Phys 778 – 2009 2. Molecular Clouds
description
Transcript of 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:
Milky Way
Extragalactic
Dwarf galaxies
Extragalactic
Luminosity-
line width relation
Extragalactic
Luminosity – size relation
CO luminosity vs virial mass
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.
More Jeans masses:
Turbulence breaks up clouds into dense cores which form before big sheets are organized…
4.9
1.14
0.1
;5.27
A
J
M
n
Cluster formation in magnetized clouds (Tilley & Pudritz, MNRAS 2007)
Close-up:
spinning cores emit flux of Alfven waves extracting some angular momentum
Stage set for magnetized collapse… and formation of jets
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)
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)
Berkeley press release, C. Heiles, ….
"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
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.
Filamentary structure: from 0.1 pc down to
sub AU scale
- Large scale filamentary collapse onto a growing disk:
x-z plane.
y-z plane along filament: same as for x-y plane - are seeing a true filamentary collapse
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..
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
cm 1010 scalesfor
3/5;)(
187
kkE
]6.2,5.1[
0.3 :vKolomogoro ;6.05.0
2/)1(;v
R
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
timeShocks: Supersonic shocks produce -2 (eg. Kritsuk et al 2007,
Vasquez-Semadeni et al 1997..) – but spectra often shallower.
2/32 or
kk
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
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:
o)(
0
)( tonormalized ;))(1( xjn
j
n
1
1ln
12
,1
1ln
2log
2
)log)(log(exp
2
1)(
2
2
2
n
n
P
)]1/(2,0[
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
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:
pttE )(
-4.5-9/2index 1,p
-2.8 17/6- index 0,pFor
)()()(
0
)26/()17(3
t ppdttRd
dP
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
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
flow upstream ofnumber Mach turbulent
shock alongnumber Mach shock
shock acrossdirection binormal
entshock tang
normalshock
strengthshock )1)(2/1(1
1/
momentum) ang. ofon conservati (III. effects) baroclinic (II. shock) along Ms of variation(I.
2/1
1
1
1
2
2
22
2
t
s
s
s
ts
s
s
M
M
nsb
s
n
M
M
bsuS
MM
MS
Mb
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
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)
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
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