Chi Orionids

Observer's Synopsis

The Northern Chi Orionids are active during November 16-December 16, with a maximum ZHR of two coming on December 10 (solar longitude=258 deg) from RA=82 deg, DEC=+23 deg. The Southern Chi Orionids are active during December 2-18, with a maximum ZHR of three coming on December 10 (solar longitude=258 deg) from RA=88 deg, DEC=+20 deg. The meteors of both streams tend to be bright, with about 14% leaving trains.

History

The Chi Orionids have been around for at least 100 years, though its weak activity at a time when the Geminids are nearing prominence should probably be blamed for its frequently being overlooked by observers. As early as the 1890's, William F. Denning included this shower in his annual list of active radiants published in The Observatory. The most important discovery came during the photographic surveys of the 1950's, when the Chi Orionids became one of several annual meteor showers recognized as split into northern and southern branches.
The earliest observations of activity from this stream extend back to the last quarter of the 19th century, when Denning (Bristol, England) observed activity during 1876 and 1877. In the former year he plotted 13 meteors during December 6-7 emanating from a radiant of RA=80 deg, DEC=+23 deg. In the latter year, 6 meteors were plotted during December 6-8 from RA=80 deg, DEC=+25 deg. The meteors were described as slow on both occasions. The first person to confirm Denning's radiant was E. F. Sawyer (Cambridge, Massachusetts), who plotted 5 meteors during December 7-12, 1879 from RA=82 deg, DEC=+23 deg.
Similar observations continued to be made during the 1880's and 1890's, but this was clearly referring to the Northern Chi Orionids. The Southern Chi Orionids were not in the same degree of prominence with only two apparent 19th century observations: Denning plotted 11 meteors during November 30-December 10, 1885 from a radiant of RA=88 deg, DEC=+19 deg, while Denning combined the plots of E. Weiss (Hungary) and Giuseppe Zezioli (Bergamo, Italy), and found 17 meteors revealing a radiant of RA=86 deg, DEC=+19 deg for the period of December 9-13.
The Southern Chi Orionids were not well observed during the first half of the 20th century, but numerous observations were made of the northern branch. Most notable was the inclusion of the Northern Chi Orionid radiant in a table of "third grade radiants" in Cuno Hoffmeister's 1948 book Meteorstrome. Ten radiants were isolated which were active around December 11 (solar longitude=259 deg) from an average radiant of RA=83 deg, DEC=+22 deg. The Author's examination of these radiants revealed an average diameter of 3 deg.
One of the more important events in the acquisition of knowledge about the Chi Orionids was the Harvard Meteor Project of 1952-1954. A few early analyses of this photographic data (JW1961 and MP1961) revealed members of the stream, but it was the computerized analysis of Bertil-Anders Lindblad (Lund Observatory, Sweden) which provided the most complete data.
The Southern Chi Orionids were best represented among the data collected by Lindblad. Eight meteors indicated a duration of December 7-14 and an average radiant of RA=85 deg, DEC=+16 deg. The Northern Chi Orionid data consisted of four meteors indicating a duration of December 4-13 and an average radiant of RA=83 deg, DEC=+26 deg.
The Author has further examined the photographic meteor orbits obtained during the 1940's, 1950's, and 1960's, in the United States and Soviet Union, and has arrived at more refined orbits for each branch of the Chi Orionid stream using stricter values of the D-criterion than used by Lindblad. The southern branch was again best represented with 12 meteors. The stream's duration was December 4-14, while the average radiant was RA=83.9 deg, DEC=+16.8 deg. Curiously, the stricter D-criterion shortened the duration of the northern branch to December 11-17. The average radiant was RA=84.4 deg, DEC=+26.1 deg.
Radio-echo surveys conducted during the 1960's were successful in detecting the Chi Orionids, but not in isolating the northern and southern branches. C. S. Nilsson (Adelaide Observatory, South Australia) detected four radio meteors during December 7-12, 1960, and gave the average radiant as RA=80.0 deg, DEC=+16.8 deg. Zdenek Sekanina detected the Chi Orionids during both sessions of the Radio Meteor Project. During the 1961-1965 session, the duration was given as December 2-18. The date of the nodal passage was given as December 9.9 (solar longitude=257.5 deg), at which time the radiant was at RA=87.1 deg, DEC=+20.6 deg. The 1968-1969 session indicated a duration of November 16-December 16. The date of the nodal passage was given as December 9.9 (solar longitude=257.5 deg), at which time the radiant was at RA=81.5 deg, DEC=+23.4 deg.
The Western Australia Meteor Section (WAMS) has recently provided very complete visual observations of both the northern and southern branches of the Chi Orionid stream.

The WAMS also detected the Chi Orionid streams during 1978. By combining the 48 meteors observed, Jeff Wood (section director) was able to provide additional details about the Chi Orionid meteors in general. The stream seems to produce fairly bright meteors with an average magnitude of 1.60, while 14.3% of the meteors produced trains. Concerning colors, it was determined that 31.4% of the meteors were yellow, while 4.5% were blue or green.

Orbit

The Author has collected photographic meteor orbits from W1954, MP1961 and BK1967. The Northern Chi Orionid orbit was based on 5 meteors, while the Southern Chi Orionid orbit was based on 12 meteors.

AOP AN i q e a
N. 275.5 261.0 2.4 0.528 0.739 2.02
S. 101.5 77.9 6.0 0.460 0.798 2.28

The stream was also detected during three radio-echo surveys of the 1960's, though the northern and southern branches could not be separated. The first two orbits were obviously influenced by the Southern Chi Orionids, while the last orbit was influenced by the Northern Chi Orionids.
AOP AN i q e a
N1964 93.7 78.6 4.6 0.556 0.70 1.852
S1973 109.0 77.5 2.6 0.420 0.765 1.790
S1976 278.7 257.5 0.2 0.515 0.711 1.783

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