Great Migrations

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Hayashi, Jeans, and the necessity of disks
ASTB21 Lecture 10
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Jeans instability the collapse of molecular
clouds
James Hopwood Jeans (1877-1946)
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How Do Stars Form?
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Oph
Giant Molecular Cloud, 160 pc away contains
numerous dark clouds
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GMCs contain dark clouds, cores, Bok
globules GMC mass / solar mass 105
V380 Ori NGC1999
Oph
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Dark clouds
L57
Barnard 68
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Jeans mass
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Jeans mass
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Following the fragmentation history, and tracking
the way M_Jeans changes w.r.t. the fragment
mass, Hoyle (1953) arrived at a concept of
opacity-limited fragmentation. When heat gets
trapped by opacity, Jeans mass increases because
T raises. The smalles mass of fragment is 0.01
M_sun 10 m_jup
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Hayashi tracks and thePre-Main
Sequenceevolution of stars
Chushiro Hayashi (1920- )
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Hayashi theory gives a nice explanation for
vertical tracks of PMS objects in the H-R
diagram, however we need to make certain
assumptions/guesses
Log L
Log T_eff
H-R diagram
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Hayashi phase of protostar contraction
Star formation in reality is a bit different
e.g., no spherical symmetry!
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Which regions of a star are convective and which
radiative? That depends on mass...
M/M_sun
Sun is radiative except for a thin subsurface
layer
10 solar mass star has convctive core and
radiative envelope
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UKAFF (UK Astroph. Fluids Facility)
supercomputer (parallel computer)
Stars are forming in these boxes.
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Matthew Symmetric initial
conditions Bate (1998)
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Realistic star formation simulations using
Smoothed Particle Hydrodynamics became possible
several years ago. (Up to) millions of
particles represent a moving, irregular,
3-D grid, and can be thought of as gas clouds
that partially overlap. Each particle
interacts with 1050 neighbors to represent
pressure forces with good accuracy. A
somewhat ad hoc treatment also endows gas with
viscosity. The gas is self-gravitating.
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Matthew Bate (2003), Bate and Benz (2003) SPH,
1.5M particles starting from turbulent gas
cloud collapse starts after turbulence dies
down and Jeans mass drops below the cloud mass.
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Brown dwarfs - a failedattempt at stardom
As seen in the simulation of molecular cloud
fragmentation, brown dwarfs (smallest objects
simulated as white points) form in large
numbers, and are mostly dispersed throughout the
Galaxy afterwards. Sometimes, they are found as
orbital companions to stars (not frequently,
hence the term brown dwarf desert by
comparison with the large numbers of planetary
companions to stars.) And there is even one BD
with its own companion of only 5 Jupiter masses!
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A strange system discovered in 2003
5 M_jup planet around a 25 M_jup Brown Dwarf in
2MASS1207
BD image has been removed by observational
technique
ESO/VLT AO HST/NICMOS, 1.6um
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Disks - a natural way to stardom
As seen in the simulation of molecular cloud
fragmentation, star formation is very
non-spherical and not even very axisymmetric it
is 3-D and leads to protostars surrounded
by accretion disks. The main physical reason is
the angular momentum (L) conservation before L
is transferred outward (e.g. by viscosity), the
gas cannot approach the rotation axis but it has
no such restriction on approaching the
equatorial plane (or midplane), where it gathers
in the form of a rotationally-supported thin disk.
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disk
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R 200 AU