Alpha Monocerotids (AMO)

Observing

The maximum of this meteor shower occurs on November 21 (λ=238.7°) from a radiant of α=109°, δ=-6°. Although weak annual activity may be present during the period of November 13 to December 2, strong activity of perhaps 100 meteors per hour may return every 10 years on the indicated date of maximum. The daily motion of the radiant is +0.8° in α and -0.4° in δ. This stream is also known as the November Monocerotids.

History

Our present knowledge of this meteor shower is primarily based on visual observations obtained in three separate years, but the implications are that this shower is of short duration and possesses an apparent period of almost exactly ten years.

The actual discovery should be credited to F. T. Bradley (Crozet, Virginia), who observed on the night of November 20/21, 1925. He began his observations at 11 a.m. (local time) and counted 37 meteors between 11:02 and 11:15. A 10-minute break was taken to obtain star charts for plotting, but when he resumed observations at 11:25 the outburst had ended. Without having had the opportunity to plot any of the meteors, no definite radiant could be determined, but judging by the meteors’ tendency to move from east to west, Bradley estimated the radiant was below Orion. Fortunately, C. P. Olivier had been working in the observatory at the University of Virginia that same night. He said he stepped outside for a few moments and “saw 3 bright meteors about 11:05 a.m. The paths of two of these were mentally noted quite accurately, the path of the third being too poorly seen, though it was parallel to that of the second.” The deduced radiant was α=97.5°, δ=+8.5°, but Olivier admitted that the position was not very accurate. After uncovering several additional reports of enhanced activity, none of which acted to shed light on the position of the radiant, Olivier concluded that the meteors “were of various colors, bright, slow, and left trains.”

No additional activity was noted from this region until November 21.75, 1935, when M. A. R. Khan (Begumpet, India) witnessed “a fine shower of meteors whose radiant appeared to be near Gamma Monocerotis.” Overall, over 100 meteors were noted in the first 20 minutes, while 11 were counted in the next 20 minutes. A few months later, Khan wrote to Olivier with a more precise radiant position of α=110°, δ=-5° (he revealed that the star Gamma Monocerotis was the same as Alpha Monocerotis on other star charts). The parabolic orbit computed by Olivier is given in the “Orbit” section. The activity and radiant of the shower were apparently confirmed by the commanding officer of the USS Canopus, then anchored in Manila harbor, who noted that meteors appeared about once every 30 seconds during one 30-minute interval.

Olivier was confident enough to say the strong returns of 1925 and 1935 were not only related, but pointed towards a probable return in 1945. Although he was the first to point out a possible ten-year period, Olivier did not rule out the possibility of this being an annual shower whose short duration and “apparently very narrow cross-section,” would make it easy to miss completely. He cited an American Meteor Society observation of November 19.17, 1904 (UT), as possibly representing a previous appearance (AMS radiant number 165, with a position of α=95.4°, δ=+10.9°).

If any attempts were made to observe activity from this stream in 1945 conditions would have been very poor due to the appearance of a full moon late on November 19. Conditions would have been a little better in 1955, with full moon coming on November 29, but no apparent searches or accidental observations were made.

In 1958 L. Kresak (Czechoslovakia) examined the 1925 and 1935 events, claiming the 1935 shower represented “a unique and highly interesting example of an extremely condensed meteor stream….” Kresak estimated the maximum ZHR reached about 2000 per hour, which he said made it “the most conspicuous meteoric event observed in the present century, with the exception of the two richest returns of the October Draconids.” Kresak said the cometary character of the two showers was “beyond doubt,” and that their very short durations indicated a very recent departure from their parent body. In trying to determine what comet was responsible, he compared the solar longitudes of the 1925 and 1935 displays (given as 238.7° and 238.7°, respectively) and, after adding details on the theoretical orbit, he said only comet van Gent-Peltier-Daimaca (1944 I) came closest to representing the meteor stream orbit. Kresak admitted to some large discrepancies between the comet and stream orbits and concluded that if comet 1944 I was not responsible, the stream “must have been generated by a body too faint to be discovered by the present means.”

This region produced no notable activity during the five decades following 1935, but on the night of November 21, 1985, two independent discoveries were made by observers in California. The first was Keith Baker, night assistant at Lick Observatory. He stepped outside around 3:00 a.m. (local time) and observed 18 meteors in 7 minutes coming from a region near Canis Minor. The meteors were of magnitude 2 to 4, rapid, of short duration, and left no trains. From Capitola, Richard Ducoty observed 27 meteors during 3:41 and 3:45 a.m., 5 during 3:45 and 3:49 a.m., 2 during 3:49 and 3:53 a.m., and 2 during 3:53 and 3:57 a.m. His estimate of the radiant position was α=109°, δ=-7°+/-5°. He said, “The brightest meteors were 0 to -2. Their speed was quite fast, a little slower than the Leonids.”

The 1985 radiant estimate by Ducoty provided an excellent confirmation of Khan’s 1935 radiant determination. The Author has utilized these observations, as well as the apparent ten-year period, to calculate an elliptical orbit. A search was then conducted among published observations of the last 100 years to see if other observations could be located. The earliest possible observation appears to be that of W. Doberck (Hong Kong Observatory), who plotted five meteors from a radiant of α=102.5°, δ=-12° during November 19-25, 1895. Although this radiant seems to support the ten-year period, Doberck gave no indication of strong activity and said all the meteors were between magnitude 4 and 5.

The next possible visual observations were made by R. M. Dole (East Lansing, Michigan) and Cuno Hoffmeister (observing during an expedition to South-West Africa). Dole plotted three meteors from α=111°, δ=-11.2° during November 17.8, 1923, while Hoffmeister plotted several meteors from α=112°, δ=-10° during November 26, 1937. Neither radiant satisfies the ten-year period, but neither radiant produced very many meteors. The Author suggests these observations might indicate the presence of weak activity for at least two years before and after the appearance of the main shower and that the stream is wide enough to produce a duration of at least November 17-26.

The Author used a D-criterion value of 0.20 and less to compare photographic and radio meteor orbits, with his computed elliptical orbit. Altogether, only four orbits were revealed: two photographic meteors from the Harvard Meteor Project (1952-1954) and two radio-echo meteors from the first session of the Radio Meteor Project (1961-1965). These meteors do not offer much evidence to support either the ten-year period or the short-duration suggested, although the photographic meteors do seem to offer additional proof supporting the persistence of activity for a period of two years before and after the expected dates of maxima. What is interesting is that the implied duration of activity is extended to November 13-December 2. The dates of the appearance of the meteors (as well as their radiant positions) are November 13, 1953 (α=101.7°, δ=-2.1°), November 26, 1954 (α=115.4°, δ=-7.4°), November 16, 1962 (α=109.1°, δ=-5.8°), and December 2, 1963 (α=118.0°, δ=-9.4°). Combining the meteor radiants with the visual observations of 1935, 1937 and 1985, the Author finds a daily motion of +0.8° in α and -0.4° in δ.

With an apparent 10-year period present, meteor observers eagerly watched for this meteor shower in 1995. On the night of November 21 observers in eastern Europe saw an outburst of around 100 meteors per hour.

Orbit

Using a radiant given by Khan for 1935, Olivier computed the following parabolic orbit.

The Author has computed the following elliptical orbit based on the stream’s apparent period of ten years.

Using two photographic meteors from papers published in 1959 and 1961, and two radio meteor orbits obtained from among the 39,145 orbits computed by Sekanina during the two sessions of the Radio Meteor Project, the Author finds the following average orbit.

The orbit of comet van Gent-Peltier-Daimaca (1944 I) is