The Reflectance of Cold Classical Trans-Neptunian Objects in the Nearest Infrared
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The Reflectance of Cold Classical Trans-Neptunian Objects in the Nearest Infrared
Tom Seccull,
1, 2
Wesley C. Fraser, and Thomas H. Puzia Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA Astrophysics Research Centre, Queen’s University Belfast, University Road, Belfast, BT7 1NN, UK Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada Institute of Astrophysics, Pontificia Universidad Cat´olica de Chile, Av. Vicu˜na MacKenna 4860, 7820436, Santiago, Chile (Received 2020 Nov 8; Revised 2021 Jan 22; Accepted 2021 Feb 6)
Submitted to PSJABSTRACTRecent photometric surveys of Trans-Neptunian Objects (TNOs) have revealed that the cold classicalTNOs have distinct z-band color characteristics, and occupy their own distinct surface class. Thissuggested the presence of an absorption band in the reflectance spectra of cold classicals at λ > . µ m.Here we present reflectance spectra spanning 0 . − . µ m for six TNOs occupying dynamically coldorbits at a ∼
44 au. Five of our spectra show a clear and broadly consistent reduction in spectralgradient above 0 . µ m that diverges from their linear red optical continuum and agrees with theirreported photometric colour data. Despite predictions, we find no evidence that the spectral flatteningis caused by an absorption band centered near 1 . µ m. We predict that the overall consistent shapeof these five spectra is related to the presence of similar refractory organics on each of their surfaces,and/or their similar physical surface properties such as porosity or grain size distribution. The observedconsistency of the reflectance spectra of these five targets aligns with predictions that the cold classicalsshare a common history in terms of formation and surface evolution. Our sixth target, which has beenambiguously classified as either a hot or cold classical at various points in the past, has a spectrumwhich remains nearly linear across the full range observed. This suggests that this TNO is a hot classicalinterloper in the cold classical dynamical range, and supports the idea that other such interlopers maybe identifiable by their linear reflectance spectra in the range 0 . − . µ m. Keywords:
Classical Kuiper belt objects, Trans-Neptunian Objects, spectroscopy INTRODUCTIONThe cold classical Trans-Neptunian Objects (TNOs)are minor planets on non-resonant, non-scattering, lowinclination ( i (cid:46) ◦ ), and low eccentricity ( e < .
24) he-liocentric orbits with semimajor axes between 40 and50 au (Gladman et al. 2008; Petit et al. 2011; Bannisteret al. 2018). The cold classicals are distinct from thedynamically excited (hot) TNOs in a number of ways,including their lack of dynamical excitation; their higherfraction of surviving, and in particular widely separated,binary pairs (Stephens & Noll 2006; Noll et al. 2008;Parker & Kavelaars 2010); their distinct size distribu-
Corresponding author: Tom [email protected] tion and its lack of large objects (
Diameter (cid:38)
400 km;e.g. Fraser et al. 2014; M¨uller et al. 2020); and their uni-modally (rather than bimodally) distributed colors andalbedos, which on average respectively sit at the very redand high ends of the color and albedo distributions ofTNOs as a whole (Tegler et al. 2003; Brucker et al. 2009;Fraser & Brown 2012; Lacerda et al. 2014; Vilenius et al.2014; Pike et al. 2017; Terai et al. 2018; Schwamb et al.2019). Notably the cold classical TNOs are typically notquite as red as the very reddest TNOs such as 225088Gonggong (2007 OR ; Brown et al. 2011). Taken to-gether the distinct properties of the cold classicals sup-port the prediction that they formed and evolved in-situ,separately from the other TNO populations, and thatthey emerged mostly unscathed from the period of giantplanet migration thought to have emplaced the dynam- a r X i v : . [ a s t r o - ph . E P ] F e b Seccull, Fraser, & Puzia ically hot TNOs into the modern trans-Neptunian Belt(Levison et al. 2008; Parker & Kavelaars 2010; Batyginet al. 2011; Wolff et al. 2012; Nesvorn´y 2015).After setting aside any interlopers sharing their or-bital parameter space (such as the blue binaries; Fraseret al. 2017), the cold classical TNOs consitute the bestpreserved samples of material derived from the outer-most reaches of the Solar protoplanetary disk at the timeof planet formation. They have been spared significantsurface evolution from most of the common sources thataffect minor planets. The delivery of collisional and ac-cretional heat to the cold classicals is limited by therelatively low frequency and velocities of the collisionsthey experience (Dell’Oro et al. 2012; Fraser et al. 2014;Greenstreet et al. 2019; Spencer et al. 2020). TNOs assmall as most cold classicals (
D <
400 km) tend to havelow densities (e.g. Grundy et al. 2007; Mommert et al.2012; Santos-Sanz et al. 2012; Vilenius et al. 2012, 2014;Lellouch et al. 2013; Kovalenko et al. 2017), suggest-ing that they may have a low rock/ice ratio, and con-sequently a lower capacity for internal radiogenic heat-ing (e.g. Coradini et al. 2008). At large heliocentricdistances of 40 −
50 au weak insolation doesn’t drivesignificant thermal evolution of planetary surfaces (e.g.Guilbert-Lepoutre et al. 2011). By residing at these dis-tances since their formation (Parker & Kavelaars 2010;Morbidelli & Nesvorn´y 2020), the cold classicals are alsopredicted to have received relatively low doses of pho-tonic and ionic radiation from both the Sun and galacticsources (Cooper et al. 2003, 2006; Hudson et al. 2008).Due to the relatively primitive nature of the materialson their surfaces, the cold classical TNOs are enticingtargets for spectroscopic study, and may provide one ofthe most direct observational routes to measurement ofthe chemical and thermal conditions that prevailed inthe outer protoplanetary disk of the young Sun. Due totheir faintness, however, they also prove to be challeng-ing observational targets. To date only four TNOs thathave been solidly classified as cold classicals have beenobserved spectroscopically (see Table 1).Within the signal-to-noise (S/N) of the observationaldata and the wavelength ranges covered so far, the re-flectance spectra of cold classicals are almost completelyfeatureless. In the optical range they show only contin-uum with a strong positive gradient, while in the near-infrared (NIR) they appear neutrally reflective or veryslighly blue (Grundy et al. 2005, 2020; Barkume et al.2008). The only absorption bands confidently identifiedin the reflectance spectrum of a cold classical were ob-served at 2 . µ m and 2 . µ m in the NIR spectrumof Arrokoth by the NASA New Horizons probe; thesebands were attributed to surface methanol ice (Grundy et al. 2020). The red optical color of the cold classicals istypically attributed to the presence of complex macro-molecular organic residues on their surfaces (e.g. DalleOre et al. 2015). Such materials are predicted to be pri-marily comprised of a highly diverse agglomeration ofcomplex hydrocarbon molecules, and are known to beproduced through radiolytic and/or photolytic process-ing of the simple molecular ices from which the TNOsare thought to have formed (e.g. Khare et al. 1989;Cruikshank et al. 2005; Brunetto et al. 2006; Matereseet al. 2014, 2015). The featureless nature of the op-tical and NIR reflectance spectra of TNOs has so farprecluded robust quantitative chemical analysis of theirsurfaces, however.Recently the reflectance properties of the cold classi-cals were reported to be distinct from those of the dy-namically hot TNOs based on photometric colors ob-tained by both the Colors of the Outer Solar SystemOrigins Survey (Col-OSSOS; Pike et al. 2017) and anindependent study conducted by Terai et al. (2018).From the colors measured by both teams, the dynami-cally hot TNOs were inferred to have average reflectancespectra with only a small decrease in gradient acrossthe range 0 . − . µ m, if any at all. By constrast,cold classicals were found to occupy a unique space in g − r / r − z color space, implying a reduced spectral gra-dient at λ ∼ . µ m compared to that across the samewavelength region for the dynamically hot TNOs withsimilar optical color (Pike et al. 2017; Terai et al. 2018).The distinct spectroscopic behaviour of the cold clas-sicals may set them apart from dynamically hot TNOswith similar optical redness in terms of composition; adistinction like this warrants spectroscopic investigation.With the broad aim of spectroscopically confirming andcharacterising the distinct spectral behaviour of the coldclassical TNOs at λ > . µ m, we set out to obtain asample of their reflectance spectra. TARGET SELECTIONOur original sample was comprised of nine targets.505476 (2013 UL ) and 511552 (2014 UE ) were se-lected because they had published colours obtained bythe Col-OSSOS survey, and were predicted by Pike et al.(2017) to have a large divergence from linearity in theirreflectance spectra at λ > . µ m. The other sevenTNOs were selected from the catalog of Peixinho et al.(2015) if they were dynamically classified as a cold clas-sical, had a full set of BV RI photometric colours, andwere bright enough (
R < .
5) that spectroscopic ob-servations would be obtainable, reducable, and usefulfor characterisation and analysis. Of the nine initiallyselected targets, six were successfully observed. he Reflectance of Cold Classical TNOs Table 1.
Reflectance Spectra of Cold Classical TNOs Reported to DateTNO Wavelength Ranges Covered, µ m References58534 Logos (1997 CQ ) 0 . − .
88 (Boehnhardt et al. 2001)66652 Borasisi-Pabu (1999 RZ ) 1 . − . ) 0 . − .
88; 1 . − . ) 1 . − . OBSERVATIONSEach spectrum presented here was observed duringa visitor mode observing run at the European South-ern Observatory’s (ESO’s) Very Large Telescope (VLT)that took place during the three nights between 2019Aug 28 and 31. Table 2 presents a log of our success-ful observations. At the VLT we used the FOcal Re-ducer/low dispersion Spectrograph 2 (FORS2; Appen-zeller et al. 1998) mounted on the 8.2 m UT1(Antu) unittelescope. FORS2 is a multipurpose optical spectro-graph, imager, and polarimeter, which we used in Long-Slit Spectroscopy mode to observe our TNOs and Solarcalibrators. FORS2 was configured with the red sensi-tive MIT CCD detector, the standard resolution colli-mator, and a longslit of width 1.6 (cid:48)(cid:48) . The non-standard
GRIS_200I+28 grism was installed in FORS2 for our ob-servations, offering a resolving power of λ/ ∆ λ = 380 at0 . µ m and a wavelength coverage of 0 . − . µ m.The extremely low S/N of the spectra at the longestwavelengths would effectively limit the extent of ouruseful data to the range 0 . − . µ m, however. The GRIS_200I+28 grism has an OG550 order separation fil-ter cemented to it, so no extra order blocking filter wasrequired.For all TNOs and Solar calibrators we used a two-point repeating spatial dither pattern between spectro-scopic exposures centered at +1 (cid:48)(cid:48) and − (cid:48)(cid:48) from the cen-ter of the spatial axis of the slit. To mitigate the effectsof atmospheric differential refraction (Filippenko 1982)we aligned the slit to the parallactic angle before com-mencing spectroscopic observations of each target, andfor each TNO the slit was realigned to the parallacticangle after every four exposures (approximately onceevery 45 minutes). All exposures were read out usingthe standard 100kHz,2x2,high readout mode.We ensured that the observations of each TNO werebracketed by observations of a Solar calibrator obtainedat a similar airmass and pointing. The spectra of thesecalibrators were used to cancel the Solar spectrum fromthose of the TNOs and derive the TNO reflectance spec-tra; they also double as telluric standards. Only onebracketing calibrator spectrum was required, but obser-vation of two provided redundancy in the event of unex- pected changes in conditions. At least one Solar calibra-tor for each TNO was selected from published catalogsof Solar twins and analogs (Ram´ırez et al. 2009, 2014).If the second calibrator was not a Solar twin or analog itwas a star with both a reported spectral type and B − V color that were close to Solar (Wenger et al. 2000). Ul-timately we were able to calibrate each TNO spectrumwith an associated Solar twin or Solar analog spectrum,and we therefore expect the gradients and shapes of theTNO reflectance spectra to be accurate. All our spec-tra were observed under clear (sky 90% cloudless at el-evation > ◦ , transmission variation < < < . DATA REDUCTIONStandard spectroscopic data reduction steps includingbias subtraction, flat-fielding, and wavelength calibra-tion were conducted for all our raw specta with the ESOFORS data reduction pipeline (v. 5.4.3 ) in the ESOReflex data processing environment (v. 2.9.1; Freudlinget al. 2013). The spectra were not corrected for instru-ment response, as the response of FORS2 is stable overa period of several days, and any response calibrationapplied to a TNO spectrum and a Solar calibrator starspectrum would only be cancelled out once the formerwas divided by the latter later on.While not a problem for the 2D spectra of the brightSolar calibrator stars, low level fringing was identifiedat longer wavelengths in the 2D spectra of the TNOs,so a fringe frame was constructed for each TNO to re-move them. First, for each 2D spectrum associated witha given TNO, a linear profile was fitted and subtractedfrom each pixel column parallel to the spatial axis to ap-proximately cancel the illumination pattern of sky emis-sion lines. Next, each spatial pixel column within each ftp://ftp.eso.org/pub/dfs/pipelines/instruments/fors/fors-pipeline-manual-5.10.pdf Seccull, Fraser, & Puzia
Table 2.
Observation LogTarget a UT Observation Date | Time T exp , s N exp
Airmass IQ, (cid:48)(cid:48) ∆, au r , au α , ◦ HD 225194 2019-08-29 | ) 2019-08-29 | HD 8291 | HD 209562 | ) 2019-08-30 | | | ) 2019-08-30 | HD 224448 | HD 142331 | | | | HD 224448 | HD 8291 | ) 2019-08-31 | | Notes:
For each target we present the UT observation date and time, the integration time per exposure (T exp ), the number ofexposures (N exp ), and the airmass at which they were observed. The range of estimated image quality (IQ) values presented foreach target is the range of Full Widths at Half Maximum measured from Moffat profiles (Moffat 1969) fitted to the set of spatialprofiles produced by median collapsing each of a target’s reduced 2D spectroscopic exposures along the dispersion axis. Forthe TNOs we present their geocentric distances (∆), heliocentric distances ( r ), and phase angles ( α ) at the time of observation.The apparent magnitude of our targets lay in the range 21 . (cid:46) r (cid:46) a Stars presented in bold are those ultimately used to calibrate the spectra of their associated TNOs.
2D spectrum was sigma-clipped at ± σ before all thesigma-clipped 2D spectra were mask-median combinedto form a fringe frame that didn’t contain spectra of anysources. This fringe frame was then subtracted fromeach 2D spectrum of the TNO to cancel the fringes. Alinear profile was then fitted to, and subtracted from,each pixel column parallel to the spatial axis again toremove the sky emission line illumination pattern. Thisprocedure makes the simple assumption that the skyline illumination has a spatially linear profile, but testsmaking use of a second order polynomial didn’t showsignificant improvement in the illumination pattern can-cellation.Sky subtraction, cosmic-ray removal, and extractionof the spectra were performed using a method similarto that used by Seccull et al. (2018). Briefly, Moffatfunctions (Moffat 1969) were fitted to the spatial profileof the 2D wavelength-calibrated spectrum at many loca-tions along the dispersion axis in order to track the wave-length dependent center and width of the spectrum’sspatial profile. Sky region boundaries were defined at ± σ were separatelyconducted in the sky and target regions to remove cos-mic rays. In each unbinned wavelength element the me-dian background was subtracted. While the TNO spec-tra had effectively been background subtracted duringfringe removal, performing a simple median sky subtrac-tion was useful in making small improvements to thequality of the background removal.Another round of Moffat fitting was conducted forthe sky-subtracted spectrum and extraction limits weredefined at ± . . Within the defined extraction limits the flux see the Standard Exraction algorithm;ftp://ftp.eso.org/pub/dfs/pipelines/instruments/xshooter/xshoo-pipeline-manual-3.3.5.pdf he Reflectance of Cold Classical TNOs f C ( λ ) = f ( λ )10 . ak ( λ ) , where f C ( λ )and f ( λ ) are respectively the extinction corrected anduncorrected spectra, a is the median airmass at whichthe spectrum was observed, and k ( λ ) is the FORS2 ex-tinction coefficient (taken from the FORS2 data reduc-tion pipeline) interpolated to the resolution of the spec-tra.Once corrected for atmospheric extinction, the 1Dspectra for each target were median stacked. ForBorasisi-Pabu, 2000 OK , 2001 QY , and 2001 HZ ,however, 1, 3, 3, and 2 spectra were respectively ex-cluded from their final stacks due to their very low S/Nor large sky emission line residuals. The stacked spec-tra of all other targets, including the solar calibrators,were produced using all of their available spectroscopicexposures. Solar calibration of the stacked TNO spec-tra was performed by dividing them by their associatedstacked Solar calibrator spectrum, resulting in the pro-duction of the reflectance spectrum of each TNO. Thereflectance spectra were then binned using the boot-strapping method described by Seccull et al. (2019) toestimate their uncertainties and boost their S/N at theexpense of wavelength resolution. Binning factors foreach spectrum are presented in Fig. 2. RESULTS & ANALYSISFigure 1 presents the reflectance spectra of the sixTNOs we observed. These are the first optical re-flectance spectra reported for any of these targets, andfive of them (excluding Borasisi-Pabu which has a NIRspectrum; Barkume et al. 2008) are the very first to bereported for these objects at any wavelength. TheseTNOs have red and nearly linear spectra at 0 . − . µ m, and all with the exception of 2000 OK (seeSection 6) have a clear transition in their reflectancespectra at λ > . µ m where their spectral gradientsdecrease towards the NIR. It is worth noting that a mod-erately strong telluric water absorption band exists inthe range 0 . − . µ m (cf. Smette et al. 2015). Whiledifferences between the strength of this telluric band inthe raw spectra of the TNOs and their solar calibratorstars have the potential to affect the shape of the re-flectance spectra at λ > . µ m, the very stable dry con-ditions (PWV < . . − . µ m telluric band cannot accountfor any curvature in the spectra at λ < . µ m. Ourobservation of this curvature spectroscopically supports inferences of the existence of such a flattening in thereflectance spectra of cold classicals from photometriccolor measurements (Delsanti et al. 2004; Doressoundi-ram et al. 2007; Fraser & Brown 2012; Pike et al. 2017;Terai et al. 2018; Schwamb et al. 2019).Ancillary plots showing the appearance of our spectraat their native wavelength resolution (Fig. 2), the con-sistent shape of the spectra of our Solar calibrators (Fig.3), and the wavelength dependent S/N of our binnedspectra (Fig. 4) can be found in the Appendix.5.1. Comparison to Published Photometry
Since no optical reflectance spectra have yet been re-ported in the literature for any of our targets we com-pared our dataset to coarse reflectance spectra producedfrom published photometric colours using methods de-scribed by Hainaut & Delsanti (2002). For Borasisi-Pabu, 2001 QY , and 2001 HZ , BV RI reflectancepoints were determined using optical colors reported byPeixinho et al. (2015). The simultaneous V − J colormeasurement of Borasisi-Pabu reported by McBrideet al. (2003) was used to estimate its J -band reflectance.The BV RIJ reflectance points for 2000 OK were de-termined using simultaneously observed BV RIJ colorsreported by Delsanti et al. (2004). grizJ reflectancepoints for 2013 UL and 2014 UE were derived fromcolors published by the Outer Solar System OriginsSurvey (Pike et al. 2017; Schwamb et al. 2019). Wemake use of updated values however, which reflect post-publication improvements to the photometry pipelinesdeveloped by that group (see Figure 1; Fraser et al.,in prep.). Solar BV RI , V − J , r − J , and griz pho-tometric colors were respectively taken from Ram´ırezet al. (2012), Casagrande et al. (2012), Schwamb et al.(2019), and the web pages of the Sloan Digital Sky Sur-vey . Overall the photometrically derived reflectances ofour targets are in good agreement with our reflectancespectra. We note that our reflectance spectra do notextend to wavelengths that are short enough to test forthe presence of the non-linearity suggested to be presentin the reflectance spectrum of 2001 HZ by its V -bandphotometric point (Peixinho et al. 2015). In addition,some difference between our spectrum and the photo-metrically derived reflectance of 2000 OK may be ex-pected, as this object has been reported to have variableoptical colors (Doressoundiram et al. 2002).5.2. Measurements and a Search for Trends
We measured the optical gradient of each reflectancespectrum across the wavelength range 0 . − . µ m Seccull, Fraser, & Puzia )0.6 0.8 1.0 1.20.51.01.52.02.5 2001 HZ ) 505476 (2013 UL )0.6 0.8 1.0 1.2138537 (2000 OK ) R e l a t i v e R e f l e c t a n c e Wavelength, m
Figure 1.
Reflectance spectra of six dynamically cold TNOs at 40 < r <
47 au. Our observed spectra are plotted in blackand cover the range 0 . − . µ m. Grey points are significantly affected by residual sky lines. Orange dashed lines show alinear fit to the spectrum in the range 0 . − . µ m extrapolated across the full wavelength coverage of the spectra, whilethe associated shaded regions denote the standard error of the linear fit. Red points are coarse reflectance spectra derived frompublished photometric colors (McBride et al. 2003; Delsanti et al. 2004; Peixinho et al. 2015; Pike et al. 2017; Schwamb et al.2019). We use the following updated Col-OSSOS colors to derive the coarse reflectance spectra presented here: g − r = 0 . ± . r − i = 0 . ± . r − z = 0 . ± . r − J = 1 . ± . g − r = 1 . ± . r − i = 0 . ± . r − z = 0 . ± . r − J = 1 . ± . . µ m. relative to the reflectance at 0 . µ m using the boot-strapping method of Seccull et al. (2019). Measuredgradients are presented in Table 3 along with the aver-age values of these measurements for this sample. Theaverage optical gradient measured from our spectra (seeTable 3) is consistent with those previously determinedfrom photometric colour measurements of cold classi-cal TNOs (e.g. Hainaut et al. 2012), and also gradientsmeasured from the limited number of previously pub-lished optical reflectance spectra of cold classicals (seeBoehnhardt et al. 2001). We did not attempt to con-strain the surface composition of our targets via use ofHapke (2012) modelling on our reflectance spectra asthey are featureless, and such analysis would thereforeonly return highly degenerate results.To estimate the central wavelength of the transitionin our spectra ( λ T ), and constrain the gradient of theNIR continuum, we fitted a dual sloped spectral model to each spectrum, including J-band spectral photome-try where available. The precision of this method wasultimately limited by our lack of spectral coverage in J band, however, resulting in poor constraint of λ T andthe NIR continuum gradient, making them insufficientfor use in further analysis. A crude estimate of the lowerlimit of λ T was made for each spectrum by finding thelowest wavelength at which all redward datapoints havereflectances below the fitted linear continuum slope ex-trapolated redward from the optical region. These lowerlimits on λ T are presented in Table 3.We also present the dynamical properties of our tar-gets in Table 3 alongside their absolute V band magni-tudes, H V . H V values for Borasisi-Pabu, 2001 QY ,and 2000 OK were taken directly from the publisheddatabase of the TNOs are Cool survey (Vilenius et al.2012, 2014). H V for 2001 HZ was calculated using the H R magnitude and V − R color of this object from Peix- he Reflectance of Cold Classical TNOs Table 3.
Cold Classical TNO PropertiesTNO S (cid:48) (0 . / (0 . µm ) λ T , µm a , au e i , ◦ q , au H V . ± . > .
81 43.98 0.092 0.563 39.95 6 . ± . ) 19 . ± . > .
87 46.77 0.143 4.877 40.04 6 . ± . )** 22 . ± . > .
87 44.17 0.085 1.547 40.42 5 . ± . ) 22 . ± . > .
80 45.90 0.101 2.025 41.26 7 . ± . ) 33 . ± . > .
75 43.73 0.066 4.504 40.84 6 . ± . . ± . > .
86 42.94 0.032 2.933 41.55 6 . +0 . − . Average 25 . ± . > . . ± . > . Notes:
For each TNO that we observed we present the properties that we compared in a search for evidence of any trendsbetween their reflectance properties, their absolute V -band magnitudes ( H V ), and their orbital semimajor axes, eccentricities,inclinations, and perihelia ( a , e , i , and q , respectively). The gradients measured from our spectra across 0 . − . µ m ( S (cid:48) (0 . λ T ). Averages of thesevalues and the standard errors of these averages are also presented for the full sample, and the full sample excluding the potentialinterloper 2000 OK . Methods used to estimate the H V values for our targets are described in section 5.2. TNOs with markedwith ** are known binaries. inho et al. (2015). Because the H R values published byPeixinho et al. (2015) are not corrected for phase dark-ening we must account for the possibility that our valueof H V is overestimated. β V = 0 . ± . mag/ ◦ wasadopted as a nominal phase coefficient by averaging thevalues of β V reported by Rabinowitz et al. (2007) forTNOs with H V >
4. From its JPL Horizons ephemeris,we determined that 2001 HZ has never been observedat a phase angle greater than α = 1 . ◦ since discov-ery. Based on this maximum phase angle and nominalphase coefficient, we predict that H V may be overesti-mated by up to 0.21 magnitudes, hence the asymmetricuncertainty of H V for 2001 HZ reported in this work.To estimate H V for our Col-OSSOS targets, 2013 UL and 2014 UE , we first used the H r and g − r valuesfrom Schwamb et al. (2019) to get H g . We then fol-lowed the example of Sheppard (2010, 2012) and used V = g − . g − r ) − .
03 from Smith et al. (2002)to determine H V . Finally, we corrected H V for phasedarkening using our nominal β V and the phase anglesof these two targets at the time they were observed byCol-OSSOS (Schwamb et al. 2019).Thorough statistical analysis of the properties of sucha small sample of TNOs would be a foolhardy effort,but we did compare their measured reflectance prop-erties to their dynamical properties and H V values byeye to see if any potential trends in the data might beglimpsed. No convincing trends were observed, how-ever. Neither do the binary TNOs, Borasisi-Pabu and2001 QY , exhibit any spectral behaviour that differ-entiates them from the rest of the objects in our sample,nor do we observe any discernible relation between thereflectance properties of our targets and dynamical res- idence within, or near to, the cold classical kernel (Petitet al. 2011; Bannister et al. 2016). INTERLOPERSPike et al. (2017) put forward the possibility that ob-servers may be able to distinguish hot classical TNOsfrom cold classical TNOs by the shape of their re-flectance spectra at λ > . µ m. Here we present thefirst spectroscopic implementation of this technique byreporting that 2000 OK is very likely to be a hot clas-sical interloper in cold classical orbital parameter space.2000 OK has been variably attributed membership inboth the hot and cold classical dynamical classes (e.g.Peixinho et al. 2015; M¨uller et al. 2020), largely depend-ing on where researchers have chosen to divide the twopopulations in terms of inclination. While our assign-ment of 2000 OK to the hot classicals is partly basedon the fact that its reflectance spectrum does not levelout toward the NIR nearly as much as those of the othercold classical targets in our sample, it is important tonote that distinction of hot classical TNOs from coldclassical TNOs cannot be done by characterisation oftheir z-band reflectance properties alone. Two of thecold classical TNOs observed by Col-OSSOS have col-ors which suggest that their reflectance spectra may beclose to linear from 0 . − . µ m (Pike et al. 2017). In-deed, Arrokoth also shares this trait, as its average op-tical spectral gradient remains approximately constantat least as far to the red as ∼ . µ m (Stern et al. 2019;Grundy et al. 2020). Our classification of 2000 OK isbased upon the combination of its near linear reflectancespectrum from 0 . − . µ m, its relatively low opti-cal redness in comparison to the average for cold classi- Seccull, Fraser, & Puzia cal TNOs (see Table 3), and its already ambiguous dy-namical classification. Note that 2000 OK would notlook out of place among the cold classicals when com-pared in terms of size and albedo ( D = 164 +33 − km and p V = 0 . +0 . − . respectively; cf. M¨uller et al. 2020).We caution that multiple properties of a classical TNOmust be considered before it may be assigned to eitherthe hot or cold dynamical classes. DISCUSSION7.1.
Silicates
Based on their finding that the cold classicals occupya unique region of g − r / r − z colour space, Pike et al.(2017) inferred that their reflectance spectra have dis-tinct behaviour at λ ∼ . µ m that might possibly re-sult from the presence of surface material on cold classi-cals that has an absorption band at those wavelengths,and is not present on the surfaces of the dynamicallyhot TNOs. We find, however, that there is no obviousabsorption band present at λ ∼ . µ m in any of ourreflectance spectra. Inclusion of published J -band pho-tometric data for our targets (McBride et al. 2003; Del-santi et al. 2004; Schwamb et al. 2019) further strength-ens this non-detection. Rather than the presence of anabsorption band, all three cold classical targets in oursample which have J -band photometric measurementsexhibit reflectance properties that are more consistentwith the existence of featureless linear continuum intheir spectra between ∼ . µ m and ∼ . µ m.The non-detection of a silicate absorption band is dis-appointing, but given the low densities so far typicallydetermined for cold classical TNOs it is not surprising(see Vilenius et al. 2014; M¨uller et al. 2020). If anhy-drous mafic silicates are present on the surfaces of thecold classicals it is plausible that their strong 1 . µ mabsorption bands may be masked by any opaque macro-molecular organics they are mixed with (e.g. de Berghet al. 2008). Laboratory studies have shown that mask-ing of the 1 . µ m band may be achieved when irradiatedorganics are present at only half the concentration ofthe mafic silicate component within a porous refractorymantle (e.g. Poch et al. 2016). Because of this possi-ble masking of the 1 . µ m silicate band, our reflectancespectra can neither rule out the presence of silicates onthe surfaces of our targets, nor their absence. IR ob-servations of these cold classical TNOs at λ > . µ mmay fare better in directly detecting the signatures ofmafic silicates on the surfaces of cold classical TNOs(e.g. Parker et al. 2016). 7.2. Carbonaceous Material
As in previous spectroscopic studies of cold classicalsand other extremely red TNOs the reflectance propertiesof our targets appear most consistent with those of ir-radiated residues comprised of complex macromolecularorganic compounds (e.g. Cruikshank et al. 1998; Barucciet al. 2006; Brunetto et al. 2006; Grundy et al. 2020).Like our targets, such materials have optical reflectancespectra that broadly consist of a linear continuum with astrong positive gradient that may curve downward closeto the boundary between the optical and NIR ranges(e.g. de Bergh et al. 2008).Delocalized π -bonded electrons within organicmolecules absorb light via π − π ∗ and n − π ∗ excitation;the energy structure of these delocalised electrons isgoverned by the size and clustering of the conjugated π -bond networks in which they reside. In turn thesefactors are influenced by the interconnected composi-tional and structural properties of the organic moleculesthemselves, such as the sp /sp bond ratio, the extentof sp bond clustering, the abundance ratios of variousconstituent elements (e.g. C/N, C/H, C/O), and thenumber of nitrogen and oxygen bearing heterocycles andauxochrome functional groups. In concert all of thesefactors determine the albedo of an irradiated organicsample or residue, and the gradient and curvature of itsreflectance spectrum from the near-UV to the nearestIR (e.g. McKay 1996; Imanaka et al. 2004; Cruikshanket al. 2005; Bernard et al. 2006; Brunetto et al. 2006; deBergh et al. 2008).Unfortunately the mapping between the optical re-flectance properties of an irradiated organic sample andthe properties of its diverse constituent macromoleculesis often intractably degenerate (Bernard et al. 2006).As a result it is not possible to use our reflectancespectra to diagnostically characterise any carbonaceousmaterial present on the surfaces of our targets. Suffi-ciently sensitive observations at λ > . µ m stand abetter chance of being useful in efforts to characteriseany refractory organics on the surfaces of cold classi-cal TNOs, through detection and characterisation of thevibrational molecular absorption bands that refractoryorganics often exhibit at IR wavelengths (e.g. Imanakaet al. 2004; de Bergh et al. 2008; Materese et al. 2014,2015). Sufficiently sensitive observations of cold classi-cal TNOs at near-UV wavelengths may also be informa-tive about the carbonaceous materials on their surfaces,as carbon-rich phases exhibit multiple near-UV spectralbehaviours that are diagnostic of both their compositionand physical state (Hendrix et al. 2016; Applin et al.2018). he Reflectance of Cold Classical TNOs The Cold Classical Surface Type
While optical reflectance spectra of organics cannotbe used for precise characterisation, it is possible to usethem to tell different classes of carbonaceous materialapart (e.g. de Bergh et al. 2008; Fraser & Brown 2012).Because organic compounds are predicted to dominatethe optical and very-NIR reflectance properties of veryred TNOs (e.g. Dalle Ore et al. 2015), the distinctlyconsistent overall shape of the reflectance spectra of allour 100 km scale cold classical targets suggests that theclass of refractory organics on their surfaces is similarfrom object to object. If this homogeneity is a propertyshared by all larger members of the cold classical pop-ulation, as suggested by their unimodal and relativelynarrow color and albedo distributions (Pike et al. 2017;Schwamb et al. 2019; M¨uller et al. 2020), it supportsthe prediction that the cold classical TNOs are likelyto have both formed under similar conditions and expe-rienced a common history of surface processing. Lab-oratory studies support this idea by repeatedly show-ing that if refractory organics are formed from differentinitial ice mixtures, processed in different ways, or pro-cessed to different extents, they will typically exhibit re-flectance spectra with large differences in shape, unlikethose of the cold classicals (Imanaka et al. 2004; Cruik-shank et al. 2005; Bernard et al. 2006; de Bergh et al.2008; Materese et al. 2014, 2015; Poston et al. 2018).While the composition and molecular structure of anyrefractory organics on the surfaces of our targets maydefine their reflectance properties, the role of the physi-cal properties of their surfaces, such as grain size distri-bution or porosity, must not be ignored. The physicalproperties of the surface of a TNO may determine theproperties of its reflectance spectrum independently ofcomposition (e.g. Poch et al. 2016; Cloutis et al. 2018).The homogenous shape of the reflectance spectra of coldclassical TNOs may therefore indicate that their surfaceshave similar physical properties, while the physical prop-erties of the surfaces of the dynamically hot TNOs maybe more diverse. Unfortunately, without high phase an-gle observations of TNOs it is not currently possible todisentangle any effects that the physical properties andcomposition of their surfaces have on their reflectancespectra. High phase angle measurements from space-craft will be required to examine the physical propertiesof TNO surfaces (see Porter et al. 2016).It is worth noting that some of the reflectance spectraof dynamically hot TNOs (here in which we include thecentaurs), also exhibit a flattening of their optical gra-dients toward the NIR (e.g. Alvarez-Candal et al. 2008;Fornasier et al. 2009; Merlin et al. 2010). In particularthe behaviour appears more commonly in the reflectance spectra of TNOs with similar optical redness to the coldclassicals. In contrast to that of the cold classicals,however, the flattening behaviour observed for dynami-cally hot TNOs is diverse in both shape and the wave-length at which it occurs. Reports show that the spec-tra of dynamically hot TNOs can begin to diverge fromtheir linear continuum slopes anywhere within the range0 . − . µ m (e.g. Cruikshank et al. 1998; Boehnhardtet al. 2004; Barucci et al. 2006; Alvarez-Candal et al.2008; Fornasier et al. 2004, 2009; Gourgeot et al. 2015;Merlin et al. 2005, 2010, 2017; Schwamb et al. 2019).By comparison, all of our cold classical reflectance spec-tra flatten within the narrower range of 0 . − . µ m.Therefore, while flattening behaviour is not a uniquequality of the reflectance spectra of cold classical TNOs,the shape of their reflectance spectra is remarkably con-sistent in comparison to those of the dynamically hotTNOs. Our cold classical reflectance spectra support theprediction of Pike et al. (2017), that the cold classicalTNOs (at least those observable from current ground-based facilities) comprise a unique surface type amongTNOs. Not only do the cold classicals have distinctdistributions in terms of optical color and albedo, theyalso appear to have reflectance spectra with a consis-tent characteristic shape from optical wavelengths to thenearest IR. 7.4. Comparison to Arrokoth
Benecchi et al. (2019) noted that their photometricmeasurements of cold classical TNOs hinted at the exis-tence of an interesting, but not statistically significant,increase in the F W − F W color of cold classicalsof decreasing size. The F814W filter used by Benecchiet al. (2019) has a bandpass covering 0 . − . µ m, andincludes the region in which our cold classical reflectancespectra flatten. An increase in F W − F W colorfor smaller cold classicals may therefore be interpreted asa shift toward longer wavelengths of the point at whicha reduction in the spectral gradient occurs.We note that not one of the five, nearly randomly se-lected, ∼
100 km scale cold classical TNOs in our sam-ple has a reflectance spectrum like that of the ∼
10 kmscale Arrokoth, which has linearly increasing reflectanceat least as far to the red as ∼ . µ m. The potentialsize-color trend hinted at by Benecchi et al. (2019), andthe absence of linear spectral behaviour in our sampleat λ > . µ m raises the question as to whether spec-tral flattening at λ > . µ m is more common in thereflectance spectra of large cold classicals than it is inthose of small ones.Our spectra unfortunately do not have sufficient NIRwavelength coverage for us to precisely measure the cen-0 Seccull, Fraser, & Puzia tral wavelength of their downward curvature at λ > . µ m. Nor is our sample large enough for statisti-cally significant analysis. As a result, further specula-tion on this suggested trend without a stronger foun-dational dataset would be reckless. We finish here bystating that the size-color trend for cold classical TNOshinted at by Benecchi et al. (2019) appears, at least inbroad qualitative terms, to be consistent with the ab-sence of linear spectral behaviour at λ > . µ m in thereflectance spectra of all our ∼
100 km scale cold classi-cal targets. CONCLUSIONSFollowing reports by Pike et al. (2017) and Terai et al.(2018) that the cold classical TNOs have distinct photo-metric color properties, and potentially even an absorp-tion band in their reflectance spectra at λ > . µ m, weobserved the reflectance spectra of six TNOs currentlyresiding in the cold classical dynamical range. Our re-flectance spectra, which cover 0 . − . µ m, are the firstto be observed in the optical range for any of our tar-gets. We find that the reflectance spectra obtained areconsistent with coarse reflectance spectra derived frompreviously published photometric colors for our targets.Five of the six targets we observed, including Borasisi-Pabu, 2001 QY , 2001 HZ , 2013 UL , and2014 UE , have reflectance spectra which are linearand red in the range 0 . < λ < . µ m, but show abroadly consistent flattening at λ > . µ m. This re-sult is consistent with the reported lower average r − z color of cold classicals in comparison to dynamically hotTNOs with similar g − r color, and the suggestion thatthe cold classicals occupy a distinct surface class (Pikeet al. 2017). We find no evidence that the observed flat-tening in our spectra is associated with the presence ofan absorption band at λ ∼ . µ m; anhydrous maficsilicates remain elusive on the surfaces of TNOs. Wepredict that the similar shape of the reflectance spectraof these five targets arises due to the presence of similarrefractory organics on their surfaces and/or similaritybetween the physical properties of their surfaces. Suchapparent consistency between the reflectance propertiesof cold classicals aligns well with their unimodal andrelatively narrow color and albedo distributions, sup-porting predictions that the cold classical TNOs formedunder similar conditions and have since experienced sim-ilar types and extents of surface evolution.We assert that the sixth TNO in our sample,2000 OK , as likely to be a hot classical TNO inter-loping in the cold classical dynamical range. While thisis partly because the reflectance spectrum of 2000 OK is close to linear across the full 0 . − . µ m range, we caution that the presence or absence of flattening at λ > . µ m in a TNO’s reflectance spectrum is insuffi-cient on it’s own for the purposes of classification. Ourassignment of 2000 OK to the hot classicals is alsobased upon its sporadic assigment to the hot classicalsin prior reports, and its relatively low optical spectralgradient in comparison to the average for cold classicalTNOs.Future sufficiently sensitive observations at near-UVand IR wavelengths will be crucial in making headwayin determining whether refractory organics are responsi-ble for the distinct reflectance properties of cold classicalTNOs, as these wavelength regions have been shown toexhibit features that are diagnostic of both the chemi-cal composition and molecular structure of complex car-bonaceous materials (e.g. Materese et al. 2014, 2015;Hendrix et al. 2016; Applin et al. 2018). Observationsof cold classical TNOs at high phase angles (which areonly feasible from vantage points at large heliocentricdistances; e.g. Porter et al. 2016) will also be requiredto explore any possible connection between the physicalproperties and the distinct reflectance properties of coldclassical surfaces.ACKNOWLEDGMENTSWe are grateful to Jonathan Smoker, Cedric Ledoux,Joe Anderson, and Francisco Belmar for sharing theirexpertise and assisting our VLT observations. Thanksalso to Xiaoyu Zhang and Dale Cruikshank for theirassistance in accessing literature on refractory organ-ics. T.S. is supported through a Gemini Science Fellow-ship by the international Gemini Observatory, a pro-gram of NSF’s OIR Lab, which is managed by theAssociation of Universities for Research in Astronomy(AURA) under a cooperative agreement with the Na-tional Science Foundation, on behalf of the Gemini part-nership of Argentina, Brazil, Canada, Chile, the Repub-lic of Korea, and the United States of America. T.S.was also supported in part by the Astrophysics Re-search Centre at Queen’s University Belfast, and theNorthern Ireland Dept. for the Economy. This workis based on observations collected at the European Or-ganisation for Astronomical Research in the SouthernHemisphere under ESO program 0103.C-0708. This re-search made use of NASA’s Astrophysics Data SystemBibliographic Services, the JPL HORIZONS web in-terface (https://ssd.jpl.nasa.gov/horizons.cgi), and dataand services provided by the IAU Minor Planet Center. Facility:
ESO: VLT-UT1(FORS2) he Reflectance of Cold Classical TNOs Software:
Astropy (Astropy Collaboration et al.2013), ESO Reflex (Freudling et al. 2013), Matplotlib (Hunter 2007), NumPy (Harris et al. 2020), SciPy (Vir-tanen et al. 2020)REFERENCES
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APPENDIX
Figure 2.
The reflectance spectra presented in Fig. 1, but prior to spectral binning, are plotted here in black. The associatedbinned spectra are plotted with yellow points. The binning factor (i.e. number of resolution elements per bin) used to producethe binned spectrum from the unbinned one is presented at the bottom of each panel. he Reflectance of Cold Classical TNOs F l u x R e l a t i v e t o . m HD 8291 - 2014 UE
HD 224448 - Borasisi-PabuHD 8291 - 2013 UL HD 142331 - 2001 HZ HD 209562 - 2001 QY
HD 224448 - 2000 OK Figure 3.
The unbinned stacked spectra of the Solar twins used to calibrate each of our TNO spectra. They are scaled tounity at 0 . µ m, but are not corrected for instrument response. The shapes of the star spectra are very consistent across ourfull wavelength coverage, with the only noteworthy variation between them occuring in bands of moderate telluric absorptionat λ ∼ . µ m and 0 . − . µ m. The consistency of our calibrator star spectra shows that any variation between thereflectance spectra of our TNO targets primarily results from differences between the intrinsic reflectance properties of theTNOs themselves. S / N Wavelength, m
Figure 4.
The wavelength dependent S/N of our reflectance spectra. Going left to right along the top row and then the bottomrow, the panels present data for 2014 UE , Borasisi-Pabu, 2013 UL , 2001 HZ , 2001 QY , and 2000 OK67