Alpha Capricornids (CAP)

Observing the Alpha Capricornids

For a short summary of this meteor shower and others visible in late July and early August of 2011, click here

The duration of this shower extends from July 15 to September 11. Maximum seems to occur during August 1 (λ=128.6°) from an average radiant of α=306.7°, δ=-8.3°. The maximum ZHR ranges from 6-14, while the meteors are generally described as slow. The shower has the reputation of producing some of the brightest meteors of the major showers, with the average magnitude being estimated as about 2.2.

History

Although the Iota Aquarids might be one of the most confused annual streams, the last three decades have revealed the Alpha Capricornids to not be far behind. This visual shower has been well known since the 19th Century, but as photographic and radio-echo surveys were conducted the shower began to appear more complex. Today astronomers generally seem to agree that two or three distinct maximums occur during the time the Alpha Capricornids are active.

This shower seems to have been discovered in 1871 as N. de Konkoly (Hungary) plotted six meteors from RA=305°, DECL=-4° during July 28-29. Before the end of the decade two additional observations were made. On July 28, 1878, W. F. Denning (England) made a probable observation of this shower when he plotted five meteors from α=305°, δ=-14° and E. Weiss (Hungary) observed the shower during July 25-28, 1879, when he plotted four meteors from α=305°, δ=-7°.

The Alpha Capricornids became known as a consistent producer of meteors during the period of late July and early August, and in 1899, Denning commented that, at the time of Perseid activity, this shower was rich with very slow and “often bright” meteors. This latter comment of bright meteors has become the trademark of this stream and Denning provided interesting facts to back this statement in 1920. He pointed out that he and his fellow English observers had determined the real paths of 25 meteors from this shower during the period of July 15-August 28, which possessed a mean radiant of α=305.2°, δ=-10.4°. He added that he had personally “seen at least 34 bright meteors from it during the period from July 27 to Aug. 5.”

Several visual surveys of the early 20th Century helped to expand the knowledge of this shower. One of the first was Ronald A. McIntosh’s 1935 paper “An Index to Southern Meteor Showers.” Primarily using observations made by observers in New Zealand during 1927-1934, McIntosh combined 15 visual radiant determinations of the Alpha Capricornids to reveal that the radiant moved from α=300°, δ=-11° to α=308°, δ=-10° during July 22-31. A second radiant was also noted which may have been the first indication of the complex nature of the Alpha Capricornids. It was said to possess a radiant that moved from α=300°, δ=-9° to α=305°, δ=-8° during July 23-31.

It is interesting that the Alpha Capricornids are not well represented among the 5406 visual radiants listed in Cuno Hoffmeister’s 1948 book Meteorströme. Subsequently it is not listed in his table of annual meteor showers. On the other hand, Hoffmeister’s analysis of the active annual showers began with a preliminary list of 238 radiants. One of those, designated number 56, was given an average activity date of July 29 (λ=126°), at which time the radiant was at α=314°, δ=-12°. Although this may not have been produced by the main Alpha Capricornid shower, it could represent yet another early clue to the complex nature of this stream.

While the German observers failed to detect convincing proof of this shower’s existence, members of the American Meteor Society were quite successful in observing activity. The Author notes that during the period of 1929-1953 no less than 21 radiants were observed. The indicated duration of the shower was July 15 to August 5, while the average radiant was at α=303.1°, δ=-12.5°.

The picture of a fairly consistent Alpha Capricornid stream became disrupted in 1956 when F. W. Wright, L. G. Jacchia and F. L. Whipple (Harvard College Observatory, Massachusetts) examined associated photographic meteors. Using 12 doubly-photographed meteors, the authors first established the overall look of the shower. The duration was given as July 16-August 22, while the date of maximum was determined as August 2 (λ=129.0°). The average radiant was α=308.5°, δ=-9.7°, while the daily motion was determined as +52’+/-2.4′ in α and -2’+/-2.6′ in δ. As can be seen, the motion of δ was quite uncertain, with the potential existing for either a northward or southward movement. The reason for this was a large scatter in the δ values of the collected photographic meteors. The authors stated that the scatter was possibly due to the existence of two or more streams producing the photographic Alpha Capricornid meteors.

The authors combined the double-station meteors with 36 single station meteors to derive a graph showing the scatter of the meteors around the radiant. They found “the mean scatter for the early part of the stream, to July 27, to be 83′. The mean scatter for ten days around August 1 is 93′, and from August 7 to August 22, there is a fairly large increase in scatter to an average of 155′, quite independent of mass (luminosity).” The authors added that an “irregular frequency distribution of meteors with date” existed. They concluded that two concentrations definitely seemed present. The first was given a duration of July 16-August 1, while the second began on August 1 and ended on August 22.

The Author has examined the lists of multiple-station photographic meteors obtained from the United States and the Soviet Union, and has isolated 29 probable members of the Alpha Capricornid stream. The two streams noted by Wright, Jacchia and Whipple are present, as well as a third stream. Details of these follow:

• *Stream I: This represents the main stream of the Alpha Capricornid complex. It is based on 17 meteors and possesses a duration of July 16 to August 29. The nodal passage occurs on August 1 (λ=128.6°) from an average radiant of α=306.7°, δ=-8.3°. The radiant’s daily motion is +0.84°±0.07° in α and +0.21°±0.02° in δ.
• *Stream II: This was the secondary stream noticed by Wright, Jacchia and Whipple. It is based on 5 meteors, which indicate a duration of August 8-21. The nodal passage occurs on August 15 (λ=141.9°), at which time the radiant is at α=322.4°, δ=-13.1°.
• *Stream III: This third stream is composed of 7 meteors, which indicate a duration of July 15-August 1. The nodal passage occurs on July 25 (λ=122.2°), at which time the radiant is at α=302.7°, δ=-12.5°. It is especially interesting that its semimajor axis of 2.069 AU is 20-25% smaller than that determined for the other two streams.

The Alpha Capricornid stream was detected during the 1960’s by both sessions of the Radio Meteor Project. From the 1961-1965 session, the duration was determined as July 30-September 11. The nodal passage came on August 20.2 (λ=146.8°), at which time the radiant was at α=326.4°, δ=-11.9°. For the 1968-1969 session, a similar duration was given as July 25-September 9, but the nodal passage was determined as August 9.6 (λ=136.6°), while the average radiant was at α=314.8°, δ=-7.1°. The orbit of each stream is listed in the “Orbit” section below. It can be seen that the stream detected in 1961-1965 bears a striking resemblance to the orbit of the Author’s “Stream II” discussed above, while the stream detected during 1968-1969 is quite similar to “Stream I,” which is the primary component of the Alpha Capricornid stream. These also happen to be the two streams detected by Wright, Jacchia and Whipple.

Numerous astronomers have tried to identify the object responsible for the formation of the Alpha Capricornid stream. The Russian astronomer E. N. Kramer was the first to examine the problem, when, in 1953, he concluded that the most likely candidate was comet 1457 II. In 1954, H. J. Bernhard, D. A. Bennett and H. S. Rice suggested a link to comet 1881 V (periodic comet Denning, now known as Denning-Fujikawa). In 1956, Wright, Jacchia and Whipple suggested that comet 1948 XII (periodic comet Honda-Mrkos-Pajdusakova) “may be a parent comet for the later α-Capricornids.” In 1973, Sekanina indicated that the comet most likely capable of producing this stream was, again, 1948 XII, but in his 1976 analysis of radio-echo data he suggested the Apollo asteroid Adonis as a candidate. Unfortunately, the question of which object produced the Alpha Capricornid stream remains unsolved. None of the suggested bodies matches the orbit of this stream perfectly and all of these authors rely on the fairly diffuse nature of this stream as an indication that it is old and far removed from the original orbit.

Although most of the information on this shower has come from observer’s in the Northern Hemisphere, the radiant does possess a fairly low altitude for most observers, thus reducing the number of meteors which can be seen. As an example, several members of the American Meteor Society in New York, Texas and Florida observed (or tried to observe) activity during July 23-August 1, 1970. Hourly rates seemed to reach a fairly consistent maximum of 2-3 between July 30 and August 1. According to the British Astronomical Association, meteor observers can expect to see a maximum ZHR of 8 on August 2 (λ=129°), from a radiant of α=309°, δ=-10°.

For the most part, it appears that the quantity of meteors visible to observers in the Northern Hemisphere prevents an accurate isolation of the individual branches of the Alpha Capricornid stream. Such does not seem to be the case in the Southern Hemisphere, where Capricornus resides quite near the zenith. During the period 1969-1980, M. Buhagiar (Perth, Western Australia) observed 20,974 meteors. Among them was included the Alpha Capricornid shower, the hourly rate of which he gave as 14. Hourly rates of this magnitude, allow a more complete examination of the visual complexity of the Alpha Capricornid shower.

The 1979 observations of the Western Australia Meteor Section (WAMS) may possibly indicate just how complex this stream might be. The most significant detail offered by these observations is that more than one peak in activity was noted. In fact, three distinct dates of activity were revealed: July 22, July 28 and August 5 (the latter being affected by increasing moonlight). The multiple radiants noted on the first two dates may or may not be significant, depending on whether they are confirmed in the future. The Author believes it may be more acceptable to simply average these radiants into one July 22 and July 28 radiant.

The first maximum of July 22 revealed three weak Alpha Capricornid radiants. The longest duration shower lasted from July 20-27, and possessed a maximum ZHR of 1.74+/-0.25 from a radiant of α=307°, δ=-11°. The other two radiants lasted 2-3 days centered on July 22 and appeared to be subcenters of the longer duration group. One had a ZHR of 1.08+/-0.18 and a radiant of α=304°, δ=-14°, while the second had a ZHR of 1.46+/-0.21 and a radiant of α=308°, δ=-9°. The second maximum of July 28 possessed two showers with durations of July 27-28. One shower was located at α=304°, δ=-12° and had a ZHR of 1.56+/-0.33, while the other shower was at a position of α=306°, δ=-11° and had a ZHR of 3.42+/-0.51. Finally, with moonlight beginning to interfer, the Australian observers found another radiant reaching maximum on August 5. The duration of this shower extended over August 3-5. The ZHR peaked at 6.20+/-1.79, while the radiant was at α=309°, δ=-10°. Using all observations of WAMS members, section director Jeff Wood has concluded that the ZHR typically reaches 5-10.

Additional details of the Alpha Capricornid meteors have been obtained by numerous observers in recent years. In the following table it will be noted that the fairly consistent estimates of the average magnitude reveal a value slightly fainter than 2. Details of the number of Alpha Capricornids possessing persistent trains are unfortunately lacking.

The observers cited are N. W. McLeod III (Florida), J. West (Texas), D. Swann (Texas), R. Hill (North Carolina), B. Gates (Florida), Western Australia Meteor Section, R. Lunsford (California), P. Roggemans (Belgium).

As with all major meteor showers, the question of the color of the meteors frequently comes up. For the Alpha Capricornids, the primary color seems to be yellow. This seems to be the consensus of the British Astronomical Association, as the shower was described as “rich in yellow fireballs,” and the Western Australia Meteor Section, where color estimates made in 1978 revealed 23.3% of the Alpha Capricornids to be yellow.