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Viewed As Simple Stellar Populations 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 Tuesday, 14 April 15 Tuesday, 14 April 15 Tuesday, 14 April 15 V old, > 10 Gyr (brightness) metal poor V-I (colour) Tuesday, 14 April 15 V old, > 10 Gyr (brightness) metal poor Some of the oldest luminous objects in the Universe Globular cluster formation V-I intimately linked with galaxy (colour) formation Tuesday, 14 April 15 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” Tuesday, 14 April 15 Introduction to Stellar Clusters Pleiades Open Clusters few - 104 Msun few Myr - few Gyr ~solar metallicity disk of the Galaxy Globular Clusters 104 - 106 Msun 10 - 13 Gyr low metallicity bulge/halo of the Galaxy M80 Tuesday, 14 April 15 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 Tuesday, 14 April 15 Surprise #1 V V-I Tuesday, 14 April 15 Surprise #1 V V-I Tuesday, 14 April 15 Surprise #1 I V “multiple populations” B-I V-I Tuesday, 14 April 15 few million Msun Surprise #2 ~12 Gyr Omega Cen - ESO Tuesday, 14 April 15 few million Msun Surprise #2 ~12 Gyr Omega Cen - ESO Antennae colliding galaxies - HST Tuesday, 14 April 15 few million Msun Surprise #2 ~12 Gyr few million Msun ~7 Myr Omega Cen - ESO Antennae colliding galaxies - HST Tuesday, 14 April 15 few million Msun Surprise #2 ~12 Gyr similar masses & sizes (densities) few million Msun ~7 Myr Omega Cen - ESO Antennae colliding galaxies - HST Tuesday, 14 April 15 few million Msun Surprise #2 ~12 Gyr globular clusters are similar masses & sizes still forming(densities) in the local universe few million Msun ~7 Myr Omega Cen - ESO Antennae colliding galaxies - HST Tuesday, 14 April 15 Young Massive Clusters (YMCs) Schweizer & Seitzer 2007 NGC 34 Cabrera-Ziri et al. 2014 Maraston et al. 2004 Bastian et al. 2006 NGC 7252 Tuesday, 14 April 15 Young Massive Clusters (YMCs) Schweizer & Seitzer 2007 NGC 34 Cabrera-Ziri et al. 2014 ~400 Myr 108 Msun ~400 Myr 107 Msun ~100 Myr 107 Msun Maraston et al. 2004 Bastian et al. 2006 NGC 7252 Tuesday, 14 April 15 Young Massive Clusters (YMCs) Schweizer & Seitzer 2007 NGC 34 Cabrera-Ziri et al. 2014 ~400 Myr ~15 Myr 108 Msun 106 Msun ~400 Myr 107 Msun NGC 1705 ~100 Myr 107 Msun Maraston et al. 2004 Bastian et al. 2006 NGC 7252 Tuesday, 14 April 15 Globular clusters are not simple stellar populations Milone et al. 2013 Cordero et al. 2014 UV [Na/Fe] NGC 6752 47 Tuc UV - U [O/Fe] Mulitple (spread) Chemical Spreads CMD features All globulars show anomalies, but all differ in the details Tuesday, 14 April 15 NGC 2808 ~60% of stars on nominal main sequence (“first generation”) I ~30% of stars are He enriched (“second generation”) not due to age or metallicity differences, only He PiottoPiotto et et al. al. 20072007 abundance can explain it B-I Tuesday, 14 April 15 D’Antona 2012 (Vatican Observatory Lectures) “enriched” “primordial” Generally, elements affected by hot hydrogen burning show deviations. Not elements related to SNe. Tuesday, 14 April 15 D’Antona 2012 (Vatican Observatory Lectures) Tuesday, 14 April 15 Relation to the field Martell et al. 2011 Stars in GCs “know about” where they form [Na/Fe] 47 Tuc [O/Fe] Tuesday, 14 April 15 Relation to the field Martell et al. 2011 ~3% of halo stars ~50% of cluster stars ??? Stars in GCs “know about” where they form [Na/Fe] ~97% of halo stars ~50% of cluster stars 47 Tuc [O/Fe] Tuesday, 14 April 15 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 Tuesday, 14 April 15 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) Tuesday, 14 April 15 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 Tuesday, 14 April 15 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 Tuesday, 14 April 15 Predictions of the AGB scenario ✤ The 2nd generation is more centrally concentrated -no, quite complicated profiles (Larsen et al. 2015, Bastian et al. in prep) ✤ 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 Tuesday, 14 April 15 Predictions of the AGB scenario ✤ The 2nd generation is more centrally concentrated -no, quite complicated profiles (Larsen et al. 2015, Bastian et al. in prep) ✤ 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) -directly contradicted by observations (Larsen et al. 2012, 2014) ✤ 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 Tuesday, 14 April 15 Predictions of the AGB scenario ✤ The 2nd generation is more centrally concentrated -no, quite complicated profiles (Larsen et al. 2015, Bastian et al. in prep) ✤ 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) -directly contradicted by observations (Larsen et al. 2012, 2014) ✤ clusters can retain ejecta and accreted gas for long periods -not observed (Bastian & Strader 2014, Cabrera-Ziri et al. 2015) ✤ 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 Tuesday, 14 April 15 Predictions of the AGB scenario ✤ The 2nd generation is more centrally concentrated -no, quite complicated profiles (Larsen et al. 2015, Bastian et al. in prep) ✤ 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) -directly contradicted by observations (Larsen et al. 2012, 2014) ✤ clusters can retain ejecta and accreted gas for long periods -not observed (Bastian & Strader 2014, Cabrera-Ziri et al. 2015) ✤ 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 let’s see.... Tuesday, 14 April 15 Are Young Massive Clusters the Same as Globular Clusters? ✤ 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. Tuesday, 14 April 15 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 CMD Peacock et al. 2013, Bastian et al. 2013a Tuesday, 14 April 15 Bastian et al. 2009 Tuesday, 14 April 15 Bastian et al. 2009 Tuesday, 14 April 15 Bastian et al.
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