Why do globular Why do globular clusters have more clusters have more

than one main than one main sequence?sequence?

Ref: Gratton et al. 2012, A&ARv, 20, 50Ref: Gratton et al. 2012, A&ARv, 20, 50

11stst, a pretty picture… , a pretty picture…

OutlineOutline The lives of starsThe lives of stars

Some basic physics Some basic physics Importance of clustersImportance of clusters

Globular clusters are really, really old!Globular clusters are really, really old! GC detailsGC details

ObservationsObservations GC detailsGC details

Models and formationModels and formation

Lives of StarsLives of Stars

What do we What do we know about know about stars?stars? They are They are

bornborn They liveThey live They dieThey die They spread They spread

their seedtheir seed

Lives of StarsLives of Stars

Stars appear Stars appear in groupsin groups

Modeling a StarModeling a Star

2

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Four coupled first-order differential equations describe stellar Four coupled first-order differential equations describe stellar

structurestructure

Physics of StarsPhysics of Stars

Need a way to Need a way to connect: pressure, connect: pressure, opacity, and opacity, and energy productionenergy production

),,(

),,(

),,(

ncompositioT

ncompositioT

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Equation of StateEquation of State4

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OpacityOpacity5.3, Tffbf

Energy generationEnergy generation

31

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Star clustersStar clusters

Globular Cluster

Open Cluster

Stellar evolutionStellar evolution

Our ideas of stellar evolution came Our ideas of stellar evolution came about because of the single population about because of the single population theory of cluster formation.theory of cluster formation.

But there are still some outstanding But there are still some outstanding issuesissues Blue stragglersBlue stragglers Mass lossMass loss AGB starsAGB stars White dwarf coolingWhite dwarf cooling

Why are clusters Why are clusters important?important?

Overview of star Overview of star formationformation

TriggerTrigger Gravitational collapseGravitational collapse

Heating/rotation/magnetic fields slow Heating/rotation/magnetic fields slow collapsecollapse

Eventual H-fusionEventual H-fusion Main sequence starMain sequence star

OB association formationOB association formation

Globular clustersGlobular clusters

Spherical collection Spherical collection of gravitationally of gravitationally bound starsbound stars > 100,000 stars> 100,000 stars

The Milky Way has The Milky Way has about 200about 200

GCs are found in GCs are found in all galaxies of large all galaxies of large sizesize

Other galaxiesOther galaxies

M87M87 More than 10,000 More than 10,000

GCsGCs Many visible in this Many visible in this

imageimage

1 Main sequence1 Main sequence

Photometric narrow MSsPhotometric narrow MSs Proof of single-age Proof of single-age Single metallicitySingle metallicity

Until…Until…

47 Tuc47 Tuc

16,700 ly 16,700 ly awayaway

120 ly 120 ly acrossacross

MMVV ~ 4.9 ~ 4.9

~ full moon ~ full moon sizesize

47 Tuc47 Tuc

Single MS?Single MS?

Cannon et al. 1998

47 Tuc47 Tuc

Anti-Anti-correlaticorrelation on between between CN and CN and CH CH moleculamolecular bandsr bands

47 Tuc47 Tuc

Why is this impressive?Why is this impressive? Primordial in originPrimordial in origin

Giant stars can mix NGiant stars can mix N MS stars can not!MS stars can not!

Anticorrelations between O and Na & Mg Anticorrelations between O and Na & Mg and Aland Al Low mass stars do not make NaLow mass stars do not make Na Nor do they destroy MgNor do they destroy Mg

Clear indication of external originClear indication of external origin

ωω Cen Cen

15,800 ly 15,800 ly awayaway

180 ly across180 ly across

~ million ~ million MMsunssuns

Core stars .1 Core stars .1 ly from each ly from each otherother

ωω Cen Cen

2 or 3 MSs2 or 3 MSs

Bedin et al. 2004

NGC 2808NGC 2808

Typical GCTypical GC

31,000 ly 31,000 ly awayaway

~12.5 Gyr ~12.5 Gyr oldold

NGC 2808NGC 2808

Triple Triple MSsMSs

Piotto et al. 2007

Not just MSsNot just MSs

Red giant branches of HR diagrams Red giant branches of HR diagrams contain multiple populations of starscontain multiple populations of stars

Horizontal branches of HR diagrams Horizontal branches of HR diagrams too.too.

GC FormationGC Formation

Understanding of the multiple Understanding of the multiple population phenomenon has population phenomenon has changedchanged OriginallyOriginally

Few peculiar clustersFew peculiar clusters Only massive clustersOnly massive clusters

Now Now Probably all GCs have multiple populations Probably all GCs have multiple populations

If 1-population theory If 1-population theory wrong…wrong…

GCs formed within large episodes of GCs formed within large episodes of star formation in Giant Molecular star formation in Giant Molecular AssociationsAssociations

Scenarios include several Scenarios include several generations of starsgenerations of stars Primordial, first, second…Primordial, first, second…

We observe second generation nowWe observe second generation now First generation “polluted” the secondFirst generation “polluted” the second

4 additional mechanisms 4 additional mechanisms to considerto consider

Original homogeneities in the Original homogeneities in the primordial materialprimordial material

Peculiar evolution of starsPeculiar evolution of stars Merging of GCsMerging of GCs GC pulling in interstellar material GC pulling in interstellar material

(“pollution”)(“pollution”)

A possible modelA possible model

N-body & hydro. simulationsN-body & hydro. simulations

11stst generation AGB star’s ejecta flows to GC generation AGB star’s ejecta flows to GC centercenter

22ndnd generation forms with “different” generation forms with “different” metallicitymetallicity Concentrated at center of GCConcentrated at center of GC

Problem: IMF either uncommon or Problem: IMF either uncommon or unknownunknown

A sticky detailA sticky detail

The polluting ejecta must be The polluting ejecta must be “diluted” by fresh gas“diluted” by fresh gas Mandatory part of the processMandatory part of the process

Problem: how much of original gas Problem: how much of original gas went to 1went to 1stst generation? generation?

Problem: how is pristine material Problem: how is pristine material gathered?gathered?

GC – Milky Way GC – Milky Way connectionconnection

Present day GCs smaller nowPresent day GCs smaller now Star loss during initial formationStar loss during initial formation

Problem: what role does dark matter Problem: what role does dark matter play?play?

Mass of GCsMass of GCs

GCs used to be largerGCs used to be larger Original proto-GCs contained 50% of Original proto-GCs contained 50% of

halo masshalo mass

Of all stars in halo 1.2% are in GCsOf all stars in halo 1.2% are in GCs Other 98.8% are free but are in a HUGE Other 98.8% are free but are in a HUGE

volumevolume

Lingering questionsLingering questions

Mechanisms of inhomogeneitiesMechanisms of inhomogeneities Nature of pollutersNature of polluters Nucleosynthesis mechanismsNucleosynthesis mechanisms Origin of diluting materialOrigin of diluting material Reliable 3-D hydrodynamical models Reliable 3-D hydrodynamical models

of formationof formation

my Contributionmy Contribution

I model 2 aspects of this problemI model 2 aspects of this problem Mean opacities necessary for stellar Mean opacities necessary for stellar

evolutionevolution Atmosphere models necessary for color-Atmosphere models necessary for color-

color diagramscolor diagrams

Example Example

NGC 6752NGC 6752 MMVV ~ 5.4 ~ 5.4 Age ~ 11.8 GyrAge ~ 11.8 Gyr ~ 17,000 Lyr~ 17,000 Lyr

2 2 atmospheratmosphere models e models comparedcompared

Dotter et al. 2015, MNRAS, 446,

1641

Another pretty picture…Another pretty picture…

dSphs?dSphs?

Are GCs just small dwarf spheroidal Are GCs just small dwarf spheroidal galaxies?galaxies? GCs are closer to main MWGCs are closer to main MW GCs lost dark matter (dSphs did not)GCs lost dark matter (dSphs did not) GCs are compactGCs are compact dSphs evolved in isolation (GCs are part dSphs evolved in isolation (GCs are part

of MW)of MW)