P. Jenniskens, T. Cooper, J. Baggaley, S. Heathcote, D. Lauretta
Abstract: The predicted new meteor shower from crossing the 1988 and 1995 dust trails of Jupiter-family type comet 15P/Finlay on September 27–30, 2021, materialized and was recorded by CAMS video-based meteoroid orbit survey networks in New Zealand and Chile. The new shower is called the “Arids”, with meteors radiating from the constellation Ara, the Altar. The median radiant position of the first 13 shower members observed was at R.A. = 262.7 deg., Decl. = -57.5 deg. (Equinox J2000.0) with geocentric entry speed Vg = 10.8 km/s, but the shower was ongoing when this report was made. A potentially more intense shower is expected during the crossing of the 2014 dust trail on October 7, 2021.
Introduction
Jupiter-family comet 15P/Finlay does not pass far from Earth’s orbit but until now did not have an associated meteor shower (e.g., Beech et al., 1999). Indeed, dust trails created following past returns to the inner solar system did not wander into Earth’s path as far back as 1965, according to calculations by M. Maslov (2009). That changed this year. Calculations by S. Shanov and S. Dubrovski, reported in Jenniskens (2006), first predicted that dust tails would be in Earth’s path on 2021 September 27 around 6:14 UT (1988 dust trail) and on September 28 18:58 UT (1995 dust trail). Since that time, better predictions were made by Maslov (2009), Sato (2009), Ye et al. (2015), and Vaubaillon (2020).
The upcoming October 6 and 7 encounters with the 2014 dust trail are especially interesting. Ye et al. (2015) pointed out that comet 15P/Finlay had two cometary outbursts of activity during that return, which could translate into a more dense dust trail. As a result, their predictions for this year’s meteor shower have ZHR peak at 600–1100 per hour. The most recent update of expected rates is given in Ye et al. (2021).
The observations
These predictions have gotten more urgency now that in the past few nights CAMS video-based meteoroid orbit survey networks in New Zealand and Chile have detected the first two of these predicted encounters. These were meteor outburst caused by Earth encountering debris ejected from comet 15P/Finlay during its perihelion passage in 1988 and 1995. The shower was clearly detected from September 27 to 30. These are the first meteors observed from comet 15P/Finlay. The shower was added to the IAU Working List of Meteor Showers under number 1130 and name “Arids” (ARD), because the meteors radiated from the southern constellation Ara, the Altar (c.f., http://cams.seti.org/FDL/ for dates of 2021 Sep. 29 and 30).
In a recent CBET telegram (Jenniskens et al., 2021), we reported that CAMS New Zealand, with stations operated by I. Crumpton, C. Duncan, and N. Frost and the network coordinated by J. Baggaley of the University of Canterbury at Christchurch, triangulated 9 Arids between 2021 Sept. 28 08:40 and 17:18 UT, while CAMS Chile, with stations operated by J. Rojas, E. Jehin and T. Abbott and the network coordinated by S. Heathcote of AURA/Cerro Tololo, triangulated 4 Arids between 2021 Sept. 28 23:49 and Sept. 29 03:45 UT. Observations continued until 09:33 UT, but at that time the radiant had long set. Other southern hemisphere CAMS networks had poor weather. At the time of writing, the outburst was ongoing.
The meteors observed to that point radiated from R.A. = 262.7 deg., Decl. = -57.8 deg. (Equinox J2000.0) with geocentric entry speed Vg = 10.8 km/s from a direction with few sporadic meteors (see figure 1). The observed median orbital elements of the 13 meteors, centered on solar longitude 185.27 deg., are given in Table 1 and are compared to the orbit of comet 15P/Finlay at the Epoch 2014-Nov-08.0 (TDB).
Table 1 – The orbit of the Arids (ARD#1130) compared to orbital elements comet 15P/Finlay had at Epoch 2014-Nov-08.0 (Equinox J2000.0).
| Arids | 15P/Finlay | |
| Epoch | 2021-Sep-28.5 | 2014-Nov-08.0 TDB |
| a | 3.53 AU | 3.49 AU |
| q | 1.0010 ± 0.0004 AU | 0.976 AU |
| e | 0.717 ± 0.042 | 0.720 |
| i | 9.10 ± 0.54 deg. | 6.80 deg. |
| ω | 356.1 ± 1.01 deg. | 347.55 deg. |
| Ω | 5.28 ± 0.29 deg. | 13.78 deg. |
Figure 1 – The Arid radiants detected by CAMS 2021 September 29/30.
These meteors were mostly faint, with a magnitude distribution index of 4.7 +/- 0.8. The shower was also detected by the SAAMER radar. Bruzzone et al. (2021) reported that activity lasted for about 3 hours and was centered on Sept. 29 03:32 UT (solar longitude 185.92 deg.). Over 100 Arids orbits were measured.
The meteors from the 1995 dust trail crossing were predicted to radiate from geocentric radiant R.A. = 261.1 deg., Decl. = -57.7 deg., with Vg = 10.8 km/s during Sept. 29 02:30 to 04:17 UT by Maslov (2009) and from R.A. = 260.8 ± 0.9 deg., Decl. = -57.4 ± 0.5 deg, with Vg = 10.807 km/s, during the peak on 2021 Sept. 29 at 08:35 UT by Vaubaillon et al. (2020). Those of the 1988 dust trail crossing were earlier, on September 27 between 13:58 and 16:22 UT according to Maslov (2009). The observed meteors by CAMS are what appear to be the 1995 dust trail crossing, with perhaps also a weak detection of the 1988 dust trail crossing.
The predictions for activity during the 2014 dust trail crossing vary a lot between the different models. Vaubaillon et al. (2020) has the 1995 dust trail crossing being the more intense. The 2008 dust trail would be crossed on 2021 Oct. 07 00:35 UT, followed by an outburst from debris ejected in 2014 centered on 2021 Oct. 07 03:55 UT.
Ye et al. (2015) expects high rates from the upcoming 2014 dust trail crossing on October 7. The expected encounter times are October 7 between 00:34 and 01:09 UT for the encounter with the first cometary outburst ejecta, and October 6 between 21:59 and 22:33 UT for the second cometary outburst ejecta.
Two Arid meteors, caused by debris from comet 15P/Finlay entering Earth’s atmosphere, captured by cameras of the Cerro Tololo station of the CAMS Chile network at 04:51 UT on 2021 September 29. Photo: P. Jenniskens/SETI Institute and S. Heathcote/AURA Cerro Tololo.
References
Beech M., Nikolova S., Jones J. (1999). “The silent world of comet 15P/Finlay.” MNRAS 310, 168–174.
Bruzzone J. S., Janches D., Weryk R., Hormaechea J. L. (2021). “Arid meteors 2021”. CBET 5046, 2021 October 1, Daniel W. E. Green, editor.
Jenniskens P. (2006). “Meteor Showers and their Parent Comets”. Cambridge University Press, Cambridge, UK, 790 pp (Table 6c).
Jenniskens P., Cooper T., Lauretta D. (2021). “Arid meteors 2021”. CBET 5046, 2021 October 1, Daniel W. E. Green, editor.
Maslov M. (2009). “Finlayids 1901–2001: Activity predictions”. Website: http://feraj.ru/Radiants/Predictions/ 1901-2100eng/Finlayids1901-2100predeng.html (last accessed 2021-09-30).
Sato M. (2009) In a deleted post on a yahoo group message board. (see: Ye et al. 2015).
Vaubaillon J. , Egal A. , Desmars J., Baillie K. (2020). “Meteor shower output caused by comet 15P/Finlay”. WGN, Journal of the International Meteor Organization, 48, 29-35.
Ye Q.-Z., Brown P. G., Bell C., Gao X., Masek M., Hui M.-T. (2015). “Bangs and meteors from the quiet comet 15P/Finlay.” Astrophys. J., 814, 79–88.
Ye Q.-Z., Vaubaillon J., Sato M., Maslov M. (2021). “Dust ejecta from comet 15P/Finlay”. Astronomers Telegram 2 Oct. 2021.
