Is the Initial Mass Function universal? Morten Andersen, M. R. Meyer, J. Greissl, B. D. Oppenheimer,...
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Transcript of Is the Initial Mass Function universal? Morten Andersen, M. R. Meyer, J. Greissl, B. D. Oppenheimer,...
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Is the Initial Mass Function universal?
Morten Andersen, M. R. Meyer, J. Greissl, B. D. Oppenheimer, M. Kenworthy, D. McCarthySteward Observatory, University of Arizona, USAH. Zinnecker, AIP, Potsdam, Germany
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● Why study the IMF?
● Why young clusters?
● Results from Mon R2, W51, and R136.
● Conclusions and outlook
Outline
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Why study the IMF?
● To understand galaxies chemical evolution
● Interpret the M/L of galaxies● Constrain contributions to baryonic
DM● Crucial information for star
formation models
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The shape of the IMF
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Chemical evolution models for Zw18
Recchi et al. 2004
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What determines a characteristic mass?
● 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 high-mass 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?
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No variations in stellar IMF locally
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Spanning the parameter space
● 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
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Why young clusters?
● 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
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Why the near-infrared?
● 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
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Monoceros R2● Distance 830 pc
● Early B star, 370 members K < 14 mag
● Roughly 1 Myr old
● HST/NICMOS 2 obs. of 1' square (0.24 pc)
● J, H, F165M, and K band observations
obtained
● Complete to 40 Mjup through Av=13 mag
More details in Andersen et al, 2006, AJ, accepted
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Field Observed
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J-H versus J CMD
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Water band absorption
● 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
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Ratio of “low mass stars ” to brown dwarfs
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The similar ratio for other regions
•Mon R2: 10.3+-5.8
•Taurus: 9.6+-3.2
•IC348: 16.8+-5.8
•Orion: 5.5+-0.8
•Chabrier:5.3
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Is the IMF different in massiveclusters?
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W51● 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
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Region surveyed
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Derived ratio
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The 30 Dor region
● Most luminous HII region in the Local Group
● Metal poor, 0.25-0.5 solar metallicity
● Distance 50kpc, 1”=0.25 pc
● Template for star bursts
● Claims the IMF flattens at 2Msun (Sirianni
et al., 2000)!
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R 136
● 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
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The area observed
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The derived IMF
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A possible explanation for the discrepancy
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Is the cluster mass segregated?
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Conclusions
● 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 indicator.
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The future:
● 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.
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Westerlund 1 The most massive young cluster in the
Galaxy?● Distance 4-5 kpc. ● Hidden by Av=10mag● 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
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2MASS image, 13 arminute times 13 arcminute
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NACO observations, FWHM=0.08”
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Rough spectral classification