Accretion%20vs%20Star%20Formation

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Accretion vs Star Formation

• The Ultimate Tug of War

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The Little Gas Particle

• What is your fate?
• Accrete to the black hole?
• Become part of a star and
• be saved...
• Forever?
• A little while longer?
• Continue into the black hole?

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Aims

1. To determine the critical radius of competition
   between accretion and star formation around a
   supermassive black hole
2. To determine if the star, once forms, accretes
   onto the black hole or maintains a stable orbit.

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Quasars and Active Galactic Nuclei (AGN)

• Galaxies have supermassive black holes (SMBH) in
  their centers
• Accretion onto the SMBH releases electromagnetic
  radiations ? creates quasars and AGN
• AGN are smaller versions of quasars

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LLAGN Sgr A (Our SMBH)

• Low Luminosity AGN (LLAGN)
• not accreting as much
• undergoing inefficient accretion
• Sgr A the SMBH in the centre of our galaxy
  the lowest luminosity LLAGN.

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LLAGN Sgr A (Our SMBH)

• Low Luminosity AGN (LLAGN)
• not accreting as much
• undergoing inefficient accretion
• Sgr A the SMBH in the centre of our galaxy
  the lowest luminosity LLAGN.

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Accretion

• Process of gathering matter onto a central body
• Gas / dust must lose angular momentum and energy
  to accrete onto the SMBH (or the Earth would
  accrete!)

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Sources of Friction for Accretion

• Friction between gas particles ? too low to
  account for quasars
• MHD (Magneto-hydrodynamic turbulence) ? magnetic
  fields within the disk allow for the transfer of
  angular momentum without direct contact between
  particles.

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Alpha

• Shakura-Sunyaev prescription ? hide all physics
  in the parameter alpha
• Alpha is between 0 and 1
• Gives us radial dependence of
• Temperature
• Density
• Radial velocity

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Star Formation Requirements

• High Densities (approx gt 10-22 kg/m3)
• Low temperatures (approx lt 100K) ? molecular
  hydrogen gas
• Possibly a trigger
• ? self-gravitating disk which can form stars

Star Formation Thresholds and Galaxy Edges Why
and Where Joop Schaye The Astrophysical
Journal, 609667-682, 2004 July 10
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Stars near a SMBH are special

• Initial Mass Function (IMF) is top-heavy ? stars
  are bigger on average
• More gas
• Less difference between the velocity of the gas
  and star ? less angular momentum to lose

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Stars near a SMBH are special

• Initial Mass Function (IMF) is top-heavy ? stars
  are bigger on average
• More gas
• Less difference between the velocity of the gas
  and star ? less angular momentum to lose

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Gap formation in an accretion disk

• Nearby gas is accreted onto the protostar
• Once the star has formed, stellar winds push gas
  away from the star ? a gap may be formed

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Gap prevents accretion of the star

• Little or no source of friction
• Stars around the SMBH become like the Earth
  around the Sun

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Collision zone

• Near the SMBH, stellar densities are very high
• Region becomes a stellar collider ? collisions
  occur frequently
• Stars pushed into an eccentric orbit out of plane
  of accretion disk
• In our Galactic Center, we see stars at
  0.002-0.04pc from Sgr Ain highly eccentric orbits
• (animation ?)

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Finding the critical radius of competition

• Want radius at which
• Densities gt 10-22kg/m3
• Temperature lt 100K

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Radial Dependence of Density
.

• ? k a-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

.
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Radial Dependence of Density
.

• ? k a-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

.
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Radial Dependence of Density
.

• ? k a-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

.
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Radial Dependence of Density
.

• ? k a-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

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Density vs Radius
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Radial Dependence of Temperature
.

• T k a-1/5 M3/10 m1/4 R-3/4 f6/5 K
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

.
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Radial Dependence of Temperature
.

• T k a-1/5 M3/10 m1/4 R-3/4 f6/5 K
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

.
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Radial Dependence of Temperature
.

• T k a-1/5 M3/10 m1/4 R-3/4 f6/5 K
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

.
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Radial Dependence of Temperature
.

• T k a-1/5 M3/10 m1/4 R-3/4 f6/5 K
• 0 lt a lt 1 Shakura-Sunyaev parameter a 0.3
• M Mass accretion rate
• mMass of the Black Hole
• f 1-(R/R)1/21/4
• R G MBH / c2 Gravitational radius

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Temperature vs Radius
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Temperature vs Radius

• Mass accretion rate 1 solar mass/yr
• Mass of Black Hole varies

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Temperature vs Radius

• Mass of Black Hole
• 108 solar masses
• Accretion rate varies

• Mass accretion rate 1 solar mass/yr
• Mass of Black Holes varies

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Critical radius

• Temperature is the critical factor
• Depends on both Mass of Black Hole and Mass
  Accretion Rate by a factor of about 0.3.
• Ranges from 10-4pc (for our galactic centre) to 4
  pc for large mass black holes with high accretion
  rates

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Our Galactic Centre
.
M 10-8 solar masses/yr m 3.3106 solar masses
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Comparison with our Galactic CentreTemperature

• Our graph and equations suggest a critical radius
  of 210-4 pc.
• Stars observed at least as close as 210-3 pc
  from the BH.
• Observations are consistent with the model

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Testable Predictions

• Prediction stars will form up to 210-4 pc from
  the black hole, but not any closer
• Experimental Test examine our Galactic Centre at
  resolutions lt 4.8 milliarcseconds.
• May be possible using
• Very Large Baseline Array (VLBA) radio
  telescope (10 microarcseconds)
• Sydney University Stellar Interferometer (SUSI)
  optical telescope (70 microarcseconds)

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Using the Very Large Baseline Array (VLBA) Radio
Telescope

• Test critical radius of star formation
• Look for spectroscopic signatures of star
  formation in the radio, most promisingly, the
  presence of molecular line emission of CO or H2.

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Using the Sydney University Stellar
Interferometer Optical Telescope

• Test Whether or not stars accrete once formed
• Look for spectroscopic signatures of stars most
  obviously of the absorption lines of young hot
  blue (O or B) stars

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Comparison with our Galactic CentreDensity

• Predicted vs Observed
• Discrepancies of around 4-5 orders of magnitude!

Distance (pc) Predicted(solar masses/pc3) Observed(solar masses/pc3)
0.04pc 4.89102 3107
0.004pc 3.66104 8108
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Why the discrepency?

• Equations make assumption of steady state
  accretion.
• Time-varying accretion rate?
• Densities are actually more consistent with a
  mass accretion rate of 0.1-1 solar mass/yr.
• If observations through VLBA revealed a different
  critical radius, it may be possible to infer the
  mass accretion rate when the stars were formed.

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Other Testing Grounds

• Nearby galaxies with large black holes and
  reasonably high accretion rates are ideal!
• Unfortunately, these 3 parameters are rarely all
  fulfilled.

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M87

• M87 is
• nearby (16Mpc)
• large black hole (3109 solar masses)
• reasonably low accretion rate (110-4 solar
  masses/yr).

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Temperature profile for M87
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M87

• Critical Radius approx 0.1pc
• Required resolution lt1.2 milliarcseconds
• This would be a good confirmation of the model!

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Competition between accretion and star formation

• Stars can form outside a certain critical radius
  (10-4 4pc)
• If mass accretion rate decreases, then star
  formation is favoured (e.g. Our GC?)
• Stars, once formed, are unlikely to accrete onto
  the black hole.

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Your Fate as the Little Gas Particle

• Outside Rcritical ? You may be saved!
• form a star
• stay as gas without accreting for about the
  lifetime of the stars in the inner regions.
• Inside Rcritical ? You will die!
• accreted to the black hole in less than a million
  years.

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Time for Inflow vs Radius
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So do stars accrete onto the black hole?

• I conclude tentatively that they dont.
• Literature seems to say either
• Assumption of continued accretion of the star,
  without extensive justification.
• Development of a gap model, which would hinder
  star accretion ... Probably.

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Why cant we be sure?

• Our model of accretion in quasars and AGN is
  still incomplete, despite recent developments.
  Until the source of accretion is known
  conclusively, we cannot say whether or not the
  star will be subject to the friction and accrete.
  
• More observations of nearby galaxy centres with
  high resolution are needed.

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Thankyou

• To my supervisor Zdenka Kuncic for all her help
  in discussing the issue and preparing the project
• Andrew Hopkins for discussions regarding star
  formation

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Accretion vs Star Formation

• The Ultimate Tug of War