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 Analyze previously obtained images of Trojan asteroids Explain what projects are -- OPPOSITIION TROJAN TROJAN LIGHTCURVE

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 Results CCD Photometry Image Processing Complications Measuring Calibration Results Preparing for a night of observing.

Trojan Asteroids Asteroids in orbit around Jupiter’s 4th and 5th Lagrange points Trojan Asteroids Main Belt Asteroids http://cseligman.com http://epsc.wustl.edu http://epsc.wustl.edu/classwork/classwork_171/SP2008/Lectures/EPSC171A-Spring08-15-Asteroids.pdf (green) http://cseligman.com/text/asteroids/trojan.htm (lagrange)

Lightcurves Lightcurve: change in brightness throughout rotation More illuminated surface area  brighter Less illuminated surface area  dimmer Lightcurve Asteroid Orbit Lightcurve

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 CCD Electrodes Conduction Band CCD:Top View Valence Band

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 Processing RAW DARK FLAT FINAL - ÷ = Explain CCD here

Dark Frame Flat Frame CCD COOLed to decrease noise DARK -- shutter closed FLAT--uniformly illuminated

Dark Subtracted Reduced Image Raw Image Flat Divided

Images

Complications Asteroid

Source = Aperture Count - Annulus Count 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 Aperture Annulus

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

STEP 4: Lightcurve Phase Lightcurve Use previously published rotation periods to plot brightness vs. phase

Results

Questions?