisson cores (∝ ρ2) and electron-scattering wings (∝ ρ) in WR stars (Hillier 1991, Moffat & Robert 1994).”

“…stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012, Mahy et al. 2009, Sana et al. 2008, 2009, 2012).”

“The best studied example is the Galactic object η Carinae, providing us with its historically observed light curve (Smith & Frew 2011), as well as its complex ejecta that contain 10-20 M⊙ and ∼1050 ergs of kinetic energy (Smith 2006, Smith et al. 2003).”

“For the Milky Way, LMC, and SMC, various studies using modern non-LTE analysis find fcl ≃ 10 (Crowther et al. 2002, Evans et al. 2004, Figer et al. 2002, Hillier et al. 2003, Massa et al. 2003, Puls et al. 2006), corresponding to Ṁ reductions by a factor of ∼3 with typical uncertainties of ∼30%.”

“…stars in clusters using radial velocities, which typically find an observed binary fraction of 20-60% (Chini et al. 2012, De Becker et al. 2006, Evans et al. 2006, Garcia & Mermilliod 2001, Gies 1987, Kiminki & Kobulnicky 2012, Kiminki et al. 2012, Mahy et al. 2009, Sana et al. 2008, 2009, 2012).”

“…although there has been some controversy about whether the 2012 event was a core-collapse SN (Mauerhan et al. 2013a, Ofek et al. 2013a,a, Prieto et al. 2013, Smith et al. 2014) or some type of extremely bright non-terminal event (Fraser et al. 2013a, Margutti et al. 2013, Pastorello et al. 2013).”

“…mass loss is less extreme than required for SLSNe IIn, some SNe IIn show strong CSM interaction that continues for years or decades (like SN 1988Z, Aretxaga et al. 1999) as well as long-lasting IR echoes from distant dust shells illuminated by the SN (Fox et al. 2011, Gerardy et al. 2002) — both…”

“…clumping is expected on theoretical grounds (Dessart & Owocki 2005, Feldmeier 1995, Owocki & Rybicki 1984, Owocki & Puls 1999, Owocki et al. 1988, Sundqvist & Owocki 2013), due mostly to the line-driven instability (this arises because the force of line driving is velocity dependent; gas parcels…”

“Note that similar considerations of UV diagnostics were noted earlier as well (Cohen et al. 1997, 2008, Drew et al. 1994).”

“No H-free WR stars are known with masses above roughly 20-25 M⊙ (see Crowther 2007, Smith & Conti 2008), so the lion’s share of mass loss is yet to come.”

“There are also a few reported transitional cases between SNe Ibn and IIn, including SN 2005la and SN 2011hw and 2005la (Pastorello et al. 2008, Smith et al. 2012), suggesting a possible continuum in progenitor H envelope stripping, similar to SNe II-P, IIb, and Ib.”

“Moreover, η Car also shows a smaller mass of extremely fast material moving at 5000 km s−1 or more (Smith 2008), which is hard for a SE wind model to achive while driving such a large amount of mass. SE winds are still viable for most LBVs, which are less extreme than η Car’s 19th century eruption.”

“For the Milky Way, LMC, and SMC, various studies using modern non-LTE analysis find fcl ≃ 10 (Crowther et al. 2002, Evans et al. 2004, Figer et al. 2002, Hillier et al. 2003, Massa et al. 2003, Puls et al. 2006), corresponding to Ṁ reductions by a factor of ∼3 with typical uncertainties of ∼30%.”

“Here we focus on the more extreme cases where the high Γ leads to instability in the star and highly time-dependent mass loss, or where advanced nuclear burning stages may play a role (Arnett & Meakin 2011).”

“…published, although there has been some controversy about whether the 2012 event was a core-collapse SN (Mauerhan et al. 2013a, Ofek et al. 2013a,a, Prieto et al. 2013, Smith et al. 2014) or some type of extremely bright non-terminal event (Fraser et al. 2013a, Margutti et al. 2013, Pastorello et…”

“The nebula around VY CMa is quite similar to that around the yellow hypergiant IRC+10420, which is regarded as the prototypical post-RSG because of its maser shell surviving around such a warm star, and its observed fast blueward evolution (Humphreys et al. 1997).”

“Rare stars that appear in the middle of the HR diagram, possibly as post-RSGs, and usually with strong mass loss (Drout et al. 2012).”

“P Cyg’s nebula has an expansion speed of ∼140 km s−1 (Barlow et al. 1994, Smith & Hartigan 2006), with an implied total kinetic energy of a few 1046 ergs.”

“Significant wind clumping is expected on theoretical grounds (Dessart & Owocki 2005, Feldmeier 1995, Owocki & Rybicki 1984, Owocki & Puls 1999, Owocki et al. 1988, Sundqvist & Owocki 2013), due mostly to the line-driven instability (this arises because the force of line driving is velocity…”

“In fact, there are currently 3 direct progenitor detections for SNe IIb (SN 1993J, 2011dh, and 2013df) that are thought to be YSGs in binary systems with inferred initial masses of 13- 17 M⊙ (Maund & Smartt 2009, Van Dyk et al. 2013a,b).”

“Consequently, their loosely bound envelopes may be susceptible to strange-mode instabilities (Glatzel & Kiriakidis 1993, Glatzel et al. 1999), runaway mass loss (a.k.a. the “Geyser” model; Humphreys & Davidson 1994, Maeder 1992), or the critical rotation limit (a.k.a. the “Ω limit”; Langer 1998,…”

“…collisions (Ofek et al. 2007, Smith & McCray 2007, Smith et al. 2007, 2010b, Woosley et al. 2007), but a number of very luminous SNe IIn have been studied in detail since then, including objects like SN 2006tf (Smith et al. 2008), SN 2003ma (Rest et al. 2011), and SN 2008fz (Drake et al. 2010).”

“There are many ways M & 8 M stars can end their lives 3 (e.g., Woosley et al. 2002; Smartt 2009; Meynet et al. 2010; Langer 2012; Nomoto et al. 2013; Smith 2014).”

“It is frequently cited as one of the most uncertain ingredients in stellar evolution, and is thought to play a crucial role in the advanced stages of stellar evolution for low-mass stars and in all phases of evolution for massive stars (see Smith 2014 for a recent review on this topic).”

“…the formation of the dense molecular clouds and stars (Heyer & Dame 2015); the evolution of stellar populations, their winds and energetic feedback (Smith 2014); the explosions of supernovae (Nomoto, Kobayashi & Tominaga 2013); the formation of supermassive black holes and the interaction of…”