Star formation

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Star formation
Suzanne Ramsay UK Astronomy Technology
Centre, Royal Observatory Edinburgh
UKIRTWFCAM infrared image of Orion
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The Challenge

• A theory of star formation requires to explain
  the origins of stars over four orders of
  magnitude in mass
• From 0.01 M? brown dwarfs powered only by
  gravitational energy
• To gt100 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?

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Stars form in molecular clouds

• 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.
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GMC chemistry

• gt100 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)
• much cloud chemistry takes place on dust grains
• Most dust mass is in grains size 1000A, 109
  atoms

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GMC chemistry

• Cosmic ray flux is key parameter in cloud
  chemistry and provide ionisation deep in clouds,
  though ionisation fraction is always low
• Magnetic field acts on ions, indirectly on gas
  and dust
• Strength of B-field 1-10 mGauss

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Properties of GMCs

• 2-4 of interstellar volume
• The rest is the atomic interstellar medium
• Lifetime, debatable but lt 107 years
• 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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• Within the Orion molecular cloud higher density
  clumps are readily identifiable

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• 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

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Star formation in clusters

• 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

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dense cores

• Bok globule b335
• typical formation site for an individual star

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Phase GMCs Clumps Cores
Mass (M?) 6x104 2x106 102 1-10
Size (pc) 20-100 0.2-4 0.1-0.4
Density (cm3) 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
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Extinction

• Some values
• AV20mag
• AK2mag
• Much higher for dense cores

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Star formation requires long wavelength astronomy

• High obscuration means that many starformation
  phenomena require long wavelength observations
• mm, submm and infrared
• Youngest sources are the most deeply embedded and
  therefore the hardest to study

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Evolution of a (low mass) protostar
Evolutionary sequence From Andre, Ward-Thompson
Barsony 1993 Extended from original by Lada
1987
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Starless cores

• Starless core or pre-stellar core
• Cold (lt15K)
• Sufficient mass for protostar envelope (0.05-30
  M?)
• Gravitationally bound, but no protostar

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Core collapse

• Considering the core as an isothermal sphere
• Density ? 1/r2
• Maximum mass for such a sphere is the Bonor Ebert
  mass
• M gt MBE, collapse starts with central core

M?
Balances surface pressure from the cloud,
velocity dispersion from temperature and gravity.
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Core collapse

• If unmediated, free fall collapse with Density ?
  1/r3/2 and vff2? 1/r1/2
• Requires additional support otherwise
• Timescales too fast
• Velocities become supersonic and core fragments

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Magnetic Support

• Clouds are known to contain magnetic fields
• These support the cloud against collapse
• Mechanism to allow slow collapse required
• Ambipolar diffusion
• 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

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Starless cores

• Observed magnetic fields inadequate for ambipolar
  diffusion model
• Turbulent support of the core required

Ward-Thompson, Motte, Andre 1999
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Class 0 sources

• Sources with a central protostar that are very
  faint/undetectable in the optical/NIR
• Lsubmm/Lbol gt 0.5
• Menvelopegtm
• Tbol lt 70K

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Class 0 sources

• First Class 0 source, VLA1623, discovered in Rho
  Ophiucus (1993)

• Andre, Ward-Thompson, Barsony 1993

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Class 0 sources

• Sources with a central protostar that are very
  faint/undetectable in the optical/NIR
• Lsubmm/Lbol gt 0.5
• Menvelopegtm
• Tbol lt 70K
• The deeply embedded protostar acquires most of
  its mass during this phase
• Bipolar molecular outflows are associated with
  Class 0 sources
• Mechanism for removing angular momentum

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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.
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Protostellar evolution

• Most of the core mass must be ejected to evolve
  from Class 0 to Class I
• During their evolution, Class 0 sources
• Increase mass from 0.3 M? to 3 M?
• Mass accretion regulated by deuterium burning
• Luminosity reaches 10-100 L?

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Class I sources IR visible protostars

• Sources with air gt 0 over the wavelength range
  from 2.2 to 10-25mm
• air is the slope on the spectral energy
  distribution
• These sources have both disks and envelopes
• 70K lt Tbol lt 650K
• Identifiable by their large infrared excess
• Infrared emission lines detectable
• Outflows, less energetic than those from Class 0

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Class 0/I sourcestimescales

• Time spent in Class I phase 1-5 105 years from
  statistical arguments on source numbers
• 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
• 10 times fewer than Class I
• Implies mass accretion rate of 10-5 M?Yr-1 to
  form half solar mass star

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Class II sourcesClassical T Tauris

• Sources with -1.5 lt airlt0 pre-main sequence
  sources with large circumstellar disks
• Optically visible
• 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

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Class III sourcesWeak line T Tauris

• Sources with airlt-1.5 pre-main sequence stars
  that are no longer strongly accreting
• Disks disspipated, so optically visible
• weak-lined - H-alpha equivalent width lt 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

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Accretion and outflow

• 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
• 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

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Accretion and outflow
HH212 (above) and HH211 (below) are class 0
sources high collimation, highly luminous
molecular outflow
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HH-30
HH-47
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Outflows and angular momentum transport

• Preferred launching mechanism for outflows is
  magnetic
• 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 stars
  surface

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High mass star formation

• Stars above 8 M? cant form by the same process
  as low mass
• 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 180620 130-190 M?

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High mass star formation

• 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

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High mass star formation

• Basic problem Kelvin Helmholtz timescale
  exceeds the free fall timescale
• tKH104 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

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HII regions as signposts

• HII regions form once Hydrogen burning ignites
  producing Lyman continuum photons
• Electron free-free emission detected in radio
• Embedded HII regions are constrained as compact
  or ultra-compact HII regions

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High mass young stellar objects

• hot cores (T100K) observed associated with or
  as precursors to UCHII regions

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High mass young stellar objects

• 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
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Outflows from HMYSOs

• Well know examples of high mass outflows have
  suggested low collimation compared with low mass
  sources
• Different mechanism for generation or low spatial
  resolution?

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IRAS201264104Varricatt et al. 2008
Outflows from high mass sources
IRAS18151-1208Davis et al. 2004
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Brown dwarfs

• Stars with insufficient mass to star hydrogen
  burning
• Mass limit 0.011-0.013 M? (12-14MJup)
• Brown dwarfs represent bridge the gap between
  stars and planets
• Stars form from collapsing cloud cores
• Planets from coagulation of material in
  circumstellar disks (during the Class II stage)

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Formation of the lowest mass stars

• Brown dwarf discoveries
• L and T dwarfs now numerous, identified from
  their very red colours through 2MASS and Sloan
  surveys
• T dwarfs M 80MJup-10MJup, Temp800K
• Surveys with e.g. WFCAM on UKIRT, VISTA promise
  the discovery of yet cooler, lower mass objects
  the (as yet) mythical Y dwarf
• NB 2-3 objects for 100s sq degrees of sky

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Formation of the lowest mass stars

• Statistics suggest that brown dwarfs have much in
  common with stars
• Possible formation mechanisms include
• photo-evaporation of cores by HII regions
• ejection from star forming cores
• fragmentation of low mass prestellar cores
• All supported by modelling which dominates?

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Outflow from 2MASSW J1207334-393254
Subarcsecond outflow detected from a 24 Jupiter
Mass brown dwarf (Whelan et al. 2007, ApJ, 659,
L45.
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The initial mass function

• From Salpeter (1955)
• The relative number of stars produced per unit
  mass interval
• Derived from the observed luminosity function
• Power law function of Mg, slope g -2.35

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Initial mass function
Example observed IMF
Salpeter mass function
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The initial mass function

• Salpeter power law slope g -2.35
• Now updated
• C(M/ M? )-1.2 0.1 lt M/ M? lt 1.0
• C(M/ M? )-2.7 1 lt M/ M? lt 10
• 0.4C(M/ M? )-2.3 10 lt M/ M?

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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
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IMF in clusters
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The initial mass function

• The IMF for field stars and those in clusters
  shows it to be the same
• 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

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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.