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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