CN Variations in Globular Clusters Jason Smolinski originally presented 08/11/2010 encore presentation 08/25/2010
Outline I. What Are We Talking About? a) Cluster Environment b) Expectations from Theory II. What Do We See? a) Observations to Date III. What Does It Mean? a) Proposed Explanations b) Implications for Stellar Evolution Theory
Cluster Formation Classical formation scenarios: all stars form in a single event from a single chemically homogeneous protocloud Stars should have same age and chemical composition Studies suggest that it s not that simple
Cluster Environment Globular Clusters ~10 5-10 6 stars Densely distributed (avg. ~100 pc -3 in the core) Old, generally metal-poor Stars typically retained long enough to evolve together Lifetimes ~10 Gyr Field stars typical density ~0.1 pc^-3 in solar neighborhood Open Clusters ~10 3 stars Loosely distributed (avg. ~10 pc -3 in the core) Younger, generally metalrich Stars may disperse before the population has evolved significantly Lifetimes ~0.5-1 Gyr
Globular Clusters What is expected Typical star undergoes H-shell burning while ascending RGB First dredge-up transports 12 C inward and 14 N outward He flash at the RGB tip, then migrates to the HB and burns He Second dredge-up afterward further increases 14 N in the envelope Ascends AGB with a C-O degenerate core and He- and H-shell burning Third dredge-up (mass dependent) may increase C abundance in the envelope
Globular Clusters What is observed Typical star undergoes H-shell burning while ascending RGB Envelope already shows abundance variations in studied clusters, even on MSTO He flash at the RGB tip, then migrates to the HB and burns He Abundance variations appear to diminish Ascends AGB with a C-O degenerate core and He- and H-shell burning Envelopes are depleted in nitrogen; most cluster stars have weak CN-features, only a few are CN-strong
Globular Clusters vs. Open Clusters CN-enhancement in cluster stars first studied in earnest on the RGB in the 1970s, SGB in the 1980s, and upper MS in the late 1990s and 2000s CN distribution seen to be bimodal in nearly every globular cluster Abundance variations in other light elements (eg. C, N, O, Na, Mg, Al) have been observed on the RGB and SGB in most clusters and even on MS for some Abundance variations are generally not seen at significant levels in open clusters
What Could Be Happening? Several possible scenarios: 1. Additional deep mixing events - Deep mixing isn t expected prior to RGB ascent - Would have to be true for GC stars but not non- GC stars 2. Self-pollution - Accretion of material from AGB stellar winds 3. Primordial variations - Assuming homogeneity may be incorrect
283 RGB stars from 47 Tuc Briley, 1997, AJ, 114, 1051
Variation on the Main Sequence 115 MS stars from 47 Tuc Harbeck, Smith, & Grebel 2003, AJ, 125, 197
Theory does not match observation CN-strong stars are observed at points earlier than expected Indicates that N-enrichment is setting in early CN bimodality is observed in nearly every GC studied Indicates that N-enrichment is not occurring uniformly in all stars And another thing Disk and bulge GCs seem to have higher N-enrichment than halo GCs
Globular and Open Cluster Sample Our sample contains SEGUE spectra for >14,000 stars from: 8 Globular clusters (GCs): M2, M3, M13, M15, M53, M71, M92, NGC 5053 5 Open clusters (OCs): M35, M67, NGC 2158, NGC 2420, NGC 6791 649 true GC member stars selected 420 true OC member stars selected ---------------------------------- Also included are 5 GCs w/ corrected data from Kayser et al. (2008) M15, M22, NGC 288, NGC 362, NGC 5286
Globular and Open Cluster Sample Our sample contains SEGUE spectra for >14,000 stars from: 8 Globular clusters (GCs): M2, M3, M13, M15, M53, M71, M92, NGC 5053 5 Open clusters (OCs): M35, M67, NGC 2158, NGC 2420, NGC 6791 649 true GC member stars selected 420 true OC member stars selected
CH G-Band CN Band 10/21/09 Jason Smolinski - Michigan State University 14
Questions 1. Is there a particular point on the CMD at which CN bimodality appears in each cluster? Investigates primordial inhomogeneities vs. additional mixing 2. How does the degree of bimodality change along the CMD, if at all? Investigates known mixing vs. additional mixing 3. How do the cluster giant CN abundances compare to those of field giants from SEGUE-2? Investigates self-pollution 4. How do the globular clusters compare to the open clusters in our sample and is that consistent with past studies?
What are we left with? Mass loss at the RGB tip prior to migration onto the HB? (Must check which stars are AGB in M13) Is the dispersion on M13 s MS significant? What sets the amount of disparity between CNstrong and CN-weak stars? Why don t we see bimodality on the MSTO and SGB like other studies have?
Implications Post-dredge-up mass loss at the RGB tip and on the AGB, along with early SNe, may pollute other stars, creating an apparent CN-strong population What does this imply about the cluster IMF? (How much pollution is needed?) May explain why open clusters lack CNstrong stars (insufficient gravity to retain enriched gas) but does NOT explain why disk GCs have higher CN enrichment than halo GCs
Stay tuned!