Transcript PROBING THE MILKY WAY’S OXYGEN GRADIENT WITH …

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
•
•
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•
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)
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Data points: 124 (122 degrees of freedom)
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Errors: 1 σ errors in both O abundances and distances
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O errors: propagated through abundance calculations
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Distance errors: standard 20%
Output:
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Correlation coefficient and its probability
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Slope (b) & intercept (a)
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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
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Photoionized and heated by young
hot central star(s)
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Radiatively cooled via emission
lines
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Te ~ 104 K
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Density ~ 10-102
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90% H, 8% He, 2% metals
Measuring Abundances: Spectra
• Emission spectrum
• Absorption spectrum