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
)()()(
r
rrGM
dr
rdP
)(4)( 2 rr
dr
rdM
32 )(44
)()()(3)(
racTr
rrrL
dr
rdT
)()(4)( 2 rrr
dr
rdL
**
*
)(
0)(
0)0(
0)0(
MrrM
rrP
rL
rM
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
ncompositioTPP
Equation of StateEquation of State4
3
1~ aTm
kTPPP radg
OpacityOpacity5.3, Tffbf
Energy generationEnergy generation
31
32
61
02
35
43exp
)(3
22
kT
E
kT
EQS
AAm
XX GG
BAH
BA
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)