Comet 252P/LINEAR: born (almost) dead? Quan-Zhi Ye (叶泉志) Department of Physics and Astronomy, The University of Western Ontario, London, 6 1 Ontario N6A 3K7, Canada 0 2 n [email protected] a J 8 2 and ] P Peter G. Brown and Paul A. Wiegert E . h Department of Physics and Astronomy, The University of Western Ontario, London, p - o Ontario N6A 3K7, Canada r t s Centre for Planetary Science and Exploration, The University of Western Ontario, London, a [ Ontario N6A 5B8, Canada 1 v 7 Received ; accepted 3 8 7 0 . 1 0 6 1 : v i X r a – 2 – ABSTRACT Previous studies have revealed Jupiter-family comet 252P/LINEAR as a comet that was recently transported into the near-Earth object (NEO) region in ∼ 1800 AD yet only being weakly active. In this Letter, we examine the “formed (almost) dead” hypothesis for 252P/LINEAR using both dynamical and observational approaches. By statistically examining the dynamical evo- lution of 252P/LINEAR over a period of 107 years, we find the median elapsed residency in the NEO region to be 4×102 years which highlights the likelihood of 252P/LINEAR as an (almost) first-time NEO. With available cometary and meteor observations, we find the dust production rate of 252P/LINEAR to be at the order of 106 kg per orbit since its entry to the NEO region. These two lines of evidence support the hypothesis that the comet was likely to have formed in a volatile-poor environment. Cometary and meteor observations during the comet’s unprecedented close approach to the Earth around 2016 Mar. 21 would be useful for the understanding of the surface and evolutionary properties of this unique comet. Subject headings: comets: individual: 252P/LINEAR – meteorites, meteors, meteoroids. – 3 – 1. Introduction Comets are small, icy objects originating from the outer solar system. They are the leftover planetesimals from the formation of the outer planets. These objects remain in the outer solar system for most of their lifetime, until perturbations sent them into the inner solar system where they become visible. It has been noted that the observable comet population displays a large diversity in ice composition which, apart from evolutionary effects, is linked to predictions that comet nuclei were formed at different places and times in the solar nebula (e.g. Whipple 1987; Bockel´ee-Morvan et al. 2004). Thus, the observation of comets provides a unique opportunity for understanding the range of chemistry in the primitive solar nebula. However, the quest is not without its obstacles. Observational interpretations are biased by the fact that more active comets tend to be easier to detect and study, meaning that less active comets are somewhat underrepresented in the sample. Dynamical investigations are limited by the chaotic nature of the orbital evolution of small bodies, which make it challenging to reconstruct the orbit of history over even modest timescales (e.g. ∼ 103 yr). The fact that the evolutionary processes of cometary nuclei are little understood makes it difficult to isolate evolutionary effects from formation diversity when addressing the volatile inventory of individual objects. Numerical integrations carried out by Tancredi (2014) indicate that Jupiter-family comet (JFC) 252P/LINEAR might have entered the near-Earth object (NEO) region1 only ∼ 50 orbits ago. This short dynamical timescale suggests that 252P/LINEAR should be a “physically young” comet, considering the typical physical lifetime of near-Earth JFCs of 150–200 revolutions (Di Sisto et al. 2009). However, with an absolute total magnitude of 1NEO region refers to the region within 1.3 AU from the Sun. – 4 – M = 18.62, 252P/LINEAR exhibits the characteristics of a weakly active comet (Ye et al. 1 2016). Considering its recent entry to the NEO region, we expect evolutionary processes to have played a relatively minor role in altering the subsurface volatile composition. One possible explanation of the weak activity in this young comet could be a volatile-poor environment at the time of formation of the nucleus. The 2016 perihelion passage of 252P/LINEAR offers an exceptional opportunity for Earth-based observers to study the comet. The comet will pass perihelion on 2016 Mar. 15 and make a close approach to the Earth on 2016 Mar. 21 at 0.036 AU, which is one of the closest cometary approaches to the Earth on record3. The Earth may also have passed through the dust trails produced by 252P/LINEAR during its past revolutions4, potentially producing meteor activity. Despite the proximity of 252P/LINEAR’s nodal point to the Earth’s orbit, meteor activity from 252P/LINEAR has not been reported. However, it is well known that meteor observations are useful in understanding of the physical history of a comet (e.g. Yeomans 1981; Jenniskens 2004). Meteoroids from 252P/LINEAR could provide clues to the recent history of the comet. In this Letter, we examine the “born (almost) dead” scenario for the case of 252P/LINEAR in preparation for the observation campaigns in Mar. 2016. We adopt two approaches: an examination of the dynamical evolution of the comet (§ 2), as well as an analysis of the available comet and meteor observations guided by an existing cometary 2From JPL Small-Body Database, http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=252P, accessed 2015 Dec. 21. 3http://www.minorplanetcenter.net/iau/lists/ClosestComets.html, accessed 2015 Dec. 21. 4M.Maslov(unpublished), http://feraj.narod.ru/Radiants/Predictions/252p-ids2016eng.html accessed 2015 Dec. 21. – 5 – dust model (§ 3). Results from both tracks are discussed and summarized in § 4. 2. Dynamical Evolution We generated 1000 “clones” of 252P/LINEAR using its orbital covariance matrix provided in JPL 285, and integrated them 105 yr backwards in time from 2000 AD. Integration was performed with the MERCURY6 package using the symplectic integrator (Chambers 1999). Clones are considered to have been ejected from the solar system when they reach a heliocentric distance of 100 AU. As shown in Figure 1, we confirm the recent (circa. 1800 AD) entry of 252P/LINEAR into the NEO region. The distribution of the clones is extremely compact until a close approach to Jupiter in 1690 AD (miss distance ∼ 0.1 AU). There have been a few recent approaches to the Earth: 0.10 AU in Mar. 2000 (when the comet was discovered), 0.08 AU in Mar. 1921, and 0.05 AU in Feb. 1847. If the activity level of 252P/LINEAR in the 19th century is comparable to what it is today, the comet would have been at +10 mag during its approach in 1847. We searched through the catalog of 19th century comets compiled by Kronk (2008) but do not find any association. Comets at this magnitude may be at the observational limit for 19th century observers (Everhart 1967), but the lack of detection also implies that the comet was probably not substantially much more active at that time compared to today. With evidence that 252P/LINEAR has possibly been very weakly active since its current entry to the NEO region, it is possible that the comet depleted most of its volatiles during a previous residency in the NEO region. The question then becomes: how long did 5From JPL Small-Body Database, http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=252P, accessed 2015 Dec. 21. – 6 – 252P/LINEAR reside in the NEO region in the past? To answer this question, we extend our integration backward and forward in time for 107 years. We find the median elapsed residency in the solar system (i.e. the time that the clone already spent as a bounded object) to be 1.3×105 years and median elapsed NEO region residency to be 4×102 years. The median dynamical lifetime (i.e. the total time that the clone has and will spend as a bounded object) and median integrated time as a NEO are found to be 2.5 × 105 years and 4 × 103 years respectively. While this dynamical and NEO lifetime is comparable to the values for the general JFC population (Fern´andez et al. 2002), we note that the elapsed residency as a NEO is virtually null as it is certain the comet has already spent ∼ 200 years in the NEO region (since its current entry in ∼ 1800 AD). This suggests that 252P/LINEAR is likely a first time (or, almost first time) visitor to the NEO region. We also examine the time that 252P/LINEAR has spent in a “visible” state (i.e. q < 2.5 AU, Quinn et al. 1990). The value of 2.5 AU is considered as the distance from the Sun that water ice sublimation is expected to start (e.g. McNamara et al. 2004). We find the elapsed and total median time of 252P/LINEAR as a visible comet to be 4×103 yr and 1.2×104 yr, the latter of which is more in line to literature values (∼ 8.5×103 yr as given by Levison & Duncan 1994) compared to that in the NEO region. This indicates that 252P/LINEAR may have been active for some time before it entered the NEO region; but the process of thermal devolatilization occurs on longer timescales for comets with larger q (e.g. Jewitt 2004). 3. Current and Past Dust Production The current and past dust production of comets can be constrained by cometary and meteor observations. A parameter commonly used as a proxy for cometary dust production – 7 – is the Afρ parameter (A’Hearn et al. 1984): Afρ = 4r2∆2ρ−1 ·100.4(m⊙−m) (1) H where r is the heliocentric distance of the comet, ∆ is the geocentric distance of the comet, H ρ is the linear radius of the photometric aperture at the distance of the comet, and m and ⊙ m are the apparent magnitudes of the Sun and the comet. Prior to its 2016 perihelion passage, 252P/LINEAR had only been observed during its 2000 and 2010 passages (the 2005 passage was not observed possibly due to poor viewing geometry). However, the 2010 passage was only observed at r = 2.5 AU on its outbound leg and the comet had likely already ceased H activity. Hence, we only use the observations from the 2000 passage which cover a section of r = 1.1 AU to 1.5 AU on the outbound leg of the comet. We obtain H a total of 80 observations from the Minor Planet Center Observation Database (http://www.minorplanetcenter.net/db_search/show_object?object_id=252P) and calculate the Afρ quantity using Equation 1. The ρ parameter accounts for the photometric aperture which differs among observers; to simplify the problem, we adopt an aperture of 13′′ from the conservative (small) end of coma diameter estimations (Shelly et al. 2000). The computed Afρ are therefore at the upper end of possible values. The temporal variation of the Afρ parameter of 252P/LINEAR during its 2000 perihelion passage is shown as Figure 2. The upward trend before t + 55 d (t is the P P perihelion epoch) is likely an artifact due to small ∆ at the time of the observation and the conservative ρ used in the calculation, resulting in an underestimation of the cometary flux (coma diameter estimation given in Shelly et al. 2000, varies from 13′′ to 1′). The median Afρ value is 0.6 cm, one of the lowest numbers ever measured for a comet and comparable with the reported Afρ of another low activity comet, 209P/LINEAR (Schleicher & Ye – 8 – 2014). We further note that the Afρ may be contaminated by the light reflected from the nucleus given the extremely low dust production of 252P/LINEAR, which may result in some overestimation of the measured Afρ; hence, the actual Afρ could be even lower. To search for any meteor activity originated from 252P/LINEAR, we make use of the Monte Carlo meteoroid stream model developed in our earlier works (c.f. Ye et al. 2015, and references therein). Integrations are performed using the 15th order RADAU integrator bundled with the MERCURY6 package (Everhart 1985). Gravitational perturbations from the eight major planets (with the Earth-Moon system represented by a single mass at the barycenter of the two bodies), radiation pressure, and Poynting-Robertson effect are included. We first integrate 252P/LINEAR back to 1800 AD (i.e. back to its entry into the NEO region) and then integrate it forward in time, releasing meteoroids in the diameter range of 0.5 mm < a < 50 mm (i.e. the diameter range that associates with visible meteors) when the comet is at each perihelion, assuming a bulk density of 1 000 kg·m−3 and a size distribution of dN/da ∝ a−2.6. Modeling of meteoroid streams is usually insensitive to the ejection model (e.g. Williams 2001; Williams & Ryabova 2011); however, since 252P/LINEAR is apparently a low activity comet, we use two different ejection models and repeat the simulation for each one of them: the “low activity scenario” described in Ye et al. (2016) and the “normal scenario” described in Brown & Jones (1998). At the completion of the integration, we examine two scenarios of possible meteor activity: (1) meteor outbursts from young meteoroid trails. This is examined by selecting meteoroids that are approaching the Earth within 0.01 AU in the period of interest; and (2) annual “background” activity, i.e. activity from older, more dispersed trails, this is examined by selecting meteoroids with Minimum Orbit Intersection Distance (MOID) with respect to the Earth’s orbit < 0.01 AU. The simulation result for 2001–2020 is listed in Table 2. We note that the result is sensitive to the choice of ejection model in contrast to other meteor showers, probably due – 9 – to the relatively young age of the trails and small encounter speed. By applying the flux estimation technique outlined in Ye et al. (2015), we find that most of these computed activities are below the detection limit of modern meteor surveys, except for the outburst cases of 2011 and 2016. In 2011 the Earth passed close to the 1984- and 1989-trail, which are very young and compact trails formed recently. However, the characteristic Minimum Orbit Intersection Distance (MOID) of both encounters are close to the selection limit (close to ∼ 0.01 AU), therefore the possibility of meteor activity is likely minimal. In 2016, the Earth will pass close to the 1915-, 1921- and 1926-trails. Assuming the dust production rate of 252P/LINEAR in the 1920s is comparable to its current value, the meteor flux is likely to be at the order of 10−4 to 10−3 km−2 ·hr−1, or less than a few meteors in terms of Zenith Hourly Rate (ZHR). Next, we conducted a search in the survey data collected by the Canadian Meteor Orbit Radar (CMOR) (c.f. Jones et al. 2005, and references therein). We apply the wavelet technique described in Brown et al. (2008) at the computed radiants to search for meteor activity, without finding any enhancement (Figure 3). Using the number of background meteors detected by CMOR within a radius of 10◦ of the expected radiant and 10% from the expected meteoroid speed, as well as the CMOR collection area as a function of the declination (Brown & Jones 1995), we estimate the upper limit of the meteoroid flux to be at the order of 10−4 km−2 ·hr−1 to a limiting mass of ∼ 10−7 kg, with an uncertainty within a factor of several due to the unconstrained mass distribution of the stream (which affects the collection area). From the simulation, we find the meteoroid delivery efficiency (i.e. the fraction of the ejected meteoroids with MOID< 0.01 AU) to be η = 17%. Assuming that the meteoroids are uniformly distributed along the orbit of 252P/LINEAR, we can estimate the past dust production since the comet’s entry into the NEO region using – 10 – FP ·∆X2 N = ηN orb where N is the meteoroid production per orbit, F = 10−4 km−2 ·hr−1 is the upper limit of meteoroid flux constrained by meteor data, P = 5 years is the orbital period of the meteoroids, ∆X = 0.01 AU is the collection area, and N = 50 is the number of orbits orb that the comet is active. By inserting numbers from previous analysis, we get N = 1012 meteoroids, or ∼ 106 kg per orbit appropriate to millimeter-sized dust (assuming a bulk density of 1 000 kg·m−3). For comparison, the dust production rate of 55P/Tempel-Tuttle (parent of the Leonid meteor shower) is of the order of 1011 kg per orbit (Vaubaillon et al. 2005). While the actual number may be off by an order of magnitude, such extremely low number illustrate 252P/LINEAR as a comet with very low dust production, as well as its lack of significant activity since its entry to the NEO region. 4. Summary The two lines of evidence – dynamical and observational – have outlined 252P/LINEAR as a comet that is likely an (almost) first-time visitor to the NEO region, yet only little active in terms of dust production. These evidences support the hypothesis that 252P/LINEAR was likely to have formed in a volatile-poor environment, as compared to other members in the visible JFC population. Cometary and meteor observations during its close approach in Mar. 2016 will likely provide more information regarding the surface and evolutionary properties of this unique comet. We thank an anonymous referee for his/her comments, as well as Michal Drahus and Davide Farnocchia for discussions. Thanks to Zbigniew Krzeminski, Jason Gill, Robert Weryk and Daniel Wong for helping with CMOR operations. Funding support from the NASA Meteoroid Environment Office (cooperative agreement NNX11AB76A) for CMOR