THE CHEMICAL ENRICHMENT OF CENTAURI

JOHN E. NORRIS

RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS

MOUNT STROMLO & SIDING SPRING OBSERVATORIES

AUSTRALIAN NATIONAL UNIVERSITY

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

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

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

Persson et al 1980, ApJ, 235, 452

C and/or O enhance-ment unique to Cen

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)

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

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

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

High resolution spectrum obtained withAAT UCL Echelle Spectrograph (UCLES)

High resolution spectra of 35 red giants(AAT UCLES, R~35,000;

Norris & Da Costa 1995, ApJ, 447, 680

[alpha/Fe] vs. [Fe/H](NB: heavily biased sample)

Enrichment by SNe II

Cen Other clusters

Norris & Da Costa 1995, ApJ, 447, 680

[neutron capture/Fe] vs. [Fe/H]

Enrichment by (intermediate-mass) AGB stars

Cen Other clusters

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

[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

Norris, Freeman & Mighell 1996, ApJ, 462, 241

[Ca/Fe] vs. radius

Abundance decreases with radial distance

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

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

Cen

Lee et al 1999,Nature, 402, 55

Ferraro et al 2004, ApJ, 603, L81Pancino et al 2000, ApJ, 584, L83

‘Third’ population

Pancino et al 2002, ApJ, 568, L101

Enrichment by SNe Ia

[Ca/Fe]

[Fe/H]

Sollima et al. 2005, MNRAS, 357, 265

To Cen’s main sequence withAAT Two Degree Field

Spectrographs

… working with Laura Stanford, Gary Da Costa & Russell Cannon(Stanford et al 2006, ApJ, 647,1075)

1998/992002

Stars observed in 2002 box Cen radial-velocity

members in 2002 box

Stanford thesis

Stanford thesis

Metallicity Distribution Function

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

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

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

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

Stanford et al. (2006, in prep)

[Sr/Fe] = +1.6[Ba/Fe] < +0.8:

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

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

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

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

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

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

Piotto et al. 2005, ApJ, 621, 777

The blue main sequence is more metal-rich by 0.3 dex!

VLT Giraffe

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?

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

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. ”

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.

98/9998/99

2002

Stanford thesis (2006, ApJ, 647, 1075)

Age-Metallicity Relation

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

Sollima et al. 2005, ApJ, 634, 332

Smith, Cunha & Lambert 1995 AJ, 110, 2827

Mixing line

[Fe/H]

[Ba/Fe]

Metallicity RangeStanford thesis work

Age-Metallicity Relation

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

Norris & Da Costa 1995 ApJ, 447, 680

[iron peak/Fe] vs. [Fe/H]

Cen Other clusters

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

Stanford thesis work

[Fe/H]

Age(Gyr)

15

10

5

-2 -1

Turnoff stars

THE CHEMICAL ENRICHMENT OF CENTAURI

JOHN E. NORRIS

RESEARCH SCHOOOL OF ASTRONOMY & ASTROPHYSICS

MOUNT STROMLO & SIDING SPRING OBSERVATORIES

AUSTRALIAN NATIONAL UNIVERSITY

AAT Two Degree Field - Plate with fibres

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