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Characterizing the strangest dwarf planet
Mutual events of (136108) Haumea and its moon Namaka
M. T.
1

1, Bannister

M. E.

2, Brown

D. A.

2, Ragozzine

W. C.

2, Fraser

P. J.

1 Francis

Research School of Astronomy and Astrophysics, Mt Stromlo Observatory, The Australian National University 2 Division of Geological and Planetary Sciences, California Institute of Technology
Satellites of Haumea

Introduction
The dwarf planet Haumea is the parent body to the only known icy collisional family in the Kuiper belt.[1] This family and Haumea's two moons formed in a primordial giant impact that left Haumea with an extremely rapid rotation, causing it to distort into an elongated ellipsoid. For the next few years the orbit of its satellite Namaka will cross the visible disk of its parent (Fig. 1). This orbital configuration, which will not repeat for more than a century, is predicted to produce complex eclipses, transits, occultations and shadow transits between Namaka and Haumea.[2] We aim to detect many of these mutual events, thus measuring multiple chords across Haumea. This will allow us to constrain its unique shape to within ~20 km, a precision not possible with other available techniques, and these constraints will provide information on the effects of Haumeas satellite-forming collision.

Observations worldwide
· 17 Haumea-Namaka events visible worldwide in 2009 · 18 telescopes of 2 m aperture on six continents observing as part of this international campaign · Five events visible from Australia in 2009

useful enough Simply combin global minimu the fit, going fr 1.10, althoug Adding the Ke the error bars a parameters. A 1- error bar mass estimate in a best-fit Na HST data alon entire Monte C dataset is 8 ratio of 2 в 10 ness ratio of ties greater th sumes that the on the solution rectly cross-cal Fig. 1.-- Relative positions of the satellites as viewed from Earth. with HST astr The outer orbit corresponds to the brighter Hi'iaka and [2] inner Figure e1. onds torthesfainttere moon.sIoftheaunteras theumea, The o bit of h Namaka n H ce me i , Ha orbit corr sp the relative pla w w c t sc nt , as e to p ecess r id croy, p c m n of 5 due 00 drahin hocoaleinusumingran ellipsoapidlss-seritioarily 00 x 1to 0 km telescopes coul (Rabieffeicz of ahe 0a6) ertmthenongiaaisa,rientedamaka.outh. now t t et t l. 2 l 0 rg wi h oo l, H i x k o on N North-S the ted parameters The apparent orbit changes due to parallax and three-body effects; This currently creates 200 . See Fi ure 2 f r model a angle does not this is the view near March the8mutualgeventoseason. nd data positions throughout the observation period (2005-2008). verified by tria the HST-only s mass of Namak error bars for each parameter are given by the standard Using the M deviation of global-best parameter fits from these synwe can also ca thetic datasets. For each parameter individually, the disrors. Using Ke occultation eclipse tributions were nearly Gaussian and were centered verygraze lite), the perio 19.3 ingress nearly on the best-fit parameters determined from the + ingress 0.083 days and actual data. The error bars were comparable to error a ratio of 2.706 19.4 bars estimated by calculating where 2 increased by 1 actual mean m from the global minimum (see Press et al. 1992). affected by the First, we consider a solution using only the observanon-spherical n 19.5 tions from HST. Even though these are taken with differThe mass rat ent instruments (ACS, NICMOS, and mostly WFPC2), ± 0.00030 and 19.6 the extensive calibration of these cameras allows the diNamaka/Hi'iak rect combination of astrometry into a single dataset. tual inclination 19.7 The best-fit parameters and errors are shown in Table where the mu 2. The reduced chi-square for this model is 2ed = 0.64 tween the two r sin iH sin iN cos (2 = 36.4 with 57 degrees of freedom). The data are 19.8 clination and l very well-fit by the three point mass model, as shown in this significant Figures 2 and 3. A reduced 2 less than 1 is an indi19.9 in Section 4.3. cation that error bars are overestimated, assuming that is the angle b they are independent; we note that using 10 separate 20 tic first point "observations" for the Feb 2007 data implies that our Expected event geometry rors in the arg observations are not completely independent. Even so, (M ) shown in 2ed values lower than 1 are typical for this kind of as20.1 r 9 10 11 13 16 nte Carlo re Mo trometric orbit fitting 12 .g., Grundy et a14 2008). 15 ach (e l. E time amete s is ecovered, thoug the N = 202.57 ± of the fit parof dayr(UT)r(hours), 13 Mayh2009 mass of inal point-mas Namaka is only detected with a 1.2- significance. NaFigure 2. Observed ai's htcusrvethe Haumea, rameter . Tdeterredictsed e ventcwas es by abo hang mak l g mas is of hardest pa May 13 to he p mine ince obs an eclipse + occitltatiuir; s deteicting mines man-Kdplerve ± erturbationsertainty.ervations, i u reqone pred cted tim ute no rke e haian p 1 hour unc years; the nonto orbit of the more massive Hi'iaka. The implications tected with ver of the orbital state of the Haumea system are described in the next section. We also list in Table 3 the initial 3.2 condition of the three-body integration for this solution. The HST data are sufficient to obtain a solution for The non-sph Hi'iaka's orbit that is essentially the same as the orbit ditional, poten obtained from the initial Keck data in B05. NevertheThe largest of less, the amount and baseline of Keck NIRC2 data is gravitational m

Analysis:

magnitude (R filter)

A troublesome affair

SSO 2.3 m observations
We observed two events on May 13 and 21 2009, using the imager on the 2.3 m telescope at Siding Spring Observatory.
We recorded a comparison lightcurve of Haumea on the preceding night. This was necessary as Haumeas intrinsic lightcurve varies on timescales of ~1 week. We obtained R filter 2 minute integrations, without dithering, for all airmasses where Haumea was above the horizon limit. Conditions were photometric with close to 1" seeing. High winds and humidity prevented us observing the event on May 21.

· Event produces a ~1% change in a lightcurve with ~20% intrinsic variation · Haumea: magnitude 17.3 in R
Differential photometry with comparison stars in the field was used to generate the lightcurve (Fig. 2). Although S/N on Haumea was >100, image stacking was required to provide adequate S/N on the comparison stars. As the duration of the event is several hours and ingress/egress is twenty minutes, this did not adversely affect the time resolution. The comparison stars were selected as Haumea-coloured through a single I filter image taken during the comparison lightcurve. The comparison lightcurve proved unusable, due to imperfect telescope focus after repeated telescope reboots following power brownouts.

Conclusion
No conclusive detections of the Haumea and Namaka mutual events have yet been made.
Further observations with the 2.3 m at SSO are scheduled for July 6, 7 and 15. Further observations are also scheduled at other telescopes during 2009.

References
1. Brown, M. E.; Barkume, K. M.; Ragozzine, D.; Schaller, E. L. 2007. A collisional family of icy objects in the Kuiper belt. Nature, 446(7133):294-296 2. Ragozzine, D; Brown, M. E. 2009. Orbits and masses of the satellites of the dwarf planet Haumea (2003 EL61). Astronomical Journal, 137(6):4766-4776

Acknowledgements
Thanks to Donna Burton for assistance during the 2.3 m observing.

Further information
http://haumea-namaka.blogspot.com/ http://web.gps.caltech.edu/~mbrown/2003EL61/mutual/

Contact: M.T. Bannister michele@mso.anu.edu.au