1 / 30

Radiation-driven Winds from pulsating luminous Stars

Radiation-driven Winds from pulsating luminous Stars. Ernst A. Dorfi Universit ä t Wien Institut f ü r Astronomie. Outline. XLA Data for stellar objects Luminous massive stars Computational approach Stellar Pulsations Dynamical atmospheres and mass loss Conclusions and Outlook.

gore
Télécharger la présentation

Radiation-driven Winds from pulsating luminous Stars

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Radiation-driven Winds frompulsating luminous Stars Ernst A. Dorfi Universität Wien Institut für Astronomie

  2. Outline • XLA Data for stellar objects • Luminous massive stars • Computational approach • Stellar Pulsations • Dynamical atmospheres and mass loss • Conclusions and Outlook

  3. XLA Data for Stellar Astrophysics • Nuclear cross sections for energy generation as well as nucleosynthesis • Stellar opacities for radiative transfer, grey or frequency-integrated (OPAL and OP-projects), new values solved a number of discrepancies between observations and theory (molecular opacities still needed) • Equation of State, hot dense plasmas (but also cold dense plasmas for ‘planets’) • Optical constants for dust particles

  4. SN-Progenitor •  Car will explode as Supernova, distance d=7500 ly • Massive object: M~120M (1M=2●1030kg) • Extremely luminous star: L~4●106L (1L=3.8●1026 W) • Observed mass loss, lobes are expanding with 2300 km/s • Central source and hot shocked gas between 3-60 ●106 K, X-ray emission • Giant eruptions between1837 and1856 • Questions: mass loss, giant eruptions, variability, rotation, binarity, ...  Car: HST/NASA  Car: CHANDRA

  5. IRS16SW WN8 WR123 Theoretical HRD Adopted from Gautschy & Saio 1996

  6. Some Properties of LBVs • LBVs are the most luminous stellar objects with luminosities up to 106L • Radiation pressure dominates most of the radial extension of the stars • LBVs are poorly observed (sampled) variable stars, small and large scale variations, large outbursts on scales of several decades, poorly determined stellar parameter • More theoretical work on variability necessary: regular pulsations of LBVs on a time scale of days or less (Dorfi & Gautschy), strange modes in the outer layers, LBV phenomenon due to dynamically unstable oscillations near the Eddington-limit (Stothers & Chin, Glatzel & Kiriakidis) • Theoretical LBVs light curves: complicated structures due to shock waves running through the stellar atmosphere

  7. Observed light curves of LBVs • Luminous Blue Variables exhibit so-called micro-variability • LBVs show outbursts on scale of several years R40 in SMC Sterken et al. 1998, y- and Hipparcos photometry

  8. MOST light curve of WR123 • Observations over 38 days • Clear signal with a period of P=9.8 h Lefèvre at al. 2005, ApJ

  9. Growth of pulsations • Pulsations initiated by a small random perturbation: 5 km/s • Initial linear growth (dotted line), stellar atmosphere can adjust on a different time scale • Final amplitude when kinetic energy becomes constant • Model WR123U: M=25M, Teff=33900 K, L=2.82 • 105L Dorfi, Gautschy, Saio, 2006

  10. Computational Requirements • Resolve relevant features within one single computation like driving zone, ionization zones, opacity changes, shock waves, stellar winds, … global simulations • Kinetic energy is small fraction of the total energy • Steep gradients within the stellar atmosphere and/or possible changes of the atmospheric stratification due to energy deposition may change boundary conditions • Long term evolution of stellar pulsations, secular changes on thermal time scales, i.e. tKH >> tdyn • Solve full set of Radiation Hydrodynamics (RHD), problem: detailed properties of convection

  11. Adaptive Grid • Fixed number of N grid points: ri, 1iN, and grid points must remain monotonic: ri<ri+1 • Grid is rearranged at every time-step • Additional grid equation is solved together with the physical equations • Grid points basically distributed along the arc-length of a physical quantities (Dorfi & Drury, 1986, JCP) • Physical equations are transformed into the moving coordinate system • Computation of fluxes relative to the moving spherical grid

  12. Computational RHD • All variables depend on time and radius, X=X(r,t) • Equations are discretized in a conservative way, i.e. global quantities are conserved, correct speed of propagating waves • Adaptive grid to resolve steep features within the flow • Implicit formulation, large time steps are possible, solution of a non-linear system of equations at every new time step • Flexible approach to incorporate also new physics

  13. Adaptive conservative RHD • Integration over finite but time-dependent volume V(t) due to moving grid points • Advection terms calculated from fluxes over cell boundaries • Relative velocities between mater and grid motion: urel = u - ugrid

  14. Equations of RHD (1) • Equation of continuity (conservation of mass) • Equation of motion (conservation of linear momentum), including artificial viscosity uQ

  15. Equations of RHD (2) • Equation of internal gas energy (including artificial viscous energy dissipation Q) • Poisson equation leads to gravitational potential, integrated mass m(r) in spherical symmetry

  16. Equations of RHD (3) • 0th - moment of the RTE, radiation energy density • 1th- moment of RTE, equation of radiative flux

  17. Advection (I) • Transport through moving shells as accurate as possible • Usage of a staggered mesh, i.e. variables located at cell center or cell boundary • Fulfil accuracy as well as stability criteria for sub- and supersonic flow • Avoid numerical oscillations, so-called TVD-schemes • Ensure correct propagation speed of waves

  18. Advection (II) • TVD-schemes are based on monotonicity criteria of the consecutive ratio R • Correct propagation speed of waves requires ψ(1)=1 • Monotonic advection scheme according to van Leer (1979) essential for stellar pulsations: 2nd-order TVD 1st-order TVD

  19. Temporal discretization • 2nd-order temporal discretization to reduce artificial damping of oscillations • Smallest errors in case of time-centered variables

  20. Linear vs. non-linear pulsations • Work integrals based on linear as well as full RHD-computations, remarkable correspondence (normalized to unity in the damping region) • Driving and damping mechanisms are identical for both approaches • Pulsations are triggered by the iron metals bump in the Rosseland-mean opacities (5.0 < log T< 5.3) • These high luminosity stars exhibit modes located more at the surface than classical pulsators • M = 30 M • L = 316000L • Teff= 31620K

  21. Pulsations with small amplitudes • M = 20M • L = 66000L • Teff = 27100K • P = 0.29days Radius [R] Synchronous motion of mass shells Time in pulsation periods

  22. Atmosphere with shock waves • M = 25M • L = 282000L • Teff= 33900K • P = 0.49days Shock wave Ballistic motions on the scale of tff

  23. Observations of stellar parameter • Effective temperature can decrease as mean radius increases • WR123R:M=25 M, log L/L=5.5, Teff_i=33000K • Teff_puls=31700K, ΔT=1300K • Rph=17.2R, Rpuls=18.7R • P = 0.72d

  24. Atmospheric dynamics • IRS16WS model: L=2.59•106L • Rotation plays important role in decoupling the stellar atmosphere from internal pulsations • Ballistic motions at different time scales introduce complex flows • vrot=220km/s, P=3.471d, T=25000K • vrot=225km/s, P=3.728d, T=24000K • Higher rotation rates lead to mass loss of about 10-4 M/yr

  25. Light curves without mass loss • P=3.728d, vrot=225 km/s, T=24000K, L=2.59•106L • Shocks, dissipation of kinetic energy, large variations in the optical depth • Looks rather irregular and pulsation can be hidden within atmospherical dynamics • Large expansion of photosphere around 10 and 20 days clearly visible • Typical amplitudes decrease from 0.5mag in U,B to less than 0.25mag in H,K

  26. Initiating mass loss • Pulsation perturbed by increase rotational velocity from 225km/s to 230 km/s • After 4 cycles outermost mass shell accelerated beyond escape velocity • Outer boundary: from Lagrangian to outflow at 400 R, advantage of adaptive grid • Gas velocity varies there around 550 km/s escape velocity

  27. Pulsation and mass loss • Pulsation still exists, very different outer boundary condition • Large photosphere velocity variations due to changes in the optical depth • Mean equatorial mass loss: 3•10-4M/yr, vext=550km/s • Total mass loss rate probable reduced by angle-dependence

  28. Motion of mass shells Episodic mass loss Photosphere Ballistic motions Shock formation Regular interior pulsations

  29. Conclusions • According to theory: All luminous stars with L[L]/M[M]>104 exhibit strange modes located at the outer stellar layers • All stars in the range of 106L should be unstable, but no simple light curves expected • Complicated, dynamical stellar atmospheres, difficulties to detect pulsations due to shocks, irregularities, non-radial effects, rotation, dM/dt ~ 10-4M/yr • In many cases the resulting light curves as well as the radial oscillations can become rather irregular and difficult to analyze • These oscillations will affect mass loss and angular momentum loss as well as further stellar evolution

  30. Computational Outlook • Include better description of convective energy and momentum transport into the code • Include Doppler-Effects in the opacities, additional opacity may cause large-scale outbursts, even without rotation • Non-grey radiative transport on a small number (about 50) of frequency points • 2-dimensional adaptive, implicit calculations based on the same numerical methods Stökl & Dorfi, CPC, 2008

More Related