Download Dynamics of Centaurs

The Centaur Population
Luke Dones and Harold F. Levison (SwRI)
Martin J. Duncan and Ramon Brasser (Queen’s University)
Paul R. Weissman (Jet Propulsion Laboratory)
Outline
1.
2.
3.
4.
5.
Chiron, the first Centaur
What is a Centaur?
Known orbits
Orbital integrations
Comparison of observations and models
1. Chiron (discovered 1977)
Circular No. 3129
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.Ss.A.
Cables: SATELLITES NEWYORK Telex: 921428
Telephone: (617) 864-5758
SLOW-MOVING OBJECT KOWAL
C. T. Kowal, Hale Observatories, reports the discovery of a slow-moving object of stellar appearance on exposures with
the 122-cm Schmidt telescope at Palomar. A prediscovery image was identified by T. Gehrels on an exposure with the same
instrument. The motion, scarcely greater than that of Uranus, is
The positions are:
1977 UT
R.A. (1950)
Oct. 11.3
2 06.8
18.38090 2 05 34.09
19.42882 2 05 22.79
1977 November 4
extraordinarily slow for an object so close to opposition.
Decl.
Mag. Observer
+12 21
19 Gehrels
+12 09 12.4 18.0 Kowal
+12 08 07.5
“
(3129)
Brian G. Marsden
From 1895, 1941, 1952, 1969, 1976 and 1977 observations:
T = 1996 Feb. 19.5345 ET
e = 0.378623
a = 13.695195 AU
i = 6.9229
q = 8.509883 AU
P = 50.68 years
1977 December 13
(3151)
Brian G. Marsden
Science
Sky Centaur
Monday, Jan. 9, 1978
What, after all, is Object-Kowal?
When Hale Observatories Astronomer Charles Kowal
announced in November that he had spotted a small object
circling the sun beyond the orbit of Saturn, some scientists
thought that he might have discovered the
solar
system's tenth planet. Now, Object-Kowal—as it is
temporarily called—has been downgraded. The body,
which is at least 160 km. (100 miles) in diameter, could be an
errant asteroid or an inactive
comet.
Re-examining old sky photographs, astronomers have found
Qbject-Kowal on plates taken by Harvard astronomers as far
back as 1895. In fact, Kowal himself had unknowingly
photographed it while making a sky survey in 1969. From
these old plates, Brian Marsden, of the Harvard-Smithsonian
Center for Astrophysics, was able to compute Object-Kowal's
orbit around the sun. Making a complete revolution once in
about 50 years, it swings as close as 1.3 billion km. (790
million miles) to the sun—which brings it to a point just inside
the orbit of Saturn—and as far out as 2.8 billion km. (1.75
billion miles), just shy of the orbit of
Uranus.
Some astronomers speculate that Object-Kowal is an escaped
asteroid that was ejected from the debris-littered asteroid belt
between Mars and Jupiter as the result of a collision. Marsden
favors another idea: that it is an "inactive" comet so far from
the sun that the solar radiation it receives is not strong
enough to vaporize its icy surface and thus produce a
characteristic comet's head and tail.
Whatever the strange body is, Kowal has suggested what he
feels is an appropriate choice for its permanent name: Chiron,
after one of the centaurs, the mythological beasts that were
half-man, half-horse. His reason: Chiron was a descendant of
the gods Saturn and Uranus, for whom the object's closest
planetary neighbors were named.
Past and Future Orbit of Chiron
Kowal et al. 1979, Oikawa and Everhart 1979, Scholl 1979, Hahn and Bailey
1990, Nakamura and Yoshikawa 1993, Gabryszewski 2002, Horner et al. 2004
•
•
•
•
Chaotic orbit in Saturn-Uranus region (Oikawa and Everhart 1979)
Likely to become Jupiter-family comet [JFC] (Oikawa and Everhart 1979,
Scholl 1979)
Was probably a JFC in the recent past [~0.1 Myr] (Hahn and Bailey 1990)
Lifetime against ejection from the Solar System 1 Myr
Kowal, Liller, and Marsden (1979)
Scholl 1979
Nature 435, 69 (5 May 2005)
D. Jewitt, Saas-Fee lectures 2005
2. What Is A Centaur?
Informal Definition: Centaurs are small bodies in heliocentric orbit that strongly
interact with the giant planets.
MPC lists Centaurs and Scattered Disk Objects together – 170 objects
http://www.cfa.harvard.edu/iau/lists/Centaurs.html
1. Jewitt 2006: Non-Trojan bodies with both perihelia (q) and semi-major
axes (a) between the orbits of Jupiter (5 AU) and Neptune (30 AU)
Under this definition, 59 known.
2. Di Sisto & Brunini 2006: Bodies on the MPC list with q < 30 AU,
regardless of semi-major axis
Adds 25 more objects  total of 84 Centaurs
3. Tiscareno & Malhotra 2003: Bodies on the MPC list with q < 33 AU
4. Elliot et al. 2005: Non-resonant objects with q < 30 AU at any time
during the next 10 Myr.
Under this definition, some “Centaurs” are Plutinos at the present time.
(65489) 2003 FX128
3. Known Orbits
(a,e)
from MPC
Centaur+SD
and TNO lists
(a, q)
(a, i)
(q, i)
Centaur Binaries
(Noll et al. 2006)
• (42355) 2002 CR46
s/RP = 28 ± 3 (2s/rH ~ 0.01)
a = 38.1 AU
q = 17.5 AU
i = 2.4o
• (65489) 2003 FX128
a = 103 AU
q = 17.9 AU
i = 22.3o
Binary with semi-major axis a is stable if closest
approach distance to planet, q, satisfies q/RP
 a/R1, where R1 is the physical radius of the
larger component (Agnor and Hamilton 2006)
Centaur Comets
C/2001 M10 NEAT
C/2004 A1 LONEOS
39P/Oterma
29P/Schwassmann-Wachmann 1
174P/Echeclus
P/2005 T3 Read
P/2005 S2 Skiff
165P/LINEAR
95P/Chiron
166P/NEAT
167P/CINEOS
q
5.303
5.463
5.471
5.724
5.826
6.202
6.398
6.830
8.454
8.564
11.788
e
0.801
0.308
0.245
0.044
0.457
0.174
0.197
0.621
0.383
0.384
0.269
i
28.1
10.6
1.9
9.4
4.3
6.3
3.1
15.9
6.9
15.4
19.1
P
138
22.2
19.5
14.7
35.1
20.6
22.5
76.4
50.7
51.9
64.8
TJ
2.586
2.963
3.005
2.984
3.032
3.045
3.076
3.095
3.356
3.285
3.527
http://www.physics.ucf.edu/~yfernandez/cometlist.html
174P/Echeclus, R = 13 AU
http://observatory.ou.edu/
Absolute Magnitude Distribution
Brightest known
Centaur with
secure orbit:
(10199) Chariklo,
H = 6.4,
d = 236 ± 12 km,
pV = 0.07±0.01
(Groussin et al.
2004; Stansberry
et al. 2005)
Donnison (2006)
SDOs: α ~ 0.4
KBOs: α ~ 0.6
4. Orbital Integrations
"I have received information psychically, which is corroborated by
scientific data, according to which on May 25, 2006 a giant tsunami will
occur in the Atlantic Ocean, brought about by the impact of a comet
fragment which will provoke the eruption of under-sea volcanoes.”
- Eric Julien
Centaurs: Lifetime ~ 0.4 Myr exp[q/7.4 AU]
(Horner et al. 2004)
or 30,000-80,000 orbits
Scattered disk:  exp[q/4.7
AU]
(Fernández et al. 2004)
5. Comparison of Observations and Models
Observed Distance Distribution
Observed Size Distribution
Simulations of Di Sisto and Brunini (2006)
Initial conditions: scattered-disk objects with
• 40 AU < a < 227 AU, f(a) da  a-2.09 da (Fernández et al. 2004)
• 30 AU < q < 39 AU
• Integrate for 4.5 Gyr or until
– a > 1000 AU
– collision with planet, or
– r < 5.2 AU  “second integration” for 100 Myr including
terrestrial planets
• For modeling discovery, assumed N(<H) = 10αH, α = 0.54
Di Sisto and Brunini (2006)
Di Sisto and Brunini (2006)
Di Sisto and Brunini (2006)
S  R1.3
S  R2.8
S ~ constant
S  R-2.5
Tiscareno and Malhotra 2003
S  R3
(?)
S  R-2.5
(?)
Levison and Duncan 1997
Di Sisto and Brunini 2006
Conclusions
• Centaurs are conveniently located TNOs
• Number of Centaurs now 84, including 11 comets, 2
binaries
• ~ One-third have a > 30 AU
• Dynamical lifetimes 1-100 Myr  population should be
1/2000 of scattered disk (Jupiter-Saturn region) to 1/20
of scattered disk (q = 25-30 AU)
• Size distribution of Centaurs may be shallower than that
of KBOs
• Observed orbital distribution of Centaurs consistent with
dynamical models, but do not yet provide details on
source regions.