arXiv subjects

Götz Gräfener

Publications and source records attributed to Götz Gräfener.

13 recordsLinked to original sources

arXiv2026

Evolution of wide O star binaries through their LBV stage. Population synthesis with mass-ejection-driven orbital evolution

Context. Long-period Wolf-Rayet (WR) star binaries produced by mass transfer are predicted to be abundant, but are observationally rare. This yields constraints on the evolution of initially wide O star binaries, including those potentially leading to the formation of gravitational-wave sources through the Common Envelope Channel. Aims. We investigate this issue in the light of a new type of orbital evolution for initially wide O star binaries, which is driven by mass ejection at periastron passage during the Luminous Blue Variable (LBV) phase. Methods. The assumption that the mass ejection occurs instantly at periastron passage allows us to analytically describe the orbital evolution. This approach is motivated by our understanding of an Eddington-limit driven LBV phase. We perform population synthesis calculations for the WR stars in the Small Magellanic Cloud (SMC), and compare them to the observed SMC WR star population. Results. Different from mass transfer, our mass ejection scenario leads to increased orbital periods and eccentricities. The Galactic system WR 140 (orbital period 2895 d, eccentricity 0.9) could be a typical result of this evolution scenario. Our models predict measurable binary space velocities, and allow for the disruption of the binary. Our SMC population synthesis model predicts statistically 5.3 close, 3.7 long-period, and further 2 runaway single WR stars. With largely increased orbital periods and eccentricities, such WR+O star binaries may not be ruled out by past radial-velocity searches. Applying our scenario to the Gaia BH1 and BH2 systems, we find that it provides viable progenitor evolution models. Conclusions. The mass-ejection-driven orbital evolution could explain why so few wide WR binaries are observed, and why some of the apparently single WR stars have high space velocities. We discuss implications for gravitational-wave sources.

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arXiv2023

Extinction towards the cluster R136 in the Large Magellanic Cloud: An extinction law from the near-infrared to the ultraviolet

The cluster R136 in the giant star-forming region 30 Doradus in the Large Magellanic Cloud (LMC) offers a unique opportunity to resolve a stellar population in a starburst-like environment. We obtain the near-infrared to ultraviolet extinction towards 50 stars in the core of R136, employing the `extinction without standards' method. To assure good fits over the full wavelength range, we combine and modify existing extinction laws. We detect a strong spatial gradient in the extinction properties across the core of R136, coinciding with a gradient in density of cold gas that is part of a molecular cloud lying northeast of the cluster. In line with previous measurements of R136 and the 30 Doradus region, we obtain a high total-to-relative extinction ($R_V = 4.38 \pm 0.87$). However, the high values of $R_V$ are accompanied by relatively strong extinction in the ultraviolet, contrary to what is observed for Galactic sightlines. The relatively strong ultraviolet extinction suggests that the properties of the dust towards R136 differ from those in the Milky Way. For $R_{V} \sim 4.4$, about three times fewer ultraviolet photons can escape from the ambient dust environment relative to the canonical Galactic value of $R_{V} \sim 3.1$ at the same $A_{V}$. Therefore, if dust in the R136 star-bursting environment is characteristic for cosmologically distant star-bursting regions, the escape fraction of ultraviolet photons from such regions is overestimated by a factor of three relative to the standard Milky Way assumption for the total-to-selective extinction. Furthermore, a comparison with average curves tailored to other regions of the LMC shows that large differences in ultraviolet extinction exist within this galaxy. Further investigation is required in order to decipher whether or not there is a relation between $R_V$ and ultraviolet extinction in the LMC.

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arXiv2022

Thermonuclear and Electron-Capture Supernovae from Stripped-Envelope Stars

(abridged) When stripped from their hydrogen-rich envelopes, stars with initial masses between $\sim$7 and 11 M$_\odot$ develop massive degenerate cores and collapse. Depending on the final structure and composition, the outcome can range from a thermonuclear explosion, to the formation of a neutron star in an electron-capture supernova (ECSN). It has been recently demonstrated that stars in this mass range may initiate explosive oxygen burning when their central densities are still below $ρ_{\rm c} \lesssim 10^{9.6}$ g cm$^{-3}$. This makes them interesting candidates for type Ia supernovae -- which we call (C)ONe SNe Ia -- and might have broader implications for the formation of neutron stars via ECSNe. Here, we model the evolution of 252 helium stars with initial masses in the $0.8-3.5$ M$_\odot$ range, and metallicities between $Z=10^{-4}$ and $0.02$. We use these models to constrain the central densities, compositions and envelope masses at the time of explosive oxygen ignition. We further investigate the sensitivity of these properties to mass loss rate assumptions using additional models with varying wind efficiencies. We find that helium stars with masses between $\sim$1.8 and 2.7 M$_\odot$ evolve onto $1.35-1.37$ M$_\odot$ (C)ONe cores that initiate explosive burning at central densities between $\rm \log_{10}(ρ_c)\sim 9.3$ and 9.6. We constrain the amount of residual carbon retained after core carbon burning, and conclude that it plays a critical role in determining the final outcome: Chandrasekhar-mass degenerate cores that retain more than $\sim 0.005$ M$_\odot$ of carbon result in (C)ONe SNe Ia, while those with lower carbon mass become ECSNe. We find that (C)ONe SNe Ia are more likely to occur at high metallicities, whereas at low metallicities ECSNe dominate.

astro-ph.HE↗

arXiv2022

The R136 star cluster dissected with Hubble Space Telescope/STIS. III. The most massive stars and their clumped winds

Context: The star cluster R136 inside the LMC hosts a rich population of massive stars, including the most massive stars known. The strong stellar winds of these very luminous stars impact their evolution and the surrounding environment. We currently lack detailed knowledge of the wind structure that is needed to quantify this impact. Aims: To observationally constrain the stellar and wind properties of the massive stars in R136, in particular the parameters related to wind clumping. Methods: We simultaneously analyse optical and UV spectroscopy of 53 O-type and 3 WNh-stars using the FASTWIND model atmosphere code and a genetic algorithm. The models account for optically thick clumps and effects related to porosity and velocity-porosity, as well as a non-void interclump medium. Results: We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities and clumping characteristics and compare these to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities. Conclusions: We confirm a cluster age of 1.0-2.5 Myr and derive an initial stellar mass of $\geq 250 {\rm M}_\odot$ for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of $f_{\rm cl}=29\pm15$. We find tentative trends in the wind-structure parameters as a function of mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped. We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduces both quantities. The prescription of Krtička & Kubát (2018) matches best the observed mass-loss rates.

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arXiv2022

Stripped-Envelope Stars in Different Metallicity Environments I. Evolutionary Phases, Classification and Populations

Massive stars that become stripped of their hydrogen envelope through binary interaction or winds can be observed either as Wolf-Rayet stars, if they have optically thick winds, or as transparent-wind stripped-envelope stars. We approximate their evolution through evolutionary models of single helium stars, and compute detailed model grids in the initial mass range 1.5 to 70 M$_{\odot}$ for metallicities between 0.01 and 0.04, from core helium ignition until core collapse. Throughout their lifetime, some stellar models expose the ashes of helium burning. We propose that models that have nitrogen-rich envelopes are candidate WN stars, while models with a carbon-rich surface are candidate WC stars during core helium burning, and WO stars afterwards. We measure metallicity dependance of the total lifetime of our models and the duration of their evolutionary phases. We propose an analytic estimate of the wind optical depth to distinguish models of Wolf-Rayet stars from transparent-wind stripped-envelope stars, and find that the luminosity ranges at which WN, WC and WO type stars can exist is a strong function of metallicity. We find that all carbon-rich models produced in our grids have optically thick winds and match the luminosity distribution of observed populations. We construct population models and predict the numbers of transparent-wind stripped-envelope stars and Wolf-Rayet stars, and derive their number ratios at different metallicities. We find that as metallicity increases, the number of transparent-wind stripped-envelope stars decreases and the number of Wolf-Rayet stars increases. At high metallicities WC and WO type stars become more common. We apply our population models to nearby galaxies, and find that populations are more sensitive to the transition luminosity between Wolf-Rayet stars and transparent-wind helium stars than to the metallicity dependent mass loss rates.

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arXiv2021

Physics and evolution of the most massive stars in 30 Dor. Mass loss, envelope inflation, and a variable upper stellar mass limit

The identification of stellar-mass black-hole mergers with up to 80 Msun as powerful sources of gravitational wave radiation led to increased interest in the physics of the most massive stars. The largest sample of possible progenitors of such objects, very massive stars (VMS) with masses up to 300 Msun, have been identified in the 30 Dor star-forming region in the Large Magellanic Cloud (LMC). The physics and evolution of VMS is highly uncertain, mainly due to their proximity to the Eddington limit. In this work we investigate the two most important effects that are thought to occur near the Eddington limit. Enhanced mass loss through optically thick winds, and the formation of radially inflated stellar envelopes. We compute evolutionary models for VMS at LMC metallicity and perform a population synthesis of the young stellar population in 30 Dor. We find that enhanced mass loss and envelope inflation have a dominant effect on the evolution of the most massive stars. While the observed mass-loss properties and the associated surface He-enrichment are well described by our new models, the observed O-star mass-loss rates are found to cover a much larger range than theoretically predicted, with particularly low mass-loss rates for the youngest objects. Also, the (rotational) surface enrichment in the O-star regime appears to be not well understood. The positions of the most massive stars in the Hertzsprung-Russell Diagram (HRD) are affected by mass loss and envelope inflation. For instance, the majority of luminous B-supergiants in 30 Dor, and the lack thereof at the highest luminosities, can be explained through the combination of envelope inflation and mass loss. Finally, we find that the upper limit for the inferred initial stellar masses in the greater 30 Dor region is significantly lower than in its central cluster R 136, implying a variable upper limit for the masses of stars.

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arXiv2020

Type Ia supernovae from non-accreting progenitors

Type Ia supernovae (SNe Ia) are manifestations of stars deficient of hydrogen and helium disrupting in a thermonuclear runaway. While explosions of carbon-oxygen white dwarfs are thought to account for the majority of events, part of the observed diversity may be due to varied progenitor channels. We demonstrate that helium stars with masses between $\sim$1.8 and 2.5 M$_{\odot}$ may evolve into highly degenerate, near-Chandrasekhar mass cores with helium-free envelopes that subsequently ignite carbon and oxygen explosively at densities $\sim(1.8-5.9)\times 10^{9}$g cm$^{-3}$. This happens either due to core growth from shell burning (when the core has a hybrid CO/NeO composition), or following ignition of residual carbon triggered by exothermic electron captures on $^{24}$Mg (for a NeOMg-dominated composition). We argue that the resulting thermonuclear runaways is likely to prevent core collapse, leading to the complete disruption of the star. The available nuclear energy at the onset of explosive oxygen burning suffices to create ejecta with a kinetic energy of $\sim$10$^{51}$ erg, as in typical SNe Ia. Conversely, if these runaways result in partial disruptions, the corresponding transients would resemble SN Iax events similar to SN 2002cx. If helium stars in this mass range indeed explode as SNe Ia, then the frequency of events would be comparable to the observed SN Ib/c rates, thereby sufficing to account for the majority of SNe Ia in star-forming galaxies.

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arXiv2018

Type IIP supernova light curves affected by the acceleration of red supergiant winds

We introduce the first synthetic light-curve model set of Type IIP supernovae exploded within circumstellar media in which the acceleration of the red supergiant winds is taken into account. Because wind acceleration makes the wind velocities near the progenitors low, the density of the immediate vicinity of the red supergiant supernova progenitors can be higher than that extrapolated by using a constant terminal wind velocity. Therefore, even if the mass-loss rate of the progenitor is relatively low, it can have a dense circumstellar medium at the immediate stellar vicinity and the early light curves of Type IIP supernovae are significantly affected by it. We adopt a simple beta velocity law to formulate the wind acceleration. We provide bolometric and multicolor light curves of Type IIP supernovae exploding within such accelerated winds from the combinations of three progenitors, 12 - 16 Msun; five beta, 1-5; seven mass-loss rates, 1e-5 - 1e-2 Msun/yr; and four explosion energies, 0.5e51 - 2e51 erg. All the light curve models are available at https://goo.gl/o5phYb. When the circumstellar density is sufficiently high, our models do not show a classical shock breakout as a consequence of the interaction with the dense and optically-thick circumstellar media. Instead, they show a delayed 'wind breakout', substantially affecting early light curves of Type IIP supernovae. We find that the mass-loss rates of the progenitors need to be 1e-3 - 1e-2 Msun/yr to explain typical rise times of 5 - 10 days in Type IIP supernovae assuming a dense circumstellar radius of 1e15 cm.

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arXiv2017

Immediate dense circumstellar environment of supernova progenitors caused by wind acceleration: its effect on supernova light curves

Type IIP supernova progenitors are often surrounded by dense circumstellar media that may result from mass-loss enhancement of the progenitors shortly before their explosions. Previous light-curve studies suggest that the mass-loss rates are enhanced up to ~ 0.1 Msun/yr, assuming a constant wind velocity. However, density of circumstellar media at the immediate stellar vicinity can be much higher than previously inferred for a given mass-loss rate if wind acceleration is taken into account. We show that the wind acceleration has a huge impact when we estimate mass-loss rates from early light curves of Type IIP supernovae by taking SN 2013fs as an example. We perform numerical calculations of the interaction between supernova ejecta and circumstellar media with a constant mass-loss rate but with a beta-law wind velocity profile. We find that the mass-loss rate of the progenitor of SN 2013fs shortly before the explosion, which was inferred to be ~ 0.1 Msun/yr with a constant wind velocity of 10 km/s by a previous light-curve modeling, can be as low as ~ 1e-3 Msun/yr with the same terminal wind velocity of 10 km/s but with a wind velocity profile with beta ~ 5. In both cases, the mass of the circumstellar medium is similar (~ 0.5 Msun). Therefore, the beginning of the progenitor's mass-loss enhancement in our interpretation is ~ 100 years before the explosion, not several years. Our result indicates that the immediate dense environment of Type II supernova progenitors may be significantly influenced by wind acceleration.

astro-ph.HE↗

arXiv2016

Two bi-stability jumps in theoretical wind models for massive stars and the implications for Luminous Blue Variable supernovae

Luminous Blue Variables have been suggested to be the direct progenitors of supernova types IIb and IIn, with enhanced mass loss prior to explosion. However, the mechanism of this mass loss is not yet known. Here, we investigate the qualitative behaviour of theoretical stellar wind mass-loss as a function of Teff across two bi-stability jumps in blue supergiant regime and also in proximity to the Eddington limit, relevant for LBVs. To investigate the physical ingredients that play a role in the radiative acceleration we calculate blue supergiant wind models with the CMFGEN non-LTE model atmosphere code over an effective temperature range between 30000 and 8800 K. Although our aim is not to provide new mass-loss rates for BA supergiants, we study and confirm the existence of two bi-stability jumps in mass-loss rates predicted by Vink, de Koter, & Lamers (1999). However, they are found to occur at somewhat lower Teff (20000 and 9000 K, respectively) than found previously, which would imply that stars may evolve towards lower Teff before strong mass-loss is induced by the bi-stability jumps. When the combined effects of the second bi-stability jump and the proximity to Eddington limit are accounted for, we find a dramatic increase in the mass-loss rate by up to a factor of 30. Further investigation of both bi-stability jumps is expected to lead to a better understanding of discrepancies between empirical modelling and theoretical mass-loss rates reported in the literature, and to provide key inputs for the evolution of both normal AB supergiants and LBVs, as well as their subsequent supernova type II explosions.

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arXiv2014

The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence

The evolution and fate of very massive stars (VMS) is tightly connected to their mass-loss properties. Their initial and final masses differ significantly as a result of mass loss. VMS have strong stellar winds and extremely high ionising fluxes, which are thought to be critical sources of both mechanical and radiative feedback in giant Hii regions. However, how VMS mass-loss properties change during stellar evolution is poorly understood. In the framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss transition region from optically thin O to denser WNh star winds, thereby testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and WNh stars, an unprecedented sample of stars with the highest masses and luminosities known. We perform a spectral analysis of optical VFTS as well as near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN to obtain stellar and wind parameters. For the first time, we observationally resolve the transition between optically thin O and optically thick WNh star winds. Our results suggest the existence of a kink between both mass-loss regimes, in agreement with recent MC simulations. For the optically thick regime, we confirm the steep dependence on the Eddington factor from previous theoretical and observational studies. The transition occurs on the MS near a luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find that - even when accounting for moderate wind clumping (with f = 0.1) - wind mass-loss rates are enhanced with respect to standard prescriptions currently adopted in stellar evolution calculations. We also show that this results in substantial helium surface enrichment. Based on our spectroscopic analyses, we are able to provide the most accurate ionising fluxes for VMS known to date, confirming the pivotal role of VMS in ionising and shaping their environments.

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arXiv2014

On the H$α$ Behaviour of Blue Supergiants: Rise and Fall over the Bi-stability Jump

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$α$ line is the most oft-used mass-loss tracer in the OB-star regime, we provide a detailed analysis of the H$α$ line for OB supergiant models over an $T_{\rm eff}$ range between 30000 and 12500K. We find a maximum in the H$α$ equivalent width at 22500 K - at the location of the bi-stability jump. The H$α$ line-profile behaviour is characterised by two branches of $T_{\rm eff}$: (i) a "hot" branch between 30000 and 22500 K, where H$α$ 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$α$ behaviour is related to the optical depth of Ly$α$, 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$α$ 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$α$ line between homogeneous and clumpy winds becomes insignificant. Moreover, we show that at the bi-stability jump H$α$ 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$α$ line profiles, but also for the reported discrepancies between theoretical and empirical mass-loss rates.

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arXiv2012

The transition mass-loss rate: Calibrating the role of line-driven winds in massive star evolution

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

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