By Paul Roggemans and Peter Cambell-Burns
CAMS reported an outburst of an unknown minor stream on 14 February 2018, listed in the IAU working list of meteor showers as #1032 FCM (α = 124°, δ = +2°, λʘ = 324°, vg = 16.5 km/s). This analysis shows that many similar orbits can be found in the time span and region in space around the reference orbit. The term ‘outburst’ is rather misleading as only few orbits were detected during several nights. A search through all public available orbit catalogues resulted in a significant number of similar orbits, but the region proves to be rich in unrelated similar sporadic meteors that fulfill low and medium low D-criteria. This case study on the possible February Hydrids did not result in a convincing evidence for the existence of this minor shower. Both the distribution of the number of similar orbits and the spreading in space indicate the possible presence of a diffuse minor shower without any distinct peak activity. This is a case of a barely detectable minor stream.
February 2018 was an exceptional favorable month for the CAMS BeNeLux network with as many as 16 nights with more than 100 orbits. 13–14 February was the most successful night with 364 orbits. Dr. Peter Jenniskens drew the network coordinator’s attention to a possible outburst for which the CAMS BeNeLux network had recorded some orbits. The new shower got listed as #1032 FCM (α = 124°, δ = 2°, λʘ = 324°, vg = 16.5 km/s). More orbits of this shower were found in the period 9–16 February as well as in previous years.
The term outburst raises the expectation that suddenly a significant number of orbits were found to identify a distinct dust trail based on similar orbits. The reality is far less spectacular. The online CAMS tool (http://cams.seti.org/FDL/index-cams.html ) allows checking the results for all the CAMS networks. CAMS BeNeLux had the best conditions with 325 orbits on 13 February and 364 orbits on 14 February; the night of 15 February was clouded out. Only a few orbits were collected as candidate FHY-meteors. CAMS California collected 255 orbits on 13 Feb., 135 on 14 Feb. and 204 on 15 Feb. and had few extra candidates. CAMS United Arab Emirates had respectively 37, 40 and 36 orbits for the three nights and one candidate orbit. CAMS Arizona suffered bad weather and had only 9 obits on 14 February with 1 candidate FHY-orbit. The other CAMS networks had bad weather or did not work these nights.
Altogether, the evidence for a new meteor shower is rather thin and therefor the authors decided to search for more evidence in the publicly available meteor orbit catalogues.
2 The available orbit data
We have the following data, status as until April 2018, 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 257010 orbits (2007–2017). SonotaCo is an amateur video network with over 100 cameras in Japan (SonotaCo, 2009).
- CAMS with 111233 orbits (October 2010 – March 2013), (Jenniskens et al., 2011). For clarity, the CAMS orbits April 2013 – April 2018 are not included in this dataset because this data is still under embargo.
Altogether we can search among 686073 video meteor orbits.
3 Preliminary orbit selection
The authors followed the procedure described in a previous similar analysis (Roggemans and Johannink, 2018) to identify possible FHY orbits. Based on the known radiant position, velocity and date of activity, we can define a sub-dataset to limit the amount of orbits in time and space to a region where related orbits might be located.
Orbits were selected in a period of 15 days before and after 14 February. All orbits within the following intervals were selected:
- Time interval: 309° < λʘ < 340°;
- Radiant area: 108° < α < 139° and –9° < δ < +12°;
- Velocity: 11 km/s < vg < 22 km/s.
In total 461 orbits occurred within these intervals, 173 from SonotaCo, 158 from EDMOND and 130 from CAMS data. These 461 orbits were obtained from meteors that appeared in the sky in a way that any single station observer would associate these meteors as FHY shower members, coming from the right direction of the radiant with the right angular velocity expected for this shower. The purpose of analyzing the orbital data is to get an idea how many of these orbits are nothing other than sporadics that contaminate the radiant area and how many of these orbits have enough similarity to form a concentration that proves the presence of a minor shower.
The median values for these 461 orbits compare very well with the orbital parameters given by Jenniskens et al. (2018). The error margins σ represents the standard deviation:
- λʘ = 321.4°
- α = 126.5 ± 8.0°
- δ = +3.72.0 ± 5.8°
- vg = 16.9 ± 2.9 km/s
- a = 2.4 ± 3.0 AU
- q =0.783 ± 0.09 AU
- e = 0.674 ± 0.08
- ω = 61.4 ± 13.9 °
- Ω = 141.4 ± 7.8°
- i = 7.2 ± 3.0°
We apply three discrimination criteria to evaluate the similarity between the individual orbits taking the median values of the 461 selected orbits as parent orbit. The D-criteria used are these of Southworth and Hawkins (1963), Drummond (1981) and Jopek (1993). We consider four different 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.
315 orbits fulfill the D-criteria compared to the median values of our 461 orbits as parent orbit. If our dataset contains a concentration of orbits for the FHY shower, the median values should be comparable. The results are shown in Table 1 and differ slightly from the reference orbit.
Table 1 – The median values for the selected orbits with four different threshold levels on the D-criteria, compared to the reference orbit from literature (Jenniskens et al., 2018).
|Low||Medium low||Medium high||High||Reference (2018)|
Table 1 shows the percentage (S) of orbits of the sample that fails to fulfill the D-criteria and must be considered as sporadic contamination of the radiant area. The remainder is an indication for the presence of a possible dust concentration within the sample.
When we plot the graph for all 461 orbits with inclination i against length of perihelion Π, we see a rather dispersed picture (Figure 2). The spreading in length of perihelion is rather large even for those orbits that fulfill the low, medium low and medium high threshold. Only orbits which fit the high threshold D-criteria show less dispersion but there is no real concentration of orbits. This could indicate that we are comparing sporadic orbits that fulfill the D-criteria by pure chance. Since we use the medium values of all selected orbits, this includes indeed some sporadic contamination.
In the next step we take the median values for the orbits that fulfill the high threshold D-criteria (DSH < 0.1 & DD < 0.04 & DH < 0.1, Table 1) as parent orbit to recalculate the D-criteria for all 461 orbits of the dataset. The results are listed in Table 2. The median values for all orbits for each level of threshold on the D-criteria differ slightly from the reference orbit given by Jenniskens et al.
Table 2 – The median values for the selected orbits with four different threshold levels on the D-criteria, using the high threshold orbit from Table 1 as parent orbit, compared to the reference orbit from literature (Jenniskens et al., 2018).
|Low||Medium low||Medium high||High||Reference (2018)|
Recalculating the D-criteria using the median values of the orbits that fulfill the high threshold criteria listed in Table 1 as a parent orbit does not change much to Figure 2. Only a few more orbits fulfill the D-criteria and some dots change color. The result is shown in Figure 3.
At this point we can conclude that the region is rich in many similar orbits, but these may be unrelated sporadic orbits. Although the inclination i for all orbits is within 8° ± 4°, the spread in the length of perihelion is too large to conclude anything about the presence of a dust trail in this region. We look a bit further at the distribution of these orbits in time.
4 Case study FHY-1032: sporadic orbits?
Activity profile and periodicity
The dataset contains orbits for each year from 2007 until 2017 and in each of these years we find a significant number of similar orbits that fulfill the low threshold D-criteria. CAMS contributed only data to 2011, 2012 and 2013 while 2017 represents only SonotaCo orbits. Figure 4 represents the proportion of similar orbits that respect the low threshold D-criteria for each year compared to the total number of orbits available for the interval 309° < λʘ < 340°. In total 28149 orbits were collected during this time span and 317 or 1.1% of this total number of orbits fulfill the low threshold D-criteria for the FHY orbit. The variation in the percentage of orbits per year can be explained as normal statistical fluctuations, except for 2013 when, remarkably, many look-alike FHY-1032 orbits were found. It is not possible to conclude that the high number of 90 possible FHY-1032 orbits in 2013 represents some enhanced activity or rather a statistical fluctuation.
When we look at the time distribution of all the orbits that fulfill the D-criteria it becomes very obvious that we find these similar orbits at each degree of solar longitude (Figure 5). The profile does not look like a typical meteor shower activity profile with a shower maximum. There is a noticable dip in the number of candidate FHY-1032 orbits at λʘ = 324° with best numbers at λʘ = 323° and
λʘ = 327°. The relative high number or orbits that fulfill the D-criteria for each time slot in this interval may also indicate the presence of many look-alike sporadic orbits that fulfill D-criteria although not being related to any dust trail in this region.
We try to detect a radiant drift relative to the reference position at α = 126.1° and δ = +4.5°, valid at λʘ = 322.4°.
It is obvious that the radiant positions that fulfill the low threshold criteria display a too large scatter. The medium low, medium high and high threshold levels cover a relevant time span and display an acceptable correlation. We use the high threshold (DD < 0.04) data to obtain the radiant drift (see Figures 6 and 7). This results in the following radiant drift:
Δα = 0.49°/ λʘ and Δδ = –0.3°/ λʘ.
The radiant distribution appears to be very diffuse (Figure 8). Some radiants of orbits that fail to fulfill any D-criteria appear close to the parent orbit position while orbits that fulfill the high threshold D-Criteria (red dots) appear very dispersed. This indicates we are in a region rich in unrelated but very similar sporadic orbits. Applying the radiant drift obtained from the high threshold D-criteria in Figure 9, we see on one hand the sporadic radiants (black dots) and some low threshold criteria radiants (blue dots) getting more dispersed while the medium high and high threshold criteria radiants (orange and red) contract towards the reference position, indicating that the radiant drift is valid for these orbits.
Other shower characteristics
The slow meteors have a median begin height of 90.0 ± 5.6 km and ending height of 79.4 ± 7.0 km. With a velocity of 16.9 km/s these are slower than the Draconids (DRA-9) with 97.7 ± 2.2 as starting height and 90.1 ± 3.4 as ending height (Roggemans, 2017). The Draconids are known to be relatively fresh cometary meteoroids which fail to penetrate deep into the atmosphere because of their fragile composition. The candidate February Hydrid meteors are only slightly slower than the Draconids but penetrate significant deeper into the atmosphere, perhaps a hint for a more compact meteoroid composition?
The analysis does not prove a distinct concentration of orbits. A rather diffuse picture emerges of possibly related orbits embedded in a sporadic background rich in look-alike but unrelated sporadic orbits. To resolve such dispersed dust trail from the rich sporadic background is at the limit of detectability and tricky to distinguish possible shower members from similar sporadic orbits.
When we use the reference orbit given by Jenniskens et al. (2018) to compare the selected 461 orbits, recalculating the D-criteria, we obtain median values for the four threshold levels of D-criteria as listed in Table 3.
The median values for 43 high threshold orbits compare very well to the reference orbit as given by Jenniskens et al. (2018). The plot of inclination i against length of perihelion Π (Figure 10) does not show a distinct concentration, but a rather diffuse picture. The picture is about the same as what we obtained in Figures 2 and 3.
Table 3 – The median values for the selected orbits with four different threshold levels on the D-criteria, using the reference orbit given by Jenniskens et al. (2018), as parent orbit, compared to the reference orbit from literature (Jenniskens et al., 2018).
|Low||Medium low||Medium high||High||Reference (2018)|
Figure 11 shows the reference orbit published by Jenniskens et al. (2018) in red with the 43 orbits of our sample in grey that fulfill the high threshold D-criteria in Table 3. The final orbit that we obtain from our 461 selected orbits is shown in green and is situated well within the orbit given by Jenniskens et al. Also the orbit we found in Table 2 (green in Figure 11) has 43 orbits that fulfill the high threshold D-criteria. The high threshold D-criteria orbits listed in Table 3, using the orbit given by Jenniskens et al. as parent orbit and those listed in Table 2, obtained from this analysis, have 20 orbits in common that fulfill the high D-criterion for both parent orbits! This paper indicates a diffuse meteor stream with more orbits further inside the reference orbits (smaller eccentricity and shorter perihelion and semi major axis).
A search on the orbital data from the major video camera networks worldwide, good for ~686000 orbits (status April 2018), resulted in a collection of very similar orbits with a significant number of orbits that fulfill the high threshold D-criteria of DD < 0.04. There is no distinct concentration but a rather diffuse trace of some weak shower embedded in a region strongly contaminated with similar sporadics orbits.
From this analysis we do not find convincing evidence to confirm the existence of the February Hydrids (FHY-1032). This study indicates that a weak and diffuse shower may be present in the data for the period 2007–2017. More attention is required in the future to assess the relevance of this discovered minor shower.
The author is very grateful to Jakub Koukal for updating the dataset of EDMOND with the most recent data, to SonotaCo Network (Simultaneously Observed Meteor Data Sets SNM2007-SNM2017), to CAMS (2010-2013) and to all camera operators involved in these camera networks.
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