Some Meteors Showers of July  

Arkadiusz Olech, Michal Jurek and Marcin Gajos

Warsaw University Observatory
Al. Ujazdowskie 4, 00-478 Warszawa, Poland
e-mail:
olech@sirius.astrouw.edu.pl
gajos@antares.astrouw.edu.pl

Abstract

We present the results of the visual observations made by Polish meteor observers during July 1995-1998. Up to now, we collected 990.6 hours of effective observing time with 4187 plotted meteors. Analyzing our data using the RADIANT software we detected three weak showers not included in the IMO Working List of Visual Meteor Showers. They are the Alpha Cygnids (described in detal by Olech et al. 1999), the Delphinids and the Pi Draconids. Additionally, in the period July 9-17, we detected a clear radiant of early Perseids.

INTRODUCTION

Each July in 1995-1998, many Polish meteor observers associated in the Comets and Meteors Workshop (CMW) watched the sky using visual, telescopic and photographic techniques. Finally they obtained 990^h38^m of effective time of visual observations. During this time a group of 40 CMW members plotted as many as 4187 meteors on the gnomonic star maps. For each of them the angular veloctity was estimated. We used 0-5 scale with 0 corresponding to stationary meteors, 1 to very slow events, 2 to slow, 3 to medium, 4 to fast, and 5 to very fast meteors. Equatorial coordinates of the beginnings and ends of these events and their velocities were entered into the RADIANT software (Arlt 1992).

ALPHA CYGNIDS

The first informations about meteors from a radiant near Alpha Cygni come from W.F. Denning (Denning 1919). In 1885--1918, he observed 50 meteors radiating from the close vicinity of Deneb.

In the comprehensive work undertaken by Dutch Meteor Society (DMS) and North Australian Planetary Observers - Meteor Section (NAPO--MS) in 1981-1991 and described in detail by Jenniskens (1994) one can read about the weak stream called Omicron Cygnids. During 98 hours of effective time of observations 8 observers noted 72 possible members of that stream. From this data Jenniskens (1994) estimated the following parameters of the stream: equatorial coordinates of the radiant during the maximum of activity alpha=305^o delta=+47^o, drift of the radiant (in units ^o/day) Delta_alpha=+0.6, Delta_delta=+0.2, maximum of activity: lambda(1950.0) = 116.0 +\- 0.5^o, population index r=2.7, maximum Zenithal Hourly Rates (ZHRs) equal to 2.5 +\- 0.8 and the entry veloctity to V=37 km/s.

The next approach to investigate July meteors radiating from Cygnus was made by Olech et al (1999). Based on the visual and telescopic observations they estimate basic parameters of the stream. According to them the activity of Alpha Cygnids lasts from around June 30 to July 31 with a clear maximum near July 18 (solar longitude lambda=116.5^o). Maximum Zenithal Hourly Rates (ZHRs) are equal to 3.6 +/- 1.2. The structure of the radiant analyzed by the RADIANT software is most compact for a entry velocity equal to 41 km/s, and for the drift of the radiant (in units ^o/day) equal to Delta_alpha=+0.6, Delta_delta=+0.2. The center of the radiant for the time of maximum is alpha=302.5^o delta=+46.3^o, and the population index r equal to 2.55 +\- 0.14.

The analysis mentioned above was made basing on 785^h41^m of effective time of the visual observations, 757 possible meteors from Alpha Cygnid shower and 4569 sporadic meteors.

During the July of 1998, we organized the fourth astronomical camp of the CMW in Ostrowik. Very good weather conditions in the period of July 13-26 allowed us to collect as many as 204.95 hours of effective time of the visual observations (1836 meteors observed) and 94.72 hours of the telescopic observations (661 meteors observed). Only our visual data are already analyzed and processed by the RADIANT software so we decided to combine the visual results form the years 1995-1997 and 1998.

Now our sample of July meteors read into the RADIANT software amounted to 4187. We processed it using the following parameters: V=41 km/s, lambda_max=116^o, Delta_alpha=+0.6, Delta_delta=+0.2. We also assumed the maximum distance of the meteors form the radiant of shower equal to 50^o and excluded meteors with angular velocities smaller than 5^o/sec and greater than 25^o/sec. The resulting picture of the radiant of Alpha Cygnid shower is shown in Figure 1. The best fit of the two-dimensional Gaussian surface to this density of the probability map gives the coordinates of a radiant at alpha=305.2^o and delta=+45.1^o, which is in very good agreement with the coordinates given by Olech et al (1999).

Fig. 1: The radiant of the Alpha Cygnids resulting from CMW visual data.

The activity profile of Alpha Cygnids in 1995-1998 is shown in Figure 2. The full dots corresponds to ZHR averaged from years 1995-1997 and presented earlier by Olech et al (1999) and open squares denote this year's results. It is clear that activity of Alpha Cygnids in 1998 was a little smaller than in 1995-1997. The highest ZHR point in 1998 is equal to 3.4 +\- 0.9 and corresponds to a solar longitude lambda=111.3^o. One can see that the accuracy of this point is certainly low. It is due to the small number of individual ZHR estimates involved in the average. Other high ZHR values equal to 2.6 +\- 0.4 and 2.4 +\- 0.3 were obtained at lambda=113.2^o and lambda_=115.2^o, respectively. The next point with ZHR = 2.0 +\- 0.3 occured at lambda=117.1^o. Unfortunately, the night of July 18/19 (lambda=116.2^o), when the maximum of activity was expected was cloudy. We hope that others of our observers who did not participate in the camp in Ostrowik were more lucky and made observations at that time.

Fig. 2: Activity of the Alpha Cygnids in 1995-97 (dots) and 1998 (open squares)

DELPHINIDS

We payed attention to the Delphinid shower after reading Valentin Velkov's article in the Proceedings of IMC in Apeldoorn (Velkov 1996). After analyzing 3975 meteors plotted by our members from July 10 to 31 in 1995-1998, we found a clear radiant near the constellation of Delphinus as presented in Figure 3. According to Velkov (1996) we assumed V=35 km/s. In the calculation, we excluded meteors with velocities smaller than 2^o/sec and greater than 22^o/sec and also the events appearing at distance larger than 50^o from the radiant. The assumed lambda is equal to 119^o. First we calculated our sample using standard daily drift of the radiant equal to Delta_lambda=1.0^o/day but better results were obtained using Delta_lambda=1.5^o/day. The best fit of the two-dimensional Gaussian distribution gives the following coordinates of the radiant: alpha=303.5^o and delta=+5.8^o.

Fig. 3: The radiant of the Delphinids resulting from CMW visual data.

Velkov (1996) found a double radiant of Delphinids with the northern part situated at alpha=309.1^o and delta=+15.5^o and the southern part placed at alpha=308.3^o and delta=+12.2^o. One can see that our radiant is certainly connected with Velkov's Southern Delphinids and there is no trace of Northern Delphinids in our data.

We also tried to calculate the activity profile of the Delphinid shower. We assumed a population index r equal to 2.6 and a zenith exponent gamma equal to 1.0. The result of our calculation is presented in Figure 4. The highest activity with ZHR = 1.7 +\- 0.5 was detected at lambda=119^o which corresponds to the night of July 21-22.

Fig. 3: Activity of the Delphinids in 1998.

PI DRACONIDS

During calculation of the radiant picture of the Alpha Cygnid shower we found a small new radiant in the constellation of Draco. The radiant was clear when 1868 meteors from period July 1-16 were analyzed and was absent during analysis of the remaining 2319 meteors from the period July 17-31. The conclusion is that the meteors radiating from Draco are active only during the first part of July.

We obtained the most compact structure of the radiant (i.e. smallest values of chi² parameter in a fit of a two-dimensional Gaussian surface) for the following calculation parameters: entry velocity V=45 km/s, angular velocity between 6 and 24^o/sec, maximum distance of the meteors from the radiant equal to 50^o and daily drift Delta_lambda=1.0^o. The radiant structure is not so clear as in case of Alpha Cygnids and Delphinids and it is possible that in fact it is not a true radiant. Nevertheless, we obtained the following coordinates of the center of the radiant: alpha=296^o and delta=66^o. The structure of this possible radiant is presented in Figure 5.

Fig. 5: The radiant of the possible Pi Draconids.

EARLY PERSEIDS

According to the IMO Working List of Meteor Showers, the Perseids start their activity on July 17. On the other hand, our members often observed meteors radiating from Perseid radiant before that date. So we tried to investigate 1673 plotted meteors observed by our members in the period of July 9-17, 1995-1998.

Using V=59 km/s, lambda=115^o, Delta_lambda=1.0^o/day and excluding meteors observed farther than 50^o from the radiant we obtained the density distribution presented in Figure 6. The coordinates of the radiant obtained by us are alpha=19^o, delta=50^o and do not differ very much from the known coordinates (alpha=15^o and delta=52^o).

Fig. 6: The radiant of early Perseids from CMW visual data.

Our results strongly suggest that activity of the Perseid shower lasts longer than was previously thought.

References

Arlt R. (1992), WGN , 20 , 62.

Denning W.F., (1919), J. British Astron. Assoc. , 29 , 161.

Jenniskens P. (1994), Astron. Astrophys. , 287 , 990.

Olech A., Gajos M., Jurek M., (1999a), Astron. Astrophys. Suppl. , 135 , 291.

Velkov V., (1997), Proceedings of the International Meteor Conference, Apeldorn, 1996, 32