27 research outputs found
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
million spectral lines. We derived the spectral line strength and
tabulated the corresponding line-distribution parameters for a range of input
densities and temperatures
. 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 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
Empirical mass-loss rates and clumping properties of Galactic early-type O supergiants
International audienceAims. We investigate the impact of optically thick clumping on spectroscopic stellar wind diagnostics in O supergiants and constrain wind parameters associated with porosity in velocity space. This is the first time the effects of optically thick clumping have been investigated for a sample of massive hot stars, using models which include a full optically thick clumping description. Methods. We re-analyse existing spectroscopic observations of a sample of eight O supergiants previously analysed with the non-local-thermodynamic-equilibrium (NLTE) atmosphere code CMFGEN. Using a genetic algorithm wrapper around the NLTE atmosphere code FASTWIND we obtain simultaneous fits to optical and ultraviolet spectra and determine photospheric properties, chemical surface abundances and wind properties. Results. We provide empirical constraints on a number of wind parameters including the clumping factors, mass-loss rates and terminal wind velocities. Additionally, we establish the first systematic empirical constraints on velocity filling factors and interclump densities. These are parameters that describe clump distribution in velocity-space and density of the interclump medium in physical-space, respectively. We observe a mass-loss rate reduction of a factor of 3.6 compared to theoretical predictions from Vink et al. (2000), and mass-loss rates within a factor 1.4 of theoretical predictions from Björklund et al. (2021). Conclusions. We confirm that including optically thick clumping allows simultaneous fitting of optical recombination lines and ultraviolet resonance lines, including the unsaturated ultraviolet phosphorus lines (P v λλ1118-1128), without reducing the phosphorus abundance. We find that, on average, half of the wind velocity field is covered by dense clumps. We also find that these clumps are 25 times denser than the average wind, and that the interclump medium is 3-10 times less dense than the mean wind. The former result agrees well with theoretical predictions, the latter suggests that lateral filling-in of radially compressed gas might be critical for setting the scale of the rarefied interclump matter
Radiation-hydrodynamics with MPI-AMRVAC: Flux-limited diffusion
Radiation controls the dynamics and energetics of many astrophysical environments. To capture the coupling between the radiation and matter, however, is often a physically complex and computationally expensive endeavor. Aims. We sought to develop a numerical tool to perform radiation-hydrodynamics simulations in various configurations at an affordable cost. Methods. We built upon the finite volume code MPI-AMRVAC to solve the equations of hydrodynamics on multi-dimensional adaptive meshes and introduce a new module to handle the coupling with radiation. A non-equilibrium, flux-limiting diffusion approximation was used to close the radiation momentum and energy equations. The time-dependent radiation energy equation was then solved within a flexible framework, fully accounting for radiation forces and work terms and further allowing the user to adopt a variety of descriptions for the radiation-matter interaction terms ("opacities"). Results. We validated the radiation module on a set of standard test cases for which different terms of the radiative energy equation predominate. As a preliminary application to a scientific case, we calculated spherically symmetric models of the radiation-driven and optically thick supersonic outflows from massive Wolf-Rayet stars. This also demonstrates our code's flexibility, as the illustrated simulation combines opacities typically used in static stellar structure models with a parametrized form for the enhanced line-opacity expected in supersonic flows. Conclusions. This new module provides a convenient and versatile tool for performing multi-dimensional and high-resolution radiative-hydrodynamics simulations in optically thick environments with the MPI-AMRVAC code. The code is ready to be used for a variety of astrophysical applications, where our first target is set to be multi-dimensional simulations of stellar outflows from Wolf-Rayet stars
Accretion, Outflows, and Winds of Magnetized Stars
Many types of stars have strong magnetic fields that can dynamically
influence the flow of circumstellar matter. In stars with accretion disks, the
stellar magnetic field can truncate the inner disk and determine the paths that
matter can take to flow onto the star. These paths are different in stars with
different magnetospheres and periods of rotation. External field lines of the
magnetosphere may inflate and produce favorable conditions for outflows from
the disk-magnetosphere boundary. Outflows can be particularly strong in the
propeller regime, wherein a star rotates more rapidly than the inner disk.
Outflows may also form at the disk-magnetosphere boundary of slowly rotating
stars, if the magnetosphere is compressed by the accreting matter. In isolated,
strongly magnetized stars, the magnetic field can influence formation and/or
propagation of stellar wind outflows. Winds from low-mass, solar-type stars may
be either thermally or magnetically driven, while winds from massive, luminous
O and B type stars are radiatively driven. In all of these cases, the magnetic
field influences matter flow from the stars and determines many observational
properties. In this chapter we review recent studies of accretion, outflows,
and winds of magnetized stars with a focus on three main topics: (1) accretion
onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and
(3) winds from isolated massive magnetized stars. We show results obtained from
global magnetohydrodynamic simulations and, in a number of cases compare global
simulations with observations.Comment: 60 pages, 44 figure
Analytic, dust-independent mass-loss rates for red supergiant winds initiated by turbulent pressure
Context. Red supergiants are observed to undergo vigorous mass loss. However, to date, no theoretical model has succeeded in explaining the origins of these objects’ winds. This strongly limits our understanding of red supergiant evolution and Type II-P and II-L supernova progenitor properties.
Aims. We examine the role that vigorous atmospheric turbulence may play in initiating and determining the mass-loss rates of red supergiant stars.
Methods. We analytically and numerically solve the equations of conservation of mass and momentum, which we later couple to an atmospheric temperature structure, to obtain theoretically motivated mass-loss rates. We then compare these to state-of-the-art empirical mass-loss rate scaling formulae as well as observationally inferred mass-loss rates of red supergiants.
Results. We find that the pressure due to the characteristic turbulent velocities inferred for red supergiants is sufficient to explain the mass-loss rates of these objects in the absence of the normally employed opacity from circumstellar dust. Motivated by this initial success, we provide a first theoretical and fully analytic mass-loss rate prescription for red supergiants. We conclude by highlighting some intriguing possible implications of these rates for future studies of stellar evolution, especially in light of the lack of a direct dependence on metallicity