Postperihelion Cometary Activity on the Outer Main-belt Asteroid 2005 XR132

We report comet-like activity on the outer main-belt asteroid 2005 XR132 discovered by the Lulin One-meter Telescope in early 2021 April. A series of follow-up observations were triggered to characterize the morphology and brightness variation of 2005 XR132. Long-term photometric data of the 2020 perihelion return reveal a 2 mag fading in 120 days, starting 20 days postperihelion, attributed to decreased cometary activity. Even though no variation indicative of the rotational period can be found in our data, we infer an a/b axial ratio of 1.32, given that the lower limit of rotational amplitude is 0.3 mag. A visible spectrum and broadband color support that 2005 XR132 has a reflectance feature similar to a BR-type Centaur object. The syndyne and synchrone simulations reveal a low-speed dust ejecta consisting of millimeter-sized dust grains released shortly after the perihelion passage. We demonstrate that 2005 XR132 has a short dynamical lifetime of 0.12 Myr, with <5% of it in the near-Earth space. Due to the strong gravitational influence from Jupiter and Saturn, the asteroid has followed a random walk orbital migrating process. We also find that since 1550 CE, the perihelion distance of 2005 XR132 has gradually decreased from 2.8 to 2.0 au, likely due to the Kozai–Lidov effect, which potentially reactivated the dormant nucleus. All these dynamical properties support a cometary origin for 2005 XR132 rather than an ice-rich main-belt object kicked out from a stable orbit, although current observational evidence has yet to confirm repeating cometary activities.


Introduction
Understanding the mechanisms of material transportation during the formative years of the solar system is crucial to studying the origins of life and the distribution of water on terrestrial planets.The D/H ratio of all comets spans a broad range from similar to Earth's water to multiple times higher, whereas the meteoritic record suggests that the D/H ratio of asteroids is more similar (Altwegg et al. 2015).The "wet" asteroids in the outer main asteroid belt may be another potential candidate (Kelley et al. 2023).
The Hilda family of asteroids and Jupiter Trojans, both of which are tightly controlled by Jupiter's 3:2 and 1:1 meanmotion resonance (MMR), respectively, are among the most prevalent asteroid populations found within the outer reaches of the main asteroid belt.They are dynamically stable over billions of years (Franklin et al. 2004) and are believed to be the remnants of planet migration (Lykawka & Horner 2010;Emery et al. 2015).In addition, the C/P/D surface reflectance feature (DeMeo & Carry 2014) can be attributed to organic-rich material with limited thermal exposing history ever since they were formed primordially or migrated from the outer solar system.Even though they are currently locked by a resonance, Di Sisto et al. (2005Sisto et al. ( , 2019) ) pointed out that a small fraction of Hildas can be kicked out to chaotic orbits and become asteroids in cometary orbits (ACOs) if they temporarily remain inactive or comets if solar radiation can trigger the sublimation process when approaching the Sun.The ACO population consists of different kinds of dynamically unstable objects including dormant comets, near-Earth objects, and escaped main-belt objects.Consequently, ACOs provide an opportunity to study the physical properties of cometary nuclei, which are usually hidden by the coma.
Gravitational perturbation by Jupiter is the major driver for transporting minor objects inward from the outer solar system or vice versa.Numerical simulation indicates that during the Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
inward orbital transferring process, some periodic comets may be shortly locked by 1:1, 3:2, or high-order orbital resonance of Jupiter not more than a thousand years (Belbruno & Marsden 1997;Koon et al. 2002;Horner & Evans 2006) and even become a temporary satellite capture (Howell et al. 2001;Emel'yanenko 2012) if it is suddenly trapped by the gravitational field of Jupiter.The quasi-Hilda comets (Kresak 1979;Toth 2006;Ohtsuka et al. 2008) are the most populated subgroup of Jupiter-family comets (JFCs) temporarily captured by the 3:2 Hilda resonance.
The Tisserand parameter is an invariant quantity to assess the gravitational perturbation in a restricted three-body system.It is an indicator of objects in high-eccentricity planet-crossing orbits in which the gravitational perturbation of the planet strongly reduces dynamical stability.In the inner solar system, the Tisserand parameter respective to Jupiter (T J ) provides a dynamical view that implicates the possible origin of the bodies.Objects with T J between 2 and 3 and T J < 2 indicate they originate from the Kuiper Belt and the Oort Cloud, respectively, whereas objects in the main asteroid belt have T J > 3, revealing that they are primordially formed (Vaghi 1973;Levison 1996).This dynamical feature is commonly used to identify objects with cometary origins in the main asteroid belt and the near-Earth population.However, Hsieh & Haghighipour (2016) noticed that T J = 3 is not a clear boundary to separate asteroids and comets.During the 2 Myr orbital integration period, some objects with 3.0 < T J < 3.1 have spent 30% of the time on the opposite side of the T J = 3.05 boundary, given an indistinct range to identify the origin of the objects.
Consequently, Tancredi (2014) proposed a rigorous selection criterion for the cometary-originated bodies considering the Tisserand parameter (T J ), minimum orbital intersection distance (MOID), and MMR respective to Jupiter.In addition, Gil-Hutton & García-Migani (2016) employed a numerical approach to identify 11 potential quasi-Hildas of cometary origin.They performed a 50,000 yr simulation of their previous orbits and used the results to select the candidates.Recently, several ACOs have been reidentified as comets with the improvement of large telescopes and wide-field surveys, such as 212P/2000 YN 30 (Cheng & Ip 2013), 282P/2003 BM 80 (Chandler et al. 2022), and 362P/2008GO 98 (García-Migani & Gil-Hutton 2018;Borysenko et al. 2019;Kokhirova et al. 2021).It is interesting to have detailed observations for understanding the origin, orbital evolution, and driver of their resurgent activities.
The asteroid 2005 XR 132 (hereafter XR 132 ) is located in the outer main asteroid belt with a highly eccentric orbit (a = 3.763 au, e = 0.432, i = 14.47°,Q = 5.389 au, T J = 2.868) close to the 13:8 high-order MMR of Jupiter.Even though XR 132 is a Jupiter crosser (Q > Q J ), the highinclination orbit makes the current MOID of Jupiter as large as 0.704 au.Following the selection criterion by Tancredi (2014), XR 132 essentially belongs to the ACO-Jupiter family, as it is a Jupiter crosser with a Tisserand parameter T J between 2 and 3.It has a diameter of 3.4 km assuming an albedo of 4% for a typical D-type object, which we derived from the broadband color.It was discovered in 2005 December by the Spacewatch project at Kitt Peak with an asteroidal appearance.There have been limited astrometric and photometric reports since the discovery in 2005 because of the unfavorable observing conditions in the 2006 and 2013 apparitions.According to the reports in the Minor Planet Center, all observations were conducted with a true anomaly ν greater than 80°( r h > 2.83 au), and no evidence of cometary activity was observed.In this paper, we use both observational evidence and dynamical simulations to characterize the coma morphology, activity, and possible origin of XR 132 .We present our photometry, spectroscopic observations, and data-mining results in Section 2. Orbital evolution and dust dynamics are presented in Sections 3 and 4. We discuss the implications in Section 5 and conclude in Section 6.

Observation
Comet-like activity in XR 132 was first detected in an observation obtained with the Lulin One-meter Telescope (LOT) in Taiwan (Cheng et al. 2021).Subsequently, we conducted a series of target-of-opportunity (ToO) observations in both imaging and spectroscopy with the Hale Telescope at Palomar Observatory, Keck I, and LOT and also analyzed archive images from several sky survey projects collected by the Canadian Astronomy Data Centre (CADC) online service13 (Gwyn et al. 2012).This platform allowed us to search the archives of the Zwicky Transient Facility (ZTF; Smith et al. 2014), the Subaru Hyper Suprime-Cam (Subaru HSC; Miyazaki et al. 2018), and the Dark Energy Camera (DECam) mounted on the Blanco telescope at the Cerro Tololo Inter-American Observatory (Flaugher et al. 2015).Given the nonsidereal motion of XR 132 , we used the 2D trail fitting method (Vereš et al. 2012;Fraser et al. 2016) to optimize the shape of the photometric aperture that can minimize the photometric uncertainty in these survey observations.We briefly introduce the instrumental setup and analytic method in the following section and summarize the observing result.Table 1 lists the observing log of the images we analyzed, except for the ZTF observations, whose observing trajectories are shown in the bottom panel of Figure 4 and can be found in the full version of the table.

LOT
The LOT is a 1 m Trebur Cassegrain Telescope (F/8) equipped with a SOPHIA 2048B 2k × 2k CCD camera manufactured by Princeton Instruments.The pixel scale of 0 768 under the bin 2 readout mode is suitable for the average seeing condition of 1 3 at the Lulin Observatory.To perform the broadband color measurement, standard Johnson BVRI filters are used.The photometric calibration was conducted using the Pan-STARRS DVO catalog (ps1 _pv3 _20170110; Magnier et al. 2020), with filter transformation coefficients by Tonry et al. (2012).We evaluated the zero-point magnitude of individual frames by cross-matching the measured flux of numerous field stars and their brightness recorded in the DVO catalog.The absolute brightness of our target can be derived through the zero-point and the total flux.We adopted this method to analyze the observing images obtained by the Palomar 200 inch Hale Telescope (P200), Keck I, and Blanco introduced in the following sections.Stacked R-band images are shown in Figure 1.We requested 2 nights of ToO observations including a 3 hr consecutive R-band observation to confirm the comet-like feature of XR 132 and try to derive the rotational properties.We verified a median signal-to-noise ratio (S/N) of around 15 for each frame under the average seeing measured between 1 7 and 1 9.

Hale Telescope at Palomar Observatory
The Wafer-Scale Imager for Prime (WaSP) instrument mounted at prime focus on the P200 was used to observe XR 132 on 2021 April 10.The WaSP detector array consists of a 6144 × 6160 Teledyne e2v array with a pixel scale of 0 19 (Nikzad et al. 2017).XR 132 was observed with the standard Sloan Digital Sky Survey (SDSS) broadband g′, r′, and z′ filters (Fukugita et al. 1996) alternatively to minimize the effects of rotational brightness variations on the color measurements.The total exposure time per filter taken with WaSP was 480 s for the g′ band, 1800 s for the r′ band, and 720 s for the z′ band.The telescope was tracked at the on-sky rate of motion of XR 132 .The images of the comet were measured to have a seeing of approximately 1 5, and the observations were conducted at an airmass of 2.2.Photometric calibration has followed the method described in Section 2.1.1.(2020,2021,2022) were applied to the observation of XR 132 .
The photometric calibration was carried out with the method described in Section 2.1.1.In addition, spectra of XR 132 were taken using the 1 0 wide slit and the 560 nm dichroic with ∼50% transmission efficiency in combination with the 400/ 3400 grating for the blue camera and the 400/8500 grating for the red camera providing a spectral resolution of 0.2 and 0.1 nm, respectively (McCarthy et al. 1998).A total exposure time of 600 s was used.To perform wavelength calibration, HgCdZn lamps were used for the blue camera, and ArNeXe lamps were used for the red camera.Flux calibration used the standard star Feige 67 for both the blue and red cameras, and a solar analog star, HD 10354, was used for slope correction.For both the imaging and spectroscopic observations, the telescope was tracked at the on-sky rate of motion of XR 132 .The observations were taken at an airmass of 1.3, and the seeing was measured at 0 7 in the images.

ZTF
The ZTF is a wide-field time-domain observing system that employs the 1.2 m Samuel Oschin Schmidt Telescope at Palomar Observatory.The ZTF camera has a 4 × 4 array of 6k × 6k CCDs (1 01 pixel −1 ) and custom g-, r-, and i-band filters (Bellm et al. 2019a;Dekany et al. 2020).The telescope system robotically executes several concurrent surveys with a range of scientific goals (Bellm et al. 2019b;Graham et al. 2019).Most data are taken with a 30 s integration time, resulting in a median 5σ detection limit of 20-21 mag,   The zero-point determination used the similar differential method mentioned in Section 2.1.1,with additional color term correction. 14Based on the photometric zero-point recorded in the image header, we roughly estimate the brightness of XR 132 when it was close to the aphelion passage (ν ∼ 175°) in 2017.

Victor M. Blanco 4 m Telescope
Using the CADC online service, we access the uncalibrated raw images obtained with the DECam, a mosaic wide-field camera with 62 scientific CCDs covering a 3 deg 2 field of view and a pixel scale of 0 263.The DECam is mounted on the prime focus of the Blanco 4 m Telescope at the Cerro Tololo Inter-American Observatory, leading a pioneer dedicated survey project in the Southern Hemisphere.A series of VRband image sets were taken about 260 days after the last perihelion return in 2013, and one was taken on 2021 March 26 in the i′ band.We estimate the i-band brightness following the method described in Section 2.1.1.Since the DECam VR-band filter is mainly designed for discovering faint transients but not for accurate photometry, moreover, this filter is not generally used in solar system observation, we did not estimate the DECam VR-band brightness taken during the 2013 perihelion passage.1.

Summary of Observations
A diffuse tail is clearly present in the stacked images taken by LOT, P200, Blanco, and Keck I from March and became fainter in 2021 May.However, XR 132 has a similar FWHM to the field stars, indicating the activity is very low at present.Due to the relatively short exposures in the ZTF observations, all the detections have an S/N lower than 10, and the tail cannot be seen directly in an individual frame.
Figure 2 shows the normalized reflectance obtained by the Keck I LRIS spectrum and P200 broadband photometry.XR 132 has a featureless spectrum with a spectral slope of (15.93 ± 0.70)%/100 nm between 480 and 760 nm.The CN and C 2 gas emission bands around 388 and 505 nm are below the 3σ threshold.The broadband color measured within a 2 16 radius of the optocenter of XR 132 is g′-r′ = 0.59 ± 0.03 and r′-z′ = 0.40 ± 0.03, corresponding to a spectral slope of (8.66 ± 0.33)%/100 nm.We did not use the LOT observation to derive the broadband color because of the low S/N and error propagation.
We analyzed our LOT and P200 photometric data sets to search for the rotation period utilizing the Lomb-Scargle periodogram.Unfortunately, since we have limited photometric measurements and cannot properly remove the scattered light from the dust coma, the folded light curve is too sparse to definitively establish the rotation property of XR 132 .The daily light curve depicted in Figure 3 suggests that a rotational amplitude of 0.3 mag is plausible.Using the simple conversion formula Δm = 2.5 log(a/b), we derived that the a/b axial ratio is 1.32.It consists of the axial ratio for cometary nuclei reported by Lamy et al. (2004).
We found no evidence of coma activity in archival data taken in 2014 and 2017.The stacked images from the 2013 perihelion return exhibit radial intensity profiles that are comparable to background stars.This similarity can be attributed to the object's inherently low activity or inactivity, or it suggests that XR 132 ceased its comet-like activity 260 days postperihelion.The Subaru images taken in the 2017 aphelion passage are too faint to search for cometary features.
The ZTF survey provides an excellent data set to monitor the active history of XR 132 after the 2020 perihelion passage.Figure 4 summarizes the trend of absolute brightness M r (1, 1, 0) variation in the r′ band obtained with multiple facilities assuming a general phase coefficient β = 0.04 (mag deg −1 ) for a cometary nucleus (Lamy et al. 2004).We convert the LOT Rband brightness into the SDSS r band according to the transformation formula given on the SDSS website. 15The Subaru HSC-R2 brightness is also labeled in the figure as a green horizontal line.Since we have only one HSC snapshot on the irregular-shaped XR 132 , the gray shading indicates the rms of the rotational amplitude and the photometric uncertainty.As we observed a short-term change in visible reflectance, color correction was adopted with the typical color of active comets  presented by Solontoi et al. (2012).The 2 mag fading trend is clear within a baseline of about 120 days shortly after the perihelion, revealing a decreasing nature in the cometary activity at a farther heliocentric distance.All the photometric measurements in this study are converted to the SDSS r′ band to ensure the consistency of the final result.

Dynamical Simulation
To understand the dynamical evolution history of XR 132 , we adopted the Mercury N-body simulation package (Chambers 1999) to calculate the short-term (2 kyr) and long-term (1 Myr) orbital evolution of 1000 massless clones in forward and backward directions.The clones were generated according to the covariance matrix of the orbital elements provided by the JPL small-body database API service. 16Table 2 lists the nominal orbital elements of XR 132 .We include the gravitational effect of eight major planets (the Earth-Moon system is seen as one object) based on the Bulirsch-Stoer algorithm and used an initial integration time step of days for better efficiency since the inner planets are not the major perturbers for the test particles.We did not consider the nongravitational force in our simulation, assuming that XR 132 is a low-activity object and such additional force can be neglected.The simulation began at the epoch date of the orbital elements (2019 April 3, JD = 2458576.5).In the long-term dynamical integration, we terminate the simulation of individual clones once they drift beyond the 100 au boundary from the Sun.Figures 5 and 6 are the simulation results.

Short-term Orbital Evolution
In the 2 kyr short-term simulation, the evolutionary tracks of each clone diverged around 700 yr after the beginning of the simulation in both forward and backward directions due to the orbital uncertainty.However, we have high confidence in the orbital evolution track from 1350 to 2850 CE. Figure 5 illustrates the short-term orbital evolution of XR 132 .The semimajor axis of XR 132 is nearly constant during the integration period, showing that XR 132 is currently in a quasi-resonant orbit.The MOID of Jupiter is as large as 0.704 au, which is insufficient to make planetary encounters during the short-term simulation.XR 132 smoothly changes its orbit through the periodic gravitational perturbation of Jupiter, as it is not exactly locked by any stable resonance and will likely leave its current orbit soon.Furthermore, we observed that the orbital eccentricity (e) and inclination (i) exchange on a timescale of a few thousand years.It can be explained by the Kozai-Lidov mechanism (Kozai 1962), which shows that the transfer between orbital eccentricity and inclination is dominated by the conservation of orbital momentum.Below is the simple relation of the momentum conservation: dynamical simulation results show that the Kozai-Lidov effect reduced the perihelion distance from 2.8 to 2.0 au in the past 500 yr.This inward migration can raise the equilibrium temperature on XR 132 ʼs surface and may drive the sublimation process of deep-layer volatiles to reactivate the comet-like activity in the recent apparitions.

Long-term Orbital Evolution
The long-term orbital evolution provides a statistical view of the orbital evolution and reveals that XR 132 is dynamically unstable.Figure 6 illustrates the long-term dynamical behavior of the object in two ways: first, a probability map that shows the object's evolution over 1 million yr in terms of its semimajor axis and perihelion distance, and second, a timedependent population ratio across different dynamic groups.These diagnostics provide insight into the object's overall dynamical evolution.In the probability map (top panels of Figure 6), XR 132 follows the random walk orbital migration pathway rarely trapped in any MMR of Jupiter.This dynamic feature is more likely to happen to active Centaurs (Bailey & Malhotra 2009).Except for the Hilda and Trojan regions, which are strongly gravitationally bound with Jupiter (near the current orbit, marked with a cyan cross), XR 132 has a minimal low probability for reaching an orbit of the main asteroid belt or inner.
The time-dependent populating fraction shown in the bottom panels of Figure 6 explores the possible origin of XR 132 .In  each simulation time step, we calculated the number of remaining clones and classified them into different dynamical groups, including near-Earth object (NEO), main-belt asteroid (MBA), JFC, Centaur, and trans-Neptunian object, during the 1 Myr orbital integration.The blue curve represents the percentage of the remaining clones since the beginning of the simulation.The half-lifetime of XR 132 is 0.12 Myr, which is comparable to the dynamic life of the short-period comet (Levison & Duncan 1994;Fernández et al. 2002;Rickman et al. 2017).During the entire 1 Myr simulation, less than 5% of the remaining clones ever reached the MBA and NEO regions.This populating fraction is consistent with the dynamic evolution of the near-Earth JFC presented by Fernández et al. (2002) and Fernández & Sosa (2015) that the JFC spent at most 1000 yr in the NEO (q < 1.3 au) compared with its median lifetime of about 0.15 Myr.This observation suggests that XR 132 may not have been primordially formed within the main asteroid belt.If it had, it would have spent more time in the region close to Jupiter's orbit before being perturbed out of its stable orbit.

Dust Evolution
We adopt the syndyne-synchrone curves to model the motion of dust grains released from a cometary nucleus (Finson & Probstein 1968).These curves were generated by considering the size of the dust grains, the gravitational force, and the solar radiation pressure while assuming a zero ejecting velocity.The β value is a simplified parameter to describe the ratio between the solar radiation pressure and the gravitational force on a freed cometary dust grain.It can be described as where C pr and Q pr are the radiation pressure coefficient and the scattering efficiency for radiation pressure, respectively.ρ d and d are the density and diameter of the dust grain.
Figure 7 shows the syndyne (cyan) and synchrone (red) curves generated by the comet-toolbox web service17 (Vincent 2014) using the orbital elements in Table 2. Assuming the ejected dust grain has a density ρ = 1000 kg m −3 , C pr = 1.19 × 10 −3 kg m −2 , and Q pr ∼ 1 (Moreno et al. 2012), we identify the dust tail of XR 132 as mostly attributed to millimeter-sized grains.This result is consistent with the in situ measurement obtained with the GIADA instrument on board the Rosetta spacecraft of a dust coma dominated by >0.1 mm dust grains (Della Corte et al. 2019).Ground-based optical observations also support millimeter-sized dust grains that can be generally observed around various types of comets, such as the Jupiter-family Comet (JFC; Ishiguro et al. 2016), the Mainbelt Comet (MBC; Kim et al. 2022), and even the Long-period Comet (LPC; Hui et al. 2023).According to the synchrone curves, the oldest dust ejecta is best fitted with the 120-140 day trails in our LOT discovery image obtained on April 5, indicating the cometary activity had begun shortly after the 2020 perihelion return.This coincides with the maximum reduced brightness observed by the ZTF photometry about 120 days before the LOT observation.A small amount of tiny dust particles continued to be released until 2021 April.
The syndyne-synchrone model assumes zero terminal ejection speeds of dust particles and therefore cannot constrain the ejection speeds of the dust.To further constrain the properties of the dust tail, we employ the dust dynamics code originally derived by Ye et al. (2016a).The model assumes isotropic ejection with the terminal ejection speed defined as v ej = V 0 β 1/2 .We assume the minimum particle size to be 0.1 mm, as evidenced from the syndyne analysis above.The maximum particle size does not impact the outcome since they stay near the nucleus.As a cursory test to broadly constrain the shape of the coma and tail, we assume the dust size follows a simple power law with a differential size index of q = −3.5, as the value of q only affects the surface brightness gradient and does not significantly affect the shape of the coma and tail.We test two scenarios, an active comet with activity driven by the sublimation of pure water ice as calculated by the Whipple (1950) model and a low-speed ejection similar to comets known to be near dormancy (Ye et al. 2016b).The input parameters are summarized in Table 3.
As shown by the middle and bottom panels of Figure 7, the observations in the left column are clearly more consistent with the low-speed scenario, as the Whipple model consistently predicts a more symmetric coma.

Discussion
From the ZTF, LOT, and P200 photometric results shown in Figure 4, the absolute r-band magnitude displays a decreasing trend from M r = 15.2 (T − T P = 20 days) to M r = 17.1 (T − T P = 140 days).The decline of 2 mag indicates a diminishing trend in XR 132 ʼs cometary activity.At the end of the observing campaign, the absolute r-band magnitude is marginally brighter (by ∼0.1 mag) than the Subaru HSC-R2 measurement at ν ∼ 172°.This suggests that XR 132 might not be completely inactive in our final ZTF observation and could still be enveloped by a modest dust coma, especially if the rotational amplitude Δm is realistically larger than 0.4 mag.We note that photometric data are undulated at around T − T P = 45 days.Taking into account the rotational amplitude and photometric uncertainty, they are still within a reasonable range and may not be a signature of outburst activity.Near the end of the ZTF observation (T − T P > 150 days), there are some "bright measurements" with large uncertainty.Those measurements are reaching the ZTF detection limit and are probably not a real brightening event.Due to the limited observing data, we cannot deduce the onset date of XR 132 ʼs activity.The activity of XR 132 persisted for over 100 days and the ejected dust was gently dissipated after the 2020 perihelion return, in good agreement with results from the synchrone dust simulation.If XR 132 has a similar level of activity in the 2013 return, it returns to the quiescent state no longer than 260 days postperihelion, as we cannot detect any active features in the Blanco observation in early 2014.However, the maximum brightness might not occur at the beginning of outburst activity.The time delay may be related to the dust ejection velocity, the filling factor of the photometric aperture, and the geocentric distance.So here we consider the artificial impact experiment on comet 9P/Tempel 1, in which brightness variation had been well recorded during the outburst by Keller et al. (2007).The maximum brightness happened within 20 hr after the impact, with a dust ejection velocity of ∼200 m s −1 , meaning that XR 132 (ejection velocity of ∼40 m s −1 derived by our simulation) may have taken 100 hr to develop a dust coma of similar size and reaching the peak brightness.The long-term light curve roughly indicates the level of outgassing activity.
Since we cannot resolve and eliminate the scattered light from the dust coma, the measured visible reflectance and broadband color may be partly contributed by dust particles but not purely from the nucleus.According to the long-term light curve obtained with ZTF and LOT (Figure 4), we estimate that the dust component may raise the brightness of XR 132 in about 0.3 mag during our P200 multicolor observation (the gap between the red and gray shadows at T − T p = 134 days), suggesting that the absolute r′ magnitude of XR 132 is M r(1,1,0) = 17.1.This means that the scattering cross-section ratio of ejected near-nucleus dust and nucleus A d /A n is 32% at most (Hsieh et al. 2011).However, the LRIS visible spectrum is rarely contributed by the dust particle because the enhancement of absolute r-band brightness is less than 0.1 mag.The photometric and spectroscopic result reveals that XR 132 has an organic-rich chemical composition and is likely primordially formed beyond the outer main asteroid belt.The broadband color measured by P200 is consistent with a D-type reflectance feature in the Bus-DeMeo system (DeMeo et al. 2009), mostly contributing to the outer main belt.In the colorcolor diagram shown in Figure 8, XR 132 has a surface color similar to other active comets reported by Solontoi et al.We notice an increasing spectral slope from 8.6% of the grz photometry to 15.9% of the LRIS spectrum between 480 and 760 nm in 36 days (Figure 2).Spectral slope can be an indicator of chemical composition, with a bluer spectral slope indicating a magnesium-silicate or water-ice-rich dust and a red slope indicating carbonaceous organics (Zubko et al. 2011).However, the apparent spectral slope may also be influenced by multiple factors such as grain size distribution, viewing geometry, and degree of cometary activity (Filacchione et al. 2020).Voitko et al. (2022) reported a significant short-term variation of the spectral slope of dust surrounding Centaur comet 29P/Schwassmann-Wachmann 1 in 2018, where the spectral slope changed by about 20% in 10 days due to outburst activity that released a specific dust size distribution.However, this explanation may not apply to XR 132 , as it is a low-activity object with no obvious outburst activity observed in the longterm light curve.A possible scenario for the observed color change is the contribution of ejected dust with a neutral color obscuring the red nucleus surface and reducing the spectral slope.The decline observed in the long-term light curve suggests that the nucleus of XR 132 likely aligns with our spectroscopic findings.This is because the broadband color could be influenced by the dust coma, which might account for approximately 32% and exhibits a neutral color.
XR 132 has a Tisserand parameter T J = 2.87, indicating it likely originated from the Kuiper Belt.Our dynamical simulation also supports this scenario that XR 132 is more likely a reactivated dormant comet rather than a floating object primordially formed in the main asteroid belt due to the following reasons.First, the dynamical lifetime of XR 132 is as short as 0.12 Myr, which is comparable to the usual lifetime of short-period comets (Levison & Duncan 1994).Second, in the long-term simulation, XR 132 spent no more than 5% of its life inside the orbit of Jupiter.This means XR 132 is almost in its innermost orbit during the journey in the solar system.Third, the orbital migration of XR 132 is dominated by the gravitation of Jupiter and Saturn.It is an important sign that the previous evolutionary step of XR 132 should be a Centaur object.Fourth, XR 132 follows the random walk migrating process in the longterm simulation.This process occurs on most active Centaurs (Bailey & Malhotra 2009).In contrast, inactive Centaurs follow the resonance-hopping migration process.
The Kozai-Lidov mechanism is believed to have reactivated the sublimation process of XR 132 .The short-term simulation indicates a significant decrease in perihelion distance over the past 500 yr, from 2.8 to 2.0 au, which could raise the surface temperature from T 0 = 233 to 276 K at the subsolar point of a simple spherical object.The standard thermal model (Harris & Lagerros 2002) was used to calculate this based on various parameters, where η is the beaming parameter, ò is the emissivity, σ is the Stefan-Boltzmann constant, S 0 is the solar constant, A is the bond albedo, and S is the solar irradiance under a given heliocentric distance R h .Here we suggest that the beaming parameter and the emissivity are unity, and the bond albedo is 0.04 for cometary nuclei (Lamy et al. 2004).However, the classical Kozai-Lidov mechanism in a simple three-body problem requires the minor body to have an orbital inclination between 39°.2 and 141°.8 (Kozai 1962), which is not the case for XR 132 ʼs initial orbital inclination.Vinogradova (2017) developed a method to characterize the Kozai-Lidov effect in the main asteroid belt considering multiple perturbers like Saturn.A clear oscillation pattern between (ω-e) and (ω-sin(i)) is shown in Figure 9, where e and i reach the maximum and minimum when the argument of perihelion ω = 270°, respectively.This provides evidence for the Kozai-Lidov effect dominating the inward perihelion migration of XR 132 .The Kozai-Lidov effect gently pushed XR 132 ʼs orbit closer to the Sun and raised the equilibrium temperature significantly in the past 500 yr.Even though XR 132 already passed the 3 au water-ice snow-line boundary, thermal conduction is a gradual process to heat the deeper layer of the nucleus over each perihelion apparition.If XR 132 is a dormant comet (i.e., cometary nucleus covered by an amount of dust mantle), the thermal energy can reach the inner ice-rich layer of the nucleus, trigger the sublimation process, and develop the dust coma near the perihelion passage.CN and C 2 gas emissions centering at 388 and 505 nm, respectively, are important but not necessary indicators of sublimation-driven cometary activity (Feldman et al. 2004), especially for the comets with low-level activity.It can infer a low gas-to-dust ratio comet, as no evidence for these molecules is seen in the spectrum.Future follow-up observations might helpfully confirm the cometary nature if the cometlike activity reappears around the next perihelion return in 2028, or spectroscopic detection of volatile species (e.g., JWST spectroscopy of the H 2 O and CO 2 bands at 2.7 and 4.3 um).
Rotational disruption is another viable mechanism to activate an asteroidal object (Jewitt 2012).The fast-rotating asteroid can reduce the angular momentum by ejecting mass along the equatorial plane.The reaccretion process of the ejected dust can produce a satellite asteroid and become a binary pair.It can be verified by time-series photometric measurement.Unfortunately, our sparse observations cannot derive any rotational properties and confirm this scenario even though a ∼0.3 mag brightness variation is shown in the daily light curve obtained in early April.
The broadly diffused tail of a comet is a morphological indication of a persistent dust-releasing process.As dust is continuously emitted from the nucleus, it creates a long-lasting dust trail that follows a wide range of orbital trajectories, particularly during the perihelion approach.In the stacked images taken in 2021 April and May, XR 132 has shown a classical broadly diffused tail for a couple of months.This is unlike the cataclysmic impact or energetic outburst event, which usually results in a more complex structure and fades away over a short timescale of several weeks.Recently, NASA's DART mission demonstrated this phenomenon by revealing a narrow tail that appeared days after the impact and persisted for hundreds of days (Li et al. 2023).In our advanced simulation assuming a nonzero ejecting velocity, the dust tail of XR 132 is more likely produced by a low-speed ejection.The long-lasting, low-velocity, broadly diffused dust trail with a smoothed brightness decay is evidence showing that the activity of XR 132 was not likely triggered by a cataclysmic impact or energetic outburst.

Conclusion
We highlight the results of this paper as follows.
1.The long-term light curve shows a 2 mag dimming history in 120 days shortly after the 2020 perihelion return, indicating a decrease in coma activity.The dustreleasing process remains after our last ZTF observation.2. XR 132 has a lower limit of the rotational amplitude of 0.3 mag, corresponding to an a/b axis ratio of 1.32.The rotation period cannot be confidently derived due to the sparse observing data.3. We identified a D-type Bus-DeMeo taxonomy and BRtype surface feature according to the P200 broadband color and Keck I LRIS visible spectrum, respectively.The LRIS spectrum is red and featureless, related to the bluer group of the Centaur population.No CN and C 2 gas emissions can be detected above the 3σ threshold.4. We observed a changing spectral slope from 8.6% to 15.9% in 36 days, probably because the fresh neutral dust cloud obscured the red nucleus.5. Dynamically, XR 132 is more likely a dormant JFC considering the Tisserand parameter and its short dynamical lifetime, random walk migrating behavior, and low probability of visiting near-Earth space.6.The Kozai-Lidov effect reduced the perihelion distance of XR 132 from 2.8 to 2.0 au and may progressively reactivate the cometary activity.7. The synchrone dust curve illustrates that XR 132 began releasing the dust after the 2020 perihelion passage.A broadly diffused dust tail belongs to a continuous dust release process, consistent with the long-term light curve obtained in the 2020 return.8.The dust morphology of XR 132 is consistent with a lowspeed dust-ejecting scenario with millimeter-sized dust grains.We presume that the activity of XR 132 is unlikely to be triggered by cataclysmic impact or outburst activity.
depending on the filter and the seeing.Images are processed with the ZTF Data System(Masci et al. 2019), which includes bias subtraction, flat-field correction, and astrometric and photometric calibration.Photometric calibration was performed against the selected stable stellar sources from the Pan-STARRS1 DR1 catalog.The color terms are also considered when deriving the zero-point of each image frame.We searched the ZTF archive (Data Release 16 and ZTF Partnership proprietary data) for images of XR 132 using the ZChecker software(Kelley et al. 2019) and found 55 images between 2020 October 7 and 2021 June 3 UTC.ZChecker analysis is based on calibrated data processed with the ZTF Data System.Photometry was measured with ZChecker on single frames within a 5″ radius aperture, assuming the average cometary colors fromSolontoi et al. (2012): g − r = 0.49 and r − i = 0.24 mag.The fixed angular aperture is best for lightcurve studies of extended sources, and this particular radius was chosen to mitigate the effects of night-to-night variations in seeing (median = 2 6).Nondetections, observations with poor seeing, and images affected by background objects or artifacts were manually removed.2.2.2.Subaru TelescopeWe obtained publicly available images through the HSC Subaru Strategic Program (HSC-SSP;Aihara et al. 2018) based on the orbital ephemeris provided by the JPL Horizon online service.HSC-SSP is a dedicated multicolor survey project covering ∼1400 deg 2 of the northern sky since 2014 March.The HSC consists of 104 2k × 4k scientific CCDs with a pixel scale of 0 168.We collect the CORR-level single-frame imaging data through the Public Data Release version three (Aihara et al. 2022).CORR refers to a calibrated image that already finished the image reduction, the World Coordinate System (WCS) alignment, distortion correction, and zero-point determination CCD by CCD with hscPipe (Bosch et al. 2018).

Figure 1 .
Figure 1.Stacked images of XR 132 in linear scale.The field of view of each image stamp is 20″ and oriented with north up.The observational details are shown in Table1.

Figure 2 .Figure 3 .
Figure 2. The visible spectrum obtained with the LRIS instrument on Keck I and the g′, r′, z′ broadband reflectance normalized at 550 nm, indicated by an orange cross.The error bars of the spectrum correspond to 1σ uncertainty.The Bus-DeMeo D-and BR-type reflectances are shown as gray and red shading, respectively.No clear features of cometary gas emissions, such as CN and C 2 , were detected.(The data used to create this figure are available.)

Figure 4 .
Figure 4.The long-term light curve in the 2020 perihelion return.The crosses, triangles, and circles indicate measurements obtained with LOT, P200, and ZTF, respectively.The g-and i-band photometry have been scaled using the mean cometary colors from Solontoi et al. (2012).An offset was added to correct the Johnson R band to the r band in the LOT observation.The 30 day moving average and Subaru HSC-R2 brightness are shown in red and green lines with an indication of rotational amplitude and uncertainty shown by shading.The solid orange and blue curves in the bottom panel are the heliocentric and geocentric distance in the given time, respectively.The black triangle indicates the date we discovered XR 132 ʼs comet-like activity.

Figure 5 .-
Figure 5.The 2 kyr forward and backward orbital evolution in semimajor axis, perihelion distance, eccentricity, inclination, Tisserand parameter, and ( ) e i 1 c o s 2 constant to assess the conservation of angular momentum.The gray and blue curves are the evolution path of individual clones and the median value of all clones, respectively.The vertical dotted line indicates the beginning of the simulation.

Figure 6 .
Figure 6.The 1 Myr backward (left) and forward (right) orbital evolution of XR 132 .The top row demonstrates the probability map along the semimajor axis and perihelion distance during the 1 Myr simulation.Different dynamic groups are labeled following the definition given in the JPL Small-Body Database Query web service.The cyan cross is the initial orbit of the simulation.The bottom row shows the fraction of remaining clones per dynamical class with an indication of halflifetime in vertical dashed lines.

Figure 7 .
Figure 7.The top row shows the syndyne and synchrone dust curves in cyan and red overlapped with the observing images taken on April 5 and May 16.The syndyne curves from left to right indicate dust grain sizes of 0.1, 1, and 10 mm, respectively, assuming the dust density is 1000 kg m −3 .The synchrone curves are separate in 20 day steps clockwise from 20 to 160 days from left to right.The middle and bottom panels are dust simulated images with the observing data obtained in 2021 April and May.Stamps on the central column are the comet assume a low-speed ejection similar to dormant comets (Ye et al. 2016b); those on the right column assuming an activity driving by the sublimation of pure water ice (Whipple 1950).All the subplots are aligned with north up and east to the left in a 20″ × 20″ field of view.

Figure 9 .
Figure9.The Kozai-Lidov oscillation of sin(i) and e from our short-term simulation.This plot is made by only the data within the reliable time period between 1000 and 3000 CE.

Table 2
Orbital Parameters of XR 132 This orbital solution obtains the observations between 2005 December 5 and 2021 May 14 retrieved from the JPL SBDB API on 2023 April 25.