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PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH PLANETARY NEBULAE

PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH PLANETARY NEBULAE. Collaborators: Karen Kwitter (Williams College) Anne Jaskot (University of Michigan) Bruce Balick (University of Washington) Mike Morrison (University of Oklahoma) Jackie Milingo (Gettysburg College). Dick Henry

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PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH PLANETARY NEBULAE

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  1. PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH PLANETARY NEBULAE Collaborators: Karen Kwitter (Williams College) Anne Jaskot (University of Michigan) Bruce Balick (University of Washington) Mike Morrison (University of Oklahoma) Jackie Milingo (Gettysburg College) Dick Henry H.L. Dodge Department of Physics & Astronomy University of Oklahoma Thanks to the National Science Foundation for partial support.

  2. Homer L. Dodge Department of Physics & Astronomy University of Oklahoma Astrophysics and Cosmology Atomic and Molecular Physics Condensed Matter Physics High Energy Physics

  3. ASTRONOMY AT Bill Romanishin Solar system Eddie Baron Supernova studies David Branch Supernova studies Karen Leighly Active Galactic nuclei John Cowan Chemical evolution Milky Way studies Supernova remnants Yun Wang Cosmology Dark matter Dark energy Dick Henry Chemical evolution Galaxies Nebular abundances

  4. ASTRONOMY AT Bill Romanishin Solar system Eddie Baron Supernova studies David Branch Supernova studies Karen Leighly Active Galactic nuclei John Cowan Chemical evolution Milky Way studies Supernova remnants Yun Wang Cosmology Dark matter Dark energy Dick Henry Chemical evolution Galaxies Nebular abundances

  5. ASTRONOMY AT Bill Romanishin Solar system Eddie Baron Supernova studies David Branch Supernova studies Karen Leighly Active Galactic nuclei John Cowan Chemical evolution Milky Way studies Supernova remnants Yun Wang Cosmology Dark matter Dark energy Dick Henry Chemical evolution Galaxies Nebular abundances

  6. OUTLINE • Introduction to chemical evolution of galaxies • Abundances and abundance gradients • Planetary Nebula abundance study • Statistics and the inferred gradient • Conclusions

  7. MILKY WAY MORPHOLOGY • Halo • Bulge • Disk • Dark Matter Halo

  8. 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

  9. CHEMICAL EVOLUTION OF A GALAXY Stars produce heavy elements Stars expel products into the interstellar medium INTERSTELLAR MEDIUM

  10. Stellar Evolution

  11. Stellar Evolution

  12. Stellar Evolution Gas pressure outward Gravity inward

  13. Stellar Evolution 4 1H --> 4He 3 4He --> 12C 12C + 4He --> 16O 16O + 4He --> 20Ne 20Ne + 4He --> 24Mg Gas pressure outward Gravity inward Stellar Nucleosynthesis Reactions

  14. Stellar Evolution 4 1H --> 4He 3 4He --> 12C 12C + 4He --> 16O 16O + 4He --> 20Ne 20Ne + 4He --> 24Mg Gas pressure outward Gravity inward Stellar Nucleosynthesis Reactions Supernova

  15. Stellar Evolution 4 1H --> 4He 3 4He --> 12C 12C + 4He --> 16O 16O + 4He --> 20Ne 20Ne + 4He --> 24Mg Gas pressure outward Gravity inward Stellar Nucleosynthesis Reactions Supernova Planetary Nebula

  16. 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

  17. Global Results of Chemical Evolution Oxygen Abundance Gradient Abundance gradient  Star formation history

  18. WHAT DO ABUNDANCE GRADIENTS TELL US? Abundance gradients constrain: Star formation efficiency Star formation history Galactic disk formation rate

  19. 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

  20. Abundance Probes of the Interstellar Medium • Stellar atmospheres: absorption lines • H II Regions: emission lines • Planetary Nebulae: emission lines

  21. 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

  22. Measuring gas phase abundances

  23. THE PN SAMPLE • Number: 124 • Location: MWG disk • Distance range: 0.9-21 kpc (~3-60 x 103ly) from center of galaxy • Data reduced and measured in homogenous fashion • Oxygen abundances for all 124 PNe • Galactocentric distances from Cahn et al. (1992)

  24. Data Gathering CTIO 1.5m KPNO 2.1m APO: 3.5m

  25. Emission Spectrum

  26. The Physics of Emission Lines • Bound-bound transition • Inelastic ion-e- collision • Radiative de-excitation • Photon production h

  27. Calculating Abundances from Emission Lines Abundance Software Measure

  28. Results: 12+log(O/H) vs. Rg

  29. 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

  30. 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

  31. 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

  32. 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

  33. RESULTS: Trial #2 • a = 9.09 (+/- .05) • b = -0.058 (+/- .006) • r = -0.54 (r2=.29) • χν = 1.00 • qχ2 = 0.49 (>.05) 2 Gradient = -0.058 dex/kpc

  34. 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

  35. Two-part Linear Fit Rg < 10 kpc gradient = -0.054+/-.013 dex/kpc Rg > 10 kpc gradient = -0.12 +/-.14 dex/kpc

  36. Quadratic Fit 12+log(O/H) = 8.81 – 0.014Rg -0.001Rg2

  37. Compare with Stanghellini & Haywood

  38. Comparisons with Other Object Types

  39. COMPARISONS

  40. CONFUSION LIMIT • Observed range in O/H gradient: -0.02 to -0.06 dex/kpc Improvement will depend upon knowing: • Better distances to abundance probes • Origin of natural scatter

  41. Is Improving Gradient Accuracy Worth the Effort? Marcon-Uchida (2010): Sensitivity to star formation threshold Fu et al. (2009): Sensitivity to the timescale for disk formation Observed gradient range: -0.02 to -0.06 dex kpc-1

  42. CONCLUSIONS • We obtain a new O/H gradient of -0.058 +/- .006 dex kpc-1. • A good linear model of the data requires the assumption of natural scatter. • Observed gradient range ~ -0.02 to -0.06 dex kpc-1. We are at the confusion limit. • Improvements will come with better distances and the understanding of the natural scatter. • The endeavor is worthwhile for understanding the evolution of our Galaxy.

  43. SN 1987A: 2/23/87

  44. Heavy element abundances Distance from galaxy’s center Disk Abundance Gradient

  45. OTHER SPIRALS

  46. NEBULAE AS PROBES OF THE INTERSTELLAR MEDIUM

  47. 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

  48. Measuring Abundances: Spectra • Emission spectrum • Absorption spectrum

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