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

● Why study the IMF? ● Why young clusters? ● Results from Mon R2, W51, and R136. ● Conclusions and outlook Outline

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

The shape of the IMF

Chemical evolution models for Zw18 Recchi et al. 2004

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?

No variations in stellar IMF locally

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

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

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

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

Field Observed

J-H versus J CMD

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

Ratio of “low mass stars ” to brown dwarfs

The similar ratio for other regions Mon R2: Taurus: IC348: Orion: Chabrier:5.3

Is the IMF different in massive clusters?

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

Region surveyed

Derived ratio

The 30 Dor region ● Most luminous HII region in the Local Group ● Metal poor, solar metallicity ● Distance 50kpc, 1”=0.25 pc ● Template for star bursts ● Claims the IMF flattens at 2Msun (Sirianni et al., 2000)!

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

The area observed

The derived IMF

A possible explanation for the discrepancy

Is the cluster mass segregated?

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.

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.

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

2MASS image, 13 arminute times 13 arcminute

NACO observations, FWHM=0.08”

Rough spectral classification