Courtesy Jason Harris, Steward Observatory

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

Michael R. Meyer

Steward Observatory

The University of Arizona

with Julia Greissl, Morten Andersen,

and Alan Aversa

Stellar Initial Mass Function (Chabrier, 2003; Kroupa, 2001)

Chabrier (2003) Initial Mass Function for unresolved binaries plotted in linear units.

No local variations in stellar IMF (e.g. Meyer et al. 2000).

What about the sub-stellar IMF ? cf. Luhman et al. PPV (2007)

HST/NICMOS Observations of Mon R2:Multi-Color Photometry and H2O Filter

Andersen, Meyer, Oppenheimer, Dougados, and Carpenter (2006)

H-R Diagrams for Sub-stellar Objects in NGC 1333

Greissl, Meyer, Wilking, Fanetti, Greene, Schneider, Young (2007)

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Bottom Line: The Sub-stellar IMF is falling! (cf. Allen et al. 2005)

Where dN/dm ~ M-

0.0 > > -2.3(with 90 % C.I.)

Brown dwarfs do not outnumber stars.

Meyer et al. (in prep)

Spatial Variations in the Ratio of Stars to Sub-stellar Objects?

The HST Orion Treasury Program(Robberto et al.)

Color-Magnitude Diagrams as a Function of Cluster Radius

0 1 2 3 (J-H) 0 1 2 3

Andersen et al. (in prep)

R=0.6-1.0 pc R=1-2 pc R=2.2-2.9 pc

MH

Color-Magnitude Diagrams as a Function of Cluster Radius

0 1 2 3 (J-H) 0 1 2 3

Andersen et al. (in prep)

R=0.6-1.0 pc R=1-2 pc R=2.2-2.9 pc

MH

Stars

Sub-stellar

No strong radial variation in IMF detected in Orion.

N(0

.01-

1.0

Mo)

/N(0

.02-

0.08

Mo)

1

2

3

4

0.8 1.0 1.2 1.4 1.6 1.8 Radius (parsecs)

Andersen et al. (in prep)

Is the IMF different in super-star clusters?

Unresolved Super Star Clusters in NGC 4038/4039

Mengel et al. (2002)

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Meyer & Greissl (2005); Greissl et al. (2007)

Integrated Spectra of Super-star Clusters:Can distinguish Chabrier (2003) from Salpeter (1955)

What is going on?

Siess et al. (2000); Ali et al. (1995)

CaI+CO(2-0)< 0.5 Msun

MgI > 1.0 Msun

1 Myr isochrone

Greissl, Meyer, Christopher, & Scoville (2007)See Poster this session!

IMF in Antennae Very Young SSC #6 Consistent with Chabrier (2003) IMF

“UD” HII Regions (Proto-SSCs)

Johnson et al. (2001)

12”

Conspectus

1) The sub-stellar IMF in young clusters is consistent with field and a turnover below 0.1 Mo! (Meyer et al. 2007).

2) 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).

3) Preliminary results suggest IMF in very young Antennae super-star clusters consistent with field star IMF

(Greissl, Meyer, Christopher, & Scoville, 2007).

The Search for Variations: A Six-Parameter IMF

1. Mean Mass

2. Variance

3. High Mass Break

4. High Mass Slope

5. Sub-stellar Break

6. Sub-stellar Slope

-2 -1 0 1 2 log[M*/Mo]

lo

g[N

*]

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 @ UofA).

NICMOS Color-Magnitude Diagram for NGC 1333

Greissl, Meyer, Wilking, Fanetti, Greene, Scheider, Young (2007)

Ratio of Stars to Sub-stellar Objects in NGC 1333

Greissl, Meyer, Wilking, Fanetti, Greene, Scheider, Young (2007)

Results

0.000.990.0110 MyrCh03K

0.000.990.0110 MyrCh03H

0.000.990.0110 MyrCh03J

0.730.230.043 MyrCh03K

0.580.370.053 MyrCh03H

0.520.440.043 MyrCh03J

0.860.070.071 MyrCh03K

0.760.140.101 MyrCh03H

0.730.180.091 MyrCh03J

0.810.070.121 MyrS55K

0.700.1250.1751 MyrS55H

0.700.170.131 MyrS55J

F_NEBF_MSF_PMSAgeIMFBand

Ca Mg CO(2-0)

3 Myr

1 Myr

3 Myr

1 Myr

Age

6.44 +/- 0.30Ch03

6.27 +/- 0.44Ch03

11.01 +/- 0.44S55

8.98 +/- 0.82S55

EW(CaI + CO(2-0))/EW(MgI)

IMF

S55 1 Myr

S55 3 Myr

Ch03 1 Myr

Ch03 3Myr

MMT-AO Engineering PSF Simulated Trapezium Observations R(Sky Noise) = 1 Rc = 0.2 pc from Close et al. 2003. using Hillenbrand & Carpenter (2000). Hcomp(at Rc) < 24 mag

R(sky noise) = 2.5 Rc = 0.5 pc R(Sky Noise) = 4 Rc = 0.8 pc R(Sky Noise) > 20 Rc = 4-5 pc Hcomp(at Rc) < 17.8 mag. Hcomp(at Rc) < 15.3 mags. Core Radius not resolved.

25 kpc 50 kpc 0.5 Mpc

5 kpcPSF 0.5 kpc

The Trapezium on the Bleeding Edge: Sensitivity vs. Confusion...

Narrow-band Filters Provide Estimates of Teff

Andersen et al. (2006)

J-H versus J CMD for MonR2

Ratio of “low mass stars ” to brown dwarfs

Andersen et al. 2006, AJ

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

Chabrier:4.3All measurements within

2sigma of each other

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?

Prologue: Why Study the IMF?