arXiv subjects

J. O. Sundqvist

Publications and source records attributed to J. O. Sundqvist.

At least 19 recordsLinked to original sources

arXiv2026

Effects on vsini determinations of O stars from 3D model atmospheres with high turbulent velocities

When studying massive stars and their life cycles, rotation plays a key role. Hence, understanding the rotation of these stars is crucial when determining their properties, or for constraining evolutionary models. We examine the reliability of the standard methods to derive projected rotation speeds vsini from photospheric spectra of hot, massive stars in the presence of large turbulent velocities. We include rotation in the spectral synthesis of O-stars by means of three-dimensional model atmospheres showing significant photospheric turbulent velocities. We then use these as mock-observations to back-test the Fourier Transform and goodness-of-fit methods commonly used for empirical determination of vsini when the turbulent velocity field is not known. When the expected vsini>v_mac, with v_mac the macroturbulent velocity, FT vsini determinations (most of the time) give reasonable results. However, if vsini < v_mac the method is no longer reliable. Results from the GOF method show that if one parameter is not significantly larger than the other, empirical best-fit values may be located in a large region of the vsini-v_mac parameter space, independent of the true values. The degenerate region follows well the empirical formula found by Howarth et al. (2007), sqrt(v_mac^2/2+vsini^2/4). In other words, only this sum can be constrained by GOF observational analysis. Our analysis shows clearly that, generally, only the sum sqrt(v_mac^2/2+vsini^2/4) can be constrained through the standard spectroscopic methods used to infer these parameters individually. Only in the case where one of the two clearly dominates, can good constraints on the dominating parameter be derived. This demonstrates that previously found empirical correlations between vsini and v_mac as well as derived statistical distributions of observed rotation rates for O-star populations will need to be re-analysed and interpreted.

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arXiv2026

X-Shooting ULLYSES: Massive stars at low metallicity XV. On the metallicity dependence of B-supergiant mass-loss rates

Context. For stellar evolution models we rely on mass-loss rate prescriptions that show features that lack empirical confirmation, such as the bi-stability jump. This jump is an increase in mass loss in the decreasing temperature regime Teff 28-21 kK. Although papers compared empirical results to prescriptions,a large observational sample of B stars for which the wind has been scrutinised over different metallicities is still lacking. Aims. By modelling of both ultraviolet (ULLYSES) and optical (XShootU) spectra, we determined the stellar and wind parameters, of 24 SMC B stars ranging in Teff from 13 to 29 kK. By combining this sample with LMC studies, we compared the wind behaviour of B stars in two different metallicity regimes. We compared our results to common mass-loss rate prescriptions to test features present in these models and their metallicity dependence. Methods. We have used the model atmosphere code fastwind and the genetic algorithm code Kiwi-GA to fit the UV and optical spectra simultaneously. This allows us to determine wind properties including clumping behaviour. Results. The metallicity trends present in the mass-loss prescriptions (Z^(0.41-1.4)) explored here overestimate the empirical metallicity dependence in the B-star regime, which appears very weak. We do not find an increase in mass-loss rate at approximately spectral type B1. We show that on average 40% of the wind mass is located in the wind medium between the clumps. We compiled a sample of 80+ O and B stars in the SMC and LMC. From a comparison we find a clear difference in O- and B-type metallicity dependence. Conclusions. The lack of a bi-stability jump in the B-star regime and a weak metallicity dependence for the same stars offers new empirical constraints to models of line-driven winds. As differences between these models are large (reaching orders of magnitude) such constraints are much needed.

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arXiv2026

Simulations of massive star atmospheres and winds during giant eruptive and quiescent luminous blue variable phases

Mass loss from massive stars located in the part of the Hertzsprung-Russell diagram (HRD) where we find luminous blue variables (LBVs) is profoundly important for stellar evolution yet poorly understood. We use time-dependent radiation-hydrodynamic (RHD) simulations to examine the atmosphere and wind properties of such massive stars, computing 2D and 1D RHD models of the coupled envelopes, atmospheres, and wind outflows, tuned to this region in the HRD. Our unified simulations start deep in the stellar envelope (well below T ~ 200 kK) and include the outflowing wind, accounting for line-driving, radiative enthalpy, and photon tiring. Mass-loss rates, wind speeds, and the radiative luminosity at the photosphere are emergent properties in the simulations. A grid of models is created by slightly increasing the stellar energy at the lower boundary. This results in a natural transition from very turbulent atmospheres with line-driven winds to effectively stationary super-Eddington massive outflows. Our sub-Eddington models are essentially blue hypergiant stars with very variable surfaces, effective mass-loss rates $\dot{M} \sim 2 - 5 \times 10^{-5}$ $M_{\odot}$/year, and wind speeds $v_{\infty} \sim 200 - 300$ km/s, resembling quiescent LBVs like P Cygni. The super-Eddington models have optically thick wind envelopes and extremely inflated yellow surfaces (Teff ~ 5000 K), $\dot{M} \sim 0.1 - 1$ $M_{\odot}$/year, and $v_{\infty} \sim 400 - 500$ km/s, resembling a massive star during a great eruption like eta Carinae's. Our models naturally reproduce the overall characteristic stellar and wind parameters inferred for massive stars in their quiescent LBV and yellow giant eruptive phases. It remains an open question whether the energy increase needed to trigger a giant eruption can be obtained solely by the internal evolution of the star itself or if it requires an external energy source.

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arXiv2026

The Tarantula massive binary monitoring VII. The nature of the eccentric O+BH binary candidate VFTS 812

Massive O-type stars ($M\gtrsim15\,M_\odot$) with an X-ray quiet black hole (BH) companion represent a crucial stage in massive binary evolution leading to binary BH mergers. The population of such binaries remains elusive, with $\lesssim5$ candidate or confirmed systems. The Tarantula nebula harbors thousands of massive stars, 2-3 % of which are expected to have BH companions. It is therefore an ideal place to hunt for such systems. Here we analyse 30 epochs of VLT/FLAMES IFU high-resolution observations of the H$δ$ region, as well as archival FLAMES spectroscopy, of VFTS 812, a 17-day single-lined spectroscopic binary with an O4V primary and a minimum secondary mass of $5.1\,M_\odot$. Following careful removal of the nebular contamination, spectral disentangling on the new data did not reveal any signature of the hidden companion. We derive $T_\mathrm{eff}=49^{+3}_{-4}$ kK, $\log L/L_\odot=5.7\pm0.1$ and $v_\mathrm{rot,max}{\rm \,sin\,}i=110^{+25}_{-35}$ km/s for the O4V component, yielding a (single star) evolutionary mass of $53^{+6}_{-5}$ $M_\odot$ and an age in the range of 0-1.6 Myr. Using injection tests of various luminous artificial companions in our data, we exhaustively rule out the presence of any luminous signature from a main sequence star more massive than $6\,M_\odot$. We discuss the possible nature of the companion, suggesting that the rejuvenated O star + BH companion is the most suitable scenario to consistently explain the location, (rejuvenated) young age, eccentricity and lack of companion signature. While this establishes VFTS 812 as a strong candidate O+BH system, follow-up observations are deemed necessary for robust confirmation and to search for accretion signatures on the O4V star.

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arXiv2025

Spectral synthesis of 3D unified model atmospheres with winds for O stars

Spectroscopic studies of massive and luminous O-type stellar atmospheres and winds have primarily been done by using 1D, spherically symmetric and stationary models. Both observations and modern theoretical models show that such stars have highly structured and variable atmospheres and winds. We present first spectral synthesis based on 3D time-dependent unified RHD model atmospheres with winds for O stars. We first carried out time-dependent, 3D simulations of unified atmospheres with winds. We then used 3D radiative transfer to compute surface brightness maps for the optical continuum as well as integrated flux profiles for select diagnostic lines. To derive occupation numbers and source functions, an aNLTE method was used, as well as scattering source functions. Our continuum intensity maps of a prototypical early O star reveal a highly variable and time-dependent surface, characterised by local emergent radiation temperature variations. Our averaged synthetic line profiles of optical absorption lines have large widths, without applying any macro- or microturbulence. From the simulations we find correspondingly large velocity dispersions in the photospheric layers. Additionally, the absorption line EWs are larger than for comparable 1D models. First results using scattering source functions further demonstrate that characteristic features like the softening of the blue edge of strong ultra-violet wind lines are qualitatively well-reproduced by our models. Our 3D simulations clearly predict a highly structured and strongly variable O star surface. First line profile results further suggest that several observed features are naturally reproduced by our models without the need to introduce ad-hoc spectral fitting parameters. We also discuss how using 3D rather than 1D simulations as a basis for future studies may affect the derivation of fundamental stellar parameters.

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arXiv2025

An on-the-fly line-driven-wind iterative mass-loss estimator (LIME) for hot, massive stars of arbitrary chemical compositions

Mass-loss rates from hot, massive stars are important for a range of astrophysical applications. We present \href{https://lime.ster.kuleuven.be/}{LIME}, a fast, efficient, and easy-to-use real-time mass-loss calculator for line-driven winds from hot, massive stars with given stellar parameters and arbitrary chemical compositions. The tool is publicly available online. We compute the line force on-the-fly from excitation and ionization balance calculations using a large atomic data base containing more than four million spectral lines. We then derive mass-loss rates from line-driven wind theory, including effects of a finite stellar disk and gas sound speed. For a given set of stellar parameters and chemical composition, we obtain predictions for mass-loss rates and for the three line-force parameters at the wind critical point. A comparison of our predicted mass-loss rates with a large sample of recent, state-of-the-art, homogeneously derived empirical mass-loss rates obtained from the XshootU collaboration project demonstrates that the simple calculator presented here performs on average as well as, or even better than, other available mass-loss recipes based on fits to restricted model grids computed from more sophisticated but less flexible methods. In addition to its speed and simplicity, a strength of our mass-loss calculator is that it avoids uncertainties related to applying fit formulae to underlying model grids calculated for more restricted parameter ranges. In particular, individual chemical abundances can be easily modified, and their effects on predicted mass-loss rates can be readily explored. This enables direct applications also to stars that are significantly chemically modified at the surface.

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arXiv2025

Improving 1D stellar atmosphere models with insights from multi-dimensional simulations II. 1D versus 3D hydrodynamically consistent model comparison for WR stars

Classical Wolf-Rayet (cWR) stars are evolved massive stars that have lost most of their H envelope and exhibit dense, extended atmospheres with strong, line-driven winds. Accurately modeling wind launching from optically thick layers remains a challenge. Two main approaches have advanced our understanding: 1D stationary atmosphere models with consistent hydrodynamics and time-dependent, multi-dimensional radiation-hydrodynamic simulations. Due to high computational demands, multi-dimensional models are limited in scope. Therefore, 1D hydrodynamically consistent models remain essential but must incorporate insights from 3D simulations. We compare averaged stratifications from recent multi-dimensional cWR models with 1D models computed using the hydrodynamically consistent PoWR$^{HD}$ code. We focus on winds driven by the hot iron opacity bump and explore how variations in 1D input parameters affect model outcomes. The 1D models reproduce the average 3D density structure well. While mass-loss rates are typically $\lesssim$0.2 dex higher in 1D models, small adjustments accounting for multi-dimensional dispersion reconcile the differences. 1D models tend to be more radially extended, with higher terminal velocities and lower effective temperatures. They reproduce the general velocity trends of 3D models but launch winds slightly further out and reach higher velocities during the hot iron bump. These differences also manifest in synthetic spectra computed from different 1D model approaches. Despite methodological variations, both 1D and averaged 3D models yield consistent stellar parameters when accounting for the variability seen in time-dependent simulations. For stars near the Eddington limit, reducing Doppler velocities in 1D models improves agreement in mass-loss rates, temperatures, and wind velocities. Matching temperature structures in optically thin layers remains an open challenge.

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arXiv2025

Structure formation in O-type stars and Wolf-Rayet stars

Turbulent small-scale structures in the envelopes and winds of massive stars have long been suggested as the cause for excessive line broadening that could not be explained by other mechanisms such as thermal broadening. However, the origin of these structures, particularly in the envelope, has not been extensively studied. We study the origin of structures seen in 2D unified stellar atmosphere and wind simulations of O stars and Wolf-Rayet (WR) stars. Particularly, we study whether the structure growth in the simulations is consistent with sub-surface convection, as is commonly assumed to be the origin of this turbulence. Using a linear stability analysis of the optically thick envelopes of massive stars, we identified multiple instabilities that could drive structure growth. We quantified the structure growth in the non-linear simulations of O stars and WR stars by computing density power spectra and tracking their temporal evolution. Then, we compared these results to the analytical results from the stability analysis. The stability analysis leads to two possible instabilities: the convective instability and an acoustic instability. Analytic expressions for the growth rates of these different instabilities are found. In particular, strong radiative diffusion damps the growth rate $ω$ of the convective instability leading to a distinct $ω\sim 1/k^2$ dependence on wavenumber $k$. From our power spectra analysis of the simulations, however, we find that structure growth rather increases with $k$ - tentatively as $ω\sim \sqrt{k}$. Our results suggest that structures in luminous O and WR star envelopes do not primarily develop from the sub-surface convective instability. Rather the growth seems compatible with either the acoustic instability in the radiation-dominated regime or with Rayleigh-Taylor type instabilities, although the exact origin remains inconclusive for now.

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arXiv2025

Improving 1D stellar atmosphere models with insights from multi-dimensional simulations I. 1D vs 2D stratifications and spectral comparison for O stars

We compare current 1D and multi-dimensional atmosphere modelling approaches for massive stars to understand their strengths and shortcomings. We calculate averaged stratifications from selected 2D calculations for O stars -- corresponding to the spectral types O8, O4, and O2 -- to approximate them with 1D stellar atmosphere models using the PoWR model atmosphere code and assuming a fixed $β-$law for the wind regime. We then study the effects of our approximations and assumptions on current spectral diagnostics. In particular, we focus on the impact of an additional turbulent pressure in the subsonic layers of the 1D models. To match the 2D averages, the 1D stellar atmosphere models need to account for turbulent pressure in the hydrostatic equation. Moreover, an adjustment of the connection point between the (quasi-)hydrostatic regime and the wind regime is required. The improvement between the density stratification of 1D model and 2D average can be further increased if the mass-loss rate of the 1D model is not identical to those of the 2D simulation, but typically $\sim0.2\,$dex higher. Especially for the early type star, this implies a significantly more extended envelope with a lower effective temperature. Already the inclusion of a constant turbulence term in the solution of the hydrostatic equation sufficiently reproduces the 2D-averaged model density stratifications. The addition of a significant turbulent motion also smoothens the slope of the radiative acceleration term in the (quasi-)hydrostatic domain, with several potential implications on the total mass-loss rate inferred from 1D modelling. Concerning the spectral synthesis, the addition of a turbulence term in the hydrostatic equation mimics the effect of a lower surface gravity, potentially presenting a solution to the ``mass discrepancy problem'' between the evolutionary and spectroscopy mass determinations.

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arXiv2024

Suppression of photospheric velocity fluctuations in strongly magnetic O-stars in radiation-magnetohydrodynamic simulations

O-stars generally show clear signs of strong line-broadening (in addition to rotational broadening) in their photospheric absorption lines (typically referred to as 'macroturbulence'), believed to originate in a turbulent sub-surface zone associated with enhanced opacities due to recombination of iron-group elements (at T~ 150-200 kK). O-stars with detected global magnetic fields also display such macroturbulence; the sole exception to this is NGC 1624-2, which also has the strongest (by far) detected field of the known magnetic O-stars. It has been suggested that this lack of additional line-broadening is because NGC 1624-2's exceptionally strong magnetic field might be able able to suppress the turbulent velocity field generated in the iron opacity peak zone. For moderately strong magnetic cases (~1 kG) the simulated atmospheres are highly structured characterised by large root-mean-square velocities, and our results are qualitatively similar to those found in previous non-magnetic studies. By contrast, we find that a strong horizontal magnetic field in excess of 10 kG can indeed suppress the large velocity fluctuations and thus stabilise (and thereby also inflate) the atmosphere of a typical early O-star in the Galaxy. On the other hand, an equally strong radial field is only able to suppress horizontal motions, and as a consequence these models exhibit significant radial fluctuations. Our simulations provide an overall physical rationale as to why NGC 1624-2 with its strong ~20 kG dipolar field lacks the large macroturbulent line broadening that all other known slowly rotating early O-stars exhibit. However, they also highlight the importance of field geometry for controlling the atmospheric dynamics in massive and luminous stars that are strongly magnetic, tentatively suggesting latitudinal dependence of macroturbulence and basic photospheric parameters.

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arXiv2024

Empirical mass-loss rates and clumping properties of O-type stars in the LMC

We constrain wind parameters of a sample of 18 O-type stars in the LMC, through analysis with stellar atmosphere and wind models including the effects of optically thick clumping. This allows us to determine the most accurate spectroscopic mass-loss and wind structure properties of massive stars at sub-solar metallicity to date and gain insight into the impact of metallicity on massive stellar winds. Combining high signal to noise (S/N) ratio spectroscopy in the UV and optical gives us access to diagnostics of multiple different physical processes in the stellar wind. We produce synthetic spectra using the stellar atmosphere modelling code FASTWIND, and reproduce the observed spectra using a genetic algorithm based fitting technique. We empirically constrain 15 physical parameters associated with the stellar and wind properties, including temperature, surface gravity, surface abundances, rotation, macroturbulence and wind parameters. We find, on average, mass-loss rates a factor of 4-5 lower than those predicted by Vink et al. 2001, in good agreement with predictions from Bjorklund et al. 2021, and the best agreement with those from Krticka et al. 2018. In the 'weak-wind' regime we find mass-loss rates orders of magnitude below any theoretical predictions. We find a positive correlation of clumping factors (fcl) with effective temperature with an average fcl = 14 +- 8 for the full sample. Above 38 kK an average 46 +- 24% of the wind velocity span is covered by clumps and the interclump density is 10-30% of the mean wind. Below an effective temperature of roughly 38 kK there must be additional light leakage for supergiants. For dwarf stars at low temperatures there is a statistical preference for very low clump velocity spans, however it is unclear if this can be physically motivated as there are no clearly observable wind signatures in UV diagnostics.

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arXiv2024

2D unified atmosphere and wind simulations of O-type stars

Massive and luminous O-star atmospheres with winds have been studied primarily using one-dimensional (1D), spherically symmetric, and stationary models. However, observations and theory rather suggest that O-star atmospheres are highly structured, turbulent, and time-dependent. As such, when comparing to observations, present-day 1D modeling tools need to introduce ad-hoc quantities such as photospheric macro & microturbulence, wind clumping, etc. We present multi-dimensional, time-dependent, radiation-hydrodynamical (RHD) simulations for O-stars that encapsulate the deeper sub-surface envelope (down to T ~ 450 kK) as well as the supersonic line-driven wind outflow in one unified approach. Time-dependent, two-dimensional (2D) simulations of O-star atmospheres with winds are performed using a flux-limiting RHD finite volume modeling technique. Opacities are computed using a hybrid approach combining tabulated Rosseland means with calculations (based on the Sobolev approximation) of the enhanced line opacities expected for supersonic flows. When compared to 1D models, the average structures in the 2D simulations display less envelope expansion, no sharp density-inversions, density and temperature profiles that are significantly less steep around the photosphere, and a strong anti-correlation between velocity and density in the supersonic wind. To qualitatively match the different density and temperature profiles seen in our multi-D and 1D models, we need to add a modest amount of convective energy transport in the deep sub-surface layers and a large turbulent pressure around the photosphere to the 1D models.

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arXiv2023

X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars

The winds of massive stars have an impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed, is a global wind parameter and an essential input for models of stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a legacy UV spectroscopic survey with HST, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. We empirically quantify the wind speeds of a large sample of OB stars, including supergiants, giants, and dwarfs at sub-solar metallicity. Using these measurements, we investigate trends of terminal wind speed with a number of fundamental stellar parameters, namely effective temperature, metallicity, and surface escape velocity. We empirically determined the terminal wind speed for a sample of 149 OB stars in the Magellanic Clouds either by directly measuring the maximum velocity shift of the absorption component of the Civ 1548-1550 line profile, or by fitting synthetic spectra produced using the Sobolev with exact integration method. Stellar parameters were either collected from the literature, obtained using spectral-type calibrations, or predicted from evolutionary models. We find strong trends of terminal wind speed with effective temperature and surface escape speed when the wind is strong enough to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03 when we compared our results to previous Galactic studies. Our results suggest that effective temperature rather than surface escape speed should be used as a straightforward empirical prediction of terminal wind speed and that the observed metallicity dependence is steeper than suggested by earlier works.

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arXiv2023

The dusty circumstellar environment of Betelgeuse during the Great Dimming as seen by VLTI/MATISSE

The 'Great Dimming' of the prototypical red supergiant Betelgeuse, which occurred between December 2019 and April 2020, gives us unprecedented insight into the processes occurring on the stellar surface and in the inner wind of this type of star. In particular it may bring further understanding of their dust nucleation and mass loss processes. Here, we present and analyse VLTI/MATISSE observations in the N-band (8 - 13 $μ$m) taken near the brightness minimum in order to assess the status of the dusty circumstellar environment. We explore the compatibility of a dust clump obscuring the star with our mid-infrared interferometric observations using continuum 3D radiative transfer modelling, and probe the effect of adding multiple clumps close to the star on the observables. We also test the viability of a large cool spot on the stellar surface without dust present in the ambient medium. Using the visibility data, we derive a uniform disk diameter of 59.02 $\pm$ 0.64 mas in the spectral range 8 to 8.75 $μ$m. We find that both the dust clump and the cool spot models are compatible with the data. Further to this, we note that the extinction and emission of our localised dust clump in the line of sight of the star, directly compensate each other making the clump undetectable in the spectral energy distribution and visibilities. The lack of infrared brightening during the 'Great Dimming' therefore does not exclude extinction due to a dust clump as one of the possible mechanisms. The visibilities can be reproduced by a spherical wind with dust condensing at 13 stellar radii and a dust mass-loss rate of (2.1 - 4.9) $\times$ 10$^{-10}$ $\mathit{M}_{\odot} {\rm yr}^{-1}$, however, in order to reproduce the complexity of the observed closure phases, additional surface features or dust clumps would be needed.

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arXiv2022

First 3D Radiation-Hydrodynamic Simulations of Wolf-Rayet Winds

Classical Wolf Rayet (WR) stars are direct supernova progenitors undergoing vigorous mass-loss. Understanding the dense and fast outflows of such WR stars is thus crucial for understanding advanced stages of stellar evolution, the dynamical feedback of massive stars on their environments, and characterizing the distribution of black hole masses. In this paper, we develop first time-dependent, multi-dimensional, radiation-hydrodynamical models of the extended optically thick atmospheres and wind outflows of hydrogen-free classical WR stars. A flux limiting radiation hydrodynamics approach is used on a finite volume mesh to model WR outflows. The opacities are described using a combination of tabulated Rosseland mean opacities and the enhanced line opacities expected within a supersonic flow. For high-luminosity models, a radiation-driven, dense, supersonic wind is launched from deep sub-surface regions associated with peaks in the Rosseland mean opacity. For a model with lower luminosity, on the other hand, the Rosseland mean opacity is not sufficient to sustain a net-radial outflow in the sub-surface regions. Rather, what develops in this case is a "standard" line-driven wind launched from the optically thin regions above an extended and highly turbulent atmosphere. We thus find here a natural transition from optically thick outflows of classical WR stars to optically thin winds of hot, compact sub-dwarfs; in our simulations this transition occurs approximately at a luminosity that is about 40% of the Eddington luminosity. Because of the changing character of the wind-launching mechanism, this transition is also accompanied by a large drop (on the low-luminosity end) in average mass-loss rate.

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arXiv2022

Method and new tabulations for flux-weighted line-opacity and radiation line-force in supersonic media

In accelerating and supersonic media, the interaction of photons with spectral lines can be of ultimate importance. However, fully accounting for such line forces currently can only be done by specialised codes in 1-D steady-state flows. More general cases and higher dimensions require alternative approaches. We presented a comprehensive and fast method for computing the radiation line-force using tables of spectral line-strength distribution parameters, which can be applied in arbitrary (multi-D, time-dependent) simulations, including those accounting for the line-deshadowing instability, to compute the appropriate opacities. We assumed local thermodynamic equilibrium to compute a flux-weighted line opacity from $>4$ million spectral lines. We derived the spectral line strength and tabulated the corresponding line-distribution parameters for a range of input densities $ρ\in[10^{-20},10^{-10}]gcm^{-3}$ and temperatures $T\in[10^4,10^{4.7}]K$. We found that the variation of the line distribution parameters plays an essential role in setting the wind dynamics in our models. In our benchmark study, we also found a good overall agreement between the O-star mass-loss rates of our models and those derived from steady-state studies using more detailed radiative transfer. Our models reinforce that self-consistent variation of the line-distribution parameters is important for the dynamics of line-driven flows. Within a well-calibrated O-star regime, our results support the proposed methodology. In practice, utilising the provided tables, yielded a factor $>100$ speed-up in computational time compared to specialised 1-D model-atmosphere codes of line-driven winds, which constitutes an important step towards efficient multi-D simulations. We conclude that our method and tables are ready to be exploited in various radiation-hydrodynamic simulations where the line force is important.

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arXiv2022

Theoretical wind clumping predictions from 2D LDI models of O-star winds at different metallicities

Hot, massive (OB) stars experience strong line-driven stellar winds and mass loss. As the majority of efficient driving lines are metallic, the amount of wind driving and mass loss is dependent on the stellar metallicity Z. In addition, line-driven winds are intrinsically inhomogeneous and clumpy. However, to date, neither theoretical nor empirical studies of line-driven winds have investigated how such wind clumping may also depend on Z. We theoretically investigated the degree of wind clumping due to the line-deshadowing instability (LDI) as a function of Z. We performed two-dimensional hydrodynamic simulations of the LDI with an assumed one-dimensional radiation line force for a grid of O-star wind models with fixed luminosity, but with different metal contents by varying the accumulative line strength Qbar describing the total ensemble of driving lines. We find that, for this fixed luminosity, the amount of wind clumping decreases with metallicity. The decrease is clearly seen in the statistical properties of our simulations, but is nonetheless rather weak; a simple power-law fit for the dependence of the clumping factor f_cl = / ^2 on metallicity yields f_cl ~ Z^(0.15 +/- 0.01). This implies that empirically derived power-law dependencies of mass-loss rate Mdot on metallicity -- which were previously inferred from spectral diagnostics effectively depending on Mdot*sqrt(f_cl) but without having any constraints on f_cl(Z) -- should be only modestly altered by clumping. We expect that this prediction can be directly tested using new data from the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) project.

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arXiv2022

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars III. Updated mass-loss rates for stellar evolution

Massive stars lose a large fraction of their mass to radiation-driven winds throughout their entire life. These outflows impact both the life and death of these stars and their surroundings. Theoretical mass-loss rates of hot, massive stars are derived to be used in applications such as stellar evolution. The behaviour of these rates in the OB-star regime is analysed, and their effects on massive-star evolution predictions is studied. Dynamically-consistent models are computed by solving the spherically symmetric, steady-state equation-of-motion for a large grid of hot, massive stars. The radiative acceleration is derived from non-local thermodynamic equilibrium radiative transfer in the co-moving frame. The resulting mass-loss rates are used to derive a simple scaling recipe with stellar parameters and to evaluate some first impacts upon evolution tracks. We provide a new prescription for steady-state, radiation-driven mass-loss from hot, massive stars depending on their fundamental parameters. The rates for O-stars are lower by about a factor ~3 than the rates typically used in previous stellar-evolution calculations, where differences generally decrease with increasing luminosity and temperature. For cooler B-giants/supergiants we find larger discrepancies. This arises because we do not find any systematic increase in mass-loss rates below the so-called bi-stability region; indeed, our results do not show any sign of a significant bi-stability jump within the parameter range covered by the grid. Due to the lower mass-loss rates we find that envelopes are not easily stripped by means of standard steady-state winds, making it difficult to create classical Wolf-Rayet stars via this channel. However, a remaining key uncertainty regarding these predictions concerns unsteady mass loss for very high-luminosity stars close to the Eddington limit as well as the impact of non-line-driven winds.

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