The Dusty View of DI from ESO Chile

Around the time of the impact of NASA’s Deep Impact (DI) mission at comet 9P/Tempel 1, in total 6 telescopes with altogether 7 different instruments, located at the La Silla (LSO) and Paranal (VLT) Observatories of the European Southern Observatory (ESO) in Chile, were used to characterize the dust properties before and after the event. The ejecta cloud expanded at an average speed of about 200 ms−1 during the first hours after the event. It reached stagnation distance of 25000 km about 3 days after impact. The pre-impact dust jet and fan activity (‘porcupine’ pattern) remained undisturbed after impact. In our measurements the jet activity can be traced to a few 100 km nucleus distance. In total 9 comastructures are identified which may originate from at least 4 regions of enhanced dust emission on the nucleus – one of this region may in fact be multiple. No obvious signatures of a new active region created by DI are found. The overall dust production during the impact compares to about 5– 10 h of normal activity. The global expansion geometry of the DI cloud is compatible with a majority of dust grains in the micron size range. Indications exist for asymmetric brightness and colour distributions of the dust in the ejecta cloud. The dust temperature rose from about 280–290 K before to 330 K one day after the event and fell to pre-impact level the day thereafter. The dust reflected sunlight was found to be linearly polarized at about 7.5% in the visible and near–IR, at constant level within about 4000 km from the nucleus. No circular polarization of the dust is detected. 148 H. Boehnhardt et al. 1 DI Dust Observations at ESO The observations of the dust, produced by the DI event at comet 9P/Tempel 1 on 4 July 2005, were performed at ESO’s La Silla and Paranal Observatories in Chile. They cover the immediate pre-impact period and extend until several days after the event. However, at the time of the impact (05:53UT) the comet had already set for the ESO sites and could only be re-observed again 14 h (mid-IR) and 16 h(visible and near-IR) thereafter. All telescopes, operated by ESO at the two sites, together with a number of instruments were ‘employed’ for the DI dust campaign: at La Silla the 2.2 m ESO/MPG telescope plus the Wide-Field-Imager (WFI), the 3.5 m New Technology Telescope (NTT) plus the EMMI and SOFI instruments and the 3.6 m telescope plus TIMMI2; at Paranal the four 8.2 m unit telescopes (UT1-4) of the Very Large Telescope (VLT) using FORS2 and ISAAC at UT1, FORS1 at UT2, VISIR at UT3 as well as NACO and SINFONI at UT4. The suite of telescopeinstrument combinations allowed a comprehensive observing program covering the visible (WFI, EMMI, FORS1, FORS2), near-IR (SOFI, ISAAC, NACO, SINFONI) and mid-IR (TIMMI2, VISIR) wavelength range (0.36 – 20 μm) with different observing techniques (imaging, spectroscopy, polarimetry). Table 1 summarizes the ESO observations obtained for the analysis of the dust component of the Deep Impact event. In parallel, ESO telescopes and instruments were also used for observations of the gas phenomena of the comet and related with DI (for an overview see [3]). The ESO web pages of the La Silla and Paranal observatories provide detailed information on the telescope and instrument equipment used.