Transcript Course Overview - McMaster Physics and Astronomy

Physics 778 (2009): Star formation
1. Overview
Ralph Pudritz
Origins Institute and Dept. of
Physics & Astronomy
Ext. 23180
[email protected]
Office; ABB 31
Turbulent ISM:
Canadian Galactic Plane Survey (CGPS): the
turbulent interstellar medium: shocks driven by supernova
explosions and stellar winds
(Atomic H map: Midplane of Milky Way - near Perseus)
Radio
Star formation in the Galaxy
The Galactic Center
in visible Light
Optical
Major questions in star formation:
Macroscopic aspects:
1. How is the filamentary and clumpy structure of selfgravitating clouds related to star formation?
2. What determines the stellar mass spectrum (the
“initial mass spectrum” IMF)? Is it universal?
3. How do star clusters form?
Micropscopic aspects:
4. How do individual stars/disks/jets form?
5. Do low and high mass stars form in the same way?
6. How did the first stars in the cosmos form?
1. Observational Overview Macroscopic Aspects: Stars form in
massive clouds of dusty, cold,
molecular gas
- To detect gas - map millimetre wave
emission from carbon monoxide
molecule.
- To detect dust - map sub-millimetre
emission from dust grains (eg. Use
James Clerk Maxwell Telescope – on
top of Mauna Kea volcano - Hawaii)
Optical images and infrared images of
the Orion Nebula
IRAS satellite: sensitive at wavelengths 10 – 100 microns
Structure of Giant Molecular Clouds (GMCs)
on 100 pc scale
(Carpenter et al 2009)
Extinction map of Orion and Mon clouds (Cambresy 1998) –right
Scuba continuum 850 micron map of 10 pc portion of cloud (Johnstone
& Bally 2006)
Taurus molecular
cloud – site of low
mass star formation
(distance: 140 pc. )
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FCRAO – molecular
survey in 12 and 13 CO
(top, bottom). Integrated
intensity maps. 45”
resolution, 1 km/sec
velocity resolution
Highly filamented
structure, with cavities
and rings also apparent
Narayan et al 2008, ApJS
Taurus properties
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Alignment of magnetic
field (measured by
optical polarimetry)
with molecular streaks
Cloud mass 2x10^4
solar masses
Low efficiency of star
formation
SFE = mass of young
stars/ molecular mass
=0.3 – 1.2 %
(General feature of
molecular clouds is
that they are in bulk
very inefficient in
forming stars.)
Goldsmith et al 2008
Extragalactic studies of clouds:
M51 – Whirlpool Galaxy
Global spiral
waves and
associated star
formation.
Molecular clouds
associated with
dust seen in HST
image - right
GMCs in M33 – Association with HI filaments
- Catalogue
of 148
GMCs,
complete
down to 1.5
x 10^5 solar
masses.
-Steep
mass
spectrum
for clouds
with index
(-2.6)
Engargiola et al, 2003, ApJS
Filaments and star
clusters:
clusters of stars form in
special places: hub filament systems (Myers
2009)
Above: Rho-Oph
Right: Pipe Nebula
The Origin of Stellar
Masses:
Formation of Molecular
Cloud Cores?
• Numerous small
dense gas “cores”
within a clump.
Individual stars form in
cores – typically 0.04
pc in size
(Motte et al 2001)
Filaments – home to cores, stars, clusters…
c2d
Spitzer
legacy
results:
90% of
stars lie
within
loose
clusters.
(Evans
et al
2009,
ApJS
Megeath et al – Spitzer data
Orion GMC - and the
Orion Nebula Cluster
Most stars form as
members of star clusters
and not in isolation:
Major clue to origin of
the Mass Spectrum of
stars (or initial mass
function, IMF)
Scale
ONC ~
1 pc
Super-massive star clusters
N II B in the Large Magellanic Cloud, Hubble Space Telescope Heritage
Core Mass Function (CMF). Note it has similar form as the
Initial Mass Function (IMF) for stars.
Stellar mass spectrum - the “initial mass function” (IMF)
- Broken power laws(Salpeter)
at high mass: - 1.35 if
plotted with log M*)
- Lognormal + power law
(Cabrier 2003, Hennebelle +
Chabrier 2007)
Link between CMF and IMF:
Alves et al 2007 (Pipe Nebula)
Kroupa 2002, Science
2. Microphysics of star formation: Gravitational
collapse and formation of a star/disk/jet
system
Infrared image Barnard 68 (Alves et al 2001): excellent fit with
Bonner-Ebert model (pressure truncated isothermal sphere)
Disks around young – and
old stars
Orion Proplyd – star in formation
Submm image of Epsilon Eridanni
Greaves et al (1998)
T-Tauri Stars – Spectral Energy Distributions
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Young T-Tauri stars
have Spectral Energy
Distributions (SEDs)
that differ from pure
photospheric models
of young star –
pronounced Infrared
Excess observed.
This is attributed to
emission from an
accretion disk.
Feature evolves with
time as disk
disappears ( over a
few million years)
D’Alessio et al 1999
Molecular (CO) outflows
Measure thrust
in swept-up CO;
FCO
 250( Lbol / 103 L ) 0.3
Lbol / c
(Cabrit &
Bertout1992)
 Correlation works
for both low and
high mass stars
For 391 outflows: Wu et al (2004) same index
High speed optical jets - are strongly correlated with disk
properties
Evidence for jet/disk coupling: (i) jet rotation
(Bacciotti et al 2003, Coffey et al 2004, Pesenti et al
2004)
jet rotation, 110 AU from source, at 6-15 km/sec
Footpoints for launch of jet *extended over disk
surface* (Anderson et al 2003) LV originates from
disk region: 0.3-4.0 AU
(ii) accretion and jet mass loss rates coupled (wide
variety of systems (eg. Hartmann et al 1998)
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M w / M a  0.1
Stellar spins: rotation properties of stars in
Orion Nebula Cluster (Herbst et al 2002)
Spins of 396 stars in cluster
measured… (spotted TTSs)
- Slow rotators correlate with
IR excess – indicates
presence of disk.
-bimodal… massive stars
spin slowly at 8 days, and
have a high spin peak at 2
days.
- Low mass stars have a
single peak, 2 days
Unified models for stellar spin
(Matt & Pudritz, ApJL, 2005)
Question: why do
young stars rotate
so slowly?
- MHD wind
maintains stellar
spin at small values
through accretion
powered wind
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 w   M w * R (rA / R* ) 2
2
HH 30 (from HST)
Star formation
and planet
formation
closely linked
Flared,
gaseous,
dusty disk
Gas Accretion & Gap-formation
Protoplanet
http://www.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html
Forming the first star….
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First stars, formed in primordial gas,
devoid of metals, dust, strong B field,…
Going back in time – to 400 million years
after the Big Bang – to the “dark era”
In what kind of objects did such stars form,
and how did they influence their
environment…
Text
Cosmic reionization
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End of the cosmic dark ages
measured by Ly alpha
absorption of background
quasars (Fan et al 2006) –
“Gunn-Peterson” effect.
First (massive) stars as
ionizers?
Spectra of quasar “probes” –
redshifts 5.74<z<6.42
Left: Lyman alpha optical
depth as function of redshift
Formation of the first star
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Physics is much simpler – no magnetic
fields, no dust or complex molecules,
primordial gas contains hydrogen and
helium.
Start with a small, cold, dark matter halo –
about a million times mass of the Sun.
Gas within it cools down to 200 K,
(molecular hydrogen is the coolant)
A 100 solar mass core forms inside a
filamentary “molecular cloud”
First stars
turned on
perhaps 400
million years
after the Big
Bang… they
started to
ionize and
enrich the
IGM. First
steps
towards
planets and
ultimately
life...
How do we observe all of this?
Canada’s new international facilities..
New Observatories:
- James Webb Space Telescope
- Atacama Large Millimetre Array
(50 telescope millimetre array)
ALMA and JCMT will
resolve disks, find forming
Jupiters and
first stars.
Birth of a Solar System: what ALMA can do…..
ALMA band 7
300 GHz = 1 mm
resolution = 1.4”
to 0.015”
~ Highest
resolution at
300 GHz = 1 mm
(0.015”)
~ Highest
resolution at
850 GHz =
350 mm