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, USA3 Centre for Stellar & Planetary Astrophysics, Monash University, Clayton VIC

Australia

Outline

1. Motivation2. Evidence for enhancement 3. Helium production in AGB stars4. The chemical evolution model5. Results6. 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)

Isochroneswith age 13 Gyrand 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 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

Simon’s yields Ventura et al. (2002) yields

Y ~ 0.29 Y ~ 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?