Is the I nitial M ass F unction u niversal?

1. 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

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

3. 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

4. The shape of the IMF

5. Chemical evolution models for Zw18 Recchi et al. 2004

6. 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 powerspectrum (Motte et al. 1998, Beuther & Schilke 2004)? • Opacity limit for fragmentation?

7. No variations in stellar IMF locally

8. 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

9. 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

10. 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

11. 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

12. Field Observed

13. J-H versus J CMD

14. 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

17. Ratio of “low mass stars ” to brown dwarfs

18. 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

19. Is the IMF different in massive clusters?

20. 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

21. Region surveyed

22. Derived ratio

23. 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)!

25. 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

26. The area observed

27. The derived IMF

28. A possible explanation for the discrepancy

29. Is the cluster mass segregated?

30. 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 MonR2 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.

31. 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.

32. 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

33. 2MASS image, 13 arminute times 13 arcminute

34. NACO observations, FWHM=0.08”

35. Rough spectral classification