Helium-enhancements in globular cluster stars from AGB pollution

1. Helium-enhancements in globular cluster stars from AGB pollution Amanda Karakas1, Yeshe Fenner2, Alison Sills1, Simon Campbell3 & John Lattanzio3 1Department of Physics & Astronomy, McMaster University Hamilton ON Canada 2 CFA, Harvard University, Cambridge MA, USA 3 Centre for Stellar & Planetary Astrophysics, Monash University, Clayton VIC Australia

2. Outline • Motivation • Evidence for enhancement • Helium production in AGB stars • The chemical evolution model • Results • Discussion

3. Motivation • Unusual horizontal branch morphology of NGC 2808, M3, M13 • Stars in the extended blue tails of this cluster have an enhanced amount of helium, Y ~ 0.32 (D’Antona & Caloi 2004) compared to the primordial (0.24) • Recent results suggest a peculiar main-sequence for 2808 too (D’Antona et al. 2005) now suggesting Y up to 0.40 • Omega Centauri has a clearly defined double main sequence (Bedin et al. 2004; Piotto et al. 2005) • Norris (2004) used isochrones with Y = 0.38 to fit the bluest stars on the MS of ω Centauri.

4. ω Centauri’s main sequence from Norris (2004)

5. Peculiar main-sequence: NGC 2808 Isochrones with age 13 Gyr and Y = 0.24, 0.30, 0.40 Images from D’Antona et al. (2005)

6. Extended blue HB stars: NGC 2808 Horizontal Branch using data from Bedin et al. (2000): from D’Antona & Caloi (2004)

7. The self-pollution scenario • Attributes a previous generation of more massive stars as being responsible for the abundance anomalies we observe today • Hot bottom burning (HBB) provides an ideal environment (at least qualitatively) to convert C and O to N, Ne to Na, Mg to Al and H to Helium • Helium suggested to have come from intermediate-mass AGB • Massive AGB models can result in final surface Y ~ 0.36 (Karakas 2003, PhD thesis) it is unclear if will result in Y > 0.30 after dilution

8. The self-pollution scenario: Our approach • We approach this problem from a global perspective • Use a globular cluster chemical evolution model to follow the evolution of the intracluster gas • Model previously to follow the evolution of Na, Mg and Al in NGC 6752 • We follow helium, C, N and O and heavy elements (in this case barium) • This time we use two independent sets of AGB yields • From Simon’s models (Campbell et al. 2004, used in Fenner et al. 2004) • Ventura, D’Antona & Mazzitelli (2002)

9. Helium production in AGB stars • Helium mixed to the surface by the first and second dredge-up as a result of the convective envelope moving into a region of partial (or complete) H-burning • The third dredge-up (TDU) and hot bottom burning (HBB) further increase Y in the envelope • The amount of 4He expelled into the IMS fromSimon’s models and Ventura et al. (2002) agree to within 30%! • The net result of hydrogen fusion is the production of 4He hence the yields are fairly robust

10. Helium production in AGB stars • Z = 0.004 models ([Fe/H] ~ -0.7)

11. The chemical evolution model • Original study: Fenner et al. (2004, MNRAS…) • Prompt initial enrichment to get the cluster gas to [Fe/H] = -1.4 using Chieffi & Limongi Pop. III SN yields • Second stage we form AGB stars out of this gas, we then follow the evolution of the gas as these AGB stars pollute the cluster • Besides using a different set of AGB yields and changing the IMF, all other parameters the same as original study • Note that Ventura et al. (2002) yields are scaled solar; whereas our models have [O/Fe] = +0.4 initially

12. The initial mass function • One of the most uncertain parameters in the chemical evolution model • Determines how many stars of a given mass contribute to the chemical enrichment of the cluster • We test varying the IMF • Salpeter with slope = 1.31 (our standard) • Using a flat Salpeter with slope = 0.3 • Intermediate-mass star bias

13. The initial mass functions used

14. Evidence (or lack of) for a top heavy IMF? • Evidence for: • D’Antona & Caloi (2004) need factor of 10 more 4 to 7 Msun stars to produce He enhancements in GC stars • To produce the observed number of C, s-element rich metal-poor stars Lucatello et al. (2005) need more 1 to 5 AGB stars in the early galaxy • Evidence against: • Bekki & Norris (2005) find a top-heavy IMF would likely result in the disintegration of the cluster (applicable to helium coming from massive OR AGB stars) • n-body simulations by Downing & Sills suggest a top heavy IMF is not supported in GCs for dynamical reasons • Tilley & Pudritz (2005) studied the IMF that results from 3D simulations with MHD turbulence, conclude IMF likely to be universal (except in Z=0 gas)

15. Results: standard IMF Y ~ 0.29 Y ~ 0.26 Simon’s yields Ventura et al. (2002) yields

16. IMS-biased IMF: Our yields Y ~ 0.35

17. IMS-biased IMF: Ventura et al. yields Y ~ 0.29

18. Discussion • Using our standard IMF, the helium abundance in the gas did not exceed Y = 0.30 • Require the IMS-biased IMF to produce Y ~ 0.35 but then note the large enhancements in CNO, barium • Result for Y largely independent of AGB yields used • Results NOT supportive of AGB stars producing the large helium enhancements • Given the difficulties in obtaining a quantitative match between AGB models and GC stars without much fine-tuning suggests that AGB stars are not the solution… • Too pessimistic? There are many model uncertainties and unknowns…

19. Uncertainties • Uncertainties concerning convective model • Rotational mixing? • Efficiency of third dredge-up could be less than we predict • Mass loss behaviour at low Z unknown • Super-AGB stars? Far from clear how they contribute • Binary interactions – how will it affect the yields? • Massive AGB stars sink toward the centre of GCs • Probability of binary interactions higher in centre • Primordial binary fraction in GCs? (Ivanova et al. suggests it was high, ~ 100%)

20. Summary • We have followed the chemical evolution of helium, CNO using two independent sets of AGB yields • Results not supportive of an AGB solution • AGB stars may have produced some helium but current models cannot account for the largest enhancements (Y  0.30) • At least, not without assuming a top-heavy IMF • This also leads to large enhancements of CNO, s-process elements • Evidence for such an IMF not overwhelming • Perhaps Bekki & Norris’s idea of pollution from outside the cluster also application to other GCs besides ω Centauri?