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(meteorobs) A-T campaign 96: Part 3
The following is the third part of an unsubmitted/unpublished paper I have
written detailing what we know of the Aries-Triangulid stream. I will be
the first to admit that this installment contains a lot more hypothetical
data that facts, but I hope it might inspire someone to set up either
photographic or video equipment in the attempt to acquire some
double-station meteors.
Part 3: An Orbit is Determined
By Gary W. Kronk
With the apparent conflict of the Kronk/Sleeter radiant with the Gliba
radiant at hand, it became apparent that an investigation into the orbit of
this stream was necessary.
As with previous investigations of minor meteor showers, Kronk began with a
parabolic orbit for each radiant. The Kronk/Sleeter radiant (K/S) was taken
as RA=30 degrees, DEC=+29 degrees--being a combination of their plots and
the highest weighted radiants taken from the lists in part 2 of this
series. The Gliba radiant (G) was taken as RA=28 degrees, DEC=+20
degrees--being a combination of Gliba's estimate and, once again, the
highest weighted radiants taken from the lists in part 2 of this series.
The resulting parabolic orbits were as follows:
AOL AN INCL PERI ECC
K/S 304.2 169.5 127.5 0.221 1.0
G 328.5 169.5 137.6 0.074 1.0
(where AOL=Argument Of Perihelion, AN=Ascending Node, INCL=Inclination,
Peri=Perihelion distance, and ECC=eccentricity)
Although these two orbits had their similarities, large descrepancies in
the AOL and perihelion distance looked menacing. I turned to the works of
previous astronomers and decided to use the D-criterion, created by Richard
B. Southworth and Gerald S. Hawkins in 1963, to compare the orbits. I also
went one step further and also used the D'-criterion created by Jack D.
Drummond in 1979. These comparisons both revealed that the two parabolic
orbits were probably not associated with one another.
Two more sets of elements were also computed for each radiant: one with an
orbital period of 6 years, which is typical of the Jupiter family of
comets, and the other with a semimajor axis of 0.9 AU, being representative
of a minor planet of the Aten class. These orbits were as follows:
Cometary Orbit
AOL AN INCL PERI ECC
K/S 314.6 169.5 122.2 0.173 0.948
G 336.7 169.5 128.6 0.048 0.985
Aten-like Orbit
AOL AN INCL PERI ECC
K/S 340.1 169.5 85.5 0.069 0.923
G 347.9 169.5 50.5 0.026 0.972
Taken together, these three sets of orbits offered a strong idea as to how
the orbits change with variations in the semimajor axis, and, therefore,
velocity.
These orbits were subsequently compared to a database of nearly 40 thousand
radio meteor orbits that were determined by Zdenek Sekanina from data
collected during the two sessions of the Harvard Radio Meteor Project
(HRMP) of the 1960s. Interestingly, Kronk immediately found a number of
meteors that came from both radiants. What is even more interesting is that
Sekanina had identified these two radiants among the data collected during
the 1969 session of the HRMP. Sekanina referred to these streams as the
Alpha Triangulids and the Alpha Arietids. The Alpha Triangulids were based
on 13 radio-echo meteors which indicated an average radiant of RA=30.4
degrees, DEC=+29.5 degrees. The Alpha Arietids were based on six radio-echo
meteor orbits which came from an average radiant of RA=32.6 degrees,
DEC=+21.8 degrees. The resulting orbits, as given by Sekanina, were as
follows:
AOP AN Incl Peri Eccen Period
Alpha Tri 345.9 165.7 38.7 0.087 0.870 0.55
Alpha Ari 324.6 165.8 117.4 0.143 0.929 2.81
What obviously became visible from these orbits was that the Alpha
Triangulids were in a distinctly Aten-like orbit, while the Alpha Arietids
were in an orbit comparable to the Earth-crossing minor planets of the
Apollo family.
Sekanina's streams aside, Kronk continued with his own search through the
nearly 40 thousand radio meteor orbits of the HRMP. He found that these
surveys covered the first half of September during 1962, 1963, 1964, and
1969. While the solar longitude of the radiant's appearance in 1993 was
169.5 degrees, Sekanina's surveys never operated while the radiant was
above the horizon beyond a solar longitude of 168.1 degrees, or more than a
day earlier than the potentially observed maximum in 1993. The result of
the search was the detection of 47 meteors which could represent as many as
five streams or five filemants of one diffuse stream. Both of Sekanina's
1969 streams (Alpha Triangulids and Alpha Arietids) were detected among
this group, as well as three potential minor radiants which produced 4
meteors or less. The Alpha Arietids appeared exclusively in 1969, so
Sekanina's orbit given above could not be improved upon. The Alpha
Triangulids produced meteors in 1962, 1963, and 1969, which subsequently
increased the overall number of radio meteors; however, it was then noted
that a strong core of 9 meteors was very apparent. The orbit of this core
was as follows:
AOP AN Incl Peri Eccen Period
Radio-echo 344.1 165.8 36.1 0.097 0.857 0.560
The resulting average radiant for solar longitude 165.8 degrees was RA=27.5
degrees, DEC=+28.8 degrees.
The Aten-like orbit seemed confirmed for the Alpha Triangulids, but a new
problem arose. Correspondence with Duncan Steele, who is currently
conducting an elaborate radio meteor survey in the Southern Hemisphere,
revealed the liklihood that the velocities of Sekanina's radio meteors were
all probably underestimated by as much as 20 percent. This meant the
semimajor axis, as well as the orbital period, of every meteor in the HRMP
was larger than indicated. Although the data contained within the HRMP
seemed to confirm the existence of activity from the Aries-Triangulid
region, it was thought best to pursue the question of the orbits from
another, unfortunately hypothetical, perspective.
Conversations between Kronk and Gliba late in 1993 and early in 1994
resulted in the mutual agreement that the stream might have been detected
in 1993 because of enhanced activity. If such was the case, what caused
this enhanced activity and has this stronger-than-normal activity been
observed in the past?
The answer to the first part of this question can be traced back to the
basic origins of meteor streams--the comets. As comets travel around the
sun, their ices are melted by the sun's heat and dust is released. This
dust basically continues along in the comet's orbit until it is either
kicked out by the gravitational field of some planet or until it encounters
another object, like a planet for instance. From studies of other
comet-meteor shower relationships, the most populated portion of a meteor
stream typically remains near the parent comet. This is why the Perseids
increase in intensity when periodic comet Swift-Tuttle is near perihelion,
and why the Leonids increase in intensity when comet Tempel-Tuttle is near
perihelion. These comparisons are not provided here to indicate a comet is
still orbiting within the orbit of the Aries-Triangulid stream, but a knot
of material could exist in the place of a dead comet. Since the
Aries-Triangulids seem to most likely be a high-inclination stream,
planetary perturbations would not greatly affect the stream's orbit.
The answer to the second question can be found from looking at part 2 of
this series. If it is true that the two independent discoveries of this
radiant in 1993 indicated enhanced activity, then similar activity might
have been responsible for the two independent discoveries that occurred in
1934, 1940, and 1951. When all four years of possible enhanced activity are
considered, it seems that an orbital period of 5.5 to 6 years is indicated.
This would imply an orbit comparable to that of an average comet of the
Jupiter family.
Does any other evidence exist to support a link to the Jupiter family of
comets? The answer is yes.
The velocity of the 1993 was not determined any further than estimates of
the meteors travelling at "moderate" speeds. Such estimates would probably
eliminate an origin from an Aten-like orbit, but let's proceed by playing
with the semimajor axis of our hypothetical orbits.
After computing the three sets of hypothetical orbits given above,
something interesting became apparent--the K/S radiant was the lower
inclination when parabolic, but it became the highest inclination when
reduced to an Aten-like orbit. Somewhere between a parabolic and Aten-like
orbit the two streams would therefore possess similar inclinations. The key
was to find out the magical semimajor axis. After playing with the numbers,
Kronk found that if the semimajor axis was assumed to be 1.8 AU for both
streams the inclinations met at about 115 degrees. In addition, the
D'-criterion of Drummond began to indicate the two orbits could now be
associated. Even more interesting is the fact, that Drummonds' criteria
were only slightly less favorable for a semimajor axis of 3.2 AU.
What all of this meant was that the diffuse radiant reported for this
stream in part 1, and subsequently the "Triangulid" stream and the
"Arietid" stream, can only work when the two streams were related and a
relationship seems most favorable when the assumed orbital periods were 2.4
and 5.7 years. The latter matches the four dates of 1934, 1940, 1951, and
1993 reasonable well, offering additional evidence that this could be a
cometary orbit with a rather notable knot of material moving within it.
Whether a comet still exists within one of these orbits is unknown, but it
should be noted that a problem does exist. Up to this time we know of no
comet within the Jupiter family that moves within an orbit with an
inclination higher than 60 degrees. The proposed inclination of the
Aries-Triangulid stream(s) is 115 degrees and this may be hard to swallow.
A key to solving the problems of the orbit of the Aries-Triangulid stream
would be the acquisition of a velocity and a precise radiant. Once these
are achieved, most of the hypothetical discussions above would be
eliminated.
The next installment will be Part 4: The 1994 Observations