Abstract: Meteor orbits with D criterion similarity to the comet like orbit Amor Asteroid 2018 LF5 are presented and their potential association with known meteor showers discussed.
A list of known Near Earth Objects (NEOs) was obtained from the Minor Planet Center and the 618 objects with inclinations of 40 degrees or greater were filtered out for assessment via the Jopek (1993) variation of the Southworth and Hawkins (1963) D criterion against publicly available meteor orbits from various sources utilizing a threshold value of 0.1.
Resultant orbits were then assessed in the cases where any particular NEO had ten or more meteor orbits matching with D criteria less than this value. The list of NEOs were also tested against themselves as a simple way of trying to avoid the situation of objects similar in orbital characteristics simply because of the commonality of being injected into their current orbit via interactions with Jupiter, although the selection of such a high inclination cut off point should have removed that selection effect. Similarly, the meteoroid orbits were also further tested against the NEO list again to ensure that they did not match any other object in the NEO dataset. The original agglomeration of publicly available datasets also contained the elements for all comets with perihelion distances of 1.3 AU or less.
Finally, the resultant stream particulars where assessed against showers detailed by the International Astronomical Union Meteor Data Center (Jopek and Kanuchova, 2017) for known streams.
Although a handful of NEOs returned more than ten meteoroid orbits per object, the vast majority only just did so with only the Amor Asteroid 2018 LF5 having markedly more.
This had a total of 42 orbit matches for D < 0.1 predominantly classified as sporadic and consisting of 2 from the Croatian Meteor Network (CMN) (Korlević et al., 2013), 3 from SonotaCo (e.g. SonotaCo, 2009), 16 from EDMOND (e.g. Kornoš et al., 2014) and 21 from CAMS 3.0 (Jenniskens et al., 2018), spread across the period 2009 to 2016. Figure 1 demonstrates the spread in D criterion in 0.01 steps whilst Figure 2 shows the number per year which is a reflection of the observing regimes rather than the shower activity. It should be noted that all these surveys bar CAMS, exactly half of the orbits, use the same analysis software which reports to a higher precision than CAMS and that despite the CAMS precision is reflecting better the reality, the rounding off of the numeric data before a mathematical analysis will give different results than rounding off after analysis. The surveys also use different technology and to some extent observing methodologies, with the EDMOND dataset being a combination of many different groups. Nevertheless, the differences in the mixed data were considered negligible and their results were analyzed as equivalent.
Table 1 – The identifier, perihelion distance (q) in Astronomical Units, eccentricity (e), inclination (i), argument of perihelion (ω), ascending node (Ω) all in degrees and D criterion values for the 42 video meteors. Their mean and median values are also included as are those for 2018 LF5.
|ID||q (A.U.)||e||i (°)||ω (°)||Ω (°)||D|
Table 2 – The Right Ascension (R.A.), Declination (Dec.), Solar Longitude (λʘ), geocentric velocity (vg), perihelion distance (q) in Astronomical Units, inclination (i), argument of perihelion (ω), ascending node (Ω) all in degrees for mean and median of the 42 meteor orbits as well as those for the zeta Draconids (ZDR#073), omicron Draconids (ODR#088) and July zeta Draconids (ZED#279).
|ID||R.A. (°)||Dec. (°)||λʘ (°)||vg (km/s)||q (A.U.)||i (°)||ω (°)||Ω (°)|
The 20 orbits with D criterion values of 0.080 and less along with that for 2018 LF5 are presented in Figure 3 for illustrative purposes. The meteor orbits’ individual elements, their mean and median, and the same elements for 2018 LF5, are presented in Table 1.
The particulars of 2018 LF5’s orbit are derived from 239 observations spanning 149 days from May to October 2018 as provided in MPO 529360 led to the object being identified as an Amor type asteroid. However, both the inclination of 41 degrees and the Tisserand parameter with respect to Jupiter (TJ) of 2.7 are more representative of a comet. However, an examination of the few publicly available images for the object during its apparition (Deen S., pers. comm.) gave no indication whatsoever of anything other than a point like object, although this is not surprising given its apparent magnitude barely rose above 18.
Examination of the Asteroid Lightcurve Photometry Database (e.g. Warner et al., 2011) revealed somewhat noisy data except for a short run near opposition where a handful of peaks very roughly 1.5 hours apart are suggested, which for a non-spheroidal object could represent a periodicity around 3 hours or so. However, the irregular shape and on occasion tumbling motion of such objects often lead to multiple peaks per rotation period, not just the two expected per rotation from an elongated object. Further such a rotation rate would not necessarily distinguish between a comet and a fast-rotating asteroid throwing off fine debris
In summary it can be said that varying degrees of circumstantial evidence can hint at 2018 LF5 being more cometary than asteroidal, especially the orbital parameters, but no definitive evidence exists.
The mean particulars for the derived meteors were checked against the IAU MDC list of showers both numerically and visually via plotting in the astronomical charting software Guide 9.0 to check for any relation to known showers. No evident association appeared to exist, and the nearest representative star would be 42 Draconis (although the more well-known Cat’s Eye Planetary Nebula lies very adjacent, and of course the mean radiant also lies very close to the North Ecliptic Pole, however such things are not included in the nomenclature guidelines).
However, three relatively adjacent showers in terms of radiant position and solar longitude do exist, namely ZDR#073 (zeta Draconids), ODR#088 (omicron Draconids) and ZED#279 (July zeta Draconids). The surveys providing the meteor orbit data all assess shower association via radiant clustering algorithms and predominantly identified their objects as sporadic meteors with no known association, albeit with EDMOND matching 4 to ODR#088, 1 to ZDR#073 and 1 to ZED#279, 6 out of the total of 16. A CMN meteor that was included in the EDMOND dataset but removed to avoid duplication before analysis had however been identified by CMN as sporadic despite being classified as ZDR#073 by EDMOND (see Koseki, 2019 with respect to SonatoCo, which is also used by CMN, and EDMOND shower look up table differences). Table 2 provides the mean particulars for this study’s orbits as well as those for the other three showers as derived from the IAU MDC database or literature search.
The shower ZDR#073 results from Lindblad (1971) and was obtained from a small handful of photographic orbits yet those orbits had been included in the analysis having been obtained from the IAU MDC (Neslušan et al., 2014), yet none from that dataset were matched to 2018 LF5. The ODR#088 Sekanina (1976) are again originally from a small number of orbits, which are in fact radar orbits, and although those orbits were also included in the analysis only one matched to 2018 LF5, although Jenniskens (2016) has cross identified CAMS meteor orbits with this shower and provided new particulars. The ZED#279 (Jenniskens, 2006) are from an assessment of potential asteroidal showers but again based on little data, certainly predating the modern era of plentiful double station video meteor orbit datasets.
In other words, none of the showers as originally presented are well defined in orbit elements, being based on few meteors, and indeed in that context are not particular that different from each other given the low and to some extent happenstance sampling rate upon which radiant positions and solar longitudes were based upon. Unfortunately, the classical minor meteor streams are often ill defined, more or less unique to their discovery papers and consequently difficult to relate to other surveys thus at times inflating the IAU MDC shower list with duplication which could well all be manifestations of the same entity. Yet on the other hand these same reasons are why there is insufficient information to show that these three classical showers and the meteors associated via D criteria with 2018 LF5 are assuredly the same. The data are insufficient to outright connect or outright reject an association between any and all of the showers or the current orbits connected to 2018 LF5, yet this would represent four not particularly different near coincidental showers in a relatively small area of the sky about at the same time of year.
Whether 2018 LF5 is associated with an unknown meteor stream with a radiant lying near the star 42 Draconis, or whether it is associated with one of the three known meteor streams, which in turn may all be themselves manifestations of the same stream only appearing different due to selection effects and limited data, is more a matter for the IAU Meteor Group nomenclaturists and taxonomists than for this paper. Within zoological and botanical nomenclature, the naming of a taxonomic group usually follows priority, that is which one was published first, which would favor the zeta Draconids of the Lindblad (1971) study.
Examination of a large number of publicly available meteor orbits revealed that 42 video survey orbits matched well with Amor Asteroid 2018 LF5’s orbit based on the results of Jopek (1993) modified Southworth and Hawkins (1963) using a D criterion threshold of 0.1. Meanwhile the same orbits revealed no association between them and neither other known NEOs nor comets. Despite a comet like orbit there was no direct evidence for 2018 LF5 being a comet so the nature of that object is not entirely clear. Assessment of the derived mean shower details with respect to known showers revealed no strong candidate but that there were three not completely dismissible showers adjacent in both time and space. These showers themselves appeared to be potentially the same entity only differentiated because of the limited datasets they were based upon, yet this same limitation ensured there was insufficient information to either confirm or dismiss them being all aspects of the same shower or stream, let alone associated with 2018 LF5, despite this circumstantial evidence.
The author decided that this however would be a parsimonious solution over declaring yet another new shower and that in terms of publication priority the ZDR#073 zeta Draconids would be the choice.
Sam Deen is acknowledged for kindly finding and pointing out archival asteroid survey images of 2018 LF5.
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