Behavior of Comet 9P/Tempel 1 around the Deep Impact event

Abstract

Context. Comet 9P/Tempel 1, the target of the Deep Impact (DI) Mission, has been monitored from June 18 until July 12, 2005, as a follow up to the long-term campaign started in Jan. 2005. Aims. The goal of this campaign is to characterize the comet before the DI event and to detect changes in activity and coma morphology produced by the DI experiment on July 04.226 UT 2005. Methods. Optical broadband images and long-slit spectroscopic measurements have been acquired with the instrument CAFOS mounted at the 2.2 m telescope at Calar Alto Observatory (CSIC-MPG) from July 1 to 11, whereas only imaging has been performed with Versarray at the 1.5 m telescope of the Sierra Nevada Observatory (IAA-CSIC) on June 26 and 29, 2005. Results. Fifteen hours after the impact, the ejecta cloud extends over ~240° in position angle (PA) with a symmetry axis at PA ~ 235°. The effect of the solar radiation pressure is already visible as a slight deviation from a fully symmetric plume and the ejecta dust is already feeding the tail. The exhaustive analysis of the broadband images has revealed that no new long-lasting coma structure is produced by the impact. The structures existing in the coma before the event are recovered after the ejecta plume has moved out. The maximum projected expansion velocity of the ejecta dust results in ~230 and ~150 m/s 15 and 40 h after impact, respectively. Surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in the north-south direction from the long-slit spectra can generally be well fit with a slope m of -0.94 $\le m \le$ -1.49 in $\log B - \log \rho$ representation. A few exceptions occur on July 2 and 8-10 when much flatter continuum profiles are detected that are possibly related to fragmentation processes and to the reported outbursts occurring around those dates. Normalized color $S^\prime$ of the dust inside the coma does not show spatial variations excluding July 04.875 UT, our first observation after the impact. At that time, the dust inside the ejecta plume is undoubtedly bluer than the surrounding coma (8.2$ \pm $0.4 % /100 nm versus 14.5 ± 0.8% /100 nm). The dust color averaged at $\rho \le$ 10 000 km returns to a ~12% /100 nm on July 07.875 UT, the same value measured a few days before the projectile impact. A lower limit to the mass in the ejecta can be given from our optical observations resulting in 1.2$\times$10 6 kg, which represents about 14 h of quiet (i.e., steady state) pre-impact activity. The value of $A f \rho$ is remarkably variable during the 18 days monitoring as several outbursts took place, beside the one induced by the DI experiment. Apart from outburst periods, $A f \rho \sim$110-12 cm. The gas activity represented by the CN, C 2 , and C 3 production rates ( Q ), are relatively constant from July 1 to 6 excluding the immediate post-impact period on July 4. The number of molecules of CN, C 2 , and C 3 produced by the DI were equal to 2.13$\times$10 29 , 2.07$\times$10 29 , and 1.49$\times$10 28 as measured 15 hrs after impact. The amount of their potential parent species detected at other wavelengths seems to indicate that a large fraction of the daughter species measured 15 h after the impact might originate from the ejected dust grains. Conclusions. General conclusions include (i) new post-impact refractory material different from that seen pre-impact was ejected during the DI experiment, i.e., these dust grains had either different optical properties or a size distribution peaking at smaller sizes, (ii) the DI event did not initiate a long-lasting period of sustained cometary activity, and (iii) in many ways the artificial impact looked very much like a natural outburst of the comet, also (iv) some fraction of gaseous daughter species within the coma after the DI experiment might have originated in the ejected dust grains.