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Re: (meteorobs) Are Xi Draconids same stream as June Lyrids?

On Fri, 25 Jun 1999, Stop Light Pollution! wrote in Meteorobs:
 
>    Is it possible that, if the Xi Draconids exist, and the June Lyrids
> are extinct, or nearly so, that they are the same meteor stream? That
> the old June Lyrids parent comet was perturbed by Jupiter or something.
> They both have similar velocities and are relatively (~20 deg.) close
> in the sky. Any comments?

Hello!

This point indeed has been raised soon after I reported my 1996 'xi 
Draconid' outburst of June 15-16 1996. A number of people pointed to the 
similarity between both streams.

Indeed, there is reason to think that the 'June Lyrids' and 'xi Draconids' 
concern the same stream. The radiants reported for 1966 (Stan Dvorak's 
June Lyrid outburst) and 1996 (my xi Draconid outburst) are not too far 
apart as mentioned by GWG above: my 1996 radiant was at RA 18h40m and dec. 
+55, with some leeway possible most notably towards a somewhat lower 
declination (see plotting chart that appeared in WGN with my report). 
This is based on 13 plotted stream members from that night. Dvorak in 
1966 noted a radiant at 18h 35m, dec. +30 (can anyone tell me if this is 
based on plottings?). Perhaps the real radiant position is somewhere 
inbetween. We have seen this kind of disagreement before with streams 
displaying either a very low level activity or short duration bursts:
note that before we published our 1995 photographic and video results on 
the alpha Monocerotids in November, the radiant positions given for that 
stream were very wide apart from source to source. Perhaps, there even is 
a real deviation of this order from return to return with these kind of 'far 
comet meteor outburst' (note that similar deviations occur in the 
reported radiants for the three Aurigid outbursts too...).

Dvorak's 1966 activity burst occured at solar longitude 85.13 (+- 0.05), 
while my activity burst from 1996 occured at 85.17 (+-0.01), which is 
virtually identical. Another reason to think that the 'two' streams are 
identical and it is 'just' a matter of some uncertainty in the correct 
radiant position.

I have grown ever more convinced that what Dvorak observed in 1966 and 
what I observed in 1996 (see note below) were modest examples of a 'far 
comet type' meteor outburst from one single stream (and I suggest to give 
the Dvorak stream name priority and hence call the stream 'June Lyrids', 
not 'xi Draconids' as I called them in 1996). There are reasons to think 
so. Peter Jenniskens (NASA/Ames) has done a lot of work on these 
enigmatic outbursts that happen without warning. He found that for the 
streams with a historic record of several of such outbursts (which he 
calls 'far comet type' (Jenniskens A&A 295 (1995), 206; A&A 317 (1997), 
953), the outburst tend to correlate with certain barycentric positions 
(see Jenniskens 1997; the barycenter is the position of the gravity 
center of the solar system; the sun shows a slight wobbling movement 
around this barycenter which is known as the 'reflex motion'). In 
essence, the outburst thus tend to occur when the two planets that 
account for the major part of the barycentric displacement (Jupiter and 
Saturn) occupy similar orbital positions. He also noted that a favourable 
condition for many historic far comet outburst examples seems to be when 
either of those two planets is in line with the nodal line of the stream in 
question (see Jenniskens' 1997 Astronomy & Astrophysics paper for 
details: 'Meteor outbursts and the reflex motion of the sun, A&A 317, 
953-961). When we look at the 1966 and 1996 'June Lyrids/xi Draconids', we 
note:

a) Saturn occupying a similar orbital position in both 1966 and 1996;
b) Jupiter in line with the nodal plane of the stream in both 1966 and 
1996, albeit in opposite directions.

Therefore, and given the similarities between the 1996 and 1996 meteor 
occurence, I feel what Dvorak observed in '66 and I observed in '96 were 
modest 'far comet' outburst from one single stream. Either 2026 or 2020 
might then give prospects for another small outburst.

- Marco Langbroek
  Dutch Meteor Society
  (observed the 1996 June Lyrid/'xi Draconid' outburst)


note: for those who do not know the history behind the 'xi Draconids' of 
1996: on the night of June 15-16 1996, during a routine meteor observing 
session from my house at Voorschoten, the Netherlands, I observed a 
-totally unexpected- burst of meteors from the radiant mentioned above. 
In 1.90h effective with Lm near +6.1, I noted (in addition to a good 
number of sporadics) 13 meteors from this radiant, most of them occuring 
within one hour. In fact, 5 of the 13 occured in rapid succesion within ten 
minutes (between 22:32 and 22:42 UT). Although rates were not 
spectacularly high, being confronted with an activity of such unexpected 
character was thrilling!
That same June, Bob Lunsford and George Zay reported (independantly from 
my observation) a low level activity over several nights from a similar 
radiant. While the stream thus was noted by more than one observer in 
1996, the surge I observed near 22:40 UT on June 15 has as far as I know 
not been observed by any other observer, which is very unfortunate 
because confirmation of these kind of events is important.
Stan Dvorak was an American observer who observed a surge of 13 meteors 
(and a low number of sporadics in addition) in 90 minutes time from a 
radiant about 20 degrees south of the 1996 radiant from his observing 
site at La Mesa (CA) on June 15 1966.

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