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(meteorobs) Excerpts from "CCNet, 64/2000 - 5 June 2000"




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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 5 June 2000
Date: Mon, 5 Jun 2000 10:26:04 -0400 (EDT)

CCNet, 64/2000 -  5 June 2000
------------------------------

[...]

(4) EXTINCT COMETS OR D-TYPE ASTEROIDS?
    M.D. Hicks et al., CALTECH,JET PROP LAB

(5) ORBITAL EVOLUTION OF ASTEROIDS DURING DEPLETION OF THE=20
    SOLAR NEBULA
    M. Nagasawa et al., TOKYO INST TECHNOL

[...]

================================================================

(4) EXTINCT COMETS OR D-TYPE ASTEROIDS?

M.D. Hicks*), B.J. Buratti, R.L. Newburn, D.L. Rabinowitz: Physical=20
observations of 1996 PW and 1997 SE5: Extinct comets or D-type=20
asteroids? ICARUS, 2000, Vol.143, No.2, pp.354-359

*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,MS 183-501,PASADENA,CA,91109

The minor planets 1996 PW and 1997 SE5 are two of the few known=20
asteroids with orbital elements typical of long-period and=20
Jupiter-family comets and as such represent strong candidates for=20
extinct cometary nuclei. We obtained filter photometry of 1996 PW and=20
filter photometry and medium-resolution CCD spectroscopy of 1997 SE5=20
during their discovery apparitions. We also observed a suite of D-type=20
asteroids as possible spectral analogs of cometary nuclei. Both 1996 PW =

and 1997 SE5 have moderately red, featureless spectra typical of the=20
D-type asteroids, cometary nuclei, and other extinct cometary=20
candidates. The photometry for 1997 SE5 was fit by a triple-peaked=20
lightcurve with a period of 9.050 +/- 0.005 h and an amplitude of 0.4=20
magnitude, suggesting a relatively complex and elongated shape. With=20
this work, 1997 SE5 and 1996 PW join the ranks of 3552 Don Quixote and=20
944 Hildago as established candidates for extinct comet nuclei. (C)=20
2000 Academic Press.

================================================================

(5) ORBITAL EVOLUTION OF ASTEROIDS DURING DEPLETION OF THE=20
    SOLAR NEBULA

M. Nagasawa*), H. Tanaka, S. Ida: Orbital evolution of asteroids during =

depletion of the solar nebula. ASTRONOMICAL JOURNAL, 2000, Vol.119,=20
No.3, pp.1480-1497

*) TOKYO INST TECHNOL,DEPT EARTH & PLANETARY SCI,MEGURO KU,TOKYO=20
   1528551,JAPAN

The pumping-up of inclinations and eccentricities caused by sweeping=20
secular resonances in the asteroid belt is studied through numerical=20
orbital integration and linear analysis. The present asteroids have=20
large mean eccentricities and inclinations that cannot be explained by=20
planetary perturbations alone. Sweeping of secular resonances=20
associated with the depletion of the primitive solar nebula has been=20
proposed for the origin of asteroids' orbits (e.g., by Ward, Colombo,=20
and Franklin in 1976). We performed three-dimensional orbital=20
integrations of asteroids under gravitational forces by Jupiter,=20
Saturn, and the solar nebula. The asteroids' motions are also affected=20
by hydrodynamic gas drag. We consider three types of nebula depletion=20
models: (1) the uniform depletion model, in which the nebula density=20
decreases exponentially with time and uniformly throughout the nebula;=20
(3) the inside-out depletion model, in which nebula gas is depleted=20
from the inside region; (3) the gap-opening model, in which a gap=20
centered at Jupiter gradually expands. Previous studies have=20
concentrated on the two-dimensional uniform depletion model. Our=20
simulation shows that inclinations of asteroids are not pumped up=20
enough in the first model to account for the observed magnitude.=20
Moreover, most asteroids spiral into the Sun by gas drag if the=20
depletion timescale is longer than 10(5) yr because pumped-up=20
eccentricity induces strong gas drag. On the other hand, in the second=20
and third models, inclinations are pumped up. Our linear analysis shows =

that the nonuniform depletion model is essential for the secular=20
resonances pumping up inclination to sweep in the asteroid belt. In the =

case of the inside-out depletion model, both eccentricity and=20
inclination are pumped up enough to be consistent with the observed=20
magnitude in the entire asteroid belt, if the nebula depletion=20
timescale (which is the time required for the nebula edge to migrate by =

the distance of 5 AU) is longer than 3 x 10(5) yr. Furthermore, since=20
secular resonances pass after nebula gas has already been depleted in=20
the passing region, gas drag does not damp the pumped-up eccentricities =

and inclinations and the semimajor axes of the asteroids. Therefore,=20
the resultant eccentricity and inclination in the inside-out depletion=20
model are consistent with observed ones for the nebula depletion time
inferred from observed T Tauri stars (10(6)-10(7) yr). Our gap-opening=20
model shows similar results, although eccentricity and inclination are=20
slightly smaller in the outer asteroid belt region. The essentially=20
required condition to pump up both eccentricity and inclination highly=20
enough in the asteroid region is that the nebula edge moves outward=20
from 5 to 10 AU. The gap formation caused by Jupiter's tidal=20
perturbations may be similar to our inside-out or gap-opening models,=20
inside 10 AU. Therefore, it would be responsible for the high=20
eccentricities and inclinations in the asteroid belt. Copyright 2000,=20
Institute for Scientific Information Inc.

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