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Time-series Photometry of Earth Flyby Asteroid 2012 DA 14

Tsuyoshi TeraiSubaru Telescope

Asteroid populations

Main-belt asteroids

Near-Earth asteroids

Dynamical evolution

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AsteroidsR

elat

ive

refle

ctan

ce S-type C-type X-type V-type D-type

P M E

HighLowReflectivity

(Bus & Binzel 2002)

Wavelength (µm)

Asteroid classification by reflectance spectra

Spectral classificationSurface composition of asteroids depends on formation environments and several evolution processes (e.g. thermal, chemical, weathering)

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Surface properties

Composition ���� spectra , reflectivity

Structure ���� light scattering characteristics

Providing important clues as to

- Internal materials / structure

- Collisional evolution

- Thermal / aqueous alterations

- Linkage between asteroids and meteorites

433 Eros

33 km

25143 Itokawa

0.5 km

� covered with small particle layer

� covered with large rocks

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Phase curve

Sun

Observer(Earth)

Asteroid

Solarphase angle

0.0

0.2

0.4

0.6

0 10 20 30

Solar phase angle (deg)

Rel

ativ

e b

right

ness

(m

ag)

Brightness increases with decreasing solar phase angles

Solar phase angle (deg)

Rel

ativ

e r

efle

ctan

ce

E-type

V-type

S-type

M-type

C-type

Muinonen et al. (2002)Phase curve

The shape of phase curve depends on

- Composition (reflectivity)

- Structure (roughness)

Useful indicator of surface properties

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Asteroid 2012 DA 14

Earth

Today

- Discovered on February 23, 2012, in Spain

- Near Earth asteroid named “Duende ”

- Tiny body with diameter of about 50 m

Earth Flyby of 2012 DA 14

Earth

Feb 15, 2013 (UT)

- Passed closely to the Earth on Feb 15, 2013

- Approached to the distance of ~28,000 km

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Earth Flyby of 2012 DA 14

- Passed closely to the Earth on Feb 15, 2013

- Approached to the distance of ~28,000 km

Earth

2012 DA14 ’s orbit

Sun

Motion Geosynchronousorbit

���� inside a geosynchronous orbit !

Brightness highly increased to V = 7 mag !

� Easy to perform high accuracy photometry

Brightening

Date (UT)

Vm

agni

tude

Feb, 2013

Closest approach

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Aim of observation

Solar phase angle varied 20O – 50O during 2 hours around the closest approach

Allowing us to measure the phase curve precisely and efficiently

Great opportunity for investigating surface properties of tiny bodies

19 20 21 22

10

20

30

40

50

60

UTS

olar

pha

se a

ngle

(de

g)

Closest approach

Minimum phase angle

Difficulty

19 20 21 22

60

50

40

30

20

10

0

UT

Sky

mot

ion

(ar

cmin

/min

)

Closest approach

(1) Extremely rapid sky motion� More than 50 arcmin min -1 in maximum

Pass through the moon within 36 sec only !

30ʹ

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Difficulty

(2) Observable within only a short time (in Japan)

Twilight start

Sunrise

Ele

vatio

n (d

eg)

JST (UT + 9)

Strategy

1. Using 0.55-m Saitama-Univ. Telescope

0.55-m telescopeat Saitama Univ.

- Bright objects are not easy to saturate

- High mobility and operability

- Allows pointing toward low elevation

2. With an 1k x 1k CCD camera mounted on the prime focus

� Wide field of view (32ʹ x 32ʹ )

3. High-rate continuous imaging with very short exposure

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Observation

- 2013 Feb 16 4:00–6:10 (JST; UT+9)

- R-band imaging with 0.5-sec exposure

- Data acquisition every 2 sec

- Under sidereal tracking

More than 2000 images have been obtained with good sky condition all over 2 hours

- Keep the target in the field-of view with manual telescope operation

32ar

cmin

Animation of acquired images

N

EJST

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Measurements

3ʹ

- Photometry with an elongated circular aperture

- Flux calibration using background USNO stars

Lightcurve

Rm

agni

tude

4 5 6

6.5

7.0

7.5

JST

Average every 1 min

The magnitude was adjusted for the heliocentric/geocentric distances to those at the closest approach.

Closest approach

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Lightcurve

10 20 30 40 50Solar phase angle (deg)

Rm

agni

tude

6.5

7.0

7.5

Before 5:00

After 5:00

Average every 1 min

The magnitude was adjusted for the heliocentric/geocentric distances to those at the closest approach.

Before 5:00

Lightcurve

10 20 30 40 50Solar phase angle (deg)

Rm

agni

tude

6.5

7.0

7.5

After 5:00

Average every 1 min

The magnitude was adjusted for the heliocentric/geocentric distances to those at the closest approach.

Phase curve

Rotational variation

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Additional observation

In the next night (Feb 16, 2013), 2012 DA14 had

- Constant solar phase angle ( 82.2O – 82.6O in the latter half night)

- Slower sky motion (~10 arcmin hr-1 )

- Fainter brightness of V ~ 15 mag

Follow-up observation with continuous R-band imaging of T exp = 10–60 sec over 5 hours

���� Possible to obtain a brightness variation due to only the asteroid rotation

Texp = 0.5 sec

R ~ 7 mag

Texp = 20 sec

R ~ 14 mag

20.5 hr

1st night vs. 2nd night

Feb 15 19:30 UT Feb 16 16:00 UT

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Lightcurve of the 2nd night

The magnitude was adjusted for the heliocentric/geocentric distances to those at the beginning of the observation.

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15

16

1 2 3 4 5

JST

Average every 3 min

Rm

agni

tude

Rotation model

-1.0

-0.5

0.0

0.5

1.0

Rm

agni

tude

Rotational phase

0.0 0.2 0.4 0.6 0.8 1.0

Fitting with a 4th-order Fourier series formulation

Rotational period = 11.0 hr

Peak-to-peak amplitude = 1.59 ±±±± 0.02 mag

+1.8–0.6

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Phase curve model

+ ����

0 10 20 30 40 50

Solar phase angle 10 20 30 40 50

Solar phase angle

6.5

7.0

7.5

Rotational phase0.0 0.2 0.4 0.6 0.8 1.0

R m

agni

tude

Rotation model Phase curve model Acquired lightcurve

Fitting

“ H-G phase function” ( Bowell et al. 1989 )

])(Φ+)(Φ)1([log5.2+)0(=)( 21 αGαGmαm -

m (α) : apparent magnitude at a phase angle αG ：：：： slope parameter (0 < G < 1)Φ1 ，Φ2 : specified phase functions

Best-fit model

10 20 30 40 50

Solar phase angle (deg)

Rm

agni

tude

6.5

7.0

7.5

G = 0.44±±±±0.08

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Solar phase angle (deg)

Rm

agni

tude

0 10 20 30 40 50

6.5

7.0

7.5

Best-fit model

Shallow phase curve slope relative to S/C-type asteroids

Spectral type

Wavelength (µm)

de León et al. (2013)

0.5 1.0 1.5 2.0

1.8

1.6

1.4

1.2

1.0

0.8Rel

ativ

e re

flect

ance

Bus & Binzel (2002)

S-type

C-type

L-type

Visible spectra + Visible/NIR colors (de León et al. 2013)

� Classified as an L-type asteroid

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S-type

C-type

Interpretation

Surface reflectivity

Pha

se c

urve

slo

pe (

mag

/deg

) Belskaya & Shevchenko (2000)

Slope of asteroid phase curve is inversely correlated with surface reflectivity (10O < α < 50O )

Previous observations

Mean reflectivity

S-type : ~ 0.23

L-type : 0.14 – 0.18

(Belskaya & Shevchenko 2000)

(Mainzer et al. 2011 ; Usui et al. 2013)

Should have steeper phase curve than S-type asteroids

���� Our observation shows the opposite result���� 2012 DA14 could have a peculiar surface property

Interpretation

Possible cause I

Possible cause II

Weak gravitational field of a tiny asteroid has difficulty in retaining fine particles on its surface

� Coated with coarse particles

� Less effect of brightness decrease with phase angle

2012 DA14 may have a high reflectivity surface

- Young age (~ Myr) � less surface modification ?

- Frequent encounters with Earth freshen the surface by tidal stress (Binzel et al. 2010) ?

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Summary

- We performed time-series observations for a tiny asteroid 2012 DA14 around its closest approach

- It is likely to rotate with a period of about 11 hours

- Our measurements show a significantly shallow phase curve, which is inconsistent with known L-type asteroids

- 2012 DA14 may be covered with a coarse particles and/or high reflectivity surface