star formation efficiencies of molecular clouds · summary we find active star formation with ɛ ff...
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Star formation efficiencies of molecular clouds in a galactic center environment1
...Erik Bertram
Zentrum für Astronomie der Universität Heidelberg (ZAH)Institut für Theoretische Astrophysik (ITA)
Universität Heidelberg...
1Bertram et al. (2015)Collaborators are Ralf S. Klessen, Simon C. O. Glover & Paul C. Clark
Credit: www.nasa.gov
Central region of the Milky Way(infrared image by the Spitzer space telescope)
„The Brick“
Credit: www.nasa.gov
CENTRAL REGION OF THE MW
● Morphology of the Galactic Center with its CMZ („Central Molecular Zone“):
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(Kruijssen et al. 2015)
CENTRAL REGION OF THE MW
● GC provides an accessible laboratory for studying physical processes under extreme conditions:(see, e.g. Longmore et al. 2013, Kauffmann et al. 2013, Johnston et al. 2014)
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Solar neighbourhood: Galactic Center:
Densities ~ 100cm-3 > 104cm-3
Temperatures ~ 20K ~ 70K
Velocities Several km/s > 10 km/s
Magnetic fields Several µG Several mG
Interstellar radiation field(ISRF)
G0 = 1 G
0 = 100-1000
Cosmic-ray flux(CRF)
3 x 10-17 s-1 10-14-10-15 s-1
Scientific question
SCIENTIFIC QUESTION
● How do stars form in the Galactic Center?● Problem: Observations show, that although the SFR in the CMZ is high, the specific SFR is low (lower by a factor of > 10) (Longmore et al. 2013)
● The SFR of the CMZ lies below the Kennicutt-Schmidt relation● What causes this reduced efficiency? - Turbulence? - Radiation pressure? - Feedback from stars / supernovae? - Magnetic support? ...
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Our study!
What we are doing
What we are doing
● Perform high resolution simulations of a number of GC clouds
● We use the AREPO code (Springel, 2010)
● Implementation of time-dependent chemistry with heating and cooling (Glover et al. 2010)
● Jeans refinement with self-gravity (8 cells per Jeans length)
● Self-consistent shielding (Clark et al. 2012)
● Sink particle implementation (Greif et al. 2011)
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Credit: Wikimedia
Credit: Jülich Supercomputing Center
Simulation setup
● Cloud parameters (as experienced by a typical GC cloud): - Cloud mass: ~105M
sun
- Standard solar abundance of Z = 1 - 1000 x ISRF, 1000 x CRF - decaying turbulent velocity field - periodic boundary conditions - clouds are homogeneous spheres at the beginning
● We then vary some environmental parameters:- Initial number densities: 100, 1000, 10000 cm-3
- Turbulence level: α = Ekin
/ |Egrav
| = 0.5, 1, 2, 4, 8
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What we are doing
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Simulation setup
Ekin=12
M tot σv2
Egrav=−3GM tot
2
5R
α=Ekin
|Egrav|
M tot=43
πR3ρ
t ff=√ 3π
32Gρ
ϵff=t ff⋅M s
M tot
What we are doing
● Simulation: n = 1000cm-3, box length: ~ 44pc
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What we are doing
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Star formation efficiencies against virial parameter:(compare to the canonical Milky Way average value of ɛ
ff ~ 1%)
What we are doing
εff ~ n
0
-0.5
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What we are doing
● We also perform radiative transfer post-processing with RADMC-3D and generate synthetic images of our clouds (α = 2.0):
Total column: 12CO: 13CO:
[OI] (63µm): [OI] (145µm): [CII] (158µm):
[K km/s]
[K km/s]
Summary● We find active star formation with ɛ
ff > 1% in all models!
● Our values are more comparable to Galaxy-wide SFEs (ɛ
ff ~ 1%) rather than to the low values found in the CMZ!
● The SFE decreases by a factor of ~4-10 as we increase the virial parameter from = 0.5 to = 8.0.α α
● Even in our most extreme models, we find SFEs of ~1%.● We conclude: High levels of turbulence with a strong ISRF /CRF cannot by themselves explain the low SFEs in the GC!
● We find that the atomic [OI] and [CII] fine structure lines are good tracers for MCs in a GC environment.
Summary
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Outlook for future work
● What is the impact of magnetic fields, SF feedback, etc. on the SFEs in the GC?
● Analyse the IMF of our clouds, compare to observed IMF● Perform large-scale runs of clouds in an orbit around the GC● ...
Outlook
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Thank you for your attention!
WWW: www.ita.uni-heidelberg.de/~ebertramE-Mail: Bertram@zah.uni-heidelberg.de
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