Electronic Telegram No. 710
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
CBAT@CFA.HARVARD.EDU (science)
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LEONID METEORS 2006
     P. Jenniskens, SETI Institute, reports that the earth will encounter the
two-revolution dust trail of comet 55P/Tempel-Tuttle around 2006 Nov.
19d04h50m UT, with a FWHM of 38 min (Jenniskens 2006, Meteor Showers and
their Parent Comets, Cambridge Univ. Press, p. 630).  A 3-percent moon in
Virgo will not interfere.  Meteors should radiate from a geocentric radiant
of R.A. = 154.32 deg, Decl. = +21.09 deg (equinox 2000.0), with velocities
of V_g = 70.80 km/s.  Each particle's orbital period shortly after ejection
differed from that of the comet by 0.94 year, which needs a high ejection
speed of 88 m/s, if ejected at perihelion in the forward direction
[http://feraj.narod.ru/Radiants/Predictions/Leonids2006eng.html], or a
strong radiation pressure force, presumably due to small size or a high
surface-to-mass ratio.  As a result, the magnitude-distribution index (number
ratio of meteors in neighboring magnitude intervals, once the observed rates
are corrected for detection probability, chi) is expected be high (chi =
3.47, but much higher according to H. Meng, Peking University, in Proc. 2005
Internat. Meteor Conf., Internat. Meteor Org.).  These meteors may have
unusual light curves that are flat at peak brightness or that peak early in
their visibility, as would happen if the meteoroids break more readily due to
a high surface-to-mass ratio.  In a 1999 paper (JIMO 27, 85), R. McNaught and
D. Asher first found that the earth will pass this trail by 0.00009 AU at
2006 Nov. 19d04h45m UT (236.615 solar longitude) for observers in southern
Spain; the peak time will be 7 minutes earlier in eastern Brazil and 6
minutes later in Scandinavia.  A recent update
[http://www.arm.ac.uk/leonid/dust2006.html] puts the peak rate at ZHR = 120
meteors/hr.  These results are confirmed by Lyytinen and van Flandern (2000,
Earth, Moon, and Planets 82/83, 149) and Vaubaillon [http://www.imcce.fr/
page.php?nav=en/ephemerides/phenomenes/meteor/DATABASE/Leonids/2006/index.php],
who have nominal peak times at 2006 Nov. 19d04h50m and 58m UT, respectively,
and predicted peak rates of 50 and 200 meteors/hr.  In 1969, the same dust
trail (one-revolution old) contained grains that had a similar high-orbital-
period difference (with respect to the comet) of 0.90 yr, passing at a similar
geocentric distance of 0.00005 AU, when observed rates peaked at ZHR = 400 +/-
50 meteors/hr, with a magnitude-distribution index (chi) of 2.96 +/- 0.11
(Jenniskens 1995, AA 295, 206).


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2006 CBAT
2006 October 31                  (CBET 710)               Daniel W. E. Green