2. THE CHEMICAL ENRICHMENT OF  CENTAURI JOHN E. NORRIS RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS MOUNT STROMLO & SIDING SPRING OBSERVATORIES AUSTRALIAN NATIONAL UNIVERSITY

4. PLAN OF ATTACK Historical review (pre ~1995) Chemical abundances on the Red Giant Branch –Metallicity Distribution Function & relative abundances –constraints on enriching stars and age spread Kinematics vs. abundance –Constraints on formation mechanisms Three populations Main sequence studies –Constraints on the population parameters Collaborators: M.S.Bessell, K.Bekki, R.D.Cannon, G.S.Da Costa, K.C.Freeman, M.Mayor, K.Mighell, G.Paltoglou, P.Seitzer, L.Stanford

5. Abundance inhomogeneity of  Cen (1960-1995) Discovery of CH star –Harding (1962) Wide giant branch –Woolley et al (1966, photographic), Cannon & Stobie (1972, photoelectric)  Cen 47 Tuc Cannon & Stobie 1972, MNRAS, 162, 207 Lee 1977, A&AS, 27, 381

6. Abundance inhomogeneity of  Cen (1960-1995) Discovery of CH star –Harding (1962) Wide giant branch –Woolley et al (1966, photographic), Cannon & Stobie (1972, photoelectric) [Ca/H] spread among RR Lyrae stars –Freeman & Rodgers (1975, low res) Large CN variations among red giants –Norris & Bessell (1975, low res), Dickens & Bell (1976, low res) Large CO spread among red giants –Persson et al (1980, IR photometry) [Ca/H] = log(N(Ca)/N(H)) * -log(N(Ca)/N(H)) o

7. Persson et al 1980, ApJ, 235, 452 C and/or O enhance- ment unique to  Cen

8. Abundance inhomogeneity of  Cen (1960-1995) Discovery of CH star –Harding (1962) Wide giant branch –Woolley et al (1966, photographic), Cannon & Stobie (1972, photoelectric) [Ca/H] spread among RR Lyrae stars –Freeman & Rodgers (1975, low res) Large CN variations among red giants –Norris & Bessell (1975, low res), Dickens & Bell (1976, low res) Large CO spread among red giants –Persson et al (1980, IR photometry) Heavy element abundance spreads –High resolution spectroscopy –Cohen (1981; 5 stars), Gratton (1982; 8), Francois et al (1988; 6), Paltoglou & Norrris (1989; 15), Brown & Wallerstein (1993; 6), Norris & Da Costa (1995; 35), Smith et al (1995; 7)

9. Norris, Freeman & Mighell 1996, ApJ, 462, 241 Ca II H&K Ca II infrared triplet ROA 253 Low resolution (R~4000) [Ca/H] from Ca II H&K and Ca II infrared triplet ROA 253

10. Norris, Freeman & Mighell 1996, ApJ, 462, 241 Ca II H&K AAT Ca II triplet 74-inch Ca II triplet 74-inch [Ca/H] abundance histograms METALLICITY DISTRIBUTION FUNCTION [Ca/H] = log(N(Ca)/N(H)) * -log(N(Ca)/N(H)) o

11. Norris, Freeman & Mighell 1996, ApJ, 462, 241 Two populations First population: [Ca/H] 0 = -1.59 = -1.29 Second population: [Ca/H] 0 = -1.09 = -0.83 Simple model, closed box approximation: metal-rich/metal-poor ~ 0.20

12. High resolution spectrum obtained with AAT UCL Echelle Spectrograph (UCLES)

13. High resolution spectra of 35 red giants (AAT UCLES, R~35,000; 

14. Norris & Da Costa 1995, ApJ, 447, 680 [alpha/Fe] vs. [Fe/H] (NB: heavily biased sample) Enrichment by SNe II  Cen Other clusters

15. Norris & Da Costa 1995, ApJ, 447, 680 [neutron capture/Fe] vs. [Fe/H] Enrichment by (intermediate- mass) AGB stars  Cen Other clusters

16. Norris, Freeman & Mighell, 1996 ApJ, 462, 241 Heavily biased sample (AAT UCLES high-res) Unbiased sample (AAT, 74-inch low-res) Normal globular clusters No counterpart elsewhere in Galaxy. Suggests causal link between populations

17. [ Fe /H] -1.5-2.0 0.0 1.0 Smith et al 2000, AJ, 119, 1239 5Mo 3Mo 1.5Mo 5Mo 3Mo 1.5Mo 5Mo 3Mo 1.5Mo [Rb/Zr] Star formation occurred over 2-3 Gyr

18. Norris, Freeman & Mighell 1996, ApJ, 462, 241 [Ca/Fe] vs. radius Abundance decreases with radial distance

19. Norris, Freeman, Mayor & Seitzer 1997, ApJ, 487, L187 Rotation vs. abundance Metal-poor sample:  V = 10.7 +/- 1.8 km/s Metal-rich sample:  V = 3.0 +/- 2.4 km/s Metal-poor population rotating more rapidly

20. Metal-poor sample kinematically hotter and rotating more rapidly. Kinematics vs. abundance Norris, Freeman, Mayor & Seitzer 1997, ApJ, 487, L187 O Not ELS collapse O Kinematically consistent with binary cluster evolution (e.g. Makino et al 1991 Ap&SS, 185, 63); but not clear this works chemically

21.  Cen Lee et al 1999, Nature, 402, 55

22. Ferraro et al 2004, ApJ, 603, L81 Pancino et al 2000, ApJ, 584, L83 ‘Third’ population

23. Pancino et al 2002, ApJ, 568, L101 Enrichment by SNe Ia [Ca/Fe] [Fe/H]

24. Sollima et al. 2005, MNRAS, 357, 265

25. To  Cen’s main sequence with AAT Two Degree Field Spectrographs

26. … working with Laura Stanford, Gary Da Costa & Russell Cannon (Stanford et al 2006, ApJ, 647,1075) 1998/99 2002

27. Stars observed in 2002 box  Cen radial-velocity members in 2002 box Stanford thesis

29. Metallicity Distribution Function Stanford et al (2006, ApJ, 647, 1075)

30. Stanford et al. (2006, ApJ, 647, 1075)

31. From - Ages of individual star in the CMD determined from YY isochrones, taking into account correlated age-metallicity errors Comparisons of Monte-Carlo CMD simulations with that of the cluster There exists an age-metallicity relation, with the more metal-rich populations being younger by 2-4 Gyr than the metal poor one

32. Stanford et al. 2006, ApJ, 647, 1075 Age ranges from the literature

33. Stanford et al. (2006, in prep) [Sr/Fe] = +1.6 [Ba/Fe] < +0.8:

34. Bedin et al. 2004, ApJ, 605, L125 (astro-ph/0403112) (also Anderson 1997, 2000, 2003 Thesis U Berkeley & ASP Proceedings) Anderson’s double main sequence HST data

35. Bedin et al. suggest: Observations and/or modelling wrong Bluer main sequence has [Fe/H] < -2.0 Bluer main sequence has higher helium (Y > 0.3) Two clusters superimposed, separated by 1-2 kpc along line of sight Majority, metal-poor population should be bluest! Note: X = hydrogen mass fraction Y = helium mass fraction Z = heavy element mass fraction

36. Pop 1st 2nd 3rd [Fe/H] -1.7 -1.2-0.6 Y 0.23 0.23 0.23 Age(Gyr) 16 16 16 Fraction 0.80 0.150.05 Revised Yale Isochrones Norris 2004, ApJ 612, L25

37. Pop 1st 2nd 3rd [Fe/H] -1.7 -1.2-0.6 Y 0.23 0.23 0.23 Age(Gyr) 16 14 12 Fraction 0.80 0.150.05 Revised Yale Isochrones Norris 2004, ApJ, 612, L25

38. Pop 1st 2nd 3rd [Fe/H] -1.7 -1.2-0.6 Y 0.23 0.35 0.38 Age(Gyr) 16 15 14 Fraction 0.80 0.150.05 Revised Yale Isochrones Norris 2004 ApJ, 612, L25

39. Bedin et al. 2004, ApJ, 605, L125 (astro-ph/0403112) (also Anderson 1997, 2000, 2003 Thesis U Berkeley & ASP Proceedings) Anderson’s double main sequence HST data

40. Piotto et al. 2005, ApJ, 621, 777 The blue main sequence is more metal-rich by 0.3 dex! VLT Giraffe

41. BUT … Canonically,  Y/  Z ~3-4, and with an increase from [Fe/H] = -1.7 to -1.2 one expects only  Y = 0.003! Suggests non-canonical evolution. OBSERVATIONALLY … Determine Y from hot blue horizontal-branch stars? Use sensitivity of HB luminosity and T eff to helium? Zero-Age HB RR Lyraes of 2nd pop should be brighter by 0.2- 0.3mag. In contrast, the observed metal-richer RR Lyraes are fainter by 0.2-0.3mag! (see also Sollima et al. 2006, ApJ, 640, L43) But … are the variables representative of the populations?

42. Ferraro et al 2004 ApJ, 603, L81 Pop 1st2nd Alt.2nd [Fe/H] -1.7-1.2-1.2 Y 0.230.35 0.23 Age(Gyr) 14 12 12 Fraction0.800.150.15 Turnoff mass (M sun )0.820.710.85 Rey et al 2004 D’Cruz et al 2000 ApJ, 530, 352 - HST UV observations “… over 30% of the HB objects are “extreme” HB or post-HB stars” see also: Lee et al., 2005, ApJ, 621, L57

43. CANDIDATES FOR PRODUCERS OF HELIUM Massive stars (~60 M o ) with rotationally driven mass loss (Maeder & Meynet astro-ph/0601425) - also produce copius C, N, and O 10-14 M o SNe (Piotto et al 2005, ApJ, 621, 777) More massive (~6-7 M o ) AGB stars Problems with self enrichment by above candidates within a closed system producing so much helium. (Pre-Maeder & Meynet) Bekki & Norris (2006, ApJ, 637, L109) suggested second population formed from gas “ejected from field stellar populations that surrounded  Cen when it was the nucleus of an ancient dwarf galaxy” Helium diffusion in protocluster phase (Chuzhoy astro-ph/0602593): “Element diffusion can produce large fluctuations in the initial helium abundance of the star-forming clouds. Diffusion time-scale … can fall below10 8 years in the neutral gas clouds dominated by collisionless dark matter or with dynamically important radiation or magnetic pressure. ”

44. SUMMARY  Cen possesses at least three distinct populations, described to first approximation by: Population First Second Third Fraction 0.80 0.150.05 [Fe/H] -1.7 -1.2-0.6 Y 0.230.35 0.38: YY Age (Gyr) 14 12 12:  (Vr) (km/s) 13 8 13 Rotation (km/s) 11 3 unknown The origin of the helium in the second population is currently not understood. System not formed in an ELS scenario, but more likely as a dwarf galaxy having multiple star-formation episodes well away from the forming Galaxy, and later being captured by it.

46. 98/99 2002 Stanford thesis ( 2006, ApJ, 647, 1075)

47. Age-Metallicity Relation Stanford et al (2006, ApJ, 647, 1075)

48. Sollima et al. 2005, ApJ, 634, 332

49. Smith, Cunha & Lambert 1995 AJ, 110, 2827 Mixing line [Fe/H] [Ba/Fe]

50. Metallicity Range Stanford thesis work

51. Age-Metallicity Relation Stanford et al (2006, ApJ, 647, 1075)

53. Norris & Da Costa 1995 ApJ, 447, 680 [iron peak/Fe] vs. [Fe/H]  Cen Other clusters

54. Stanford thesis work Observations Simulations of populations: [Fe/H] Fraction First -1.7 0.80 Second-1.2 0.15 Third-0.6 0.05 0 Gyr 6 Gyr 4 Gyr 2 Gyr Age spread

55. Stanford thesis work [Fe/H] Age(Gyr) 15 10 5 -2 Turnoff stars

56. THE CHEMICAL ENRICHMENT OF  CENTAURI JOHN E. NORRIS RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS MOUNT STROMLO & SIDING SPRING OBSERVATORIES AUSTRALIAN NATIONAL UNIVERSITY

57. AAT Two Degree Field - Plate with fibres

58. D’Cruz et al 2000 ApJ, 530, 352 - HST UV observations ‘Normal’ Horizontal Branch EHB “… over 30% of the HB objects are “extreme” HB or post-HB stars” V ~ 16