Sternentstehung

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Sternentstehung
Ralf Klessen
Christian Fendt
Phases of the ISM Properties of Molecular Clouds
Uni Potsdam, WS2002/03
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Phases of the ISM
Phases of interstellar Matter As the ISM is
dominated by hydrogen, the classification of
different phases follows the State of
H Ionized atomic hydrogen HII (H) Neutral
atomic hydrogen HI (H) Molecular hydrogen
H2 (Dust 1) The three phases
consist to almost 100 of the corresponding
component, the Transtition regions between the
phases HII, HI, and H2 are very thin. Most of
the volume of the ISM in galactic disks ( 80)
are HI und HII regions of very Low density. H2 is
in cold dense clouds. These contain about 50 of
the gas mass, and are often associated with
(surrounded by) HI and/or HII. In molecular
clouds there is an important contribution from
dust (important for heating and cooling, and for
chemical reactions)
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Phases of interstellar matter
Das molekulare Gas H2, CO, ...

• Transitions of two-atomic molecules
• Rotational transitions (needs dipole moment)
• Ro-vibrational transitions
• Electronic Ro-vibrational transitions

c) b)
a)
Niedrigste Rotations- und Schwingungsübergänge
J 1 0
n 1 - 0
Frequenz Wellenlänge T
Frequenz Wellenlänge T H2
3,87 THz 77 µm 185 K 131
THz 2,28 µm 6300 K 12CO
115 GHz 2,6 mm 5,5 K 64
THz 4,63 µm 3100 K
Adopted from B. Stecklum Physik der
Sternentstehung
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Phases of interstellar matter
Molecular Gas CO as tracer of molecular
H2 (analysis of density and velocity structure)
Observations in mm and sub-mm
ESO Swedish European Sub-mm Telescope SEST
Surface density maps of emision from Orion A (in
13CO) at different velocities (velocity bandwidth
is 1 km/s). The size of the region is 2o x 5o.
Chart of CO spectra at different locations in a
MC. With this type of survey one obtains
position-position-velocity cubes (i.e. surface
density at different velocity bands). Velocity
information allows for separation of different
clouds or cloud components (which are thought to
have different relative velocities. BUT problems
with deprojection (i.e. solutions are not unique
and interpretation often misled)
Adopted from B. Stecklum Physik der
Sternentstehung
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Phases of interstellar matter
Molecular Gas More than 100 different molecules
observed
List of some of the most abundant molecules
observed.
Spectrum of a hot core in the Orion molecular
cloud observed around 1.3 mm (atmospheric
window). More than 800 molecular lines can be
identified. Molecular clouds are highly
dynamical. Individual structural elements (e.g.
Clumps and core) are transient and so is their
chemical state (as inferred from time-dependent
chemical network models)
Adopted from B. Stecklum Physik der
Sternentstehung
B. Stecklum Physik der Sternentstehung III (8)
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Phases of interstellar matter
Molecular Gas Global properties of molecular
clouds
Temperature Density
Radius Mass velocity gradient
Erot/Epot diffuse molecular clouds T 40
... 80 K n 100 cm-3 (10 ... 50 of total
H2 mass)

Dark clouds/globules T 20 ...
40 K n 103 ... 104 cm-3 R 0,1 ...
5 pc 1 ... 10 M? 0,5 ... 4 km/s/pc
10-3... 0.3 Giant molecular clouds T
10 ... 50 K n 104 ... 106 cm-3 R
10 ... 100 pc 103 ... 106 M? 0,1 ... 0,2
km/s/pc 10-4... 0.1 Hot cores in MCs T
100 ... 300 K n gt 107 cm-3
R lt 0,1 pc 10 ... 100 M? Giant
molecular clouds are strongly concentrated in the
galactic plane and towards the center of the
Galaxy (similar holds for external galaxies)
CO Survey of Milky Way (Dame et al. 2001)
Adopted from B. Stecklum Physik der
Sternentstehung
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The multi-phase ISM
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Life-cycle of ISM
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Phases of interstellar matter
Dust

• Nature of dust particles
• Formation and desctruction
• Extinction
• Reddening
• Polarisation
• Application Mass determination

Bok globule Barnard 68
(Alves, Lada, Lada 2001)
Reddening of B68. The upper left image is
obtained in the infrared, a superposition of
three images at 1.2, 1.6 and 2.2 µm. The lower
left image is in visible wavebands. This
information can be used to obtain the column
density distribution (right).
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Phases of interstellar matter
Dust

• Formation is thought to occur in the outflowing
  gas surrounding cool stars (red giants,
  supergiants, OH/IR stars)
• Depending on chemistry oxygen rich stars (Mira)
  or carbon rich stars (Carbon stars)
• Most of the Carbon Oxygen forms CO ? Graphites
  and silicates can then form from the more
  abundant remaining atoms
• Possibly fractal growth
• Power-law size districtutiondN(s) s-3.5ds for
  s5250nm
• There are hints for very small particles (slt2nm)
  which could contribute to 10-50?m emission and
  which are not in thermal equilibrium.
• Also PAHs (polycyclic aromatic hydrocarbon) have
  been observed (these are small graphite particles
  with H atoms attached, about 50100 atoms).

• Grains will be destroyed by collisions with fast
  moving particles (sputtering) in hot gas (e.g. in
  shocks or around hot stars).

Cosmic dust particle
fractal growth model
Images from B. Stecklum Physik der
Sternentstehung
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Dust extinction
Wolf Diagram
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Dm
dark cloud
4
comparison field
log N(m)
2
0
m1 distance to the cloud
m1
m2
m2 - m1 size of the cloud
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10
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m
Wolf diagram Dependency of the cumulative star
count N(m) as function of apparent magnitude for
a dark cloud and a comparison field (Wolf 1923).
The dark cloud dims stars in the background
(extinction Am2-m1).
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Phases of interstellar matter
Dust Extinction

• Recall astronomical brightness definition m1
  m2 2.5 log (I1/I2), with m1 and m2 being
  the apparent brightness of stars 1 and 2, and
  with I1 and I2 being the received intensity.
• Absolute luminosity M apparent luminosity of a
  star in a (assumed) distance of 10pc
• Distance modulus (recall Ir-2) m M 5 log
  (r/10) 5 log (r) 5
• Extinction adds distance m M 5 log
  (r/10) 5 log (r) 5 A

comparison field
dark cloud
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Phases of interstellar matter
Reddening

• Extinction due to wavelength-dependent
  scattering of light away from line-of-sight.
• Blue light scatters more strongly than red one.
• ? Distant stars appear redder than nearby ones
• ?The degree of reddening is proportional to the
  amount of extinction or scattering
• Reddening is measured in terms of color excess
  (say compared to star 0)
• Extinction is defined as
• Extinction normalized to visual waveband V and
  color (B-V)

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Phases of interstellar matter
Reddening
What is the normalization, i.e. what is the
relation between AV and EB-V? From wavelength
dependency of extinction ? extinction curve. In
the optical, they follow a general behavior, and
we see
for
and therefore
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Phases of interstellar matter
Observing dust Thermal emission
Direct observation by thermal emission Dust
temperature is Ts 10...100 K ? optical thin
emission in submm and mm wavebands
I? ?? B? (Ts) ??
is proportional to the number of dust grains
along the line of sight ? column density Nd In
submm/mm ???-b with b1...2, also B? ?2
Therefore I? NdB? (Ts) Nd ?2-b If emission
only from object of interest and if we assume
that it is spherically symmetric (i.e. Size on
sky size along line of sight) then I? Md ?2-b
r-2 The ratio of dust mass to molecular hydrogen
mass is about 1100. There is a relation
between extinction AV and hydrogen column
density N(H) AV 1 mag
(N(H)/ 2.0 x 1021 cm-2) Spectral energy
distribution of protostar IRAS 230116126. The
dust mass Md is about 0.25 M (Chini et al.
2000). Unit of radiation intensity Jansky 1
Jy 10 26 W m 2 Hz 1
I? Jy
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Phases of interstellar matter
Observing dust Polarisation
Polarisation in the Taurus molecular cloud
Dust grains are elongated and are alignted
perpendicular to magnetic field vectors.
Unlikely alignment
Unpolarized star light
Observed partial polarisation
Most likely alignment of dust grains in magnetic
field
Circular symmetric polarisation patter around an
O star that creates the ultra-compact HII region
G5.89-0.39. The cross in the center marks the
position of the light source which is invisible
at 2.2µm.
Vector of polarisation (E component) is parallel
to the projected field lines
Adopted from B. Stecklum Physik der
Sternentstehung
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Properties of Molecular Clouds

• Spatial structure
• Velocity structure
• Thermal structure
• Magnetic field structure
• Larson relations
• The Virial Theorem applied to MCs
• Dynamical evolution

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Spatial structure
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Chamaeleon 1
Distribution of molecular gas in the Chamaeleon
region
Mizuno et al. (2001)
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Chamaeleon 2
Distribution of molecular gas in the Chamaeleon
region sampled in different velocity bins.
Mizuno et al. (2001)
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Chamaeleon 3
5 x 10 pc
Distribution of molecular gas and young stars in
the Chamaeleon region (only stars with proper
motions from Hipparchos)
Frink et al. (1998)
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Taurus
Distribution of molecular gas and young stars in
Taurus
20 pc
Hartmann (2002)
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? Ophiuchus
0.5 pc
Motte, Andre, Neri (1998)
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Hierarchical density structure
Molecular clouds exhibit hierarchical density
distribution and exhibit complex (possibly
fractal) structure down to the smalles scales
observable. What causes this complex structure?
(mostly due to interstellar turbulence) How is
it related to the velocity field? (compressible
turbulence) How can it be quantified (measured)
in a statistically meaningful sense? (e.g.
fractal indices, ? variance, principle component
analysis PCA, structure functions, etc. ? NONE
OF THESE MEASURES IS REALLY GOOD!)
Falgarone et al. (1992, IRAM Key Project)
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Fractal density structure?

• Attempts to fit power laws to the observed size
  distribution (clump-size spectrum) reveal varying
  (and non-integer) slopes.
• Related to Hausdorff definition of fractal index.
• However
• what is really measured? (limited dynamic
  range of tracer molecule)
• 2D projection of real 3D

Elmegreen Falgarone (1997)
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Clump mass spectra

• Clump mass spectra exhibit power-law behavior
• dN/dM M-1.5
• Two main schemes
• gaussclump (gaussian decomposition)
• clumpfind (identification of connected
  regions)
• BUT problems with projection render clump mass
  spectra meaningless or at least extremely
  difficult to interpret physically. (see examples)

Kramer et al. (1998)
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Gaussian decomposition
Kramer et al. (1998)
Gaussclump by Stutzki et al. (1990)
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Search for connected PPV regions
See drawing on black board
Clumpfind by Williams, deGeus, Blitz (1995)
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Problems with clump mass spectra
Projection problem deconvolution PPV ?
PPP clumps identified in PPV may correspond to
several real physical gas clumps in
PPP Radiation transfer problem only tracer
molecules observed (limited dynamic range in
density)
Ballesteros-Paredes Mac Low (2002)
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? Variance (observed)
Polars Flare Cirrus Cloud orbserved at
various scales in 12CO
Bensch et al. (2001)
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? Variance (observed) 2
?-variance for the Polaris Flare on different
spatial scales
?-variance statistical measure to quantify
the structure on different scales using
wavelet functions.
Bensch et al. (2001)
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? Variance (observed) 3
?-variance for different star forming molecular
clouds
density structure of molecular clouds is
dominated by large- scale modes (?-variance
carries most power on lare scales, ? ? -3)
Bensch et al. (2001)
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? Variance (some thoughts)
?-variance statistical measure to quantify the
structure on different scales using wavelet
functions. In gravitational collapse the
structure dominating scale shifts from large to
small scales. This can be seen in the sub-mm
(dust) in star forming regions. It cannot be seen
in molecular lines due to limiting dynamic
range!!!
(from Ossenkopf, Klessen, Heitsch 2001)
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? Variance (some thoughts)
Structure of self-gravitating turbulent media
(from Ossenkopf, Klessen, Heitsch 2001)
(data from Test Sargent 1998)
model of large-scale driven turbulence at
different times
sub-mm dust emission interfero- metry map of a
star forming region in Serpens
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? Variance (some thoughts)
Structure of self-gravitating turbulent media
Molecular line observations trace only a very
small dynamic range (1 to 1.5 decades) due to
under-excitation at low-density end and optical
depth effects at high densities. ? best observed
in dust emission!
(from Ossenkopf, Klessen, Heitsch 2001)
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Velocity Structure of MCs

• Supersonic linewidth on all scales (except maybe
  on the smallest)
• Associated kinetic energy equals (even may
  exceed) self-gravity
• Supersonic turbulence ? velocity determines
  density
• QUESTION What causes interstellar turbulence?
• Observed linewidths increase with size of
  measured region ? Larson relation
• Supersonic turbulence is driven on large scales

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Linewidth size relation (example)
Linewidth size relation for the Polaris Flare
Ossenkopf Mac Low (2002)
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Thermal Structure of MCs

• Approximate energy balance between heating
  (cosmic rays) and cooling (molecular lines)
  processes lead to roughly constant temperature of
  10K.

Magnetic Fields in MCs

• Magnetic field structure and its importance for
  our under-standing of molecular cloud evolution
  will be discussed in the next lecture.

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Larson Relations

• Larson (1981) found the following relations
  between linewidth and size and mean density and
  size
• ? ? R? ?? -1 density size
  relation (1) ? ? R? ?? 1/2
  linewidth size relation (2)
• In virial equilibrium ?? -1, ?? ½
• Molecular clouds appear gravitationally bound.
  (3)
• Values
• ? (0.720.07) km/s (R/pc)0.50.05
  (Solomon et al.)
• ? 0.55 km/s (R/pc)0.51
  (Caselli Myers)
• ?NH? (1.50.3)x1022 cm-2 (R/pc) 0.00.1 (Solomon
  et al.)
• Only two of the three statements (1,2,3) are
  independent.

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Larson Relations
Richard Larson (1981, MNRAS, 184, 809)
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Larson Relations

• Only ONE of the two Larson relation appears real
  (in the sense that it exists for the real 3D
  clumps)
• Density size relation is likely not to exist in
  3D data, but is only observed in projected data
  due to limited dynamic range of tracer molecules
  (corresponding to a roughly constant column
  density)
• Velocity size relation may exist in real 3D data
  (but may only be marginal).

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Larson Relations
density size relation
Linewidth size relation
Ballesteros-Paredes Mac Low (2002)
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Larson Relations