Title: Exploring the surface properties of Transneptunian Objects and Centaurs with polarimetric FORS1/VLT observations
Authors: S. Bagnulo, H. Boehnhardt, K. Muinonen, L. Kolokolova, I. Belskaya, M.A. Barucci
Polarisation is a powerful remote-sensing method to investigate solar system bodies. It is an especially sensitive diagnostic tool to reveal physical properties of the bodies whose observational characteristics are governed by small scatterers (dust, regolith surfaces). For these objects, at small phase angles, a negative polarisation is observed, i.e., the electric vector E oscillates predominantly in the scattering plane, contrary to what is typical for rather smooth homogeneous surfaces. The behaviour of negative polarisation with phase angle depends on the size, composition and packing of the scatterers. These characteristics can be unveiled by modelling the light scattering by the dust or regolith in terms of the coherent backscattering mechanism.
The researchers have investigated the surface properties of TNOs and Centaurs by means of polarimetric observations with FORS1 of the ESO VLT. TNOs Ixion and Quaoar, and Centaur Chiron show a negative polarization surge. The Centaur Chiron has the deepest polarisation minimum (-1.5 - 1.4%). The two TNOs show differing polarisation curves: for Ixion, the negative polarisation increases rapidly with phase; for Quaoar, the polarisation is relatively small (~ -0.6%), and nearly constant at the observed phase angles. For all three objects, modelling results suggest that the surface contains a real mixture of at least two components with different single-scatterer albedos and photon mean-free paths.
Visible and near-infrared spectroscopic observations carried out on August 1st 2005 by a group leaded by the ING-IAC astronomer Javier Licandro1 using the William Herschel Telescope (WHT) and the Telescopio Nazionale Galileo (TNG) at El Roque de Los Muchachos Observatory show that the recently discovered trans-neptunian object (TNO) 2005 FY9 is very similar to Pluto. Results have been published in the journal Astronomy & Astrophysics.
Visible spectroscopy was obtained using the ISIS spectrograph on the WHT, and near-infrared spectroscopy was obtained using the NICS spectrograph on the TNG. The complete visible and near-infrared spectrum is shown in the graph and compared with the spectrum of Pluto and that of pure methane ice. The graph clearly shows that the spectra of both TNOs are very similar. They are dominated by strong absorption bands produced by methane ice. In fact, the absorption bands in the spectrum of 2005 FY9 are deeper than in the spectrum of Pluto as a result of the larger abundance of methane ice in 2005 FY9. Also the colour of the surface of the TNO is red (indicated by the slope of the spectrum), similar to that of Pluto. This shows the presence of complex organic compounds in the surface.
The discovery of a Pluto 'twin' in the trans-neptunian belt is relevant as Pluto is the only known TNO possessing a small atmosphere. The similar size and surface composition of 2005 FY9 are facts that suggest it can also have such a tenuous bound atmosphere.
Until now only one known TNO, Pluto, showed the presence of strong methane ice absorption bands in the spectrum. However, apart from 2005 FY9, recently these bands were also observed in the spectrum of the largest yet known TNO, 2003 UB313
IN GOOD COMPANY: Astronomers have spotted a moon orbiting the newly discovered tenth planet, 2003 UB313, or Xena, as it has been codenamed after the TV warrior princess. The moon, dubbed Gabrielle for the TV Xena's sidekick, is estimated to be one tenth the size of the planet, which itself is believed to measure around 2,700 kilometers across. Scientists plan to observe Gabrielle again in November and December using the Hubble Space Telescope, which should enable them to ascertain the moon's orbit and mass.
The new Trans-Neptunian objects mentioned in the press earlier this year are being formally announced this week at a planetary conference in Cambridge, England. Bearing the extremely temporary names 'Xena,'(2003 UB313) 'Santa,' (2003 EL61) ( and 'Easterbunny,'(2005 FY9) the new objects are quite interesting in their own right (Santa is cigar-shaped, rotates end-over-end every 3.9152 hours, and has a 60-mile-diameter moon). The conference in Cambridge represents the first formal, scientific disclosure of the objects.
"They're just weird, and it appears that they must have been thrown into their unusual positions by Neptune's gravity jolting them like a slingshot" - Michael E. Brown
The three astronomers are Michael E. Brown of the California Institute of Technology, Chadwick A. Trujillo of the University of Hawaii and David Rabinovitz of Yale University. Trujillo and Rabinovitz are discussing their finds at a planetary conference this week in Cambridge, England, but Brown stayed home because he promised his wife he'd help care for their new baby.
Informally, Brown's team named Xena after TV's Warrior Princess; Santa because it was discovered last Dec. 28, just after Christmas, and Easterbunny because it appeared just before Easter this year. The light-hearted names are unlikely to be recognized officially