Download - An HST/ACS Survey of the Kuiper Belt
An HST/ACS Survey of the Kuiper Belt
Gary Bernstein (University of Pennsylvania)
#ith co-I’sLynne A&en (UBC), Mike Brown (Cal Tech), Mat'
Holman (CfA), Renu Malhotra (LPL), & David Tri&ing (Penn➙Steward)
and for Astrometry: N. Zacharias (USNO) & J. A. Smith (LANL)
Background Image: section of summedACS TNO search-field data.☀Funded by GO-9433 grants
The Kuiper Belt as an accretion and dynamics laboratory
Design and Execution of the ACS Survey
TNO Discoveries & Followup
Implications for Solar System Evolution
Light Curves and Astrometry
Planet Formation Scenario:
Nebular/dust phases areobserved around other stars
Giant planets have beendetected around otherstars
Planetesimal phaseis observable onlyin our outer "fossilized"Solar System - theKuiper Belt.
Collapse of gas/dust cloudto form protostar and nebulardisk.
Coagulation of grains, runaway gravitational growthof planetesimals
Oligarchic growth of planetesimals.
Gas accretion onto largest bodies,dissipation of gas, ejection of smallbodies
Mature planetary system
Theoretical Expectation: Observational Evidence:
Known Outer Solar-System Bodies (as of 2003 April 19)
Key
Plutino
Scattered
Other TNO
Comet
Asteroid
Giant Planet
Centaur
Pluto
Neptune
Uranus
Open symbols denote orbits uncertain due to short arcs.
20 30 40 50 60 70 80 90 1001000
0.2
0.4
0.6
Semimajor Axis (AU)
Outer Solar System Objects (as of 4/03)
Neptune-crossing
Eccen
tric
ity
Uranus
Neptune
Pluto
Resonant
3:2
2:1
5:2
1:1(Trojan)
KBO Resonances and Planetary Migratio(
Pluto and many KBOs (”Plutinos”) are locked in 3:2 resonance with Neptune, crossing Neptune’s orbit yet avoiding ejection.
Malhotra (1993) explains “accident” of Pluto’s resonance by positing an outward migration by ~8 AU of Neptune during the clearing of the planetesimal disk. Resonances “sweep” through the population, acting like phase-space traps. Prediction is that many KBOs should be found in 3:2 and 2:1 resonances
KBOs have been found in following resonances (Chiang et al ‘03):1:1 (”Trojan”), 5:4, 4:3, 3:2 (”Plutino”), 5:3, 7:4, 2:1, 7:3, 5:2Resonance sweeping can only explain some of these, obviously not 1:1.
3:2 Resonances inNeptune frame(Malhotra 1995)
Orbit of 2001 QR322 in Neptune frame(Chiang et al 2003)
20 30 40 50 60 70 80 90 1001000
0.2
0.4
0.6
Semimajor Axis (AU)
Outer Solar System Objects (as of 4/03)
Neptune-crossing
Eccen
tric
ity
Uranus
Neptune
Pluto
Resonant
3:2
2:1
5:2
1:1(Trojan)
Centaurs
"Scattered
Disk"
"Classical" Belt
Cold Primordial Disk?
CAUTION:Strong
Selection Effects
Recall that distant TNOs have
Traditional response to fainter target is longer integration, but S/N gain for moving objects stops when object trails across sky background. TNOs move few arcsec per hour.
Non-sidereal tracking of telescope only works for one preselected motion vector, not appropriate for a survey
Ground-based exposures limited to ~10 minutes before trailing, or
Digital Tracking: Take short exposures, shift and add digitially to follow any candidate orbit, then run detection algorithm.
Used successfully from ground in 1992 by GMB, with Cochran et al WFPC2 data, many other times since.
Finding Faint TNOs
f ∝ r−4
mR<∼ 24
a) Original Single Exposure (c) Single Exposure, Fixed Objects Subtracted/Masked
(b) Combined ImageTrack at Sidereal Rate
(d) Combined ImageTrack at KBO rate
R=23.5
R=24.0
R=23.0
R=24.5
R=25.0
R=25.5
Co
rrec
tly tra
ck
ed
Tra
ck
ed
at
wro
ng
rate
6.5
7
8
9
10
11
Below detection th
reshold
(
R>26)
10 20 30 40 50 60 70
Heliocentric Distance (AU)
Dia
me
ter
of
Ob
ject
(km
)
300
200
100
0
Earth
Jupiter
Saturn
Uranus
Neptune
Pluto
H
Ground-Based Survey of 1.5 Square DegreesA&en, Bernstein, & Malhotra (2001)
Size Distribution of KBOs
20 22 24 26 28 30
R Magnitude
100 10Diameter at 35 AU (km, albedo=0.04)
0.01
0.1
1
10
100
1000
104
105
log
N(<
m)
pe
r squ
are
de
gre
e
Number vs Mag (~size) for
Trans-Neptunian Objects
ABM01
GL98/01
LJ98
JLT98
TJL01
CB99
Fa01
La01
CLSD/WFPC2
???
Keck data
For fixed distance, mag distribution is the size distribution.
Faintest two points on this plot known to be contaminated by false positives!
Data seem consistent with a single power law,
N(< m) = 100.63(m−23)
⇒ dN
dr∝ r−q, q = 4.0± 0.5
Theoretical Expectations for Size Distributio(
Single power law must break down to avoid divergent mass, surface brightness.
Modelling of collisional accretion and erosion requires detailed numerical study, assumptions about fracturing, dynamics, etc.
Dohnanyi shows that q=3.5 is a point of steady-state collisional cascade under scale-free fracturing laws. Expect this below some disruption scale.
t = 0 Myr
t = 5 Myr t = 25 Myr
t = 50 Myr t = 100 Myr
t = 130 Myr t = 265 Myr
t = 1000 Myr
Does this size break exist? Kuiper Belt is perhaps our only opportunity to test an accretion model!
Model by Davis et al
Outstanding Kuiper Belt Questions
What is the size (magnitude) distribution of objects?
Are the dynamical classes physically distinct?
What event(s) or process(es) heated the planetesimal disk?
Is there truly a truncation? What caused it?
Are any of the dynamical classes viable source reservoir for the observed 1-10 km Jupiter family comets?
What can we learn about the history of the solar system, and planetary systems in general?
Plan for the ACS SurveySix ACS fields, 0.02 square degrees (12x HDF)
Observe each field 22 ksec over 5 days
Sum flux over 30 exposures in candidate orbits
Wait 5 days; most TNOs pass stationary points and reverse
Observe another 15 ksec to confirm discoveries
Detection limits: m<29.2 in F606W
Expect 85 detections under extrapolation.
≈ 1014
20 22 24 26 28 30
R Magnitude
100 10Diameter at 35 AU (km, albedo=0.04)
0.01
0.1
1
10
100
1000
104
105
log N
(<m
) per
square
degre
e
Number vs Mag (~size) for
Trans-Neptunian Objects
ABM01
GL98/01
LJ98
JLT98
TJL01
CB99
Fa01
La01
Expected
ACS Result
break t
o Dohnanyi
slope?
A& it takes is
116 orbit Cycle 11 allocation (executed in 125 orbits, thanks Tony Roman!)
30,000 lines of C++ code
New Fourier-domain PSF characterization methodPSF interpolation across ACSImage registration & distortion (Anderson)Image combining with reduced PSF distortionImage subtractionDigital tracking addition & detectionOrbit calculation and trailed-PSF fitting directly to images.
Several CPU-years on P4 processors
Schedul)
Feb 2002: Phase II request
Aug 2002: Preparations begin in earnest
Jan 26 2003: First TNO observation
Feb 09 2003: Last observation
April 14 2003: All candidates identified
April 29 2003: Keck retrieval run (Magellan clouded out)
August 2003: Deep search complete
I&ustration of ACS Data Quality
140
150
160
170
180
-60 -40 -20 0 20-3
-2
-1
0
1
2
X Position of 2000 FV53
in part of ACS Data
...linear trend removed
Positio
n (
arc
sec)
-70 -65 -60 -55 -50
Time (Hours)
0.4
0.2
0.0
-0.2
-0.4predicted parallax from HST orbit at 39 AU
TNO Discoveries in the ACS Survey2000 FV53
m=23.4
2003 BF91
m=26.95
2003 BG91
m=28.15
2003 BH91
m=28.38
Single Exposure
400 seconds
Summed Orbit
2000 seconds
Summed Visit
6000 seconds
All Data
38,000 seconds
S/N=73 S/N=26 S/N=23
S/N=90
1.5 arcsec
40.26 AU2.5 deg
0.0744 km
Distance:Inclination:
Eccentricity:Diameter:
42.14 AU1.5 deg
??28 km
42.55 AU2.0 deg
??25 km
Faintest solar system objects ever detected!1 photon per 5 seconds at HST
Analysis of a& survey data
All TNOCKBExcited
0.001
0.01
0.1
10
100
18 20 22 24 26 28
R Magnitude
0.01
0.1
10
1
1
0
1
100
1000
10
Σ(m
) (m
ag
-1 d
eg
-2)
Se
arc
h A
rea
(d
eg
2)
De
tectio
ns
LarsenTB
ABM
TJL
Gladman
ACSCB
38AU ≤ d ≤ 55AU
i < 5◦
Combine data from all large, well-characterized surveys within 3 degrees of the invariable plane.
Deviation from single power law at high confidence.
Split the TNO sample into “Classical” KB and “Excited” populations. CKBs defined as:
LFs of dynamical classes distinct at 96% confidence.
Double-power-law fits to the Magnitude Distributio(
18 20 22 24 26 28
0.1
1
R Magnitude
Σ(m
) x 1
0-0
.6(m
-23
) (m
ag
-1 d
eg
-2)
Excited
CKB
(Mass p
er
mag)
Bright-end slope α1
Fa
int-
en
d s
lop
e α
2
CKBO
Excited
0.4
0.6
0.2
0.
-0.2
-0.4
0.6 0.8 1.0 1.2 1.4
What does it mean?
t = 0 Myr
t = 5 Myr t = 25 Myr
t = 50 Myr t = 100 Myr
t = 130 Myr t = 265 Myr
t = 1000 Myr
Stern (1995) gives sizes below ~200 km as subject to disruption in current collisional environment:
Crudely, this is size of potential break in size distribution.
Resembles >1 Gyr phases in Davis simulation - Excited older than CKB
Implications
14 16 18 20 220
0.2
0.4
0.6
0.8
1
R magnitude
Pro
ba
bili
ty o
f A
ny T
NO
s <
RQuaoar
2000 KX14
Pluto
Total mass of the CKB: (for p=0.04, <d>=42 AU, ρ=1000 kg/m3):
Extrapolation of Excited bright end predicts largest object in the Pluto-Quaoar range. Pluto is uniquely but not anomalously large.
Note largest CKBO is predicted (and is, so far) 60x lower mass.
0.010 M⊕ ± 15%
Are KBOs a viable source reservoir for Jupiter-family comets?
One astronomer’s noise is another’s signal
Cycle 13 Observations
Additional 6 orbits granted and executed in May 2004 (GO-10268, PI Trilling)
Retrieve the 3 new objects to determine eccentricity - really CKB objects?
Additional astrometry will give sufficient orbital info to locate these objects with JWST in 2010.
Recovery of 2003 BG91
Phased photometry of ACS TNOs
0.4
0.6
0.8
1
1.2
flu
x (
elec
tro
n/s
ec)
0.2
0.4
0.6
flu
x (
elec
tro
n/s
ec)
0 0.2 0.4 0.6 0.8 1phase
0
0.2
0.4
flu
x (
elec
tro
n/s
ec)
0.2 0.4 0.6 0.8 1phase
18
19
20
21
22
23
flu
x (
elec
tro
n/s
ec)
P=4.2 h P=9.1 h
P=2.8 h P=3.79 h
2003 BG91
2003 BF91
2003 BH91
2000 FV53
>1 mag variation!
0.07 mag variation!
Trilling & Bernstein (submitted to AJ)
A Model for 2003 BF91
Is 2000 FV53 a strengthless body?
0 0.04 0.08 0.12 0.16 0.2 0.24 0.28τ ( = K / W )
0
0.5
1
1.5
2
2.5
3
3.5
4ρ
( g/c
m3 )
(Varuna)
Pluto
Mathilde
Eros
P = photom. period
P = twice photom. period
P(θ)
JacobianMaclaurin
b ≈ cb > ca ≈ b ≈ ca = b a >> b
Exclud
ed:
Too Asym
metric
Oblate:
no light c
urve!
Too dense?
Too light?
Spots?
Rubble-pile solutions
0 0.2 0.4 0.6 0.8 1b / a
0
15
30
45
60
75
90φ (
deg
rees
)
P(θ
) =
1.0
P(θ
) =
0.1
density = 0.5
density = 1.0
density = 2.0Too
asym
metric
Ang
le o
f Rep
ose
Acceptable!
Astrometric Residuals for 2000 FV53
Bernstein, Zacharias, & Smith (in prep)
0
0
N-S
Err
or
(mill
iarc
sec)
E-W
Err
or
(mill
iarc
sec)
-4
-2
2
4
-4
-2
2
4
0. 0.5 1. 11. 11.513.3. 3.5 13.5
Time (days)
Is mi&iarcsecond KBO astrometry just a neat trick?
N. Zacharias (USNO): 5 mas tie to ICRS very practical, better over 1-degree patches?
1 mas at 40 AU = 30 km = 5 seconds of HST motion
Detectable photocenter motion on 300-km spotted body
Displacement from undiscovered solar system masses is
∆x = 1 km(
M
M⊕
) (T
1 yr
)2 (D
100 AU
)−2
Future DevelopmentsSky to R=21 is mostly covered by Palomar/Quest
NOAO Deep Ecliptic Survey, CFH Legacy survey to expand sky coverage for R<24
Full sky to R=24 will be done by Pan-STARRS, LSST - 10,000’s of objects with full orbital dynamics
Could reach R=26 over limited regions of sky
Some further coverage from ACS archive (PI: Tamblyn) or another Large program.
SNAP could repeat ACS survey to get 10,000’s of R=28 objects, detail dynamics/size correlations.
Occultation surveys (TAOS) explore <10km regime
New Horizons probe flies by 2 (?) KBOs c. 2017
Summary
ACS works flawlessly for TNO search. Background-limited detection for 29th mag moving objects.
Clear detection of a feature in the size distribution, with dependence upon dynamical state of subpopulations.
No sign of cold population beyond 50 AU, for D>40 km
Scattered disk a feasible JFC source, but still a stretch?
Suprising light curves, consistent with rubble pile in 200 km body 2000 FV53
Additional info forthcoming from followup & astrometry.
0.1 10 100 10000.1
1
10
100
1000
Distance at Discovery (AU)
1
104
105
Bo
dy R
adiu
s (
km
)
Solar System Discovery Space
Naked Eye (<1610)JupiterSaturn
Venus
Mercury
Mars
Sun
Moon
0.1 10 100 10000.1
1
10
100
1000
Distance at Discovery (AU)
1
104
105
Bo
dy R
adiu
s (
km
)
Solar System Discovery Space
Naked Eye (<1610)
Telescopic (1610-1900)
Ceres (1801)Pallas(1802)
Juno (1804)
Vesta (1807)
GalileanMoons(1610)
PhobosDeimos(1877)
Uranus(1781)
Neptune(1846)
Titan (1655)
0.1 10 100 10000.1
1
10
100
1000
Distance at Discovery (AU)
1
104
105
Bo
dy R
adiu
s (
km
)
Solar System Discovery Space
Naked Eye (<1610)
Telescopic (1610-1900)
Photo (1900-1990)
Pluto (1930)
Charon (1978)
Voyager Moons(1978-1989)Eros
NEA's
0.1 10 100 10000.1
1
10
100
1000
Distance at Discovery (AU)
1
104
105
Bo
dy R
adiu
s (
km
)
Solar System Discovery Space
Naked Eye (<1610)
Telescopic (1610-1900)
Photo (1900-1990)
1992 QB1
Quaoar (2002)
Kuiper Belt
CCD (1990-2002)
0.1 10 100 10000.1
1
10
100
1000
Distance at Discovery (AU)
1
104
105
Bo
dy R
adiu
s (
km
)
Solar System Discovery Space
Naked Eye (<1610)
Telescopic (1610-1900)
Photo (1900-1990)
New (2003+)
Sedna
2003 Bx91
CCD (1990-2002)
0.1 10 100 10000.1
1
10
100
1000
Distance at Discovery (AU)
1
104
105
Bo
dy R
adiu
s (
km
)
Solar System Discovery Space
Pan-STARRSLSST
Space Occultation Survey
Wid
e-Fi
eld
Spac
e Im
agin
g
Naked Eye (<1610)
Telescopic (1610-1900)
Photo (1900-1990)
New (2003+)
Sedna
2003 Bx91
CCD (1990-2002)