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THE CHEMICAL ENRICHMENT OF w CENTAURI

THE CHEMICAL ENRICHMENT OF w CENTAURI

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THE CHEMICAL ENRICHMENT OF w CENTAURI

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

  2. 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

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

  4. 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) Abundance inhomogeneity of w Cen (1960-1995) [Ca/H] = log(N(Ca)/N(H))* -log(N(Ca)/N(H))o

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

  6. 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) Abundance inhomogeneity of w Cen (1960-1995)

  7. Norris, Freeman & Mighell1996, ApJ, 462, 241 Low resolution (R~4000) [Ca/H] from Ca II H&K and Ca II infrared triplet ROA 253 Ca II H&K Ca II infrared triplet ROA 253

  8. Norris, Freeman & Mighell1996, ApJ, 462, 241 METALLICITY DISTRIBUTION FUNCTION Ca II H&K AAT Ca II triplet 74-inch [Ca/H] abundance histograms Ca II triplet 74-inch [Ca/H] = log(N(Ca)/N(H))* -log(N(Ca)/N(H))o

  9. Norris, Freeman & Mighell1996, ApJ, 462, 241 Two populations Simple model, closed box approximation: First population: [Ca/H]0 = -1.59 <[Ca/H]> = -1.29 Second population: [Ca/H]0 = -1.09 <[Ca/H]> = -0.83 metal-rich/metal-poor ~ 0.20

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

  11. High resolution spectra of 35 red giants (AAT UCLES, R~35,000; ~1993)

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

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

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

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

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

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

  18. Norris, Freeman, Mayor & Seitzer 1997, ApJ, 487, L187 Kinematics vs. abundance Metal-poor sample kinematically hotter and rotating more rapidly. 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

  19. w Cen Lee et al 1999, Nature, 402, 55

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

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

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

  23. To w Cen’s main sequence withAAT Two Degree Field Spectrographs

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

  25. Stanford thesis Stars observed in 2002 box • Cen radial-velocity members in 2002 box

  26. Stanford thesis

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

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

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

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

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

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

  33. Majority, metal-poor population should be bluest! 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 Note: X = hydrogen mass fraction Y = helium mass fraction Z = heavy element mass fraction

  34. Norris 2004, ApJ 612, L25 Revised Yale Isochrones 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 0.15 0.05

  35. Norris 2004, ApJ, 612, L25 Revised Yale Isochrones 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 0.15 0.05

  36. Norris 2004 ApJ, 612, L25 Revised Yale Isochrones 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 0.15 0.05

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

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

  39. BUT … • Canonically, DY/DZ ~3-4, and with an increase from [Fe/H] = -1.7 to -1.2 one expects only DY = 0.003! • Suggests non-canonical evolution. OBSERVATIONALLY … • Determine Y from hot blue horizontal-branch stars? • Use sensitivity of HB luminosity and Teff 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?

  40. Ferraro et al 2004 ApJ, 603, L81 Rey et al 2004 D’Cruz et al 2000 ApJ, 530, 352 - HST UV observations Pop 1st 2nd Alt.2nd [Fe/H] -1.7 -1.2 -1.2 Y 0.23 0.35 0.23 Age(Gyr) 14 12 12 Fraction 0.80 0.15 0.15 Turnoff mass (Msun) 0.82 0.71 0.85 “… over 30% of the HB objects are “extreme” HB or post-HB stars” see also: Lee et al., 2005, ApJ, 621, L57

  41. CANDIDATES FOR PRODUCERS OF HELIUM • Massive stars (~60 Mo) with rotationally driven mass loss (Maeder & Meynet astro-ph/0601425) - also produce copius C, N, and O • 10-14 Mo SNe (Piotto et al 2005, ApJ, 621, 777) • More massive (~6-7 Mo) 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 w 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 below108 years in the neutral gas clouds dominated by collisionless dark matter or with dynamically important radiation or magnetic pressure. ”

  42. SUMMARY • w Cen possesses at least three distinct populations, described to first approximation by: Population First Second Third Fraction 0.80 0.15 0.05 [Fe/H] -1.7 -1.2 -0.6 Y 0.23 0.35 0.38: YY Age (Gyr) 14 12 12: s(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.

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

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

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

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

  47. Stanford thesis work Metallicity Range