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PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH PLANETARY NEBULAE Dick Henry H.L. Dodge Department of Physics & Astronomy University of Oklahoma Collaborators: Karen Kwitter (Williams College) Anne Jaskot (University of Michigan) Bruce Balick (University of Washington) Mike Morrison (University of Oklahoma) Jackie Milingo (Gettysburg College) Thanks to the National Science Foundation for partial support. Homer L. Dodge Department of Physics & Astronomy University of Oklahoma Astrophysics and Cosmology Atomic and Molecular Physics Condensed Matter Physics High Energy Physics ASTRONOMY AT Eddie Baron Supernova studies David Branch Supernova studies John Cowan Chemical evolution Milky Way studies Supernova remnants Dick Henry Chemical evolution Galaxies Nebular abundances Bill Romanishin Solar system Karen Leighly Active Galactic nuclei Yun Wang Cosmology Dark matter Dark energy ASTRONOMY AT Eddie Baron Supernova studies Bill Romanishin Solar system Karen Leighly Active Galactic nuclei John Cowan Chemical evolution Milky Way studies Supernova remnants Dick Henry Chemical evolution Galaxies Nebular abundances Yun Wang Cosmology Dark matter Dark energy ASTRONOMY AT Eddie Baron Supernova studies Karen Leighly Active Galactic nuclei Dick Henry Chemical evolution Galaxies Nebular abundances Yun Wang Cosmology Dark matter Dark energy OUTLINE 1. 2. 3. 4. 5. Introduction to chemical evolution of galaxies Abundances and abundance gradients Planetary Nebula abundance study Statistics and the inferred gradient Conclusions MILKY WAY MORPHOLOGY • • • • Halo Bulge Disk Dark Matter Halo Galactic Chemical Evolution The conversion of H, He into metals over time Stars produce heavy elements Stars expel products into the interstellar medium New stars form from enriched material CHEMICAL EVOLUTION OF A GALAXY Stars produce heavy elements Stars expel products into the interstellar medium INTERSTELLAR MEDIUM Stellar Evolution Stellar Evolution Stellar Evolution Gas pressure outward Gravity inward Stellar Evolution 4 1H --> 4He Gas pressure outward Gravity inward 3 4He --> 12C 12C + 4He --> 16O 16O + 4He --> 20Ne 20Ne + 4He --> 24Mg Stellar Nucleosynthesis Reactions Stellar Evolution 4 1H --> 4He Gas pressure outward Gravity inward 3 4He --> 12C 12C + 4He --> 16O 16O + 4He --> 20Ne 20Ne + 4He --> 24Mg Stellar Nucleosynthesis Reactions Supernova Stellar Evolution 4 1H --> 4He Gas pressure outward Gravity inward 3 4He --> 12C 12C + 4He --> 16O 16O + 4He --> 20Ne 20Ne + 4He --> 24Mg Stellar Nucleosynthesis Reactions Supernova Planetary Nebula Local Results of Galactic Chemical Evolution 1. INTERSTELLAR MEDIUM BECOMES RICHER IN HEAVY ELEMENTS 2. NEXT STELLAR GENERATION CONTAINS MORE HEAVY ELEMENTS Heavy element abundances Age-Metallicity Relation Time Global Results of Chemical Evolution Oxygen Abundance Gradient Abundance gradient Star formation history WHAT DO ABUNDANCE GRADIENTS TELL US? Abundance gradients constrain: 1. Star formation efficiency 2. Star formation history 3. Galactic disk formation rate Project Goal •Measure the oxygen gradient in the ISM of the Milky Way disk •Employ planetary nebulae as abundance probes •Perform detailed statistical treatment of data Abundance Probes of the Interstellar Medium •Stellar atmospheres: absorption lines •H II Regions: emission lines •Planetary Nebulae: emission lines PLANETARY NEBULAE •Planetary Nebula •Expanding envelope from dying star •Contains O, S, Ne, Ar, Cl at original interstellar levels •C, N altered during star’s lifetime •Heated by stellar UV photons •Cooled through emission line losses THE PN SAMPLE • Number: 124 • Location: MWG disk • Distance range: 0.9-21 kpc (~3-60 x 103 ly) from center of galaxy • Data reduced and measured in homogenous fashion • Oxygen abundances for all 124 PNe • Galactocentric distances from Cahn et al. (1992) Data Gathering CTIO 1.5m KPNO 2.1m APO: 3.5m Emission Spectrum The Physics of Emission Lines • Bound-bound transition • Inelastic ion-ecollision • Radiative deexcitation • Photon production h Calculating Abundances from Emission Lines I(el) I(H) Abundance Software Measure I(el) N(el) f (t,n) C I(H) N(H) N (el) N(H) Results: 12+log(O/H) vs. Rg Statistical Analysis Least squares fitting Input: • Stats program: fitexy (Numerical Recipes, Press et al. 2003) • Data points: 124 (122 degrees of freedom) • Errors: 1 σ errors in both O abundances and distances • O errors: propagated through abundance calculations • Distance errors: standard 20% Output: • Correlation coefficient and its probability • Slope (b) & intercept (a) • Χ2, reduced X2, and X2 probability RESULTS: Trial #1 • a = 9.15 (+/- .04) • b = -0.066 (+/.006) • r = -0.54 (r2=.29) • χν = 1.46 • qχ2 = 0.00074 (<.05) Gradient = -0.066 dex/kpc Improving the Linear Model • Assume statistical errors don’t account for all of the observed scatter in O abundances • Add natural scatter to statistical O/H abundance errors • σtotal = 1.4 x σstat Natural Scatter • Poor mixing of stellar products in the ISM • Stellar diffusion: stars migrate from place of birth to present location • Age spread among PN progenitors RESULTS: Trial #2 • a = 9.09 (+/- .05) • b = -0.058 (+/.006) • r 2= -0.54 (r2=.29) • χν = 1.00 • qχ2 = 0.49 (>.05) Gradient = -0.058 dex/kpc Different Models •Gradient steepens in outer regions (Pedicelli et al. 2009; Fe/H) •Gradient flattens in outer regions (Maciel & Costa 2009; O/H) 2-part linear quadratic Two-part Linear Fit Rg < 10 kpc gradient = -0.054+/-.013 dex/kpc Rg > 10 kpc gradient = -0.12 +/-.14 dex/kpc Quadratic Fit 12+log(O/H) = 8.81 – 0.014Rg -0.001Rg2 Compare with Stanghellini & Haywood Comparisons with Other Object Types COMPARISONS CONFUSION LIMIT • Observed range in O/H gradient: -0.02 to -0.06 dex/kpc Improvement will depend upon knowing: 1. Better distances to abundance probes 2. Origin of natural scatter Is Improving Gradient Accuracy Worth the Effort? Marcon-Uchida (2010): Sensitivity to star formation threshold STAR FORMATION THRESHOLD (M pc-2) PREDICTED GRADIENT (dex kpc-1) 7.0 -0.059 4.0 -0.025 Fu et al. (2009): Sensitivity to the timescale for disk formation DISK FORMATION TIMESCALE PREDICTED GRADIENT RANGE (dex kpc-1) Begins at galaxy formation, disk-wide -0.009 to -0.027 Increases with distance from center -0.056 to -0.091 Observed gradient range: -0.02 to -0.06 dex kpc-1 CONCLUSIONS 1. We obtain a new O/H gradient of -0.058 +/- .006 dex kpc-1. 2. A good linear model of the data requires the assumption of natural scatter. 3. Observed gradient range ~ -0.02 to -0.06 dex kpc-1. We are at the confusion limit. 4. Improvements will come with better distances and the understanding of the natural scatter. 5. The endeavor is worthwhile for understanding the evolution of our Galaxy. SN 1987A: 2/23/87 Heavy element abundances Distance from galaxy’s center Disk Abundance Gradient OTHER SPIRALS NEBULAE AS PROBES OF THE INTERSTELLAR MEDIUM H II REGIONS • Photoionized and heated by young hot central star(s) • Radiatively cooled via emission lines • Te ~ 104 K • Density ~ 10-102 • 90% H, 8% He, 2% metals Measuring Abundances: Spectra • Emission spectrum • Absorption spectrum