Delta Aurigids

Observer's Synopsis

Meteors from this stream may be observed during the interval of September 22 to October 23. Maximum occurs between October 6 and 15 from an average radiant of RA=84.8 deg, DEC=+51.9 deg. The radiant's daily motion is +1.2 deg in RA and +0.1 deg in DEC.

History

This meteor shower was discovered in 1979, while Jack D. Drummond, Robert K. Hill and Herbert A. Beebe (New Mexico State University) were determining the orbits of 13 meteors doubly photographed during 1976 and 1977 at the NASA-NMSU Meteor Observatory in southern New Mexico. Two of the 13 meteors had nearly identical orbits and had been photographed on October 13 and 18, 1977. It was concluded that the possible date of maximum might have occurred during October 15-16, 1977, from an average radiant of RA=95.7 deg, DEC=+52.5 deg.
The first observations of this shower occurred during October 1980, as Norman W. McLeod, III (Florida) and Drummond (New Mexico) carried out independent visual surveys. McLeod's observations began early as he noted 1 meteor in 4.6 hours on October 6, 1 in 0.8 hours on the 10th and 0 in 2.7 hours on the 12th. Both men observed on the 13th, with McLeod noting 2 in 1 hour and Drummond detecting 3 in 2 hours. Drummond's observations of the 14th and 15th revealed 2 in 2 hours and 5 in 2 hours, respectively---the latter being the highest determined hourly rate of 2.5 per hour. Observations continued from the 16th to the 22nd, with the Delta Aurigids being either nonexistent or possessing hourly rates of 1 or less.
During 1981, Drummond isolated 5 probable members of this stream that had been photographed by American and Russian surveys conducted between 1950 and 1962. Combining these 5 with the 2 meteors from the 1977 NASA-NMSU survey, Drummond concluded that the shower's duration might be from September 29 to October 18. The mean date of activity was October 8, with the radiant then being RA=87.8 deg, DEC=+50.2 deg. The shower's daily motion was given as +1.0 deg in RA and +0.1 deg in DEC. Lastly, the stream's mean orbit, based on the 7 photographic meteors, is a retrograde orbit with a period of 115 years.
From both the photographic studies and the visual survey of 1980, Drummond pointed out that only the second half of the indicated shower duration had been covered and that the determination of the shower's true maximum "must await further observation from the first half."
The Author has recently searched through the 39,145 radio meteor orbits obtained during the two sessions of the Radio Meteor Survey and isolated 24 probable Delta Aurigids. The orbital details will be further discussed in the "Orbit" section below, but several interesting features can be discussed here.
First, while comparing the radio meteor orbits with the photographic orbits, it becomes apparent that the radio meteors seem to reach peak concentration about one week before the photographic meteors. The radio meteors also possess a much smaller semimajor axis of 2.3 AU, compared to the photographic meteors' semimajor axis of 18.7 AU. At first glance, it might seem that there is a significant indication of mass distribution within this stream, but when all of the data was combined and reexamined it appears that the following very distinct filaments may be present in this stream:

Searches for previous sightings of the Delta Aurigid shower have not proved extremely successful, though one very promising observation occurred during October 14-17, 1876, when William F. Denning plotted 6 meteors from an average radiant of RA=90 deg, DEC=+58 deg. The meteors were described as rapid, with streaks. This radiant is the closest 19th century match for the Delta Aurigids, but is only borderline according to the D-criterion.
More positive observations of the Delta Aurigid stream are found in Cuno Hoffmeister's Meteorstrome. Three excellent radiants are given for October 1, 1910 (RA=86 deg, DEC=+57 deg), October 9, 1920 (RA=84 deg, DEC=+48 deg), and October 7, 1935 (RA=83 deg, DEC=+48 deg). The first radiant seems a good example of a filament "A" shower, while the latter two radiants probably belong to filament "B". Seven additional radiants also appear, but are strictly borderline, according to the D-criterion. These seven radiants might belong to filaments "A", "B" and "C". Filament "D" continues to be elusive in the records of visual observations and may be composed mostly of small, telescopic particles---if it exists at all.
Interestingly, despite these indications of visual observations, there is no appearance of this shower in the records of the American Meteor Society, or most other catalogs of visual radiants.

Orbit

Combining 10 photographic meteors obtained from W1954, MP1961, B1963, C1964, BK1967 and D1980, with 24 radio meteor orbits obtained from the orbital data obtained during the two sessions of the Radio Meteor Project, the Author found this stream to possess the following average orbit.

AOP AN i q e a
227.1 190.9 125.9 0.859 0.720 3.068

Some differences were noted between the photographic and radar data as the following two orbits indicate:

AOP AN i q e a
Photo 229.5 195.3 131.1 0.823 0.956 18.71
Radar 226.4 188.6 123.9 0.878 0.617 2.292

Finally, despite the slight differences between the radar and photographic orbits, the Author combined all of the orbital data and found the following four distinct orbits which may represent filaments within the Delta Aurigid stream.

AOP AN i q e a
A 210.2 186.5 123.4 0.951 0.621 2.509
B 229.4 193.2 129.5 0.848 0.810 4.463
C 243.5 199.3 132.6 0.753 0.793 3.638
D 245.2 188.2 116.4 0.773 0.648 2.196

Stream "A" was based on 11 meteors, "B" on 13 meteors, "C" on 4 meteors and "D" on 5 meteors. As can be seen, these four branches cross the ecliptic between solar longitudes of 186.5 deg and 199.3 deg, or, roughly between September 30 and October 13. Streams "A" and "B" apparently produce the strongest activity and have maximums between September 30 and October 7.
Only one known comet seems to have an orbit similar to that of the Delta Aurigids---comet Bradfield (1972 III). The link should not be considered definite, but its orbit is given here as follows:

AOP AN i q e a
1972 III 257.7 159.6 123.7 0.927 0.998 494.8

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