Eta Virginids

Observer's Synopsis

Observations of this shower indicate a duration of February 24 to March 27. Maximum is not prominent, but seems to fall on March 18 (solar longitude=358 deg), from a radiant of RA=185 deg, DEC=+3 deg. The maximum hourly rate reaches about 1 to 2. A possible southern branch of this stream seems to exist about 10 deg to the south.

History

The Eta Virginids appear to be a fairly diffuse branch of the Virginid complex of February to April. Although visual radiants are not particularly abundant in the literature, this stream has produced its share of photographic meteors and has appeared in each of the sessions of the Radio Meteor Project conducted at Havana, Illinois, by Zdenek Sekanina during the 1960's.
The first detection came during 1961-1965, when 11 meteors were detected during March 12-27. The nodal passage came on March 18 from a radiant of RA=184.9 deg, DEC=+2.7 deg. The second detection came during the 1968-1969 survey, when 26 meteors were detected during February 24-March 14. Nodal passage came on March 10 from an average radiant of RA=182.2 deg, DEC=+13.8 deg.
Sekanina's data indicates radiants separated by about 10 deg between the two sets of data. A look at the orbital elements reveals that this difference is entirely due to the inclination, since all the other elements are nearly identical. The Author has examined the individual radio meteor orbits used in Sekanina's two analyses and finds a diffuse stream, which might possess a radiant diameter of 10-12 deg. There is also a possibility that this stream may itself be composed of two filaments---one being of high inclination, while the other is of low inclination. This latter hypothesis could explain the differences in duration and dates of maximum activity among the two radio-meteor streams detected by Sekanina. Support is also apparent among the recent observations of the Western Australia Meteor Section (WAMS).
Observations of both Eta Virginid showers were made by members of the WAMS during 1979 and 1980. In the former year the early stream was observed during February 23-March 5, with a maximum ZHR of 2.20+/-0.52 coming on March 5 from a radiant of RA=184 deg, DEC=+1 deg. The second stream was detected during March 16-24, with a maximum ZHR of 3.33+/-1.66 coming on March 20 from RA=183 deg, DEC=+2 deg. In 1980, the early Eta Virginids were seen during February 23-March 10, with a maximum ZHR of 2.27+/-0.34 coming on March 9 from RA=184 deg, DEC=+1 deg. The second shower was observed during March 14-23, with a maximum ZHR of 3.22+/-1.07 coming on March 21 from RA=187 deg, DEC=0 deg.
When all things are considered, it is interesting that the shower that occurs in the second half of March is very similar to the stream detected by Sekanina during 1961-1965. The February-March activity noted by the WAMS does not, however, match the 1968-1969 radar data of Sekanina in the declination of the radiant. In fact, there is about a 10 deg difference at the time of maximum.
The Author has conducted a mathematical determination of the radiant ephemeris of this stream and revealed a daily motion of +0.9 deg in RA and -0.4 deg in DEC. Confirmation of this was given when the 37 radio meteor orbits were grouped according to inclination and then subjected to a least-squares fit.
In 1980, Sam S. Mims (Baton Rouge, Louisiana) suggested a relationship between this stream (he simply referred to it as the Virginids) and a comet discovered by Dunlop (Parramatta) on September 30, 1833. The comet was only observed 15 times in 16 days, by a few observers in South Africa and Australia, so that the orbit is considered as somewhat uncertain. Nevertheless, during 1888, Schulhof showed that the eccentricity could be as small as 0.8. Mims pointed out that the orbits of the comet and meteor stream are close except for the longitude of perihelion, which is about 50 deg off. He suggested that, if the comet was indeed of short period, it may "have been perturbed (by Jupiter) before 1833...." Although realizing that several assumptions would have to be accepted to support some of his suggestions, Mims added, "it is interesting to think that we could learn much more today about a comet observed for only a short period in the early 19th century...."
One final point about this shower's activity is that a third radiant with a position about 10 deg to the south may be present throughout the month of March. The strongest support exists in the data collected by Sekanina during 1961-1965, when a radiant referred to as the "Southern Eta Virginids" was described as having a duration extending from March 9 to April 9. The date of the nodal passage was given as March 22, at which time the radiant position was RA=178.9 deg, DEC=-8.2 deg.

Orbit

The orbits determined by Sekanina during the two sessions of the Radio Meteor Project are as follows:

AOP AN i q e a
S1973 282.4 357.3 3.7 0.496 0.707 1.691
S1976 281.8 349.3 11.3 0.501 0.703 1.690

As noted above, Sam S. Mims has suggested this stream is related to comet Dunlop (1833). The orbit of this comet is given in Brian G. Marsden's Catalog of Cometary Orbits (1983) as

AOP AN i q e a
1833 259.6 324.9 7.3 0.458 1.0 ---

Although the orbit is given here as parabolic, primarily due to the comet's being observed for only 16 days, the original investigation by Schulhof in 1888, revealed the possibility of both a hyperbolic and an elliptical orbit, with the latter fitting the observations better. Schulhof indicated that it was not impossible that the eccentricity could be as small as 0.8.

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