PROBING THE MILKY WAYS 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 Yun Wang Cosmology Dark matter Dark energy Karen Leighly Active Galactic nuclei

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 Yun Wang Cosmology Dark matter Dark energy Karen Leighly Active Galactic nuclei

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 Yun Wang Cosmology Dark matter Dark energy Karen Leighly Active Galactic nuclei

OUTLINE 1.Introduction to chemical evolution of galaxies 2.Abundances and abundance gradients 3.Planetary Nebula abundance study 4.Statistics and the inferred gradient 5.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

Gas pressure outwardGravity inward

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

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

Stellar Evolution Gas pressure outwardGravity inward 4 1 H --> 4 He 3 4 He --> 12 C 12 C + 4 He --> 16 O 16 O + 4 He --> 20 Ne 20 Ne + 4 He --> 24 Mg 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 Time Heavy element abundances Age-Metallicity Relation

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

Abundance gradients constrain: 1. Star formation efficiency 2. Star formation history 3. Galactic disk formation rate WHAT DO ABUNDANCE GRADIENTS TELL US?

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 Nebula Expanding envelope from dying star Contains O, S, Ne, Ar, Cl at original interstellar levels C, N altered during stars lifetime Heated by stellar UV photons Cooled through emission line losses PLANETARY NEBULAE

THE PN SAMPLE Number: 124 Location: MWG disk Distance range: kpc (~3-60 x 10 3 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-e - collision Radiative de- excitation Photon production h

Calculating Abundances from Emission Lines Abundance Software Measure

Results: 12+log(O/H) vs. R g

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) Χ 2, reduced X 2, and X 2 probability

RESULTS: Trial #1 a = 9.15 (+/-.04) b = (+/-.006) r = (r 2 =.29) χ ν = 1.46 q χ2 = (<.05) Gradient = dex/kpc

Improving the Linear Model Assume statistical errors dont account for all of the observed scatter in O abundances Add natural scatter to statistical O/H abundance errors σ total = 1.4 x σ stat

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

a = 9.09 (+/-.05) b = (+/-.006) r = (r 2 =.29) χ ν = 1.00 q χ2 = 0.49 (>.05) 2 RESULTS: Trial #2 Gradient = 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 = /-.013 dex/kpc Rg > 10 kpc gradient = /-.14 dex/kpc

Quadratic Fit 12+log(O/H) = 8.81 – 0.014R g R g 2

Compare with Stanghellini & Haywood

Comparisons with Other Object Types

COMPARISONS

CONFUSION LIMIT Observed range in O/H gradient: to 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? STAR FORMATION THRESHOLD (M pc -2 )PREDICTED GRADIENT (dex kpc -1 ) Observed gradient range: to dex kpc -1 Marcon-Uchida (2010): Sensitivity to star formation threshold DISK FORMATION TIMESCALEPREDICTED GRADIENT RANGE (dex kpc -1 ) Begins at galaxy formation, disk-wide to Increases with distance from center to Fu et al. (2009): Sensitivity to the timescale for disk formation

CONCLUSIONS 1.We obtain a new O/H gradient of /-.006 dex kpc A good linear model of the data requires the assumption of natural scatter. 3.Observed gradient range ~ to 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

Distance from galaxys center Heavy element abundances 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 T e ~ 10 4 K Density ~ % H, 8% He, 2% metals

Measuring Abundances: Spectra Emission spectrum Absorption spectrum