1 / 31

Accretion disks

Lecture 4. AST3020. Accretion disks. Flaring shape jets. Outflows disappear before the disks do. High!. (on the other hand, in debris disks which don’t have a lot of gas and much less dust as well, both the opacity of dust and the

clint
Download Presentation

Accretion disks

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. Lecture 4. AST3020 Accretion disks

  2. Flaring shape jets Outflows disappear before the disks do

  3. High! (on the other hand, in debris disks which don’t have a lot of gas and much less dust as well, both the opacity of dust and the surface density of matter are much lower, so that the optical depth is tau_0 << 1 in every direction.)

  4. [accretion heating active disks; illumination heating passive disks] Since the flux F also equals sigma * T^4, and c ~ T^(1/2), we have that in disks where Sigma*nu = const. (stationary thin disks far from the stellar surface) F ~ r^(-3) ~ T^4 ==> T ~ r^(-3/4) z/r ~ c / v_K ~ r^(+1/8), a slightly flaring disk.

  5. Diffusion equation for the viscous evolution of an accretion disk cf. Pringle (1981 in Ann Rev Astr Astoph) [accretion heating active disks; illumination heating passive disks]

  6. The ratio of viscous to dynamical time is called Reynolds number and denoted Re. It always is a very large number in astrophysics.

  7. The analytical solutions (Pringle 1981)

  8. *** *** - there is another solution…which??

  9. ANOMALOUS VISCOSITY IN DISKS

  10. Problem: convection transports angular momentum inwards

  11. - disks Shakhura-Sunyayev (1973) Non-dimensional parameter c = soundspeed z = disk scale height Idea: gather all uncertainties in alpha-parameter: l = Specific angular momentum because Reynolds number: (spiralling of gas very much slower than v_k, Keplerian vel.)

  12. Magneto-rotational instability (MRI) as a source of viscosity in astrophysical disks. Velikhov (1959), Chandrasekhar (1960), and re-discovered by Balbus and Hawley (1991). Disk conditions:gas ionized; magnetic field dragged with gas magnetic field energy and pressure << gas energy,pressure differential rotation (angular speed drops with distance) 2-D and 3-D simulations of Magnetic turbulence inside the disk

  13. Chris Reynolds et al. Results: alpha computed ab initio, sometimes not fully self-consistently often not in full 3-D disk: alpha ~ several * 1e-3 Charles Gammie et al.

  14. VISCOUS EVOLUTION SEEN IN DISKS

  15. PPIV = Protostars and Planets IV book (2000) Observations of dM/dt as a function oflog age [yr] M_sun/yr log age [yr]

  16. Observed dM/dt ~ 1e-6 M_sun/yr for ~0.1 Myr time ==> total amount accreted ~0.1 M_sun Observed dM/dt ~ 1e-7 M_sun/yr for ~Myr time ==> total amount accreted ~0.1 M_sun

  17. Mass of the dust in disks (around A-type and similar stars) Primordial solar nebulae Debris disks = beta Pic disks, zodiacal light disks Natta (2000, PPIV)

  18. gas (T Tau stars) dM/dt [M_sun/yr] log age [yr] PPIV = Protostars andPlanets IV book (2000)

  19. Observations Modeling of observations Compares OK Ab-initio calculations (numerical)

  20. Percentage of optically thick “outer disks” (at~3AU) From: M. Mayers, S. Beckwith et al. Conclusion: Major fraction of dust cleared out to several AU in 3-10 Myr 0.1 1 10 100 1000 Myr Age

  21. SED = Spectral En. Distrib. If part of the disk missing => SED may show a dip => possible diagnostic of planets. flux If this ring missing frequency

  22. Z0

  23. Summary of the most important facts about accretion disks: Found in: • quasars’ central engines, • active galactive nuclei (AGNs), galaxies, • around stars (Cataclysmic Var., Dwarf Novae, T Tauri, b Pic), • around planets. Drain matter inward, angular momentum outside. Release gravitational energy as radiation, or reprocess radiation. Easy-to-understand vertical structure with z/r ~ c/v_K Radial evolution due to some poorly known viscosity, parametrized by alpha <1. Best mechanism for viscosity is MRI (magneto-rotational instability), an MHD process of growth of tangled magnetic fields at the cost of mechanical energy of the disk. Simulations give alpha= a few * 1e-3

More Related