the origin of multiple populations within stellar clustersbastian et al. 2013a range where the...

The Origin of Multiple Populations within Stellar Clusters

An Unsolved Problem

Nate Bastian(Liverpool, LJMU)

Ivan Cabrera-Ziri (LJMU), Katie Hollyhead (LJMU), Florian Niederhofer (LMU/ESO)

Tuesday, 14 April 15

V-I

V

(colour)

(brightness)

old, > 10 Gyr

metal poor


V-I

V

(colour)

(brightness)

old, > 10 Gyr

metal poor Some of the oldest luminous

objects in the Universe

Globular cluster formation intimately linked with galaxy formation


Globular Clusters as Tools: GCs have been used to....

• Determine the structure of the Milky Way (Shapely 1918)• Understand (and calibrate) stellar evolution and stellar

populations (Eddington 1926)

• Constrain the formation and evolution of the Milky Way (Searle & Zinn 1978)

• ...as well as other nearby galaxies (Brodie & Strader 2006)• Find and study “stellar exotica”


M80

Pleiades

104 - 106 Msun 10 - 13 Gyr low metallicity bulge/halo of the Galaxy

few - 104 Msun few Myr - few Gyr ~solar metallicity disk of the Galaxy

Globular Clusters

Open Clusters

Introduction to Stellar Clusters


Clusters historically viewed as simple stellar populations

• All stars have the same age (very small spread, < 1-2 Myr)

• All stars have the same abundances• Range of stellar massses (and potentially rotation

rates)

• Quintesential “simple stellar populations”• GCs could only form in the special conditions of the

early Universe


V-I

V

Surprise #1


V-I

V

B-I

I

Surprise #1

“multiple populations”


Omega Cen - ESO

few million Msun~12 GyrSurprise #2


Omega Cen - ESO

Antennae colliding galaxies - HST

few million Msun~12 GyrSurprise #2


Omega Cen - ESO


few million Msun~12 Gyr

few million Msun~7 Myr

Surprise #2


Omega Cen - ESO




similar masses & sizes (densities)

Surprise #2


Omega Cen - ESO




similar masses & sizes (densities)

globular clusters are still forming in the local

universe

Surprise #2


Young Massive Clusters (YMCs)

Maraston et al. 2004Bastian et al. 2006 NGC 7252

NGC 34Schweizer & Seitzer 2007Cabrera-Ziri et al. 2014



Maraston et al. 2004Bastian et al. 2006

~400 Myr108 Msun

~400 Myr107 Msun

NGC 7252

NGC 34

~100 Myr107 Msun

Schweizer & Seitzer 2007Cabrera-Ziri et al. 2014



Maraston et al. 2004Bastian et al. 2006

~400 Myr108 Msun

~400 Myr107 Msun

NGC 7252

NGC 34

~100 Myr107 Msun

~15 Myr106 Msun

NGC 1705

Schweizer & Seitzer 2007Cabrera-Ziri et al. 2014


[O/Fe][N

a/Fe

]

Cordero et al. 2014

47 Tuc

Milone et al. 2013

UV

UV - U

NGC 6752

Mulitple (spread) CMD features Chemical Spreads

All globulars show anomalies, but all differ in the details

Globular clusters are not simple stellar populations


Piotto et al. 2007

NGC 2808

~60% of stars on nominal main sequence

(“first generation”)

~30% of stars are He enriched

(“second generation”)

not due to age or metallicity differences, only He

abundance can explain it

B-I

I

Piotto et al. 2007


D’Antona 2012 (Vatican Observatory Lectures)

Generally, elements affected by hot hydrogen burning show deviations. Not elements related to SNe.

“primordial”

“enriched”


D’Antona 2012 (Vatican Observatory Lectures)


[O/Fe]

[Na/

Fe]

Martell et al. 2011

47 Tuc

Stars in GCs “know about”

where they form

Relation to the field


[O/Fe]

[Na/

Fe]

Martell et al. 2011

47 Tuc

Stars in GCs “know about”

where they form

~3% of halo stars~50% of cluster stars

~97% of halo stars~50% of cluster stars

???

Relation to the field


Observables✤ Na-O anti-correlation

✤ Large Al spread, small (or no) Mg spread. Some spread in other light elements (e.g C, N). Little/no spread in Fe.

✤ Discrete/spread main sequences/turn-offs, sub-giant branches, presumably due to discrete He abundances and/or CNO abudances

✤ Found in red (metal rich) and blue (metal poor) clusters

Sources of the enriched material✤ Only certain stars produce the right abudances. SNe can’t do it.

✤ AGB stars (3-8 Msun - although wrong Na-O correlation), Rapidly rotating high mass stars, interacting massive binaries, extremely massive stars


Models of multiple populations in GCs

✤ Multiple epochs of star formation

✤ The ejecta of 1st generation stars, mixes with primordial material and forms a 2nd generation

✤ Only certain stars produce the “correct” abundance ratios - AGBs, massive stars

✤ Extremely ad-hoc, many adjustable parameters, mix of theory and ‘fixes’ to fit observations

✤ Generally assume that a cluster can hold on to gas expelled from stars (and accrete new gas at just the right time) for 10s to 100s of Myr (Bekki & Norris 2006, Decressin et al. 2007, D’Ercole et al. 2008, Vesperini et al. 2009, Conroy & Spergel 2011, Krause et al. 2013)


AGB scenario✤ The 1st generation forms

✤ T > 30 Myr, AGBs begin shedding material which collects in the centre of the cluster

✤ The cluster accretes (a lot) of “pristine gas” from the surroundings.

✤ The 2nd generation forms in the cluster center

✤ Most of the 1st generation (~95%) is lost


Predictions of the AGB scenario✤ The 2nd generation is more centrally concentrated

✤ in order to have enough material to form the 2nd generation, GCs must have been 10-100 times more massive that presently seen (mass budget problem)

✤ clusters can retain ejecta and accreted gas for long periods

✤ Massive clusters should show an age spread (or multiple bursts) - i.e. we should be able to find young massive clusters (>10 Myr) with ongoing star formation






-no, quite complicated profiles(Larsen et al. 2015, Bastian et al. in prep)






-directly contradicted by observations (Larsen et al. 2012, 2014)









-not observed (Bastian & Strader 2014, Cabrera-Ziri et al. 2015)








-not observed (Bastian & Strader 2014, Cabrera-Ziri et al. 2015)

let’s see....


✤ While Globular Cluster formation (at high-z) may have been fundamentally different from massive clusters forming today, all main theories for the origin of multiple populations predict that it should be happening in young clusters today.

✤ i.e. current theories do not invoke any special conditions/physics for GC formation.

Are Young Massive Clusters the Same as Globular Clusters?


Evidence for extended star formation histories in other clusters?

• 140 clusters with ages between 10-1000 Myr and masses between 104-108 Msun, from the literature, with integrated optical spectroscopy or resolved stellar photometry

• clusters in spirals, dwarfs, starbursts/mergers• Look for emission associated with the clusters (Hβ,

O[III]) or O-stars in the CMDPeacock et al. 2013, Bastian et al. 2013a


Bastian et al. 2009


Bastian et al. 2013a

Range where the “AGB scenario” expects the 2nd generation to be forming

(30-200 Myr)Conroy & Spergel (2011)

Given the relative ‘sketchiness’ of the scenarios put forward, there is not an agreed upon age (or even continuous vs. discreet bursts)

where the 2nd generation should form. The obs here rule out continuous (>7 Myr duration) and disfavour discreet bursts.

No ongoing star-formation detected

in any cluster


Star formation history of clusters NGC 34 Cluster 1

Schweitzer & Seitzer 2007


Star formation history of clusters NGC 34 Cluster 1

Schweitzer & Seitzer 2007

Cabrera-Ziri, NB et al. 2014

Single population - 100 Myr2 x 107 Msun


NGC 7252: W3570 Myr, ~108 Msun

Star formation history of clusters

no star-formation for the past ~400 Myr

Cabrera-Ziri et al. 2015b, in prep.

5 more clusters under study to sample the full

30-200 Myr range


ALMA observations of the AntennaeCabrera-Ziri, NB, et al. 2015

Whitmore et al. 2014

50-200 Myr1-3 * 106 Msun

No gas detected(

Summary of young clusters✤ Appear to be gas free at young ages (107 Msun) shows no evidence for multiple bursts or extended SFHs (Cabrera-Ziri et al. 2014, 2015b)

✤ No gas/dust found in YMCs, which would be required to form further generations of stars

✤ Models with multiple star formation events are disfavoured by observations of YMCs

-a problem for the FRMS scenario - Hollyhead et al. 2015

Previous (popular) models do not agree with observations of YMCs

Bastian & Strader 2014; Cabrera-Ziri et al. 2015a

Niederhofer et al. 2015


New Model: The Early Disc Accretion Scenario (Bastian et al. 2013)

Single star-formation event

Uses the nuclear burning products of high mass stars and accretes them onto some low-mass stars

within the clusters

Matches observations of YMCs


Can Self-Enrichment Scenarios Work? Basic Abundance Predictions

Observations now routinely measure Na, O and He (Y) spreads in GCs.

Can self-erichment scenarios match the observations?



Bastian, Cabrera-Ziri, Salaris 2015

For a given position in Na-O

space, dilution models give a

direct prediction of He (Y) spreads




AGB ejectaY=0.38

PristinematerialY=0.25







AGB ejectaY=0.38

PristinematerialY=0.25




Constant He value




Allowed spread based on He

Observations

AGB stars over-produce He

Expected He spread of 0.10 based on Na/O


Disagreement with observations

of GCs



AGBs AGBs

FRMS Binaries



AGBs AGBs

FRMS Binaries

Allowed range



AGBs AGBs

FRMS Binaries

Allowed rangeAll suffer from the same basic problem, over-production of He.


Can Self-Enrichment Scenarios Work? Doomed to Fail.


[O/Fe] [O/Fe] [O/Fe]Similar spread in Na-O, huge differences in

their He abundance spreads.


Can Self-Enrichment Scenarios Work? Doomed to Fail.


[O/Fe] [O/Fe] [O/Fe]Similar spread in Na-O, huge differences in

their He abundance spreads.

Can never be accounted for in current self-enrichment scenarios.


Take away messages✤ Globular clusters host “multiple populations” within them, seen in

chemistry and in their strange CMDs

✤ GCs still forming today (YMCs)

✤ No evidence for age spreads or multiple bursts in young clusters, either through resolved photometry or integrated spectroscopy

✤ Obs of YMCs appear to rule out the “popular scenarios” for the multiple populations in GCs.

✤ Obs of abundance trends in GCs appear rule out all self-enrichment scenarios.

✤ New ideas needed! (ISM physics? Crazy stellar evolution?....)

Back to square one.


Larsen et al. 2015

Radial profile of the multiplepopulations in M15

Primoridal population more centrally concentrated


MV

log

(del

ta(Y

))

Milone 2014

Other problems for self-enrichment scenarios

He spread and [N/Fe] are functions of cluster mass Schiavon et al. 2013

Galactic GCs

M31 GCs


NGC 18062*105 Msun

1.5 GyrLMC cluster

Mucciarelli et al. 2014

No abundance spreads!


• First generation forms at t=0, cluster remains embedded for 20-30 Myr

• 2nd generation stars form in the decretion discs around high mass stars, also fed from the primordial material that is left in the embedded cluster (5-10 Myr)

Prediction: YMCs with ages < 20 Myr should still be embedded

Testing the Fast Rotating Massive Star Scenario

Krause et al. 2012, 2013


Bastian, Hollyhead, & Cabrera-Ziri 2014

ESO 338-IG04 - Cluster 23

t = 6+4-2 MyrAv = 0M~ 1x107 MsunRbubble ~ 120-200pcZ = 0.2 Zsun

Östlin et al. 2007

200pc100pc


Bastian, Hollyhead, & Cabrera-Ziri 2014

ESO 338-IG04 - Cluster 23

t = 6+4-2 MyrAv = 0M~ 1x107 MsunRbubble ~ 120-200pcZ = 0.2 Zsun

• Bubble began expanding 1-3 Myr after formation• Efficiently removed any pristine material out to hundreds of parsecs (still expanding at 40 km/s)• Metallicity below that of Galactic globular clusters that show anomalies

Östlin et al. 2007

200pc100pc


No clusters, older than 10 Myr, were found with signs of ongoing star formation

Bastian et al. 2013a

resolved photometry

integratedspectroscopy

new integrated spectra


New model✤ One burst of star formation (i.e. an SSP) - as observed in young

clusters

✤ High mass stars (binaries) mass segregated

✤ interacting binaries and spin-stars eject (low velocity) material into the cluster - this material has been processed by the high mass stars (70% of high mass stars are in binaries that will interact)

✤ low mass stars keep their discs for 5-10 Myr, which can entrain material as they move in the cluster

✤ the material eventually accretes onto the young star (

cluster

core

low mass PMS star + disc

interacting high-mass binaries


1) Source of the enriched material

de Mink et al. 2009

Interacting high mass binaries

✤ The ejected envelope is very He rich✤ Many/most abundances reproduced (trends and quantitatively)

(Cassisi & Salaris 2014) - may have a lithium problm (Salaris & Cassisi 2014)

✤ Ejected at low velocities (

Summary of the model

✤ The entire volume core is swept out every ~2 Myr

✤ Without invoking multiple episodes of SF, the model 1) accounts for the enrichment patterns observed, 2) Potentially quantised abundances (and fractions), 3) doesn’t have a “mass budget problem” (other models miss the mass budget by 10-100, even in the best cases)

✤ Explains why young massive clusters are observed to be gas free from young ages (~1-2 Myr) and don’t show evidence for extended SFHs

✤ Same effects should be visible in young massive clusters (haven’t been tested yet)


Caveats and Predictions

✤ The model requires a low coupling between SNe and dense cool matter in the discs of young stars and in the ejecta material from binaries. Supported by radiation-hydrodynamical models - e.g., Rogers & Pittard (2013)

✤ Circumstellar discs in massive clusters need to survive for 5-10 Myr.

✤ Leads to different kinematic predictions (in progress)

✤ Different radial profiles, with the degree of concentration a direction function of the amount of enrichment.


Conroy 2012

fraction of current mass that has been processed by

AGB stars

prediction


ALMA observations of the AntennaeCabrera-Ziri, NB, et al. submitted

Whitmore et al. 2014

50-200 Myr1-3 * 106 Msun

No gas detected(

2) Mass budget problem?

stars

that

are s

till o

n th

e m

ain

sequ

ence

toda

y

stars tha

t accre

ted pr

ocessed material

stars that have evolved off the main sequence

enriched gas potentially returned to the inter stellar medium (13%)

spin stars

AGB stars

binaries, dynamical stripping

M (M¤)0.1 1.0 10 100

0.6

0.5

0.4

0.3

0.2

0.0

0.1

M d

N/d

logM

0.8 M¤ 2.0 M¤

enriched gas accreted by pre main sequence

stars (~10%)

But since a star is already in place to accrete the material, we can make the approximation that a star accretes

3) Distribution of the stars in the cluster


Problems with the spin-star scenario✤ Need primordial and processed gas to stay within the

cluster for 5-10 Myr

✤ Should not find any massive young clusters (

2) Quantisation of He and other elements?

only 45% of stars ever enter the core


Caveats and Predictions✤ The model requires a low coupling between SNe and dense cool

matter in the discs of young stars and in the ejecta material from binaries. Supported by radiation-hydrodynamical models - e.g., Rogers & Pittard (2013)

✤ Circumstellar discs in massive clusters need to survive for 5-10 Myr.

✤ Need to invoke stellar mergers to explain Mg depletion in ~15% of clusters (although all models have some difficulties with the chemistry)

✤ Leads to different kinematic predictions (in progress)

✤ Different radial profiles, with the degree of concentration a direction function of the amount of enrichment.


Do clusters have extended star formation histories?✤ Young massive (104 - 105 Msun) clusters (Westerlund 1, NGC

3603, R136) do not, < 1-2 Myr (Kudryavtseva et al. 2012)

✤ Resolved clusters in the LMC (200-300 Myr) and 105 Msun do not, < 30 Myr (Bastian & Silva-Villa 2013)

V-I

MV

Bastian & Silva-Villa 2013


July 14th - 18th 2014

A Critical Look at Globular Cluster Formation Theories: Constraints from Young Massive Clusters


[O/Fe]

[Na/

Fe]

r/rh

Cordero et al. 2014


Larsen, Strader, Brodie 2012

Metallicity distribution of stars in Fornax


~15 Myr106 Msun

NGC 1705


Do clusters have extended star formation histories?

• Young massive (104 - 105 Msun) clusters (Westerlund 1, NGC 3603, R136) do not, < 1-2 Myr (Kudryavtseva et al. 2012)

• There have been suggested of extended SFHs (200-500 Myr) in intermediate age (1-2 Gyr) clusters in the LMC/SMC based on extended main sequence turn-offs

✤ Mackey & Brobie Neilson (2007)✤ Mackey et al. (2008) - 3 clusters✤ Milone et al. (2008) - 15 clusters✤ Glatt et al. (2008) - 1 cluster in SMC✤ Goudfrooij et al. (2009, 2011a,b)


Intermediate age clusters in the LMC

Mackey & Brobie Neilson 2007

~1.5 Gyr

V-I

mv


Intermediate age clusters in the LMC

Mackey & Brobie Neilson 2007

~1.5 Gyr

300 Myr age difference

V-I

mv


Goudfrooij et al. 2011a

observed modelled


Goudfrooij et al. 2011b



< 500 Myr1-3 Gyr clusters with claimed age

spreads

If an age spread, and a common feature of clusters, this should be seen in younger clusters with similar properties


Cabrera-Ziri et al. in prep

VLT/X-Shooter mini-survey of 6 YMCs (106 - 108 Msun) with ages between 15 - 500 Myr to search for age spreads


NGC 1856 NGC 1866

Age ~ 280 Myrmass ~ 105 msunAv ~ 0.8 mag

Age ~ 180 Myrmass ~ 105 msun

Av ~ 0.15 magpublished HST photometry in Brocatto et al. 2001 and 2003


To avoid background contaminations, only inner 4.8pc were used in both clusters

MV

V-I


NGC 1856

Quantitative results: use the FITSFH (Silva-Villa & Larsen 2010) code to derive the SFH of the clusters

-Takes into account photometric errors-Adopts Salpeter IMF (above 1 Msun)-Uses Padova isochrones (Bressan et al. 2012) at z=0.008

MV

B-V


Intermediate age clustersshifted to 200 Myr


upper limits on the width


Intermediate age clustersshifted to 200 Myr


upper limits on the width

No evidence for extended star formation histories


Reported Age Spreads in Clusters

• Young clusters (

Mayya et al. 2008

Westmoquette et al. 2009

91

Konstantopoulos et al. 2009

clusters with ages from 1-300 Myr

and masses from 103 to 106 Msun

also found in spirals, dwarfs, irregulars


Problems with the spin-star scenario✤ Need primordial and processed gas to stay within the

cluster for 5-10 Myr

✤ Should not find any massive young clusters (

the origin of multiple populations within stellar clustersbastian et al. 2013a range where the...

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