arXiv subjects

L. Delbroek

Publications and source records attributed to L. Delbroek.

6 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

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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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

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

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 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 $\beta-$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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