Alpha Virginids

Observing

The strongest branch of the Virginid Complex is the Alpha Virginids, which may represent the actual core of the very diffuse Virginid stream. The Alpha Virginids are of long duration--spanning the period of March 10 to May 6--and the radiant is rather diffuse. Maximum hourly rates typically reach 5 to 10 during April 7 to 18, with the average radiant being α=204°, δ=-11°. The meteors are generally slow.

History

This display was apparently first detected by A. S. Herschel during April 10-17, 1895, when 8 meteors were plotted from a radiant of α=209°, δ=-7°. Aside from 6 fireballs that were noted from this radiant between 1896 and 1915, no other activity was reported until April 12-15, 1915, when W. F. Denning saw a "rich shower" from α=209°, δ=-10°.

Over the years, observations were occasionally made to try to understand the complexity of the Virginid Complex, but no coordinated attempts were conducted to better understand the Alpha Virginids. A first step came in the early 1950s, when several meteors of the Virginid Complex were detected during the Harvard Meteor Project. Although these indicated a very diffuse radiant, a fairly concentrated core was noted, which was the Alpha Virginids.

Support for the possibility that the Alpha Virginids represented the actual core of the Virginid Complex came in 1973, when E. I. Kazimirchak-Polonskaya and A. K. Terent'eva published a paper examining the evolution of the Virginid Complex. They took the orbit of one of the Alpha Virginid photographic meteor orbits to represent the orbit of the Virginid Complex, and then created 10 groups of hypothetical meteors at various positions along the stream's orbit. The stream was then subjected to perturbations by 7 planets (Venus to Neptune) over the interval 1860-2060. What the study revealed was that the groups underwent large changes in both their radiant position and date of maximum. In some cases, the radiant moved around an elliptical area measuring 15° by 31°, while the date of maximum moved back and forth across a period spanning 51 days. Even more interesting was their belief that each of the radiants would probably remain active, even after the primary radiant, now known as the Alpha Virginids, moved on.

The first big meteor sky survey to provide information about the Alpha Virginids was the 1968-1969 session of the Radio Meteor Project. Z. Sekanina determined the duration as March 10-May 6. The date of the nodal passage was given as April 8.7, at which time the radiant was at α=203.6°, δ=-11.7°. This survey revealed the Alpha Virginids to be in a very low inclination (1.5°) orbit and this seems to explain the Author's puzzling findings when searching for photographic members of this stream years later.

The Author has identified 12 probable members of the Alpha Virginids among the 2,529 meteors listed in a 1961 paper written by R. E. McCrosky and A. Posen. The puzzle that developed involved the argument of perihelion and ascending node of the meteor orbits, because when the meteors were averaged it was unclear whether these two orbital elements should be 104.7° and 200.3°, or 284.7° and 20.3°, respectively. Since 75% of the meteor orbits indicated the latter set of elements, the Author trusted these as indicating the true orbit, despite the fact that the former set agrees well with the radar data.

The first coordinated visual study of the Alpha Virginids was made by the Western Australia Meteor Section in 1979 and 1980. Jeff Wood said observers obtained excellent observations of this shower. In the former year the ZHR reached 4.11+/-1.01, while the 1980 maximum reached 6.09+/-0.59. Wood summarized the Australian observations by pointing out that the Alpha Virginids typically produce 5-10 meteors per hour, with a maximum that more commonly falls on April 11. The duration of activity extends from March 30 to April 17.