Two Tails of a Distribution: The Initial Mass Functions of Extreme Star Formation

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Two Tails of a DistributionThe Initial Mass
Functions of Extreme Star Formation
Michael R. Meyer Steward Observatory The
University of Arizona with Julia Greissl, Morten
Andersen, and Alan Aversa
Courtesy Jason Harris, Steward Observatory
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Stellar Initial Mass Function (Chabrier, 2003
Kroupa, 2001)
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No local variations in stellar IMF (e.g. Meyer
et al. 2000).
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What about the sub-stellar IMF ? cf. Luhman et
al. PPV (2007)
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HST/NICMOS Observations of Mon R2Multi-Color
Photometry and H2O Filter
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Is the IMF different in super-star clusters?
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Unresolved Super Star Clusters in NGC 4038/4039
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Integrated Spectra of Super-star ClustersCan
distinguish Chabrier (2003) from Salpeter (1955)
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What is going on?
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IMF in Antennae Very Young SSC 6 Consistent
with Chabrier (2003) IMF
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UD HII Regions (Proto-SSCs)
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Conspectus

• The sub-stellar IMF in young clusters is
  consistent with field and a turnover below 0.1
  Mo! (Meyer et al. 2007).
• No strong evidence for radial variation in ratio
  of stars to sub-stellar objects in Orion between
  0.8-1.8 parsecs (Andersen et al. 2007).
• Preliminary results suggest IMF in very young
  Antennae super-star clusters consistent with
  field star IMF
• (Greissl, Meyer, Christopher, Scoville, 2007).

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The Search for Variations A Six-Parameter IMF
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Epilogue

1. Need surveys for the sub-stellar IMF down below
   minimum mass for fragmentation. Will require
   surface gravity information (multi-object near-IR
   spectra) to sort out background stars (Gorlova et
   al. 2003 Mohanty et al. 2004) and kinematic
   studies to probe dynamics.
2. Determine companion mass ratio distribution as a
   function of primary star mass/separation down to
   planetary mass regime (e.g. Joergens, 2006
   Metchev Hillenbrand, 2005 Apai et al.). Could
   this help discern the difference between brown
   dwarfs and planets?
3. Surveys for ratio of high to low mass stars as a
   function of Fe/H, B-field, and ISM pressure in
   Milky Way and local group galaxies (Andersen et
   al., Meyer et al.) to search for variations in
   Jeans Mass.
4. Further modelling of integrated light in
   ultra-compact HII regions in M33 (Fe/H vs.
   Galactocentric radius) and very young Super-Star
   Clusters in starburst galaxies (J. Greissl, PhD
   thesis _at_ UofA).

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Results
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J-H versus J CMD for MonR2
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Ratio of low mass stars to brown dwarfs
Andersen et al. 2006, AJ
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The similar ratio for other regions

• Mon R2 8.5-6.4
• Taurus 6.9-2.0
• IC348 11.6-3.4
• Orion 4.3-0.6
• Chabrier4.3

All measurements within 2sigma of each other
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Prologue Why Study the IMF?

• To understand chemical evolution.
• Interpret the integrated light of other galaxies.
  
• Constrain contribution to baryonic dark matter

• The shape of the initial mass function provide
  crucial information concerning the origins of
  stellar masses.
• Are there characteristic masses?
• Is the IMF truly universal?