AGB stars in Galactic Globular Clusters –
Are They Chemically Distinct to Their Fellow
RGB and HB Stars?
Simon Campbell
1. Universitat Politecnica de Catalunya, Barcelona, Spain
2. CSPA, Monash University, Melbourne, Australia
M5: SDSS
Collaborators
RSAA, Mt Stromlo, Australia:
David Yong
Elizabeth Wylie de Boer
Monash University, Australia:
John Lattanzio
Richard Stanciffe
George Angelou
University of Aarhus, Denmark:
Frank Grundahl
University of Texas, USA:
Chris Sneden
Disclaimer
Warning: Theorist talking about observing stuff...! :)
• 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
Part 1:
Background: AGBs in GCs
+ CN Bimodality
AGBs in Globular Clusters
M5 (SDSS)
M5 (Sandquist & Bolte 2004)
100 AGBs!
I
• High quality
photometry is now
making the AGB
accessible in GCs,
we can now get
good numbers of
AGBs.
B-I
Quantifying Cyanogen Abundance:
The S(3839) CN Index
• 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.
'Continuum'
Blue CN Index:
CN Weak
Norris et al. (1981)
CN Strong
• 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 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
No. Stars
6752,
This NGC
is not
observed in halo field stars!
Norris et al 1981
Langer et al,on
1992)
Bimodal (see
CN eg.
distribution
RGB
• Note trend
with Temp.
Mag
References: eg. Bell & Dickens 1974, Da Costa & Cottrell 1980, Norris et al. 1981.
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??
NGC 6752,
Norris et al 1981
AGBs: all CN-weak
CN Index
• Note trend
with Temp.
Mag
References: eg. Bell & Dickens 1974, Da Costa & Cottrell 1980, Norris et al. 1981.
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
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)
BGB=RGB+AGB
NGC 6752
v
AGB
RGB
(yellow = raw data)
v-y
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
• The cluster that Norris et al 1981 investigated.
• RGB nicely bimodal, as expected.
• And on the AGB....
Strong to Weak
Ratios
*ALL AGBs CN-weak!*
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.
Ext-Blue HB
Red 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!
Thanks to the producers
& maintainers of these
tools:
The End :)
-- 2MASS
-- IRAF
-- ViZieR & Aladdin
-- SIMBAD
-- 2dFdr, the 2dF data
reduction software
19
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
CN Bimodality on
MS/SGB
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
47 Tuc
B-VCannon et al, 1998
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 Tuc
Strong 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
• 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..
CN Index
More complex than NGC 6752...
M5 photometry: Sandquist & Bolte 2004.B_Mag
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
• 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.
(Ivans et al, 2004)
(Smith & Norris,1993)
(Mallia 1978 + ?)
(Sunzeff, 1981)
M5 & 47 Tuc– the 'Contrary' GCs
(Suntzeff, 1981)
(Smith & Norris 1993)
(Briley et al., 1993)
(Lee, 2000)
• So, the data
generally points to
CN-weak AGBs,
but there is also
evidence for CNstrong AGBs...
• Note however that
the sample sizes
are quite small...
Which Telescope/Instrument?
• 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.
2dF Field Plate
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
Background 2: GC Weirdness; O-Na Anticorrelation
• However, the light elements
were soon found to be not so
uniform.
• Where this nucleosynthesis
occurs is still a matter of
debate.
Field
[Na/Fe]
• 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)
[Na/Fe]
• For example, an O-Na
anticorrellation exists in many
GCs.
O-Na anticorrellation is not
observed in halo field stars!!
Cluster
[O/Fe]
Gratton et al,
2000
Part 2:
Cyanogen in GC AGBs – Also
Bimodal?
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.
Fe I
Fe II
Sc II
VI
M4: Lisa Elliott 2003
Kraft, et al., 1992: M3, M13
•
•
•
•
•
•
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.
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:
Smith et al. 2005 (RGBs)
✓This is also observed in halo field stars (eg. Gratton et al, 2000)
Excellent Photometry Needed
• 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.

AGBs
!
I


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!

B-I
They identify
105 AGB stars!
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]
[N/Fe]
✓This is also observed in halo field stars. (eg. Gratton et al, 2000)
M3 RGBs, Smith
2002
Mag
Mag
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...
CN
CH
Observed:
Synthesised:
Norris et al 1981; NGC6752
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