This case study focuses on fast moving meteors with radiants in the constellation of Taurus during late September until begin of October. The Zeta Taurids (ZTA#226) seem to be an erroneous combination of early Orionid orbits with a weak concentration of distinctly different types of orbits, listed in the IAU Working List of Meteor Showers as the Phi-Taurids (PTA#556). In total 173 orbits were identified as PTA#556 shower members within the activity interval in solar longitude from 175° until 206°. These orbits are characterized by a small perihelion distance q of ~ 0.24 ± 0.02 AU and an eccentricity of ~0.95 ± 0.04. Based on this case study it is recommended to remove the ZTA#226 entry from the IAU Working List of Meteor Showers.

 

1 Introduction

Browsing the IAU Working list of meteor showers, I noticed an entry that raised questions because the two shower orbits mentioned are very different. Only the inclination i is similar. The data from the IAU Meteor Data Center is summarized in Table 1.

Table 1 – The data as listed in the IAU working list of meteor showers for the Zeta Taurids (ZTA#226 status May 2019).

Jenniskens (2006) Sekanina (1976)
λʘ 196° 193.5°
αg 86.1° 71.5°
δg +14.7° +28.2°
vg 67.2 km/s 56.5 km/s
a 21.3 AU 1.632 AU
q 0.715 AU 0.231 AU
e 0.966 0.858
ω 70.9° 311.8°
Ω 16.5° 193.5°
i 162.4° 163.1°
N 3 6

 

The online shower list mentions a third entry without any orbital elements to support the existence of this shower. This source is based on single station video observations (Molau and Rendtel, 2009). The use of single station data is inappropriate for detections of weak minor meteor streams for which orbits are essential to identify a more reliable shower association. The authors mention that their radiant is based on 294 meteor trails. However, the rich meteor activity around that time and the presence of early Orionid radiants nearby will generate many meteor paths with a suitable angular velocity lined up with an assumed radiant just by chance. The real radiant for single station meteor paths cannot be determined.

The difference between the two orbits is that large, that if the Zeta Taurids exist, only one of both reference orbits can be valid for this shower. The number of orbits on which both reference orbits are based are too few to define any statistical relevant reference orbit. This case study was made to clarify this confusion.

 

2 Available orbit data to search

We have the following orbit data collected over 12 years, status as until May 2019, available for our search:

  • EDMOND EU+world with 317830 orbits (until 2016). EDMOND collects data from different European networks which altogether operate 311 cameras (Kornos et al., 2014).
  • SonotaCo with 284138 orbits (2007–2018). SonotaCo is an amateur video network with over 100 cameras in Japan (SonotaCo, 2009).
  • CAMS with 110521 orbits (October 2010 – March 2013), (Jenniskens et al., 2011). For clarity, the CAMS BeNeLux orbits since April 2013 are not included in this dataset because this data is still under embargo.

In total 712489 video meteor orbits are publicly available. Our methodology to detect associated orbits has been explained in a previous case study (Roggemans et al., 2019).

 

3 The ζ-Taurids reference by Jenniskens

Dr. Peter Jenniskens mentioned this shower without providing further details (Jenniskens, 2006). He associates the ζ-Taurid orbit he obtained from 3 orbits with the orbit published earlier by Sekanina (Table 1) as well as a third orbit published by Kashcheyev and Lebedinets (1967):

  • αg = 88°
  • δg = +12°
  • vg = 57 km/s
  • a = 1.48 AU
  • q = 0.38 AU
  • e = 0.74
  • ω = 119°
  • Ω = 23°
  • i = 152°

This third reference orbit has been omitted when the shower was listed in the IAU Working List of Meteor Showers. The ζ-Taurid shower has not been detected in later meteor shower searches neither on CAMS orbits (video) nor on CMOR orbits (radar) which may be an indication that this stream perhaps does not exist at all.

To verify if this reference orbit is similar to any known other meteor stream we calculate the discrimination criteria according to Hawkins (1963), referred to as DSH, Drummond (1981) referred as DD and Jopek (1993) referred as DH. The results are listed in Table 2.

If we search the 712489 orbits available to find orbits that fulfil the similarity criteria with this ζ-Taurid reference orbit of Jenniskens, we find as many as 8298 orbits with a low threshold, 1963 with medium low, 431 with medium high and 90 with a high threshold similarity. However, most of these orbits were identified before as either Orionids or Eta Aquariids.

The reference orbit mentioned by Jenniskens (2006) for the ζ-Taurids (ZTA#226) seems to be based on 3 early Orionid orbits which were not recognized as Orionids. If the
ζ-Taurids (ZTA#226) exist as a distinct meteor shower, this orbit is probably not related to it.

Table 2 – The five reference orbits found in the IAU working list of meteor showers that fulfil the similarity discrimination criteria for association with the ZTA reference orbit given by Jenniskens.

  ORI ETA SOO (1) SOO (2) SOO (3)
λʘ 207.5° 46.9° 185.6° 185.7° 187°
αg 94.7° 338° 79.2° 80.4° 80.9°
δg +15.5° –2° +12.1° +10.6° +12.5°
vg 66.4 – 67.6 66.9 67.6
a 9.71 36 10.8 6.4 8.1
q 0.597 0.612 0.774 0.792 0.777
e 0.9385 0.983 0.928 0.876 0.911
ω 80.1° 101.5° 58.1° 56.5° 57.6°
Ω 27° 45.8° 5.6° 5.7° 7.7°
i 163.6° 165.5° 159.3° 156.5° 159.9°
N 1297 11 18 20 40
DSH 0.13 0.19 0.12 0.18 0.13
DD 0.09 0.09 0.05 0.09 0.06
DH 0.11 0.17 0.11 0.17 0.13

 

The first orbit (ORI) resulted from the CMOR stream search (Brown et al., 2008) as reference orbit for the Orionids. The second orbit (ETA) was found from photographic orbits as reference for the Eta Aquariids (Lindblad, 1990). The remaining three orbits are for the September omicron Orionids (SOO#479) a not yet confirmed shower. The first SOO (1) reference orbit based on 18 orbits has the best similarity criteria (Rudawska and Jenniskens, 2014). The second SOO (2) reference orbit based on 20 orbits was obtained from the EDMOND database (Kornoš et al., 2014). The third SOO (3) reference orbit based on 40 orbits was obtained from CAMS orbits 2010–2013 (Jenniskens et al., 2016).

The first reference orbit for the ZTA#226 seems to be more related to the unconfirmed SOO#479 shower, and most likely all these orbits are nothing else than early Orionids. The tendency to split out main meteor showers into several minor showers inflated the number of shower entries in the IAU Working List of Meteor Showers. The confusion and inconsistencies in the IAU Working List have been discussed by Masahiro Koseki (2016, 2018).

 

4 The ζ-Taurids reference by Sekanina

Sekanina mentions this orbit as tau Taurids, a result of a shower search on radar orbits obtained during the Radio Meteor Project at Havana, Illinois, U.S. in 1961–1965 and 1968–1969. For a correct interpretation of the stream search the limited accuracy of these radar orbits should be taken into account. Moreover, the threshold of the similarity criterion of Southworth and Hawkins (1963) has been taken very optimistic. For instance, from all similar radar orbits only one fulfils the low threshold in our analyzes, mainly because several orbits that pass the test with Southworth and Hawkins fail on the Drummond (1981) test.

Checking the ZTA reference orbit of Sekanina with all other reference orbits listed in the IAU Working List results in two matches with two reference orbits listed for the phi Taurids (PTA#556). The orbits are compared in Table 3. All three orbits have no similarity with the Orionids.

Table 3 – The two reference orbits found in the IAU working list of meteor showers that fulfill the discrimination criteria for association with the ZTA reference orbit given by Sekanina.

  ZTA
Sekanina 1976		 PTA
Andreic et al., 2014		 PTA
Jenniskens et al., 2018
λʘ 193.5° 193° 188.8°
αg 71.5° 63.9° 58°
δg +28.2° +29.1° +28°
vg 56.5 60.2 60.1
a 1.632 8.7 12.3
q 0.231 0.234 0.22
e 0.858 0.973 0.984
ω 311.8° 303.7° 305.2°
Ω 193.5° 193.1° 188.8°
i 163.1° 156.3° 155.6°
N 6 22 38
DSH 0.21 0.19
DD 0.09 0.09
DH 0.21 0.19

 

The weak similarity between the ZTA reference orbit as given by Sekanina (1976) and the PTA reference orbits suggests that these are related. The orbit obtained by Sekanina is based on orbits that fulfil the discrimination criteria with the PTA orbit as reference. It is very likely that both are physically related to each other.

5 Is there any meteor shower?

To make an independent check for possible similar orbits among our database with 712489 orbits with the reference orbit given by Sekanina for the ZTA#226 shower, we check if and how many similar orbits we have. As many as 115 orbits fulfil the low threshold criteria when we take the orbit of Sekanina as reference. These similar orbits are distributed over the following interval in solar longitude, radiant area and velocity range:

  • Time interval: 172° < λʘ < 210°;
  • Radiant area: 45° < αg < 100° & +21° < δg < +34°;
  • Velocity: 50 km/s < vg < 62 km/s.

Next, we select all 89573 orbits we have in this time interval and from these we take the orbits with a radiant and velocity within the above-mentioned range. This results in a selection of 1097 orbits. In a first approach we calculate the average orbit for this set, using the calculation method explained by Jopek et al. (2006). For all 1097 orbits we calculate the discrimination criteria according to Hawkins (1963), referred to as DSH, Drummond (1981) referred as DD and Jopek (1993) referred as DH and we consider the following similarity threshold classes:

  • Low: DSH < 0.25 & DD < 0.105 & DH < 0.25;
  • Medium low: DSH < 0.2 & DD < 0.08 & DH < 0.2;
  • Medium high: DSH < 0.15 & DD < 0.06 & DH < 0.15;
  • High: DSH < 0.1 & DD < 0.04 & DH < 0.1.

Table 4 – The average values for the final selections of orbits for the four different threshold levels on the D-criteria, compared to the orbit for the shower PTA#556, with their similarity values for DD listed below each average orbit.

  Low Medium low Medium high High PTA Andreic et al., 2014h
λʘ 188.1° 188.2° 187.4° 187.7° 193°
αg 60.4° 60.4° 59.4° 59.3° 63.9°
δg +28.5° +28.3° +28.3° +28.6° +29.1°
vg 60.0 60.2 60.5 60.6 60.2
a 4.3 5.0 6.2 7.0 8.7
q 0.257 0.258 0.262 0.264 0.234
e 0.940 0.948 0.958 0.962 0.973
ω 301.4° 301.1° 300.3° 299.9° 303.7°
Ω 187.8° 188.0° 187.2° 187.5° 193.1°
i 157.0° 157.8° 157.5° 156.7° 156.3°
N 164 94 42 13 22
DD 0.05 0.05 0.06 0.06

 

We select the orbits that fulfil the medium high criterion, recalculate the average orbit and repeat this process. After some iteration we have an average orbit. The results are shown in Table 4 for the different thresholds of similarity.

The sample of orbits which we selected on bases of the orbit given by Sekanina for the ZTA#226 shower effectively include a concentration of orbits that define a meteor shower. However, the average orbit for this shower is similar to the orbit listed for PTA#556 with medium high threshold levels.

Assuming that the orbit given by Sekanina is likely not the best starting point, the selection interval was slightly changed, based on 193 orbits which are similar to the PTA#556 orbit among our 712489 orbits.

  • Time interval: 174° < λʘ < 210°;
  • Radiant area: 43° < αg < 85° & +22° < δg < +35°;
  • Velocity: 56 km/s < vg < 65 km/s.

Table 5 – The average orbital elements at each iteration when approaching the most likely average orbit for the concentration included in the sample.

q e ω Ω i
0.302272 0.887987 294.2602 188.2856 159.372
0.298632 0.919714 296.526 189.02 158.9835
0.293532 0.928027 296.8326 187.9376 158.528
0.288686 0.931124 297.3361 187.1088 157.9038
0.285071 0.933901 297.7646 186.9801 157.3177
0.280027 0.934187 298.4372 186.8108 156.7733
0.273227 0.93638 299.265 186.7455 156.2642
0.268345 0.937086 299.878 187.0397 156.2198
0.265031 0.938128 300.2929 187.0279 156.3707
0.260453 0.940173 300.857 187.2763 156.5485
0.256141 0.940977 301.3942 187.3674 156.6071
0.253252 0.940951 301.7957 187.5985 156.4208
0.251696 0.941017 302.0131 187.7667 156.493
0.250212 0.94182 302.1847 187.9417 156.6415
0.249229 0.942413 302.2956 188.0193 156.6136
0.24823 0.9427 302.4522 188.2577 156.5657
0.246357 0.94339 302.6816 188.4263 156.6904
0.244159 0.943669 303.0149 188.914 156.7515
0.24335 0.943486 303.0961 189.0144 156.9089
0.242471 0.943893 303.1838 189.2693 157.0215
0.241615 0.944677 303.2316 189.4679 157.04
0.241149 0.945055 303.2613 189.4819 157.0684
0.240392 0.945186 303.37 189.4996 157.131
0.23928 0.94489 303.5443 189.7237 157.2393

 

86069 orbits are available in this time bin, and 1093 orbits have the radiant and velocity in the selected range. Starting with the average orbit for these 1093 orbits using the method by Jopek et al. (2006). Next, we calculate the discrimination criteria for all 1093 orbits. Then the average orbit is calculated based on those orbits that fulfill the low threshold criteria and again new discrimination criteria are calculated for all 1093 orbits. This way we approach the concentration of similar orbits using an iterative procedure. At each step a better representative average orbit is obtained, outliers are rejected, more similar orbits included and this until the result converges to a final average orbit. This happens after the 23rd recombination. The intermediate steps are listed in Table 5 to show how the best fitting selection of orbits is approached.

After 23 iterations we cannot find any better selection of orbits. For the final orbit we consider the different average orbits for each threshold level in Table 6. Each of these orbits fulfils the high threshold similarity criterion with the PTA#556 orbit as reference (Andreic et al., 2014).

Table 6 – The average values for the final selections of orbits for the four different threshold levels on the D-criteria, compared to the orbit for the shower PTA#556, with its similarity values for DD listed below each average orbit.

  Low Medium low Medium high High PTA Andreic et al., 2014
λʘ 189.7° 190.2° 189.8° 189.5° 193°
αg 61.3° 61.9° 61.4° 60.3° 63.9°
δg +28.5° +28.5° +28.8° +28.7° +29.1°
vg 59.9 60.1 60.2 60.6 60.2
a 4.3 4.8 5.3 7.2 8.7
q 0.239 0.240 0.241 0.241 0.234
e 0.945 0.950 0.954 0.967 0.973
ω 303.5° 303.5° 303.3° 303.0° 303.7°
Ω 189.7° 190.1° 189.3° 189.3° 193.1°
i 157.2° 156.9° 156.6° 156.2° 156.3°
N 173 102 46 11 22
DD 0.03 0.02 0.03 0.02 –

 

6 The φ Taurids (PTA#556) shower

Our search for orbits similar to the ZTA#226 resulted in a selection of orbits that confirm the PTA#556 meteor shower. With the available information we can try to obtain some more characteristics of this shower.

PTA orbits were detected in the time interval in solar longitude λʘ [175°, 206°], or September 19 until October 20. This is much longer than what Andreic et al. (2014) published with [187°, 198°]. The Right Ascension of 61.3° and Declination +28.5° is close to the position given by Andreic et al. (2014).

Figure 1 – Plot of the ecliptic latitude β against the Sun centered longitude λ – λʘ. The different colors represent the 4 different levels of similarity.

 

Figure 2 – Plot of the ecliptic latitude β against the Sun centered longitude λ – λʘ for the 921- orbits from the selection that failed in the similarity criteria.

Figure 3 – Plot of the ecliptic latitude β against the Sun centered longitude λ – λʘ (°) for the 173 PTA orbits that fulfill the low threshold similarity criteria with a color gradient to display the variation in the velocity vg.

 

Andreic et al. (2014) mention a rather compact radiant, but that does not really emerge from this case study. Figure 1 shows the radiant plot for different threshold levels of the D-criteria. Figure 2 shows the sporadic background. The region near the ecliptic is very rich in dust which means there is a risk to find similar orbits that have no physical relationship. We reduce this risk by combining three different similarity criteria. Our selection includes 97 orbits that were identified as PTA#556 meteors by UFOCapture as these fulfil the Southworth & Hawkins criteria with DSH < 0.25, but fail in our Drummond criterion with DD > 0.105. Our selection contains probably more orbits that belong to the PTA meteor shower, but which were not identified because of the rather strict selection criteria we apply. Using only the Southworth & Hawkins criteria is not recommended as such less strict criterion could include unrelated sporadics.

All the sporadic radiants in Figure 2 produced meteors that for a perfect single station observer or video station would line up with the assumed radiant position, with the right angular velocity although most of the orbits fail in all discrimination criteria. Therefore, single station meteor shower searches are not reliable for too weak showers.

 

Figure 4 – The plot of inclination i (°) against the length of perihelion П (°) for the 1093-selected possible PTA-orbits. The colors mark the different threshold levels of the D-criteria.

 

Figure 5 – The plot of inclination i (°) against the length of perihelion П (°) for the 921 orbits from the selection that failed in the similarity criteria.

 

Figure 6 – Close-up on the plot of inclination i (°) against the length of perihelion П (°) for the 173 PTA orbits that fulfill the low threshold similarity criteria with a color gradient to display the variation in the velocity vg.

 

Figure 3 shows the velocity variation on the orbits in the radiant plot for the 173 PTA orbits that fulfil our criteria. All these orbits are characterized by a short perihelion distance q and high eccentricity e.

Figures 4, 5 and 6 show the same distributions but for inclination i (°) against the length of perihelion П (°).

The number of orbits allows to look at the radiant drift. Table 7 lists the results for all four threshold levels. The number of orbits with the high threshold level is too small, but the radiant drift for the three other levels is in perfect agreement with Andreic et al. (2014).

Table 7 – Radiant drift with ± σ for the φ Taurids obtained from the orbits for each threshold level of the D-criteria compared with a reference from literature.

 

Threshold/source PTA – 556
Δα / λʘ Δδ / λʘ
Low 1.12 ± 0.02 +0.18 ± 0.02
Medium low 1.16 ± 0.03 +0.19 ± 0.03
Medium high 1.23 ± 0.09 +0.33 ± 0.08
High 1.39 ± 0.35 –0.04 ± 0.29
Andreic et al. (2014) 1.15 +0.20

 

The activity period given by Andreic et al. (2014) differs from this case study. We have orbits for this shower during a longer period with λʘ = 190° about at the middle. The entire period of time is very well documented with an average of about 2000 orbits available for each degree in solar longitude. The percentages of PTA orbits in these datasets are very small, less than 1%. Looking at these percentages in time bins of 1° in solar longitude to reconstruct the activity profile, strong fluctuations are visible which is most likely just statistical flutter due to the low number of PTA orbits detected. No distinct maximum is visible.

Looking at the number of PTA orbits detected each year it is obvious this is an annual shower (Figure 8). The numbers vary between 0.2% and 0.4% of all available orbits during the activity period 175° < λʘ < 206°. Most of the orbit data comes from EDMOND and SonotaCo. Only for 2011 and 2012 CAMS data is publicly available and no EDMOND data is available for 2017 and 2018.

Figure 7 – The relative number of PTA orbits collected per 1° of solar longitude in steps of 0.5° during the years 2007–2018, with blue for DD < 0.105, green for DD < 0.08, orange for DD < 0.06 and red for DD < 0.04, as percentage compared to the total number of non-PTA orbits without any Orionids, collected in the same time span.

 

Figure 8 – The percentage of PTA orbits relative to the total number of non-PTA orbits obtained per: Total activity period
175° < λʘ < 206.5° (blue).

 

7 Conclusion

The ζ-Taurids (ZTA#226) are most likely an erroneous combination of a few early Orionid orbits that were linked to another minor shower, τ-Taurids, with a different type of orbit. A stream search among all available orbits indicates that there is a weak concentration of similar orbits that are related to the τ-Taurids detected by Sekanina (1976), which is already listed in the IAU Working List of Meteor Showers as the φ-Taurids (PTA#556) detected by Andreic et al. (2014).

Based on this case study, we recommend to remove the record of the ZTA#226. The PTA#556 data is confirmed by this case study, with a longer activity period
175°< λʘ< 206°, centered around λʘ = 190° without a distinct maximum.

Acknowledgment

The author is very grateful to Jakub Koukal for the dataset of EDMOND with the data until 2016, to SonotaCo Network (Simultaneously Observed Meteor Data Sets SNM2007–SNM2018), to CAMS (2010–2013) and to all camera operators involved in these camera networks.

I thank Denis Vida for providing a tool to plot a color gradient to show the dispersion in velocity and an algorithm to calculate the average meteor shower orbit according to the method published by Jopek et al. (2006).

EDMOND[1] includes: BOAM (Base des Observateurs Amateurs de Meteores, France), CEMeNt (Central European Meteor Network, cross-border network of Czech and Slovak amateur observers), CMN (Croatian Meteor Network or HrvatskaMeteorskaMreza, Croatia), FMA (Fachgruppe Meteorastronomie, Switzerland), HMN (HungarianMeteor Network or Magyar Hullocsillagok Egyesulet, Hungary), IMO VMN (IMO Video Meteor Network), MeteorsUA (Ukraine), IMTN (Italian amateur observers in Italian Meteor and TLE Network, Italy), NEMETODE (Network for Meteor Triangulation and Orbit Determination, United Kingdom), PFN (Polish Fireball Network or Pracownia Komet i Meteorow, PkiM, Poland), Stjerneskud (Danish all-sky fireball cameras network, Denmark), SVMN (Slovak Video Meteor Network, Slovakia), UKMON (UK Meteor Observation Network, United Kingdom).

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[1] https://fmph.uniba.sk/microsites/daa/daa/veda-a-vyskum/meteory/edmond/