Accretion and Binary Mergers in the Formation of Massive Stars

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Accretion and Binary Mergers in the Formation of
Massive Stars
Ian A. BonnellUniversity of St Andrews
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Massive stars context

• Found in cores of young clusters
• Need to explain in context of
  
  low-mass SF
• N gt several 100 stars
• Densities 104 stars pc-3
• Separations 5000 AU
• Interact every 104 years
• Mjeans 0.3 Mo
• Commonly in binaries
• Separations few R to 100s AU
• With massive companion

Mark McCaughrean
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Stellar clusters and massive star formation

• Observed correlation of stellar density with mass
  of most massive star
• Physical requirement?
• Chance sampling of IMF?
• Any O stars formed in isolation?
• Not runaways?
• De Wit et al. 2005

Testi et al. 1997
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Problems I

• Difficult to form massive stars
• Require high accretion rates 10-4 Mo / yr
• Generally in dense cluster cores
• Not much room for proto-massive star
• Radiation pressure on dust grains
• Reverses infall once M gt 10-40 Mo
• Sets (low) upper mass limit?
• Yorke 1993, Wolfire Casinelli 1986
• Eddington limit 100 Mo
• Electron scattering

need
J. Bally
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Radiation pressure Solutions

• Disc Accretion Yorke Sonnhalter 2002
• Mass accretion through disc
• Partially shielded
• Stellar Radiation beamed away from disc
• Due to rapid rotation
• High accretion gt 10-3 Mo/yr McKee Tan 2003
• Overwhelm radiation pressure
• Need very dense initial conditions
• Rayleigh-Taylor Instabilities Krumholz et al
  2005
• Locally increase accretion rate

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Radiation pressure Solutions II

• Destroy dust in accretion flow Keto 2003
• Hyper-compact HII region
• Dust destroyed (just) before radiation pressure
  imparts momentum
• Stellar Collisions Bonnell, Bate
  Zinnecker 1998
• Dense stellar cluster n gt 108 stars /pc3
• Intermediate mass stars hit and merge
• No problem with radiation pressure
• Binaries tidal capture
• Ultra dense cluster due to accretion

Bally Zinnecker 2005
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Collapse and Massive star formation
Yorke Sonnhalter 2002
Radiation pressure halts infall Sets mass limit
Even with radiation beaming and
Macc 10-3 Mo/yr
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Problems II

• Difficult to form close binary systems
• Separation lt 10 AU
• Even for low-mass stars
• Fragmentation requires
• Each self-gravitating part has
• Close systems should be low-mass

Boss 1986
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Low-Mass Close Binaries
Bate, Bonnell Bromm 2003

• Close

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Low-Mass Close Binaries

• Stars form well separated and at low masses
• Stars brought together by
• Gas accretion
• Adds mass to stars
• Deepens potential
• Circum-binary discs remove
  angular momentum
• Stellar interactions
• Hardens binaries
• Exchanges
• Can this work for high-mass stars?

Pringle 1991
Bate Bonnell 1997
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Gas Accretion and Separation

• Depends on angular momentum of accreted gas
• For a binary
• If accreted gas has zero net angular momentum
• L constant
• If accreted gas has constant specific angular
  momentum
• L M

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Accretion and Cluster Contraction
Stellar Density as f(n) of time

• Accretion onto individual stars increase their
  binding energy to cluster
• Cluster contracts Increases stellar density
• Density in core increases by gt 105
• 1000 108 stars/pc 3
• Stellar collisions
• For zero angular momentum accretion

Maximum density In core of cluster
Density at half-mass radius
Bonnell Bate 2002
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Fragmentation and cluster formation
Bonnell, Bate Vine, 2003

• 1000 Mo in 0.5 pc radius
• SPH simulation (5 x 105 particles)
• Min resolution 0.1 Mo
• Sink-particles model stars
• Supported by turbulence
• Jeans mass is 1 Mo
• Macc 10-6 Mo/yr
• No feedback or radiation pressure
• No magnetic fields

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Fragmentation and the formation of a stellar
cluster
Fragmentation of cloud with M 1000 Msun , R
0.5 pc

• Fragmentation forms stars 0.5 Msun
• Accrete gas higher masses
• Binaries 3-body capture in small-N systems

Bonnell, Bate Vine 2003
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Global Properties

• Forms 419 stars
• in 2.5 tff (5 x 105 years)
• Maximum stellar mass is 30 Mo
• Macc 10-4 Mo/yr
• Median stellar mass is 0.5 Mo

Stellar mass
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Stellar Products
Bonnell, Bate Vine 2003

• 419 stars formed
• In 0.5 million years
• Most massive stars 30 Mo
• Full IMF
• shallow for low-mass stars
• Salpeter for high-mass stars
• High-mass stars in cores of clusters

G -1
mass limit
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Where does the mass come from
Bonnell et. al. 2004
Tracing the mass of a high-mass star
Massive stars Accretion from outside cluster
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Massive star formation lined to forming stellar
cluster

• Massive star grows by accreting gas that falls
  into cluster
• Gas is accompanied by low-mass stars
• Forms cluster around massive star.

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Discs around massive stars
Bonnell, Bate 2005

• Disc formation around central massive binary
• Scale 1000s AU

• Stellar interactions inside disc

• Disc is disturbed, semi-transient structure

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Accretion and massive binaries

• Stars form with low-mass and well separated
• Form binary system due to 3-body (and gas)
  capture
• Accretion increases masses
• Hardens binary
• Stellar interactions harden binary
• Extrapolation lt 1 AU?

Evolve
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Forming close binary systems

• Calculate real separations from spec. angular
  momentum and energy of binary
• Semi-major axis of 0.1 to 10 AU
• Periastron separations of lt0.01 to gt 1 AU
• But stellar radii are 0.03 AU
• collisions

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Final Binary Properties

• Red semi-major axis and binary mass
• Blue peri-astron separation and stellar mass
• More massive stars in closer binaries
• Periastron separation lt stellar radii!
• Binary mergers likely

Radius of star
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Binary Mergers

• Many binaries have rperi lt rstar
• Binary mergers
• Mergers gt double
  stars
  mass
• Stellar density need
  not be
  so high
• Require encounters
  at R
  rapastron gtgt rstar
• For R10 AU,
• need n 106 stars/pc3

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Binary separation
Red Mgt8 Mo
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Stellar Interactions and Mergers

• Binary hardens due to accretion and interactions
  with 3rd star (No softening)
• Interactions can force mergers (at 2 AU)

Separation of binary system
Distance to next closest (3rd) star
Mass of most massive star
Stellar mergers
Bonnell Bate 2002
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Modeling stellar mergers

• Little mass loss (Dale Davies 2005, Davies et
  al 2005)
• Rapid rotation
• Energies 1048 to 1051 ergs
• Tidal capture
  from close
  flybys
• Tidal shredding of
  lower-mass star
• Disc formation
  from
  debris
• Disc capture of
  
  next close passage

(Bally Zinnecker 2005
Davies et al 2005
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External Collimation of winds from massive stars

• Intrinsically spherical wind
• SPH Particle injection
• Collimated by external density structure
• From a previous simulation of massive star
  formation

• Produces collimated outflow

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Circumstellar structure and Outflows

• Circumstellar structure

• Collimated winds

x-y
x-z
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Feedback from Star Formation

Winds from massive stars Interacting with
cluster gas
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Feedback from OB stars

Ionisation from massive stars Interacting with
cluster gas 18 to 75 of UV photons escape
Dale et al 2005
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Conclusions

• Massive star formation due to accretion in
  clusters
• Gives relatively high accretion rate 10-4 Mo /
  yr
• linked to the formation of a stellar cluster
• Binaries form at wide separations and low-mass
• Evolve to high-mass and small separations
• Due to accretion and stellar interactions
• Binary mergers likely
• From stellar encounters perturbing orbit
• Require moderate stellar densities
• Most massive stars single?
• Feedback from massive stars
• Collimation by environment
• Need to include radiation pressure in massive
  star formation