AGB stars in Galactic Globular Clusters – - PowerPoint Presentation

Slide1

AGB stars in Galactic Globular Clusters –

Are They Chemically Distinct to Their Fellow RGB and HB Stars?

M5: SDSS

Simon

Campbell

1.

Universitat

Politecnica

de

Catalunya

, Barcelona, Spain

2. CSPA,

Monash

University, Melbourne, AustraliaSlide2

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 Slide3

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...! :) Slide4

Part 1:

Background: AGBs in GCs+ CN BimodalitySlide5

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!Slide6

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:Slide7

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 RGB

This

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.Slide8

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.Slide9

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...Slide10

Part

2: Observing CN in GC AGBs+ Preliminary ResultsSlide11

v-y

v

NGC 6752

AGB

RGB

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)Slide12

Spectra Collected: Number of AGBs

5 nights on AAT Multi-object spectrograph 2dF/

AAOmegaData collected for

241

AGB stars across 9 clusters (plus many RGB & HB stars).Slide13

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 = 0:100Slide14

Results: GC

Pair ComparisonNGC 288 and 362 have similar metallicities ([Fe/H] ~ -1.2) but different HB morphologies  compare CN behaviour

.

Red HB

Ext-Blue HBSlide15

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.Slide16

Results: NGC 362 (Red HB)

Strong to Weak

Ratios

RGB = 60:40

AGB = 40:60 to 60:40



E

ither a CN-weak dominated AGB,

or no change from

RGB (hard to define the bimodal split)

NGC 362 photometry:

Bellazzini

et al., 2001.Slide17

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

.Slide18

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!Slide19

19

The End :)

Thanks to the producers

& maintainers

of these

tools:

-- 2MASS

-- IRAF

--

ViZieR

& Aladdin

-- SIMBAD

-- 2dFdr, the 2dF data reduction softwareSlide20

References for Photometric Studies

NGC 362: Bellazzini et al. 2001NGC 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. 1998Slide21

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.

SIMBADSlide22

GCs – Not so Simple After all!

NGC 1851

Han et al. 2008Slide23

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 TucSlide24

Results: NGC 6752 – CMD

Where did all the CN-Strong stars go??!!

NGC 6752 photometry: Grundahl et al., 1999.Slide25

Bellazzini et al. 2001, 122, 2569 Slide26

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?Slide27

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.Slide28

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.Slide29

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.Slide30

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.Slide31

M10: [Fe/H] = -1.1, Blue HBSlide32

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'

GCsSlide33

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.Slide34

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.... Slide35

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!!Slide36

Part 2:

Cyanogen in GC AGBs – Also Bimodal?Slide37

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 ISlide38
Slide39

288: [Fe/H] = -1.2, BHB only

362: [Fe/H] = -1.2, RHB onlyM5: [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.Slide40

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)Slide41

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.Slide42

Background 3:

The C-L AnticorrelationHowever 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]

Mag

Mag

[N/Fe]

M3 RGBs, Smith 2002

✓

This is also observed in halo field stars.

(eg. Gratton et al, 2000)Slide43

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!Slide44

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):Slide45

M5 – all program starsSlide46

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.Slide47

CN Bands Cont...

Norris et al 1981; NGC6752

CN

CH

Observed:

Synthesised:Slide48

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 .............. :(Slide49
Slide50

Comparing with 2MASS