Transcript A new mechanism for heating the corona

About the 8 keV plasma at the
Galactic Center
High Energy Phenomena in the Galactic Center
17th June 20005
• CEA, Saclay
Belmont R.
Tagger M.
• UCLA
Muno M.
Morris M.
Cowley S.
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The Galactic
Center:
R ≤ 150-180 pc
(~ Central Molecular Zone)
X-ray and radio
observations:
- SN remnants
- discrete point sources,
- gas, clouds…
- Arcs, Filaments
Pervasive, vertical,
magnetic field
(Morris & Serabyn 1996)
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6.7 keV
6.9 keV
Spectral
components:
(Muno et al. 2004)
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Soft phase:
Hot phase :
Ionized lines + bremstrahlung
 T ~ 0.8 keV
Patchy distribution
= SN remnants
6.7 + 6.9 keV + bremstrahlung
 T ~ 8 keV,
Diffuse
large scale: 300pc*200pc (and more)
The hot phase as a diffuse plasma
at 8 keV

Origin of the hard diffuse emission: (Muno et al. 2004)
– Non thermal emission ?
– discrete point sources ?
– Chandra: Diffuse plasma.

Diffuse plasma ? (Kaneda et al., 97)
–
–
–
–

Vertical magnetic field.
Cs > 1500km/s ≥ vescape ~ 1100 km/s  not bound to the galactic plane…
Very fast escape: esc~ 40 000 yr
Heating source must be very efficient (> 30 SNe / yr in the Galaxy !!)
Also: heating mechanism ?
I. The confinement problem…
(submitted)

Elements with different weight behave differently:
– Protons alone must escape (vth > vesc)
– Other ions alone would not escape (vth < vesc)

What happens for H+He ?
– Can protons drag other ions ?
– Faint (0.1 cm-3) + hot :  ~ e ~ 105 yr > esc
– Collisionless escape => No drag.

Conclusion: plasma of helium and metals
A Hot Helium plasma ?

Too hot => no H- or He lines

New estimates for inferred plasma parameters:
– Lower densities and abundances:
– n(He) ~n(H)/3
– [Fe]/[He] for He plasma ~ 1/3*([Fe]/[He] for H plasma)
Fe trapped in grains in molecular clouds ?
H-like Argon line ?

Radiative cooling time ~ 108 yr = long time scale…
– Reasonable energy requirement
II. A possible heating mechanism

Gravitational energy of molecular clouds
– ~100 of them
– ~10 pc size
– ~100 km/s relative velocity
B
Galactic plane
(Bally et al. 87, Oka et al. 98…)

Viscosity: (Braginskii 65)
B => No shear viscosity: bulk/shear ~ 1017 !!
The bulk viscosity acts on compressional motion:
Efficiency:
–Subsonic motion: vc < cs < va => weak compression
– Very high viscosity:  ~ T5/2
=> high 
– Depends on the exact flow around the clouds…
The wake of a cloud:
Drell et al. 65,
Neubauer 80,
Wright & Schwartz 90,
Linker 91…
(in a low- plasma)
B
Alfvén wing
-> wing flux: FA
But incompressible !
V
Fast MS perturbation:
Slow MS wing:
-> wing flux: FS
And compressible
- 2D toy model
- asymptotic expansion in vc/va
-> dissipated power: QF
In the Central Region (h*d = 200*300 pc2):
Cloud number: ~ 100
hot component luminosity: ~ 5. 1037 erg/s
Fast:
Too weak…
Slow:
OK…
Alfvén:
1% dissipation would be sufficient… (irregularities, curvature…)
And more:
+ complex clouds structures
+ intermittent accretion

An intriguing coincidence:
– The hotter, the more viscous: ~ T5/2
– The hotter, the less collisional: coll ~ T3/2
– for coll >> 0 the efficiency drops  most efficient for coll ~0
– For the clouds: = r/v ~ 5 104 yr ~ He-He = optimal regime
– Coincidence or self regulation mechanism ?

Consequence on accretion:
– emission of Alfvén waves = associated drag (cf artificial satellites)
– => loss of gravitational energy and accretion
Conclusions:

In the conditions deduced from observations, H
must escape whereas heavier elements may remain.
This solves the energetics problem.

A possible heating mechanism is the dissipation of
the gravitational energy of molecular clouds by
viscosity.

The associated drag on the cold clouds would help
in accreting matter to the central object.

more analytical work + simulations
THANK YOU !
Wings
Braginskii Viscosity

Viscosity: =~l2/ ~ P ~ nkT
~ nvv
– Perfect gas: v~cst
~ n-1T-1/2  ~ T1/2
– Ionized gas: v~v-4
~ n-1T3/2  ~ T5/2

Magnetized plasma  Braginskii viscosity
(1965):
– Bulk viscosity:
Fi = 0 didjvj
– Shear viscosity:
Fi = 1 dj2vi
– Shear / bulk = 10-20