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Home , 20000 Varuna, 28978 Ixion, 38628 Huya, Eris (dwarf planet), Haumea, Opposition (astronomy)

arXiv:0810.2714v1 [astro-ph] 15 Oct 2008 ob bqiospeoeo o lntr ufcsa smal at surfaces planetary for phenomenon ubiquitous f later a and be observations, to lunar in (1929) wa Lyot by polarization Rayleigh-scatterin discovered linear single Negative simple model. the Fresnel-reflection from contras p expected in is component, where what the perpendicular light to the over parallel scattered predominates component polarized vector field linearly electric partially of case seod,stlie fmjrpaes n trans-Neptuni and exhibit , (TNOs) com major like jects th objects of , solar-system satellites the where between , ), angle the the and is angle object, phase (the angles phase l of mechanism the to scattering. linked are heterogeneity refracti and complex porosity, size, of surfaces particle the e.g., be of bodies have classification solar-system and polarization study linear the of for measurements exploited time, long a For Introduction 1. aaa bevtr ne rgam D18C03 P:A B A. (PI: 178.C-0036 ID programme under Paranal oebr3 2018 3, November ⋆ Astrophysics & atclryitrsigpeoeo sosre tsmall at observed is phenomenon interesting particularly A ae nosrain aewt S eecpsa h aSill La the at ESO with made observations on Based nl ag.I mle bet uha,eg,IinadVaru and e.g., as, such ∼ objects smaller In range. angle mle objects. smaller frtiigvltlssc sC,N CO, as such retaining of uha,eg,PuoadEi,so ml oaiaini th in polarization small a show , and e.g., as, such eevd 2008-09-08 Received: losoet eemn hte h agtabd slwo hi or low words. is Key target the whether determine to one allows 5 6 olt dniydi identify to tool Conclusions. exhi that objects of classes two exist there that discovered Results. in Methods. study to us Aims. allow They system. solar evolution. the of phase cretional Context. 1 di 4 ( 2 3 ∼ > ff EI,Osraor ePrs ,p.JlsJnsn FR-9219 Janssen, Jules pl. e-mail: 5, Paris, de Observatoire LESIA, nvriyo ayad olg ak D S.e-mail: USA. MD, Park, College Maryland, of University rahOsraoy olg il rahB6 D,Norther 9DG, BT61 Armagh Hill, College Observatory, Armagh NF-Os srfiiod rer,LroE em ,I515F I-50125 5, Fermi E. Largo Arcetri, di Astrofisico Oss. - INAF srnmclOsraoyo hri ainlUniversity,e-mail: National Kharkiv of Observatory Astronomical bevtr,P o 4 01 nvriyo esni Finla Helsinki, of University 00014 14, Box PO Observatory, i h cteigpae tpaeage1 angle phase at plane) scattering the (in % 1 .Bagnulo S. 0A) h bevdrneo hs nlsi iie oabout to limited is angles phase of range observed the AU), 30 rn auso hi hs nl ie,teagebtenth between angle the (i.e., angle phase their of values erent eepotteueo oaierctcnqe saremote-se a as techniques polarimetric of use the exploit We [email protected] [email protected] [email protected], [email protected] ehv nlzdornwosrain ffieTO,adtoeo those and TNOs, five of observations new our analyzed have We icvr ftodsic oaiercbhvor of behaviours polarimetric distinct two of Discovery rn-etna bet TO)cnantems primitive most the contain (TNOs) objects Trans-Neptunian sn OS fteEOVT ehv bandlinear-polariza obtained have we VLT, ESO the of FORS1 Using uprBl wr lnt oaiain–Scattering – Polarization – planets dwarf – Belt Kuiper nsieo h eylmtdrneo bevbepaeangles phase observable of range limited very the of spite In 1 .Belskaya I. , ff eaiepolarization negative rn rpriso h ufcso Ns esgetta si a that suggest We TNOs. of surfaces the of properties erent / cetd 2008-10-10 Accepted: aucitn.Dual no. manuscript 2 .Muinonen K. , 2 n CH and rn-etna objects trans-Neptunian hsi peculiar a is This . 4 oho hs at a elne oterdi their to linked be can facts these of Both . ◦ h agrojcshv ihrabd hntesalrone smaller the than albedo higher a have objects larger The . 3 ..Tozzi G.P. , eindex, ve nob- an arucci) first s small i di bit ound cteigpae( plane scattering e ABSTRACT a ierplrzto hne ail ihtepaeangl phase the with rapidly changes polarization linear na, lane or g [email protected] ight to t ets, the u,teojc,adteErh.Det h ag itneof distance large the to Due Earth). the and object, the Sun, e 5Smk t. 12 hri,Ukraine. Kharkiv, 61022 str., Sumska 35 gh. en edncdx France. cedex, Meudon 5 a- erltosbtenvroscasso ml ois thei bodies, small of classes various between terrelations e d e-mail: nd. l ff rn oaiercbhvor bet ihadiameter a with Objects behaviour. polarimetric erent 0 rne tl.e-mail: Italy. irenze, 4 rln,UK e-mail: U.K. Ireland, n ◦ sn olt hrceietesraeo TNOs. of surface the characterize to tool nsing ..Barucci M.A. , h lp fteplrmti curve; polarimetric the characteristic of important slope three least the phase at of range identify extended and an gles, for polarization the of behaviour interpr the observations. for one TNO relevant of most tion the is 2004) scatteri backscatt Muinonen coherent from (e.g. which ex among polarization media, that particulate negative by mechanisms of light physical existence several the are plain There angles. phase ihntevr arwrneo h bevdpaeangles phase observed the Ixion. of 28978 range for narrow (2004) (0 very al. et the Boehnhardt by Within out carried genera were in possible, not is it minimum. and pa polarization always plane, the estimate is scattering light the dista polarized to large observed lel the The to objects). (due these range of phase-angle limited very a can only TNOs of observations polarimetric plane. ground-based scattering trast, the l to at perpendicular becomes, values, and angle plane phase scattering the to parallel being from tion; − . 2 2 ◦ o ayslrsse bet,i spsil omauethe measure to possible is it objects, solar-system many For h rtplrmti bevtoso N ecp Pluto) (except TNO a of observations polarimetric first The ◦ n hral es-rcse aeil rmteeryac- early the from materials least-processed thermally and . iii) − [email protected] inmaueet nteBessell the in measurements tion rudbsdplrmti bevtosaeapowerful a are observations polarimetric ground-based , 1 nte orTO bandi rvoswrs and works, previous in obtained TNOs four another f h neso nl twihteplrzto changes polarization the which at angle inversion the . ∼ 3 ◦ 0 ,te eeldapoone eaiepolariza- negative pronounced a revealed they ), geplrmti bevto tpaeangle phase at observation polarimetric ngle . )wihsol hne nteosre phase observed the in changes slowly which %) 5 5 [email protected] .Kolokolova L. , ff [email protected] rn oaiercbhvorcmae to compared behaviour polarimetric erent ⋆ 6 n .Fornasier S. and , ,adhv h capability the have and s, R ii) le o v Nsat TNOs five for filter h iiu polariza- minimum the ,adreaches and e, > rgnand origin r h targets the 00km, 1000

c S 2018 ESO ∼ 5 ,i.e.: s, 1 ncon- In ◦ gof ng cover ering arger ,to l, eta- nce ral- an- i) - 2 S. Bagnulo et al.: Discovery of two distinct polarimetric behaviours of trans-Neptunian Objects

Table 1. of five trans-Neptunian objects in Bessell R band. PQ and PU are the Stokes parameters transformed such that PQ is the flux perpendicular to the plane Sun-Object-Earth (the scattering plane) minus the flux parallel to that plane, divided by the sum of the two fluxes.

Date Time (UT) Exp Object PQ PU (yyyy mm dd) (hh:mm) (sec) (DEG) (%) (%)

2008 01 09 06:07 6000 26375 (1999 DE9) 1.412 −1.39 ± 0.12 0.00 ± 0.12 2008 03 09 06:07 6000 0.110 −0.20 ± 0.11 0.08 ± 0.11 2008 03 29 03:19 6400 0.519 −0.66 ± 0.11 −0.16 ± 0.11 2008 07 08 09:20 2880 136199 Eris 0.600 −0.11 ± 0.05 0.04 ± 0.05 2008 09 07 03:40 2880 0.379 0.03 ± 0.05 0.12 ± 0.05 2007 05 10 01:54 3760 0.614 −0.73 ± 0.07 −0.04 ± 0.07 2007 05 18 04:19 3760 0.831 −0.58 ± 0.07 0.08 ± 0.07 2007 07 17 01:44 3760 1.984 −1.61 ± 0.07 0.00 ± 0.07 2008 03 05 07:40 6000 1.624 −1.27 ± 0.06 0.05 ± 0.06 2008 05 30 04:54 5120 1.115 −1.10 ± 0.06 0.04 ± 0.06 2006 11 26 07:05 5800 20000 0.91 −1.04 ± 0.12 0.08 ± 0.11 2006 12 14 06:03 5800 0.572 −0.45 ± 0.15 −0.11 ± 0.14 2007 01 13 02:55 5800 0.135 −0.22 ± 0.10 0.04 ± 0.10 2008 03 29 01:10 7200 1.301 −1.18 ± 0.13 0.06 ± 0.13 2008 08 09 23:59 960 136108 0.987 −0.68 ± 0.06 −0.02 ± 0.06

tion changing rapidly as a function of the phase angle. Since blocks were planned so as to avoid too bright a background due then, three other TNOs have been the subject of a detailed to lunar illumination, and to avoid epochs when targets were too study: 29981 (1999 TD10) (Rousselot et al. 2005), 50000Quaoar close to bright . (Bagnulo et al. 2006), and 136199 Eris (Belskaya et al. 2008). Polarimetric observations were generally performed with the Belskaya et al. (2008) noted that the available polarimetric data retarder waveplate at all positions between 0◦ and 157.5◦, at for TNOs show negative polarization with two different trends 22.5◦ steps. For each observation, the exposure time cumulated at small phase angles. Only a small fraction of linear polariza- over all exposures varied from 24 minutes (for 136199 Eris) tion is measured in the largest objects Pluto, 136199 Eris, and to 2h (for ). Raw data were then treated as ex- 50000 Quaoar, with only subtle changes as a function of phase plained in Bagnulo et al. (2006), and our measurements are re- angle. This is in contrast with the steep gradient of about −1 % ported adopting as a reference direction the perpendicular to the per that was measured for . great circle passing through the object and the Sun. This way, In this paper we present 13 observations of another four PQ represents the flux perpendicular to the plane Sun-Object- TNOs, and two new observations of 136199 Eris. These new Earth (the scattering plane) minus the flux parallel to that plane, data allow us to generalize the finding by Belskaya et al. (2008). divided by the sum of these fluxes. For symmetry reasons, PU values are always expected to be zero, and inspection of their values allows us to perform an indirect quality check of the PQ 2. Observations values. Fifteen new broadband linear polarization measurements of Our new polarimetric measurements are given in Table 1. five TNOs were obtained from November 2006 to September These data have enlarged the number of TNOs for which polari- 2008 at the ESO Very Large (VLT) with FORS1 metric observations are available by a factor of two. Previously (Appenzeller et al. 1998). Using the R Bessell filter, we ob- published data, that will be also considered in our analysis, in- tained two new measurements of Eris, that was already observed clude seven measurements of Pluto (Breger & Cochran 1982), by Belskaya et al. (2008), and thirteen measurements of 26375 nine measurements of 28978 Ixion (Boehnhardt et al. 2004), five (1999 DE9), 38628 Huya, 136108 Haumea (2003 EL61), and measurements of 29981 (1999 TD10) (Rousselot et al. 2005), 20000 Varuna, that were never before observed in polarimetric five measurements of 50000 Quaoar (Bagnulo et al. 2006), and mode. four measurements of 136199 Eris (Belskaya et al. 2008). For The heliocentric distance of the observed TNOs is ∼> 30 AU. the sake of consistency, data of 29981 (1999 TD10) were re- As a consequence, the observable phase-angle range is very lim- reduced adopting exactly the same reduction procedureas forthe ited, compared to what can be achieved for most asteroids and remaining observations obtained with FORS1, leading to values . The phase angle range that was sampled is ∼< 2◦ for all only slightly different from the previously published ones. newly observed TNOs. All data considered here, except for Pluto, were obtained We aimed at obtaining polarization measurements with error with the FORS1 instrument with the Bessell R filter. Pluto’s bars between 0.05% and 0.1%, which requires a signal-to-noise polarimetry refers to a filter similar to Bessell V. We note that ratio between 1000 and 2000 (cumulated on both beams and all Bagnulo et al. (2006) obtained polarimetric measurements of the positions of the retarder waveplate). Since our targets are faint Chiron in the Bessell B, V, and R bands at six differ- (R ∼ 18−20), these observations were possible only by using an ent phase angles. At each phase angle, the polarimetric mea- 8-m telescope. In order to optimize the phase-angle sampling, surements obtained in the three different bands appear rela- our observations were scheduled in service mode. Observing tively consistent among themselves. A similar behaviour was S. Bagnulo et al.: Discovery of two distinct polarimetric behaviours of trans-Neptunian Objects 3

in error bar units (for most of new data between 0.05% and 0.12%). Due to the relatively limited number of measurements (38) we cannot expect to reproduce a Gaussian distribution. Yet, the fact that the PU distribution is roughly centered at zero, and all points are within −3 ≤ PU /σU ≤ 3, fully supports the relia- bility of the polarimetric measurements. Figure 2 shows the PQ values as a function of the phase an- gle measured for all data reported in Table 1, and those previ- ously published listed above. The left panel of Fig. 2 refers to the larger TNOs 136199 Eris, Pluto, 50000 Quaoar, and 136108 Haumea. The right panel shows the polarization phase angle de- pendence for the remaining objects: 28978 Ixion, 38628 Huya, 26375 (1999 DE9), 29981 (1999 TD10), and 20000 Varuna.

3. Results and discussion Figure 2 shows that larger TNOs exhibit a small fraction of neg- ative linear polarization roughly constant in the observed phase angle range. As far as 136108 Haumea is concerned, the only conclusion that can be drawn from its single measurement is Fig. 1. Distribution of the PU values normalized to their error that the polarization is very similar to that measured for Quaoar bars. The null detection of PU values serves as a quality check and higher (in absolute value) than that of Pluto and 136199 for the PQ measurements of Fig. 2. Eris. Whether its phase angle dependenceresembles that of other large objects will have to be checked with new observations. Our new polarimetric measurements of Eris, which were ob- found for 29981 (1999 TD10) that was observed both in the Bessell R and V filters by Rousselot et al. (2005). Additional tained with a higher accuracy than before, confirmed our previ- measurements of Pluto obtained by Kelsey & Fix (1973) and ous findings about its small negative polarization (Belskaya et al. Avramchuk et al. (1992) with no filter are also consistent with 2008) and expanded the observed phase angle range to the max- those by Breger & Cochran (1982). This suggests that even imum range presently reachable for this distant object. In par- ff ticular, a previous observation obtained at the phase angle of though Pluto polarimetry was obtained in a di erent band than ◦ the other TNOs, its comparison with new data obtained in the R 0.35 showed the strongest negative polarization for this object band is still meaningful. (∼−0.3%, see Belskaya et al. 2008), whereas our new measure- It should also be recalled that Pluto is in fact a double sys- ment obtained at a similar phase angle is practically consistent tem. Yet, in the observed phase-angle interval, Pluto’s intrinsic with zero. Therefore we cannot confirm the presence of a nega- polarization is confined within the range −0.35% and −0.1 %, tive polarization surge around that phase angle. for the following reason. We denote with Q the Stokes param- The new observations of the classical TNOs 20000 Varuna, eter not normalised to the intensity I, and with P(P) = Q(P)/I(P) 38628 Huya and the scattered-disk object 26375 (1999 DE9) re- Q veal a pronounced negative polarization changing rapidly with (C) (C) (C) and PQ = Q /I Pluto’s and ’s intrinsic polarization, phase angle and reaching about −1% at the phase angle of respectively. We can assume that, at each phase angle, Pluto, 1◦. Similar polarization behaviour was previously found for the Charon, the Sun, and the Earth define an identical scattering resonant object 28978 Ixion by Boehnhardt et al. (2004), who (P) (C) pointed out that this object exhibits the most pronounced neg- plane, so that PQ and PQ are expressed in the same reference system. This allows us to write, for the observed polarization of ative polarization measured for a solar-system body so far, and the double system, P(P+C) = (Q(P) + Q(C))/(I(P) + I(C)). Taking raised the question of whether this was a unique case or typical Q for TNOs. Our new observations have proved that the high nega- into account that the ratio between the reflected light of the two tive polarization at small phase angles is quite typical for TNOs, objects is ∼ 0.17, we deduce that as three newly observed objects have shown a polarimetric be- P(P) = P(P+C) + 0.17(P(P+C) − P(C)) . (1) haviour similar to that of Ixion. In fact, all four “small” TNOs Q Q Q Q together show a strikingly similar polarimetric behaviour that is < (C) < practically indistinguishable within the error bars. Assuming that Charon has −1.5 % ∼ PQ ∼ 0%, and consid- ering for the total polarization its mean value of −0.3%, Eq. (1) Data for 29981 (1999 TD10) (Rousselot et al. 2005) extend up to phase angle ∼ 3◦. In the phase-angle range 0◦ − 2◦, the tells us that P(P) ranges between −0.35%(inthecaseof P(C) = 0) Q Q observed polarization behaviour is consistent with those of the (C) and ≃−0.1% (in the case of PQ = −1.5%). other small objects, but this should be confirmed by a more re- Some polarimetric measurements are close to the limit fined sampling of this range. At phase angles 2◦ and 3◦ the ob- of instrumental polarization, which in FORS1 is ∼< 0.04 % served polarization is about −1%, which suggests a polarization (Fossati et al. 2007). Instrumental polarization is difficult to sub- minimum in that range. Observations of Chiron ob- tract fromscience data, since it dependson the instrumentsetting tained at larger phase angles compared to TNOs reveal a polar- and telescope orientation but, for the same reason, we can as- ization minimum in the range ∼ 1.5◦ − 2◦ (Bagnulo et al. 2006), sume that instrumental polarization does not introduce any sys- and new unpublished observations suggest a similar behaviour tematic offset. An indirect confirmation that instrumental polar- for Centaur Pholus (Belskaya et al., in preparation). All this ization does not introduce systematic offsets comes from inspec- leads us to speculate that the minimum of polarization of TNO ◦ tion of the measured PU values. Figure 1 shows the distributions phase curves are at phase angles slightly larger than 1.5 , per- ◦ ◦ of the PU values for all objects observed with FORS1 expressed haps between 1.5 and 3 , which would be noticeably different 4 S. Bagnulo et al.: Discovery of two distinct polarimetric behaviours of trans-Neptunian Objects

Fig. 2. Linear-polarization measurements of nine TNOs as a function of the phase angle. from that of asteroids and comets, that show a minimum between of the coherent-backscattering surges for large TNOs 7◦ and 10◦ (see, e.g., Penttil¨aet al. 2005). as compared to those observed for satellites and asteroids (see In spite of the very limited observed phase angle range, po- discussion in Belskaya et al. 2008). Note that, according to lab- larimetric observations of TNOs reveal two different behaviours. oratory measurements, some bright powdered samples also do TNOs with a diameter smaller than 1000km exhibit a nega- not show a negative-polarization surge in the phase-angle range tive polarization that rapidly increases (in absolute value) with of 0.2-4 deg (see Shkuratov et al. 2002). the phase angle, reaching about 1% at the phase angle of 1◦. Perhaps the most important difference between the surface Larger TNOs exhibit a small fraction of negative linear polariza- characteristics of the objects that exhibit a different polarimet- tion (∼< 0.7%) which does not noticeably change in the observed ric behaviour is that the TNOs with small and constant nega- phase angle range. tive polarization are supposed to have the capability of retain- It is quite natural to associate the two different behaviours ing volatiles such as CO, N2 and CH4 (Schaller & Brown 2008). of polarization phase dependencies with a different composition 136199 Eris and Pluto have rich surfaces. The other and/or structure of the surfaces of the objects. The two groups two objects 136108 Haumea and 50000 Quaoar, with slightly of objects with different polarimetric properties differ not only higher polarization (in absolute value), exhibit dominantly in size but also in surface albedo, which is higher in the larger spectra, and are believed to be in a transition phase where not objects than in the smaller ones. Although uncertainties in TNO all volatiles are lost yet (Brown 2008). This evolutionary phase albedo determination are quite large, it is evident that darker sur- may explain their different albedo, as well as their different po- faces exhibit higher negative polarization than brighter surfaces. larimetric behaviour compared to smaller objects, which have This trend resembles the dependence of the polarization mini- certainly lost the volatile components. mum on the albedo found for asteroids, but the lack of accurate albedo data prevents us from attempting to obtain firm relation- ships between albedo and polarization such as those obtained for References asteroids (e.g., Lupishko & Mohamed 1996). Yet, even a single measurement of linear polarization of a TNO at phase angle ∼ 1◦ Appenzeller, I., Fricke, K., Furtig, W., et al. 1998, The Messenger, 94, 1 Avramchuk, V. V., Rakhimov, V. I., Chernova, G. P., & Shavlovskii, N. M. 1992, can provide at least a distinction between high- and low-albedo Kinemat. Fiz. Nebesn. Tel, 8, N4, 37. surfaces. Bagnulo, S., Boehnhardt, H., Muinonen, K., et al. 2006, A&A, 450, 1239 At first glance, our results seem contradictory to the pre- Belskaya, I., Bagnulo, S., Muinonen, K., et al. 2008, A&A, 479, 265 Boehnhardt, H., Bagnulo, S., Muinonen, K., et al. 2004, A&A, 415, L21 dictions of the coherent-backscattering mechanism considered Breger, M., & Cochran, W. D. 1982, Icarus, 49, 120 to be the most probable cause of negative polarization at small Brown, M.E. 2008, in: The beyond . Barucci A.M., phase angles. The coherent-backscattering mechanism results in Boehnhardt, H., Cruikshank, D.P., & Morbidelli, A. (eds.), Univ. Arizona a sharp surge of negative polarization accompanied by narrow Press, p. 335 brightness opposition peaks that should be more prominent for Fossati, L., Bagnulo, S., Mason, E., & Landi Degl’Innocenti, E. 2007, in: C. Sterken (ed.), ASP Conference Series, vol. 364, p. 503 the brighter surfaces. Such surges were found for bright satellites Kelsey, J. D., & Fix, L. A. 1973, AJ, 184, 633 and asteroids with a peak polarization of about −0.4% centered Lupishko D.F., & Mohamed R.A. 1996, Icarus, 11, 209 atthe phaseangleof0.2◦−1◦ (fora review,see Mishchenko et al. Lyot, B. 1929, Ann. Obs. Paris, vol. 8, 1 2006). The observations of Eris and Pluto do not show oppo- Mishchenko, M. I.,Rosenbush, V. K., & Kiselev, N. N. 2006, Applied optics, 45, sition surges in polarization or brightness in the phase-angle 4459 ◦ ◦ Muinonen, K. 2004, Waves in Random Media, vol. 14, Iss. 3, p. 365 ranges covered (down to 0.15 and 0.80 , respectively). The ob- Penttil¨a, A., Lumme, K., Hadamcik, E., Levasseur-Regourd, A.C. 2005, A&A, servations can, however, be well explained by a narrower width 432, 1081 S. Bagnulo et al.: Discovery of two distinct polarimetric behaviours of trans-Neptunian Objects 5

Rousselot P., Levasseur-Regourd A.C., Muinonen K., & Petit J.-M. 2005, Earth, , & Planets, 97, 353 Schaller, E.L., & Brown, M.E. 2008, ApJ, 659, L61 Shkuratov Yu., Ovcharenko A., Zubko E., et al. 2002, Icarus, 159, 396