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Sternentstehung - Star Formation

Sternentstehung - Star Formation. Sommersemester 2006 Henrik Beuther & Thomas Henning. 24.4 Today: Introduction & Overview 1.5 Public Holiday: Tag der Arbeit 8.5 --- 15.5 Physical processes, heating & cooling, cloud thermal structure

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Sternentstehung - Star Formation

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  1. Sternentstehung - Star Formation Sommersemester 2006 Henrik Beuther & Thomas Henning 24.4 Today: Introduction & Overview 1.5 Public Holiday: Tag der Arbeit 8.5 --- 15.5 Physical processes, heating & cooling, cloud thermal structure 22.5 Physical processes, heating & cooling, cloud thermal structure (H. Linz) 29.5 Basic gravitational collapse models I 5.6 Public Holiday: Pfingstmontag 12.6 Basic gravitational collapse models II 19.6 Accretion disks 26.6 Molecular outflow and jets 3.7 Protostellar evolution, the stellar birthline 10.7 Cluster formation and the Initial Mass Function 17.7 Massive star formation, summary and outlook 24.7 Extragalactic star formation More Information and the current lecture files: http://www.mpia.de/homes/beuther/lecture_ss06.html beuther@mpia.de

  2. Summary last week • Ambipolar diffusion • Magnetic reconnection • Turbulence • Cloud collapse of singular isothermal sphere and Bonnor-Ebert sphere • Inside out collapse, rarefaction wave, density profiles, accretion rates • Accretion shock and accretion luminosity • Observational features: SEDs, Infall signatures • Rotational effects: • - magnetic locking of “outer” dense core to cloud. • - further inside it causes the formation of accretion disks

  3. Star Formation Paradigm

  4. Rotational effects II • In dense core centers magnetic bracking fails because neutral matter • and magnetic field decouple with decreasing ionization fraction. • matter within central region can conserve angular momentum. Since Fcen grows faster than Fgrav each fluid element veers away from geometrical center. --> Formation of disk The larger the initial angular momentum j of a fluid element, the further away from the center it ends up --> centrifugal radius wcen wcen = W02R4/GM =m03atW02t3/16 = 0.3AU (T/10K)1/2 (W0/10-14s-1)2 (t/105yr)3 wcen can be identified with disk radius. Increasing with time because in inside-out collapse rarefaction wave moves out --> increase of initial j.

  5. 1.3mm IRAS NIR 1.3mm IRAS NIR Full line: no inner whole Dashed line: Inner whole Beckwith et al. 1990 Early disk indications and evidence Early single-dish observations toward T-Tauri stars revealed cold dust emission. In spherical symmetry this would not be possible since the corresponding gas and dust would extinct any emission from the central protostar. --> Disk symmetry necessary!

  6. HH30, one of the first imaged disks

  7. Optical disk examples

  8. The Butterfly star Schmetterlingsstern Wolf et al. 2003

  9. 5  1013 m 1016 m 300 AU 7  104 AU Circumstellar dust disk Molecular cloud core Approximate disk size-scales

  10. Disk masses • Theories of early solar system require disk masses between 0.01 and 0.1Msun. • --> Typical disk systems apparently have enough disk mass to produce • planetary systems. Beckwith et al. 1990, Andre et al. 1994

  11. Disk ages Haisch et al. 2001

  12. T ~ r-3/4, Ldisk ~ 1/4 L* Model SEDs Simple case: flat, black disk Beckwith et al. 1996, 1999

  13. Effects of gaps on disk SED Full line: no gapLong-dashed: gap 0.75 to 1.25 AU Short-dashed: gap 0.5 to 2.5 AU Dotted: gap 0.3 to 3 AU To become detectable gap has to cut out at least a decade of disk size.

  14. Additional FIR excess • Larger inner or smaller outer disk radii even increase the discrepancy. • Data indicate that outer disk region is hotter than expected from • flat, black disk model --> Disk flaring

  15. Disk flaring The scale height h of a disk increases with radius r because the thermal Energy decreases more slowly with increasing radius r than the vertical Component of the gravitational energy: Evert, grav ~ h/r * GM*/r ~ Etherm ~ kT(r) with T(r) ~ r-3/4 --> h ~ k/GM* r5/4

  16. Hydrostatic equilibrium, radiative transfer models for flared disks I Chiang & Goldreich 1997

  17. Hydrostatic equilibrium , radiative transfer models for flared disks II nvert ~ exp(z2/2h2) h Chiang & Goldreich 1997

  18. Hydrostatic equlibrium, radiative transfer models for flared disks III Chiang & Goldreich 1997

  19. disk Class I protostar Infalling envelope Outflow cavity Class II T Tauri star Flat spectrum disks • Flat-spectrum sources have too much flux • to be explained by heating of protostar only. • In very young sources, they are still embedded • infalling envelope --> this can scatter light • and cause additional heating of outer disk. • Flat spectrum sources younger than typical class II T Tauri stars. Calvet et al. 1994, Natta et al. 1993

  20. . Maccµ M*1.8+-0.2 Dimensionless rotation rate of initial core: w = W0/Wbreak Disk accretion rates • The centrifugal radius wcen is: • wcen = W02R4/GMcore =GMcore/2at2w with Wbreak = 2√2 at3/GMcore • larger mass cores form larger disks • accretion rate spread is dominated by initial rotation of cores, imprint of initial core properties Dullemond et al. 2006

  21. Disk dynamics: Keplerian motion Simon et al.2000 DM Tau Guilloteau et al. 1998 Offset Ohashi et al. 1997 Velocity For a Keplerian supported disk, centrifugal force should equal grav. force. Fcen = mv2/r = Fgrav = Gm*m/r2 --> v = (Gm*/r)1/2

  22. SMA, Lin et al. 2006 SMA, Lin et al. 2006 CO(3-2) 345GHz continuum Non-Keplerian motion: AB Aur • Central depression in • cold dust and gas • emission. • Non-Keplerian velo- • city profile vµr -0.4+-0.01 • Possible explanations • Formation of low- • mass companion or • planet in inner disk. • Early evolutionary • phase where • Keplerian motion is • not established yet • (large envelope). PdBI, Pietu et al. 2005

  23. Disk chemistry Bergin et al. 2006

  24. Strassmeier et al. 2005 Accretion and mass transport Equilibrium between Fcen and Fgrav: mrw2 = Gmm*/r2 => w = (Gm*/r3)1/2 --> no solid body rotation but a sheared flow --> viscous forces --> mass transport inward, angular momentum transport outward, heating The inner disk is warm enough for large ionization: matter and magnetic field are coupled well --> accretion columns transport gas from disk to protostar

  25. Particle growth in disks • At (sub)mm wavelength the flux Fnµ B(n,T) k(n) • with the dust absorption coefficient k(n) proportional to nb. • --> In the Rayleigh-Jeans limit we get Fnµ n2+b • For typical interstellar grains b = 2. • Rocks are opaque at mm wavelengths --> b = 0. • --> Grain growth could be witnessed by decreasing b. Tempting to interpret as grain growth. However, there are caveats, e.g., the temperature dependence not considered.

  26. Disks in massive star formation IRS5 in M17, Chini et al. 2006 26Msun mass star with 0.5MJup dust IRAS20126+4104, Cesaroni et al. 1997, 1997, 2005 Keplerians disk around central protostar of ~7Msun • Still deeply embedded, large distances, clustered environment --> confusion • Current obs. status largely “Velocity gradient perpendicular to outflow”. • (Sub)mm interferometry important to disentangle the spatial confusion. • The right spectral line tracer still missing which can distinguish the disk • emission from the surrounding envelope emission.

  27. Summary • Disks are expected from angular momentum considerations. • The SEDs of disk sources show strong FIR excess. • SEDs allow to analyze various disk aspects: • Radial and vertical disk morphology, flaring of disks • Evolutionary stages • Inner holes • Gaps maybe due to planets • Observable in NIR in absorption and in (sub)mm line and • continuum emission. • Disk lifetimes a few million years. • T Tauri disks usually in Keplerian motion. • Younger disks like AB Aur deviate from Keplerian motion • In young sources, the outer disks can also be irradiated by • scatter light from the surrounding envelope • Strong vertical and radial chemical evolution. • Sites of grain growth and later planet formation. • Massive disks far less understood.

  28. Bob Shobbrook, Siding spring, 2 hours after sunset. Etienne Leopold Trouvelot (1827-1895) Zodiacal light

  29. Sternentstehung - Star Formation Sommersemester 2006 Henrik Beuther & Thomas Henning 24.4 Today: Introduction & Overview 1.5 Public Holiday: Tag der Arbeit 8.5 --- 15.5 Physical processes, heating & cooling, cloud thermal structure 22.5 Physical processes, heating & cooling, cloud thermal structure (H. Linz) 29.5 Basic gravitational collapse models I 5.6 Public Holiday: Pfingstmontag 12.6 Basic gravitational collapse models II 19.6 Accretion disks 26.6 Molecular outflow and jets 3.7 Protostellar evolution, the stellar birthline 10.7 Cluster formation and the Initial Mass Function 17.7 Massive star formation, summary and outlook 24.7 Extragalactic star formation More Information and the current lecture files: http://www.mpia.de/homes/beuther/lecture_ss06.html beuther@mpia.de

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