THE CHEMICAL ENRICHMENT OF CENTAURI JOHN E. NORRIS RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS...
Transcript of THE CHEMICAL ENRICHMENT OF CENTAURI JOHN E. NORRIS RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS...
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THE CHEMICAL ENRICHMENT OF CENTAURI
JOHN E. NORRIS
RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS
MOUNT STROMLO & SIDING SPRING OBSERVATORIES
AUSTRALIAN NATIONAL UNIVERSITY
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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
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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
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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
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Persson et al 1980, ApJ, 235, 452
C and/or O enhance-ment unique to Cen
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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)
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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
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Norris, Freeman & Mighell 1996, ApJ, 462, 241
Ca II H&K AAT
Ca II triplet74-inch
Ca II triplet74-inch
[Ca/H] abundance histograms
METALLICITY DISTRIBUTION FUNCTION
[Ca/H] = log(N(Ca)/N(H))*
-log(N(Ca)/N(H))o
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Norris, Freeman & Mighell 1996, ApJ, 462, 241
Two populations
First population: [Ca/H]0 = -1.59 <[Ca/H]> = -1.29
Second population: [Ca/H]0 = -1.09 <[Ca/H]> = -0.83
Simple model,closed box approximation:
metal-rich/metal-poor ~ 0.20
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High resolution spectrum obtained withAAT UCL Echelle Spectrograph (UCLES)
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High resolution spectra of 35 red giants(AAT UCLES, R~35,000;
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Norris & Da Costa 1995, ApJ, 447, 680
[alpha/Fe] vs. [Fe/H](NB: heavily biased sample)
Enrichment by SNe II
Cen Other clusters
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Norris & Da Costa 1995, ApJ, 447, 680
[neutron capture/Fe] vs. [Fe/H]
Enrichment by (intermediate-mass) AGB stars
Cen Other clusters
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Norris, Freeman & Mighell, 1996 ApJ, 462, 241
Heavily biased sample(AAT UCLES high-res)
Unbiased sample(AAT, 74-inch low-res)
Normal globular clustersNo counterpartelsewhere in Galaxy. Suggestscausal link between populations
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[Fe/H]-1.5-2.0 -1.0
0.0
0.0
0.0
1.0
1.0
1.0
Smith et al 2000, AJ, 119, 1239
5Mo
3Mo1.5Mo
5Mo3Mo1.5Mo
5Mo
3Mo1.5Mo
[Rb/Zr]
[Rb/Zr]
[Rb/Zr]
Star formation occurred over 2-3 Gyr
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Norris, Freeman & Mighell 1996, ApJ, 462, 241
[Ca/Fe] vs. radius
Abundance decreases with radial distance
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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
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Metal-poor sample kinematically hotter and rotating more rapidly.
Kinematics vs. abundance
Norris, Freeman, Mayor & Seitzer 1997, ApJ, 487, L187
O Not ELS collapseO Kinematically consistent with binary cluster evolution (e.g. Makino et al 1991 Ap&SS, 185, 63); but not clear this works chemically
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Cen
Lee et al 1999,Nature, 402, 55
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Ferraro et al 2004, ApJ, 603, L81Pancino et al 2000, ApJ, 584, L83
‘Third’ population
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Pancino et al 2002, ApJ, 568, L101
Enrichment by SNe Ia
[Ca/Fe]
[Fe/H]
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Sollima et al. 2005, MNRAS, 357, 265
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To Cen’s main sequence withAAT Two Degree Field
Spectrographs
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… working with Laura Stanford, Gary Da Costa & Russell Cannon(Stanford et al 2006, ApJ, 647,1075)
1998/992002
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Stars observed in 2002 box Cen radial-velocity
members in 2002 box
Stanford thesis
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Stanford thesis
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Metallicity Distribution Function
Stanford et al (2006, ApJ, 647, 1075)
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Stanford et al. (2006, ApJ, 647, 1075)
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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
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Stanford et al. 2006, ApJ, 647, 1075Age ranges from the literature
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Stanford et al. (2006, in prep)
[Sr/Fe] = +1.6[Ba/Fe] < +0.8:
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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
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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
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Pop 1st 2nd 3rd[Fe/H] -1.7 -1.2 -0.6Y 0.23 0.23 0.23Age(Gyr) 16 16 16Fraction 0.80 0.15 0.05
Revised Yale Isochrones Norris 2004, ApJ 612, L25
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Pop 1st 2nd 3rd[Fe/H] -1.7 -1.2 -0.6Y 0.23 0.23 0.23Age(Gyr) 16 14 12Fraction 0.80 0.15 0.05
Revised Yale Isochrones Norris 2004, ApJ, 612, L25
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Pop 1st 2nd 3rd[Fe/H] -1.7 -1.2 -0.6Y 0.23 0.35 0.38Age(Gyr) 16 15 14Fraction 0.80 0.15 0.05
Revised Yale Isochrones Norris 2004 ApJ, 612, L25
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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
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Piotto et al. 2005, ApJ, 621, 777
The blue main sequence is more metal-rich by 0.3 dex!
VLT Giraffe
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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 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?
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Ferraro et al 2004 ApJ, 603, L81
Pop 1st 2nd Alt.2nd[Fe/H] -1.7 -1.2 -1.2Y 0.23 0.35 0.23Age(Gyr) 14 12 12Fraction 0.80 0.15 0.15Turnoff mass (Msun) 0.82 0.71 0.85
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
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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 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. ”
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SUMMARY• 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: (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.
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98/9998/99
2002
Stanford thesis (2006, ApJ, 647, 1075)
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Age-Metallicity Relation
Stanford et al (2006, ApJ, 647, 1075)
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Sollima et al. 2005, ApJ, 634, 332
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Smith, Cunha & Lambert 1995 AJ, 110, 2827
Mixing line
[Fe/H]
[Ba/Fe]
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Metallicity RangeStanford thesis work
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Age-Metallicity Relation
Stanford et al (2006, ApJ, 647, 1075)
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Norris & Da Costa 1995 ApJ, 447, 680
[iron peak/Fe] vs. [Fe/H]
Cen Other clusters
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Stanford thesis work
Observations
Simulations of populations: [Fe/H] FractionFirst -1.7 0.80Second -1.2 0.15Third -0.6 0.05
0 Gyr
6 Gyr
4 Gyr
2 Gyr
Age spread
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Stanford thesis work
[Fe/H]
Age(Gyr)
15
10
5
-2 -1
Turnoff stars
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THE CHEMICAL ENRICHMENT OF CENTAURI
JOHN E. NORRIS
RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS
MOUNT STROMLO & SIDING SPRING OBSERVATORIES
AUSTRALIAN NATIONAL UNIVERSITY
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AAT Two Degree Field - Plate with fibres
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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