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Very massive stars (VMS) up to 200-300 $M_\odot$ have been found in the Local Universe. If they would lose little mass they produce intermediate-mass black holes or pair-instability supernovae (PISNe). Until now, VMS modellers have extrapolated mass-loss vs. metallicity ($Z$) exponents from optically-thin winds, resulting in a range of PISN thresholds that might be unrealistically high in $Z$, as VMS develop optically-thick winds. We utilize the transition mass-loss rate of Vink and Gr\"afener (2012) that accurately predicts mass-loss rates of Of/WNh ("slash") stars that characterize the morphological transition from absorption-dominated O-type spectra to emission-dominated WNh spectra. We develop a wind efficiency framework, where optically thin winds transition to enhanced winds, enabling us to study VMS evolution at high redshift where individual stars cannot be resolved. We present a MESA grid covering $Z_\odot/2$ to $Z_\odot/100$. VMS above the transition evolve towards lower luminosity, skipping the cool supergiant phase but directly forming pure He stars at the end of hydrogen burning. Below the transition, VMS evolve as cooler luminous blue variables (LBVs) or yellow hypergiants (YHGs), naturally approaching the Eddington limit. Strong winds in this YHG/LBV regime -- combined with a degeneracy in luminosity -- result in a mass-loss runaway where a decrease in mass increases wind mass loss. Our models indicate an order-of-magnitude lower threshold than usually assumed, at $Z_\odot/20$ due to our mass-loss runaway. While future work on LBV mass loss could affect the PISN threshold, our framework will be critical for establishing definitive answers on the PISN threshold and galactic chemical evolution modelling., Comment: Accepted for publication in MNRAS, 18 pages, 8 figures, 2 appendix

Stars strongly impact their environment, and shape structures on all scales throughout the universe, in a process known as ``feedback''. Due to the complexity of both stellar evolution and the physics of larger astrophysical structures, there remain many unanswered questions about how feedback operates, and what we can learn about stars by studying their imprint on the wider universe. In this white paper, we summarize discussions from the Lorentz Center meeting `Bringing Stellar Evolution and Feedback Together' in April 2022, and identify key areas where further dialogue can bring about radical changes in how we view the relationship between stars and the universe they live in., Accepted to the Publications of the Astronomical Society of the Pacific

The Vera C. Rubin Legacy Survey of Space and Time holds the potential to revolutionize time domain astrophysics, reaching completely unexplored areas of the Universe and mapping variability time scales from minutes to a decade. To prepare to maximize the potential of the Rubin LSST data for the exploration of the transient and variable Universe, one of the four pillars of Rubin LSST science, the Transient and Variable Stars Science Collaboration, one of the eight Rubin LSST Science Collaborations, has identified research areas of interest and requirements, and paths to enable them. While our roadmap is ever-evolving, this document represents a snapshot of our plans and preparatory work in the final years and months leading up to the survey's first light., Comment: 202 pages (in book format) 34 figures plus chapter heading figures (13)

The ages of stars have historically relied on isochrone fitting of standardised grids of models. While these stellar models have provided key constraints on observational samples of massive stars, they inherit many systematic uncertainties, mainly in the internal mixing mechanisms applied throughout the grid, fundamentally undermining the isochrone method. In this work, we utilise the M-L plane of Higgins & Vink as a method of determining stellar age, with mixing-corrected models applying a calibrated core overshooting alpha_ov and rotation rate to fit the observational data. We provide multiple test-beds to showcase our new method, while also providing comparisons to the commonly-used isochrone method, highlighting the dominant systematic errors. We reproduce the evolution of individual O stars, and analyse the wider sample of O and B supergiants from the VLT-FLAMES Tarantula Survey, providing dedicated models with estimates for alpha_ov, Omega/Omega_crit, and ultimately stellar ages. The M-L plane highlights a large discrepancy in the spectroscopic masses of the O supergiant sample. Furthermore the M-L plane also demonstrates that the evolutionary masses of the B supergiant sample are inappropriate. Finally, we utilise detached eclipsing binaries, VFTS 642 and VFTS 500, and present their ages resulting from their precise dynamical masses, offering an opportunity to constrain their interior mixing. For the near-TAMS system, VFTS 500, we find that both components require a large amount of core overshooting (alpha_ov ~ 0.5), implying an extended main-sequence width. We hence infer that the vast majority of B supergiants are still burning hydrogen in their cores., 13 pages, 13 figures. Accepted for publication in MNRAS. Comments welcome

The most massive stars are thought to lose a significant fraction of their mass in a steady wind during the main-sequence and blue supergiant phases. This in turn sets the stage for their further evolution and eventual supernova, with consequences for ISM energization and chemical enrichment. Understanding these processes requires accurate observational constraints on the mass-loss rates of the most luminous stars, which can also be used to test theories of stellar wind generation. In the past, mass-loss rates have been characterized via collisional emission processes such as H$\alpha$ and free-free radio emission, but these so-called "density squared" diagnostics require correction in the presence of widespread clumping. Recent observational and theoretical evidence points to the likelihood of a ubiquitously high level of such clumping in hot-star winds, but quantifying its effects requires a deeper understanding of the complex dynamics of radiatively driven winds. Furthermore, large-scale structures arising from surface anisotropies and propagating throughout the wind can further complicate the picture by introducing further density enhancements, affecting mass-loss diagnostics. Time series spectroscopy of UV resonance lines with high resolution and high signal-to-noise are required to better understand this complex dynamics, and help correct "density squared" diagnostics of mass-loss rates. The proposed Polstar mission easily provides the necessary resolution at the sound-speed scale of 20 km s$^{-1}$, on three dozen bright targets with signal-to noise an order of magnitude above that of the celebrated IUE MEGA campaign, via continuous observations that track structures advecting through the wind in real time., Comment: white paper

Massive stars have strong stellar winds that direct their evolution through the upper Hertzsprung-Russell diagram and determine the black hole mass function. Secondly, wind strength dictates the atmospheric structure that sets the ionising flux. Thirdly, 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 co-moving frame computations, revealing a rich multi-variate behaviour of the mass-loss rate dM/dt in terms of M, L, Eddington Gamma, Teff , and chemical composition Z. Concerning the latter, dM/dt 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 behaviour, there are several transitions in the Hertzsprung-Russell diagram, involving bi-stability jumps around Fe recombination temperatures, leading to quasi-stationary episodic, and not necessarily eruptive, Luminous Blue Variable and pre-SN mass loss. * Moreover, there are kinks. At 100 Solar Masses, a high Gamma 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 ionising radiation that could play a key role in local star formation as well as at high-redshift., Comment: Invited Annual Review of Astronomy and Astrophysics (ARA&A 2022) - 49 pages, 13 figures (now Link to Review in Advance first posted online on May 13, 2022)

As sources of chemical enrichment, ionizing radiation and energetic feedback, massive stars drive the ecology of their host galaxies despite their relative rarity, additionally to yielding compact remnants, which can generate gravitational waves. The evolution of massive stars is crucially informed by their detailed mass-loss history; however, wind structures on a variety of scales cause important uncertainties on their mass-loss rates. Binary systems can place further constraints on the mass-loss properties of massive stars, especially colliding-wind binaries. In this paper, we review how the proposed MIDEX-scale mission Polstar can critically constrain the physics of colliding winds (and hence radiatively-driven winds in general) with ultraviolet spectropolarimetric observations, providing an unprecedented improvement on the accuracy of the determination of both mass-loss rates and the velocity structure of the winds of massive stars. We propose a sample of 17 targets that will allow us to study a variety of wind-colliding systems spanning a large parameter space using the spatial information yielded by both spectroscopic and polarimetric data obtained with Polstar., Comment: 21 pages, 12 figures 1 table, Whitepaper

We find that applying a theoretical wind mass-loss rate from Monte Carlo radiative transfer models for hydrogen-deficient stars results in significantly more leftover hydrogen following stable mass transfer through Roche-lobe overflow than when we use an extrapolation of an empirical fit for Galactic Wolf-Rayet stars, for which a negligible amount of hydrogen remains in a large set of binary stellar evolution computations. These findings have implications for modelling progenitors of Type Ib and Type IIb supernovae. Most importantly, our study stresses the sensitivity of the stellar evolution models to the assumed mass-loss rates and the need to develop a better theoretical understanding of stellar winds.

A longstanding quest in studies of luminous, massive stars has been to understand the role of environment on their evolution. The abundance of metals in their atmospheres has a significant impact on their physical properties, strongly influencing the feedback they have on their surroundings and the nature of their explosive deaths. To date we have been unable to study massive stars with metallicities below 10% that of the Sun. The low oxygen abundance (3% solar) and relative proximity (~1.6 Mpc) of Leo P, a low-luminosity dwarf galaxy discovered in 2013, provides a tantalising opportunity to investigate massive stars with near-primordial compositions. Here we introduce observations of Leo P with the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT, which have revealed its spectroscopic content for the first time., Published in The Messenger vol. 174, pp. 24-27, December 2018.

The Wolf-Rayet (WR) phenomenon is widespread in astronomy. It involves classical WRs, very massive stars (VMS), WR central stars of planetary nebula CSPN [WRs], and supernovae (SNe). But what is the root cause for a certain type of object to turn into an emission-line star? In this contribution, I discuss the basic aspects of radiation-driven winds that might reveal the ultimate difference between WR stars and canonical O-type stars. I discuss the aspects of (i) self-enrichment via CNO elements, (ii) high effective temperatures Teff, (iii) an increase in the helium abundance Y, and finally (iv) the Eddington factor Gamma. Over the last couple of years, we have made a breakthrough in our understanding of Gamma-dependent mass loss, which will have far-reaching consequences for the evolution and fate of the most massive stars in the Universe. Finally, I discuss the prospects for studies of the WR phenomenon in the highest redshift Ly-alpha and He II emitting galaxies., Comment: Invited review, in "International Workshop on Wolf-Rayet Stars" (eds. W.R. Hamann, A. Sander, and H. Todt), Universit\"atsverlag Potsdam), 5 pages, 4 figures

Recent studies suggest the existence of very massive stars (VMS) up to 300 solar masses in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 solar masses, it is timely to evaluate the physics specific to VMS, which is currently missing. For this reason, we decided to construct a book entailing both a discussion of the accuracy of VMS masses (Martins), as well as the physics of VMS formation (Krumholz), mass loss (Vink), instabilities (Owocki), evolution (Hirschi), and fate (theory -- Woosley & Heger; observations -- Smith)., 8 pages, Introductory Chapter to the Book on Very Massive Stars in the Local Universe, Springer, Ed. Jorick S. Vink

The evolutionary state of blue supergiants is still unknown. Stellar wind mass loss is one of the dominant processes determining the evolution of massive stars, and it may provide clues on the evolutionary properties of blue supergiants. As the H$��$ line is the most oft-used mass-loss tracer in the OB-star regime, we provide a detailed analysis of the H$��$ line for OB supergiant models over an $T_{\rm eff}$ range between 30000 and 12500K. We find a maximum in the H$��$ equivalent width at 22500 K - at the location of the bi-stability jump. The H$��$ line-profile behaviour is characterised by two branches of $T_{\rm eff}$: (i) a "hot" branch between 30000 and 22500 K, where H$��$ emission becomes stronger with decreasing $T_{\rm eff}$, and (ii) a "cool" branch between 22500 and 12500 K, where the line becomes weaker. Our models show that this non-monotonic H$��$ behaviour is related to the optical depth of Ly$��$, finding that at the "cool" branch the population of the 2nd level of hydrogen is enhanced in comparison to the 3rd level. This is expected to increase line absorption, leading to weaker H$��$ flux when $T_{\rm eff}$ drops from 22500 K downwards. We also show that for late B supergiants (at $T_{\rm eff}$ below ~15000 K), the differences in the H$��$ line between homogeneous and clumpy winds becomes insignificant. Moreover, we show that at the bi-stability jump H$��$ changes its character completely, from an optically thin to an optically thick line, implying that macro-clumping should play an important role at temperatures below the bi-stability jump. This would not only have consequences for the character of observed H$��$ line profiles, but also for the reported discrepancies between theoretical and empirical mass-loss rates., Accepted for publication in A&A

Utrecht has a long tradition in both spectroscopy and mass-loss studies. Here we present a novel methodology to calibrate mass-loss rates on purely spectroscopic grounds. We utilize this to predict the final fates of massive stars, involving pair-instability and long gamma-ray bursts (GRBs) at low metallicity Z., Comment: 4 pages, 2 figures, 370 Years of Astronomy in Utrecht

We evaluate the place of Eta Carinae amongst the class of luminous blue variables (LBVs) and show that the LBV phenomenon is not restricted to extremely luminous objects like Eta Car, but extends luminosities as low as log(L/Lsun) = 5.4 - corresponding to initial masses ~25 Msun, and final masses as low as ~10-15 Msun. We present a census of S Doradus variability, and discuss basic LBV properties, their mass-loss behaviour, and whether at maximum light they form pseudo-photospheres. We argue that those objects that exhibit giant Eta Car-type eruptions are most likely related to the more common type of S Doradus variability. Alternative atmospheric models as well as sub-photospheric models for the instability are presented, but the true nature of the LBV phenomenon remains as yet elusive. We end with a discussion on the evolutionary status of LBVs - highlighting recent indications that some LBVs may be in a direct pre-supernova state, in contradiction to the standard paradigm for massive star evolution., Comment: 27 pages, 6 figures, Review Chapter in "Eta Carinae and the supernova imposters" (eds R. Humphreys and K. Davidson) new version submitted to Springer

For the occasion of the official retirement of Henny Lamers, a meeting was held to celebrate Henny's contribution to mass loss from stars and stellar clusters. Stellar mass loss is crucial for understanding the life and death of massive stars, as well as their environments. Henny has made important contributions to many aspects of our understanding of hot-star winds. Here, the most dominant aspects of the stellar part of the meeting: (i) O star wind clumping, (ii) mass loss near the Eddington limit, and (iii) and the driving of Wolf-Rayet winds, are highlighted., Comment: 5 pages, Conference summary on stellar winds to appear in "Mass loss from stars and the evolution of stellar clusters". Proc. of a workshop held in honour of H.J.G.L.M. Lamers, Lunteren, The Netherlands. Eds. A. de Koter, L. Smith and R. Waters (San Francisco: ASP)

The topic of wind-clumping has been the subject of much activity in recent years, due to the impact that it can have on derived mass-loss rates. Here we present an alternative method of investigating wind-clumping, that of polarimetry. We present simulations of the polarization produced by a clumpy wind, and argue that the observations may be reproduced just by statistical deviations from spherical symmetry when the outflow is only slightly fragmented. Here, the polarization scales with $\dot{M}$, which is consistent with observations of LBVs, WRs and O supergiants. Finally, we find clumping factors in the inner 2$R_{\star}$ of $\sim 2-3$, and speculate as to the clumping stratification of hot stars., Comment: 8 pages, 6 figs, to appear in proceedings of 'Mass loss from stars and the evolution of stellar clusters'

We present the first systematic spectropolarimetric study of Luminous Blue Variables (LBVs), and find that at least half those objects studied display evidence for intrinsic polarization -- a signature of significant inhomogeneity at the base of the wind. Furthermore, multi-epoch observations reveal that the polarization is variable in both strength and position angle. This evidence points away from a simple axi-symmetric wind structure \`{a} la the B[e] supergiants, and instead suggests a wind consisting of localised density enhancements, or `clumps'. We show with an analytical model that, in order to produce the observed variability, the clumps must be large, produced at or below the photosphere, and ejected on timescales of days. More details of LBV wind-clumping will be determined through further analysis of the model and a polarimetric monitoring campaign., Comment: 7 pages, 4 figures. To appear in the proceedings "Stars with the B[e] phenomenon"

The amount of mass loss is of fundamental importance to the lives and deaths of very massive stars, the input of chemical elements and momentum into the interstellar and intergalactic media, as well as the emitted ionizing radiation. I review mass-loss predictions for hot massive stars as a function of metal content for groups of OB stars, Luminous Blue Variables, and Wolf-Rayet stars. Although it is found that the predicted mass-loss rates drop steeply with decreasing metal content (Mdot ~ Z^{0.7-0.85}), I highlight two pieces of physics that are often overlooked: (i) mass-loss predictions for massive stars approaching the Eddington limit, and for (ii) stars that have enriched their own atmospheres with primary elements such as carbon. Both of these effects may significantly boost the mass-loss rates of the first stars - relevant for the reionization of the Universe, and a potential pre-enrichment of the intergalactic medium - prior to the first supernova explosions., Comment: 12 pages, 6 figures. Invited Review in Tartu workshop (Aug 2005). Proc "Stellar Evolution at Low Metallicity: Mass-Loss, Explosions, Cosmology" (eds: H. Lamers, N. Langer, T. Nugis), ASP Conf Series

We present Halpha spectropolarimetry of a large sample of pre-main sequence (PMS) stars of low and intermediate mass, and argue that the technique is a powerful tool in studying the circumstellar geometry around these objects. For the intermediate mass (2 -- 15 Msun) Herbig Ae/Be stars we find that 16 out of 23 show a line effect, which immediately implies that flattening is common among these objects. Furthermore, we find a significant difference in Halpha spectropolarimetry behaviour between the Herbig Be and Ae groups. For the Herbig Be stars, the concept of an electron scattering disc is shown to be a useful concept to explain the depolarizations seen in this spectral range. At lower masses, more complex Halpha polarimetry behaviour starts to appear. The concept of a compact source of Halpha emission that is formed close to the stellar surface, for instance by hot spots due to magnetospheric accretion, is postulated as a working hypothesis to qualitatively explain the Halpha spectropolarimetry behaviour around Herbig Ae and lower mass (M < 2 Msun) T Tauri stars. The striking resemblance in spectropolarimetric behaviour between the T Tauri star RY Tau and the Herbig Ae stars suggests a common origin of the polarized line photons, and hints that low and higher mass pre-main sequence stars may have more in common than had hitherto been suspected., 8 pages; 8 figures; to appear in:"Open Issues in Local Star Formation and Early Evolution"

We predict mass-loss rates for the late evolutionary phases of low-mass stars, with special emphasis on the consequences for the morphology of the Horizontal Branch (HB). We show that the computed rates, as predicted by the most plausible mechanism of radiation pressure on spectral lines, are too low to produce EHB/sdB stars. This invalidates the scenario recently outlined by Yong et al. (2000) to create these objects by mass loss on the HB. We argue, however, that mass loss plays a role in the distribution of rotational velocities of hot HB stars, and may -- together with the enhancement of heavy element abundances due to radiative levitation -- provide an explanation for the so-called ``low gravivity problem. The mass loss recipe derived for hot HB (and extreme HB, sdB, sdOB) stars may also be applied to post-HB (AGB-manque, UV-bright) stars over a range in effective temperatures between 12500 -- 40000 K., Comment: 11 pages; Accepted by A&A

We now understand how low mass stars evolve through cloud collapse and disc accretion, but whether higher mass stars are also disc accretors is as yet unknown. Spectropolarimetry observations can help in answering this basic question, as they probe the first few stellar radii around young stars., Comment: 2 figures, 2 pages, to appear in: "The Earliest Phases of Massive Star Birth"