J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 1

Accretion of Stellar Winds Accretion of Stellar Winds in the Galactic Centrein the Galactic Centre

Jorge Cuadra, S. Nayakshin, Jorge Cuadra, S. Nayakshin, V. Springel, T. Di MatteoV. Springel, T. Di Matteo

MPA, GarchingMPA, Garching

MNRAS 360, L55 (2005)MNRAS 360, L55 (2005)MNRAS 366, 358 (2006)MNRAS 366, 358 (2006)

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 2

(From the GC group in Köln)

Bright Stars around a Dim Black Hole

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 3

Young Massive Stars in the GC● ~ 30 Wolf-Rayets at distances < 0.5 pc

– Strong winds, up to few × 10 - 4 Msun / yr / star.– Distributed in two discs.

Genzel et al 2003

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 4

Revnivtsev et al 2004

Sgr A* Luminosity

● Very dim (~1036 erg/s)– Caused by low mass supply

and radiatively inefficient accretion.

● But it was brighter before.– Hard X-ray reflection from

Sgr B2 indicates high luminosity just 350 yr ago.

– Star formation Myrs ago, in an AGN-like disc.

(eg, Nayakshin & Cuadra 2005)

Narayan 2002

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 5

Previous Models of Stellar Winds in the Galactic Centre

● Coker & Melia (1997) fixed grid hydrodynamics. ● Rockefeller et al (2004) SPH simulation.

– Finite number of fixed sources (do not follow orbits).

● Quataert (2004) 1-d analytical model.– Infinite number of sources, isotropically distributed.

● In all these models neither cooling nor angular momentum are important.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 6

Springel 2005

Gadget-2: SPH / N-body code

● Solves gravitational and hydrodynamical forces.

● Lagrangian code.● New version has sink

particles: accretion.● We added source

particles: wind emission.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 7

Simulations with Moving Stars: Importance of Angular Momentum

● Disc and spherical configurations.

stars

An

gula

r M

omen

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the

Gas

Acc

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on R

ate

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 8

● WR winds ~ 1000 km/s.

● Cooling time: tcool

vwind

5.4

● New observations: wind velocities ~ 300 km/s for some stars.– t

cool ~ 15 years <

dynamical time scale! – These winds can cool.

Cooling of Winds

Paumard et al 2001, also Martins et al 2006

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 9

Simulating the Galactic Centre

● Use 30 Wolf-Rayets / LBV candidates. (Paumard et al 2006)

– Measured 2d positions and 3d velocities.● 3d positions set putting stars in the discs.

– Stellar wind properties measured for some stars.(Paumard et al 2001, Martins et al 2006)

● Total mass loss rate ~ 10 -3 MSun / yr .● Try different assumptions for stars not analysed yet.

– Start the simulations ~1200 yr ago and let it evolve until the present time.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 10

Simulating the Galactic Centre:Accretion Rate ~ few10 -6 MSun/yr, but Variable

• Particles in the inner 0.05'' are accreted.

• Variability caused by the stellar orbits.

• Even with circular orbits, cold clumps produce a variable accretion rate.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 11

Simulating the Galactic Centre:Variable Luminosity on 10 - 100 yr Scales

● Viscous time-scale will smooth the accretion rate, but peaks survive.

● Due to non-linear accretion physics, may give rise to strong variability in X-rays.

Yuan et al (2004)Yuan et al (2004)

50 years sampling

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 12

Simulating the Galactic Centre:Paschen alpha emission

Scoville et al 2003

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 13

Uncertainty on the mass loss rates

Decreasing the outflow from the “slow wind stars” from 10 -5 to 10 -6 MSun/yr.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 14

Future extensions

● Even more realistic model of the stellar population.– IRS 13E as a cluster, 16SW as a binary.– LBV variability?

● Include the mini-spiral.

● Feedback from the black hole.– Different AGN modes?

● Use these results as outer boundary conditions for studies of the inner accretion flow.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 15

Conclusions● Observations allow us, for the first time, to model

the mass feeding of a super-massive black hole. We have developed a method to do so.

● Dynamics of the stellar system and stellar winds properties have a strong influence on the accretion onto the black hole.

● Cool gas clumps coexist with the hot X-ray emitting gas in the inner arc-second.

● Variable accretion rate: Sgr A* probably is energetically important for the Galactic centre on long time-scales.

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 16

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 17

Simulating the Galactic Centre:Cold and Hot Gas in the Inner Region

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 18

J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 19

Star Formation in a Disc

• AGN discs become grav unstable at large radius.(Paczyński; Kolykhalov & Sunyaev; Shlosman & Begelman; Collin & Zahn; Goodman et al; Levin)

• In the GC, need Md ~ 104 MSun.

Star formation for Q < 1.

Nayakshin & Cuadra ‘05

Nayakshin, Cuadra, Springel, in prep