Theta Ophiuchids

Observing

The duration of this shower extends from May 21 to June 16. A relatively flat maximum of 5 days occurs centered on June 10 (λ=77.8°) from α=265°, δ=-28°, with a ZHR of 10.

History

The discovery of this stream seems attributable to C. Hoffmeister and is a major segment of his vast “Scorpius-Sagittarius-System” discussed in his 1948 book Meteorströme. Hoffmeister noted that numerous active areas appeared in this region of the sky beginning in late April and ending in mid-July. Around June 10 (λ=77.8°), a radiant at α=263.2°, δ=-28.3° becomes active. Hoffmeister referred to this radiant as representing the “distinct core” of the Scorpius-Sagittarius system and he described the activity as a “flat maximum at 75 deg to 80 deg.”

The first true confirmation of this stream was made in 1963, when Richard B. Southworth and Gerald S. Hawkins identified 5 meteors photographed during the Harvard Meteor Project (1952 to 1954) as members of a “new” stream called the “Theta Ophiuchids.” The duration of this stream was given as May 21 to June 16, and it was determined that 0.37 meteors could be photographed every hour. The average radiant was given as α=264.0°, δ=-26.8°.

In 1971, Bertil-Anders Lindblad essentially confirmed Southworth and Hawkins’ findings—with one extra meteor being found. Lindblad had conducted a computerized search among 865 precise photographic meteor orbits and noted the same duration, but with the average radiant being at α=265°, δ=-27°. Further proof of the stream’s existence was obtained by Lindblad during a second computer stream search among 2401 photographic meteors. The duration on this occasion was determined to be June 4-16, while the average radiant was α=266°, δ=-28°.

This shower has appeared in the observational records for a great number of years, with the name changing depending on the observer or researcher handling the data. It has been variously referred to as the “Alpha Scorpiids,” “Scorpiids-Sagittariids” and “Delta Sagittariids.” The name of “Theta Ophiuchids” was attached during the early 1960’s by Southworth and Hawkins and, in effect, the radiant lies midway between Delta Sagittarii and Alpha Scorpii. In addition, it is closer to Theta Ophiuchi at the time of maximum.

The stream plays an important part in the understanding of the structure of meteor showers. This stream, as well as the Virginid complex of April, has typified the structure of an ecliptic meteor stream—a stream continuously affected by planetary perturbations. According to Hoffmeister, this stream (under the name of the “Scorpius-Sagittarius-System”) represented the “typical case of a dissolved current. There is in times no defined radiant appreciable, the meteors radiating uniformly from an area up to 30° in diameter. But sometimes very pronounced centers use to appear, vanishing again within 24 hours.”

During 1961-1965, Zdenek Sekanina directed the Harvard Radio Meteor Project and found a stream with a duration covering May 7 to June 28, with an average radiant at α=262.5°, δ=-19.8°. Although at first glance this radiant seems to be too far north to be related to the Theta Ophiuchids, it should be noted that the radiant for this stream lies almost precisely on the ecliptic. This causes two apparently different orbits to appear, however, aside from the shift of 180 deg in both ω and Ω, the orbits are very nearly in the same plane and are the same shape. Thus, this stream possesses some very similar characteristics to those already discussed for the Ophiuchids and it probably would not be too presumptuous to say that both the Ophiuchids and the Theta Ophiuchids share a common origin. A major difference, however, is that while the Ophiuchids usually originate from a point north of the ecliptic, the Theta Ophiuchids tend to originate south of the ecliptic.

Detailed observations of this shower are hard to come by except for occasional radiant determinations. During 1978, members of the West Australia Meteor Section variously reported hourly rates of 3-4 during June 3/4 and 4-10 during June 10/11. The British Astronomical Association lists the ZHR of this stream as 10. Observers in the United States, however, rarely detect more than 1 to 2 meteors per hour on the night of maximum.

Orbit

The orbit of this stream was first established by Hoffmeister, based on visual data, and is given as orbit “A”, while orbit “B” was determined by Lindblad using six photographic meteors found during the Harvard Meteor Program. They are as follows and may refer specifically to a southern branch:

The possible northern branch of this stream detected by Sekanina’s Radio Meteor Project during 1961-1965, is as follows: