PhD course originally developed by René Liseau Updated to Master level course by Alexis Brandeker

Transcription

1 Part III Early stages in stellar tll evolution PhD ii ll d l db R éli PhD course originally developed by René Liseau Updated to Master level course by Alexis Brandeker

2 Early stages in stellar tll evolution L15 General Overview / Pre collapse phase I L16 Pre collapse phase II L17 Collapse phase I L18 Collapse phase II L19 Circumstellar disks I L20 Pre main sequence evolution L21 Circumstellar disks II debris disks

3 Circumstellar disks Inevitable consequence of star formation: Ω 1 r 12 = L 1 = L 0 = Ω 0 r 0 2 Ω 1 = Ω 0 (r 0 /r 1 ) 2 Collapse compresses scale r from r pc = AU to r 0 R sol = AU r 0 /r and Ω Ω 0 Assumes conservation of angular momentum

4 Magnetic braking Stahler & Palla : the centrifuga l radius ϖ cen T 2 Ω t = 0.3AU K 10 s 10 yr

5 Predictions of disks PS P.S. Laplace 1796, 1799 Exposition du systeme du monde Mechanique celeste I. Kant 1755 Allgemeine Naturgeschichte und Theorie des Himmels

6 Predictions of disks Lynden Bell & Pringle 1974, MNRAS 168, 603: Keplerian Disk Differential Rotation ti Mass Transport Inwards + Viscosity Angular Momentum Transport Outwards

7 Dust grain opacities See Ph. Thebault s lecture Beckwith et al. 2000, PP IV

8 40 observed SEDs of T Tauri Stars and `mean model of star+disk HABE Disk Structure: Dullemond & Dominik 2004 D Alessio et al. 1999

9 Gas Disks Structure Models Boundary Conditions Steady Disks around Single Stars Gas Disks Structure Models R in : boundary layer, magnetosphere, hole? R out : ad hoc?, interstellar turbulence? Viscosity MHD/rotation (Hawley & Balbus 1995) Opacity κ = κ(ρ, T,, XYZ,..., ζ 0,..., χ ν...) Models Adams & Shu 1986 (flat) [examples]kenyon & Hartmann 1987 (flared) Malbet & Bertout 1991 (vertical structure) D Allessio et al. 1998, Aikawa & Herbst 1998 (chemistry) Nomura 2002 (2D) Wolf 2003 (3D)

10 Two categories of disks observed T Tauri Disks: around young stars ( Myr) of half a solar mass (0.1 1 M sol ) at 150 pc distance ( pc) in and/or near molecular clouds Accretion Disks Debris Disks: around young ms stars ( Myr) of about a solar mass (1 2 M sol ) at 20 pc distance (3 70 pc) in the general field Vega excess stellar tll disks

11 Frequency of disks High Rate of occurence around young stars NGC % Trapezium cluster 80% IC %

12 Frequency of disks From Haisch, Lada & Lada 2001, ApJ, 553, 153

13 Observed disk sizes T Tauri/HABE disks AU Dust: mm continuum interferometry AU Dust: scattered stellar light 300AU Gas: CO lines (evidence for Kepler rotation) Silhouette disks (``proplyds ) up to 1000 AU Dust: scattered stellar light

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15 Observing disk masses continuum H 2 Gas Directly CO and Dust blueshifted CO redshifted CO

16 Gas disk evolution time scales t ~ ~ dyn α t therm α (H/R) 2 t visc t dyn ~ 1/Ω Kepler α ~ H/R << 1 if T ~ R 1/2, t visc ~ R t ~ 5 1 visc 10 yr (α/0.01) (R/10 AU)

17 Disk dispersal mechanisms SE = Stellar Encounter (tidal stripping) WS = Stellar wind stripping evap E = photoevaporation external star evap c = photoevaporation central star All for Trapezium conditions Physical Mechanisms Hollenbach et al PPIV

18 Accretion signatures From Camenzind (1990)

19 Accretion signatures residual = profile average

20 Disk lifetimes Average Error Bar Mass accretion evolution Calvet et al PPIV

21 Disk lifetimes Jayawardhana et al. (2006)

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23 Gas rich to gas poor evolution? f dust = ΔL IR /L vs stellar age (F)IR excess Stellar luminosity (bolometric)

24 Disk evolution? Spangler et al. 2001

25 Discovery of debris disks From Backman & Parece 1993, PPIII, 1253

26 Discovery of debris disks From Backman & Parece 1993, PPIII, 1253

27 β Pictoris Discovery imageby Smith & Terrile 1984 (Sci. 226, 1421)

28 Properties Large (up to 1000 AU) dusty disks found around young main sequence stars ~ 10 Myr 1 Gyr Dust disk mass M ~ 10 3 to a few M Earth. Essentially freeof gas Cold; typically K Dominating dust emission from μm to mm sized grains

29 Observational difficulties 29

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34 Late gas disk evolution? Young disks: gas/dust ~100 (?) Old disks: dust/gas ~100 (?) M / M Earth [Pollack et al. 1993] Total mass Core mass Gas mass 10 6 yr yr

35 More observational difficulties Energy diagram of H 2 Credit: Inga Kamp K] Energy [K

36 Favourable case: β Pictoris β Pictoris found to be shell star by Slettebak (1975, ApJ, 197:137) Lagrange et al. 1998, A&A 330:1091

37 Favourable case: β Pictoris FUSE (Lecavelier et al. 2001, Nature 412:76) Upper limits (3σ): N(H 2 ) < cm -2

38 mm/submm observations SEST T Liseau & Artymowicz 1998, A&A 335, 935

39 Olofsson, Liseau & Brandeker 2001, ApJL 563 Gas in emission Sodium D1/2 lines toward β Pictoris

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41 β Pictoris Brandeker et al. 2004, A&A 413:681

42 Conclusions Circumstellar disks are a consequence of star formation Disks are the formation environments for planets Disk lifetimes i dictate the timescale available for planet formation Disks start out gas rich and end up dust rich Accretion is intimately linked to outflow