Studying stars in M31 GCs using NIRI and GNIRS

Transcription

1 Studying stars in M31 GCs using NIRI and GNIRS Ricardo Schiavon Gemini Observatory GSM 2012 San Francisco July 19, 2012

2 Collaborators Andy Stephens (Gemini) Nelson Caldwell (SAO) Matthew Shetrone (HET) Katia Cunha (NOAO) Verne Smith (NOAO) Carlos Allende Prieto (IAC Spain)

3 Goals Determination of CNO abundances of old M31stars for the first time Detection of multiple stellar populations in an M31 GC for the first time Pioneering ELT science, pushing AOassisted NIR spectroscopy to the limit

4 Chemical Compositions Burstein et al. (1984) Edvardsson et al. 1993

5 Clusters in Andromeda Spectroscopic sample Young Old Focus on Old GCs FOV of Hectospec

6 MW vs M31 MDFs Metallicity Distribution Functions of M31 and the Galaxy are substantially different Different histories of star formation (at least as far as GCs are concerned) Caldwell et al. (2011)

7 MW vs M31 in index-index space Indices sensitive to age, horizontal-branch morphology and abundances of carbon and magnesium M31 and MW GCs occupy same locus in index-index space. Probably same age and chemical compositions At face value, this is inconsistent with Conclusion 1!

8 M31 Abundance Ratios vs [Fe/H] Abundance patterns of M31 GCs suggest enrichment by SN II Schiavon et al. (2012, in prep)

9 Motivation: Abundance ratios Abundance analysis based on high resolution integrated spectroscopy yields promising results. But samples are small. M31 GCs are the green dots. Colucci et al. (2009)

10 M31 MW Abundances vs mass We find evidence for the presence of multiple populations in M31 clusters No correlation between Fe, Mg, or Ca, suggests enrichment by ejecta of low mass stars Schiavon et al. (2012, in prep)

11 C and N in GalacCluster Stars Spreads in C and N abundances in MW GC stars have been known for many decades now They are currently understood as being due to chemical evolution of the GC Ventura & D Antona 2008

12 Sample Spectra Hδ CN Ca CH Fe Fe C 2 Hβ Mg Fe Metal-rich clusters

13 Integrated Spectroscopy M31 cluster B008 (H band) 1 FWHM ~ 0.5

14 Imaging with NIRI M31 cluster B008 (H band, Gemini/NIRI/ALTAIR-NGS) GN-2009B-Q-109, GN-2010A-Q-39 1 FWHM ~ 0.1

15 Abundances of M31 GC Stars Use AO-fed, medium resolution, NIR spectroscopy of resolved stars in M31 GCs to determine their C, N, and O abundances O from OH at H band, C from CO at K band, N from CN at J band Comparison between these abundances and those obtained for massive young stars provide a direct assessment of the chemical evolution in M31 over the Hubble time Detecting CN or CO band variations in stars of same evolutionary state would provide evidence for chemical evolution in M31 GCs

16 Target Selection Principles Intermediate metallicity: -1.0 [Fe/H] -0.4 Massive, not too compact Minimize field contamination --- avoid disk and bulge Near a good star for AO correction (NGS) Availability of HST imaging a plus ~6 clusters, from which we selected: => B008: [Fe/H] ~ -0.7, M V =16.52, M Sun

17 Imaging with NIRI M31 cluster B008 (H band, Gemini/NIRI/ALTAIR-NGS) GN-2009B-Q-109, GN-2010A-Q-39 1 FWHM ~ 0.1

18 Color-Magnitude Diagram Well defined RGB Thickness dominated by photometric error and crowding Allows for safe target selection Quality of correction in J band too poor HST imaging available and will be used.

19 GNIRS Configuration Long Camera, cross dispersed Pixel scale slit 10 l/mm grating R ~ 1,700 Align slit to maximize number of objects

20 Target Selection GN-2011B-Q-74, GN-2012B-Q-88 Projected slit width of 0.3 and 7 slit length Include cluster center for an easy RV check for membership

21 Spectra CO bands detected in spectrum of individual star Membership confirmed S/N ~ 15/pixel for H~18 in K band 2.5 hr integration --- only 1/10 of the total requested time Should be able to meet S/N requirement with 2012B time allocation weather permitting Spectra of MW GC stars from NIRI GN-2009A-Q-41

22 Proof of Concept: M71 Spectrum synthesis of CN-strong star in M71

23 Conclusions Preliminary results very promising! Should be able to obtain C and O abundances from H and K band spectra Poor quality of J band AO correction makes N abundances challenging Background subtraction and telluric correction need work

24 Proof of Concept: M71 NIRI spectra of CN-strong and CN-weak stars in M71

25 Fundamentals of SP synthesis Age Effects on Spectra Balmer lines are stronger in the spectra of younger stellar populations

26 Fundamentals of SP synthesis Metallicity Effects on Spectra Metal lines are stronger in the spectra of metal-rich stellar populations

27 Models The Models Integrated spectra are interpreted using stellar population synthesis models. Models predict colors, spectra, and line indices of single stellar populations for various ages, IMFs, and the abundances of Fe, Mg, C, N, and Ca For an extensive account of the method, see Schiavon (2007, ApJS, 171, 146) Schiavon (2007) models widely used by the community

28 Line Indices: Mg & <Fe> Both MW and M31 GCs have [Mg/Fe] > 0 This tells us that both GC systems were formed in less than ~ 1 Gyr

29 Line Indices: C 2 & <Fe> C is sensitive to total metalicity and to the abundance of C [C/Fe] < 0 for old MW and M31 GCs This is probably indicative of presence of CNOenriched material

30 Line Indices: CN & <Fe> CN 1 is sensitive to the abundances of C and N, as well as to the total metallicity It seems like there is a correlation between [N/Fe] and [Fe/H] in both families of GCs

31 Abundance pattern of M31 clusters Sample consists of ~ 180 M31 clusters with ages older than ~ 4 Gyr and -1.3 < [Fe/H] < +0.2 [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

32 Abundance pattern of M31 clusters Abundance pattern of M31 GCs different from Sun [Ca / Fe] [C / Fe] [Fe/H] [Fe/H] [Mg,Ca/Fe] > 0 suggests that the bulk of the GCs were formed in less than ~ 1 Gyr That s because SNe that contribute Fe only explode after ~ 1 Gyr, whereas those that contribute Mg explode very quickly (10s of Myr) [Mg / Fe] [N / Fe]

33 Ages and Abundances Impact of blue HB stars on abundances is minor, and virtually zero on abundance ratios Age (Gyr) [C/Fe] [Mg/Fe] [N/Fe]

34 Ages and Abundances Young metal-rich clusters have low [Mg/Fe] and [N/Fe], and disk kinematics. Consistent with longer star formation history Age (Gyr) [C/Fe] [Mg/Fe] [N/Fe]

35 MW vs M31 Mostly similar, but note small differences in [C/Fe] and [Ca/Fe] Zero points are uncertain, though CONCLUSION 2: M31 and MW clusters have similar abundance patterns (to first order) [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

36 Abundances vs cluster mass

37 Abundances vs cluster luminosity

38 Abundances vs cluster luminosity

39 CONCLUSION 3: We find evidence for the presence of multiple populations in M31 clusters

40 C and N in Cluster stars Models of selfenrichment by intermediate-mass AGB stars fit abundance ratios in metal rich and poor GCs (but see Conroy 2011) Hard to disentangle original from enriched composition on the basis of integrated light Ventura & D Antona 2008

41 Conclusions Different MDFs suggest different SFHs for MW and M31 GC systems, yet abundance ratios are similar, suggesting similar SFHs. Mg and Ca tell us that M31 GCs were formed in less than 1 Gyr. C and N abundances suggest presence of selfenrichment in M31 GCs.

42 Fundamentals of SP synthesis Metallicity Effects At fixed age, more metal-rich single stellar populations are redder

43 Fundamentals of SP synthesis The Age-Metallicity Degeneracy Age and Metallicity have similar effects on the colors of old (> 1 Gyr) stellar populations, making it almost impossible to disentangle the two effects on the basis of broad-band colors alone Spectroscopy is needed to estimate both mean ages and metal abundances

44 Models Variable Abundance Ratios An example: Carbon Solar Scaled Models

45 Models Variable Abundance Ratios An example: Carbon [X/Fe] = 0 [C/Fe] = +0.3

46 SDSS Abundance Pattern [Fe/H] [Mg/Fe] [C/Fe] [N/Fe] [Ca/Fe] Mean Age (Gyr) M r M r

47 Diagnostic Plots Estimating cluster ages and metallicities from comparison of line index measurements with model predictions for single stellar populations Age [Fe/H]

48 Models EZ_Ages EZ_Ages matches ages and abundances of known Galactic globular clusters to within dex (Graves & Schiavon 2008)

49 Line Indices Hβ is mostly sensitive to age and the morphology of the horizontal branch, whereas <Fe> is mostly sensitive to [Fe/H]

50 Line Indices Hγ is also very sensitive to age and horizontal branch morphology

51 Line Indices Focus on OLD clusters Poor [Fe/H]= Gyr Young Old 14 Gyr Rich [Fe/H]=+0.2

52 Line Indices No age differences detected between MW and old M31 clusters

53 Line Indices No age differences detected between MW and M31 old clusters Young Old

54 Line Indices: Mg & <Fe> Mg b is sensitive to Mg b is sensitive to total metallicity and to the abundance of Mg

55 Ages and Metallicities Results from the application of EZ_Ages (Graves & Schiavon 2008) to index measurements of 179 M31 clusters Note population of intermediate-age metal-poor clusters

56 Issues with metal-poor clusters Ages can t be trusted for clusters with [FeH] < -1 due to the presence of blue horizontal branch stars

57 Issues with metal-poor clusters Theoretical isochrones employed do not include blue HB stars

58 Issues with metal-poor clusters Theoretical isochrones employed do not include blue HB stars

59 Ages and Metallicities Systematic effect due to blue HB stars creates a population of artificially younger metal-poor clusters

60 Differential effects Blue HB stars contribute more light at lower λ, thus affecting Hδ and Hγ more strongly than Hβ

61 Ages and Metallicities Due to HB stars, ages inferred from Hδ are younger than those based on Hβ for clusters with [Fe/H] < -1

62 Ages and Metallicities Due to HB stars, ages inferred from Hδ are younger than those based on Hβ for clusters with [Fe/H] < -1

63 Ages and Metallicities Old GCs have an age of 11.8 ± 2 Gyr But beware of zeropoint uncertainties

64 Issues with metal-poor clusters Ages can t be trusted for [FeH] < -1 due to the presence of blue horizontal branch stars

65 Issues with metal-poor clusters Ages can t be trusted for [FeH] < -1 due to the presence of blue horizontal branch stars

66 Ages and Abundances Results from the application of EZ_Ages (Graves & Schiavon 2008) to the index measurements

67 The Carbon-Nitrogen Conundrum What do we make of the [N/Fe] correlation? It is indicative of secondary enrichment [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

68 C and N in MW Cluster stars Stars in Galactic GCs have a range of C and N abundances. Also main-sequence stars, so not due to internal mixing Poorly understood, but self-enrichment is most likely scenario Carretta et al. 2005

69 Self-enrichment in M31 clusters? Assumption: more massive clusters should self-enrich more efficiently. [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

70 4 < log (M/Mo) < 5 Low mass clusters Ca, Mg, and C: same behavior as whole sample (very scattered) NO CORRELATION in Nitrogen! [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

71 Intermediate-mass clusters 5 < log (M/Mo) < 6 Ca, Mg, and C: same behavior as whole sample NOISY CORRELATION in Nitrogen! [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

72 6 < log (M/Mo) Massive clusters Ca, Mg, and C: same behavior as whole sample (more scattered) TIGHT CORRELATION in Nitrogen! [Ca / Fe] [C / Fe] [Mg / Fe] [N / Fe]

73 Nitrogen in MW Field Figure stolen from a talk by Nikolas Prantzos

74 Figure stolen from a talk by Nikolas Prantzos Nitrogen in MW Field [N/Fe] [Fe/H] In the field, no correlation is found between [N/Fe] and [Fe/H], indicating a primary origin

75 Observations MMT/Hectospec 1 FOV 300 fibers, 1.5 arcsec diameter 270 l/mm grating, resolution ~5Å over > 5500 Å, blue MMT Hectospec focal surface

76 Motivation: MDFs Constraining the history of star formation and chemical enrichment in the nearest MW giant neighbor Brodie & Huchra (1991) M31 Abundances and ages from integrated spectra of globular clusters Interesting results from early efforts (Brodie & Huchra 1990,1991), Puzia et al. (2005), many others suggesting that: MW - M31 and MW clusters have bimodal metallicity distributions [Fe/H]

77 Spectroscopy of ~1,200 objects with MMT/Hectospec, plus many background (M31 field) spectra 25 pointings Observations Resolution ~5Å, Å Median S/N ~ 75/Å at 5200Å 450 clusters, plus stars, possible stars, background galaxies New catalog and details in Caldwell et al Study of young clusters in Caldwell et al. 2009, metallicities and ages of old clusters in Caldwell et al. 2011, kinematics of metal-rich clusters in Morrison et al , line strength comparisons with MW GCs by Schiavon et al. 2012

78 Imaging with NIRI M31 cluster B008 (H band, Gemini/NIRI/ALTAIR) 1 CNO abundances of M31 cluster stars with ALTAIR/GNIRS (on-going multisemester program at Gemini-N (Schiavon, Stephens, Caldwell, Shetrone, Allende-Prieto, Cunha, VV Smith)

79 MW vs M31 in index-index space Indices sensitive to age, horizontal-branch morphology and abundances of carbon and nitrogen

80 MW vs M31 in index-index space Indices sensitive to age, horizontal-branch morphology and abundances of calcium and carbon

81 MW vs M31 in index-index space Indices sensitive to age, horizontal-branch morphology and abundances of carbon and magnesium