Observatório do Valongo/UFRJ, Ladeira Pedro Antonio 43, CEP 20.080-090 Rio de Janeiro – RJ, Brazil e-mail: email@example.com
2 Observatório Nacional/MCT, R. General José Cristino 77, CEP 20921-400 Rio de Janeiro - RJ, Brazil
3 Observatoire de Paris/LESIA, Meudon, France
4 Université Pierre et Marie Curie, Institut Universitaire de France, Paris, France
5 Southwest Research Institute, 1050 Walnut St, Boulder, CO 80302, USA
6 Centro Universitário Estadual da Zona Oeste, Av. Manual Caldeira de Alvarenga 1203, CEP: 23.070-200 Rio de Janeiro - RJ, Brazil
Accepted: 4 February 2010
Context. We investigate transneptunian objects, including Pluto and its satellites, by stellar occultations.
Aims. Our aim is to derive precise, astrometric predictions for stellar occultations by Pluto and its satellites Charon, Hydra and Nix for 2008-2015. We construct an astrometric star catalog in the UCAC2 system covering Plutos sky path.
Methods. We carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. We made the astrometry of 110 GB of images with the Platform for Reduction of Astronomical Images Automatically (PRAIA). By relatively simple astrometric techniques, we treated the overlapping observations and derived a field distortion pattern for the WFI mosaic of CCDs to within 50 mas precision.
Results. Positions were obtained in the UCAC2 frame with errors of 50 mas for stars up to magnitude R = 19, and 25 mas up to R = 17. New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. We generated 2252 predictions of stellar occultations by Pluto, Charon, Hydra and Nix for 2008-2015. An astrometric catalog with proper motions was produced, containing 2.24 million stars covering Plutos sky path with width. Its magnitude completeness is about R = 18–19 with a limit about R = 21. Based on the past 2005–2008 occultations successfully predicted, recorded and fitted, a linear drift with time in declination with regard to DE418/plu017 ephemerides was determined for Pluto and used in the current predictions. For offset (mas) = A * (t (yr) - 2005.0) + B, we find A = +30.5 ± 4.3 mas yr-1 and B = -31.5 ± 11.3 mas, with standard deviation of 14.4 mas for the offsets. For these past occultations, predictions and follow-up observations were made with the 0.6 m and 1.6 m telescopes at the Laboratório Nacional de Astrofísica/Brazil.
Conclusions. Recurrent issues in stellar occultation predictions were addressed and properly overcome: body ephemeris offsets, catalog zero-point position errors and field-of-view size, long-term predictions and stellar proper motions, faint-visual versus bright-infrared stars and star/body astrometric follow-up. In particular, we highlight the usefulness of the obtained astrometric catalog as a reference frame for star/body astrometric follow-up before and after future events involving the Pluto system. Besides, it also furnishes useful photometric information for field stars in the flux calibration of observed light curves. Updates on the ephemeris offsets and candidate star positions (geometric conditions of predictions and finding charts) are made available by the group at www.lesia.obspm.fr/perso/bruno-sicardy/.
Key words: astrometry / occultations / planets and satellites: individual: Pluto / planets and satellites: individual: Charon / planets and satellites: individual: Nix / planets and satellites: individual: Hydra
Tables of predictions for stellar occultations by Pluto, Charon, Nix and Hydra for 2008–2015 and Catalog of star positions for 2008–2015 sky path of Pluto are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A32
Also based on observations made at the Laboratório Nacional de Astrofísica (LNA), Itajubá-MG, Brazil.
Associate researcher at the Observatoire de Paris/IMCCE, 77 Avenue Denfert Rochereau 75014 Paris, France.
© ESO, 2010