Gestazione e travaglio delle stelle, tra turbolenza e campi magnetici

1. Gestazione e travaglio delle stelle, tra turbolenza e campi magnetici Daniele Galli Osservatorio Astrofisico di Arcetri WIYN Image: T.A. Rector (NOAO/AURA/NSF) and Hubble Heritage Team (STScI/AURA/NASA)

2. Orion Nebula (part of Orion GMC) The initial conditions: Orion GMC From: CfA Harvard, Millimeter Wave Group

3. Initial conditions: the molecular gas • Molecular gas is concentrated in the galactic plane; • accounts for as much as ½ all interstellar matter; Orion GMC • most of it is concentrated in GMCs, the largest and most massive objects in the Galaxy. Dame et al. 2001

4. Initial conditions: dense gas in GMCs CS (n ≈104 cm-3) CO (n ≈103 cm-3) IR clusters (E. Lada et al. 1991) Star formation confined to dense gas (n >104 cm-3, AV > 10 mag). About 10% of the mass in GMCs is characterized by n(H2) > 104 cm-3 This gas is organized into dense, gravitationally bound cores

5. Spitzer Infrared Image: A. Noriega-Crespo (SSC/Caltech) Cores with stars Cores with stars HH 46-47 flow poking out of a globule, optical (DSS)

6. Cores without stars 0.44 mm 2.16 mm Alves, Lada & Lada 2001

7. Starless or Prestellar cores: condensations with no known internal luminosity source Visual image CO Emission 5000 AU

8. Radial Density Profile, with Critical Bonnor-Ebert Sphere Fit Extinction Map Density profile Alves, Lada & Lada (2001)

9. An evolutionary sequence? time Shirley et al (2005)

10. Final Thoughts • Fundamental question: • how does matter arrange itself within interstellar molecular clouds? • The role of magnetic fields and turbulence is critical. • Ultimate questions: • why is star formation efficiency ~1%? • how are stellar masses determined?

11. Energy densities in the galaxy NGC6946 R (kpc) Beck (2003)

12. The ISM as a magnetized fluid Galactic magnetic fields L ~ kpc, B ~1-10 mG Interstellar magnetic fields L ~ 1 pc, B ~10-102mG Stellar magnetic fields L ~ R8, B ~1-103 G

13. The ISM as a turbulent fluid

14. Supersonic line widths in molecular clouds: evidence for turbulent motions • observed Dv ~ 2-4 km/s • thermal Dv very narrow: • example: CO at T=10 K ’Dvth= 0.13 km/s • Turbulence as isotropic “pressure” contributing to cloud support: • Pturb~r Dvturb2 Falgarone & Phillips (1990)

15. Kolmogorov incompressible turbulence Energy input . v2(l) E(l) ~ = const. l/v(l) ’ v(l) ~ l1/3 inertial range dissipation by viscosity

16. Larson’s (1981) scaling law sv (km/s) ~ (l/pc)1/2 sv ~ l1/2 sv ~ l1/2 sv ~ l1/3 sv ~ l1/3 Heyer & Brunt (2004)

17. Very little turbulence inside low-mass cores Supersonic motions in the outer parts Subsonic motions in the interior Intrinsic FWHM (km/s) thermal line width distance from core centre Barranco & Goodman (1998)

18. sonic scale molecular clouds massive cloud cores supersonic subsonic The ISM as a turbulent cascade log E transfer L-1 ηK-1 log k energy source and scale not known(supernovae, winds, spiral density waves?) dissipation scale not known (ambipolar diffusion, molecular diffusion?)

19. The modeling of this process requires supercomputer simulations AMR: Stone & Norman (1992) SPH: Nordlund & Padoan (2002)

20. Formation of cores and stars in a turbulent cloud

21. molecular clouds are threaded by the Galactic magnetic field

22. cloud cores and protostars are magnetized Girart, Rao, Marrone (2006) Lai (2002)

23. in agreement with theoretical core models: Shu et al. (2000), Galli et al. (2001) Li & Shu (1996), Galli et al. (1999)

24. Effects of the magnetic field: • Suppress fragmentation • Suppress rotation (magnetic braking) Fundamental parameter for stability: the critical mass-to-flux ratio Chandrasekhar & Fermi (1953), Mestel & Spitzer (1956)

25. M/F > (M/F)crit can collapse diffuse clouds molecular clouds M/F < (M/F)crit cannot collapse

26. B = 0 Hennebelle & Teyssier (2008)

27. B ≈ 1/50 BISM

28. B ≈ 1/20 BISM

29. B ≈ 1/5 BISM

30. Catastrophic magnetic braking no B field with B field (B≈1/ 3 BISM) Price & Bate (2007)

31. Summary • The fact: stars are born in turbulent and magnetized molecular clouds; • To allow the birth of a star, a cloud must loose its turbulent and magnetic support: turbulence decay; magnetic field dissipation; • For massive protostars, radiative feedback non negligible.

32. Star formation: radiation magnetohydrodynamics with self-gravity and turbulence. The worst one can have!

34. Han et al. (2006)

35. Han et al. (2006)

36. Magnetic field strength in the Galaxy Beck (2003)

37. Breg from Zeeman effect in molecular clouds Btot from synchrotron emission of diffuse gas (+ equipartition)

38. Let l = (M/F)/(M/F)crit = 2p G1/2(M/F) subcritical, cannot collapse pressure l < 1 supercritical, can collapse l >1 volume

39. Subcritical clouds where the dimensionless mass-to-flux ratio … < 1 cannot undergo gravitational collapse/fragmentation (Mestel & Spitzer 1956) • Are HI clouds precursors to molecular clouds? (Allen et al. 2004)

40. The problem of star formation separates into: • a) how proto-cloud cores evolve from sub-critical l < 1 to super-critical l>1 • b) how supercritical cores subsequently gravitationally collapse and fragment

41. (a) • Early investigations of these processes assumed conditions of laminar flow: possibly rotation, but no turbulence • Nakano (1979), Lizano & Shu (1989), Desch & Mouschovias (2001): cores form by ambipolar diffusion

42. +) McKee (1989): far-UV radiation penetrates up to Av = 4, keeping trace elements ionized (t_AD > t_Univ.). In Ophiuchus, cores don’t exist where Av < 7 (Johnstone et al. 2004 for Ophiuchus) • -) the time-scale for core formation is too long by 1 order of magnitude in comparison with the statistic of low-mass cores with and without embedded stars. The inclusion of turbulent velocity fields alleviate the difficulty (Zweibel 2002; Li & Nakamura 2004)

43. -) difficult to form massive cores without turbulent flows

44. The ISM of galaxies is turbulent on almost all observable scales (Elmegreen & Scalo 2004) • Turbulence drops to subsonic levels in cloud cores (Goodman et al. 1998) • Magnetic field is coherent from sub-pc to kpc scales

46. Han et al. (2006)

47. Why Magnetic Fields? Q. Why no large scale electric field? • Overall charge neutrality in plasma means that E is shorted out rapidly by moving electric charges. In contrast, the required currents for large scale B can be set up by tiny drifts between electrons and ions. Maxwell’s equations: a B field of 3 muG requires e-i drift of only 10-3 cm/s Finally, once large scale B is set up, it cannot be shorted out by (nonexistent) magnetic monopoles, nor can the very low resistivity dissipate the currents in a relevant time scale.

48. Flux Freezing Self-inductance In a highly conducting plasma cloud, contraction generates currents that make the magnetic field inside grow stronger, so that magnetic flux is conserved. The magnetic field lines are effectively “frozen” into the matter.

49. Pressure Balance in Barnard 68 Pthermal / PNT = a2 / sNT2 Barnard 68 is a thermally supported Cloud!

50. ~70% of all stellar systems are composed of single stars!