Helium-enhancements in globular cluster stars from AGB pollution Amanda Karakas 1, Yeshe Fenner 2, Alison Sills 1, Simon Campbell 3 & John Lattanzio 3 1 Department 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

Outline 1.Motivation 2.Evidence for enhancement 3.Helium production in AGB stars 4.The chemical evolution model 5.Results 6.Discussion

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.

ω Centauri’s main sequence from Norris (2004)

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

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

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

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)

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 4 He expelled into the IMS from Simon’s models and Ventura et al. (2002) agree to within 30%! The net result of hydrogen fusion is the production of 4 He hence the yields are fairly robust

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

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

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

The initial mass functions used

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)

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

IMS-biased IMF: Our yields Y ~ 0.35

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

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…

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%)

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?