Phoenicids (PHO)

Observing

The duration of this shower extends from November 29 to December 9. Estimates of the ZHR seem to remain above one during December 2-7, while a sharp rise to five occurs on December 5 (λ=253°). The radiant is located at α=15°, δ=-52° at the time of maximum. The meteors possess an average magnitude of 3.27, while only 2% leave trains. The shower is most notable for producing a rate of 100 meteors per hour in 1956—the year marking its discovery. The stream is probably produced by the lost period comet D/1819 W1 (Blanpain).

History

On December 5, 1956, observers in New Zealand, Australia, the Indian Ocean, and South Africa detected strong activity from the southern constellation Phoenix. Investigations by H. B. Ridley revealed maximum visual hourly rates of 100 and a radiant at α=15°, δ=-45°, while C. A. Shain estimated visual hourly rates greater than 60 and a radiant of α=15°, δ=-58°.

A complete investigation of the visual observations was made by Ridley during 1962. He said the activity was first detected by R. Lynch (Auckland, New Zealand) at 10:10 UT, and that it continued to become visible “as sunset progressed westwards,” until last detected by S. C. Venter (Pretoria, South Africa) at 22:45 UT. Ridley’s analysis could not clearly pinpoint the time of maximum, but he estimated “it must have lain somewhere between 17h and 21h UT.” For orbital calculations, he adopted 19h UT (λ=253° 33′) as the time of maximum. One of the most impressive features of the shower was the apparent presence of exploding fireballs, as observers were frequently comparing the meteors “to the Moon, Venus, Jupiter, Sirius, etc.” The meteors were also frequently described as reddish and yellow. From a table listing the magnitude distribution of 61 meteors, the Author finds an average magnitude of 2.39.

Radio-echo observations were also made from Adelaide Observatory (South Australia) on December 5, 1956, with A. A. Weiss determining a maximum rate of 30 per hour and a radiant of α=15°+/-2°, δ=-55°+/-3°. The hourly rate was considered a puzzle by Weiss. For major showers, such as the Delta Aquarids and Geminids, the radio-echo rate was always greater than the visual rate, while the radio-echo Phoenicids were only one-third the strength shown by visual observations. One possible theory put forth was that “Earth was still on the fringe of the stream” when radio observations were made. Another suggested explanation was the possibility that slow meteors possessed a lower ionizing efficiency that faster meteors—a condition never satisfactorily tested prior to the appearance of the Phoencids.

A search through the major catalogs of Southern Hemisphere meteor stream radiants reveals no prior observation of the Phoenicids. For the 30 years following the shower’s sudden appearance in 1956, no return of the prominent activity was noted; however, visual observations were been made on numerous occasions since the early 1970’s. According to R. A. Mackenzie (British Meteor Society), activity was detected by observers in the Southern Hemisphere during 1972, 1973, 1976, and 1977, with maximum ZHRs of 4, 5, 2, and 5, respectively.

The Western Australia Meteor Section (WAMS) has made excellent observations of this stream in recent years. In 1977, Phoenicids were noted during December 2-5, with a maximum ZHR of 4.68+/-1.91 coming on December 5 from a radiant of α=15°, δ=-57°. In 1979, activity was observed during December 5-6, with a maximum ZHR of 5.67+/-4.01 coming on December 5 from α=15°, δ=-51°. In 1980, activity was detected during November 29-December 9, with a maximum ZHR of 2.58+/-0.37 coming on December 4 from α=17°, δ=-52°. During 1983, 17 amateur astronomers compiled 62 man-hours and detected Phoenicids during December 1-10. The ZHR was above 1 during December 2-7, with a maximum of 5.6+/-1.3 coming on the night of December 4/5. The meteors had an average magnitude of 3.27, while only 2% left trains. Another extensive observation program was conducted in 1985, as 25 observers compiled 122 man-hours over 9 nights. The maximum ZHR reached 8, while the average magnitude was 2.38 and 4.8% of the meteors possessed trains. Finally, during 1986, 16 members of the WAMS observed for 40 man-hours during December 2-7, with a maximum ZHR of 4.6+/-1.6 coming on December 5/6. The average magnitude was 2.88, while 5.3% of the meteors left trains.

One of the most intriguing aspects of the Phoenicid stream is its apparent link to the lost periodic comet D/1819 W1 (Blanpain). The link was first recognized by Ridley, during 1957, after he had computed a parabolic orbit. Weiss essentially confirmed the results later that year, but the small difference between the orbits was caused by the respective radiants Ridley and Weiss chose as representing the shower’s activity—the adopted radiants differing by 10° in declination. Ridley indicated that the small change in the longitude of perihelion between the stream and comet orbits, as well as the smaller ascending node of the stream, logically fit the expected evolution of a low-inclination comet orbit. Unfortunately, the comet was only observed for 59 days during 1819-1820, so that an error of several months exists in the revolution period. This uncertainty, combined with probable perturbations by Jupiter, makes it quite impossible to establish how different the current comet’s orbit is from the Phoenicid orbit.

Orbit

Ridley computed the first orbits for this stream based on a radiant of α=15°, δ=-45°. The first orbit is parabolic, but after noting a similarity to the orbit of the lost periodic comet D/1819 W1 (Blanpain), Ridley computed the second orbit with an assumed period of 5.1 years.

Weiss computed the following two orbits based on the radio-echo determination of the radiant position as α=15°, δ=-55°. The first orbit parabolic, while the second orbit was computed with an assumed period of 5.1 years.

Visual observations of weak activity from this radiant in recent years reveals Weiss’ radiant and calculations to best represent the stream’s orbit.

The 1819 orbit of comet D/1819 W1 (Blanpain) was