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(meteorobs) IMO-NEWS: Linearids / Gamma Ursae Majorids
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Clear skies,
Lew Gramer <owner-meteorobs@jovian.com>
------- Forwarded Message
Date: Wed, 27 Oct 1999 17:41:43 +0100 (MET)
From: Mailing List Operator <news@charlie.luc.acdot be>
To: imo-news@imodot net
Subject: Linearids
---------- Doorgestuurd bericht ----------
Date: Mon, 25 Oct 1999 17:45:52 -0700
From: reinder j bouma <rjbouma@wxsdot nl>
Subject: Gamma ursae Majoriden
COMET 1999 J3 (LINEAR) and the GAMMA URSAE MAJORIDS.
This comet was one of several faint comets discovered by the
LINEAR program in May 1999. At discovery it was an inconspicu-
ous object of 17th magnitude, but when a first orbit became
available, it was soon realized that this comet could become
an interesting object both for visual comet observers, AND
meteor observers. A preliminary orbit indicated that periheli-
on was due on September 20, only 0.98 AU from the Sun.
Moreover, it appeared that the descending node of the high
inclination orbit was only 0.01-0.02 AU outside the Earth's
orbit, suggesting that dust particles associated with this
comet could possibly be observed around 11 November as fast
meteors emanating from a point near Gamma Ursae Majoris.
Now that the comet has been observed for almost 5 month, and a
fairly accurate orbit has been published, it is possible to
give a better assessment of what to expect of the associated
meteors.
Visual observations of this comet since discovery show that
this is an intrinsically faint comet, with Ho ~ 9.
The very steep rise in brightness with n = 6-8 ('normal' is 3-
4) strongly suggests that this is not a dynamically new comet,
but rather an old one, that is only active during a short
period around perihelion passage. Although a short (gas)tail
developed, it is also clear that this comet is not producing a
lot of dust. Nevertheless, it was a nice object in the morning
sky, reaching magnitude 7.5-8 in the middle of October.
Currently it is moving to high southern declination and star-
ting to fade.
The most recent orbital elements published in MPC 35814 also
show that C/1999 J3 is not a 'new' comet; the indicated period
is ~63000 year. It should be noted though that this figure is
only a rough indication. Strictly speaking the period is only
valid for a choosen time close to perihelion, and planetary
perturbations may have influenced this figure considerably,
sothat the 'original' period could easily be a few ten thou-
sends years more, or less. But it now appears safe to assume
that this comet has passed the inner parts of the solar system
before, although the period is quite long.
The aforementioned orbital elements show that comet 1999 J3
passed its descending node on October 2.86 (UT), at 1.0013 AU
from the Sun, only 0.0115 AU outside the Earth's orbit. The
Earth itself is crossing the orbital plane on November 11.82
(UT), 40 days 'behind' the comet. This is also the best time
that associated meteors can be observed, with a radiant near
R.A. 176 deg and Dec. +54 deg. (J2000.0). This position is
very close to second magnitude Gamma Ursae Majoris.
This sort of observing geometry has generally been found the
best for observing (high) activity of any possible shower.
Unfortunately, at the time around orbital plane crossing the
radiant is due north for European observers, and consequently
low over the horizon, but the further east one goes through
(Northern) Europe and Asia the better. Possible meteors are
swift, much like the Perseids.
Remains the important question what sort of activity we can
most likely expect in this particular case...
I don't think that here any comparison with most known meteor
showers is appropriate, even the ones that have similar dis-
tances to the Earth's orbit and a faint parent comet, like
P/Giacobini-Zinner/Draconids and P/Tempel-Tuttle/Leonids,
because of the big difference in time scale. One may expect
that any dust that has been ejected during a previous perihe-
lion passage of C/1999 J3 has been largely dissipated after
several tens of thousends of years, even when planetary per-
turbations are very minor, because of the high inclination
long period orbit, and thus not complicating the matter as in
the case of showers that are mainly residing in the inner
solar system, and more or less permanently subject to planeta-
ry perturbations, particularly by Jupiter.
But some (heavy) particles may have survived in the wake of
the comet, and some of these we may encounter...
It should be noted further that no long period comet in an
orbit that comes very close to the Earth has ever produced a
major shower. A good example is C/IRAS-Araki-Alcock that
came very close to the Earth in May 1983; the Earth passed
the plane of the comet near the descending node only 2 days
BEFORE the comet, and nothing significant was seen then.
However, careful monitoring by Dutch observers Langbroek and
Miskotte , as well as some other observers, in subsequent
years, showed a low annual activity of associated Eta Lyrids,
with a ZHR of ~2. This comet has an absolute brightness simi-
lar to C/1999 J3 with H10 ~ 9, but its period is considerably
shorter, near 1000 years.
Even a big comet like 1995 O1 (Hale-Bopp) that released huge
amounts of dust, only produced a few possible meteors in early
January 1998, although admittedly its miss distance is 10
times larger than that of C/1999 J3.
Considering all the pro's and con's I find it unlikely that
C/1999 J3 will produce a significant shower this year, despite
the favourable geometry. Nevertheless, it may be useful to be
on the alert, particularly within 24 hours on both sides of
the orbital plane passage, for fast meteors coming from the
Big Dipper. In would not be the first time that meteor showers
have a surprise for us after all!
---------------------------------------------------
Reinder J. Bouma email: rjbouma@wxsdot nl
Bekemaheerd 77 or: rjbouma@planetdot nl
9737PR Groningen
The Netherlands phone: +31 (0)50-5418227
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