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Advances in Space Research 40 (2007) 238–243

Observations of dwarf planet (136199) Eris and otherlarge TNOs on Lulin Observatory

H.-W. Lin a,*, Y.-L. Wu a, W.-H. Ip a,b

a Institute of Astronomy, National Central University, No. 300, Jhongda Road, Jhongli City, Taoyuan County 320, Taiwanb Institute of Space Science, National Central University, No. 300, Jhongda Road, Jhongli City, Taoyuan County 320, Taiwan

Received 1 November 2006; received in revised form 5 June 2007; accepted 6 June 2007

Abstract

To provide comprehensive observations and in-depth standy of the physical property of the trans-neptunian objects (TNOs), theLulin Observatory in Taiwan has started a program to study the time variabilities of lightcurves of large TNOs and dwarf planets.Our initial results show that (50000) Quaoar has a significant brightness variation of Dm � 0.3 with a rotation period of about18.84 h while (136199) Eris (formerly 2003 UB313) has very little brightness variation (Dm � 0.05). The rotation period of Eris is hencehighly uncertain even though a periodicity of 3.55 h is suggested by our measurements.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Trans-neptunian objects; Lulin Observatory; Lightcurves; Rotation periods; Spacecraft exploration

1. Introduction

The recent discoveries of several large trans-neptunianobjects (TNOs) have completely changed our view of thedynamical structure of the outer solar system and hencethe origin of the solar system as a whole. Up to now, 5TNOs with their sizes comparable to or even larger thanthat of Pluto have been found (see Table 1). Accordingto theoretical calculations, there should be quite a few ofsuch large objects still to be discovered (Stern, 1991). Thefinding that several of them (2003 EL61 and Eris) also havesmall satellites (Brown et al., 2006a) has important implica-tions on the impact histories of the TNOs (Stern et al.,2006). The immediate scientific questions to be answeredare: (a) how did these large TNOs form; (b) how were theyplaced in the present orbital configurations; (c) what istheir relation to the Kuiper belt and the outer planets; (d)what are their dynamical evolutionary histories; and (e)how about their accretional and collisional fragmentation

0273-1177/$30 � 2007 COSPAR. Published by Elsevier Ltd. All rights reserv

doi:10.1016/j.asr.2007.06.009

* Corresponding author.E-mail addresses: sevenlin123@yahoo.com.tw (H.-W. Lin), wingip@

astro.ncu.edu.tw (W.-H. Ip).

histories? But first of all, we would need to characterizetheir physical properties including shapes, rotation periods,orbital inclinations, surface chemical composition, andinternal structures so that the whole issue can be viewedin perspective. For example, if the rotation period andthe shape of a certain object are known, we can applyhydrostatic equilibrium consideration to give some con-straint on its average density (Sheppard and Jewitt,2002). The albedos and color could provide useful informa-tion on the possible chemical makeup of the objects whencompared to asteroids and comets. The presence orabsence of surface color variations could also be used toinfer the surface heterogeneity and even the potential for-mation of a transient atmosphere (Brown and Trujillo,2004). This means ground-based photometric measure-ments could play a useful role in providing some key phys-ical parameters of the TNOs. With this in mind, we haveinitiated an observational program since 2003 at the LulinObservatory, National Central University in Taiwan, witha view to determine the rotation periods and colors of largeTNOs which are accessible to our one-meter telescope. Wehope to extend this study to smaller objects once the newtwo-meter telescope is in place on Lulin in 2009. There is

ed.

Table 1The largest TNOs

Name Equatorial diameter Semimajor axis (AU) Eccentricity (e) Inclination (degree) Rotational period (hours)

Eris, 2003 UB313* 2400 ± 100 67.7 0.442 44.187 3.55?Pluto 2320 39.4 0.249 17.141 153136472, 2005 FY9* 1800 ± 200 45.7 0.159 28.963 TBD136108, 2003 EL61 �1500 43.3 0.189 28.19 3.9Sedna, 2003 VB12 1180–1800 525.6 0.855 11.934 10.273Charon, S/1978 P1 1205 39.4 0.249 17.141 153Orcus, 2004 DW* �1500 39.4 0.226 20.55 20.16Quaoar, 2002 LM60* 1260 ± 190 43.5 0.034 7.983 17.86Ixion, 2001 KX76* 400–550 39.6 0.241 19.6134 TBD55636, 2002 TX300 <709 43.1 0.123 25.9 TBD55565, 2002 AW197 650–750 47.4 0.131 24.3498 17.7455637, 2002 UX25 �910 42.5 0.142 19.482 TBDVaruna, 2000 WR106* 450–750 43 0.051 17.2 6.34

Note: KBOs observed at Lulin are marked by *.

Table 2Observational log of Quaoar and Eris

Target D(AU)

rh a(deg)

V Exposuretime (s)

Observingdates

Number ofsnap shots

Quaoar 42.398 43.405 0.2 19 300 June 3,2003

21

Quaoar 42.398 43.405 0.2 19 300 June 4,2003

36

Eris 96.129 96.906 0.4 18.7 300 Sept 6,2005

44

Eris 96.129 96.906 0.4 18.7 300 Sept 7,2005

46

Eris 96.129 96.906 0.4 18.7 300 Sept 8,2005

21

H.-W. Lin et al. / Advances in Space Research 40 (2007) 238–243 239

also a very interesting possibility that the comprehensivesurvey can be carried out by using the most valuable data-set from the Pan-STARRS Project. A coordinated cam-

Fig. 1. The lightcurves of Quaoar from Lulin observations. The obser-vations were performed in two night in total, 57 data points were obtained.

paign of our Lulin observations and the Pan-STARRSmeasurements could lead to the discovery of new TNOssuitable for close flyby observations by the New Horizonspacecraft to Pluto. We also will search for possible candi-dates for future deep space missions like the Rama Inter-stellar Probe proposed to ESAs Cosmic Vision Program.In this report, we will give a short summary on the workand results achieved so far.

2. Observations

The Lulin Observatory is operated by the Institute ofAstronomy, National Central University. The observatoryis located in the middle of Taiwan at an altitude of 2862 m.For more information on the facilities, instrumentation

Fig. 2. The lightcurves of Eris from Lulin observations. We obtained 111data points in three nights observations. To increase signal-to-noise ratio,we have combined three points to one.

240 H.-W. Lin et al. / Advances in Space Research 40 (2007) 238–243

and observational projects, please see http://www.lulin.ncu.edu.tw. The Lulin one-meter telescope (LOT) isequipped with a Princeton Instruments 1340 · 1300 pixelCCD and Apogee AP8 1K · 1K CCD, giving a field ofview of �11 0 · 11 0. The AP8 CCD has a readout noise of14.7e� and a gain of 4.4 and PI has 4.4e� readout noiseand 2.0 gain (Kinoshita et al., 2005). Seven TNOs havebeen observed between 2003 and 2005. These include50000 Quaoar, Ixion, 2004 DW, Varuna, 2003 TX300,2002AW197, and Eris. Some of the LOT photometric mea-surements were designed to measure the surface colors ofQuaoar, Ixion, and 2004 DW. Special efforts have beenmade to perform time-series R-band observations to deter-mine the lightcurve variations of Quaoar with diame-ter = 1260 ± 190 km (Brown and Trujillo, 2004) and Eris

0

5

10

15

20

25

0 2 4

pow

er

frequenc

0

1

2

3

4

5

6

7

0 5 1

pow

er

Frequen

Fig. 3. The periodograms of (a) Quaoar and (b) E

with diameter = 2400 ± 100 km (Brown et al., 2006b).The observational results and analysis are reported here.

Table 2 gives a summary of the observational log ofQuaoar and Eris at Lulin. The data reduction includedbias and dark current correction and flat-fielding basedon standard packages and procedures in the NOAOIRAF (V2.12). The weather condition was not goodenough for standard calibration, we therefore used differ-ential photometry instead. Differential magnitudes of thetarget TNOs were measured using the DAOPHOT pack-age with reference stars in the fields of view. The light-curves are shown in Fig. 1. We estimated the apparentmagnitude of Quaoar to be mR � 19.0, and that of Eristo be mR � 18.7 by compare with known stars in thesame images. The typical error bars of the individual

6 8 10y (1/day)

"LSP_Quaoar_2003.dat" using 1:9

0 15 20cy (1/day)

’LSP_Eris_2005.dat’ using 1:9

ris. For Quaoar, the maximum peak is 9.42 h.

H.-W. Lin et al. / Advances in Space Research 40 (2007) 238–243 241

exposures (time = 300 s)are about 0.05 mag. The peak-to-peak brightness variation is about Dm � 0.3 for Quaoarand about Dm � 0.05 for Eris. The lack of significantbrightness variation has been noted by Brown et al.(2005).

3. Results

The Lulin data from two nights of observations havebeen analysed using the Lomb technique (Lomb, 1976).The corresponding periodograms are shown in Figs. 2and 3. The maximum for Quaoar at 2.55 cycle/day (or per-iod = 9.42 h) has a confidence level larger than 99.9%.

4.55

4.6

4.65

4.7

4.75

4.8

4.85

4.9

4.95

5

0 0.1 0.2 0.3 0.4

Rel

ativ

e Br

ight

ness

(mag

s)

P

4.55

4.6

4.65

4.7

4.75

4.8

4.85

4.9

4.95

5

0 0.1 0.2 0.3 0.4

Rel

ativ

e Br

ight

ness

(mag

s)

P

Fig. 4. The phase diagram of 50000 Quaoar. For 9.42 h period, the diagram wthe curves will look like (b).

Note that Ortiz et al. (2003) reported a periodicity ofPL = 8.84 h according to their observations of larger sam-ples at Sierra Nevada Observatory in 2003. Our indepen-dent Lulin measurements thus tend to confirm the SierraNevada results. Figs. 4 and 5 show the phase diagramsof Quaoar and Eris, respectively. Ortiz et al. (2003) alsodiscussed the likelihood that the lightcurve of Quaoarmight actually be double-peaked such that the rotationperiod should be twice the value determined by periodo-grams. In this event, we will have P = 18.84 h for Qua-oar’s rotation period according to the Lulin observationsand PL = 17.68 h according to the Sierra Nevada observa-tions. More detailed measurements will help to resolve this

0.5 0.6 0.7 0.8 0.9 1hase

0.5 0.6 0.7 0.8 0.9 1hase

as shown in (a). If the lightcurves actually be double-peaked, PL = 18.84,

1.55

1.6

1.65

1.7

1.75

1.8

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Rel

ativ

e Br

ight

ness

(mag

s)

Phase

1.55

1.6

1.65

1.7

1.75

1.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Rel

ativ

e Br

ight

ness

(mag

s)

Phase

Fig. 5. The phase diagram of Eris. For single-peaked, the Lomb period PL = 3.55 h, the diagram was shown in (a). For double-peaked, the diagram wasshown in (b).

242 H.-W. Lin et al. / Advances in Space Research 40 (2007) 238–243

discrepancy. Note that Ortiz et al. (2003) has pointed outthat Quaoar should be mechanically stable with such arotation period. For the case of Eris, its Lomb period isPL = 3.55 h according to our analysis (see Fig. 4). How-ever, the confidence level is only 50% due to the very smallmagnitude variations Dm � 0.05. The critical period (Pc, inhours) for a rotating body as a function of density andlightcurve amplitude (Dm, in magnitude) can be approxi-mated as P c ¼ 3:3

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1þ DmÞ=q

p(Pravec and Harris,

2000; Trilling and Bernstein, 2006). For q � 1–2 g cm�3,Pc � 2.3–3.3 h, which is close to the Lomb period ofPL = 3.55 h. This line of thinking might suggest that adouble-peak value for the rotation period of 7.10 h is pre-ferred for Eris.

4. Discussion

Our Lulin observations have shown clearly that Quaoaris not a fast rotator like 2003 EL61 which rotation period isas short as 3.9154 h (Rabinowitz et al., 2006). From thispoint of view, it is interesting to note that Eris may havea spin shorter period 3.55 h. In view of the relatively largeuncertainty, it is not possible to draw the conclusion onwhether Eris is indeed rotating at a period of 3.55 h ortwice the value of 7.10 h. According to Carraro et al.(2006), a period of larger than 5 days might even be possi-ble. There could be several interesting consequences ofthese new observational results. First, if Eris has a spin per-iod = 3.55 h, it will be the fastest-spinning object in the

H.-W. Lin et al. / Advances in Space Research 40 (2007) 238–243 243

solar system. Second, with Dm � 0.05, it should be near-spherical shape. This is probably not an expected resultsince Eris’ size is larger than Pluto (Bertoldi et al., 2006;Brown et al., 2006a,b). Following the trend g size-depen-dence the y bulk density of TNOs (Takahashi and Ip,2004; Lacerda and Jewitt, 2007), Eris could have an aver-age density of 2 g cm�3. Along this line of reasoning, itwould also mean that Eris’ surface is relatively homoge-neous in color and albedo. This property contrasts stronglywith Pluto’s large albedo variation. Would this be the phys-ical consequence of surface outgassing activity? Finally, thelack of brightness variation could be the result of a nearpole-on viewing geometry at the present time. In thisway, no surface variations will be revealed. A long termmonitoring program will be required to bright new insightto this enigmatic dwarf planet.

Acknowledgement

This work was supported in part by NSC Grant: NSC94-2111-M-008-033 and Ministry of Education under theAim for Top University Program NCU.

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