1. Accretion vs Star Formation
The Ultimate Tug of War
Accretion vs Star Formation

2. 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?

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

4. 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

5. 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.

6. 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!)

7. 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.

8. 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

9. Star Formation Requirements
High Densities (approx > kg/m3)*
Low temperatures (approx < 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, 609: , 2004 July 10

10. 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

11. 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

12. Gap prevents accretion of the star
Little or no source of friction
Stars around the SMBH become like the Earth around the Sun

22. Finding the critical radius of competition
Want radius at which
Densities > 10-22kg/m3
Temperature < 100K

23. Radial Dependence of Density.
ρ = k α-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius .

24. Radial Dependence of Density.
ρ = k α-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius .

25. Radial Dependence of Density.
ρ = k α-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius .

26. Radial Dependence of Density.
ρ = k α-7/10 M11/20 m5/8 R-15/8 f11/5 g/cm3
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius

27. Density vs Radius

28. Radial Dependence of Temperature.
T = k α-1/5 M3/10 m1/4 R-3/4 f6/5 K
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius .

29. Radial Dependence of Temperature.
T = k α-1/5 M3/10 m1/4 R-3/4 f6/5 K
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius .

30. Radial Dependence of Temperature.
T = k α-1/5 M3/10 m1/4 R-3/4 f6/5 K
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius .

31. Radial Dependence of Temperature.
T = k α-1/5 M3/10 m1/4 R-3/4 f6/5 K
0 < α < 1: Shakura-Sunyaev parameter: α = 0.3
M: Mass accretion rate
m:Mass of the Black Hole
f = [1-(R*/R)1/2]1/4
R*= G × MBH / c2 :Gravitational radius

32. Temperature vs Radius

33. Temperature vs Radius Mass accretion rate = 1 solar mass/yr
Mass of Black Hole varies

34. Temperature vs Radius Mass of Black Hole
= 10^8 solar masses
Accretion rate varies
Mass accretion rate = 1 solar mass/yr
Mass of Black Holes varies

35. 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

36. Comparison with our Galactic Centre: Temperature
Our graph and equations suggest a critical radius of 2*10-4 pc.
Stars observed at least as close as 2*10-3 pc from the BH.
Observations are consistent with the model

37. Testable Predictions
Prediction: stars will form up to 2*10-4 pc from the black hole, but not any closer
Experimental Test: examine our Galactic Centre at resolutions < 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)

38. 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.

39. 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.

40. 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

41. Accretion vs Star Formation
The Ultimate Tug of War
Accretion vs Star Formation 56