Download Accretion of Stellar Winds in the Galactic Centre

Accretion of Stellar Winds
in the Galactic Centre
Jorge Cuadra, S. Nayakshin,
V. Springel, T. Di Matteo
MPA, Garching
MNRAS 360, L55 (2005)
MNRAS 366, 358 (2006)
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 1
Bright Stars around a Dim Black Hole
(From the GC group in Köln)
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 2
Young Massive Stars in the GC
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~ 30 Wolf-Rayets at distances < 0.5 pc
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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. 3
Sgr A* Luminosity
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Very dim (~1036 erg/s)
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Narayan 2002
Caused by low mass supply
and radiatively inefficient
accretion.
But it was brighter before.
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Hard X-ray reflection from
Sgr B2 indicates high
luminosity just 350 yr ago.
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Star formation Myrs ago, in
an AGN-like disc.
(eg, Nayakshin & Cuadra 2005)
Revnivtsev et al 2004
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 4
Previous Models of Stellar
Winds in the Galactic Centre
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Coker & Melia (1997) fixed grid hydrodynamics.
Rockefeller et al (2004) SPH simulation.
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Quataert (2004) 1-d analytical model.
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Finite number of fixed sources (do not follow orbits).
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. 5
Gadget-2: SPH / N-body code
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Solves gravitational and
hydrodynamical forces.
Lagrangian code.
New version has sink
particles: accretion.
We added source
particles: wind emission.
Springel 2005
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 6
Simulations with Moving Stars:
Importance of Angular Momentum
Disc and spherical configurations.
stars
Accretion Rate
Angular Momentum of the Gas
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J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 7
Cooling of Winds
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WR winds ~ 1000 km/s.
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Cooling time: tcool vwind5.4
Paumard et al 2001, also Martins et al 2006
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New observations: wind
velocities ~ 300 km/s for
some stars.
– tcool
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~ 15 years
< dynamical time scale!
These winds can cool.
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 8
Simulating the Galactic Centre
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Use 30 Wolf-Rayets / LBV candidates.
(Paumard et al 2006)
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Measured 2d positions and 3d velocities.
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3d positions set putting stars in the discs.
Stellar wind properties measured for some stars.
(Paumard et al 2001, Martins et al 2006)
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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. 9
Simulating the Galactic Centre:
Accretion Rate ~ few10 -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. 10
Simulating the Galactic Centre:
Variable Luminosity on 10 - 100 yr Scales
50 years sampling
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Viscous time-scale will
smooth the accretion rate,
but peaks survive.
Yuan et al (2004)
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Due to non-linear accretion
physics, may give rise to
strong variability in X-rays.
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 11
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. 12
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. 13
Future extensions
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Even more realistic model of the stellar population.
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IRS 13E as a cluster, 16SW as a binary.
LBV variability?
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Include the mini-spiral.
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Feedback from the black hole.
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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. 14
Conclusions
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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. 15
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 16
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. 17
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 18
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.
Nayakshin, Cuadra, Springel, in prep
Star formation
for Q < 1.
Nayakshin & Cuadra ‘05
J. Cuadra – Accretion of Stellar Winds in the Galactic Centre – IAU General Assembly – Prague – p. 19