Helium-enhancements in globular cluster stars from agb pollution

Helium-enhancements in globular cluster stars from AGB pollution

• Amanda Karakas1, Yeshe Fenner2, Alison Sills1,
• Simon Campbell3 & John Lattanzio3
• 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

• Motivation

• Evidence for enhancement

• Helium production in AGB stars

• The chemical evolution model

• Results

• 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

Peculiar main-sequence: NGC 2808

Extended blue HB stars: NGC 2808

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

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 from Simon’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

Helium production in AGB stars

• Z = 0.004 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

IMS-biased IMF: Our yields

IMS-biased IMF: Ventura et al. yields

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

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?