The Centaur Population

1. 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)

2. Outline • Chiron, the first Centaur • What is a Centaur? • Known orbits • Orbital integrations • Comparison of observations and models

3. 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 extraordinarily slow for an object so close to opposition. The positions are: 1977 UT R.A. (1950) Decl. Mag. Observer Oct. 11.3 2 06.8 +12 21 19 Gehrels 18.38090 2 05 34.09 +12 09 12.4 18.0 Kowal 19.42882 2 05 22.79 +12 08 07.5 “ 1977 November 4 (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

4. 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.

5. 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

6. Kowal, Liller, and Marsden (1979) Scholl 1979

7. Nature435, 69 (5 May 2005)

8. D. Jewitt, Saas-Fee lectures 2005

9. 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 AUat any time during the next 10 Myr. Under this definition, some “Centaurs” are Plutinos at the present time.

10. (65489) 2003 FX128

14. 3. Known Orbits

15. (a,e) from MPC Centaur+SD and TNO lists

16. (a, q)

17. (a, i)

18. (q, i)

19. 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)

20. Centaur Comets q e i P TJ C/2001 M10 NEAT 5.303 0.801 28.1 138 2.586 C/2004 A1 LONEOS 5.463 0.308 10.6 22.2 2.963 39P/Oterma 5.471 0.245 1.9 19.5 3.005 29P/Schwassmann-Wachmann 1 5.724 0.044 9.4 14.7 2.984 174P/Echeclus 5.826 0.457 4.3 35.1 3.032 P/2005 T3 Read 6.202 0.174 6.3 20.6 3.045 P/2005 S2 Skiff 6.398 0.197 3.1 22.5 3.076 165P/LINEAR 6.830 0.621 15.9 76.4 3.095 95P/Chiron 8.454 0.383 6.9 50.7 3.356 166P/NEAT 8.564 0.384 15.4 51.9 3.285 167P/CINEOS11.788 0.269 19.1 64.8 3.527 http://www.physics.ucf.edu/~yfernandez/cometlist.html

21. 174P/Echeclus, R = 13 AU http://observatory.ou.edu/

22. 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

23. 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

26. 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)

28. 5. Comparison of Observations and Models

29. Observed Distance Distribution

31. Observed Size Distribution

33. 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

34. Di Sisto and Brunini (2006)

35. Di Sisto and Brunini (2006)

36. Di Sisto and Brunini (2006)

37. S R1.3 S R-2.8 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

38. 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.