Photometric Analysis of Asteroids

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Photometric Analysis of Asteroids

• Sara Barber
• Acknowledgements
• Dr. Bill and Erin Cooper

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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

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Motivation

• My Goal Create lightcurves for Trojan asteroids
• Future Goal Combine lightcurves throughout
  asteroids orbit to determine 3-D shape
• Shape Spin Rate ? Density
• The density could put a limit on the asteroids
  composition.
• Trojan Composition v.s. Main-Belt Composition
• Different origins within the solar system?
• Better understanding of solar systems evolution

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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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Trojan Asteroids

• Asteroids in orbit around Jupiters 4th and 5th
  Lagrange points

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Lightcurves

• Lightcurve change in brightness throughout
  rotation
• More illuminated surface area ? brighter
• Less illuminated surface area ? dimmer

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Photometry

• Photometry technique for measuring an objects
  brightness
• Steps
• Take exposures
• Process images
• Measure objects brightness
• Calibrate measurements
• Create lightcurve

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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

CCD
Electrodes
Conduction Band
CCDTop View
Valence Band
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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

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STEP 2 Image Processing
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Dark Subtracted
Raw Image
Flat Divided
Reduced Image
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Images
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Complications
Asteroid
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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

Star Field
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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

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STEP 4 Lightcurve

• Plot brightness vs. exposure time

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STEP 4 Lightcurve

• Phase Lightcurve
• Use previously published rotation periods to plot
  brightness vs. phase

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Results
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Questions?