A new meteor shower from comet 46P/Wirtanen expected in December 2023

Context. Comet 46P/Wirtanen is a near-Earth object (NEO) for which no associated meteor shower has ever been reported. Aims. This study is aimed at improving our understanding of why there has been no observed shower activity for this NEO to date, as well as to consider whether any past activity could be uncovered from the post-prediction results. Methods. The usual dynamic tools for meteoroid streams were used to describe the behavior of the particles ejected by the comet. The resulting modeled meteoroid stream was thoroughly inspected for collisions between the stream and the Earth. Results. The results show a possible encounter forecast for December 12, 2023, between 8:00 and 12:30 UT. The slow entry velocity is typically known to cause dim meteors. The activity level of the shower is highly uncertain due to the absence of reported past showers. Conclusions. Overall, the most optimal observations on the forecasted day would be achieved from Eastern Australia, New Zealand, and Oceania. These observations will help constrain the size distribution of meteoroids from comet 46P/Wirtanen in the millimeter range.


Introduction
Comet 46P/Wirtanen was discovered in January 1948 by C.
Comet 46P was the initial target of the Rosetta mission (Rickman & Jorda 1998), until the probe launch delay led to the focus to be shifted onto comet 67P/Churyumov-Gerasimenko instead.
Its orbit causes relatively close encounters with Jupiter, with the latest taking place in 1972 at 0.276 au from the giant planet.
Another very close encounter with the Earth in 2018 (0.077 au) allowed for detailed characterizations to be performed.These revealed the comet is a hyper-active comet (Moulane et al. 2023), meaning that its active surface area is higher than expected.
46P is a near-Earth comet and, as such, it may potentially be the parent body of a meteor shower (Ye & Jenniskens 2022).
Moreover, an optical trail was detected by Farnham et al. (2019).Maslov & Muzyko (2017) did look for possible showers from the modelling of the associated meteoroid stream generation and subsequent evolution, but no confirmation was performed by significant observation.The goal of this paper is to re-investigate the possibility of a meteor shower caused by 46P, based on several different models and approaches.We find an encounter between the Earth and the stream in December 2023.Section 2 describes the method used.Section 3 provides the results of the 26 dynamics of the stream and the forecast shower in 2023.

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The methods used are based on similar previous studies of the 29 dynamics of meteoroid streams and its application to the fore-30 cast of meteor showers (Ye et al. 2021;Ye & Vaubaillon 2022).31 In this case, several different models are considered.For each 32 model, simulated meteoroids were ejected from 46P/Wirtanen at 33 each return of the comet and integrated until the present epoch 34 to analyze the current dust distribution near the Earth.Details on 35 the simulation parameters used are presented in Table 1.
One model used was developed by Ye et al. (2016); this 37 model tested a classical pure-ice model (Whipple 1950) with 38 ejection speeds multiplied by 0.1×, 1×, and 5× in order to cover 39 different activity levels of the parent comet.For each scenario, 40 about 1 million particles were generated and simulated.

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Another model is based on Vaubaillon et al. (2005a,b).The 42 comet perihelion passages taken into account here are restricted 43 to the observed ones, i.e. since 1954.The dust production rate 44 was calculated using the measured A f ρ = 150 cm, where A f ρ 45 is a proxy of dust production (A'Hearn et al. 1984).A total of 46 650E + 03 particles were simulated for this model.Lastly, the model by Watanabe et al. (2005) was also used to 62 assess the possibility of a meteor shower from 46P in 2023.

Past apparitions
Like many JFC meteoroid streams, the dynamics shows a rather complicated behavior, with the reversal process (Vaubaillon et al. 2004) at play.Perturbed meteoroid trails were found to intersect the Earth's orbit in the past, although the spatial density of the dust is not easy to constrain.Unlike Maslov & Muzyko (2017), no significant encounter was found in 2012 or 2017 in the QY and JV simulations.In the AE and DM models, thin particle trails formed by particles ejected from 46P between 1925 and 1950 were found to intersect the Earth's orbit in 2012 and 2017.However, the spatial density of the trails makes any past detection of the shower unlikely.Differences between the models can be explained by different consideration of the non-gravitational forces affecting the comet's and meteoroids' motion, as well as the ejection conditions.In contrast, all models were found to produce some encounters with the Earth in 2007 and 2018, mainly caused by the 1974 trail, which may have been recorded by dedicated meteor detection networks.
The orbit of the comet causes the geocentric velocity of the associated meteoroids to be "low," namely: V g ∼ 10.2 km.s −1 (see also table 2).Due to the low geocentric velocity of the meteoroids, we expect only mm-sized meteoroids to be detectable with video networks, while particles smaller than a millimeter may be observed with radio instruments.A similar situation occurred in 2021, with the first apparition of the Arid meteor shower.Despite the low geocentric velocity of the meteoroids (V g ≤ 16 km/s), the Arids was detected by the SAAMER (Southern Argentina Agile Meteor Radar-Orbital System Janches et al. 2023).Although the meteor radiants were located at low declinations in the Southern hemisphere, multiple Arids were even recorded by the Canadian Meteor Orbit Radar (CMOR Brown et al. 2008Brown et al. , 2010) ) because of the gravitational bending of their trajectory close to the Earth.
Encouraged by the Arids detection, we searched the CMOR database for possible apparitions of the shower produced by 46P in 2007, 2012, 2017, and 2018.We performed a wavelet analysis of the radar observations based on the modelled characteristics of the shower, as described in Brown et al. (2010); Egal et al. (2022).Unfortunately, we found no conclusive detection of the shower in past CMOR observations.To our knowledge, no observation of the shower has ever been reported in the literature.

Predictions for December 2023
All the independent models presented in Section 2 predict an encounter between the Earth and the trail ejected in 1974.This trail shows a rather complicated dynamical evolution; in 1984, a close encounter of the stream with Jupiter caused a dramatic change in the orbital elements of the meteoroids.Figure 2 highlights such change by comparing the orbital elements before and after the encounter.The direct consequence is the leading position of the trail with respect to the comet, in particular in 2023 (ibid).
The expected meteoroid spatial density is very low, although it is not easy to constrain, and highly depends on the considered model.In particular, a relatively high ejection velocity (QY3) brings more particles to the Earth than a regular ejection process.The size distribution of the meteoroids released at usual ejection velocities suggest that only the smallest particles (≤ 1 mm) intersect the Earth's orbit in 2023, while meteoroids up to a few centimeters in radius approach the Earth within 0.05 AU (cf.figure 3).  1. Physical characteristics of the 46P nucleus (diameter D and fraction of active area f a ) and meteoroid ejection parameters considered for the simulations.(The QY models do not use this information.)Meteoroids were ejected from the nucleus within the limiting heliocentric distance r h ≤ 3 AU, with speeds following the model of CR97 (Crifo & Rodionov 1997), JB96 (Jones & Brown 1996), or W50 (Whipple 1950).The model by Watanabe et al. (2005) explores the ejection velocity needed to bring the meteoroids to Earth for a given year, so the ejection velocity is tuned for a specific prediction.163 On the other hand, the dynamical history of comet 46P may argue in favor of an enhanced meteoroid production in the past.Moulane et al. (2023) found the nucleus to be hyperactive, meaning that an equivalent of 40% of its surface is active.This is a huge value compared to other comets and may indicate an elevated meteoroid production rate, assuming the dust production rate is proportional to the water production rate, as reported in the study cited here.In addition, the dynamical analysis in Section 3.1 shows that the perihelion distance was larger in the past (at least for the time period considered here).This argues in favor of a quite high activity during the 1974 passage, namely, right after the perihelion distance decreased following the close encounter with Jupiter in 1972 (similarly to comet 240P/NEAT, see Kelley et al. 2019).However, the absence of detection of any trail using an IR telescope (Ye & Jenniskens 2022) is puzzling, since a high production rate would create a dense meteoroid stream that may be detected.
Attempts to reproduce the possible high activity of the comet in the past were performed by modifying the fraction of active area, f a , (from 20% to 40%) in the simulations.Different f a values were found to have a significant influence on the predicted time, but only a moderate impact on the meteor rates (cf.table 2).
Table 2. Predictions of the 2023 shower from different models.∆ 0 : distance between the trail and the Earth at the time of maximum.Symbols: V geo is the geocentric velocity; α and δ are the location of the radiant; and ZHR is the expected level of the shower.New Zealand, and Indonesia (see Fig. 5).

Conclusion
After examining the dynamics of comet 46P/Wirtanen and its associated meteoroid stream, we have shown that the orbit of the comet is stable enough for the time span considered here.Several past encounters with the Earth and the stream were found, but no observations were reported to our knowledge.This is probably due to the quite unusual ejection velocity needed to bring large particles to the Earth.We predict the birth of a new shower on December 12, 2023, between 8:00 and 12:30 UT.The exact time of the maximum depends on the considered size frequency distribution and may vary by a few hours.The location of the radiant is close to the λ-Sculptoris star, so a possible name for the shower is λ-Sculptorids.The observation and subsequent report of this shower would be of tremendous scientific interest since it would put constraints on the size frequency distribution of large particles for the comet.However, observations will be challenging due to the low entry velocity and the relatively small sizes of the meteoroids.Nevertheless, we strongly encourage meteor enthusiasts to perform scientific observations and send their reports to the International Meteor Organization (IMO).

Fig. 1 .
Figure 1 the backward evolution of the comets' semi-major axis.72

Fig. 2 .
Fig. 2. Dynamics of the 1974 trail: Semi-major axis vs eccentricity before (left) and after (right) the close encounter with Jupiter.The star represents the position of comet 46P.

Fig. 3 .
Fig. 3. Geocentric radiant of the simulated meteoroids in 2023 (AE1), color-coded as function of the particle's radius.Sub-mm particles are represented by crosses, while mm-sized particles that should be visible to radio and video instruments are represented by filled circles.Particles approaching Earth below ∆r (AU) are retained as potential impactors.Depending on the ∆r threshold considered, several clusters of radiants may be detectable in December 2023.However, only ≤ mm-sized particles intersect the Earth's orbit in the AE model. 200

Table 2
195Despite the uncertainties on the activity level of the comet 196 since 1900, all the predictions of in Table 2 point toward a notice-197 able meteor activity on December 12, 2023.The shower will be 198 best visible from North and West Australia, Papua New Guinea,