Download Is the initial mass function universal?

Is the Initial Mass Function
universal?
Morten Andersen, M. R. Meyer, J. Greissl, B. D. Oppenheimer, M. Kenworthy, D. McCarthy
Steward Observatory, University of Arizona, USA
H. Zinnecker, AIP, Potsdam, Germany
Outline
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Why study the IMF?
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Why young clusters?
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Results from Mon R2, W51, and
R136.
Conclusions and outlook
Why study the IMF?
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To understand galaxies chemical
evolution
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Interpret the M/L of galaxies
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Constrain contributions to baryonic DM
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Crucial information for star formation
models
The shape of the IMF
Chemical evolution models for
Zw18
Recchi et al. 2004
What determines a characteristic
mass?
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Does magnetic field play role (Shu et al.
2004)?
The polytropic index changes at a critical
density, does that determine the
characteristic mass (Larson 2005)?
Clump mass spectrum in low-mass and highmass regions covers the whole mass
spectrum. is the IMF a product of the cloud
power spectrum (Motte et al. 1998, Beuther
& Schilke 2004)?
Opacity limit for fragmentation?
No variations in stellar IMF locally
Spanning the
parameter space
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Clusters with different mass to magnetic
flux ratios
Clusters with different metallicity to test
for variations due to the critical density
Variations in cluster mass
Why young clusters?
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Less affected by dynamical evolution
The whole mass range of the IMF can be
studied.
All the objects are coeval (?)
Relatively compact structures relative to
older open clusters.
The low mass objects are relatively bright in
young clusters
Why the near-infrared?
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Young clusters often embedded (Av=10 mag
or more)
Low mass objects are relatively brighter in
the near-IR relative to high mass stars
Disadvantages: (still) Relatively small field of
view and high sky background
Monoceros R2
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Distance 830 pc
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Early B star, 370 members K < 14 mag
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Roughly 1 Myr old
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HST/NICMOS 2 obs. of 1' square (0.24 pc)
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J, H, F165M, and K band observations
obtained
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Complete to 40 Mjup through Av=13 mag
More details in Andersen et al, 2006, AJ, accepted
Field Observed
J-H versus J CMD
Water band absorption
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Late type objects have strong water
absorbtions bands in their spectra
The strength of the absorbtion band
can be used as an effective
temperature indicator
Method useful in the temperature
range 2700K-3300K
Ratio of “low mass stars ” to
brown dwarfs
The similar ratio for other
regions
•Mon R2: 10.3+-5.8
Taurus: 9.6+-3.2
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IC348: 16.8+-5.8
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Orion: 5.5+-0.8
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Chabrier:5.3
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Is the IMF different in massive
clusters?
W51
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The most luminous HII region in the
Galaxy
Distance of 7 kpc
MMT/ARIES AO H and K band
data have been obtained.
0”14 resolution obtained
Preliminary study, relatively shallow
observations
More details in Andersen, et al, 2005
Region surveyed
Derived ratio
The 30 Dor region
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Most luminous HII region in the Local Group
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Metal poor, 0.25-0.5 solar metallicity
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Distance 50kpc, 1”=0.25 pc
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Template for star bursts
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Claims the IMF flattens at 2Msun (Sirianni et al.,
2000)!
R 136
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The centre of the most luminous HII region in
the local group.
NIC 2 F160W observations of the central 1'
square (3*3 mosaic).
Resolution, 0.15”, integration time 3600
seconds
Sensitive to pre-main sequence stars down
to 1 solar mass.
Andersen et al., to be submitted
The area observed
The derived IMF
A possible explanation for the
discrepancy
Is the cluster mass segregated?
Conclusions
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For the young massive metal-poor cluster R
136, the IMF is found to be “normal” to 1
solar mass.
The-sub stellar IMF in the galactic cluster
Mon R2 is consistent with the field IMF. Little
evidence for variations in the IMF locally.
Tentative signs of a slightly bottom light IMF
in W51. However, not as bottom light as the
Arches
We find the use of water vapor in late type
stars to be a useful effective temperature
The future:
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Probe the IMF to the opacity limit for
fragmentation.
Requires effective temperature and surface
gravity estimation to sort out background
stars.
Deeper studies of the most massive clusters
in the Galaxy, e.g. Westerlund 1.
Studies of metal poor clusters within the
galaxy.
Westerlund 1 The most massive
young cluster in the Galaxy?
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Distance 4-5 kpc.
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Hidden by Av=10mag
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Numerous WR stars, giants and hypergiants.
(plus one neutron star)
Age estimated to be 3-5 Myr
Total mass possible as high as 10^5 solar
masses
2MASS image, 13 arminute times 13 arcminute
NACO observations, FWHM=0.08”
Rough spectral classification