000008101 001__ 8101
000008101 005__ 20260324143154.0
000008101 0247_ $$2DOI$$ahttps://doi.org/10.1051/0004-6361/202348206
000008101 037__ $$aSCART-2026-0140
000008101 100__ $$aDebnath, Dwaipayan
000008101 245__ $$a2D unified atmosphere and wind simulations of O-type stars
000008101 260__ $$c2024
000008101 520__ $$aContext. Massive and luminous O-type star (O star) atmospheres with winds have been studied primarily using one-dimensional (1D), spherically symmetric, and stationary models. However, observations and theory have suggested that O star atmospheres are highly structured, turbulent, and time-dependent. As such, when making comparisons to observations, present-day 1D modeling tools require the introduction of ad hoc quantities such as photospheric macro- and microturbulence, wind clumping, and other relevant properties. Aims. We present a series of 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. Our overarching aim is to develop a framework that is free from the ad-hoc prescriptions that plague present-day 1D models. Here, we start with an analysis of a small set of such multi-dimensional simulations and then compare them to atmospheric structures predicted by their 1D counterparts. Methods. We performed time-dependent, two-dimensional (2D) simulations of O star atmospheres with winds using a flux-limiting RHD finite volume modelling 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. The initial conditions and comparison models were derived using similar procedures as those applied in standard 1D stationary model atmosphere with wind codes. Results. Structure starts appearing in our simulations just below the iron-opacity peak at ~200 kK. Local pockets of gas with radiative accelerations that exceed gravity then shoot up from these deep layers into the upper atmosphere, where they interact with the line-driven wind outflow initiated around or beyond the variable photosphere. This complex interplay creates large turbulent velocities in the photospheric layers of our simulations, on the order of ~30–100km s−1, with higher values for models with higher luminosity-to-mass ratios. This offers a generally good agreement with observations of large photospheric ‘macroturbulence’ in O stars. When compared to 1D models, the average structures in the 2D simulations display less envelope expansion and no sharp density-inversions, along with 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. Although the wind initiation region is complex and highly variable in our simulations, our average mass-loss rates agree well with stationary wind models computed by means of full co-moving frame radiative transfer solutions. Conclusions. The different atmospheric structures found in 2D and 1D simulations are likely to affect the spectroscopic determination of fundamental stellar and wind parameters for O stars as well as the empirical derivation of their chemical abundance patterns. To qualitatively match the different density and temperature profiles seen in our multi-dimensional 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.
000008101 594__ $$aNO
000008101 6531_ $$ahydrodynamics 
000008101 6531_ $$ainstabilities 
000008101 6531_ $$amethods: numerical 
000008101 6531_ $$astars: atmospheres
000008101 6531_ $$a stars: massive 
000008101 6531_ $$astars: winds
000008101 6531_ $$aoutflows
000008101 700__ $$aSundqvist, Jon
000008101 700__ $$aMoens,  
000008101 700__ $$aVan der Sijp, Casandra
000008101 700__ $$aVerhamme, Olivier
000008101 700__ $$aPoniatowski, Luka 
000008101 773__ $$pA&A$$v684,$$y2024
000008101 8560_ $$fluka.poniatowski@ksb-orb.be
000008101 85642 $$ahttps://www.aanda.org/articles/aa/full_html/2024/04/aa48206-23/aa48206-23.html
000008101 905__ $$apublished in
000008101 980__ $$aREFERD