Recurring Activity Discovered on Quasi-Hilda 2009 DQ118

We have discovered two epochs of activity on quasi-Hilda 2009 DQ118. Small bodies that display comet-like activity, such as active asteroids and active quasi-Hildas, are important for understanding the distribution of water and other volatiles throughout the solar system. Through our NASA Partner Citizen Science project, Active Asteroids, volunteers classified archival images of 2009 DQ118 as displaying comet-like activity. By performing an in-depth archival image search, we found over 20 images from UT 2016 March 8–9 with clear signs of a comet-like tail. We then carried out follow-up observations of 2009 DQ118 using the 3.5 m Astrophysical Research Consortium Telescope at Apache Point Observatory, Sunspot, New Mexico, USA and the 6.5 m Magellan Baade Telescope at Las Campanas Observatory, Chile. These images revealed a second epoch of activity associated with the UT 2023 April 22 perihelion passage of 2009 DQ118. We performed photometric analysis of the tail and find that it had a similar apparent length and surface brightness during both epochs. We also explored the orbital history and future of 2009 DQ118 through dynamical simulations. These simulations show that 2009 DQ118 is currently a quasi-Hilda and that it frequently experiences close encounters with Jupiter. We find that 2009 DQ118 is currently on the boundary between asteroidal and cometary orbits. Additionally, it has likely been a Jupiter family comet or Centaur for much of the past 10 kyr and will be in these same regions for the majority of the next 10 kyr. Since both detected epochs of activity occurred near perihelion, the observed activity is consistent with sublimation of volatile ices. 2009 DQ118 is currently observable until ∼mid-October 2023. Further observations would help to characterize the observed activity.


INTRODUCTION
The active asteroids are a population of small solar system bodies on asteroidal orbits, but which show signs of comet-like activity, such as tails (Jewitt et al. 2015).Fewer than 50 of these intriguing objects are known (Jewitt & Hsieh 2022), and their relative sparseness in the overall asteroid population (>10 6 ) remains unexplained.
The quasi-Hildas (also known as quasi-Hilda asteroids, quasi-Hilda objects, or quasi-Hilda comets, whether or not they display cometary activity) are related to the active asteroids.They orbit between the outer edge of the main asteroid belt and the Jupiter Family Comets (JFCs).Quasi-Hildas are also characterized as being near, but not within, the 3:2 interior mean-motion orbital resonance with Jupiter (Toth 2006); the Hilda asteroid group is defined as being within this resonance.The quasi-Hildas have short dynamical lifetimes and some of them likely migrated to their current orbits from the outer solar system through interactions with the giant planets (Gil-Hutton & García-Migani 2016).Additionally, relatively few quasi-Hildas (<15) have been found to exhibit comet-like activity (Chandler et al. 2022) out of the ∼300 identified so far (Gil-Hutton & García-Migani 2016), with activity on many of these objects being discovered in recent years, for example, 2008 GO 98 (García-Migani & Gil-Hutton 2018), P/2010 H2 (Jewitt & Kim 2020), 282P (Chandler et al. 2022), and 2018 CZ 16 (Trujillo et al. 2023).Shoemaker-Levy 9, a comet known for its well-observed impact with Jupiter, was also likely a quasi-Hilda (Ohtsuka et al. 2008).
Comet-like activity on traditionally non-cometary bodies, such as asteroids, has revealed the presence of a previously unrecognized reservoir of volatile ices in our solar system (Hsieh et al. 2015).The distribution of this material throughout the solar system is poorly understood and further study may shed light on pathways for delivery of these volatiles to Earth (Morbidelli et al. 2000;O'Brien et al. 2018).Additionally, volatile ices on small solar system bodies may provide crucial resource reservoirs for future space exploration (see Chandler 2022, and references therein).
To study activity on asteroids and other bodies throughout the solar system, we created Active Asteroids1 , a NASA Partner Citizen Science project hosted on the Zooniverse2 citizen science platform (Chandler 2022).Activity was discovered on 2009 DQ 118 as a result of this project (Oldroyd et al. 2023).
In this work we will summarize the initial detection of activity on 2009 DQ 118 through the Active Asteroids project followed by a description of our archival search for additional images containing activity.Next, we discuss our follow-up observations and photometric analysis of 2009 DQ 118 , as well as the discovery of a second epoch of cometary activity.We also present a dynamical analysis of the orbital evolution of 2009 DQ 118 and compare it with other known active quasi-Hildas.Finally, we discuss mechanisms that could cause activity on 2009 DQ 118 , as well as future observing opportunities regarding this object.

CITIZEN SCIENCE DISCOVERY
In order to better study the active asteroids, we seek to discover more of them through our Active Asteroids Citizen Science project.For this project, we retrieve publicly-available images of known asteroids and other small solar system bodies from the Dark Energy Camera (DECam) archive.The wide field of view of DECam (2.2 • × 2.2 • ) on the 4 m Blanco telescope (DePoy et al. 2008) is excellently situated for detecting activity.After employing our automated vetting process described in Chandler et al. (2018) and Chandler (2022), images passing our data quality metrics are examined by Active Asteroids volunteers and classified by them as either active or inactive.
Images identified as containing activity by citizen scientists are then reviewed by our science team to further validate candidate detections.Next, we perform an indepth archival search on promising candidates from this activity identification process (Chandler 2022).These searches yield additional images displaying activity for some candidate objects, allowing us to further study potential mechanisms for activity.
As a result of the Active Asteroids project, we discovered comet-like activity originating from 2009 DQ 118 (as reported in our preliminary announcement Oldroyd et al. 2023).Once volunteers had identified 2009 DQ 118 as active, our archival search produced over 20 images of 2009 DQ 118 displaying a tail.All of these images showing activity were from UT 2016 March 8-11, just 4 months before its 2016 perihelion passage (heliocentric distance r h = 2.55 au, true anomaly f = 322 • ).We also identified ∼10 images without readily apparent signs of activity.All of these inactive images were taken more than a year away from 2009 DQ 118 perihelion passages.
Representative active images from this search are shown in Figure 1.

FOLLOW-UP OBSERVATIONS
We acquired follow-up observations of 2009 DQ 118 using the Astrophysical Research Consortium Telescope Imaging Camera (ARCTIC) on the Apache Point Observatory (APO) 3.5 m Astrophysical Research Consortium (ARC) telescope in Sunspot, New Mexico, USA (Huehnerhoff et al. 2016).On UT 2023 February 24 we took 12 300 s VR-band images of 2009 DQ 118 (Prop.ID 2Q2023-UW08; PI Chandler; observer C. Chandler).The conditions were poor, with intermittent clouds and a seeing of ∼2.7 ′′ .On this date, 2009 DQ 118 was approaching perihelion (r h = 2.456 au, f = 343.0• ) and we saw faint indications of a tail (Figure 1(c)).
In order to confirm this second epoch of activity, we acquired follow-up observations of 2009 DQ 118 using the Inamori-Magellan Areal Camera & Spectrograph (IMACS) on the 6.5 m Magellan Baade telescope at Las Campanas Observatory, Chile (Dressler et al. 2011).Our observations, taken on UT 2023 April 22 (PI: S. Sheppard), were comprised of four 150 s images in the broad WB4800-7800 filter (similar to a VR filter) with seeing between 0.8 ′′ and 0.9 ′′ and a pixel scale of 0.2 ′′ /pixel.These observations were timed so that 2009 DQ 118 was at perihelion (r h = 2.430 au, f = 359.9• ) since this is an ideal time to check for signs of cometary activity.
Our observations show a faint tail originating from 2009 DQ 118 and oriented in the direction of the antisolar and negative heliocentric velocity vectors projected to the on-sky plane (Figure 1).We performed photometry on images from both epochs in order to compare the tail between the apparitions.Epoch 1 DECam data were calibrated using Pan-STARRs DR1 r-band data, whereas epoch 2 Magellan data were calibrated to Gaia EDR3 G-band measurements (r-band catalogs were unavailable for this location) and transformed to the equivalent Sloan r-band (a calibration proxy used for comparing IMACS WB4800-7800 data with other data sets, see Pravec et al. 2022) using the G BP − G RP colors as described in the Gaia Early Data Release 3 Documentation3 .We then compared the resulting magnitudes with the expected extinction corrected magnitudes reported by JPL Horizons (Giorgini et al. 1996), thus accounting for phase correction, transforming from Vband to r-band assuming solar colors as in Jewitt et al. (2019), using the transformations given by Jordi et al. (2006).In epoch 1, 2009 DQ 118 had an r-band magnitude of 20.7, and it had an equivalent r-band magnitude of ∼20.3 during epoch 2; 0.4 magnitudes brighter than expected in both epochs.The tail was roughly 18 ′′ long during epoch 1 and it had a surface brightness of 24.4 mag/arcsec 2 .During epoch 2, 2009 DQ 118 was in a crowded field which complicated measurement of the tail.We place a lower limit of 9 ′′ on the length of the tail in epoch 2 with a maximum length of approximately 21 ′′ , and a surface brightness of 24.3 mag/arcsec 2 .Hence, the tail had a similar apparent length and surface brightness during both epochs.
The detection of two separate epochs of activity on 2009 DQ 118 likely points to sublimation of volatile ices as the primary activation mechanism.Because of the proximity of 2009 DQ 118 to its perihelion passage on UT 2023 April 22, observations during this observing window will be particularly useful for further characterization of the tail if they reach a depth of V ≳ 23 (sufficiently deep to detect the tail).2009 DQ 118 will be observable through ∼mid October 2023 (especially from the southern hemisphere), and then again in mid 2024, albeit, not near perihelion (f ≈ 100 • ).

DYNAMICAL ANALYSIS
In order to further study potential causes for activity on 2009 DQ 118 , we performed a set of dynamical simulations to examine its short term (1-10 kyr) orbital history and future.For these simulations we utilized the IAS15 integrator (Rein & Spiegel 2015) from the REBOUND N -body integration package (Rein & Liu 2012) in Python.To account for observational uncertainties in its orbit, we created 500 dynamical clones of 2009 DQ 118 .These clones were drawn from Gaussian distributions using the orbital elements and uncertainties (see Table 1) from the JPL Horizons Small Body Database (Giorgini et al. 1996).We integrated each orbital clone for ±10 kyr along with the Sun and planets (except Mercury, which has a negligible impact and requires much more computation time to properly simulate, see, for example, Brown & Rein 2023;Hernandez et al. 2022) with a timestep of 0.02 yr, sufficient for resolving the orbit of Venus (see Hernandez et al. 2022;Wisdom 2015, for discussion on adequate orbital resolution).
As a result of our simulations, we find that 2009 DQ 118 experiences frequent close encounters with Jupiter over a ±1,000 yr timescale.Many of these encounters are within 2-3 Hill radii of Jupiter (Figure 2), where the Hill radius (Hill 1878) is computed as where a, e, and m are the semi-major axis, eccentricity, and mass of the secondary body respectively (Jupiter in this case), and M is the mass of the primary body (the Sun).For Jupiter, the Hill radius is r H,J ≈ 0.34 au.An object passing within a few Hill radii of a planet will be subject to strong gravitational perturbations that will likely alter the orbit of the small body.This is the case for 2009 DQ 118 , which has had recent changes in its orbit due to these encounters and will continue to have orbit-changing encounters in the near future as shown in Figure 2. The proximity of the orbits of 2009 DQ 118 and Jupiter is also connected to the Tisserand parameter with respect to Jupiter T J of 2009 DQ 118 .The Tisserand parameter with respect to Jupiter is a mostly-constant metric for the strength of the gravitational effect of Jupiter on the orbit of another body.It is defined as where a, e, and i are the semi-major axis, eccentricity, and inclination, respectively, of the small body and a J is the semi-major axis of Jupiter.Small bodies are often categorized based on T J , with objects that have T J > 3 being classified as asteroids (which do not cross the orbit of Jupiter) while those with T J < 3 are con-sidered comets (Jupiter orbit crossing; Levison 1996).
2009 DQ 118 has a T J of 3.004, right on the traditional T J = 3 boundary between asteroidal and cometary orbits.Additionally, although T J is typically thought of as constant for a given object (Kresák 1972), close encounters with Jupiter cause minor changes to the T J of 2009 DQ 118 .These small changes cause 2009 DQ 118 to cross T J = 3 dozens of times over the course of ±1,000 yr.However, these T J crossings do not represent a dramatic orbital shift from one dynamical class to another, but rather serve to muddle the classification of 2009 DQ 118 as seen by the abrupt jumps near t = 0 in Figure 2 (d).
The results shown in Table 2 highlight that although 2009 DQ 118 is currently on an asteroidal orbit (T J > 3), it is likely that it was either a JFC or even a Centaur (a Jupiter < q < a Neptune ; as in, for example, Tiscareno & Malhotra 2003) for much of the past 10 kyr.Additionally, 2009 DQ 118 will most likely be on a JFC or Centaur orbit for much of the coming 10 kyr (Figure 2

(d))
. There is a non-negligible probability, however, that 2009 DQ 118 has been an asteroid for over 10 kyr and that, after becoming a JFC in the next 1,000 yr, it may transition back onto an asteroidal orbit or even become a near-Earth object.These values correspond to those given in Table 2.

Dynamical Classification
In addition to its residence on the tenuous T J = 3 asteroid-comet boundary, 2009 DQ 118 exhibits many dynamical similarities to other active quasi-Hildas, such as 282P (Chandler et al. 2022).At a ≈ 3.6 au, 2009 DQ 118 sits slightly outside of the quasi-Hilda semi-major axis range of ∼3.7 au < a < ∼4.2 au given by Toth (2006), placing it closer to the Cybele asteroid group than to the Hildas.However, 2009 DQ 118 experiences shortterm dynamical evolution we find to be characteristic of the quasi-Hilda population.One simple way of visualizing the similarities between quasi-Hildas, in contrast with objects of other nearby dynamical classes, is by examining the orbits of these objects in the corotating frame with Jupiter.In Figure 3, we show the orbits of objects in a frame that rotates at the same rate as the orbital motion of Jupiter so that Jupiter remains on the xaxis.Objects in separate dynamical classes appear quite different from one another in this frame, while objects in the same dynamical class have obvious similarities.Since 2009 DQ 118 clearly resembles other quasi-Hildas when examined in the frame corotating with Jupiter, we classify 2009 DQ 118 as a quasi-Hilda.

Activity Mechanisms
Among the various mechanisms for causing cometary activity on a small solar system body, the most wellstudied is sublimation of volatile ices.Sublimation is the primary driver of activity on comets throughout the solar system.It is also used as a primary method for  Note: NEO stands for Near-Earth Object and QH for Quasi-Hilda.Percentages are calculated based on the number of orbital clones within the corresponding orbital class at the given times.Because quasi-Hilda is a non-exclusive pseudoclass, objects can be classified as either a quasi-Hilda and an asteroid, a quasi-Hilda and a JFC, or not a quasi-Hilda, but potentially still a JFC or asteroid.
distinguishing between main-belt comets, which by definition have activity that is primarily sublimation driven, and other active asteroids which do not (e.g., Hsieh et al. 2015;Agarwal et al. 2017;Jewitt & Hsieh 2022).
Thermal fracture is primarily applicable for near-Earth asteroids that experience large temperature gradients (several hundred degrees) over their orbits (see Chandler et al. 2022;Jewitt & Hsieh 2022, and references therein).Additionally, observations of objects that are likely candidates for thermal fracture driven activity have found either a lack of evidence for activity for 2005 UD (Kueny et al. 2023), or, for (3200) Phaethon, that the observed activity is likely associated with gas emission rather than thermal fracture (Hui 2023); hence, this is an unlikely mechanism to explain the activity seen on 2009 DQ 118 .
While both impact and rotational instability are difficult to rule out as drivers for activity (especially because the rotational period is unknown), neither of these mechanisms are directly correlated with perihelion.Hence, due to our discovery of activity on 2009 DQ 118 at or near two separate perihelion passages, we conclude that sublimation of volatiles is the most likely cause for the observed activity on 2009 DQ 118 (see, for example, Hsieh et al. 2012).

SUMMARY AND FUTURE WORK
Through our NASA Partner Citizen Science project Active Asteroids (described in Chandler 2022), we have discovered cometary activity emanating from quasi-Hilda 2009 DQ 118 (Oldroyd et al. 2023).This activity occurred near the perihelion passage of 2009 DQ 118 in 2016.Following this discovery, we conducted followup observations of 2009 DQ 118 using the 3.5 m ARC telescope at Apache Point Observatory, Sunspot, New Mexico, USA and the 6.5 m Magellan Baade telescope at Las Campanas Observatory, Chile.From these observations, we discovered a second epoch of activity on 2009 DQ 118 .This new epoch of activity occurred during the 2023 perihelion passage of 2009 DQ 118 , approximately one orbital period after the first epoch detected.We performed a photometric analysis of the tail and find that it had similar apparent lengths and surfaces brightnesses in both epochs.Representative images from both epochs of activity are shown in Figure 1.
We conducted dynamical simulations of 2009 DQ 118 using N -body integration of orbital clones to determine probable orbital outcomes.Our simulations show that 2009 DQ 118 experiences frequent close encounters with Jupiter over ±1,000 yr (Figure 2 (b)).These encounters perturb the orbit of 2009 DQ 118 causing slight changes in its Tisserand parameter with respect to Jupiter allowing it to cross the traditional asteroid-comet boundary of T J = 3 dozens of times on this timescale.This causes a largely-superficial change in the orbital class of 2009 DQ 118 over a 1,000 yr time period, with the potential for more substantial orbital migration over 10 kyr.During this time, JFC orbits are the most common over ±10 kyr, with Centaur orbits being nearly as probably (Table 2).
Currently, 2009 DQ 118 sits slightly outside of the quasi-Hilda semi-major axis range given in Toth (2006).However, because it is dynamically similar to other known quasi-Hildas, we classify 2009 DQ 118 as a quasi-Hilda (Figure 3).
We find the most probable cause for the activity on 2009 DQ 118 is sublimation of volatile ices.While other mechanisms, such as rotational instability, could potentially cause the observed activity they are not correlated with perihelion.Therefore, since both epochs of detected activity are closely associated with perihelion passages, sublimation is the most likely cause.
Further observations of 2009 DQ 118 will be particularly useful for characterizing the tail; for example, obtaining colors, monitoring its photometric evolution, and measuring surface brightness profiles.The remainder of this observing window, until ∼mid October 2023, is an ideal time for studying activity on 2009 DQ 118 , since it is near perihelion.Additionally, future observations in coming years when 2009 DQ 118 is not expected to be active will also be useful for comparative studies of the tail and nucleus, as well as for obtaining colors of the nucleus and measuring its rotational period.2009 DQ 118 will next reach perihelion in January 2030.We predict that 2009 DQ 118 , after a period of inactivity following the recent perihelion passage, will reactivate as it approaches this date.and/or services provided by the International Astronomical Union's Minor Planet Center.This research has made use of data and services provided by JPL Horizons (Giorgini et al. 1996).This work made use of As-tOrb, the Lowell Observatory Asteroid Orbit Database astorbDB (Bowell et al. 1994;Moskovitz et al. 2021).World Coordinate System corrections were facilitated by Astrometry.net (Lang et al. 2010).This research has made use of The Institut de Mécanique Céleste et de Calcul des Éphémérides SkyBoT Virtual Observatory tool (Berthier et al. 2006).This research is based on data obtained from the Astro Data Archive at NSF's NOIRLab.These data are associated with observing programs 2016A-0189 (PI A. Rest) and 2015A-0121 (PI A. von der Linden).NOIRLab is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.Based on observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium.This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

Figure 1 .
Figure 1.Images of 2009 DQ118 (green dashed arrows) displaying a cometary tail (white arrows).Frames (a) and (b) are from the first activity epoch and resulted from our Active Asteroids citizen scientist project and archival search.Frame (c) is an Apache Point Observatory (APO) follow-up image showing faint signs of activity resulting in the tentative discovery of the second epoch of activity.In frames (a) through (c), the negative heliocentric velocity (black arrow outlined in red) and anti-solar (yellow arrow) directions projected to the on-sky plane coincide with each other and the direction of the tail.Frame (d) is a stack of our Magellan follow-up observations confirming the discovery of the second activity epoch.In this frame, the tail is oriented between the anti-solar (yellow arrow) and negative heliocentric velocity (black arrow outlined in red) directions projected to the on-sky plane.North is up and East is left in each image (solid green arrows) and all directions are referenced to the ephemeris location of 2009 DQ118 (which is centered in each image) at the time of observation as given by JPL Horizons (Giorgini et al. 1996).(a): 300 s VR-band Dark Energy Camera (DECam) image taken with the 4 m Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO), Chile on UT 2016 March 8 (Prop.ID 2016A-0189; PI Rest; observers A. Rest, DJJ).(b): 200 s r-band DECam image, UT 2016 March 9 (Prop.ID 2015A-0121; PI von der Linden; observer A. von der Linden).(c): 300 s VR-band image taken with the Astrophysical Research Consortium Telescope Imaging Camera (ARCTIC) on the APO 3.5 m Astrophysical Research Consortium (ARC) Telescope, UT 2023 February 24 (Prop.ID 2Q2023-UW08; PI Chandler; observer C. Chandler).(d): A co-added stack of four 150s WB4800-7800-band images taken with the Inamori-Magellan Areal Camera & Spectrograph (IMACS) on the 6.5 m Magellan Baade Telescope at Las Campanas Observatory, Chile on UT 2023 April 22 (PI S. Sheppard; observer S. Sheppard).

Figure 2 .
Figure 2. Dynamical evolution of 2009 DQ118 orbital clones indicating changes to its orbit and dynamical class over short timescales.(a): Orbits of 2009 DQ118 and the planets at t = 0, UT 2023 April 11.Note the proximity of the orbits of 2009 DQ118 and Jupiter.(b): Log distance between 2009 DQ118 orbital clones and Jupiter as a function of time.Distances of 5, 3, and 1 Hill radii are marked to emphasize increasingly strong perturbations from close encounters.Semi-major axes of two Jovian moons are given for reference.Note that strong downward spikes, representing close encounters, correspond with changes in the orbit.Also note the onset of dynamical chaos before ∼ −750 yr and after ∼ 600 yr.(c): Heliocentric distance of 2009 DQ118 orbital clones.Note how 2009 DQ118 clones begin to cross within the perihelion distance of Jupiter (shaded orange region below the orange line) after ∼ 600 yr.(d): Dynamical class of 2009 DQ118 over ±10 kyr (vs ±1 kyr for panels (b) and (c)).These values correspond to those given in Table2.

Figure 3 .
Figure 3. Orbits of representative bodies (blue curves) from eight dynamical classes in the corotating frame with Jupiter (orange line) illustrating the similarities between 2009 DQ118 and other quasi-Hildas.Each subplot shows 200 yr of orbital integration in this reference frame.(a): Active asteroid 2015 VA108 orbits in the main asteroid belt and is a candidate mainbelt comet (Chandler et al. 2023).(b): Near-Earth binary asteroid (65803) Didymos-Dimorphos was the target of the NASA Double-Asteroid Redirection Test (DART) mission.It is the first artificial active asteroid (Li et al. 2023).(c): Active Centaur (2060) Chiron (95P) resides between the orbits of Jupiter and Uranus.(d): Jupiter family comet 67P/Churyumov-Gerasimenko crosses the orbits of Jupiter and Mars.It was visited by the ESA Rosetta spacecraft.(e): Long period comet C/2014 UN271 (Bernardinelli-Bernstein) is currently inbound from the Oort cloud and will reach its perihelion, near the orbit of Saturn, in January 2031.Because this comet is highly inclined (i ≈ 95 • ), it appears to be interior to the orbit of Jupiter in part of this X-Y projection.(f ): Trojan asteroid (3548) Eurybates in a characteristic Trojan tadpole orbit indicative of a 1:1 mean-motion resonance with Jupiter.Eurybates is a target of the NASA Lucy spacecraft mission.(g): Asteroid (153) Hilda in its iconic 3:2 interior mean-motion resonance with Jupiter.Hilda asteroids are defined as being in this resonance and also display this trilobate pattern in this frame.(h): Active quasi-Hilda 282P/(323137) 2003 BM80 displays a typical asymmetric quasi-Hilda corotating pattern (Chandler et al. 2022).(i): 2009 DQ118 with a quasi-Hilda orbit similar to 282P.