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AGB stars in Galactic Globular Clusters – Are They Chemically Distinct to Their Fellow RGB and HB Stars?

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:

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AGB stars in Galactic Globular Clusters – Are They Chemically Distinct to Their Fellow RGB and HB Stars?

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  1. AGB stars in Galactic Globular Clusters – Are They Chemically Distinct to Their Fellow RGB and HB Stars? Simon Campbell 1. UniversitatPolitecnica de Catalunya, Barcelona, Spain 2. CSPA, Monash University, Melbourne, Australia M5: SDSS

  2. 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

  3. 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

  4. Part 1: Background: AGBs in GCs+ CN Bimodality

  5. I B-I AGBs in Globular Clusters M5 (SDSS) M5 (Sandquist & Bolte 2004) 100 AGBs! • High quality photometry is now making the AGB accessible in GCs, we can now get good numbers of AGBs.

  6. Norris et al. (1981) 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. • 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. 'Continuum' Blue CN Index: CN Weak CN Strong

  7. NGC 6752, Norris et al 1981 No. Stars Bimodal CN distribution on RGB CN Index CN-Strong CN-Weak Mag 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. This is not observed in halo field stars! (see eg. Langer et al, 1992) • Note trend with Temp. References: eg. Bell & Dickens 1974, Da Costa & Cottrell 1980, Norris et al. 1981.

  8. NGC 6752, Norris et al 1981 AGBs: all CN-weak CN Index CN-Strong CN-Weak Mag 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?? • Note trend with Temp. References: eg. Bell & Dickens 1974, Da Costa & Cottrell 1980, Norris et al. 1981.

  9. 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...

  10. Part 2:Observing CN in GC AGBs+ Preliminary Results

  11. v v-y Excellent Photometry Needed NGC 6752 • 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 AGB RGB (yellow = raw data)

  12. 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).

  13. Results: NGC 6752 • 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 *ALL AGBs CN-weak!*

  14. Results: GC Pair Comparison • NGC 288 and 362 have similar metallicities ([Fe/H] ~ -1.2) but different HB morphologies  compare CN behaviour. Red HB Ext-Blue HB

  15. 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.

  16. 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.

  17. Summary/Discussion • Our preliminary results clearly show there issomething 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.

  18. Future Work • Finish analysingthe 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 19

  20. 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

  21. 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

  22. GCs – Not so Simple After all! NGC 1851 Han et al. 2008

  23. Cannon et al., 1998 V B-V 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. 47 Tuc Cannon et al, 1998

  24. Results: NGC 6752 – CMD • Where did all the CN-Strong stars go??!! NGC 6752 photometry: Grundahl et al., 1999.

  25. Bellazzini et al. 2001, 122, 2569

  26. 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?

  27. 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.

  28. 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.

  29. CN Index B_Mag 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.. More complex than NGC 6752... M5 photometry: Sandquist & Bolte 2004.

  30. 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.

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

  32. (Ivans et al, 2004) (Smith & Norris,1993) (Mallia 1978 + ?) M5 & 47 Tuc– the 'Contrary' GCs (Sunzeff, 1981) (Suntzeff, 1981) (Smith & Norris 1993) (Briley et al., 1993) (Lee, 2000) Literature search for CN in GC AGB Stars • Intrigued that the AGB may be showing very strange behaviourin 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...

  33. 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

  34. 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....

  35. [Na/Fe] Field [Na/Fe] Cluster [O/Fe] Gratton et al, 2000 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-Na anticorrellation is not observed in halo field stars!!

  36. Part 2:Cyanogen in GC AGBs – Also Bimodal?

  37. Fe I Fe II Sc II V I Kraft, et al., 1992: M3, M13 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.

  38. 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.

  39. The anticorrelation is explicable in terms of the C-N cycle, where C is ‘burnt’ to N14: 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. ✓This is also observed in halo field stars (eg. Gratton et al, 2000)

  40. AGBs! I • They identify 105 AGB stars! B-I 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. • 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!

  41. [N/Fe] [C/Fe] M3 RGBs, Smith 2002 Mag Mag 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) ✓This is also observed in halo field stars. (eg. Gratton et al, 2000)

  42. 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!

  43. 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):

  44. M5 – all program stars

  45. 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.

  46. CN Bands Cont... CH CN Observed: Synthesised: Norris et al 1981; NGC6752

  47. 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 .............. :(

  48. Comparing with 2MASS

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