1 / 27

Phys 778 – 2009 2. Molecular Clouds

Phys 778 – 2009 2. Molecular Clouds. Ralph Pudritz McMaster University. Extragalactic Molecular Clouds (Bolatto et al 2008) BIMA, OVRO, Plateau de Bue, SEST, used. Extragalactic size – linewidth Relation:

lionel
Download Presentation

Phys 778 – 2009 2. Molecular Clouds

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. Phys 778 – 2009 2. Molecular Clouds Ralph Pudritz McMaster University

  2. Extragalactic Molecular Clouds (Bolatto et al 2008) BIMA, OVRO, Plateau de Bue, SEST, used Extragalactic size – linewidth Relation: Milky Way Extragalactic Dwarf galaxies

  3. Extragalactic Luminosity- line width relation

  4. Extragalactic Luminosity – size relation

  5. CO luminosity vs virial mass

  6. Conclusions • Small differences between extragalactic and Milky Way GMCs down to 0.2 Zsolar • Larson type relations valid independent of environment • Departures: GMCs in dwarfs slightly larger than GMCs in galaxy • Largest departure: SMC (most metal poor) • CO/H2 = xco is constant, over factor of 5 in metallicity • Photoionization regulaged star formation (McKee 1989) appears ruled out.

  7. Cluster formation in magnetized clouds (Tilley & Pudritz, MNRAS 2007) More Jeans masses: Turbulence breaks up clouds into dense cores which form before big sheets are organized…

  8. Close-up: spinning cores emit flux of Alfven waves extracting some angular momentum Stage set for magnetized collapse… and formation of jets

  9. Core, mass to flux distributions: local core Gamma is always *reduced* from initial uniform gas distribution! Ranges from supercritical to sub-critical Arises from fragmentation of the gas.. (consider formula for Gamma)

  10. Compare with distribution of B fields measured in cores (see Crutcher et al 2007) • Distribution of values of mass to flux values for cores is now supported by the data. • Take home: initial clumps strongly supercritical – some cores become strongly magnetized - later, and even sometimes subcritical. (see PN02 and earlier)

  11. "You can think of this structure as a giant, magnetic Slinky wrapped around a long, finger-like interstellar cloud,'' said Timothy Robishaw, a graduate student in astronomy at the University of California, Berkeley. "The magnetic field lines are like stretched rubber bands; the tension squeezes the cloud into its filamentary shape.'' - Support for Fiege & Pudritz helical field model Berkeley press release, C. Heiles, ….

  12. II: Filamentary accretion and disk formation: FLASH – Adaptive Mesh Refinement (AMR) hydro simulation (Banerjee, Pudritz, & Anderson 2006): - Grid adjusts dynamically to resolve local Jeans length (Truelove et al 1997); we use 12 pixels - We added a wide variety of coolants including molecular + dust cooling, H2 formation and dissociation, heating by cosmic rays, radiative diffusion, etc.

  13. Filamentary structure: from 0.1 pc down to sub AU scale - Large scale filamentary collapse onto a growing disk: x-z plane.

  14. y-z plane along filament: same as for x-y plane - are seeing a true filamentary collapse

  15. Cut through disk midplane (x-y) • accretion from an off-centre sheet of material disk provides angular momentum of the disk. • highest resolution shows spiral wave structure • sheet formation – notice velocity shear..

  16. III Turbulence: spectral energy distributions of diffuse ISM and molecular clouds “Big power law in the sky”: radio observations - Kolomogorov- like spectrum over 11 decades: Astrophysical power law spectra observed in range GMCs: Size – linewidth relation (Heyer & Brunt 2004) Armstrong et al 1995

  17. Is a Kolmogorov turbulent cascade picture relevant for ISM? • Many different physical processes at work over these scales • Many different energy sources for “turbulence”: - galactic spiral shocks, supernovae, cosmic ray streaming, expanding HII regions, K-H and R-T instabilities, gravitational and thermal instabilities …. (eg. review Elmegreen & Scalo 2004) • Variety of solutions of incompressible MHD turbulence weak turbulence: or constant flux, indices -1 to -3 (Galtier et al 2002) • Damping rate of MHD turbulence very fast – eddy turnover time Shocks: Supersonic shocks produce -2 (eg. Kritsuk et al 2007, Vasquez-Semadeni et al 1997..) – but spectra often shallower.

  18. Mass spectra of cores – turbulent box simulations • Gas cores • Self gravitating cores • collapsed cores Bottom 3 models driven until gravity turned on, driving scale indicated Lognormal fits to collapsing objects Klessen, 2001

  19. Density structure formation • Assume density changes primarily due to shock compression – after n shock passages: • Consider shock strengths to be identically distributed random variables, in interval • Take log of both sides, apply central limit theorem. Get a log-normal distribution for density PDF:

  20. Rapid generation of lognormal density PDFs Convergence rapid – 3 or 4 shock passages suffices. Seen in most simulations Mean and width grow with number of shock passages (mean RMS Mach number increasing?) Broadest distributions for nearly isothermal gas. In self gravitating medium, collapse sets in for dense enough fluctuations Kevlahan & Pudritz 2009

  21. Spherical blast wave into log-normal medium.. • Most clusters impacted by spherical shock waves from SN, massive stellar winds, HII regions. • Density PDF = convolution = lognormal * PDF of spherical blast • For Sedov-Taylor with sustained energy injection; p=0 classical point SN explosion (instant shock) p=1 steady wind (injection shock) – Dokuchaev 2002 • Density PDF of blast wave:

  22. Generation of power-law PDF at high densities Initial lognormal distribution ---- Instant and injection shocks (-17/6 and -9/2) Point: power law tail may be the result of “feedback” from massive star by blast wave

  23. Shock generated vorticity (Kevlahan & Pudritz 2009) • Curved shock waves generate vorticity on all scales – this alone may explain structure and spectra – leading towards IMF. • Vorticity jump normal to shock (Kevlahan 1997): • Term I. = 0 for spherical shocks. • Shock focusing: Ms larger in concave curvature than conve -> shock strength grows and focus at regions of minimum curvature

  24. Vortex sheets… • Term I produces kinks – strong jet-like vortex sheets develop *downstream* of kink . • Have index of -2 and produce K-H turbulence exponentially fast • IMPORTANT; -2 index associated with downstream flow – not associated with shock itself. Focused shock: Solid line is shock strength, ---- shock profile. Discontinuities in strength produce vortex sheets downstream

  25. Spectral energy distribution – kinematic approach • Semi analytic approach: vorticity jump equation used to calculate vorticity generation after each shock – velocity etc. calculated using FFT on computational domain with periodic B.C. • Computational grid for spectral code: 256^3 • Ignore internal dynamic of flow – energy redistribution due to shock • Shock’s shape and strength distribution fixed • Random shocks: direction and phase random for each passage. (see example Elliott & Majda, 1995)

  26. Multiple passages of focused shocks:energy spectra • Initial Mach No: M0=6 • Top: initial uniform flow • Bottom: initial Gaussian energy spectrum Results independent of initial condition Energy spectrum progressively shallower with each shock passage Re-distribution of energy to smaller scales consequence of baroclinic terms

  27. Multiple passages of spherical shocks:energy spectra Due to symmetry, Ms is constant Assume downstream is irrotational with Gaussian energy spectrum (upper) instant p=0 (lower) injection p=1 A few shock passages create shallow Kolomogorov -like slopes

More Related