Recurring Activity Discovered on Quasi-Hilda 2009 DQ118

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 (>10⁶) 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 GO₉₈ (García-Migani & Gil-Hutton 2018), P/2010 H2 (Jewitt & Kim 2020), 282P (Chandler et al. 2022), and 2018 CZ₁₆ (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 Asteroids, ¹⁴ a NASA Partner Citizen Science project hosted on the Zooniverse ¹⁵ citizen science platform (Chandler 2022). Activity was discovered on 2009 DQ₁₁₈ as a result of this project (Oldroyd et al. 2023).

In this work we will summarize the initial detection of activity on 2009 DQ₁₁₈ 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 DQ₁₁₈, as well as the discovery of a second epoch of cometary activity. We also present a dynamical analysis of the orbital evolution of 2009 DQ₁₁₈ and compare it with other known active quasi-Hildas. Finally, we discuss mechanisms that could cause activity on 2009 DQ₁₁₈, as well as future observing opportunities regarding this object.

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 (22 × 22) 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 in-depth 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 DQ₁₁₈ (as reported in our preliminary announcement Oldroyd et al. 2023). Once volunteers had identified 2009 DQ₁₁₈ as active, our archival search produced over 20 images of 2009 DQ₁₁₈ 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 DQ₁₁₈ perihelion passages. Representative active images from this search are shown in Figure 1.

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We acquired follow-up observations of 2009 DQ₁₁₈ 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 DQ₁₁₈ (Prop. ID 2Q2023-UW08; PI: Chandler; observer: C. Chandler). The conditions were poor, with intermittent clouds and a seeing of ∼27. On this date, 2009 DQ₁₁₈ was approaching perihelion (r_h = 2.456 au, f = 3430), 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 DQ₁₁₈ using the Inamori-Magellan Areal Camera and 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 08 and 09 and a pixel scale of 02 pixel⁻¹. These observations were timed so that 2009 DQ₁₁₈ was at perihelion (r_h = 2.430 au, f = 3599) since this is an ideal time to check for signs of cometary activity.

Our observations show a faint tail originating from 2009 DQ₁₁₈ 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 Documentation. ¹⁶ We then compared the resulting magnitudes with the expected extinction-corrected magnitudes reported by Jet Propulsion Laboratory (JPL) Horizons (Giorgini et al. 1996), thus accounting for phase correction and transforming from V-band 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 DQ₁₁₈ had an r-band magnitude of 20.7, and it had an equivalent r-band magnitude of ∼20.3 during epoch 2, 0.4 mag 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 DQ₁₁₈ 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 DQ₁₁₈ likely points to sublimation of volatile ices as the primary activation mechanism. Because of the proximity of 2009 DQ₁₁₈ 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 DQ₁₁₈ will be observable through ∼mid-October 2023 (especially from the southern hemisphere), and then again in mid-2024, albeit, not near perihelion (f ≈ 100°).

In order to further study potential causes for activity on 2009 DQ₁₁₈, 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 DQ₁₁₈. 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, Hernandez et al. 2022; Brown & Rein 2023) with a time step of 0.02 yr, sufficient for resolving the orbit of Venus (see Wisdom 2015; Hernandez et al. 2022 for discussion on adequate orbital resolution).

As a result of our simulations, we find that 2009 DQ₁₁₈ experiences frequent close encounters with Jupiter over a ±1000 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

$$\begin{eqnarray}&&{r}_{{\rm{H}}}\approx a(1-e){\left(m/3M\right)}^{1/3},\end{eqnarray} \tag{ 1 }$$

where a, e, and m are the semimajor 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 DQ₁₁₈, 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.

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The proximity of the orbits of 2009 DQ₁₁₈ and Jupiter is also connected to the Tisserand parameter with respect to Jupiter T_J of 2009 DQ₁₁₈. 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

$$\begin{eqnarray}&&{T}_{{\rm{J}}}=\displaystyle \frac{{a}_{{\rm{J}}}}{a}+2\cos (i)\sqrt{\displaystyle \frac{a}{{a}_{{\rm{J}}}}\left(1-{e}^{2}\right)},\end{eqnarray} \tag{ 2 }$$

where a, e, and i are the semimajor axis, eccentricity, and inclination, respectively, of the small body, and a_J is the semimajor 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 considered comets (Jupiter orbit–crossing; Levison 1996). 2009 DQ₁₁₈ 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 DQ₁₁₈. These small changes cause 2009 DQ₁₁₈ to cross T_J = 3 dozens of times over the course of ±1000 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 DQ₁₁₈ as seen by the abrupt jumps near t = 0 in Figure 2(d).

The results shown in Table 2 highlight that although 2009 DQ₁₁₈ 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 DQ₁₁₈ 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 DQ₁₁₈ has been an asteroid for over 10 kyr and that, after becoming a JFC in the next 1000 yr, it may transition back onto an asteroidal orbit or even become a near-Earth object.

5.1. Dynamical Classification

In addition to its residence on the tenuous T_J = 3 asteroid-comet boundary, 2009 DQ₁₁₈ exhibits many dynamical similarities to other active quasi-Hildas, such as 282P (Chandler et al. 2022). At a ≈ 3.6 au, 2009 DQ₁₁₈ sits slightly outside of the quasi-Hilda semimajor 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 DQ₁₁₈ experiences short-term 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 x-axis. 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 DQ₁₁₈ clearly resembles other quasi-Hildas when examined in the frame corotating with Jupiter, we classify 2009 DQ₁₁₈ as a quasi-Hilda.

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5.2. Activity Mechanisms

Through our NASA Partner Citizen Science project Active Asteroids (described in Chandler 2022), we have discovered cometary activity emanating from quasi-Hilda 2009 DQ₁₁₈ (Oldroyd et al. 2023). This activity occurred near the perihelion passage of 2009 DQ₁₁₈ in 2016. Following this discovery, we conducted follow-up observations of 2009 DQ₁₁₈ 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 DQ₁₁₈. This new epoch of activity occurred during the 2023 perihelion passage of 2009 DQ₁₁₈, 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 DQ₁₁₈ using N-body integration of orbital clones to determine probable orbital outcomes. Our simulations show that 2009 DQ₁₁₈ experiences frequent close encounters with Jupiter over ±1000 yr (Figure 2(b)). These encounters perturb the orbit of 2009 DQ₁₁₈, 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 DQ₁₁₈ over a 1000 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 probable (Table 2).

Currently, 2009 DQ₁₁₈ sits slightly outside of the quasi-Hilda semimajor axis range given in Toth (2006). However, because it is dynamically similar to other known quasi-Hildas, we classify 2009 DQ₁₁₈ as a quasi-Hilda (Figure 3).

We find the most probable cause for the activity on 2009 DQ₁₁₈ 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 DQ₁₁₈ 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 DQ₁₁₈ since it is near perihelion. Additionally, future observations in coming years when 2009 DQ₁₁₈ 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 DQ₁₁₈ will next reach perihelion in 2030 January. We predict that 2009 DQ₁₁₈, after a period of inactivity following the recent perihelion passage, will reactivate as it approaches this date.

We thank our anonymous reviewer who provided feedback that enhanced this work. We thank Hal Levison (SwRI) for helpful comments regarding dynamical classifications. We express our gratitude to Mark Magbanua (UCSF) for frequent feedback to the Active Asteroids project. We also thank Elizabeth Baeten (Leuven, Belgium) for moderating the Active Asteroids forums and Cliff Johnson (Zooniverse) and Marc Kuchner (NASA) for their support, guidance, and feedback throughout this project. We thank Chris Coffey (NAU) and the NAU High Performance Computing Support team, who have made this work possible. We thank Jessica Birky (UW) and David Wang (UW) for contributing telescope time at APO.

We are grateful to the thousands of Active Asteroids volunteers who are critical in this process of discovery. We specifically thank those who helped to classify 2009 DQ₁₁₈: Al Lamperti (Royersford, USA), Angelina A. Reese (Sequim, USA), Dr. Brian Leonard Goodwin (London, UK), Clara Garza (West Covina, USA), C. M. Kaiser (Parker, USA), Dawn Boles (Bakersfield, USA), Eric Fabrigat (Velaux, France), Ernest Jude P. Tiu (Pototan, Philippines), Frederick Hopper (Cotgrave, UK), Henryk Krawczyk (Czeladż Poland), Ivan A. Terentev (Petrozavodsk, Russia), Ivan Vladimirovich Sergienko (Sergiyev Posad, Russia), J. Williams (Swainsboro, USA), Jayanta Ghosh (Purulia, India), Jose A. da Silva Campos (Portugal), Konstantinos Dimitrios Danalis (Athens, Greece), Martin Welham (Yatton, UK), Marvin W. Huddleston (Mesquite, USA), Michele T. Mazzucato (Florence, Italy), Milton K. D. Bosch MD (Napa, USA), Robert Pickard (Grove Hill, USA), Robert Zach Moseley (Worcester, USA), Sarah Grissett (Tallahassee, USA), Shelley-Anne Lake (Johannesburg, South Africa), Somsikova Liudmila Leonidovna (Chirchik, Uzbekistan), Stikhina Olga Sergeevna (Tyumen, Russia), Thorsten Eschweiler (Übach-Palenberg, Germany), Tiffany Shaw-Diaz (Dayton, USA), Virgilio Gonano (Udine, Italy), and @WRSunset (Shaftesbury, UK). We also thank super-classifier C. J. A. Dukes (Oxford, UK).

This work was funded in part by NASA grant 80NSSC21K0114 (W.J.O. and C.A.T.). C.O.C., H.H.H., and C.A.T. acknowledge support from the NASA Solar System Observations program (grant 80NSSC19K0869). This material is based upon work supported by the NSF Graduate Research Fellowship Program under grant No. 2018258765 and grant No. 2020303693. This work was supported in part by NSF award 1950901 (NAU REU program in astronomy and planetary science). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF. This work was made possible in part through the State of Arizona Technology & Research Initiative Program.

Computational analyses were run on Northern Arizona University's Monsoon computing cluster, funded by Arizona's Technology and Research Initiative Fund. This research has made use of NASA's Astrophysics Data System. This research has made use of data 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 AstOrb, 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 m telescope, which is owned and operated by the Astrophysical Research Consortium. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Facilities: ARC (ARCTIC) - , Blanco (DECam) - , Magellan:Baade (IMACS) - .

Software: acronym (Weisenburger et al. 2017), astropy (Robitaille et al. 2013), Matplotlib (Hunter 2007), NumPy (Harris et al. 2020), pandas (Reback et al. 2022), Photutils (Bradley et al. 2023), REBOUND (Rein & Liu 2012; Rein & Spiegel 2015), SAOImageDS9 (Joye 2006), SciPy (Virtanen et al. 2020), termcolor (https://pypi.org/project/termcolor/), tqdm (da Costa-Luis et al. 2020), VizieR (Ochsenbein et al. 2000).