Paul Goudfrooij Collaborators: Thomas Puzia (HIA U. Catolica de Chile)

July 28, 2010Paul GoudfrooijHotSci @ STScI

ACS Imaging of Intermediate-Age Star Clusters in the Large Magellanic Cloud:

Clues to the Nature of Multiple Stellar Populations in Globular Clusters

Paul Goudfrooij

Collaborators: Thomas Puzia (HIA U. Catolica de Chile) Vera Platais (STScI)Rupali Chandar (U. Toledo) Jason Kalirai (STScI)Leo Girardi & Stefano Rubele (Obs. Padova - INAF) Leandro Kerber (U. Estadual de Santa Cruz, Brazil)


Motivation•Assembly of Stellar Populations in Galaxies one of the most popular questions in current astronomy

•Globular Clusters constitute important “fossil records” of galaxy assembly, tracing major epochs of star formation

•Good understanding of galaxy assembly requires good understanding of formation and evolution of globular clusters and their relation with field population

Antennae (Whitmore et al. 2010)

NGC 1316 (Goudfrooij et al. 2001, 2004)

M87 (e.g., Peng et al. 2006)


GCs are “Simple” Stellar Populations (?)•SSP: Assembly of coeval stars born out of gas with a single

chemical composition•Star clusters constitute best example of SSP

HST/ACS CMD of NGC 6397

(Richer et al. 2008)

CMDs of several Galactic GCs render them “text book” examples of SSPs, even when using > 100 HST orbits to “put them to the test”


The most massive Galactic GCs are not “simple”

•Exceptional case: Omega Centauri

•Most massive Galactic GC, 4106 M currently

•Large spread in [Fe/H] among RGB stars (known since seventies)

•Main Sequence also split into two “main” ones (Bedin et al. 2004)

•Higher [Fe/H] population associated with higher He abundance (Piotto et al. 2007)

Villanova et al. (2007)

Bedin et al. (2004)


M54 (third most massive Galactic GC)

• In center of Sgr dwarf galaxy which is being tidally disrupted by Milky Way (currently close to edge of bulge)

•Several populations in CMD– Younger ones associated with

dwarf galaxy

•RGB spectroscopy: Ensemble of stars shows element abundance ratios very similar to that of Cen (Carretta et al. 2010)

• Cen and other very massive (and metal-poor) GCs likely had similar history and were able to capture/retain stars from former dwarf galaxy hosts

Siegel et al. (2007)


Star-to-Star Variations of LightLight ElementsElements

• Most GC stars have constant [Fe/H], but significant variations in light elements (C,N,O,Na,Mg,Al)

• Known since the early 80’s (e.g., Pilachowski et al. 1983)

• “Na-O anticorrelation”

• Produced by CNO-cycle H burning

• But: Amplitude of effect not typically seen among field stars

Carretta et al. (2010)

Field giants


Star-to-Star Variations of LightLight ElementsElements

Field giants

Carretta et al. (2006)

• RGB stars

• MS stars

• NGC 2808NGC 2808

•Na-O anticorrelation also also present among MS stars present among MS stars in GCsin GCs

•Abundance variations must have been “primordial”

•P-capture processes require T ~ 40-70 x 106 K– HBB regions of 3-8 M AGB

stars (D’Ancona, Ventura, Denissenkov)

– Massive stars, e.g. in binaries (de Mink et al. 2009)

•GCs able to retain winds from such stars (10 - 20 km/s) and experience secondary star formation?


Photometric evidence of CNO abundance variations

• Likely effect of N abundance (CN, NH) due to p-capture process

• M4 “only” 8 x 104 M

– Vesc ~ 18 km/s, enough to retain AGB winds– Mass was much higher when 3-8 M AGB stars

were around!

M4: Marino et al. (2008)

WFC3 Filters

CNNH Goudfrooij et al. (2010)


Why look at Intermediate-Age GCs?

•Old age of Galactic GCs precludes direct insight into nature of secondary population(s)– Only know that it happened within the first few Gyr

•Half-mass dynamical trelax < 1.5 Gyr for Galactic GCs (except Cen)– Any initially dynamically separated populations now well-mixed

•Magellanic Clouds host several 1-2 Gyr old clusters– Some are as massive as typical Galactic GCs– Age ≤ Relaxation time, so 2nd population might still be recognizable

spatially if it exists– Advent of HST/ACS allowed deep enough photometry to study this


• First indication of presence of multiple populations in intermediate-age GCs in LMC: Wide (claimed bimodal) MSTO in NGC 1846 (Mackey & Broby-Nielsen 2007; ACS snapshot program)

• Two other LMC GC candidates discussed by Mackey et al. (2008) and Goudfrooij et al. (2009)

• Eight additional candidates with ages 1-2 Gyr shown by Milone et al. (2009)

• Appears to be common feature

• Are we witnessing the secondary population(s) responsible for the Na-O anticorrelations in Galactic GCs?


“Not so Fast…”

•Bastian & de Mink (2009): Broad MS turnoff regions in 1-2 Gyr old populations can be produced by stellar rotation– Many late A - early F stars (1.2 < M/M < 1.7) known to rotate rapidly

– Rotating stars have reduced gravity and hence lower Teff

– Simulate CMD with random orientations and various distributions> Note: Only MS stars affected significantly


ACS CMDs of Intermediate-Age LMC GCsGoudfrooij et al. (2009, 2010)


Radial Distributions of “Young” & “Old” MSTO stars• If wide MSTO due to 2 (or more) populations:

– Secondary populations formed from matter shed in slow winds of AGB stars or massive (binary) stars after cooling flow to cluster center (D’Ercole et al. 2008)

– Expect younger population(s) to be more centrally concentrated than first one (if age < trelax)

• If due mainly to stellar rotation:– faster rotating stars initially more centrally concentrated due to higher gas

density and more rapid collapse (McKee & Tan 2003)

“old”

“young”

RGB/RC/AGB (young + old)

Blue: “young” or “slow rotator”

Red: “old” or “fast rotator”

Black: Mixture


Radial Distributions of “Young” & “Old” MSTO stars

Blue: “young” or “slow rotator”

Red: “old” or “fast rotator”

Black: Mixture

•Less massive GCs: no significant difference

•More massive GCs: “younger” stars clearly more centrally peaked

•Evidence for age effect

Goudfrooij et al. (2010)


•Calculated GC masses using SSP models (for mean age & [Fe/H]) and Salpeter IMF

•Estimated masses at birth using stellar evolution models & GC disruption model of Fall & Zhang (2001) plus effect of (evolving) mass density– Stellar evolution: 40% of mass lost within first 300 Myr– Two-Body Relaxation as function of GC mass density

(McLaughlin & Fall 2008)

•Calculate escape velocities vesc (M/rh)1/2

Effects of Mass, Radius, and GC Dissolution


• Wind speeds for rapidly rotating or binary massive stars:

– v ~ 10-100 km/s

• Intermediate-mass (3–8 M) AGB stars:

– v ~ 10-20 km/s



• Wind speeds for rapidly rotating or binary massive stars:

– v ~ 10-100 km/s

• Intermediate-mass (3–8 M) AGB stars:

– v ~ 10-20 km/s

• The more massive LMC clusters were likely able to retain that materialThe more massive LMC clusters were likely able to retain that material• Same scenario for Galactic GCs?



Concluding Remarks and Future Work

•Intermediate-age GCs very useful benchmarks to study nature of secondary populations in GCs

•Wide MSTO in (at least the more dense) intermediate-age GCs most likely due to spread in agespread in age– Projected vesc values high enough to retain winds from AGB stars

or material shed by massive binaries or rotating massive stars

•Are we witnessing the cause of the Na-O anticorrelations in Galactic GCs?

•Cycle 18 HST/WFC3 program to get deep U, g, I images of 17 intermediate-age GCs in SMC & LMC (10 ‘new’ GCs)

– Search for presence of light-element abundance variations as predicted based on the models

– Establish robust fraction of such GCs hosting wide MSTOs


MSTO Regions Incompatible with SSP(s)

•1 or 2 SSPs with binaries cannot properly reproduce MSTO morphology (Goudfrooij et al. 2009, 2010)


Age Spread among Subpopulations in Cen•Very hard to constrain for such

old populations•Photometric and spectroscopic

study of MS turnoff stars, using [Fe/H] and He constraints on isochrones

•Age dispersion 2 – 4 Gyr, only significant for most metal-rich stars

Stanford et al. (2006)


Radial Distributions of “Young” & “Old” MSTO stars

Blue: “young” or “slow rotator” Red: “old” or “fast rotator” Black: Mixture

Paul Goudfrooij Collaborators: Thomas Puzia (HIA U. Catolica de Chile)

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