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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STEP 4: Lightcurve• Phase Lightcurve

– Use previously published rotation periods to plot brightness vs. phase

1143 Odysseus

-0.45

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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

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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

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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

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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

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Oct. 26

Questions?