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Star formation
Suzanne Ramsay
UK Astronomy Technology Centre,
Royal Observatory Edinburgh
UKIRT+WFCAM infrared image of Orion
The Challenge
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A theory of star formation requires to explain
the origins of stars over four orders of
magnitude in mass
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From 0.01 M brown dwarfs powered only by
gravitational energy
To >100 M stars with lifetimes around 1million years
The typical star has mass ~1 M
So, what do we know and how do we know it?
Stars form in molecular clouds
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although stars are generally not in clusters
young stars are and so these are identified as
the sites of star formation
From Dame, Hartmann and Thaddeus 2001.
GMC chemistry
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>100 molecules discovered in MCs
H2 most abundant
CO commonly studied at 10-4 of H2 abundance
since it emits from cold GMC which H2 does not
complex molecules detected include
formaldehyde, amino acids.
Important constituent (1% of ISM) is dust (C,
Si) –
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much cloud chemistry takes place on dust grains
Most dust mass is in grains size ~1000A, 109 atoms
Properties of GMCs
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2-4% of interstellar volume
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Lifetime, debatable but <~ 107 years
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The rest is the atomic interstellar medium
Free-fall timescale ~ 106 years
Typically dispersed by radiation from massive stars,
timescale ~107 years
Supported by magnetic fields and turbulence
due to motion of clumps
Observed galactic star formation rate 3 Myr-1
Star formation in clouds is relatively inefficient:
1-3% of the cloud ends up as stars
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Orion B
Monoceros R2
Orion A
Within the Orion
molecular cloud
higher density
clumps are readily
identifiable
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Orion B
Monoceros R2
Orion A
Stars form from yet
smaller structures cores
OMC has stars of
various ages
At 460pc, the Orion
Nebula is our
closest laboratory
for studying massive
star formation
Star formation in clusters
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Embedded clusters
T associations e.g. Taurus
R associations (AB stars) e.g. Mon
R1
OB associations (massive stars
e.g. BN-KL in Orion)
Open clusters (e.g. Hyades,
Pleiades) can be very old
dense cores
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Bok globule b335
typical formation
site for an individual
star
Phase
GMCs
Clumps
Cores
Mass (M)
6x104 –
2x106
20-100
102
1-10
0.2-4
0.1-0.4
Density (cm—3) 100-300
103-104
104-105
Temp (K)
15-40
7-15
10
B (mG)
1-10
3-30
10-50
Line width
(kms-1)
6-15
0.5-4
0.2-0.4
Dynamical life
(years)
3 x 106
106
6x105
Size (pc)
Extinction
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Some values
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AV~20mag
AK~2mag
Much higher
for dense
cores
Star formation requires long
wavelength astronomy
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High obscuration means that many starformation
phenomena require long wavelength
observations
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mm, submm and infrared
Youngest sources are the most deeply
embedded and therefore the hardest to study
Evolution of a (low mass) protostar
Evolutionary sequence
From Andre,
Ward-Thompson
& Barsony 1993
Extended from original by
Lada 1987
Starless cores
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Starless core or pre-stellar core
Cold (<~15K)
Sufficient mass for protostar + envelope (0.05-30 M)
Gravitationally bound, but no protostar
Core collapse
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Considering the core as an isothermal sphere
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Density  1/r2
Maximum mass for such a sphere is the Bonor Ebert
mass
M > MBE, collapse starts with central core
M BE  1.18
2
3
(G 2 Ps )
1
M BE
2
1
Ps
T 2
2
 0.96(
) (
)
10 K 2 x10 11 dyncm  2
M
Balances surface pressure from the cloud, velocity dispersion from temperature
and gravity.
Core collapse
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If unmediated, free fall collapse with Density 
1/r3/2 and vff2 1/r1/2
Requires additional support otherwise
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Timescales too fast
Velocities become supersonic and core fragments
Magnetic Support
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Clouds are known to contain magnetic fields
These support the cloud against collapse
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Ambipolar diffusion
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Mechanism to allow slow collapse required
Neutral particles immune to magnetic field drift to the
centre of the core
Ionised particles remain fixed by the field lines
Once the core mass reaches critical level, collapse
proceeds
AD timescales are too long for standard initial
conditions
Effect of AD increased by turbulence
Starless cores
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Observed magnetic
fields inadequate for
ambipolar diffusion
model
Turbulent support of
the core required
Ward-Thompson, Motte, Andre 1999
Class 0 sources
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Sources with a central protostar that are very
faint/undetectable in the optical/NIR
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Lsubmm/Lbol > 0.5%
Menvelope>m*
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Tbol < 70K
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Class 0 sources
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First Class 0 source, VLA1623, discovered in Rho
Ophiucus (1993)
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Andre, Ward-Thompson, Barsony 1993
Class 0 sources
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Sources with a central protostar that are very
faint/undetectable in the optical/NIR
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Lsubmm/Lbol > 0.5%
Menvelope>m*
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Tbol < 70K
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The deeply embedded protostar acquires most
of its mass during this phase
Bipolar molecular outflows are associated with
Class 0 sources
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Mechanism for removing angular momentum
B335 revisted
• Contains embedded source of 3 L
• Contains a disk, radius 100AU
• Density profile – inner region of r-1.5
and outer envelope r-2 (to 5000AU)
• Inner density profile consistent with
gravitational free fall
H2CO map from Choi.
A bipolar outflow is detected from the
embedded young source
Harvey et al 2003 sub-mm imaging
reveals. Disk of radius ~100AU.
Protostellar evolution
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Most of the core mass must be ejected to evolve
from Class 0 to Class I
During their evolution, Class 0 sources
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Increase mass from ~ 0.3 M to 3 M
Mass accretion regulated by deuterium burning
Luminosity reaches 10-100 L
Lbol
Mdot
M*
R* 1
 63( 5
)(
)(
) Lsol
1
10 M sol yr
M sol 5Rsol
Class I sources
IR visible
protostars
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Sources with ir > 0 over the wavelength range
from 2.2 to 10-25mm
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ir is the slope on the spectral energy distribution
These sources have both disks and envelopes
70K < Tbol < 650K
Identifiable by their large infrared excess
Infrared emission lines detectable
Outflows, less energetic than those from Class 0
Class 0/I sources
timescales
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Time spent in Class I phase – 1-5 105 years from
statistical arguments on source numbers
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This works under assumption that the various classes
are an evolutionary trend
10 times fewer than Class II
Timescale for Class 0 - 104 years in Rho Oph
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10 times fewer than Class I
Implies mass accretion rate of 10-5 MYr-1 to form
half solar mass star
Class II sources
Classical T Tauris
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Sources with -1.5 < ir<0 –
pre-main sequence sources
with large circumstellar disks
Optically visible
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H-alpha and forbidden lines from
outflow
Stellar photospheric features, but
often veiled by disk/dust
continuum
Ages 1-4 x 106yr
T Tauri.
2MASS Atlas Image mosaics by E. Kopan,
R. Cutri, and S. Van Dyk (IPAC).
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Strong infrared excess initially hypothesised as an
obsuring disk, with later observational confirmation
Class III sources
Weak line T Tauris
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Sources with ir<-1.5 – pre-main sequence stars
that are no longer strongly accreting
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Disks disspipated, so optically visible
‘weak-lined’ - H-alpha equivalent width < 10 Å
Ages 1-20 x 106yr
Final state for our low mass protostar
Somewhat ambiguous definition as e.g. not all
stars with disks have strong H-alpha and vice
versa
Accretion and outflow
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Outflows and jets are a ubiquitous phenomenon
associated with star formation
They appear during all phases, but with trends
in their evolution with protostellar class
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Class 0 – highly collimated, luminous
Class 1, lower collimation, less energetic
Momentum flux of outflow predicted by
modelling to be proportional to mass accretion
so Class 0 sources have higher accretion than
Class 1
Accretion and outflow
HH212 (above) and HH211 (below) are class 0 sources:
high collimation, highly luminous molecular outflow
HH-30
HH-47
Outflows and angular momentum
transport
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Preferred launching mechanism for outflows is
magnetic
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Capable of explaining high degree of collimation and
outflow strength
Material ejected along magnetic field lines from
the disk
Field geometry is crucial, but a succesful model
can remove a large fraction of angular
momentum with a small amount of material
Launch sites: disk; disk-star interface; star’s
surface
High mass star formation
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Stars above 8 M can’t form by the same
process as low mass
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Hydrogen burning ignites during accretion phase
Yet they conspicuously exist, though in small
numbers compared with low mass stars
Extreme examples
Eta Carinae: 100 M; the Pistol ~150 M;
LBV 1806–20 ~130-190 M
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High mass star formation
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Fundamental difficulties in observing high mass
star formation is due to the rarity of the sources,
the distance of the nearest examples
Recent intense effort is providing larger samples
of candidate HMYSOs based on infrared colours,
radio data
High mass star formation
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Basic problem – Kelvin Helmholtz timescale
exceeds the free fall timescale
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tKH~104 years for an O star (~107 for the Sun)
Contraction proceeds faster than accretion of
material from the cloud and hydrogen burning
begins while still embedded in the cloud
Alternative formation mechanism? E.g.
coagulation from lower mass stars
HII regions as signposts
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HII regions form once Hydrogen burning ignites
producing Lyman continuum photons
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Electron free-free emission detected in radio
Embedded HII regions are constrained as
compact or ultra-compact HII regions
High mass young stellar objects
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‘hot cores’ (T~100K) observed associated with or as
precursors to UCHII regions
High mass young stellar objects
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Sub-mm imaging reveals dense cluster of
sources analogous to the Trapezium cluster in
Orion
Outflow activity in the region
– SiO jet
Beuther et al. 2007
Outflows from HMYSOs
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Well know examples of high mass outflows have
suggested low collimation compared with low
mass sources
Different mechanism for generation or low
spatial resolution?
Outflows from high
mass sources
IRAS18151-1208
Davis et al. 2004
IRAS20126+4104
Varricatt et al. 2008
Brown dwarfs
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Stars with insufficient mass to star hydrogen
burning
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Mass limit ~0.011-0.013 M (12-14MJup)
Brown dwarfs represent bridge the gap between
stars and planets
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Stars form from collapsing cloud cores
Planets from coagulation of material in circumstellar
disks (during the Class II stage)
Formation of the lowest mass
stars
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Brown dwarf discoveries
‘L’ and ‘T’ dwarfs now numerous, identified from
their very red colours through 2MASS and Sloan
surveys
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T dwarfs: M – 80MJup-10MJup, Temp~800K
Surveys with e.g. WFCAM on UKIRT, VISTA
promise the discovery of yet cooler, lower mass
objects – the (as yet) mythical Y dwarf
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NB 2-3 objects for 100s sq degrees of sky
Formation of the lowest mass
stars
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Statistics suggest that brown dwarfs have much
in common with stars
Possible formation mechanisms include
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photo-evaporation of cores by HII regions
ejection from star forming cores
fragmentation of low mass prestellar cores
All supported by modelling – which dominates?
Outflow from 2MASSW
J1207334-393254
Subarcsecond outflow detected from a
24 Jupiter Mass brown dwarf (Whelan et al.
2007, ApJ, 659, L45.
The initial mass function
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From Salpeter (1955)
The relative number of stars produced per unit
mass interval
Derived from the observed luminosity function
Power law function of M*g, slope g  2.35
Initial mass function
Example observed IMF
Turn off at brown dwarf
Masses where sources are faint
and hard to find
Salpeter mass function
The initial mass function
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Salpeter power law slope g  2.35
Now updated –
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C(M/ M )-1.2 0.1 < M*/ M < 1.0
C(M/ M )-2.7 1 < M*/ M < 10
0.4C(M/ M )-2.3
10 < M*/ M
Determining the Initial mass
function using clusters
Low end of the IMF needs deep
IR observations and observations of open
clusters
Establishing slope for high
Mass stars requires observations
Of OB associations
IMF in clusters
The initial mass function
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The IMF for field stars and those in clusters
shows it to be the same
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confirmation that the stars did form in clusters.
More recently, the core mass function found to
be consistent with the stellar IMF
The IMF is robust to a variety of clusters and
environments, but so far lacking theoretical
basis
The end
These stars
provide most of
the mass in the
galaxy
These stars dominate energy
feedback and chemical enrichment
These stars provide
most of the luminosity
in the galaxy.