AGB stars in Galactic Globular Clusters – Are They Chemically Distinct to Their

Fellow RGB and HB Stars?

M5: SDSS Simon Campbell1. Universitat Politecnica de Catalunya, Barcelona, Spain

2. CSPA, Monash University, Melbourne, Australia

CollaboratorsRSAA, Mt Stromlo, Australia:

David YongElizabeth Wylie de Boer

Monash University, Australia:John Lattanzio

Richard StanciffeGeorge Angelou

University of Aarhus, Denmark:Frank Grundahl

University of Texas, USA:Chris Sneden

Disclaimer

• Normally I work on stellar modelling of low- metallicity stars (low & intermediate mass, including AGBs):

– GCs, Z ~ 1e-3

– EMPs, Z ~ 1e-7

– Primordial, Z = 0

Warning: Theorist talking about observing stuff...! :)

Part 1: Background: AGBs in GCs

+ CN Bimodality

AGBs in Globular Clusters

• High quality photometry is now making the AGB accessible in GCs, we can now get good numbers of AGBs.

M5 (SDSS)

B-I

I

M5 (Sandquist & Bolte 2004)

100 AGBs!

Quantifying Cyanogen Abundance:The S(3839) CN Index

• So basically you see how much flux is missing in a wavelength region due to CN absorption by comparing to piece of 'continuum' nearby. Only need fairly low (~2 Ang) resolution.

CN Weak

CN Strong

'Continuum'

Norris et al. (1981)

• Cyanogen (CN) is a molecule whose abundance is thought to track that of Nitrogen. It absorbs over a few regions in the spectrum. Here we consider the Blue CN bands.

Blue CN Index:

CN Bimodality in GC Giants• Early observations of GC giants showed that the molecule Cyanogen

(CN) has a bimodal distribution.• This suggests that each population – “CN-weak” and “CN-strong” have

different nitrogen abundances.

CN

Index

NGC 6752, Norris et al 1981

Mag

CN-Strong

CN-Weak

No. Stars

Bimodal CN distribution on RGBThis is not observed in halo field stars!

(see eg. Langer et al, 1992)

References: eg. Bell & Dickens 1974, Da Costa & Cottrell 1980, Norris et al. 1981.

• Note trend with Temp.

More GC Weirdness? – CN in AGBs• Norris et al. 1981 noted that their sample of AGB stars in NGC 6752

were all CN-weak (triangles = AGBs).• Could this be chance, due to the small sample size – or is something

strange happening on the AGB??

CN

Index

NGC 6752, Norris et al 1981

Mag

CN-Weak

CN-Strong

References: eg. Bell & Dickens 1974, Da Costa & Cottrell 1980, Norris et al. 1981.

AGBs: all CN-weak

• Note trend with Temp.

An Interesting Proposition• It is an interesting proposition that the AGBs of GCs could be

chemically distinct from the RGBs (and MS, etc). • This is not predicted by standard stellar evolution – there is no

known evolutionary phase between the RGB and AGB that changes the surface composition (note that these are generally EAGB stars -- so no TDU yet).

• Furthermore, taking into account Deep Mixing on the RGB, which tends to increase N, the (apparent) general trend seen in the AGBs is the opposite to that which would be expected...

However all this speculation is based on small samples of AGB stars, so we can't say for sure – an in-depth study is needed to settle the issue – hence our observing project :)This is easier said than done, since there are so few AGB stars in GCs, plus it is difficult to separate them in colour from the RGBs...

Part 2: Observing CN in GC AGBs

+ Preliminary Results

v-y

v

NGC 6752

AGBRGB

BGB=RGB+AGB

Excellent Photometry Needed• Excellent photometry is needed in order to split the two giant

branches.• v versus v-y CMD gave good AGB-RGB splitting fro NGC 6752. (NGC 6752 data from Frank Grundahl)

(yellow = raw data)

Spectra Collected: Number of AGBs• 5 nights on AAT Multi-object spectrograph 2dF/AAOmega• Data collected for 241 AGB stars across 9 clusters (plus

many RGB & HB stars).

Results: NGC 6752

*ALL AGBs CN-weak!*

• The cluster that Norris et al 1981 investigated.• RGB nicely bimodal, as expected.• And on the AGB....

Strong to Weak Ratios

RGB = 80:20

AGB = 0:100

Results: GC Pair Comparison• NGC 288 and 362 have similar metallicities ([Fe/H] ~ -1.2)

but different HB morphologies compare CN behaviour.

Red HBExt-Blue HB

Results: NGC 288 (Blue HB)• The normal CN bimodality is seen on the RGB.• And on the AGB....

Strong to Weak Ratios

RGB = 50:50

AGB = 0:100

A totally CN-weak AGB! – just like NGC 6752, which also has a very blue HB…

NGC 288 photometry: Grundahl et al., 1999.

Results: NGC 362 (Red HB)Strong to Weak

Ratios

RGB = 60:40

AGB = 40:60 to 60:40

Either a CN-weak dominated AGB, or no change from RGB (hard to define the bimodal split)

NGC 362 photometry: Bellazzini et al., 2001.

Summary/Discussion• Our preliminary results clearly show there is something strongly

effecting the numbers of CN-strong and CN-weak stars between the RGB, HB and AGB.

• It appears to be related to the HB morphology of the GCs.• GCs with red HBs show little or no change in the ratio of CN-strong

to CN-weak stars going from the RGB to AGB.• However in GCs with very blue HBs it is amazing to find that there

are zero CN-strong stars on the AGB (eg. 6752, 288) – the CN-strong stars seem to ‘disappear’ when moving from the RGB to AGB.

• So what is happening??– Maybe the CN-strong stars don't ascend the AGB at all? (an idea

also suggested by Norris et al. 1981). The fact that this feature is (mainly) seen in GCs with blue HBs suggests this may be the case, since the blue HB stars should have low masses.

– Primordial abundance variations (eg. He, N) may affect mass loss or other evolution.

Future Work• Finish analysing the data for the other GCs.• Get more chemical information from current

spectra (Al, NH, CH, maybe Li?)• Analyse our HB data – clues as to where things

change?• Use these sets of AGB stars for higher resolution

observations, to check for additional abundance variations/correlations (Na, O, Mg, etc.)

• Models to explain this strange change between the RGB and AGB!

19

The End :)Thanks to the producers & maintainers of these tools: -- 2MASS -- IRAF -- ViZieR & Aladdin -- SIMBAD -- 2dFdr, the 2dF data reduction software

References for Photometric Studies• NGC 362: Bellazzini et al. 2001• NGC 6752: Grundahl (private comm.)• NGC 288: Grundahl (private comm.)• M4: Mochejska et al. 2002• Omega Cen: Sollima et al. 2005• NGC 1851: Walker 1992• M2: Lee 1999• M10: Pollard 2005• M5: Sandquist & Bolte 2004• 47 Tuc: Kaluzny et al. 1998

Thanks! :)• Thanks heaps to the observers!

– Richard Stancliffe– Elizabeth Wylie de Boer– George Angelou– David Yong

• Thanks to the producers & maintainers of these tools which made life much easier:– 2MASS – an excellent resource for accurate astrometry.– IRAF– ViZieR & Aladdin.– 2dFdr, the 2dF data reduction software.– SIMBAD

GCs – Not so Simple After all!

NGC 1851

Han et al. 2008

More recent work has shown that the CN bimodality extends down to the Main Sequence, suggesting that the bimodal composition has primordial origens.

Cannon et al, 1998

V

B-V

Cannon et al., 1998

CN Bimodality on MS/SGB

47 Tuc

Results: NGC 6752 – CMD• Where did all the CN-Strong stars go??!!

NGC 6752 photometry: Grundahl et al., 1999.

Bellazzini et al. 2001, 122, 2569

Results: M2 -- Monomodal RGB??• RGB seems almost totally CN-weak, in contrast to other GCs

which show bimodal behaviour.• It looks as if the AGB is more CN-weak, so same process has

happened here despite odd RGB?

Metal-Rich GC: 47 TucStrong to Weak

Ratios

RGB = 30:70 (CN-weak)

HB = 40:60 (intermediate)

AGB = 70:30 (CN-strong!!)

47 Tuc photometry: Kaluzny et al., 1998.

Outline of Talk

Part 1:Brief background on globular cluster abundance anomalies.

Part 2: Background to our observing proposal.

Part 3: Some preliminary results.

Part 4: Summary & future work.

Results: M5 – The 'Contrary' GC

B_Mag

CN

Index

• M5 was thought to have a CN-strong dominated AGB, however this was based on 8 stars (Smith & Norris 1993).

• Our data shows it certainly has CN-strong stars on the AGB, but it actually seems to be dominated by CN-weak stars..

More complex than NGC 6752...

M5 photometry: Sandquist & Bolte 2004.

Results: NGC 1851(Red+Blue HB)Strong to Weak

Ratios

RGB = 60:40

AGB = 60:40 to 50:50

=> Either no change or a paucity of CN-strong AGB stars compared to RGB.

M10: [Fe/H] = -1.1, Blue HB

Literature search for CN in GC AGB Stars

(Ivans et al, 2004)

(Suntzeff, 1981)

(Sunzeff, 1981)

(Smith & Norris,1993)

(Mallia 1978 + ?)

(Smith & Norris 1993)

(Briley et al., 1993)

(Lee, 2000)

• Intrigued that the AGB may be showing very strange behaviour in GCs, we conducted a literature search to see if the same had been found in other GCs (Campbell et al., 2006).

• It appears some GC AGBs had been looked at, but none in any detail.

• AGB stars were generally a side issue in the studies, due to their low numbers and the difficulty in identifying them.

• So, the data generally points to CN-weak AGBs, but there is also evidence for CN-strong AGBs...

• Note however that the sample sizes are quite small...

M5 & 47 Tuc– the 'Contrary' GCs

Which Telescope/Instrument?

2dF Field Plate

• Need low/mid resolution only (R ~ 3000, CN bands are huge), but want to look at many stars.

• => Our good old friend the AAT, with its multi-fibre-fed spectrographs – AAOMega (2 degree field, 400 stars at once)

• One strong benefit of this study is that all the data is homogeneous.

• Moreover all stars observed were 2MASS objects.

GC Abundance Summary• Figuring out why things are different in GCs as

compared to the field is a long-standing, difficult problem.

• MS observations have now been made – many of the abundance anomalies are found there too, suggesting a primordial origin (but some anomalies are certainly evolutionary, eg. RGB Extra Mixing).

• So we have abundance anomalies at each stage of evolution – MS, SGB, RGB, HB.

• However, it seems that the AGBs haven't really been looked at in detail – because it is difficult to identify AGB stars & also there are not many of them due to their short lifespans....

Background 2: GC Weirdness; O-Na Anticorrelation• However, the light elements

were soon found to be not so uniform.

• For example, an O-Na anticorrellation exists in many GCs.

• This anticorrelation is readily explained by hot hydrogen burning, where the ON and NeNa chains are operating - the ON reduces O, whilst the NeNa increases Na (T~45 million K)

• Where this nucleosynthesis occurs is still a matter of debate.

[O/Fe]

Cluster

Field

[Na/Fe]

[Na/Fe]

Gratton et al, 2000

O-Na anticorrellation is not observed in halo field stars!!

Part 2: Cyanogen in GC AGBs – Also

Bimodal?

M4: Lisa Elliott 2003

Background 1: Spectroscopic/Chemical Anatomy of GCs: Fe Group

• Most globular clusters (GCs) have a very uniform distribution of Fe group elements - all the stars have the same [Fe/H].

• This indicates that the cluster was well mixed when the stars formed.

Kraft, et al., 1992: M3, M13

Fe I

Fe II

Sc II

V I

• 288: [Fe/H] = -1.2, BHB only• 362: [Fe/H] = -1.2, RHB only• M5: [Fe/H] = -1.3, RHB+BHB• M2: [Fe/H] = -1.6, EBHB• 6752: [Fe/H] = -1.6, EBHB• 1851: [Fe/H] = -1.2, no Blue HB tail.

Smith et al. 2005 (RGBs)

Background 2: The C-N 'Anticorrelation'• In contrast to the Fe group, it has been known since the early

1970s that there is a large spread in Carbon and Nitrogen in many GCs.

• The first negative correlation (anticorrelation) was found 25 years ago -- C is low when N is high.

• The anticorrelation is explicable in terms of the C-N cycle, where C is ‘burnt’ to N14:

✓This is also observed in halo field stars (eg. Gratton et al, 2000)

Excellent Photometry Needed

Eg: Sandquist et al. (2004) have done a nice photometric study of M5.

The set is 'complete' out to 8-10 arcmin.

They tabulate all the stars according to evolutionary status (RGB, HB, AGB..) -- Very handy for us!

I

B-I

AGBs!

They identify 105 AGB stars!

• Excellent photometry is needed to split the RGB and AGB.• It seems photometric studies have recently reached high enough

accuracies to enable a good separation between the giant branches.

Background 3: The C-L Anticorrelation• However it has also been observed that the C abundance

decreases with L on the RGB (and N increases). This is known as the C-L anticorrelation:

Evolutionary Effect => Deep/Extra Mixing must exist! (at least on RGB)

[C/Fe]

MagMag

[N/Fe]

M3 RGBs, Smith 2002

✓This is also observed in halo field stars. (eg. Gratton et al, 2000)

Photometry Search cont... Further ADS photometry foraging uncovered

decent samples for other clusters:

47 Tuc: ~40 stars (best previous study = 14)

M3: ~70 stars (best previous study = 8)

So, since it seems possible to get a significant sample of ABGs, it is worth (trying) to do!

Observing Project - Origin of the Idea:

• While reading up on GC abundances we came across an interesting note in an old paper by Norris et al. (1981, on NGC 6752):

M5 – all program stars

Observing 101: CN Bands• CN is a molecule (12C14N or 13C14N I think.

C2N2 for chemists) which forms when there is sufficient Nitrogen in the atmosphere (and temp is right I guess).

• CN absorbs radiation over wide spectral bands (ie. covering many wavelengths, I huess this is due to the complexity of the nuclear structure).

• Since the CN Bands are so large, we don't need a high resolution spectrograph.

CN Bands Cont...

Norris et al 1981; NGC6752

CN CH

Observed:

Synthesised:

Summary of the Proposal• Get low-resolution spectra for statistically significant

sample of AGB stars in 3 GCs.• Observe some HB stars also, as this may let us

know when/if the stars decide to go up the AGB.• RGBs will be the control stars as they are very

well studied already - and have similar temps (etc) to AGBs.

• Try for Al – might have high enough resolution (?)• We will be able to get CH (which is a proxy for C)

but we can't get NH (for N) because the range of the spectrograph doesn't go that blue.

• Proposal submitted (was well rated) – but was for service time so still haven't got any results yet .............. :(

Comparing with 2MASS