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Star Form
ation
Melvin Hoare
University of Leeds
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Overview
•Sites of star form
ation
•Gravitational collapse
•• •Disks and Outflows
•Evolutionary Stages
•Massive star form
ation
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Sites of Star Form
ation
•Stars for in spiral arm
s
•Compression of H I gas via
spiral density wave
•Form
s Giant Molecular
•Form
s Giant Molecular
Clouds GMCs
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Molecular Clouds
The Milky Way as seen in Integrated 13CO Maps
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Molecular Clouds
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Courtesy of the Galactic Ring Survey (http://www.bu.edu/galacticring)
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Molecular Clouds
•Filamentary, clumpy, hierarchical on wide range of scales:
•massive clumps, several pc and masses ~1000 Mo
•small dense cores , 0.1 pc and masses of order 1 Mo,
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Initial Conditions
7Submm Dust emission
C18O emission
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Cloud Support
•Clouds are
dominated by non-
8
dominated by non-
therm
al motions
-Turbulence
-Magnetic support
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Physical Conditions
•Cold (20 K), dense 104cm
-3, ice form
s on grains
Deep absorption feature at 9.7µm
99
Water ice -found in dense clouds
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Gravitational Collapse
The critical mass is known as the Jeans criterion i.e.
The balance between therm
al support and gravity leads to
The critical mass is known as the Jeans criterion i.e.
Free-fall tim
e
10
8 Mo cloud collapse in ~105yr
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Fragmentation
The Initial Mass Function
-Initial collapse is
isothermal
-
Orion Nebula Cluster (ONC)
Pleiades
Average Galactic Field IMF
-Jeans m
ass decreases
-Smaller fragments
become unstable
-Stops when clumps
become optically thick
and no longer
isothermal
M35
11
isothermal
Kroupa 2002, Science, 295, 82
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Progress of collapse
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Effects of Rotation
•We can determ
ine the
radius where F
C= F
G
-this is referred to as the
centrifugal radius
13
centrifugal radius
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Effects Magnetic Fields
B Field
Ambipolar Diffusion
14
-Neutrals can drift relative to the magnetic field opposed
by only collisions with ions
-Timescale for this process is typically longer than tff
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Effects of Magnetic Fields
Magnetic Fields
B Field
15
-Collapse of magnetically supported cloud should lead
to hourglass shape
-Need to loose large amount of magnetic flux during
collapse
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•SMA observations of
polarized sub-m
m
emission (Girat et al
2006)
2006)
•Shows hourglass field
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Accretion Discs
Rotation leads to accretion via a disc
17
Mannings Nature 388, 555-557 (7 August 1997)
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Accretion Disc Spectrum
Viscous dissipation in disc in optically thick disc gives
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Accretion Discs Observed
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Accretion Discs Observed
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Collapse of a 100 solar mass protostellar
core to a massive star (Krumholz, Klein, &
McKee 2007)
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Jets and Outflows
•Highly collim
ated
jets are invariably a
part of star
form
ation
form
ation
•Extend over pc
distances and end
in a bow shock -
Herbig-Haro
emission
•Some episodic and
22
•Some episodic and
precessing
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Jets and Outflows
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NICMOS
WFPC2
VLA 8 GHz
•Ionized gas
observed but
thought to be
24
thought to be
mostly 90%
neutral
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Jet Proper Motions
Material moving at around 500 km s-1
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Bipolar Molecular Outflows
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MHD Driving and Collimation Mechanisms
27
•Disc wind
model (Ouyed et
al. 2003)
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X XXX- ---wind model
wind model
wind model
wind model (Shu et al. 1997)
(Shu et al. 1997)
(Shu et al. 1997)
(Shu et al. 1997)
•X-w
ind interaction between rotating stellar field and disc
•Infall channelled along m
agnetic fields
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X XXX- ---ray Activity in Orion Cluster
ray Activity in Orion Cluster
ray Activity in Orion Cluster
ray Activity in Orion Cluster
29
www.astro.psu.edu/co
up
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Most (all?) stars form
in clusters
Most (all?) stars form
in clusters
Most (all?) stars form
in clusters
Most (all?) stars form
in clusters
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Evolution
•Form
ing stars
evolve from
deeply
embedded
embedded
phases to
optically visible
•Adams et al
(1987), Andre et
al. (1993)
classification
31
scheme Class 0,
I, II, III
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Disc Dispersal
•Disc disperses
over time as
fraction of
objects with IR
objects with IR
excess due to
discs decreases
with age of
cluster (Haisch
et al. 2001)
•Likely due to
planet form
ation
32
planet form
ation
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Massive Star Form
ation
Kelvin-Helmholtz Timescale
•Massive star starts core hydrogen burning whilst still
deeply embedded and accreting
•Radiation pressure on infalling dust is very high
•Accretion at high rates via a disc overcomes this
33
Krumholz et al 2005
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Evolution in Massive Star Form
ation
34
Infrared dark clouds –
opaque at 8 m
icrons
(Rathborne et al.
2005)
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IRDCs
35
Spitzer IRDC
24 µm
450 µm
Rathborne et al. 2005
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Massive Young Stellar Objects
Massive Young Stellar Objects
Massive Young Stellar Objects
Massive Young Stellar Objects
•Mid-IR bright point
sources
•Luminous but not
yet ionizing
yet ionizing
surroundings
•Bipolar outflows
36
www.ast.leeds.ac.uk/R
MS
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UCHII regions
UCHII regions
UCHII regions
UCHII regions
•Cometary
UCHIIs imply
stars born in
stars born in
density gradient
ie off-centre
37
Arthur & Hoare 2006
www.ast.leeds.ac.uk/Co
rnish
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Triggered Star Form
ation
Triggered Star Form
ation
Triggered Star Form
ation
Triggered Star Form
ation
•Expanding H II regions
compress surrounding cloud
triggering further
gravitational collapse
gravitational collapse
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Danger: Massive stars
about
39