PROBING THE MILKY WAYS OXYGEN GRADIENT WITH PLANETARY NEBULAE Dick Henry H.L. Dodge Department of...
Transcript of PROBING THE MILKY WAYS OXYGEN GRADIENT WITH PLANETARY NEBULAE Dick Henry H.L. Dodge Department of...
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PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH PLANETARY NEBULAE
Dick HenryH.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.
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Homer L. Dodge Department of Physics & AstronomyUniversity of Oklahoma
Astrophysics and CosmologyAtomic and Molecular Physics
Condensed Matter PhysicsHigh Energy Physics
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ASTRONOMY AT
Eddie BaronSupernova studies
David BranchSupernova studies
John CowanChemical evolutionMilky Way studiesSupernova remnants
Dick HenryChemical evolutionGalaxiesNebular abundances
Bill RomanishinSolar system
Yun WangCosmologyDark matterDark energy
Karen LeighlyActive Galactic nuclei
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ASTRONOMY AT
Eddie BaronSupernova studies
John CowanChemical evolutionMilky Way studiesSupernova remnants
Dick HenryChemical evolutionGalaxiesNebular abundances
Bill RomanishinSolar system
Yun WangCosmologyDark matterDark energy
Karen LeighlyActive Galactic nuclei
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ASTRONOMY AT
Eddie BaronSupernova studies
Dick HenryChemical evolutionGalaxiesNebular abundances
Yun WangCosmologyDark matterDark energy
Karen LeighlyActive Galactic nuclei
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OUTLINE
1. Introduction to chemical evolution of galaxies
2. Abundances and abundance gradients3. Planetary Nebula abundance study4. Statistics and the inferred gradient5. Conclusions
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MILKY WAY MORPHOLOGY
• Halo• Bulge• Disk• Dark Matter Halo
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Galactic Chemical EvolutionThe conversion of H, He into metals over time
Stars produce heavy elements
Stars expel products into the interstellar medium
New stars form fromenriched material
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CHEMICAL EVOLUTION OF A GALAXY
Stars produce heavy elements
Stars expel products into the interstellar medium
INTERSTELLARMEDIUM
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Stellar Evolution
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Stellar Evolution
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Stellar Evolution
Gas pressure outward Gravity inward
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Stellar Evolution
Gas pressure outward Gravity inward
4 1H --> 4He
3 4He --> 12C
12C + 4He --> 16O
16O + 4He --> 20Ne
20Ne + 4He --> 24MgStellar Nucleosynthesis Reactions
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Stellar Evolution
Gas pressure outward Gravity inward
4 1H --> 4He
3 4He --> 12C
12C + 4He --> 16O
16O + 4He --> 20Ne
20Ne + 4He --> 24MgStellar Nucleosynthesis Reactions
Supernova
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Stellar Evolution
Gas pressure outward Gravity inward
4 1H --> 4He
3 4He --> 12C
12C + 4He --> 16O
16O + 4He --> 20Ne
20Ne + 4He --> 24MgStellar Nucleosynthesis Reactions
Supernova
Planetary Nebula
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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
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Global Results of Chemical EvolutionOxygen Abundance Gradient
Abundance gradient Star formation history
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Abundance gradients constrain:
1. Star formation efficiency
2. Star formation history
3. Galactic disk formation rate
WHAT DO ABUNDANCE GRADIENTS TELL US?
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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
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Abundance Probes of the Interstellar Medium
•Stellar atmospheres: absorption lines
•H II Regions: emission lines
•Planetary Nebulae: emission lines
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•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
PLANETARY NEBULAE
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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)
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Data Gathering
CTIO 1.5m KPNO 2.1m APO: 3.5m
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Emission Spectrum
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The Physics of Emission Lines
• Bound-bound transition
• Inelastic ion-e- collision
• Radiative de-excitation• Photon production
h
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Calculating Abundances from Emission Lines
I(el)
I(H) f (t,n)C
N(el)
N(H)
I(el)
I(H)
N(el)
N(H)
Abundance Software
Measure
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Results: 12+log(O/H) vs. Rg
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Statistical AnalysisLeast 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
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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
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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
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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
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• a = 9.09 (+/- .05)• b = -0.058 (+/- .006)• r = -0.54 (r2=.29)• χν = 1.00
• qχ2 = 0.49 (>.05)
2
RESULTS: Trial #2
Gradient = -0.058 dex/kpc
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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 linearquadratic
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Two-part Linear Fit
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Quadratic Fit
12+log(O/H) = 8.81 – 0.014Rg -0.001Rg2
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Compare with Stanghellini & Haywood
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Comparisons with Other Object Types
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COMPARISONS
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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 probes2.Origin of natural scatter
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Is Improving Gradient Accuracy Worth the Effort?
STAR FORMATION THRESHOLD (M pc-2) PREDICTED GRADIENT (dex kpc-1)
7.0 -0.059
4.0 -0.025
Observed gradient range: -0.02 to -0.06 dex kpc-1
Marcon-Uchida (2010): Sensitivity to star formation threshold
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
Fu et al. (2009): Sensitivity to the timescale for disk formation
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CONCLUSIONS1. 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.
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SN 1987A: 2/23/87
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Distance from galaxy’s center
Heavy element abundances
Disk Abundance Gradient
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OTHER SPIRALS
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NEBULAE AS PROBES OF THE INTERSTELLAR MEDIUM
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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
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Measuring Abundances: Spectra
• Emission spectrum
• Absorption spectrum