Photometric Analysis of Asteroids
Sara Barber
Acknowledgements:
Dr. Bill and Erin Cooper
Project Evolution• Old Project: Opposition Effect of Dark Asteroids
– Goal: • Make photometric observations of asteroids with low
reflectivity near opposition• Create lightcurves for these asteroids
– Problem:• CCD malfunction
• New Project: Photometric Analysis of Trojan Asteroids– Goal:
• Analyze previously obtained images of Trojan asteroids• Create lightcurves for these asteroids
Motivation• My Goal: Create lightcurves for Trojan
asteroids
• Future Goal: Combine lightcurves throughout asteroid’s orbit to determine 3-D shape– Shape & Spin Rate Density
• The density could put a limit on the asteroid’s composition.
– Trojan Composition v.s. Main-Belt Composition• Different origins within the solar system?• Better understanding of solar system’s evolution
Outline• Trojan Asteroids
• Lightcurves
• Photometry Steps– CCD Photometry– Image Processing– Complications– Measuring– Calibration– Lightcurves
• Results
Preparing for a night of observing.
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http://epsc.wustl.edu
Trojan Asteroids• Asteroids in orbit around Jupiter’s 4th and 5th
Lagrange points
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Trojan Asteroids
Main Belt Asteroids
http://cseligman.com
LightcurveLightcurve
Lightcurve
Lightcurves• Lightcurve: change in brightness throughout
rotation– More illuminated surface area brighter– Less illuminated surface area dimmer
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Asteroid Orbit
Photometry• Photometry: technique for measuring an
object’s brightness
• Steps– Take exposures– Process images– Measure object’s brightness– Calibrate measurements– Create lightcurve
STEP 1: CCD Photometry• Charged Coupled Device (CCD)
– Photon hits Si substrate & photoexcites e-
• 1 photon = 1 e-
– Electrons trapped in “pixels” by electrodes w/
applied voltage– Get series of numbers
that are reconstructed to
make image
Valence Band
Conduction Band
CCD Electrodes
CCD:Top View
STEP 2: Image Processing• Want uniform background
• Sources of Background Inhomogeneity:– Thermal Signal Thermal energy is enough to kick
electrons into conduction band (CCD not cooled uniformly have gradient of thermal signal)
• Dark Frame
– Pixel-to-Pixel Variations Flaws on CCD chip, dust shadows
• Flat Frame
STEP 2: Image ProcessingRAW DARK FLAT
FINAL
- ÷
=
Dark Frame
Flat Frame
Raw ImageDark SubtractedFlat DividedReduced Image
Images
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Complications
Asteroid
STEP 3: Measuring• Measure electron count
within aperture
• Only want electron count
from source– Need to subtract count from
background (scattered moonlight, city lights, etc.)
• Aperture Source + Background
• Annulus Background
Source = Aperture Count - Annulus Count
Annulus
Aperture
Star Field
STEP 3: Calibration• We have electron counts, want physical
magnitudes
• Observe flux standard stars (stars of well known magnitude)
• Measure e- counts for these stars
• Use linearity of CCD (double e- count = double flux) to calibrate source– Source e- count Source magnitude
STEP 4: Lightcurve• Plot brightness vs. exposure time
1143 Odysseus
-0.43
-0.38
-0.33
-0.28
-0.23
0 10 20 30 40 50Hrs from 1st Exposure
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Oct. 27 Oct. 28 Oct. 29
STEP 4: Lightcurve• Phase Lightcurve
– Use previously published rotation periods to plot brightness vs. phase
1143 Odysseus
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
0 0.2 0.4 0.6 0.8 1Phase
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Oct. 27
Oct. 28
Oct. 29
Results
659 Nestor (P = 15.1 h)
-0.26
-0.21
-0.16
-0.11
-0.06
-0.01
0.04
0.09
0.14
0.19
0 0.2 0.4 0.6 0.8 1 1.2Phase
Arb
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lux D
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Oct. 27Oct. 28Oct. 29
3596 Meriones (P = 12.9 h)
-1.6
-1.55
-1.5
-1.45
-1.4
-1.35
-1.3
-1.25
-1.2
-1.15
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Phase
Arb
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lux D
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Oct. 25
Oct. 26
Oct. 29
1143 Odysseus (P = 10.1251 h)
-0.55
-0.5
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
0 0.2 0.4 0.6 0.8 1Phase
Arb
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lux D
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Oct. 27
Oct. 28
Oct. 29
3540 Protesilaos (P = 8.9450 h)
-0.9
-0.85
-0.8
-0.75
-0.7
-0.65
-0.6
-0.55
-0.5
-0.45
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Phase
Arb
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lux D
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Oct. 25
Oct. 26
Questions?