Robert M. West

ESO large program on physical studies of Transneptunian Objects and Centaurs: Visible photometry – First results

We present the first results of BVRI photometry of Transneptunian Objects (TNOs) and Centaurs obtained through the ESO Large Program on physical studies of these icy bodies in the outer solar system. In total 28 objects were observed of which 18 are new measurements. Combining our new BVRI photometry with the data summary published by Hainaut & Delsanti (2002) results in a database of 94 objects: 45 Cubewanos, 22 Plutinos, 13 scattered disk objects, 14 Centaurs. The reddening range seems to be similar among the four dynamical classes (− 5t o 55%/l00 nm) and only one outlier (1994 ES2) exists. The spectral gradient distribution of the Cubewanos peaks between 25 to 35%/l00 nm, while for the three other types the maximum seems to fall below 20%/l00 nm. A clustering of red Cubewanos with perihelia beyond ∼41 AU in low eccentricity and low inclination orbit suggests that these objects are less affected by the physical processes that potentially produce neutral colors, i.e. resurfacing by collision and by intrinsic activity. For Cubewanos and scattered disk objects, the range of reddening increases with decreasing perihelion distance and with increasing orbital excitation. A correlation of the spectral slope with inclination is present for Cubewanos and scattered disk objects, and is non-existent for the other dynamical types. It is unclear whether these trends (or their absence) are discriminative for the correctness of the resurfacing scenarios. If intrinsic activity is responsible for resurfacing, the start of the effect inside ∼41 AU from the Sun may be indicative for the driving agent, while in the collision scenario the survival of the red Cubewano cluster in the central region of the Kuiper-Belt argues for the existence of a population of bodies the surface of which is heavily radiation processed without impact resurfacing.

Visible and near-infrared spectroscopy of the Centaur 32532 (2001 PT13). ESO Large Programm on TNOs and Centaurs: First spectroscopy results

We present photometric and spectroscopic data obtained through visible and near-infrared observations of the Centaur 32532 (2001 PT13) performed with FORS1 and ISAAC at the Very Large Telescope (VLT-ESO) within the framework of an ESO large program on the Trans-Neptunian objects (TNOs) and Centaurs. The results show evidence for a difference in the near-infrared spectral behaviour obtained during two observations carried out one month apart. In one spectrum there is the possible presence of signatures of water ice in small amounts. Two models have been proposed to interpret the surface composition of this Centaur.

VLT observations of comet 46P/Wirtanen

Comet 46P/Wirtanen, prime target of ESAs ROSETTA mission, was successfully observed at the Very Large Telescope Observatory in Chile: on 17 May 1999 with the Test Camera at the Cassegrain focus of the 8.2-m VLT Kueyen telescope and on 8 December 2001 with FORS1 at Unit Telescope 4 Yepun. May 1999: no coma was detected at heliocentric distance r = 4.98 AU. From the measured brightness in the Bessell R-filter, a mean nucleus radius of 555 ′ 40 m is derived (for a geometric albedo of 0.04 and a phase darkening of 0.04 mag/deg). The nucleus signal varies during the 2.7 h observing interval and a peak-to-peak amplitude of ∼0.38 mag is determined. The measured lightcurve is in agreement with a rotation period of 6-7.5 hours and a ratio of the main nucleus axes of at least 1.4. The non-detection of a coma allows one to put an approximate upper limit for Afp of <0.45 cm (suggesting a dust production rate of 0.05 kg/s). December 2001: a weak and condensed coma seems to be present in the seeing disk of the comet at 2.9 AU inbound, causing a higher brightness than expected from the previous size estimates of the nucleus. The colour of the comet appears very red (V - R spectral gradient ∼47%/100 nm). The Afp value of the comet was 6.5 ′ 2 cm (equivalent to a dust production rate of about 1 kg/s).

Near Earth Asteroid search and follow-up beyond 22nd magnitude. A pilot program with ESO telescopes

We have performed a Near Earth Asteroid search and follow-up test beyond 22nd magnitude with the 2.2-m MPG/ESO and the New Technology Telescope (NTT) facilities at La Silla. The experiment comprised a total num- ber of 4 nights at the 2.2-m telescope and 3 nights at the NTT on two separate runs. In addition to the discovery of two NEAs and the recovery of many more, this pilot program has shown the advantages as well as the problems of a dedicated program using much larger facilities than the ones currently used worldwide. We confirm the results of Jedicke et al. (2003), that by observing at fainter magnitudes and finding objects at larger distances, such a system will discover km-sized NEAs with higher orbital e and i as well as a larger proportion of the smaller NEAs; moreover, it will shorten the time needed to reach 90% com- pleteness for km-sized objects. The pilot program also evidenced the need for follow-up facilities compatible with the discovery telescopes.