Theta Virginids

Observing

The duration of this shower extends from March 10 to April 21. A maximum of 1-3 per hour seems to occur around March 20, from a radiant of RA=194°, DEC=-2°. The radiant possesses a daily motion of +0.90° in RA and -0.31° in DEC.

History

The first person to recognize that this stream produces an active annual shower was Cuno Hoffmeister. In his 1948 book, Meteorstrome, his analysis of visual radiants revealed a large group of ecliptic showers visible during March and April, which he referred to as the Virginid complex. He demonstrated that this large, diffuse stream was composed of nine distinct radiants, with distinct dates of maximum. Although it had long been known that abundant activity was present from the Virginid area during March and April, Hoffmeister’s study was the first to attempt to identify the individual streams. The strongest supported shower of this complex belongs to the Theta Virginids and was based on 12 individual radiant determinations. It was most active on March 15 (λ=355.7°), from RA=192.4°, DEC=-1.5°.

The first apparent observation of the Theta Virginids was made by Robert P. Greg and A. S. Herschel during the interval of 1850-1867, when it became apparent that a radiant was active during March 5-17, from RA=190 deg, DEC=+1 deg. Ronald A. McIntosh also recognized this stream in his classic paper “An Index to Southern Meteor Showers.” Although relying on only two visual radiants, McIntosh noted a shower, which he referred to as the “46 Virginids,” was active during March 19-31 from an average radiant of RA=194°, DEC=-3°.

Recent observations of the Theta Virginids were made by the Western Australian Meteor Section. In 1977, meteors were observed during March 18-19. Maximum came on the latter date, when the ZHR was 3.17+/-2.24 and the radiant was at RA=196°, DEC=-1°. In 1979, the shower was observed during March 16-31. Maximum came on the 20th, when the ZHR peaked at 2.61+/-0.87 from an average radiant of RA=196 deg, DEC=-1 deg. During 1980, observations of the shower were made during March 19-23. At maximum on March 21, the ZHR reached 1.39+/-0.46, while the average radiant was determined as RA=196°, DEC=-1°.

The orbit of this stream was finally determined by Zdenek Sekanina, while evaluating the radio-echo data collected during the two sessions of the Radio Meteor Project. The 1961-1965 data revealed a stream, called the “Northern Virginids,” which possessed a duration of March 9-April 5. The nodal passage came on March 20.6, at which time the radiant was at RA=201.6°, DEC=-3.3°. Curiously, Sekanina’s computer analysis revealed two streams among the 1968-1969 data, which possessed nearly identical orbits (see below). The first stream, designated the “Southern Virginids,” possessed a duration of March 10-April 17. The stream’s nodal passage came on March 16.4, at which time the radiant position was RA=196.0°, DEC=-1.0°. The second stream, called the “Northern Virginids,” possessed a duration of March 10-April 21. Its nodal passage was given as April 2.1, with the average radiant being at RA=210.5°, DEC=-8.4°.

The Author examined the raw orbital data from both the 1961-1965 and 1968-1969 surveys and subjected it to a fairly restrictive D-criterion of 0.10. The result was 31 radio meteors that are definite members of the Theta Virginids. These meteors indicate the radiant’s daily motion is +0.90° in RA and -0.31° in DEC. This motion seems to connect the three radiant positions given above in Sekanina’s data. At first glance, this might indicate multiple peaks in the activity levels. However, the Author notes that, in 1969, there were two critical periods when the radar equipment was inactive. These periods were March 20-22 and March 29-April 6, and, since these were situated during the time the stream should have been producing its maximum activity, the two 1969 streams should probably be combined.

Orbit

Three orbits were included in Sekanina’s two papers which may relate to the Theta Virginids. The first orbit represents that of the stream detected during the 1961-1965 survey, which was referred to as the “Northern Virginids.” The other orbits are from the 1968-1969 survey and possess nearly identical durations, though the dates of nodal passage and the radiant positions were different. As discussed earlier, the motion of the radiant almost perfectly connects the radiants of each stream.

The Author’s examination of the radio-meteor orbits from both the 1961-1965 and 1968-1969 revealed 31 members of the Theta Virginids.