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(meteorobs) Excerpts from "CCNet 107/2000 - 20 October 2000"
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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 20 October 2000
Date: Fri, 20 Oct 2000 11:43:33 +0100
CCNet 107/2000 - 20 October 2000
--------------------------------
(1) LEONIDS 2000
Rob McNaught <rmn@aaocbn.aaodot gov.au>
[...]
(6) THERMAL EVOLUTION OF THE CENTAUR OBJECT 5145 PHOLUS
EKOnews@boulder.swridot edu
[...]
===========================================================================
(1) LEONIDS 2000
>From Rob McNaught <rmn@aaocbn.aaodot gov.au>
Dear all,
David Asher and I have written a piece for the next issue of WGN that
summarizes our expectations for 2000. We have discussed/critiqued some
other approaches to ZHR predictions and made clear the limitations of
our method. This note is being sent now to meteorobs for two reasons.
Firstly Sky and Telescope 2000 Nov., p 111 makes two statements about
our work that are erroneous, and I want to correct these before anyone
gives up on the 2000 Leonids. The second is that there are two
wonderful coincidences in this year's Leonids that will allow the
derivation of valuable data about dust trail structure from visual
observations. WE NEED YOUR OBSERVATIONS! (well, via the IMO!)
"... David Asher and Robert McNaught, foresee just a 'normal' display of
up to 100 meteors per hour ..." S&T 2000 Nov. p 111
Whilst our nominal predictions is of ~100, we do not discount, and never
have discounted, the possibility of storm activity. Our analysis relies
heavily on historical ZHR measures *for dust trail encounters*, and none
exist for the geometry being encountered in 2000 November (4 and 8-rev
trails). We suspect activity will be "low", but could still be the best
meteor shower many observers will have seen.
"This year will truly put the Asher-McNaught meteor-trail theory to the
test ..." S&T 2000 Nov. p 111
>From the comments above, it is clear that we make no strong prediction
so unless substantial activity occurs well away from a dust trail
encounter, there will be no test of our theory this year. Only Ferrin,
amongst those who have made predictions, has done so without consideration
of the existence of dust trails.
I've seen various values for our ZHR predictions floating about. Our
latest predictions are those that appeared in Sky and Telescope 2000 June,
p 32. The "?" appearing after the predicted ZHRs of 100 for this year are
there for a very good reason, as stated above. Hopefully we'll have another
look at representing the lower activity extremes of dust trails in the next
couple of weeks, but with such limited data available for dust trail
encounters, such extrapolations will always be unreliable. However,
this year will provide much needed data in this regard, so even if
predictions are questionable, the observed ZHRs from dust trail
encounters this year *will be extremely valuable*.
The following dust trail encounters are within 0.0050 AU of the Earth
distance ZHR
2000 Nov. 17 07:53 UT 2-rev -0.0012 AU ?
08:22 1-rev +0.0031 0
18 03:44 8-rev +0.0008 100?
05:51 6-rev +0.0030 0
06:44 5-rev +0.0028 0
07:51 4-rev +0.0008 100?
For miss distances of between 0.0000 AU to -0.0007 AU, the time of
prediction appears to be accurate to around 5 minutes. This indicates that
the dust trails are basically flat sheets, and this is in fact the first
observational evidence to that effect (see R.H. McNaught, D.J. Asher,
Meteorit. Planet. Sci. 34, 1999, pp. 975-978). The timing for more distant
trails may be less precise, but as we argued in the above paper, there is
no observational evidence for distant dust trail signatures in the 1965 or
1998 observations (nor does the IMO analysis indicate this for 1999).
We do not expect any young dust trail encounters to give a broad maximum
(FWHM >~1 hour), but this year will be a good opportunity to gather
suitable data.
In the above table, only the miss distances are given for several dust
trail encounters. Other relevant parameters are the minimum ejection
velocities (represented by da0 in our original paper) and the dilution
of the trail density with age (fM in our original paper). We feel confident
that the 1, 5 and 6-rev trails will produce activity lower than the
likely background activity, but 3 trails are worthy of special effort.
Several authors have previously published predictions of detectable
activity from the 2-rev trail in 2000. We certainly consider this as a
possibility, although believe such activity will be much lower than the
4 and 8-rev trails. If the 2-rev trail has significant activity in
relation to these other two trails, this will indicate a substantial
asymmetry in the dust trail profile in the sunward/anti-sunward direction
and/or a notable aging effect additional to trail stretching (the only
aging factor we believe is important). [In the upcoming note in WGN, David
and I comment on four errors in the analysis of Jenniskens et al from the
April-June 2000 WGN, as mentioned in the June 2000 S&T p. 32, which had
suggested there was an error in our assumed position for the core of the
dust trails.]
The two coincidences in this year's trail encounters are
1) the 4-rev and 8-rev trails are encountered at the same geometry of
+0.0008 AU. This would mean that differences in the observed ZHR are
caused by aging factors alone. Or at least they will be if the disruption
of the 8-rev trail due to perturbations is not major! [Magnitude index
is included in our predictions in a roundabout way.] Analysis of this
may be difficult, but the European longitude observations of the 8-rev
trail and the 4-rev from the Americas four hours later will prove
interesting.
2) the 2-rev trail and the 4-rev trail encounters occur exactly 1 day
apart (well, within 2 minutes sidereal time!). This means that observers
using the same location, observing in the same direction and, hopefully,
in the same conditions (Moon will make a minor difference) will get
directly comparable data on these two trails. Both the relative intensity
and the magnitude index will be important results.
So my advice is to get out and observe, not that readers of meteorobs need
any such encouragement! Also, if you are not at European or American
longitudes your observations are just as valuable. Any outburst must be
related to the background activity and who knows, something unusual
might happen. Don't forget the occasional Leonid (or Taurid!) fireball
and those wonderful long duration trains. Perhaps 2000 won't amount to
much, but if such is the case, you should feel well satisfied that your
observations will go towards refining the various theories of the
structure of the Leonids.
Good luck!
Cheers, Rob
Robert H. McNaught
rmn@aaocbn.aaodot gov.au
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(6) THERMAL EVOLUTION OF THE CENTAUR OBJECT 5145 PHOLUS
>From EKOnews@boulder.swridot edu
Thermal Evolution of the Centaur Object 5145 Pholus
M.C. De Sanctis1, M.T. Capria1, A. Coradini1, and R. Orosei1
1 Istituto di Astrofisica Spaziale CNR, Area di Ricerca di Roma Tor Vergata,
via del Fosso del Cavaliere, 100, 00133 Roma, Italia
We present the results obtained by the simulations of different thermal
models of 5145 Pholus, one of the known Centaurs. Pholus orbit is highly
eccentric, similar to that of comets but its dimension is more similar to
larger asteroids. Pholus cannot be clearly numbered into either class. The
most likely source of Centaurs is the Edgeworth-Kuiper Belt: dynamical
studies and physical properties suggest that Pholus recently entered in the
planetary zone. Here we assume that the nature of Pholus is that of a
cometary body made by different ices and dust. We have computed the thermal
evolution of this object under different conditions: as a ``new'' object,
namely an undifferentiated body, and an ``old'' one, differentiated and aged
in the Kuiper Belt. We have tried to see also the effects induced by the
presence of an organic dust on the overall evolution. Both the ``new'' and
the ``old'' object show low, but different, levels of gas activity.
Published in: Astronomical Journal, 120, 1571 (2000 September)
For preprints, contact M.C. De Sanctis at cristina@ias.rm.cnrdot it
or on the web at
http://www.journals.uchicagodot edu/AJ/journal/issues/v120n3/200048/200048.html
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