Characterizing the Manx Candidate A/2018 V3
Caroline Piro, Karen J. Meech, Erica Bufanda, Jan T. Kleyna, Jacqueline V. Keane, Olivier Hainaut, Marco Micheli, James Bauer, Larry Denneau, Robert Weryk, Bhuwan C. Bhatt, Devendra K. Sahu, Richard Wainscoat
DDraft version January 19, 2021
Typeset using L A TEX twocolumn style in AASTeX63
Characterizing the Manx Candidate A/2018 V3
Caroline Piro, Karen J. Meech, Erica Bufanda, Jan T. Kleyna, Jacqueline V. Keane, Olivier Hainaut, Marco Micheli,
3, 4
James Bauer, Larry Denneau, Robert Weryk, Bhuwan C. Bhatt, Devendra K. Sahu, andRichard Wainscoat Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei M¨unchen, Germany ESA NEO Coordination Centre, Largo Galileo Galilei, 1, 00044 Frascati (RM), Italy INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy University of Maryland, Dept. of Astronomy, College Park, MD 20742-2421 USA Indian Institute of Astrophys., II Block, Koramangala, Bangalore 560 034, India
Submitted to The Planetary Science JournalABSTRACTManx objects approach the inner solar system on long-period comet (LPC) orbits with the consequenthigh inbound velocities, but unlike comets, Manxes display very little to no activity even near perihe-lion. This suggests that they may have formed in circumstances different from typical LPCs; moreover,this lack of significant activity also renders them difficult to detect at large distances. Thus, analyz-ing their physical properties can help constrain models of solar system formation as well as sharpendetection methods for those classified as NEOs. Here, we focus on the Manx candidate A/2018 V3 aspart of a larger effort to characterize Manxes as a whole. This particular object was observed to beinactive even at its perihelion at q = 1.34 au in 2019 September. Its spectral reflectivity is consistentwith typical organic-rich comet surfaces with colors of g (cid:48) − r (cid:48) = 0 . ± . r (cid:48) − i (cid:48) = 0 . ± .
02, and r (cid:48) − z (cid:48) = 0 . ± .
02, corresponding to a spectral reflectivity slope of 10 . ± . ≈ (cid:46) .
68 g s − ),but not enough to lift optically significant amounts of dust. Finally, we discuss Manxes as a constrainton solar system dynamical models as well as their implications for planetary defense. Keywords: asteroids: individual (A/2018 V3) — comets: general INTRODUCTIONMany models of early solar system dynamics in-clude periods where the giant planets may have shiftedabout radially before settling into their present posi-tions (Gomes et al. 2005; Walsh et al. 2011; Raymond& Izidoro 2017). In the process, small solar system ob-jects were scattered from their original locations. How-ever, the models vary in both the magnitude and timingof planetary migration necessary to result in the solarsystem arrangement we see today. In addition, dynam-
Corresponding author: Karen J. [email protected] ical formation models disagree on how much materialmay have originally been present in the inner solar sys-tem and what percentage may have been ejected to theOort cloud during the periods of giant planet migration(Raymond & Izidoro 2017).Manx objects may help us distinguish among these so-lar system models. Manxes are small solar system bodiesthat blur the line between asteroids and comets. Theseobjects follow long-period comet (LPC) orbits but ex-hibit very little, if any, of the cometary activity expectedfrom an LPC. The term “Manx” was coined by Meechet al. (2014) after the breed of tailless cat to describethe comet C/2013 P2 (PANSTARRS). Despite a typi-cal LPC orbit, C/2013 P2 displayed orders of magnitudeless activity than typical LPCs, much less than even low- a r X i v : . [ a s t r o - ph . E P ] J a n Piro et al. activity short period comets. Even near its perihelionat 2.83 au, C/2013 P2 exhibited only a minimal tail. Anumber of other low-activity objects on LPC orbits havesince been observed (Stephens et al. 2017; Bufanda et al.2020) or identified as such in literature searches (Weiss-man & Levison 1997; Sekiguchi et al. 2018).Most intriguingly, the Manx object C/2014 S3(PANSTARRS) was observed to have a spectral reflec-tivity with the 1 . µ m absorption typical of inner so-lar system rocky asteroids and inconsistent with an in-active comet (Meech et al. 2016). The stony S-typeasteroids dominate the population of the inner aster-oid belt (Gaffey et al. 1993; Raymond & Izidoro 2017).This surface type had never been seen on objects on anLPC orbit, although dynamical models predict that in-ner solar system material can be scattered to the Oortcloud. Even more striking, the minerals that form the1 . µ m absorption characteristic of S-types cannot formin the presence of water, yet the activity seen in bothC/2013 P2 and C/2014 S3 was consistent with the pres-ence of water ice sublimation (Meech et al. 2016). Itis hoped that the fraction of Manx objects that displayS-type spectral reflectivity, among a sample of Manxes,can be extrapolated to the Oort cloud as a whole andthus help constrain the models of early solar system dy-namics (Meech et al. 2016; Shannon et al. 2019).We have studied a Manx discovered by the PanoramicSurvey Telescope and Rapid Response System (Pan-STARRS2) on Haleakal¯a, Maui (Hoegner et al. 2018).This object, later designated A/2018 V3, was first de-tected at r = 3 .
99 au on 2018 November 2 with g -bandmagnitude of 21.9, suggesting a km-scale nucleus. Fur-ther observations over the course of the month alloweda good orbit to be determined. It is on a long-periodcomet orbit ( a = 121 .
59 au; e = 0 . q = 1 .
34 au; Q = 241 . i = 165 ◦ ; P = 1341 yr at the time ofperihelion ), but no activity was observed even at dis-tances where significant activity would be expected for atypical LPC. Thus, under the criteria set out by Meechet al. (2016), A/2018 V3 currently qualifies as a Manxobject.With a perihelion of q = 1 .
34 au, A/2018 V3 falls justoutside the definition of a Near-Earth Object (NEO).NEOs are typically defined as any comet or asteroidwith a perihelion q < . , but this particular ob-ject piqued our interest when it passed perihelion andremained inactive. LPCs can pose significant hazards to JPL Small-Body Database Browser: https://ssd.jpl.nasa.gov/sbdb.cgi NASA/JPL/CNEOS definitions of NEO groups: https://cneos.jpl.nasa.gov/about/neo groups.html
Earth because of their high velocity (Meech et al. 2017);Manx objects pose an even greater hazard. Their lack ofactivity, difficulty of detection, and LPC orbits could allresult in very little warning time for a possible impact(Chodas 1996; Weissman 1997; Nuth et al. 2018).A/2018 V3 passed perihelion in 2019 September, butgiven the nature of Manxes, there are likely many othersuch inconspicuous objects traveling through the innersolar system. Characterizing A/2018 V3 will allow usto investigate the role of Manxes in the overall contextof solar system dynamics and will sharpen our abilityto detect more of these small, dark solar system objectsthat still carry sufficient kinetic energy to inflict signifi-cant impact damage. OBSERVATIONS AND DATA REDUCTIONPhotometry for A/2018 V3 was obtained using datafrom the telescopes shown in Table 1; a complete list ofthe observing geometry is presented in Table 3. All ofthe imaging data except for those from PS1 and CFHTMegacam were flattened in a standard manner using ourimage reduction pipeline. Our photometric calibrationaccesses the Pan-STARRS (Magnier et al. 2013), SDSS(Fukugita et al. 1996; Doi et al. 2010), and Gaia2 (Jordiet al. 2010) databases to provide a photometric zero-point for each image using published color correctionsto translate photometric bands. The pipeline identifiesimage files by their instrument and generalizes accessto their widely varying metadata. The image reductioncomponent bias-subtracts and flattens the data, appliesthe Terapix tool SCAMP (Bertin 2006) to fit a preciseWorld Coordinate System to the frame, and finally usesSExtractor (Bertin & Arnouts 1996) to produce multi-aperture and automatic aperture target photometry.
Table 1.
ObservationsTelescope/Instrument Gain RN (cid:48)(cid:48) /pix Nts † Gemini/GMOS 2.27 3.32 0.161 4 23CFHT/MegaCam 1.634 3.00 0.185 11 32Pan-STARRS1/GPC1 1.26 7.46 0.260 4 11Pan-STARRS2/GPC2 1.0 7.46 0.257 9 38HCT/HFOSC 2 0.254 5.80 0.180 1 8ATLAS/STA-1600 2.0 11.0 1.86 26 ‡ MPC ... ... ... 14 ‡ Note — † Number of CCD images; ‡ Received photometry (cal-ibrated from ATLAS, uncalibrated from the MPC).
ASTEX Manx Candidate A/2018 V3
Gemini North 8m Telescope
We obtained 23 images from the Gemini North tele-scope taken with the Gemini Multi-Object Spectro-graph (GMOS) (Hook et al. 2004), a mosaic of three2048 × ×
2. Thedata were obtained through SDSS filters using queueservice observing and were processed to remove the in-strumental signature using DRAGONS, Gemini Obser-vatory’s Python-based data reduction software (Labrieet al. 2019). Values in regions outside of the mosaic ofdetectors were manually converted to “Not a Number”(NaN) as necessary so that these areas were not countedas sky background. For nights where the outer chipswere non-photometric due to guide probe vignetting, weextracted only the central chip of the mosaic so that thezeropoint calibration would not be affected.Data were obtained over four days: 2019 September22; 2020 January 23 and 25; and 2020 July 22. Weperformed manual aperture photometry for each imageby first utilizing a curve of growth to capture 99.5%of the light for field stars with a similar brightness tothe object. We then measured the object through arange of apertures at 1-pixel step sizes and corrected thephotometry to a 5 (cid:48)(cid:48) -radius aperture using the curve ofgrowth. This minimized the sky-dominated backgroundnoise for the object in each image.2.2.
Canada-France-Hawai‘i Telescope (CFHT)
We obtained an additional 54 images using theCFHT MegaCam wide-field imager, an array of forty2048 × . (cid:48)(cid:48)
185 perpixel and a 1.1 square degree FOV. These images wereobtained through queue service observing and covered14 days spanning 2018 December through 2020 July.Images were taken through the SDSS r (cid:48) filter ( λ eff =0 . µ m, ∆ λ = 0 . µ m) and CFHT’s wideband gri filter ( λ eff = 0 . µ m, ∆ λ = 0 . µ m). The data werepre-processed through CFHT’s Elixir pipeline (Magnier& Cuillandre 2004) to remove the instrumental signa-ture and then further processed through the pipelinedescribed at the beginning of Section 2 for astrometricand photometric calibrations.We manually inspected each image for issues. Of the54 images we received, 22 were not usable since the ob-ject fell too close to other sources in the frame. The 32images analyzed here consisted of 18 taken through the r (cid:48) -band filter and 14 with the gri filter.2.3. Pan-STARRS1 (PS1) and 2 (PS2)
We received images and photometry data from Pan-STARRS1 (2019 January through 2020 April) and Pan-STARRS2 (2018 November to 2020 May). Images were taken through the Pan-STARRS broadband w filter( λ eff = 0 . µ m, ∆ λ = 0 . µ m), covering their g , r , and i bandpasses. All Pan-STARRS data were re-duced by the Pan-STARRS Image Processing Pipeline(IPP; Magnier et al. 2020) and calibrated against thePan-STARRS database (Flewelling et al. 2020).2.4. Asteroid Terrestrial-impact Last Alert System(ATLAS)
ATLAS provided us with 82 data points of photome-try for A/2018 V3. The data covered 2019 June through2019 September and were taken through their broad-band cyan ( λ eff ,c = 0 . µ m, ∆ λ = 0 . µ m) andorange ( λ eff ,o = 0 . µ m, ∆ λ = 0 . µ m) filters. Fornights where multiple data points were taken withina short observing interval ( (cid:46)
30 minutes), we used aweighted average of the recorded magnitudes due to lowsignal-to-noise and large scatter. These ATLAS data,averaged by date, are listed in Table 3.2.5.
Himalayan Chandra Telescope
We obtained eight images taken on 2020 February 26from the 2.01 m Himalayan Chandra Telescope (HCT)at Mt. Saraswati, Hanle, India. The images were takenwith the Himalaya Faint Object Spectrograph and Cam-era (HFOSC) and the new 4k ×
4k e2V CCD with theBessell/Cousins filter system. These were then pro-cessed through the pipeline described at the beginningof Section 2. We performed manual aperture photome-try for each image and used the weighted average of alldata points taken for the night.2.6.
NEOWISE
We searched the Canadian Astronomy Data Centre’s(Gwyn et al. 2012) archive using their Solar System Ob-ject Image Search (SSOIS) tool to search for data fromthe NEOWISE survey (Mainzer et al. 2014). A total of150 images of A/2018 V3 were found during four vis-its. The first visit, 2010 March 21-24, was during thecryogenic mission and the W1-W4 bands were available.Without any cryogens after the first visit, only the twoshort-wavelength channels at 3 . µ m (W1) and 4 . µ m(W2) were available for the subsequent visits.During visits on 2010 October 5-6, 2014 March 18-19, and 2014 October 1-2, A/2018 V3 was at heliocen-tric distances, r = 22 .
43, 15 .
98, and 14 .
85 au, respec-tively. Because of the large distance and the inability ofWISE to detect small objects at these distances, deter-mining a meaningful upper limit to the size would notbe possible from these observations and the data werenot reduced. However, there were two additional datapoints taken at much smaller heliocentric distances pre-perihelion, on 2019 January 2 ( r = 3 .
49 au, ∆ = 3 . Piro et al. au, TA = − . ◦ ), and on 2019 July 20 ( r = 1 .
53 au,∆ = 1 .
16 au, TA = − . ◦ ).2.7. Other Data Sources
We also gathered photometry data for A/2018 V3from the Minor Planet Center’s (MPC) database of ob-servations. R-band data came from nine different obser-vatories and covered 14 days spanning 2018 Novemberto 2020 April. We averaged the reported magnitudes byobservatory and by date and have assumed an averageerror on each measurement of ± .
25. These are includedin the heliocentric light curve (see Figure 4) and the ta-ble of observational geometry (see Table 3). However,the data only included the observed magnitude and fil-ter, with no information on error, filter system, or pho-tometry aperture. For this reason, the MPC data wereused only in constructing the heliocentric light curve,and not in the calculations and analysis. ANALYSIS AND RESULTS3.1.
Classification and Taxonomy
Nucleus Colors
Comparing the color of A/2018 V3 with those of othercomets and asteroids allowed us to infer some of its sur-face properties. Colors were calculated using the 5 (cid:48)(cid:48) -radius aperture magnitudes from the 2020 January 23and 25 Gemini data taken through SDSS g (cid:48) , r (cid:48) , i (cid:48) , and z (cid:48) filters. These are shown in Table 2. The ( g (cid:48) − r (cid:48) )value from 2020 January 25 was then used to transformthe data taken using other filter systems. Magnitudesconverted to SDSS r (cid:48) from other filters are denoted as¯ r (cid:48) .To convert Pan-STARRS w P filter magnitudes to ¯ r (cid:48) ,we used the transformation given by Tonry et al. (2012):¯ r (cid:48) = w p − . − . g (cid:48) − r (cid:48) ) (1) σ r (cid:48) = σ w p + ( − . σ ( g (cid:48) − r (cid:48) ) ) + 0 . (2)Transformations from the ATLAS filter system mag-nitudes to SDSS r (cid:48) are found in Tonry et al. (2018).Equations 3 and 4 refer to the ATLAS c -band values,while Equations 5 and 6 deal with the o -band magni-tudes. Again, the ( g (cid:48) − r (cid:48) ) value calculated from the2020 January 25 Gemini data was used in the transfor-mation. ¯ r (cid:48) = c − . g (cid:48) − r (cid:48) ) (3) σ r (cid:48) = σ c + (0 . σ ( g (cid:48) − r (cid:48) ) ) + 0 . (4)¯ r (cid:48) = o + 0 . g (cid:48) − r (cid:48) ) (5) σ r (cid:48) = σ o + (0 . σ ( g (cid:48) − r (cid:48) ) ) + 0 . (6)To convert the CFHT gri magnitudes to ¯ r (cid:48) , we used thefollowing transformation from CADC (2019) along withthe ( g (cid:48) − i (cid:48) ) color calculated from the 2020 January 25Gemini data.¯ r (cid:48) = gri + 0 . − . g (cid:48) − i (cid:48) ) + 0 . g (cid:48) − i (cid:48) ) (7) σ r (cid:48) = σ gri + (0 . σ ( g (cid:48) − i (cid:48) ) ) +(0 . g (cid:48) − i (cid:48) ) σ ( g (cid:48) − i (cid:48) ) ) (8)Finally, we used the following transformation given byLupton (2005) to convert the Bessell/Cousins magni-tudes to ¯ r (cid:48) .¯ r (cid:48) = R + (0 . g (cid:48) − r (cid:48) )) + 0 . σ r (cid:48) = σ R + (0 . σ ( g (cid:48) − r (cid:48) ) ) + 0 . (10) Table 2.
A/2018 V3 colors2020 Jan 23 2020 Jan 25( g (cid:48) − r (cid:48) ) 0 . ± .
031 0 . ± . g (cid:48) − i (cid:48) ) 0 . ± .
031 0 . ± . r (cid:48) − i (cid:48) ) 0 . ± .
028 0 . ± . r (cid:48) − z (cid:48) ) 0 . ± .
025 0 . ± . i (cid:48) − z (cid:48) ) 0 . ± .
026 0 . ± . Spectral Reflectivity
We computed the spectral reflectivity, R λ , at eachwavelength normalized to the g (cid:48) filter using the colorvalues calculated above along with the following equa-tions R λ = 10 − . m λ − M λ (cid:12) ) − . m ref − M ref (cid:12) ) (11) σ Rλ = 0 . R λ (cid:2) σ λ + σ + σ λ (cid:12) + σ (cid:12) (cid:3) (12)Here, m λ is the object’s magnitude in a specific filterwith uncertainty σ λ . M λ (cid:12) is the Sun’s absolute mag-nitude for the same filter with σ λ (cid:12) uncertainty. In thereference bandpass, m ref is the object’s magnitude and σ ref its uncertainty, while M ref (cid:12) and σ ref (cid:12) are the solarmagnitude and error in the reference filter.For the solar magnitudes in the SDSS filters we used g (cid:48)(cid:12) = 5 . ± . r (cid:48)(cid:12) = 4 . ± . i (cid:48)(cid:12) = 4 . ± . ASTEX Manx Candidate A/2018 V3 z (cid:48)(cid:12) = 4 . ± . . We then used the ( g (cid:48) − r (cid:48) )color index calculated from the 2020 January 25 GeminiNorth data to normalize the spectral reflectivities to λ =0 . µ m. Spectral reflectivity values across the twodays for which we had g (cid:48) r (cid:48) i (cid:48) z (cid:48) magnitudes are shown inFigure 1 and are consistent with the organic-rich redsurface reflectivities of comets (Li et al. 2013; Kelleyet al. 2017).The normalized reflectivity slope S (cid:48) for this object wascalculated from the averaged reflectivity values for thetwo days of Gemini data. We used the method fromJewitt & Meech (1986) S (cid:48) (cid:20) %100nm (cid:21) = (cid:18) λ (cid:19) . m − . m + 1 (13) σ S (cid:48) = σ ∆ m (cid:34) . · . m ∆ λ (10 . m + 1) (cid:35) (14)where ∆ m represents the difference between the object’scolor and the Sun’s color for the bandpass ∆ λ , to cal-culate a gradient of S (cid:48) = 10 . ± . S (cid:48) values for asteroidalD spectral types (Hartmann et al. 1987; Fitzsimmonset al. 1994; Hicks et al. 2000; Bus & Binzel 2002). N o r m a li z e d Re f l e c t i v i t y Wavelength [microns]
TNOs
Comets1/23/20201/25/2020C-typeD-type
S-type
Figure 1.
The spectral reflectivity as calculated from Gem-ini data is consistent with the red color of comets and the Dspectral type. Data normalized to 1.0 at λ = 0 . µ m. Searching for Activity
Surface Brightness Analysis Dust Production Limits
We compared stellar surface brightness profiles withthat of A/2018 V3 in deep stacked images to estimate upper limits on any dust production that could be pro-duced by undetected ice-sublimation (Meech & Weaver1996). The analysis was based on the heliocentric andgeocentric locations of the object and assumed an aver-age cometary dust grain size of 1 µ m (Richter & Keller1995; H¨orz et al. 2006; Bentley et al. 2016; Levasseur-Regourd et al. 2018). A/2018 V3 Reference star J u l y J u l y NE -v - ⨀ -v - ⨀ Figure 2.
Composite stacked images using CFHT frames;each image is 20 (cid:48)(cid:48) × (cid:48)(cid:48) . 5 (cid:48)(cid:48) ≈ (cid:48)(cid:48) ≈ ,
000 km at a geocentricdistance of 4.32 au in 2020. The image of A/2018 V3 is elon-gated E-W on 2020 July 13, roughly in the direction that atail would be expected.
We used the IRAF (Tody 1986) software to stack 4frames taken 2019 July 7 with CFHT’s MegaCam. Theframes were taken through r (cid:48) -band filters with exposuretimes of 90 seconds each. A/2018 V3 was at a heliocen-tric distance of 1.62 au and moving inbound toward theSun. The resulting image, seen in Figure 2, shows theobject as a point source with no visibly-discernible activ-ity, with a star from the same frame displayed alongsideas reference.We computed an azimuthally-averaged radial surfacebrightness profile for both the object and each of threefield stars of comparable brightness. The field star fluxeswere normalized and averaged at each radial distance toform an average stellar profile for comparison with theprofile of A/2018 V3. The resulting surface brightnessprofiles for the object and for the average of the threefield stars are shown in Figure 3A. No activity was ap-parent in either the composite stacked image or in a Piro et al. difference between the surface brightness profiles. Fromthis, we calculated an average upper limit for dust pro-duction of ≈ − at a distance of 1.2 (cid:48)(cid:48) from the ob-ject’s core, using the method described in Meech et al.(2003). The full curve for the calculated dust productionlimit is shown as the dashed line in Figure 3A. [A][B] Figure 3. [A] A/2018 V3’s pre-perihelion (TA = − . ◦ )surface brightness profile is indistinguishable from that ofstars, which is consistent with a point source showing no dustactivity. [B] Post-perihelion (TA = 110 . ◦ ), however, thesurface brightness profile of A/2018 V3 is broader, consistentwith the extension seen in Figure 2. The arrow indicatespossible low-level dust production for the object near 0.9 (cid:48)(cid:48) . We created a second image stack of 3 frames fromCFHT data taken 2020 July 13 using the gri filter. In-dividual frames showed a slight east-west elongation inthe object, whereas stellar profiles in the same framewere round, as seen in the lower row of Figure 2. Theobject was moving north to south across the telescope’sfield of view and as such, significant east-west trailingwould have been unlikely. Figure 3B shows the surfacebrightness comparison for the 2020 July 13 data. Here, the object’s radial profile is compared against the aver-aged profile for nine field stars.The small bump in the object’s brightness profile (in-dicated by the arrow in Figure 3B) at 0 . (cid:48)(cid:48) likely cor-responds to the small east-west elongation noted in thedeep image stack. However, since both brightness pro-files fall within their respective margins of error, we can-not say with certainty that this indicates activity.We calculated an upper limit on dust production of ≈ − at a distance of 1 . (cid:48)(cid:48) from the object’s core,again using the method from Meech et al. (2003). Thisis indicated by the dashed line in Figure 3B. Using thisupper limit on dust production, we calculated an activefractional area of 7 . × − for a spherical nucleus withaverage radius r ≈ § that could be active.If A/2018 V3 was indeed active and sublimatingaround 2020 July 13, this amount of activity was notsufficient to unambiguously lift optically significant dustgrains. The lack of significant deviation from either lightcurve in Figure 4 (discussed in § Sublimation Model
We constructed a heliocentric light curve (Figure 4)from the SDSS r (cid:48) magnitudes as a function of the ob-ject’s true anomaly. This served as the basis for under-standing whether or not A/2018 V3 was active, particu-larly around perihelion and immediately following. Wealso used the light curve to find any data points thatdeviated significantly from the trend. We conducteda careful examination of the corresponding images torule out contamination from background objects, cos-mic rays, or bad pixels. The curves fit to the data pointswere determined using the surface ice sublimation modelas in Meech et al. (1986).The sublimation model enabled us to explore the lightcurve and to search for and quantify any possible activ-ity. This model has been used successfully to explainthe behavior of comets where we do not have a lot ofdetailed information, and has proven very successful inpredicting and explaining the behavior of well-observedmission targets (Meech et al. 2011; Snodgrass et al. 2013;Meech et al. 2017). The sublimation model computesthe amount of gas sublimating from an icy surface ex-posed to solar heating, as described in detail in Meechet al. (2017). This escaping gas flow can drag dustgrains from the nucleus and thereby modify the observedbrightness of the object. By analyzing the shape of theheliocentric light curve, the model can distinguish be- ASTEX Manx Candidate A/2018 V3 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 H e li o c e n t r i c d i s t a n c e [ a u ] r ' m ag n i t u d e [ " ap e r t u r e ] True anomaly [degrees]
CFHTGemini ‒ NATLASPan ‒ STARRSHCTMPCinactive nucleusslight activityHeliocentric distance ±0.029±0.015±0.054 ±0.051 ±0.038±0.250
Figure 4.
This heliocentric light curve shows that our observations are consistent with only a very weakly active nucleus. Thepeak brightness near TA = − ◦ coincides with the object’s closest geocentric distance. The average errors for each data setare indicated in the legend. tween H O, CO, and CO driven activity based on thetotal brightness within a fixed aperture. This brightnessincludes radiation scattered from both the nucleus andany of the escaping dust grains. Model free parame-ters include nucleus properties (radius, density, albedo,emissivity), dust properties (grain size, density, phasefunction), and the fractional active area.The light curves in Figure 4 show the results oftwo model runs based on the 2020 July 13 data: oneassuming no activity (i.e. representing the bare nu-cleus), and the other consistent with the dust productiondetermined from the surface brightness analysis (Sec-tion 3.2.1). Both models assumed an average cometaryalbedo of 0.04 (Li et al. 2013) an average dust grain sizeof 1 µ m (Levasseur-Regourd et al. 2018), and a dust togas mass ratio of 1 (Marschall et al. 2020). By using thesublimation and dust models in tandem, we graduallyadjusted the free parameters of each until the resultingmodel curves visually matched the data. We then usedleast-squares fitting to calculate an average nucleus ra-dius of ≈ ≈ . × − . Thecurve fit to this weakly-active nucleus implied a waterproduction rate of 2 . × molec/s.3.3. NEOWISE Size Estimate
We also used the
NEOWISE data to determine theeffective radius of A/2018 V3. As mentioned in Sec- tion 2.6, images were available only in the W1 andW2 bands. The WISE data have all been processedthrough the WISE science data pipeline (Wright et al.2010) to bias-subtract and flatten the images, and toremove artifacts. The images are then stacked using thecomet’s apparent rate of motion (Bauer et al. 2015).Aperture photometry is converted to fluxes using the
WISE zeropoints and appropriate color temperaturecorrections (Wright et al. 2010). These corrections aretemperature-dependent, and an initial temperature esti-mate is required based on the expected blackbody tem-perature for the heliocentric distance of the observation.The data from 2019 January 2 show no signal in eitherband down to a 1- σ magnitude of 18.4, which yields a3- σ upper limit of 12 km for the nucleus radius (which isunconstraining). However, detections were identified indata from 8 exposures taken on 2019 July 20, centered at01:23:30.585 UT (Figure 5). A/2018 V3 was clearly notactive at the time of these observations as shown by theprofile comparison in Figure 6. Using the NEATM ther-mal model (Harris 1998; Masiero et al. 2017; Mainzeret al. 2019) along with the observed fluxes, we deriveda radius of 2 . ± . ∼ . +0 . − . , consistent with the derived value used inSection 3.2.2. Piro et al. NE a r c m i n a r c m i n Figure 5.
The W1 and W2 co-added images from 2019January 2 and July 20. There is no significant signal ineither of the January frames, but the July frames clearly bothshow high signal-to-noise, with the W2 image showing thegreatest. The images show no evidence of extended emission.
AK18V030 W2 B r i gh t ne ss objectPSF
0 10 20 30 40
Aperture (arcsec) Re l a t i v e B r i g h t n e ss Figure 6.
The NEOWISE W2 profile of A/2018 V3 relativeto the instrumental profile in the same bandpass. Both pro-files have been normalized to the peak counts. The profilesare nearly a perfect match, indicating no coma detection.4.
DISCUSSIONFigure 7 shows how the large survey projects such asPan-STARRS and the Catalina Sky Survey have dra-matically increased the number of known small solarsystem bodies in the last 20 years. These surveys haveenabled detections of an increasing percentage of ever-smaller solar system bodies in Earth’s neighborhood.No longer is there such a heavy sampling bias towardslarge, bright objects; but this also means that there must be many more undetected, low-albedo objects in near-Earth space.
Discovery year H e li o c e n t r i c d i s t a n c e [ a u ] Catalina Sky Survey
All othersManx objects
Figure 7.
The advent of large all-sky surveys in the mid-1990s increased both the discovery rate and the distanceat which comets are discovered. Because Manx comets arefainter and harder to detect than LPCs, most are discoveredonly within ∼ Both Mainzer et al. (2011) and Harris & D’Abramo(2015) have estimated that about 90% of near-Earth as-teroids (NEAs) larger than one kilometer in diameterhave been discovered. However, the latter study alsonoted the inherent bias against detecting NEAs withhigh eccentricities and long orbital periods. Manxesand LPCs have just such orbital parameters; thus, long-period objects that also qualify as NEAs are likely un-derrepresented in asteroid surveys.Such high-eccentricity orbits also imbue Manxes andLPCs with much higher velocities near perihelion thanthe usual near-Earth object. At its closest geocentricapproach (0.37 au), A/2018 V3 was moving at 64 km/srelative to Earth ; the average NEO velocity is approxi-mately 21 km/s relative to Earth (Shannon et al. 2015).A/2018 V3 has passed both Earth and the Sun withoutincident and will not return to the inner solar system foranother 1300 years, but given that Manxes are likely un-derrepresented in surveys, the threat from long-periodobjects may be higher than previously believed.Figure 7 demonstrates that most of the Manx objectsdiscovered thus far have only been detected within ∼ JPL Small-Body Database Browser: https://ssd.jpl.nasa.gov/sbdb.cgi
ASTEX Manx Candidate A/2018 V3
Piro et al.
This research has also made use of data and/orservices provided by the International AstronomicalUnion’s Minor Planet Center.
Table 3 . Observing Geometry and PhotometryUT Date JD a r b ∆ b α b TA c Filt ± σ d r mag (cid:48) ± σ d Gemini North data (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . CFHT data (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . (cid:48) . ± . . ± .
015 20 . ± . (cid:48) . ± . (cid:48) . ± . . ± .
034 21 . ± . . ± .
030 21 . ± . . ± .
055 21 . ± . Pan-STARRS1 data p . ± .
086 20 . ± . p . ± .
014 17 . ± . p . ± .
046 20 . ± . p . ± .
087 20 . ± . Pan-STARRS2 data p . ± .
052 20 . ± . p . ± .
004 17 . ± . p . ± .
006 16 . ± . p . ± .
051 19 . ± . p . ± .
063 20 . ± . p . ± .
067 20 . ± . Table 3 continued
ASTEX Manx Candidate A/2018 V3 Table 3 (continued)
UT Date JD a r b ∆ b α b TA c Filt ± σ d r mag (cid:48) ± σ d p . ± .
034 20 . ± . p . ± .
060 20 . ± . p . ± .
055 20 . ± . HCT data B . ± .
037 20 . ± . ATLAS data . ± .
093 18 . ± . . ± .
112 18 . ± . . ± .
088 18 . ± . . ± .
161 18 . ± . . ± .
144 18 . ± . . ± .
033 18 . ± . . ± .
033 17 . ± . . ± .
034 18 . ± . . ± .
031 17 . ± . . ± .
015 17 . ± . . ± .
015 16 . ± . . ± .
010 16 . ± . . ± .
010 16 . ± . . ± .
010 15 . ± . . ± .
007 14 . ± . . ± .
006 15 . ± . . ± .
010 15 . ± . . ± .
009 15 . ± . . ± .
014 16 . ± . . ± .
050 16 . ± . . ± .
110 17 . ± . . ± .
028 17 . ± . . ± .
094 18 . ± . . ± .
050 18 . ± . . ± .
066 18 . ± . . ± .
142 18 . ± . MPC data † Table 3 continued Piro et al.
Table 3 (continued)
UT Date JD a r b ∆ b α b TA c Filt ± σ d r mag (cid:48) ± σ d a Julian Date -2450000.0 b Heliocentric, geocentric distance [au]; and phase angle [deg] c True anomaly [deg], the position along orbit; TA at perihelion = 0 ◦ d Magnitude and error through 5 (cid:48)(cid:48) radius aperture; and converted to SDSS r (cid:48) as described in the text
Note — † MPC data did not include error; assumed ± .
25 error for light curve calculations
REFERENCES