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The Astrophysical Journal,632:L45–L48,2005October10

KECK OBSERVATORY LASER GUIDE STAR ADAPTIVE OPTICS DISCOVERY AND CHARACTERIZATION OF A SATELLITE TO THE LARGE KUIPER BELT OBJECT2003EL

61 M.E.Brown,1A.H.Bouchez,2,3D.Rabinowitz,4R.Sari,1C.A.Trujillo,5M.van Dam,2R.Campbell,2 J.Chin,2S.Hartman,2E.Johansson,https://www.wendangku.net/doc/6f2825449.html,fon,2D.Le Mignant,2P.Stomski,2D.Summers,2

and P.Wizinowich2

Received2005July28;accepted2005September2;published2005October3

ABSTRACT

The newly commissioned laser guide star adaptive optics system at Keck Observatory has been used to discover

and characterize the orbit of a satellite to the bright Kuiper Belt object2003EL

61.Observations over a6month

period show that the satellite has a semimajor axis of49,500?400km,an orbital period of49.12?0.03days, and an eccentricity of0.050?0.003.The inferred mass of the system is(4.2?0.1)#1021kg,or～32%of the mass of Pluto and28.6%?0.7%of the mass of the Pluto-Charon system.Mutual occultations occurred in1999 and will not occur again until2138.The orbit is fully consistent neither with one tidally evolved from an earlier closer con?guration nor with one evolved inward by dynamical friction from an earlier more distant con?guration.

Subject headings:comets:general—infrared:solar system—minor planets,asteroids

1.INTRODUCTION

The properties of the orbits of Kuiper Belt object(KBO)

satellites hold keys to fundamental insights into the masses and

densities of KBOs,the interaction history of the early solar

system,the internal structure of distant ice-rock bodies,and

the genesis of the Pluto-Charon binary.Progress in character-

izing the orbits of KBO satellites has been slow,owing to the

fact that observations of closely orbiting satellites have required

long–lead-time observations from the Hubble Space Telescope,

while observations of distant satellites require long baselines

to see a full orbital period.

To date,only a small number of relatively small KBO sat-

ellites have had their orbits determined(Veillet et al.2002;

Osip et al.2003;Noll et al.2004a,2004b).The total masses

of the systems range from4.2to37#1018kg,the orbital

eccentricities range from0.31to0.82,orbital periods range

from46to almost900days,and the?ux ratio between the

primary and secondary ranges from1.17to1.66.The Pluto-

Charon binary,in contrast,has a mass almost4000times higher

than the next closest object,an eccentricity close to zero,an

orbital period of only6.4days,and a?ux ratio between Pluto

and Charon of about20.The great difference in satellite char-

acteristics between the small KBOs and Pluto suggests different

mechanisms operating in the different mass regimes.

One of the goals of our ongoing survey for bright KBOs

(Trujillo&Brown2003)is to?nd higher mass satellite systems

that can be used to examine the mass range between these two

extremes.The newly discovered KBO2003EL

61is one of the

brightest known KBOs and thus likely among the most massive. It also has the advantage that it is one of the only currently known KBOs accessible to the newly commissioned laser guide star(LGS)adaptive optics(AO)system at the W.M.Keck 1Division of Geological and Planetary Sciences,Mail Code170-25,Cali-fornia Institute of Technology,1200East California Boulevard,Pasadena,CA 91125;mbrown@https://www.wendangku.net/doc/6f2825449.html,.

2W.M.Keck Observatory,65-1120Mamalahoa Highway,Kamuela,HI 96743.

3Caltech Optical Observatories,Mail Code105-24,California Institute of Technology,1200East California Boulevard,Pasadena,CA91125.

4Yale Center for Astronomy and Astrophysics,Yale University,P.O.Box 208121,New Haven,CT06520.

5Gemini Observatory,670North A‘ohoku Place,Hilo,HI96720.

Observatory.Below we describe observations of2003EL

and the discovery and orbital characterization of its satellite with the LGS AO system.

2.OBSERVATIONS

Observations of2003EL

and its satellite were obtained with the LGS AO system(Wizinowich et al.2005)at the W.M.Keck Observatory using the NIRC2infrared imager.

The LGS AO system works similarly to the natural guide star adaptive optics system(Wizinowich et al.2000),except that rather than measuring the wave-front aberrations using the light from a natural star,they are determined from observation of laser light resonantly scattered off of sodium atoms in a layer at approximately90km altitude in Earth’s mesosphere.One limitation of the LGS AO technique is the need to obtain absolute tip-tilt measurement from a natural reference within approximately1?of the target.At the current level of Keck LGS AO performance,this tip-tilt star is required to have R!

18.5mag to achieve nearly diffraction-limited images at

2.1m m(M.van Dam et al.2005,in preparation).Faint KBOs

are dif?cult to observe for long periods with LGS AO,as appropriate tip-tilt stars are close enough only sporadically.The object2003EL

,with a V magnitude of17.5(Rabinowitz et al.2005),is the only known KBO that is bright enough to use as its own tip-tilt source,relieving the dif?culties of otherwise highly time-constrained observations.

The observations of2003EL

were performed during six nights of LGS AO commissioning time between2005January 26and June30,by the Keck Observatory AO engineering team.

The object was?rst acquired by the avalanche photodiode tip-tilt sensor,which kept2003EL

centered in the?eld by con-tinuously driving the fast tip-tilt mirror.The sodium-dye laser was then projected toward the target,where it was acquired by the fast wave-front sensor.The signal from this fast wave-front sensor was then used to control the256-actuator deformable mirror to correct for atmospheric aberrations.The laser power output was12.0–13.5W,creating a reference beacon of equiv-alent magnitude11.0!V!12.5and allowing the high-order loop to be run at400Hz.Finally,the focus and high-order image-sharpening control loops were closed between the low-bandwidth wave-front sensor(a separate high-order wave-front

L45

L46BROWN ET AL.Vol.

632

Fig.1.—Composite LGS AO image of2003EL

61and its satellite from the

night of2005June30.The faint source directly south of2003EL

61is a faint

background star,blurred by the motion of2003EL

61.With the17.5mag

2003EL

61used as a tip-tilt reference,the LGS AO achieved an on-axis Strehl

ratio of0.21and a FWHM of62mas.

TABLE1

Separation of2003EL

and Its Satellite Date

(UT)

Mean

Time

R.A.Offset

(mas)

Decl.Offset

(mas) 2005Jan26.......15:5135?14?629?14 2005Mar1........12:10293?23?1004?23 2005Mar4........11:36339?20?1263?20 2005May27......07:39?62?10604?10 2005Jun29.......07:26?197?5520?10

sensor observing the tip-tilt reference)and the fast wave-front sensor.This sensor corrects focus offsets due to the variable altitude of the atmospheric sodium layer and quasi-static ab-errations caused by the apparent elongation of the laser beacon as seen by the fast wave-front sensor.

While the above acquisition steps were being performed,the NIRC2imager was con?gured with a K ?lter(1.948–2.299m m), and0?.009942pixel?1plate scale.Once the LGS AO feedback-loop bandwidths and gains were optimized for the seeing con-ditions,we recorded several30s or60s integrations with NIRC2at each of three dither positions separated by2?–5?on the detector,for a minimum of360s total integration.

The images were corrected for sky and instrumental back-ground by subtracting the median of the images in each dither pattern.They were then?at-?elded using twilight-sky?ats,and known bad pixels were corrected by interpolation.On one night of attempted observation,poor natural seeing prevented ade-quate LGS AO correction,but on every other night from the discovery of the satellite on2005January26,the satellite is readily detected in individual30or60s exposures.Figure1 shows an image from the most recent night of observation(2005 June30).Taken under excellent seeing conditions,the June30 images had a median Strehl ratio of0.21and full width at half-maximum of62milliarcseconds.

To measure the position of the satellite with respect to the primary,we?rst?tted the primary with a two-dimensional Gaussian.While a Gaussian is a poor approximation to the actual shape of the image,the center position will still be mea-sured quite accurately.We then?tted the secondary to a two-dimensional Gaussian with the same width as found for the primary.This?tting is performed independently for each in-dividual observation,and the mean and standard deviation of the individual measurements are taken as the offset and mea-surement error for each night.Table1gives a summary of the positions of the satellite over the course of the observations. Any changes in the detector position angle or plate scale over time will contribute to additional error.Long-term monitoring has shown that the position angle is stable to within0?.2,and the plate scale to within0.3%(S.A.Metchev2005,private communication).Both of these contributions are smaller than the random error.

3.ORBIT FITTING

Fitting the orbit of a solar system satellite requires correctly accounting for the changing viewing geometry due to the mo-tion of Earth and the primary in addition to the orbit of the satellite.With a limited number of observations,the orbital solutions can be plagued by aliases of different orbital periods. Luckily,the spacing of our data rules out spurious aliases.Two of the observations were taken3days apart,and the satellite moved0?.26between the two.The solution that we present below forces the motion to be only a fraction of an orbit be-tween these two dates,rather than a full orbit(or more)plus a small fraction.If we were to allow these short-period aliases, the mass of the primary object would have to be at least1025kg, or about2Earth masses,which we reject as unreasonable. To check for similarity with the Pluto-Charon system,we ?rst attempt to force a circular orbit to the positions of the satellite.The?t uses a Powell x2minimization to?nd the optimal orbital parameters.For a circular?t,the?ve parameters are semimajor axis,orbital period,inclination,longitude of the ascending node,and mean anomaly.The best?t?nds a semi-major axis of4.95#104km and an orbital period of49.1days, but it has a x2value of292,or a reduced x2for5degrees of freedom(10x-y coordinates minus?ve orbital parameters)of 58.The probability of a x2value this high due to chance is minuscule.We thus reject a circular orbit as a viable solution. Allowing a full eccentric orbital solution requires adding eccentricity and longitude of perihelion to the orbital param-eters.The best eccentric?t is shown in Figure2and Table2 and has a x2value of0.73,or a reduced x2for3degrees of freedom(10coordinates minus seven orbital parameters)of 0.24.The?t has a marginally lower x2value even than ex-pected,which suggests that we have overestimated the error bars in the positional measurements by approximately a factor of2.To be conservative,we maintain the current error bars and accept the slightly larger errors.The extremely low value of x2gives us con?dence,however,in our previous rejection of the circular orbit and our acceptance of this orbital solution. Observations of a projected orbit frequently have degener-acies between different solutions that appear identical re?ected across the plane of the sky.The changing vantage point of Earth during these observations breaks this degeneracy,how-ever.The difference in predicted positions between the two degenerate solutions differs by more than0?.05across the ob-serving season,well above the measurement uncertainties.The best-?t re?ected orbit can be rejected at the more than99.9% con?dence level;thus,we are con?dent that we have found the single viable orbital solution.

No.1,2005SATELLITE OF EL

L47

Fig.2.—Relative position of the satellite compared with2003EL

61.The

very small crosses inside the circles show the LGS AO observations along with their error bars,while the circles show the best-?t orbital solution’s pre-dicted locations at the times of the observations.The ellipse shows one full orbit surrounding the mean date of the observations.The slight discrepancies between this projected orbit and the positions of the predictions is caused by

the changing Earth–2003EL

61viewing geometry across the6months of

observation.

TABLE2

Orbital Parameters

Parameter Value Semimajor axis.....................49,500?400km Inclination..........................234?.8?0?.3 Period...............................49.13?0.03days Eccentricity.........................0.050?0.003 Argument of perihelion............278?.6?0?.4 Longitude of ascending node......26?.1?0?.4

Time of pericenter passage........JD2,453,458.40?0.02 Note.—Relative to J2000ecliptic.

We determine errors in the individual parameters through Monte Carlo simulation.We perform1000iterations of orbit optimization where we add Gaussian noise with j equal to the measurement errors of the position measurements and solve for new orbital parameters.We de?ne the1j error bars on the parameters to be the range containing the central68%of the data.Table2gives the ecliptic orbital elements of the satellite orbit.

The uncertainties on the individual orbital parameters are not independent,and thus uncertainties on quantities obtained from multiple parameters need to be determined separately through Monte Carlo analysis.The mass,in particular,depends on both semimajor axis and period.We calculate the retrieved mass independently in each Monte Carlo simulation and de?ne the uncertainty in the?nal mass identically to the manner for the individual parameters above.The?nal retrieved mass of

the2003EL

61–plus–satellite system is(4.2?0.1)#1021kg,

or～32%of the mass of Pluto and28.6%?0.7%of the mass of the Pluto-Charon system.From relative photometry on2005 June30,the satellite is3.3mag fainter than the primary,so for a similar density and albedo,it contributes only1%of the mass and can thus be neglected.

One method of obtaining detailed information on the shapes of the objects in such a system is through observations of mutual eclipses,analogous to those observed for Pluto and Charon in the1980s(Binzel&Hubbard1997).The satellite system is currently only4?from being viewed edge-on.Un-fortunately,the system is moving away from its edge-on con?guration.The orbit was last edge-on in late1999and will not be again for133years,in2138.

4.DISCUSSION

The2003EL

61system contains more than1000times the

mass of the next most massive measured Kuiper Belt satellite system and is within a factor of4of the mass of the Pluto-Charon system.The0.050eccentricity of the satellite is much closer in character to the circular orbit of the Pluto-Charon system than the higher eccentricities of the other systems.One signi?cant difference between the2003EL

system and the Pluto-Charon system,however,is that while Pluto and Charon are locked into6.3day rotation periods commensurate with their6.3day orbit,2003EL

has a high-amplitude double-peaked3.9hr rotation period,which is signi?cantly shorter than the49.1day orbital period of the satellite(Rabinowitz et al.2005).For all physically possible values of its density, 2003EL

’s spin angular momentum dominates by orders of magnitude the orbital angular momentum of the system.The object’s spin is slowed little by its satellite,owing to the likely very small relative mass of the satellite.

The fast spin of the primary and the near-circular orbit of the satellite suggest formation by impact.Detailed simulations of the higher mass Pluto-Charon–forming impacts show that satellites with a few percent of the mass of the primary can be formed in many different types of collisions(Canup2005).If such a collision occurs,the satellite orbit then must tidally evolve to its present position.We can estimate the expected orbital period of the satellite after4.5Gyr of evolution as

3/13?5/13

k/1.5r

P p(58days)90q,(1)

()()?3

Q/1001g cm

where k is the tidal Love number,q is the ratio of the satellite’s to the primary’s mass,and r is the density of the primary.For a purely?uid body with k p1.5,this58day estimate is in reasonably good agreement with the observed49day orbital period,but for any reasonable value of strength the period drops well below that observed.

Tidal evolution will affect the eccentricity as well as the period.The amount and direction of eccentricity evolution depends on the strengths of2003EL

and the satellite.If 2003EL

is strengthless while the satellite is not,eccentricity evolves mostly as a result of tides on2003EL

,which cause eccentricity to increase on the same timescale as the semimajor-axis increase.Such an increasing eccentricity is inconsistent with the low eccentricity of the satellite orbit.If both bodies are strengthless,then tides on the satellite dominate the eccen-tricity evolution and cause it to damp.The timescale for ec-centricity damping can be estimated as

˙e/e m r k Q r

p p p

s s

p p,(2)

()()()()()

F F

˙a/a2m r k Q2r

s p p s s

where the second equality assumes that both bodies have similar quality factors and similar densities.The eccentricity-damping timescale is about15times shorter than the orbital

L48BROWN ET AL.Vol.632 evolution timescale,which is by assumption the age of the

system.Therefore,the eccentricity of the satellite damps every

300Myr and should be expected to be extremely small.The

0.05eccentricity of the system thus appears dif?cult to explain

from orbital evolution alone.Stern et al.(2003)considered

perturbations from surrounding bodies as a method to excite

the putative eccentricity of the Pluto-Charon system.Scaling

their detailed calculations to the circumstances of2003EL

61,

we?nd that typical eccentricities for the2003EL

61system

should be on the order of e≈0.003,much smaller than the observed eccentricity.While a recent strong perturbation cannot be ruled out,the probability of such an event is extremely low. An alternative to tidal evolution of the system is that the system formed by capture and the semimajor axis was de-creased by dynamical friction(Goldreich et al.2002).While bodies of equal mass can evolve to become contact binaries, bodies of unequal mass evolve to have a semimajor axis of approximately the primary radius times the mass ratio.The estimated period is approximately a factor of3higher than that measured here,but the estimate is likely good only to an order of magnitude and so may be consistent.It is unknown how the eccentricity should evolve in this case.Given our current un-derstanding of the outer solar system,neither formation sce-nario is entirely satisfying for explaining both the moderate orbital period and small but signi?cant eccentricity of the system.

This research is supported by a Presidential Early Career Award from the NASA Planetary Astronomy Program.Data presented herein were obtained at the W.M.Keck Observatory, which is operated as a scienti?c partnership among the Cali-fornia Institute of Technology,the University of California,and the National Aeronautics and Space Administration.The observatory was made possible by the generous?nancial sup-port of the W.M.Keck Foundation.The authors wish to rec-ognize and acknowledge the very signi?cant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.We are most fortunate to have the opportunity to conduct observations from this mountain.

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