Cometary Activity Discovered on a Distant Centaur: A Nonaqueous Sublimation Mechanism

Centaurs are minor planets thought to have originated in the outer solar system region known as the Kuiper Belt. Active Centaurs enigmatically display comet-like features (e.g., tails, comae) even though they orbit in the gas giant region where it is too cold for water to readily sublimate. Only 18 active Centaurs have been identified since 1927 and, consequently, the underlying activity mechanism(s) have remained largely unknown up to this point. Here we report the discovery of activity emanating from Centaur 2014 OG392, based on archival images we uncovered plus our own new observational evidence acquired with the Dark Energy Camera (Cerro Tololo Inter-American Observatory Blanco 4 m telescope), the Inamori-Magellan Areal Camera & Spectrograph (Las Campanas Observatory 6.5 m Walter Baade Telescope), and the Large Monolithic Imager (Lowell Observatory 4.3 m Discovery Channel Telescope). We detect a coma as far as 400,000 km from 2014 OG392, and our novel analysis of sublimation processes and dynamical lifetime suggest carbon dioxide and/or ammonia are the most likely candidates for causing activity on this and other active Centaurs. We find 2014 OG392 is optically red, but CO2 and NH3 are spectrally neutral in this wavelength regime so the reddening agent is as yet unidentified.

In 1949 the discovery of the first active asteroid, (4015)Wilson-Harrington (also designated 107P), blurred the dividing line between asteroid and comet (Cunningham 1950). In 1977(2060Chiron was discovered (Kowal & Gehrels 1977), the first member of the population now known as Centaurs. (2060)Chiron was later found to be active, making it the first object to be identified as a Centaur prior to activity discovery (Meech & Belton 1990).
We adopt the Centaur classification system (Jewitt 2009) that defines Centaurs as objects (1) with perihelia and semimajor axes between the orbits of Jupiter (∼5 au) and Neptune (∼30 au) and (2) not in 1:1 mean-motion resonance with a giant planet (as is the case for the Trojans). We distinguish between Centaurs and Jupiter-family comets (following Levison & Duncan 1994) via the Tisserand parameter with respect to Jupiter, given by

Mining Archival Data
In order to overcome the observational challenges discussed in Section 1 we began by searching archival images captured with the 0.5gigapixel Dark Energy Camera (DECam) on the Blanco 4m telescope at the Cerro Tololo Inter-American Observatory in Chile. Archival data from this facility allow the detection of faint activity because of the relatively large aperture and because a large number of objects serendipitously imaged by the instrument can be searched.
We identified Centaurs in our own proprietary database cataloging the NSFs National Optical-Infrared Astronomy Research Laboratory (NSFs OIR Lab, formerly NOAO) public DECam archive following the methodology outlined in Chandler et al. (2018). Our general approach was to correlate image celestial coordinate and temporal data with object ephemeris services such as NASA JPL Horizons (Giorgini et al. 1996) and IMCCE SkyBot (Berthier et al. 2006; see also the acknowledgments).
We (1) extracted event information from the entire DECam public archive database, (2) submitted objects to SkyBot or matched against ephemerides produced via the Minor Planet Center and/or Horizons, and then (3) carried out a database query to identify potential images containing Centaurs.
After (4) downloading the data, we (5) checked each chip for the presence of the Centaur to ensure the object was visible and free of imaging complications (e.g., gaps between chips, scattered light from bright stars, cosmic rays). Finally, we (6) adhered to the routine outlined in Chandler et al. (2018) where, following image file retrieval of 2014OG 392 from the archive, we extracted Flexible Image Transport System (FITS) and Portable Network Graphics (PNG) thumbnails (480×480 pixel images). We subjected these thumbnails to image processing techniques in order to assist by-eye analysis.
While examining each Centaur PNG thumbnail image by eye we flagged any with apparent activity for later analysis. FITS thumbnail images corresponding to those flagged were subjected to additional image processing techniques in an effort to enhance image quality, especially comae contrast.
To ascertain potential heliocentric distance effects we made use of a simple metric (Chandler et al. 2018),  % T q , which describes how close to perihelion (q) an object's distance (d) is relative to its aphelion distance (Q): From DECam archival data we extracted ∼20 thumbnail images of 2014OG 392 ; Figure 2 shows the number of thumbnails obtained along with the predicted apparent V-band magnitude and observability of 2014OG 392 . In images from 2017 July and August, we spotted what appeared to be activity emanating from 2014OG 392 (see gallery; Appendix B); at that time the object was 10.60au from the Sun.

Follow-up Observing
To confirm the presence of activity we used the same DECam instrument and made additional observations on UT 2019 August 30. Figure 1 shows 2014OG 392 with a telltale coma revealed by a combined 1000s exposure. Appendix B contains a gallery showing the four constituent 250 s DECam exposures, plus two images where isophotal contours were overplotted to help identify coma extent for each of the first two exposures (Appendix B).
We made use of three observatories for follow-up observations of 2014OG 392 : (1) NSF's OIR Labs DECam with VR filter on the Blanco 4m telescope at the Cerro Tololo Inter-American Observatory in Chile (2) WB4800-7800 filtered imaging with the Magellan 6.5m Walter Baade Telescope equipped with the Inamori-Magellan Areal Camera & Spectrograph (IMACS) at the Las Campanas Observatory on Cerro Manqui, Chile, and (3) g, r, and i filter images taken with the Large Monolithic Imager (LMI) at the Lowell Observatory 4.3m Discovery Channel Telescope (DCT) in Arizona, USA. Galleries showing our Magellan images and DCT images are shown in Appendices B.4 and B.5, respectively. A log of observations is provided in Appendix A. Astrometric calibration was performed using the astrometry.net (Lang et al. 2010) and/or PhotometryPipeline (Mommert 2017) software packages.

Simulating Dynamical Lifetime
Determining the total mass loss possible for different volatiles requires knowledge of the dynamical lifetime of 2014OG 392 in the Centaur region (where both perihelion distance and semimajor axis are between 5 and 30 au). To this end we made use of the REBOUND N-body integrator to model the orbits of 2014OG 392 and giant planets Jupiter, Saturn, Uranus, and Neptune (Rein et al. 2019). We also carried out 25 simulations of 2014OG 392 , each with an orbital clone derived from the orbital uncertainties published by the Minor Planet Center. From these dynamical integrations, we found that the lifetime of 2014OG 392 spans the range of 13,000-1.8 million years, roughly in agreement with prior work (Liu & Ip 2019).

Sublimation Modeling
In order to better assess potential processes responsible for 2014OG 392 activity, we computed equilibrium temperatures and modeled mass-loss rates for seven astrophysically relevant ices: ammonia (NH 3 ), carbon dioxide (CO 2 ), carbon monoxide (CO), methane (CH 4 ), methanol (CH 3 OH), nitrogen (N 2 ), and water (H 2 O). Object distance is the primary factor in determining potential ice sublimation effects. We began with a simple sublimation model (Hsieh et al. 2015) well suited to gaining broad insight into the observed activity from 2014OG 392 ; we expanded the procedure to apply more generally to other volatile ices. As we do not know the composition of 2014OG 392 we cannot make use of a more comprehensive model that includes effects of, for example, porosity, tortuosity, or crystal structure (Schorghofer 2008). Moreover, 2014OG 392 is undoubtedly not composed of a single ice, and mixtures of ices can exhibit behavior uncharacteristic for any lone constituent (Grundy & Stansberry 2000). For the limiting case of an inert graybody orbiting at a distance R from the Sun (measured in au) where the fiducial solar flux F e is 1360 W m −2 , A is the Bond albedo (we choose 0.1 as representative for Centaurs; Peixinho et al. 2020), ò is the infrared emissivity of the ice (set here as 0.9), T eq is the equilibrium temperature of the body, and σ is the Stefan-Boltzmann constant (5.670×10 −8 W m −2 K −4 ).
Here χ is a factor that describes the rotational and axial tilt effects on how much flux is received from the Sun: χ=1 indicates the maximum heating scenario where the body is a "slab" facing the Sun at all times; χ=π describes a body that rotates quickly with no axial tilt with respect to the Sun; and χ=4, which we adopt here, is used for a fast-rotating (on the order of a few hours) isothermal body in thermodynamic equilibrium. Here "fast-rotating" means that the rotation period of the object is short compared to the thermal wave propagation time (Schorghofer 2008;Hsieh et al. 2015). We next consider an energy balance that incorporates sublimation in addition to blackbody radiation (Hsieh et al. 2015): where f D is the "diffusion barrier factor" that describes how much emission is blocked by overlaying material (e.g., regolith), and L the latent heat of sublimation. The mass-loss rate m T S ( )  is given by with μ the SI mass of one molecule, and k the Boltzmann constant of´-1.38069 10 23 J K −1 . The vapor pressure (in Pa) of the substance can be related to temperature by the Clausius-Clapeyron relationship in which e S is the saturation vapor pressure (in Pa) of the substance at the triple-point temperature T triple , ΔH subl is the heat of sublimation of the substance (in kJ mol −1 ), and R g is the ideal gas constant ( --8.341 J mol K 1 1 ). Notes. P: orbital period; a: semimajor axis; q: perihelion distance; Q: aphelion distance; r: heliocentric distance;  % T q : fractional perihelion-aphelion distance (Equation (2)); M V : apparent V-band magnitude. Q computed via Q=a(1+e) when otherwise unavailable. Asteroid parameters provided by the Minor Planet Center. Heliocentric distance and apparent magnitude courtesy of JPL Horizons (Giorgini et al. 1996). a Original value(s) from activity discovery epoch adopted where available; otherwise, values adopted from more recent epoch(s). Reference points to a source that discusses activity of the object. Solving Equation (4) for heliocentric distance R (in au) yields (Luna et al. 2014) and triplepoint temperatures and pressures (Fray & Schmitt 2009) were incorporated as needed. To validate our model we computed the mass-loss rate for (2060)Chiron assuming χ=4, an albedo of 0.057, a diameter of 206 km, and an orbit ranging from 8.47 au at perihelion to 18.87au at aphelion. Our (2060)Chiron model validation results were in rough agreement with the 0.5-20kg s −1 mass-loss rate reported by Womack et al. (2017).

Energy of sublimation values
We use our computed dynamical lifetime to circumstantially constrain the molecule(s) responsible for the sublimation of 2014OG 392 . Figure 3 shows, over the orbit of 2014OG 392 , the mass-loss rates for the different ices determined via modeling and validated through laboratory measurements. If 2014OG 392 has an albedo of 10%, similar to that measured for other Centaurs (see review; Peixinho et al. 2020), then the body is about 20km in diameter (see Section 7). Assuming a spherical body of low density in the range of 1-3g cm −3 suggests a reasonable body mass of (4.2-12.6)×10 15 kg and a surface area of 3.1×10 8 m 2 . Thus, the 13,000-1.8 Myr dynamical lifetime of 2014OG 392 suggests a maximum orbitaveraged mass-loss rate in the range of 7.1×10 −7 -3.3 ---10 kg m s 5 2 1 (horizontal dashed lines in Figure 3) before the body would be entirely lost due to sublimation.

Colors
The archival data and our confirmation observations did not contain enough information to determine colors, so we obtained six 300s exposures of 2014OG 392 in a g-r-i filter sequence at the DCT (Section 3). We made use of the PhotometryPipeline software package (Mommert 2017) to automate astrometry using SCAMP (Bertin 2006), which made use of the Vizier catalog service (Ochsenbein et al. 2000),  Prior to analysis we examined all thumbnail images showing activity emanating from 2014OG 392 to ensure no significant background sources were blended with the nucleus. To help us identify unseen contaminators we measured and modeled surface brightness radial profiles of 2014OG 392 ( Figure 4) and a nearby solar-type star, using the Aperture Photometry Tool. The radial profile itself (i.e., not the model) was used to identify flux contribution by unseen background sources; we rejected images in which the nucleus or nearby coma was significantly contaminated. We note that we identified at least one background source within the coma in all of our images, although for color measurement we were able to use an aperture small enough (5 pixel radius) to exclude all resolvable background objects.

Absolute Magnitude and Diameter Estimation
To gauge the overall spatial extent of the coma we examined the radial surface brightness profiles of 2014OG 392 and nearby solar-type star J004840.66-022335.6 (see Section 6). We fit the profiles to the model ( ) ( ) as described in Gwyn et al. (2012).
After subtracting the sky flux from each profile and each model we scaled the star to the peak flux of the 2014OG 392 radial profile. Figure 4 shows the radial profiles and their corresponding models plotted; we estimate the coma returns to sky background flux levels at ∼60 pixels from the aperture center, thus the coma extent is ∼4.3×10 5 km. The FWHM of 2014OG 392 was 13.62±0.37 pixels (3.2±0 09), whereas the star FWHM was 6.05±0.05 pixels (1.45±0 012).
As reported in Section 5, the coma is likely present throughout the orbit of 2014OG 392 . As a result, prior absolute (H) magnitude estimates would have included the excess flux caused by the coma, as evinced in Figure 4. To estimate the absolute nuclear magnitude of 2014OG 392 we compared the ratio of the total (nucleus + coma) flux (blue line and circles, Figure 4) to the scaled stellar flux (orange line and triangles, Figure 4). We estimate the coma accounts for 0.75 and 1.1 magnitudes of the observed r-band and g-band fluxes, respectively, implying the nucleus apparent magnitudes are m r =21.9 and m g =23.1. . Mass-loss rates for seven different astrophysically relevant ices on an isothermal (χ=4) body; water (H 2 O) and methanol (CH 3 OH) ices have been detected on Centaurs. Orbital distances of Jupiter, Saturn, Uranus, and Neptune are indicated about the top axis. The current 10.11au heliocentric distance of 2014OG 392 is indicated by a vertical black bar, bracketed by perihelion (9.97 au) and aphelion (14.40 au) distances (leftmost and rightmost dashed vertical lines, respectively). Over the course of one orbit (between the vertical dashed lines), water and methanol never appreciably sublimate and carbon monoxide (CO), methane (CH 4 ), and molecular nitrogen (N 2 ) sublimate at high and relatively constant rates; we rule out all of these molecules as potential causes of activity. (The shallow slopes of CO, CH 4 , and N 2 extend beyond 50 au [not shown], which informs us the mass loss would have begun long before 2014OG 392 became a Centaur.) However, over the course of one orbit the sublimation rates for CO 2 and NH 3 vary substantially, presumably producing significant variation in visible activity. Order-of-magnitude estimates of mass-loss-rate upper limits for the dynamical lifetime of 2014OG 392 are shown as horizontal dotted lines. Only CO 2 and NH 3 have sublimation rates near these limits.
The absolute magnitude of an asteroid, H, is commonly used to estimate the size of small bodies in the solar system. H is defined as equal to the apparent V-band magnitude of an object observed at a heliocentric distance R=1 au, a geocentric distance Δ=1 au, and a phase angle α=0°. Here we employ the International Astronomical Union defined (Swings 1986) H−G magnitude system approximated from Bowell et al. (1989) We make use of the relationships put forth by Jester et al. (2005) to derive Johnson V=22.4 from our g and r nuclear magnitudes. The JPL Horizons ephemerides service (Giorgini et al. 1996) provided G=0.150 (the standard assumed slope for dark surfaces), r=10.10 au, Δ=10.01 au, and α=5°.58 for UT 2019 December 30. Via Equation (9) we find H=11.3, 0.5 magnitudes fainter than reported by the Minor Planet Center and JPL Horizons.
Harris & Harris (1997) provide a convenient method to approximate object diameter D, which, for 2014OG 392 , gives D≈20 km.

Coma Dust Analysis
To facilitate comparing our 2014OG 392 dust-related metrics with other works we adopt the instrument and aperture-independent cometary dust production parameter described by A'Hearn et al. (1984). The metric, Afρ (units of cm), combines the mean albedo A of ejecta grains within an aperture of radius ρ (in cm), scaled by the filling factor f (unitless), which describes how much of the aperture area (πρ 2 ) is filled by N grains of cross section area σ (in cm 2 ), r s pr We measured Afρ ( Recall the quantity Afρ, here, is a measured value, so the quantities Aρ do not cancel in Equation (16). Four of our observations, Images 15-18 (details in Appendix A) were suitable for directly measuring Afρ. We found Afρ=487±12 cm with an aperture of 4.3×10 5 km. With the albedo adopted for our sublimation modeling (A=0.1) and a 1mm radius grain, the coma around 2014OG 392 is composed of roughly 5.8×10 17 particles. Assuming a grain density of 1g cm −3 the total coma mass is ∼2.4×10 15 g.

Discussion
The activity we observed spans more than two years, which rules out impact-driven activity. We determined that the two ices previously detected on Centaurs, water and methanol, would not appreciably sublimate at any point in 2014OG 392 ʼs orbit and so should still be present in solid form on the surface (Figure 3). Moreover, CO, N 2 , and CH 4 are highly volatile and sublimate at temperatures low enough that their supply is likely depleted, though reservoirs could still be trapped below the surface. We reiterate our model encompasses single-species ices subjected to the thermodynamic conditions outlined in Section 5; heterogeneous ice environments may alter sublimation chemistry (see, e.g., Grundy & Stansberry 2000), as can single-species state transitions (e.g., energy released during crystallization of amorphous water ice; see, e.g., Jewitt 2009). We find that the molecule(s) most likely to drive the observed activity is either CO 2 and/or possibly NH 3 . Neither would have sublimated appreciably at Kuiper Belt distances prior to 2014OG 392 becoming a Centaur. Interestingly, both of these substances sublimate at rates that vary by over two orders of magnitude over the course of a 2014OG 392 orbit, peaking at perihelion. As a result we predict 2014OG 392 will become less active post-perihelion. This further implies that all other active Centaurs should follow this trend, with peak sublimation near perihelion and a significant drop in outgassing for most of their orbits.
We determined 2014OG 392 is at present roughly one magnitude redder than the Sun at visible wavelengths. However, we were only able to obtain two images in each filter, so uncertainty could be improved upon with additional observations. Our color measurements inexorably included the coma; future observations during a quiescent period (should one exist) would allow for color measurements of the bare nucleus. We did, however, attempt to better estimate the H magnitude by subtracting the coma measured in the radial surface brightness profiles. We found 2014OG 392 has H≈11.3, 0.5 magnitudes fainter than previously reported. The H magnitude implies a radius of about 20km when assuming a slope parameter G=0.15 as is typical for a dark surface.
In our images of 2014OG 392 background sources were typically present in the coma and/or blended with the nucleus, but from four images we were able to directly measure dust properties. Assuming a 10% albedo and a grain radius of 1mm we estimate the coma contains roughly 5.8×10 17 particles. If the grain density is 1g cm −3 , the total mass is ∼2.4×10 15 g, or ∼0.01% the total mass of 2014OG 392 . If the coma mass is indeed of this scale, 2014OG 392 must be eroding very quickly, undergoing new activity, or the ejecta is accumulating faster than it is escaping. Our measured Afρ of 487±12cm is comparable to other Centaurs active at the same orbital distance as 2014OG 392 : C/2011P2(PANSTARRS) with Afρ=161±4 cm at ∼9au (Mazzotta Epifani et al. 2017), and for 166P(NEAT) Afρ= 288±19 cm at ∼12au (Shi & Ma 2015).
Centaurs are sometimes classified as either gray or red depending on whether the object has a B−R color closer to ∼1.2 or ∼1.7, respectively (see Tegler et al. 2008 andPeixinho et al. 2020 reviews for in-depth discussions). We find our derived B−R color of 1.64±0.4 consistent with the red classification. Notably both molecules we find viable for sublimation are spectrally neutral in visible wavelengths so the reddening agent is as yet unidentified. 2014OG 392 will remain observable through 2020 February and will again be observable beginning around 2020 August. We anticipate imaging and spectroscopy will yield further insight into the nature of these rare objects. We wish to emphasize further lab work is needed to characterize sublimation processes of volatiles under low pressure and temperature regimes. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. We also employed their solar system Object Search (Gwyn et al. 2012). 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 NASA's Astrophysics Data System. This research has made use of the The Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) SkyBoT Virtual Observatory tool (Berthier et al. 2006 This work is based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory (Prop. IDs 2019A-0337, PI: Trilling; 2014B-0404, PI: Schlegel), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.
These results made use of the Discovery Channel Telescope at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. The Large Monolithic Imager was built by Lowell Observatory using funds provided by the National Science Foundation (AST-1005313). This Letter includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Thumbnail Gallery Figure B1 shows six of the archival images in which we originally spotted what appeared to be activity emanating from 2014 OG_392. We obtained confirmation first through DECam observations ( Figure B2); the coma is more readily apparent in the isophotal contours shown in Figure B3. Figure B4 shows two additional observations we took at Magellan provided additional confirmation. Figure B5 shows six images of 2014 OG_392 we captured a the DCT which enabled us to perform color measurement and radial surface brightness profiling.  (1) dashed red arrow points to 2014OG 392 , (2) green arrows highlight the comae if visible, (3) observing date was UT 2019 August 30, (4) filter was VR, and (5) exposure time was 250 s. The apparent decrease in coma prominence was the result of increasing background noise as images were taken into twilight. Figure B3. Isophotal Contours Gallery. Isophotal contours indicate the extent and irregularity of the coma emanating from 2014OG 392 (dashed arrows), especially when contrasted with background objects (yellow arrows) presenting relatively symmetric radial profiles. These two 250 s VR-band exposures were taken at 9:54 (left) and 9:58 (right) during our UT 2019 August 30 follow-up campaign. Figure B4. New Magellan Observations Gallery. 2014OG 392 imaged 2019 December 27 via the Magellan 6.5 m Baade Telescope using the WB4800-7800 filter on the IMACS at Las Campanas Observatory on Cerro Manqui, Chile. The three images reveal an apparent coma (green arrows) emerging from the object (red dashed arrow) and were taken at 300s (left and center) exposures and one 600s exposure (right).