Articles

Proceedings of the IMC 2000
  Early and Late Perseids  

Marcin Kiraga and Arkadiusz Olech

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

Abstract

We present an analysis of the activity of the Perseid meteor stream in the periods July 5-16 (early Perseids) and August 25-31 (late Perseids) based on data collected by the observers of the Comet and Meteor Workshop (CMW) during 1996-2000. Perseids activity before July 13 was at the level ZHR <= 1.4 and increased to ZHR = 2.3 at by July 14, (solar long. = 112o.). We cannot find a clear radiant position related to the Perseid shower before July 10. The activity of the late Perseids was at the constant level ZHR = 1.2 in the period from August 25 to 31. For the late Perseids we detect a probable radiant position at alpha = 50o, delta = +56o, i.e. about 10o SW from the expected place.

INTRODUCTION

Perseids activity is usually assumed to last from July 17 to August 24 [1]. Zenithal Hourly Rate (ZHR) profiles published for this period (for instance Figure 7 in [2]) indicate that, at its limits, activity is still noticeable, at the level of ZHR ~ 3. There are also some Perseid observations outside assumed activity period (for instance Figure 3 in [3] and Figure 3 in [4]). Thus we decide to search for such events in the data collected by observers of the Polish Comets and Meteors Workshop (CMW) . We selected nights from July 5 to 16 and from August 25 to 31. We analyze such periods in five consecutive years: 1996 to 2000.

CMW OBSERVATIONS AND DATA ANALYSIS

In our analysis we used visual observations with plotted tracks collected during 1996-2000 in the periods July 5-16 and August 25-31, selected according to the following criteria:

effective observing time Teff >= 30 min,
limiting magnitude LM >= 5.00,
radiant altitude hR >= 20o,
cloud correction factor F <= 1.50.

We classified meteors as Perseids according to following (well known) criteria:

The distance (lbr) from the beginning of the meteor track to the radiant position should be larger than 1.8 times the meteo track lenght (lt).
The distance (x) from the back-projected line of the meteor track to the radiant position should be smaller than limiting value (xmax) which depend on the lbr. For xmax we assumed xmax = 7o for lbr <= 15o, xmax = 7o + 0.1*(lbr-15o) for 15o < lbr <= 30o , xmax = 8.5o + 0.075*(lbr-30o) for 30o < lbr <= 70o and xmax = 11.5o for lbr > 70o.
The meteor angular velocity omega in degrees per second, calculated from the equation omega = 0.573 * V * sin hb * sin ler, colud differ by at most one velocity class from the velocity of the velocity class estimated by the observer. Here V is the geocentric velocity of the meteoroid stream in km/s, hbis the altitude of the beginning of the meteor track, and ler is the distance between the end of the meteor track and the radiant position. The velocity classes are defined by 0o/s < A <= 5o/s < B <= 10o/s < C <= 15o/s < D <= 20o/s < E.

We assumed that the Perseid radiant at 0h00m UT on August 13 was at alpha = 46o and delta = 58o, that its daily drift in ecliptic longitude was Delta_lambda = 1o, and its geocentric stream velocity V=59 km/s.

This leaves some ambiguity, because according to the above criteria some meteors belong to two different showers. Many of these double assignations were for Alpha Aurygids and late Perseids. In such cases we selected the most probable shower according to a valocity criterion. When both showers were equally good, for instance when the calculated valocity in both cases differed by one velocity class from that estimated by the observer, the meteor was not counted as a Perseid.

Tables 1 and 2 give information about the effective observing time (Teff), the number of observers (Nobs), the number of Perseids according to the above criteria (NPER), and the number of sporadic meteors (Nspor), for individual years and for all years aggregated.


Table 1: Early Perseids (July 5-16)

Year

Teff [h]

Nobs

NPER

Nspor

1996

68.44

23

58

462

1997

64.99

23

45

366

1998

75.70

25

55

444

1999

219.44

22

149

1411

2000

30.66

8

8

105

Total

459.23

78

312

2344


Table 2: Late Perseids (August 25-31).

Year

Teff [h]

Nobs

NPER

Nspor

1996

10.83

2

2

38

1997

31.60

8

18

244

1998

17.53

3

14

81

1999

60.05

4

15

236

2000

66.77

9

61

372

Total

186.78

18

110

971

Activity profile

In calculating the ZHR for early and late Perseids we assumed thet r=2.6, similar to the value presented in Figure 1 of [6]. If the population idex is changed to r = 2 the calculated ZHR is 10-30% lower, depending on the limiting magnitude of the observations. We divided the observations into sutable solar longitude bins, and for every bin we estimated ZHR as: ZHR = (NPER + 1)/TEFF, where TEFF=Teff*sin hR/(r6.5-LM*F). One sigma error bas was estimated as .............

Radiant position

The equatorial coordinates of start and end points and the velocity classes of meteors from analyzed periods were obtained using the CooReader package [7] and input into the RADIANT software [5]. To obtain the radiant probability distribution (RPD) maps presented, we used the following parameters: geocentric stream valocity V = 59 km/s, meteor angular velocity 1o/s < omega < 30o/s, maximum distance between start of meteor track and radiant position lmax=50o.

EARLY PERSEIDS

The most important year for early Perseid ZHR estimation was 1999 (see Table 1), but we present the average ZHRs from all years. The results are presented in Figure 1.



Fig. 1: Mean ZHR estimated for eraly Perseid for all years aggregated. We assume a population index r=2.6.



Fig. 2: Map of the radiant probability distribution for early (July 5-16) Perseids for all years aggregated. From the total of 1978 meteors presented in the CMW database we use only those with distance between the radiant position and the start of the track smaller than 50o.

One can see that early Perseid activity is at the level of ZHR ~ 1 before sol. long. = 112o, and by sol. long. = 112o is incerasing to ZHR = 2.4 +\- 0.3.

LATE PERSEIDS

References

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

Babadzhanov P.B., Kramer E.N., (1965), Smits. Contr. Astrophys. , 11 , 67.

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

Molau S., (2000), Proceedings of the International Meteor Conference, Frasso Sabino, Italy, 1999, 31-37.

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

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

Olech A., Jurek M., Gajos M., (1999b), Proceedings of the International Meteor Conference, Stara Lesna, Slovakia, 1998, 58

Olech A., Juerk M., (2000), Proceedings of the International Meteor Conference, Frasso Sabino, Italy, 1999, 83.

Samujllo M., Olech A., (1999), Proceedings of the International Meteor Conference, Stara Lesna, Slovakia, 1998, 65
 

© 2001 Pracownia Komet i Meteorów.