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

● 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: 10.3+-5.8

•Taurus: 9.6+-3.2

•IC348: 16.8+-5.8

•Orion: 5.5+-0.8

•Chabrier:5.3

Is the IMF different in massiveclusters?

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, 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)!

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