Chemical tagging with Ricardo Schiavon Stellar Populations Newton Meeting São Paulo, November 30, 2015
Chemical tagging with Discovery of a new stellar population in the Galactic bulge
Astrophysics Research Institute Liverpool John Moores University (LJMU) 23 staff! 8 postdocs! Cosmology! Galaxy formation/evolution! Star formation! Stellar astrophysics! Time domain 29 grad students Theory! Observations! Instrumentation! Public Outreach
Outline 1. Multiple populations in globular clusters! 2. APOGEE! 3. Discovery of nitrogen rich stars in the bulge field! 4. Possible scenarios
Piotto et al. 2002 GCs as single stellar populations
GCs as multiple stellar populations Multiple populations identified in the color magnitude diagrams of MW globular clusters High precision photometry and combination of UV and optical filters required Piotto et al. 2015
GCs as multiple stellar populations Abundances of C and N in GC stars vary and are anti-correlated Known for 30+ years Other anti-correlations also present, including Na-O, Mg-Al Carretta et al. 2005
GCs as multiple stellar populations Secondary populations dominate the mass in GCs today SP/PP ~ 2 Secondary! pop Primary! pop Carretta et al. 2005
Features of GC multiple pops Renzini et al. 2015 GC specificity Ubiquity Variety SP Predominance Discreteness Chemical diversity in light elements No SNe enrichment (in most GCs) Mass budget problem Carretta et al. 2005
Features of GC multiple pops Renzini et al. 2015 GC specificity! Ubiquity Variety SP Predominance Discreteness Chemical diversity in light elements No SNe enrichment (in most GCs) Mass budget problem Carretta et al. 2005
Scenarios 1. A history of SF and chemical evolution. Possible nucleosynthetic sources: Supermassive stars (10 4 MSun) Fast rotating massive stars Massive interacting binaries AGB stars 2. Early disk accretion None of the models accounts for the data!
The mass budget problem Today: SP/PP ~ 2 For any reasonable IMF, there weren t enough PP stars to generate the observed mass in enriched elements All models invoke a much larger mass in PP stars in the past Past: SP/PP ~ 10-100 Secondary! pop Primary! pop Carretta et al. 2005
Weak chemical tagging Martell & Grebel (2010), Carretta et al. (2010): CN-strong stars in the halo field (see also Lind et al. 2015)! Up to 17% of halo mass originated in GCs! Gratton et al. (2012): Essentially all the halo made from GC dissolution! Key difference is assumption of SP/PP Carollo et al. (2013)
Apache Point Observatory Galactic Evolution Experiment
APOGEE: an infrared, high resolution spectroscopic survey of the stellar populations of the Galaxy! BOSS: will measure the cosmic distance scale via clustering in the large-scale galaxy distribution and the Lyman-α forest! SEGUE-2: will map the structure, kinematics, and chemical evolution of the outer Milky Way disk and halo! MARVELS: will probe the population of giant planets via radial velocity monitoring of 11,000 stars
APOGEE at a Glance Bright time SDSS-III survey, 2011.Q2 to 2014.Q2 300 fiber, R 22,500, cryogenic spectrograph, 7 deg 2 FOV H-band: 1510 1690 nm (A H /A V ~ 1/6) Typical S/N = 100/pixel @ H=12.2 for 3-hr integration RV uncertainty spec < 500 m/s in 3 hr Actual < 100 m/s in 1hr 0.1 dex precision abundances for 15 chemical elements (including C, N, O, Fe, other α, odd-z, possibly neutron-capture) 10 5 2MASS-selected [(J-K) > 0.5] predominantly giant stars, probing all Galactic populations
Why APOGEE makes a difference High resolution * Near Infrared * 10 5 giant stars
Why APOGEE makes a difference High (enough) resolution * Near Infrared * 10 5 giant stars
APOGEE Field Plan
Target Selection Main sample is selected from 2MASS (J-K)0 0.5, 7 < H < 13.8 => giants (RGB, AGB, RC) are 80% of the sample Open and globular clusters targeted for science and calibration purposes Ancillary science programs cover a variety of science targets (young Galactic clusters, M dwarfs, M31 GCs in integrated light) For details, see Zasowski et al. (2013)
Spatial Coverage - DR12 Bulge Disk
APOGEE Spectral Analysis Example: spectral fits around CO lines For details, see Holtzman et al. (2015), García Pérez et al. (2015), Majewski et al. (2015)
Performance Survey quality spectra for ~ 150,000 stars (50% over initial goal) RV precision of 100 m/s, exceeding original requirement. Accuracy 1 km/s Abundances pipeline (ASPCAP) delivers precision of 100 K in Teff, 0.15 dex in log g, 0.03-0.08 dex in [Fe/H], and 0.03 dex in [α/fe] Pipeline delivers abundances of 15 elements with 0.1 dex precision, 0.2 dex accuracy (in most cases). Issues with a few elements. Work in progress. Look out for DR13!! Data available publicly as part of Data Release 12 (DR12) since January 2015.
Slide from Gail Zasowski Science Working Group Chair: Jo Bovy (IAS) Text
C and N abundances in the bulge A population of stars with high N and an N-C anticorrelation was identified in the bulge
C and N abundances in the bulge They occupy the same locus as GC stars in this diagnostic plot
Al vs N correlation These N-rich stars also occupy the same locus as GC stars in [Al/Fe] vs [N/Fe] space!!!!
Al vs N correlation A correlation between the abundances of N and Al is also found, both in the bulge and in the GC sample!!!!
Hypothesis These stars are secondary populations from globular clusters, which got somehow lost to the field 1. What fraction of the bulge/inner halo is contributed by dissolved GCs? 2. How much mass is there in dissolved vs existing GCs? In order to answer those questions, we need to know what the ratio SP/PP was in the dissolved GCs
SP/PP Ratio To answer these questions, we need to know the value of the SP/PP ratio in dissolved GCs. 1. Today: SP/PP ~ 2 2. Models require: SP/PP ~ 10-100 in the past
However, our observations only identify the analogs of so called secondary populations SP/PP ratio
SP/PP ratio S However, our observations only identify the analogs of so called secondary populations S
SP/PP ratio The primary populations are indistinguishable from field stars of same [Fe/H]! We have to assume a SP/PP ratio P S S P
Minimum mass in dissolved GCs Assuming SP/PP has always been as observed in GCs today: ~ 2/1 Assuming same ratio for bulge field, one gets: M ~ 1.5% of M Bulge ~ 1.5-3 10 8 M Sun M ~ 17% of M Inner Halo (within 2 kpc) M ~ 5-10 x M GC system
Metallicity distributions Bulge Field N-rich stars Galactic GCs
Metallicity Distributions 1. Bulge field and N-rich MDF are DIFFERENT: hard to build the bulge from GCs (not surprising)
Metallicity distributions Bulge Field N-rich stars Galactic GCs
Metallicity Distributions 1. Bulge field and N-rich MDF are DIFFERENT: hard to build the bulge from GCs (not surprising) 2. Bulge field vs N-rich MDF at [Fe/H] < -1: primary population in GCs was at most ~90% of total stellar mass in the past
Metallicity distributions Bulge Field N-rich stars Galactic GCs
Metallicity Distributions 1. Bulge field and N-rich MDF are DIFFERENT: hard to build the bulge from GCs (not surprising) 2. Bulge field vs N-rich MDF at [Fe/H] < -1: primary population in GCs was at most ~90% of total stellar mass in the past 3. GCs and N-rich MDFs are DIFFERENT: contribution from tidal evaporation of existing GCs to N-rich population is negligible. If these stars ever lived in GCs, they were completely destroyed
Summary Assuming GC origin Newly discovered population, with GC chemistry, homogeneously distributed across the bulge At least 5-10 times more mass in destroyed than existing globular clusters Mass in primary populations cannot have been larger than ~90% of the total GC mass. It was probably less than that Cannot build bulge (or halo?) from GCs New population results from destruction of a large population of early GCs
But what if these stars were never associated with GCs to begin with?
Features of GC multiple pops Renzini et al. 2015 GC specificity Ubiquity Variety SP Predominance Discreteness Chemical diversity in light elements No SNe enrichment (in most GCs) Mass budget problem Carretta et al. 2005
Features of GC multiple pops Renzini et al. 2015 GC specificity! Ubiquity Variety SP Predominance Discreteness Chemical diversity in light elements No SNe enrichment (in most GCs) Mass budget problem Carretta et al. 2005
But what if these stars were never associated with GCs to begin with? N-rich stars were first found in GCs. Doesn t mean they necessarily can only form there GCs and N-rich MDFs don t match, suggesting different origin If N-rich stars can be generated outside GCs, no need to devise complicated destruction mechanisms But mass budget problem persists
Summary We have discovered a population of stars in the bulge that look chemically like those found in GCs Spatial distribution indistinguishable from the rest of the bulge If they come from GCs, then a very large population of early GCs were completely destroyed In that scenario, a very small fraction of the bulge mass was contributed by GC dissolution (few %). If in halo, about 20% of stellar mass in inner halo comes from GC dissolution. Our results point to a worsening of the mass budget problem. There may not be a genetic link between PP and SP.