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

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?

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.

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

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.

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!)

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.

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

Star Formation Requirements High Densities (approx > 10-22 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, 609:667-682, 2004 July 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

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

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

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

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 .

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 .

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 .

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

Density vs Radius

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 .

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 .

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 .

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

Temperature vs Radius

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

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

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

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

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)

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.

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.

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

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