Ocultações
estelares por Varuna e Ceres apresentadas em encontro nos EUA
11/10/2010
- Breno Loureiro Giacchini
Na semana passada (3 a 8 de Outubro de 2010) ocorreu em Pasadena,
Califórnia (EUA), a 42ª reunião anual da
Divisão de Ciência Planetária da Sociedade
Astronômica Americana. Nesta reunião foram apresentados
dois trabalhos que tiveram a participação da
Seção de Ocultações da REA e de outros
grupos
astronômicos brasileiros - amadores e profissionais.
Um dos trabalhos foi um pôster apresentado
pelo Felipe Braga Ribas (ON/Observatório de Paris-Meudon),
acerca da recente ocultação de uma estrela pelo
planeta-anão (1) Ceres, ocorrida no dia 17/08 e registrada dos
observatórios CEAMIG/REA, Pico dos Dias/LNA, INPE, UFSC, UEPG.
O resumo deste trabalho segue abaixo e o
pôster pode ser visualizado aqui.
The 17 August 2010
stellar occultation by the dwarf-planet 1 Ceres.
Felipe Ribas1, F. Braga
Ribas2, R. Vieira Martins2, B. Sicardy1,
J. I. B. Camargo2, M. Assafin3, G. B. Rossi2,
D. N. da Silva Neto4, A. Andrei2, C. Jacques5,
E. Pimentel5, M. Emilio6, L. Mehret6,
A. C. Milone7, W. Schoenell8, A. J. T. Mello8,
F. R. Herpich8
1Observatoire de Paris-Meudon, France, 2Observatorio
Nacional, Brazil, 3Observatorio do Valongo / UFRJ, Brazil, 4Universidade
Estadual da Zona Oeste, Brazil, 5Centro de Estudos
Astronômicos de Minas Gerais, Brazil, 6Universidade
Estadual de Ponta Grossa, Brazil, 7Instituto Nacional de
Pesquisas Espaciais, Brazil, 8Universidade Federal de Santa
Catarina, Brazil.
We will present preliminary results of the
occultation of Tycho star 6833-00163-1 by the dwarf-planet 1 Ceres. The
11.8 visual
magnitude star was occulted in the night of 17 August 2010, with the
shadow passing through south, south-east and central part of
Brazil. The observation was a challenge because of the drop in
magnitude was only 0.07 for a visual magnitude of 8.4 foreseen for
Ceres at the time of the occultation. We obtained data from five sites,
with successful detections of the event from four of them.
Occultation chords span from the extreme north portion of the asteroid
to the south of it. As this is a recent event, analysis is still in
progress. Preliminary results will be presented on Ceres' radius, and
possibly, on its oblateness
O outro
trabalho, apresentado por Bruno Sicardy (Observatório de
Paris-Meudon), tratou da ocultação estelar pelo
transnetuniano (20000) Varuna, ocorrida na noite do 19/02/10 e que
mobilizou observadores em vários estados brasileiros (clique aqui para ver a página da
campanha). O resumo do trabalho The
2010, February 19 stellar occultation by Varuna segue abaixo:
The 2010, February 19 stellar occultation by Varuna
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.
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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