Presentation Abstract

Title The 2010, February 19 stellar occultation by Varuna
Author Block Bruno Sicardy1, F. Colas1, L. Maquet1, F. Vachier1, A. Doressoundiram1, F. Roques1, T. Widemann1, J. Ortiz2, M. Assafin3, F. Braga-Ribas4, J. I. B. Camargo3, R. Vieira-Martins3, A. Andrei3, D. da Silva Neto3, R. Behrend5, F. Hund6, M. Hauser7, S. Wagner7, A. Slotegraaf8, D. Willasch8, M. de Jesus Costa9, P. Melo Sousa10, E. Faustino11, A. Prazeres11, S. Machado12, J. Dias do Nascimento13, E. Souza Barreto14, A. Amorim15, P. da Rocha Poppe16, M. Geraldete Pereira16, C. Jacques17, B. Loureiro Giacchini17, A. Collucci18, W. Ferreira da Costa19, V. Martins de Morais20
1Obs. de Paris, France, 2Inst. Astrof. Andalucia-CSIC, Spain, 3Obs. Valongo & Obs. Nacional, Brazil, 4Obs. Valongo, Obs. Nacional & LESIA Obs. Paris, France, 5Obs. de Geneve, Switzerland, 6Hakos farm, Namibia, 7Landessternwarte, Koenigstuhl 12, Germany, 8Stellenbosch Flying Field, South Africa, 9OBAFIS-UEMA, Brazil, 10SAMA, Brazil, 11AAP-SAR, Brazil, 12Quixada, Brazil, 13Natal, Brazil, 14Obs. Genival Leite, Brazil, 15Florianopolis, Brazil, 16Obs. Antares, Brazil, 17CEAMIG-REA, Brazil, 18Vespasiano, Brazil, 19Brasilia, Brazil, 20Est. Astronomica PieGise, Brazil.
Abstract On 2010, February 19, Varuna occulted UCAC2 star 41014042, as seen from regions in southern Africa and north-eastern Brazil.
No occultation was observed neither from the station deployed in South Africa, nor from three other stations in Namibia. Out of twelve stations deployed in Brazil, seven had clouds or instrument malfunctions. Three were negative, one possibly positive (visual) is still being analyzed, and one is definitely positive from Sao Luis (CCD). This makes Varuna the most remote solar system object observed to date through a stellar occultation, with a geocentric distance of 6.4 billions km.
The Sao Luis occultation has a duration of 52.5 +/- 0.5 sec, corresponding to a chord length of 1003 +/- 9 km projected in the plane of the sky. No atmospheric signature is apparent in the light curve. Since the closest observation to Sao Luis is negative at a transversal distance of 225 km (Quixada, CCD), a significantly elongated shape is required for Varuna.
We will discuss our results in view of current independent estimations of Varuna's diameter combining IR thermal and visible data, with a typical value of ~1050 km +/- ~200 km (Lellouch et al. 2002, Stansberry et al., 2008). We will also discuss the fact that the occultation occurred near Varuna's maximum brightness along its 6.4 hr rotational (0.4-mag amplitude) light curve. Thus Varuna was observed at maximum apparent surface area, which also corresponds to maximum apparent oblateness for an ellipsoidal shape.
Lellouch et al, Astron. Astrophys. 391, 1133-1139 (2002).
Stansberry et al., in The Solar System beyond Neptune (eds Barucci, M. A.,Boehnhardt, H., Cruikshank, D. P. & Morbidelli, A.) 161-179 (Univ. Arizona Press, 2008).
This work is partially supported by french ANR 08-BLAN-0177 "Beyond Neptune".

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42nd DPS Program published in BAAS volume 42 #4, 2010.