(meteorobs) Comment on the Bootid radiant

Dear Roger, Adam and list

I'm delighted that some Australian observers saw this meteor outburst. We had
some cameras of the Fireball Network in operation, so we may have recorded
some of the brighter ones.

There is little doubt that what was observed was the June Bootids from
comet 7P/Pons-Winnecke.  In fact your observations ARE quite consistent.
The radiant you gave

1998 June 27 ~13:15UT  RA 230d Dec +35 (2000)  => Az 336  El 13

Thus the radiant was lower than you suggested.  This is also confirmed by the
bright meteor that had a motion parallel to the horizon at 10 deg elevation.
The radiant of that meteor had to be at less than 10 deg elevation.

Due to the low velocity, zenithal attraction will be quite marked.  Using a
value of 14 km/sec, this gives ~20 deg for the time of your observations,
placing the actual radiant 7 degrees below the horizon!  Your observed 
radiant converts to

Az 336 El 13-~20 =~-7   => RA~218 Dec~+53

and this is quite consistent with the observations from overseas and with
the June Bootids.  In fact one can take this a step further.  The Italian
observers saw the radiant at RA 224 Dec +50.  Assuming this was at ~22:30 UT
the radiant elevation would have been ~60 deg.  One can solve these two
observations of the apparent radiant, to give the velocity of the meteors.
A solution with an atmospheric velocity of 15 km/sec fits well, and gives
the apparent radiant (corrected for zenithal attraction) of RA 215 Dec +49.
Without precise observations of the radiant, one cannot derive an accurate
velocity from the apparent radiants, but it would appear that 13 km/sec is
too low for the above observations, and 19 km/sec too high.

P/Pons-Winnecke has had 10 encounters with Jupiter within 1 AU in the last
two centuries, and had a dramatic orbital evolution.  The current orbit
of the comet has a perihelion distance of 1.26 AU, so the observed meteors
are not associated with any recent passage of the comet.  However the
perturbations on the comet will be quite different from those on meteoroids
separated from the comet in the orbit.  These undertake a quite separate
orbital evolution and there is no reason to believe that what was observed
this year was not part of the 7P/Pons-Winnecke debris.

This event once again goes to show that the meteor sky is in continual flux
and that amateur monitoring is still of great value.

Rob McNaught
rmn@aaocbn.aaodot gov.au

>ASV Meteor Section Report, June Bootids Outburst, June 1998
>Outburst of meteor activity was witnessed by four observers from Melbourne, 
>Australia between 13 UT and 13:30 UT on the 27th June
>1998.  The meteor activity was found to radiate from a region around Corona
>Borealis around 15h20m R.A and +35d Declination.  The
>radiant position was about 20d above the local horizon.  The brief outburst
>produced two fireballs of -3 and -4 magnitude and
>another seven meteors of magnitude +2 to +3.  The meteors were
>characteristically very slow, did not leave trains and were
>predominantly white with an orange/red hue.
>The observing session on the 27th of June was conducted by Adam Marsh,
>Geoff Carstairs, Roger Vodicka and David Girling covering
>the period between 13 U.T and 16h40m U.T.  The outburst was confined to
>the period 13 U.T to 13:30 U.T although a -4 magnitude
>fireball was witnessed by David Girling at 9h30m U.T earlier that evening.  
vThe sky conditions were excellent with a limiting
vmagnitude of +6.5 or better.  The observers faced towards the North about
>45d East of the suspected radiant position.
>Observations conducted on the 26th June 1998 by Adam Marsh did not show
>any activity between a period of 14h to 18h U.T.
>Observations on the 29th June between 11 U.T and 12:30 U.T by Roger
>Vodicka also showed no activity from this region.
>It is possible that the meteors witnessed were not from the June Bootid
>radiant which has a Declination of +58d since this puts
>the radiant 10d below the local horizon. The -3 fireball witnessed had
>a path almost parallel to the horizon about 10d above it.
>This fact and the trajectory of other meteor paths seen points to the
>possibility of a radiant position around Corona Borealis.
>Report by Roger Vodicka (Section Director) and Adam Marsh (Asst Section