Upsilon Pegasids

History

[This is a rather controversial meteor radiant. Although weak evidence does exist to suggest activity has emanated from the region of Upsilon Pegasi early in August, too much hype over the years has produced a greatly distorted view of the Upsilon Pegasids. After reading the following, please keep in mind that the radiant produces weak activity that may never attain rates higher than one or two meteors per hour, which makes it difficult to detect against the normal sporadic background. In addition, the radiant may no longer produce activity as there have been no confirmed observations since the mid-1980s.]

H. R. Povenmire (Florida, USA) discovered this shower on August 8, 1975, while observing the Perseids. Three meteors were noted in a very short time which originated from within the Great Square of Pegasus, but clouds prevented more detailed observations. The next evening, Povenmire again detected the shower and was able to determine the radiant as α=350°, δ=+19°.

Moonlight interfered with observations during 1976, but moonless nights prevailed during early August 1977. An analysis by Sky & Telescope said observers detected hourly rates as high as 8 to 13 on the evening of August 12 and 13, and Povenmire was quoted as saying that meteors were detected up to a week before and after these dates. On the other hand, brief details published in Meteor News seemed to cast doubt on the high rates and the results of 7 observers were summarized as indicating a maximum of only 1.71 per hour.

Moonlight was again no problem in 1978, but the confirming reports of activity were also accompanied by reports with a negative tone. From Los Padres National Forest in California, M. Davis and D. Gutierrez observed 20 meteors which originated within 20 deg of the Great Square of Pegasus during the morning of August 12. On the other hand, R. Nolthenius (Alpine, California, USA) made the comment, "Only six Pegasids were seen during 2 1/4 hours of observing on the morning of the 13th. Due to the high rate of sporadic meteors this year, my impression was that the number radiating from the Great Square was not much more than would be expected by chance." Povenmire said that about 700 persons around the world contributed observations, with some of the detailed observations including color estimates of primarily yellow to white, and a radiant diameter of about three degrees.

As these examples demonstrate, there were widely varying estimates of the strength of Upsilon Pegasid activity. Subsequently, these conflicting observations brought controversy and veteran meteor observer N. W. McLeod III (Miami, Florida) questioned the shower's very existence in the January 1979 issue of Meteor News.

McLeod listed at least 12 major and minor meteor showers which were active during early August and in his evaluation of Upsilon Pegasid activity he said, "Several of the named showers can account for the bulk of such meteors." He went on to discuss his own observations, which generally revealed "only about 3 meteors/night from the Pegasid radiant" in early August. He added that his best night was August 4/5 when 7 meteors were observed and that during the Perseid maximum "only one Pegasid/night was seen."

The Author believes the Upsilon Pegasids were victim to a series of unfortunate events which led many people to believe they were a potentially major shower which peaked at the time of the Perseid maximum. This was far from the truth. First of all, Sky & Telescope unfortunately did not discuss the observations to their fullest, as the Upsilon Pegasid discussion occurred at the end of a multi-page article dealing with the Perseids. The fact was that the high rates quoted came from inexperienced observers who, as McLeod pointed out above, simply confused meteors from several active showers with the Upsilon Pegasid activity. What the magazine should have noted was that experienced observers tended to reveal hourly rates of about 0-2 during the time of the Perseid maximum and it is this fact that leads to the next problem. Despite Povenmire's discovery of notable activity during August 8 and 9, 1975, observers in 1977-1978 rarely observed any earlier than August 11. Since it is now apparent that the Upsilon Pegasids rapidly weaken after their August 8 maximum, no one would have expected these observers to see more than a few of these meteors each night the Perseids were active.

Although a few meteor observers have continued to make this shower a controversial subject, many others have helped to refine the current knowledge of the Upsilon Pegasid stream. In the years following 1978, observers became more aware of the earlier maximum for the Upsilon Pegasids and the picture soon developed of a shower which produces maximum rates of 2-3 per hour.

The most significant visual study of this shower was made during July 30-August 16, 1983 by the Hungarian Meteor and Fireball Observation Network. In the publication Meteor, Tepliczky István collated the observations and revealed some interesting statistics concerning this shower's members. First of all, the meteor colors were tabulated into the following percentages: 7% blue, 14% blue-white, 28% white, 5% yellow-white, 32% yellow, 14% orange. Secondly, a good record of the train activity of the shower was kept. Of 155 Upsilon Pegasids observed, 20, or 13%, left persistent trains. Finally, ZHRs were computed for each day, with a maximum of 5 being recorded on three occasions---August 1/2, 6/7 and 16. Maximum was plainly indicated as occurring during August 1-7, with ZHRs remaining between 4 and 5, and the average ZHR being 4.6.

Aside from the estimates of hourly rates, one of the next most important observations of any meteor shower is the determination of the average brightness of its meteors. The average magnitude of the Upsilon Pegasids tends to be fainter than for the other showers active during the first half of August and this indicates a proportionately higher percentage of faint meteors than normal. McLeod found the average magnitude to be 3.62 based on meteors he observed during the period of 1977 to 1985. R. Lunsford (California, USA) determined the average magnitude as 3.11 in 1979, 3.73 in 1980, 3.10 in 1981, and 3.59 in 1986. P. Parviainen (Finland) determined it as 2.90 in 1981. Data on the percentage of meteors leaving persistent trains has not been frequently given, but, as noted earlier, it seems no higher than 13%.

As with all meteor showers, observations become more significant when photographs are involved, and Povenmire's annual request for photographs of the region have revealed several good specimens. Most notable has been the detection of two meteors that appeared nearly stationary, thus giving an indication of the general correctness of the original radiant estimate. In 1980, S. Hunt (Kansas City, Missouri, USA) recorded a meteor "coming nearly head-on from the radiant," while, in 1983, L. Venn (Bartlesville, Oklahoma, USA) photographed a nearly stationary Upsilon Pegasid which, when farthest from the radiant, was only one degree away.

One of the best examples of a photographic Upsilon Pegasid was not photographed by a regular watcher of this shower, but, instead, it was detected by five cameras of the European Meteor Network on August 19, 1982. Network director, Z. Ceplecha (Ondrejov Observatory, Czechoslovakia), made a careful study of this fireball and proved that it was an Upsilon Pegasid. The meteor was designated EN190882 and attained a maximum magnitude of -13.8, or brighter than the full moon! Ceplecha's subsequent calculation of a precise orbit gave the first indication of what the orbital plane of this stream might look like (see "Orbit" section below).

Despite the evidence that has accumulated since this shower's discovery, continued controversy as to the reality of this stream inspired the Author to conduct an extensive search through publications of the last 100 years to uncover past appearances. Overall, seven radiants are present which indicate a duration extending at least from August 3 to August 11. The two earliest observations were made by P. Denza and Alexander S. Herschel. The former observer plotted several meteors from α=350°, δ=+24° on August 11, 1869, while Herschel's observations of 1860-1881 revealed a radiant at α=345°, δ=+15° on August 11. C. Hoffmeister's 1948 book Meteorströme revealed three radiants. The first was located at α=347°, δ=+16° on August 3, 1913. The second radiant was at α=352°, δ=+18° on August 9, 1921. The third radiant was at α=352°, δ=+18° on August 4, 1937. Finally, American Meteor Society records reveal two radiants: one at α=347.5°, δ=+22° on Augsut 4, 1924, and the other at α=342°, δ=+22° on August 8.7, 1964.

Finally, in addition to searches for visual radiants, the Author has also searched photographic records for double-station meteors. These meteors not only offer reliable radiants, but, in many cases, they even offer orbital data. What may be the very first double-station Upsilon Pegasid appears to have been detected on August 3, 1902, during the Yale Photographic Meteor Survey, and emanated from α=346.4 deg, δ=+17.8 deg. The inability to compute a velocity prevented the calculation of an orbit and the radiant is not considered absolutely certain. The next probable photographic Upsilon Pegasid appears among the 2529 double-station meteor orbits computed by Richard McCrosky and Annette Posen from data gathered during the Harvard Meteor Project. Designated 3607, the meteor was detected on August 18.3, 1952, from α=349°, δ=+22°. Its magnitude was 0.1, while the semimajor axis was computed as 13.47 AU. On August 13.8, 1958, a photographic survey conducted at Dushanbe (USSR) revealed a meteor from α=344.5°, δ=+18.4°. The subsequent orbital calculation by Dr. P. B. Babadzhanov revealed a slightly hyperbolic orbit with an eccentricity of 1.04.