CAMS BeNeLux recorded 7 orbits in a two hours’ time lapse that were identified as 15 Bootids (FBO#923) orbits. A stream search to assess activity of this stream in earlier years failed to confirm annual activity. Instead of a confirmation of the FBO#923 activity, the stream search converged to a dominant number of orbits that are similar to the orbit of the MPV#454 shower, a meanwhile removed entry in the IAU meteor shower working list. This justifies a reconsideration of the presence of some source related to the removed MPV#454 shower.
During the night of April 21–22 the CAMS BeNeLux network registered 7 orbits that were identified as belonging to the unconfirmed shower of the 15 Bootids (FBO#923). All seven meteors were registered in a short time span of two hours. The mini outburst was confirmed by the CAMS network of the United Arab Emirates where 4 more orbits of the shower were registered during the same two-hour interval.
This weak barely detectable shower was first noticed during the stream search on the CAMS dataset 2010–2016 (Jenniskens et al., 2018). The shower remained unnoticed before; hence this is a good opportunity to check if and what we can find about this shower in the EDMOND and SonotaCo orbit databases.
2 Available orbit data to search
We have the following orbit data collected over 12 years, status as until June 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 A preliminary search
To locate the position where a concentration of 15 Bootids (FBO#923) orbits may be found, we take some sample FBO orbits to determine the range in time, radiant area and velocity interval where we can find these orbits within a low threshold similarity criterion. This first test results in:
- Time interval: 13° < λʘ < 51°;
- Radiant area: 197° < αg < 229° & 0° < δg < +23°;
- Velocity: 22 km/s < vg < 32 km/s.
The D-criteria that we use are these of Southworth and Hawkins (1963), Drummond (1981) and Jopek (1993) combined. We define five different classes with specific threshold levels of similarity:
- 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.
- Very high: DSH < 0.05 & DD < 0.02 & DH < 0.05.
In total we have 37223 orbits available within the time span in solar longitude of 13° until 51°. 331 of these orbits have their radiant position and velocity within the above-mentioned range.
In a first approach we calculate the average orbit for this set, using the calculation method explained by Jopek et al. (2006). Table 1 lists the resulting average orbit for each similarity threshold level in our preliminary sample of orbits. Figure 2 shows the radiant scatter for these orbits in Sun centered ecliptic coordinates.
However, the search for a concentration of orbits in this dataset of 331 orbits does not converge to the expected FBO#923 orbit. After a number of iterations, the search ends with a complete different average orbit. The cause is with a dominant number of orbits with lower geocentric velocity, lower inclination and smaller eccentricity. It looks like another group of similar orbits exists in this region. A check through the IAU working list of meteor showers has a positive match with medium low values for the discrimination criteria for the May phi Virginids (MPV#454). However, this shower has been removed meanwhile from the IAU working list of meteor showers. The motivation for the removal was that this meteor stream (MPV#454) did not show up in a later meteor shower search when more CAMS data was available.
The question arises as whether or not the removal of this shower was justified. Our pre-selection on activity period, radiant area and velocity range was derived from a sample that fit for a sample of FBO#923 orbits and this will definitely not be a perfect sample to search for MPV#454 orbits. Our selection may contain only part of such similar orbits. It might be useful to reconsider the MPV#454 case based on another specific selection for this stream.
Table 1– The average values for the preliminary selection of orbits for the four different threshold levels on the D-criteria, compared to a reference orbit from literature for the shower MPV#454.
|Low||Medium low||Medium high||High||Rudawska & Jenniskens (2014)|
The plot of the Sun centered ecliptic coordinates (Figure 2) indicates that likely more similar orbits can be found with more southern radiant positions. The plot of the inclination i against the length of perihelion Π in Figure 3 suggests more similar orbits may be found with lower inclination and larger length of perihelion.
Whatever sample orbit we take to start our stream search, the iterative search routine ends with the orbit type as listed in Table 1. The population with orbits similar to MPV#454 is too dominant in this region and makes it impossible to pin-point any other weaker source in this area. The only alternative to look for FBO#923 orbits in this region is to use the average orbit for the 2019 outburst of the FBO#923 (Jenniskens, 2019):
- λʘ = 31.24°–31.34°
- αg = 213.7° ± 0.2°
- δg = +11.3° ± 0.2°
- vg = 27.7 ± 0.3 km/s
- a = 25 AU
- q = 0.634 ± 0.004 AU
- e = 0.975 ± 0.038
- ω = 254.2° ± 0.5°
- Ω = 31.3° ± 0.2°
- i = 19.8° ± 0.5°
Only 56 orbits of our 712489 video meteor orbits fulfill the low threshold similarity criterion with the above orbit as reference. Table 2 lists the averaged orbits for each threshold level. The extreme low number of similar orbits, together with the presence of a more dominant source explain why this shower escaped attention in many older meteor stream searches.
Table 2– The average values for the selection of FBO-orbits for the five different threshold levels on the D-criteria.
|Low||Medium low||Medium high||High||Very high|
Using similarity criteria to identify orbits requires caution as similarity does not prove any physical relationship. When looking at the number of FBO orbits per year, there is no convincing proof for annual activity. Years with a single or few possible FBO orbits may be explained by sporadic orbits which are similar by chance. Looking at the radiant plot in Figure 4 and the plot of inclination i in function of the length of perihelion Π in Figure 5 there is a considerable spread while the 2019 FBO orbits displayed a very compact radiant.
A short, two hours long activity period can have been easily missed in the past. Until few years ago the global coverage was still poor and events such as the 2019 FBO activity could be easily missed.
Table 3 – Number of possible FBO#923 orbits and total number of orbits available per year within the time interval 13 < λʘ < 49.
Our attempt to identify past FBO#923 orbits in the public available video meteor orbits did not provide convincing proof for some annual activity. Our meteor stream tool does not detect the FBO#923 shower. Each search iterates towards an average orbit that is similar to the MPV#454, a meanwhile removed meteor shower from the IAU working list. The selection interval is definitely not optimal to search for MPV#454 orbits, but the dominant presence of similar orbits seems to indicate the presence of many similar orbits in this region and time range. Perhaps the initial determination of the MPV#454, based on an early available CAMS dataset wasn’t representative and reason why this shower was not found in later stream searches? Another possible explanation is the presence of a large number of unrelated sporadic orbits in this rich area near the ecliptic.
A possible association with comet C/539 W1 remains to be proven. The orbital elements of this comet were:
- q = 0.16 AU
- e = 1.0
- ω = 246°
- Ω = 33°
- i = 19°
The author is very grateful to Jakub Koukal for maintaining EDMOND, 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 me with scripts to compute the average orbit according to the method of Jopek et al. (2006).
EDMOND (https://fmph.uniba.sk/microsites/daa/daa/veda-a-vyskum/meteory/edmond/) 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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