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Titania's radius and an upper limit on its atmosphere from the September 8, 2001 stellar occultation |
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| Institution: | a Observatoire de Paris, LESIA, 5, Place Jules Janssen, 92195 Meudon cedex, France b Université Pierre et Marie Curie et Institut Universitaire de France, 103 boulevard Saint-Michel, F-75005 Paris, France c European Asteroidal Occultation Network (EAON), 60 impasse des Lauriers, F-84400 Apt, France d Instituto Superior de Ciencias Astronómicas, and Liga Iberoamericana de Astronomía, Buenos Aires, Argentina e International Occultation Timing Association, European Section (IOTA/ES), Bartold-Knaust-Strasse 8, 30459 Hannover, Germany f International Occultation Timing Association (IOTA), PO Box 6356, Kent, WA 98064-6356, USA g IOTA, 4535 Cedar Ridge Trail, Houston, TX 77059, USA h Observatoire de Paris, IMCCE, 77 Avenue Denfert-Rochereau, 75014 Paris, France i Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA j Observatoire de Bordeaux, 2, Rue de l'Observatoire, BP 89, 33270 Floirac, France k Simoco Digital Systems Ltd, Radio House, St Andrews Road, Cambridge, Cambridgeshire, CB4 1GR, United Kingdom l British Astronomical Association, Asteroids and Remote Planets Section, Lower Earley, Reading, RG6 4AZ, United Kingdom m Starlight Xpress, Foxley Green Farm, Ascot Road, Holyport, Berkshire, SL6 3LA, United Kingdom n Osservatorio di Trieste, Via G.B. Tiepolo 11, I-34143 Trieste, Italy o Astrocam, F-06790 Aspremont, France p Observatoire du Tim, Chemin La Chapelle, 04700 Puimichel, France q Centre d'Astronomie, 04870 St.-Michel l'Observatoire, France r Observatoire de Haute-Provence/CNRS, 04870 St.-Michel l'Observatoire, France s Association Marseillaise d'Astronomie, 213, Avenue de Montolivet, F-13012 Marseille, France t Laboratoire de Mécanique et d'Acoustique, UPR 7051, 31 chemin Joseph-Aiguier, F-13402 Marseille cedex 20, France u ASCT Section Astronomie, 3 rue du 19 mars 1962, F-76400 Toussaint, France v Home observatory, F-13300 Salon de Provence, France w Astronomie en Crau, F-13310 Saint-Martin de Crau, France x Dutch Occultation Association, Boerenweg 32, NL 5944 EK Arcen, The Netherlands y Impasse de Biaune, F-21160 Corcelles les Monts, France z Observatoire du Collège de l'Etang de l'Or, UAI 180, 34130 Mauguio, France aa Observatoire de Malibert, F-34360 Saint-Chinian, France ab Association des Utilisateurs de Détecteurs Electroniques (AUDE), c/o F. Colas, 45, Avenue Reille, 75014 Paris, France ac Observatoire d'astronomie de Saint-Caprais, F-81800 Rabastens, France ad Observatoire Jolimont, 1 Avenue Camille-Flammarion, 31500 Toulouse, France ae Société d'astronomie populaire poitevine (SAPP), Centre Socio culturel du Clos Gaultier, F-86000 Poitiers, France af Observatoire de Saint-Savinien sur Charente, F- 17350 Saint-Savinien sur Charente, and Association Nationale pour la Protection du Ciel Nocturne (A.N.P.C.N), c/o S.A.F, 3 rue Beethoven, F-75016 Paris, France ag Observatoire de Dax, rue Pascal Lafitte, F-40100 Dax, France ah Centre de Documentation Spécialisé en Astronomie - CDSA Martinique, 34 Boulevard Amilcar Cabral, Cluny - BP 411, F-97200 Fort-de-France, France ai Observatory of Algiers, CRAAG, Route de l'Observatoire, BP 63, Bouzaréah 16340, Algiers, Algeria aj Agrupación Astronómica de Barcelona, Carrer Aragó 141-143, 2-E, 08015 Barcelona, Spain ak Departamento de d'Astronomia i Meteorologia, IEEC-Universitat de Barcelona, Avenue Diagonal 647, 08028 Barcelona, Spain al Agrupación Astronómica de Sabadell, C. Prat de la Riba, s/n, Apdo 50, Sabadell, Barcelona, Spain am Asociación Valenciana de Astronomía, 16 Bajo Valencia, 46014 Valencia, Spain an Sociedade Portuguesa de Astronomia, Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal ao Associação de Astrónomos Amadores da Madeira (AAAM), Villas da Bela Vista, Moradia X, 9125-103 Caniço, Madeira, Portugal ap Instituto de Astrofísica de Andalucía, Apdo. 3004, 18080 Granada, Spain aq Agrupación Astronómica Isla de La Palma (AAP), Apartado de Correos 449, 38700 Santa Cruz de La Palma, Provincia de Santa Cruz de Tenerife, Islas Canarias, Spain ar Associação Portuguesa de Astrónomos Amadores (APAA), Rua Alexandre Herculano 57-4° Dto, 1250-010 Lisboa, Portugal as Associação Nacional de Observação Astrónomica (ANOA), Apartado 582, 2430 Marinha Grande, Portugal at Centro de Observação Astronómica no Algarve (COAA), Poio, 8500 Portimão, Portugal au Núcleo Açoriano da Associação Portuguesa de Astrónomos Amadores (NAAPAA), Açores, Portugal av Universidade da Madeira, Departamento de Matemática e Engenharias, Grupo de Astronomia, Campus da Penteada, 9000-390 Funchal, Madeira, Portugal aw Barbados Astronomical Society, Harry Bayley Observatory, Bridgetown, St. Michael, Barbados ax Trinidad & Tobago Astronomical Society, Sir Frank Stockdale Building, The University of the West Indies (UWI), St-Augustine Campus, Trinidad and Tobago ay Observatorio Arval, Santa Teresa, Caracas, Venezuela az Asociación Larense de Astronom?a (ALDA), Barquisimeto, Lara, Venezuela ba American Association of Variable Star Observers, 49 BayState Rd., Cambridge, MA 02138, USA bb Universidad de Los Andes, Facultad de Ciencias, 5101, Mérida, Venezuela bc Cité des Sciences et de l'Industrie, 30 avenue Corentin-Cariou, 75930 Paris cedex 19, France bd Observatorio Astronómico de Quito, Parque de la Alameda, PO Box 17-01-165, Quito, Ecuador be Universidad de Monterrey, Departamento de Física y Matemáticas, Avenue I. Morones Prieto, 4500 Pte., San Pedro Garza García, N.L. 66238, Mexico bf Asociación Panamena de Aficionados de la Astronomia, Panama bg George Observatory, Brazos Bend State Park, Houston, TX 77030, USA bh Universidad Autónoma de Nuevo Leon, Facultad de Ciencias Físico Matemáticas, Avenue Universidad s/n, Ciudad Universitaria, San Nicolás de los Garza, N.L. 66451, Mexico |
| Abstract: | On September 8, 2001 around 2 h UT, the largest uranian moon, Titania, occulted Hipparcos star 106829 (alias SAO 164538, a V=7.2, K0 III star). This was the first-ever observed occultation by this satellite, a rare event as Titania subtends only 0.11 arcsec on the sky. The star's unusual brightness allowed many observers, both amateurs or professionals, to monitor this unique event, providing fifty-seven occultations chords over three continents, all reported here. Selecting the best 27 occultation chords, and assuming a circular limb, we derive Titania's radius:  (1-σ error bar). This implies a density of  using the value  derived by Taylor Taylor, D.B., 1998. Astron. Astrophys. 330, 362-374]. We do not detect any significant difference between equatorial and polar radii, in the limit  , in agreement with Voyager limb image retrieval during the 1986 flyby. Titania's offset with respect to the DE405 + URA027 (based on GUST86 theory) ephemeris is derived: ΔαTcos(δT)=−108±13 mas and ΔδT=−62±7 mas (ICRF J2000.0 system). Most of this offset is attributable to a Uranus' barycentric offset with respect to DE405, that we estimate to be:  and ΔδU=−85±25 mas at the moment of occultation. This offset is confirmed by another Titania stellar occultation observed on August 1st, 2003, which provides an offset of ΔαTcos(δT)=−127±20 mas and ΔδT=−97±13 mas for the satellite. The combined ingress and egress data do not show any significant hint for atmospheric refraction, allowing us to set surface pressure limits at the level of 10-20 nbar. More specifically, we find an upper limit of 13 nbar (1-σ level) at 70 K and 17 nbar at 80 K, for a putative isothermal CO2 atmosphere. We also provide an upper limit of 8 nbar for a possible CH4 atmosphere, and 22 nbar for pure N2, again at the 1-σ level. We finally constrain the stellar size using the time-resolved star disappearance and reappearance at ingress and egress. We find an angular diameter of 0.54±0.03 mas (corresponding to  projected at Titania). With a distance of 170±25 parsecs, this corresponds to a radius of 9.8±0.2 solar radii for HIP 106829, typical of a K0 III giant. |
| Keywords: | Occultations Uranus satellites Satellites shapes Satellites dynamics Ices Satellites atmospheres |
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