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

Effects of Mass Loss on Stellar Evolution

Why a Meeting on Angular Momentum and Mass Loss for Hot Stars?.- Angular Momentum and Mass Loss and Stellar Evolution.- The Effects of Rotation on Stellar Structure and Evolution.- New Evolutionary Aspects of Mass Loss and Angular Momentum.- Mass Loss During the Evolution of Massive Stars.- Pre-Main Sequence Stages.- Angular Momentum Loss in Pre-Main Sequence Objects and the Initial Angular Momentum of Stars.- The Herbig Ae and Be Stars: Mass and Angular Momentum Losses.- Eccentric Spiral Modes in Disks Associated with Young Stellar Objects.- Evidence that Wolf-Rayet Stars are Pre-Main Sequence Objects.- A and F Stars near the Main Sequence.- Rotation, Pulsation and Atmospheric Phenomena in A-Type Stars.- Pulsation Studies of a 1.8 M? Delta Scuti Model.- Post-Main Sequence Evolution of Binary Am Stars.- A Statistical Study of Main Sequence A and F Stars: Testing the Main Sequence Mass Loss Hypothesis.- O, B and Be Stars.- Basic Magnetic Rotator Theory with Application to the Angular Momentum Driven Winds of B[e] and Wolf Rayet Stars.- The Connection between Rotation and the Winds of Be Stars.- UV Glimpse of OB Stars.- Nonspherical Radiation Driven Wind Models Applied to Be Stars.- A Simple Criterion to Identify Rapidly Rotating Stars Viewed at Small to Intermediate Inclination Angles.- Intensive Photometric Campaign on Be Stars: Behaviour of Short-Term Periodic Variations and its Relationship to Pulsation and Mass Loss.- Constraints on the Thickness of Be Star Disks Derived from Combined IR Excess and Optical Polarimetry Data.- On the Correlation between Pulsation Amplitude and Shell Activity in the Be star ? Eridani.- B[e] Supergiants: Continuum Polarization by Electron Scattering in Rotationally Distorted, Radiation Driven Stellar Winds.- Rotational Evolution of Hot Stars due to Mass Loss and Magnetic Fields.- New Facts About the Variability of 45 ? Persei.- Long-Term Study of Stellar-Wind Variability of O Stars.- HeII ?1640 as a Diagnostic for Assessing the Extent of Rapid Rotation in Be Stars.- Some Examples of the Role of Stellar Rotation in Hot Star Winds.- HD 193077 -- A Fast Rotating Wolf-Rayet Star.- How Effective is Rotation in Enhancing the Rate of Mass Loss in Early Type Stars?.- Rotation and Pulsation-Mode-Selection in B-Type Stars.- The Angular Momentum-Loss and the Differential Rotation in B and Be Stars.- Very Luminous and Very Massive Stars.- The Role of Axial Symmetry in the Upper Part of the HRD: B[e] Supergiants and LBVs.- Effects of Mass Loss on Late Stages of Massive Star Evolution.- V444 Cygni and CQ Cephei, Key Wolf-Rayet Binary Stars.- Rotation of Hot Stars After They Cool Off.- Winds, Mass Loss and Rotation in Central Stars of Planetary Nebulae.- White Dwarf Mass Loss, Rotation, Individual Masses and the Identification of the White Dwarf Remnants of Upper Main Sequence Stars.- Evolved Stars as Probes of Main Sequence Angular Momentum and Mass Loss.- The Bizarre Kinematics of Planetary Nebula NGC 7009, and Some Thoughts on the Transfer of Stellar Angular Momentum to Planetary Nebulae.- Chromospheric H? Activity in ? ORI.- Stellar Winds in A-Type Supergiants.- Special Section: A Debate Concerning the Nature of Wolf-Rayet Stars.- The Evidence that Wolf-Rayet Stars are in a Late Stage of Evolution.- Why Wolf-Rayet Stars Should Not Be Considered To Be Evolved Cores of Massive Stars.- Author index.- Keyword index.- Astronomical index.

Quiescent mass loss during the red supergiant (RSG) phase has been shown to be far lower than prescriptions typically employed in single-star evolutionary models. Importantly, RSG winds are too weak to drive the production of Wolf-Rayet (WR) stars and stripped-envelope supernovae (SE-SNe) at initial masses of roughly 20–40 M ⊙. If single stars are to make WR stars and SE-SNe, this shifts the burden of mass loss to rare dust-enshrouded RSGs (DE-RSGs), objects claimed to represent a short-lived, high-mass-loss phase. Here, we take a fresh look at the purported DE-RSGs. By modeling the mid-IR excesses of the full sample of RSGs in the Large Magellanic Cloud, we find that only one RSG has both a high mass-loss rate ( ≥ 10−4 M ⊙ yr−1) and a high optical circumstellar dust extinction (7.92 mag). This RSG is WOH G64, and it is the only one of the 14 originally proposed DE-RSGs that is actually dust enshrouded. The rest appear to be either normal RSGs without strong IR excess, or lower-mass asymptotic giant branch stars. Only one additional object in the full catalog of RSGs (not previously identified as a DE-RSG) shows strong mid-IR excess. We conclude that if DE-RSGs do represent a pre-SN phase of enhanced in single stars, it is extremely short-lived, only capable of removing ≤2 M ⊙ of material. This rules out the single-star post-RSG pathway for the production of WR stars, luminous blue variables, and SE-SNe. Single-star models should not employ -prescriptions based on these extreme objects for any significant fraction of the RSG phase.

Based on their relatively isolated environments, we argue that LBVs must be primarily the product of binary evolution, challenging the traditional single-star view wherein LBVs mark a brief transition between massive O stars and Wolf-Rayet (WR) stars. If the latter were true, then LBVs should be concentrated in young clusters and found alongside main-sequence stars with similarly high inferred initial mass. This is decidedly not the case. Examining locations of LBVs compared to O stars in our Galaxy and the Magellanic Clouds reveals that LBVs systematically avoid clusters of O stars, and many reside over 100 pc from any O star. In the LMC, LBVs are statistically much more isolated than O-type stars, and (perhaps most surprisingly) even more isolated than most WR stars. This makes it impossible for LBVs to be 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 SNeIbc correspond to the mass donors, while LBVs (and their lower-mass analogs like B[e] supergiants, which we show to be even more isolated) are the mass gainers. LBVs are essentially the late evolutionary stage of 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 SNeIIn and the observed isolation of SNe~IIn and SN impostors. We argue that environmental trends of various SN subtypes are influenced more by binarity and SN kicks, rather than tracing initial mass as is generally assumed. Mergers or Thorne-Zykow objects might also give rise to LBVs, but these scenarios may have a harder time explaining why LBVs avoid clusters.

Recent observational evidence for initial mass function (IMF) variations in massive quiescent galaxies at z = 0 challenges the long-established paradigm of a universal IMF. While a few theoretical models relate the IMF to birth cloud conditions, the physical driver underlying these putative IMF variations is still largely unclear. Here we use post-processing analysis of the Illustris cosmological hydrodynamical simulation to investigate possible physical origins of IMF variability with galactic properties. We do so by tagging stellar particles in the simulation (each representing a stellar population of ) with individual IMFs that depend on various physical conditions, such as velocity dispersion, metallicity, or star formation rate, at the time and place in which the stars are formed. We then follow the assembly of these populations throughout cosmic time and reconstruct the overall IMF of each z = 0 galaxy from the many distinct IMFs it is composed of. Our main result is that applying the observed relations between IMF and galactic properties to the conditions at the star formation sites does not result in strong enough IMF variations between z = 0 galaxies. Steeper physical IMF relations are required for reproducing the observed IMF trends, and some stellar populations must form with more extreme IMFs than those observed. The origin of this result is the hierarchical nature of massive galaxy assembly, and it has implications for the reliability of the strong observed trends, for the ability of cosmological simulations to capture certain physical conditions in galaxies, and for theories of star formation aiming to explain the physical origin of a variable IMF.

We suggest that the mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds. In order for a massive star to become a Wolf-Rayet star, it must shed its hydrogen envelope, but new estimates of the effects of clumping in winds from O-type stars indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 M☉, which do not become red supergiants and for which the best alternative is mass loss during brief eruptions of luminous blue variables (LBVs). Our clearest example of this phenomenon is the 19th century outburst of η Carinae, when the star shed 12-20 M☉ or more in less than a decade. Other examples are circumstellar nebulae of LBVs and LBV candidates, extragalactic η Car analogs (the so-called supernova impostors), and massive shells around supernovae and gamma-ray bursters. We do not yet fully understand what triggers LBV outbursts or what supplies their energy, but they occur nonetheless, and they present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuum-driven wind or a hydrodynamic explosion, both of which are insensitive to metallicity. As such, eruptive mass loss could have played a pivotal role in the evolution and ultimate fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would also profoundly affect the chemical yield and types of remnants from early supernovae and hypernovae thought to be the origin of long gamma-ray bursts.

Context. Without a doubt, mass transfer in close binary systems contributes to the populations ofWolf-Rayet (WR) stars in the MilkyWay and the Magellanic Clouds. However, the binary formation channel is so far not well explored. Aims. We want to remedy this by exploring large grids of detailed binary and single star evolution models computed with the publicly available MESA code, for a metallicity appropriate for the Large Magellanic Cloud (LMC). Methods. The binary models were calculated through Roche-lobe overflow and mass transfer, until the initially more massive star exhausted helium in its core. We distinguish models of WR and helium stars based on the estimated stellar wind optical depth. We used these models to build a synthetic WR population, assuming constant star formation. Results. Our models can reproduce the WR population of the LMC to significant detail, including the number and luminosity functions of the main WR subtypes. We find that for binary fractions of 100% (50%), all LMC WR stars below 106 L⊙ (105.7 L⊙) are stripped binary mass donors. We also identify several insightful mismatches. With a single star fraction of 50%, our models produce too many yellow supergiants, calling either for a larger initial binary fraction, or for enhanced mass loss near the Humphreys-Davidson limit. Our models predict more long-period WR binaries than observed, arguably due to an observational bias toward short periods. Our models also underpredict the shortest-period WR binaries, which may have implications for understanding the progenitors of double black hole mergers. Conclusions. The fraction of binary-produced WR stars may be larger than often assumed and outline the risk to miscalibrate stellar physics when only single star models are used to reproduce the observed WR stars.

Context. Massive stars emit copious amounts of radiation, profoundly affecting their environment in galaxies and contributing to the reionization of the Universe. However, their evolution and thus their ionizing feedback are still not fully understood. One of the largest gaps in current stellar evolution calculations is the lack of a model for the mass ejections that occur when the stars reach the Eddington limit, such as during a Luminous Blue Variable (LBV) phase. Aims. Here, we aim to remedy this situation by providing a physically motivated and empirically calibrated method applicable in any 1D stellar evolution code to approximate the effect of such mass loss on stellar evolution. Methods. We employed the 1D stellar evolution code MESA, in which we implement a new mass-loss prescription that becomes active when stellar models inflate too much when reaching the Eddington limit. We used lines of constant inflation factors in the Hertzsprung-Russell diagram (HRD) for a simple empirical calibration of the threshold value. We calculated synthetic massive-star stellar populations using grids of single-star models with this mass loss prescription compared them with the observed populations in the Large and Small Magellanic Clouds. Further, with already computed grids of binary evolution models, we investigated the impact of binarity on our predictions. Results. Our single-star models reproduce key features of the observed stellar populations, namely, (i) the absence of stars located beyond the Humphreys-Davidson limit; (ii) an upper limit of red supergiant (RSG) luminosities; (iii) the faintest observed single Wolf-Rayet (WR) stars; (iv) the absolute number of O-stars, WRs, and RSGs; (v) WO stars in low metallicity environments; and (vi) the positions of LBV stars in the HRD. We show that binarity still plays an important role in explaining the observed WR stars. However, a large fraction of the binary population can also be explained via self-stripping. At the same time, our binary population explains the 70% binary fraction of O-stars and the 40% binary fraction of WR stars. However, our synthetic population also has caveats, such as an overproduction of bright H-free WN stars. Conclusions. Our results show that the effect of the Eddington-limit induced mass ejections on the structure and evolution of massive stars can remove the tension between predicted and observed massive star populations. A more fundamental treatment of these effects, particularly for hydrogen-poor stars, is needed to fully comprehend massive star evolution.

Mass loss plays a key role in the evolution of massive stars and their environment. High mass-loss events are traced by complex circumstellar ejecta and intricate line profiles across the upper Hertzsprung-Russell diagram for massive stars in different evolutionary stages. The basic physics of radiation-driven stellar wind for hot stars is well understood. However, the driving mechanisms and related instabilities for their enhanced mass-loss episodes and the driving mechanisms for the mass loss of cool stars are still debated. In this review, the mass-loss characteristics and the possible mechanisms will be surveyed for an observational set of prominent massive stellar populations that experience outflows, strong stellar winds, and periods of enhanced and eruptive mass loss; massive young stellar objects, OB-type stars, red supergiants, warm hypergiants, luminous blue variables, and Wolf-Rayet stars.

A debate has arisen regarding the importance of stationary versus eruptive\nmass loss for massive star evolution. The reason is that stellar winds have\nbeen found to be clumped, which results in the reduction of unclumped empirical\nmass-loss rates. Most stellar evolution models employ theoretical mass-loss\nrates which are already reduced by a moderate factor of ~2-3 compared to\nnon-corrected empirical rates. A key question is whether these reduced rates\nare of the correct order of magnitude, or if they should be reduced even\nfurther, which would mean that the alternative of eruptive mass loss becomes\nnecessary. Here we introduce the transition mass-loss rate (dM/dt)_trans\nbetween O and Wolf-Rayet (WR) stars. Its novelty is that it is model\nindependent. All that is required is postulating the spectroscopic transition\npoint in a given data-set, and determining the stellar luminosity, which is far\nless model dependent than the mass-loss rate. The transition mass-loss rate is\nsubsequently used to calibrate stellar wind strength by its application to the\nOf/WNh stars in the Arches cluster. Good agreement is found with two\nalternative modelling/theoretical results, suggesting that the rates provided\nby current theoretical models are of the right order of magnitude in the\n~50Msun mass range. Our results do not confirm the specific need for eruptive\nmass loss as Luminous Blue Variables, and current stellar evolution modelling\nfor Galactic massive stars seems sound. Mass loss through alternative\nmechanisms might still become necessary at lower masses, and/or metallicities,\nand the quantification of alternative mass loss is desirable.\n

The mass-loss rate (MLR) is one of the most important parameters in astrophysics, because it impacts many areas of astronomy, such as ionizing radiation, wind feedback, star-formation rates, initial mass functions, stellar remnants, supernovae, and so on. However, the most important modes of mass loss are also the most uncertain, as the dominant physical mechanisms that lead to this phenomenon are stull largely unknown. Here we assemble the most complete and clean red supergiant (RSG) sample (2121 targets) so far in the Small Magellanic Cloud (SMC) with 53 different bands of data to study the MLR of RSGs. In order to match the observed spectral energy distributions (SEDs), we created a theoretical grid of 17 820 oxygen-rich models (“normal” and “dusty” grids are half-and-half) using the radiatively driven wind model of the DUSTY code, covering a wide range of dust parameters. We select the best model for each target by calculating the minimal modified chi-square and visual inspection. The resulting MLRs from DUSTY are converted to real MLRs based on the scaling relation, for which a total MLR of 6.16 × 10−3 M⊙ yr−1 is measured (corresponding to a dust-production rate of ∼6 × 10−6 M⊙ yr−1), with a typical MLR of ∼10−6 M⊙ yr−1 for the general population of the RSGs. The complexity of mass-loss estimations based on the SED is fully discussed for the first time, and our results indicate large uncertainties based on the photometric data (potentially up to one order of magnitude or more). The Hertzsprung-Russell (HR) and luminosity versus median-absolute-deviation (MAD) diagrams of the sample indicate the positive relation between luminosity and MLR. Meanwhile, the luminosity versus MLR diagrams show a “knee-like” shape with enhanced mass loss occurring above log10(L/L⊙)≈4.6, which may be due to the degeneracy of luminosity, pulsation, low surface gravity, convection, and other factors. We derive our MLR relation using a third-order polynomial to fit the sample and compare our results with previous empirical MLR prescriptions. Given that our MLR prescription is based on a much larger sample than previous determinations, it provides a more accurate relation at the cool and luminous region of the HR diagram at low metallicity compared to previous studies. Finally, nine targets in our sample were detected in the UV, which could be an indicator of OB-type companions of binary RSGs.

All measurements of cosmic star formation must assume an initial distribution of stellar masses-the stellar initial mass function-in order to extrapolate from the star-formation rate measured for typically rare, massive stars (of more than eight solar masses) to the total star-formation rate across the full stellar mass spectrum 1 . The shape of the stellar initial mass function in various galaxy populations underpins our understanding of the formation and evolution of galaxies across cosmic time 2 . Classical determinations of the stellar initial mass function in local galaxies are traditionally made at ultraviolet, optical and near-infrared wavelengths, which cannot be probed in dust-obscured galaxies2,3, especially distant starbursts, whose apparent star-formation rates are hundreds to thousands of times higher than in the Milky Way, selected at submillimetre (rest-frame far-infrared) wavelengths4,5. The 13C/18O isotope abundance ratio in the cold molecular gas-which can be probed via the rotational transitions of the 13CO and C18O isotopologues-is a very sensitive index of the stellar initial mass function, with its determination immune to the pernicious effects of dust. Here we report observations of 13CO and C18O emission for a sample of four dust-enshrouded starbursts at redshifts of approximately two to three, and find unambiguous evidence for a top-heavy stellar initial mass function in all of them. A low 13CO/C18O ratio for all our targets-alongside a well tested, detailed chemical evolution model benchmarked on the Milky Way 6 -implies that there are considerably more massive stars in starburst events than in ordinary star-forming spiral galaxies. This can bring these extraordinary starbursts closer to the 'main sequence' of star-forming galaxies 7 , although such main-sequence galaxies may not be immune to changes in initial stellar mass function, depending on their star-formation densities.

Massive stars have strong stellar winds that direct their evolution through the upper Hertzsprung–Russell diagram and determine the black hole mass function. Furthermore, wind strength dictates the atmospheric structure that sets the ionizing flux. Finally, the wind directly intervenes with the stellar envelope structure, which is decisive for both single-star and binary evolution, affecting predictions for gravitational wave events. Key findings of current hot star research include: ▪ The traditional line-driven wind theory is being updated with Monte Carlo and comoving frame computations, revealing a rich multivariate behavior of the mass-loss rate [Formula: see text] in terms of M, L, Eddington Γ, Teff, and chemical composition Z. Concerning the latter, [Formula: see text] is shown to depend on the iron (Fe) opacity, making Wolf–Rayet populations, and gravitational wave events dependent on host galaxy Z. ▪ On top of smooth mass-loss behavior, there are several transitions in the Hertzsprung–Russell diagram, involving bistability jumps around Fe recombination temperatures, leading to quasi-stationary episodic, and not necessarily eruptive, luminous blue variable and pre-SN mass loss. ▪ Furthermore, there are kinks. At 100 [Formula: see text] a high Γ mass-loss transition implies that hydrogen-rich, very massive stars have higher mass-loss rates than commonly considered. At the other end of the mass spectrum, low-mass stripped helium stars no longer appear as Wolf–Rayet stars but as optically thin stars. These stripped stars, in addition to very massive stars, are two newly identified sources of ionizing radiation that could play a key role in local star formation as well as at high redshift.

Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. Results. AzV 14 is a close binary system with a period of P = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M1, 2 ≈ 32 M⊙. Both stars have weak stellar winds with mass-loss rates of log Ṁ/(M⊙ yr−1) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with Teff ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with Teff ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity.

Star formation and abundance gradients in the galaxy