Monitoring of 9P/Tempel 1, the Deep Impact target, from CAHA

L.M. Lara (IAA-CSIC, Spain), H. Boehnhardt (MPS, Germany), R. Gredel (CAHA, Spain), P.J. Gutiérrez (IAA-CSIC, Spain), J.L. Ortiz (IAA-CSIC, Spain), R. Rodrigo (IAA-CSIC, Spain) & M.J. Vidal-Nuñez (IAA-CSIC, Spain)

Telescope: 2.2 m at Calar Alto Observatory (CSIC-MPG) [Code 493]
Coordinates: 2°32'44.9''W, 37°13'24.5''N, 2172.62 m
Instruments: CAFOS - R and I Johnson filters, medium-resolution spectrophotometry with grism B200- and BUSCA

 

In January 2005 NASA's Deep Impact (DI) spacecraft was launched to perform a cratering experiment at Comet 9P/Tempel 1 on 4 July 2005: A 360 kg impactor has hit the cometary nucleus at about 10km/s speed to excavate a crater and to initiate new activity of the nucleus. The scientific goals of this unique experiment are - apart from studying crater physics - to characterize the target, 9P/Tempel 1, as a cometary nucleus, i.e. as a representative of the primordial bodies from the formation period of the planetary system. Due to the very limited instrument equipment on-board the fly-by spacecraft (two cameras and a spectrometer) that was expected to follow the impact for a bit more than 10 minutes, a significant science contribution is expected from Earth-based observations. Important conclusions result from the comparison of post-impact results with observations on the pre-impact status of the comet. The published measurements on the gas and dust production of the comet (for instance Osip et al. 1992, Cochran et al. 1992 , Storrs et al. 1992)  were obtained in the last two decades of the last century, i.e. more than two orbital revolutions of the comet ago. In preparation of the DI mission, more recent results on the comet (for instance nucleus parameters like size and rotation), collected during the past perihelion-to-aphelion arc of the orbit, are published by Meech et al. (2005) and Belton et al. (2005).

 

Comet 9P/Tempel 1, the target of the Deep Impact Mission, was intensively monitored from the 2.2 m telescope at Calar Alto Observatory (MPG-CSIC) since Jan. 2005 to July 12, 2005. Most of the observations have been made in service mode, excluding to runs "in situ" of 5 and 12 days in mid-April and beginning of July, respectively . The comet has been imaged either in R or I broadband filters every 3 days to compute Afρ -a parameter equivalent to the production rate of small dust particles- and to determine the existence of structures and its evolution along the orbit. Additionally, once every month, spectroscopic measurements were done in order (i) to derive the gas (CN, C2, C3 …) production rate and colour of the dust as the comet approaches perihelion and (ii) to analyse the behaviour of the surface brightness profile (B) versus the projected cometocentric distance (ρ) as a function of cometocentric and heliocentric distance. This has provided us with a unique set of consistent data spanning the pre-impact phase, impact event and post-impact phase.

 

Most of the analysis is still going on and here we only show some of the acquired images with no conclusions as every individual result must be analysed in the frame of the whole experiment results.

 

The complete set of images to be shown are acquired in R Johnson with CAFOS, N is up E to the left, FOV is 5'x5' for the frame containing the comet image as seen by the instrument, and  53”x53” for the laplace filtered one.

 

Beside the increase of activity as the comet approached the Sun, the most outstanding results come from the morphological follow up of the dust structures existing in the coma since the beginning of the monitoring, i.e. January 2005. These structures are hidden in the nearly isotropic coma and they can be enhanced by simply applying a laplace filtering.

In January and early February, the only morphological structure noticed in the coma is a short straight feature at position angle (PA) ~250º, i.e. more than 60º out off the direction of the dust tail.  The Sun position is indicated by a yellow circle.

 

 

 

From mid Feb., the structure in the southwest coma quadrant can be resolved into a coma fan.  In early March, further straight (or slightly curved) features appear in the southern coma hemisphere and in April the coma resembled very much a porcupine pattern (similar to C/1995 O1 Hale Bopp) with features identified in PA ranging from 110º to 300º.

 

 

During May and June, the porcupine pattern is reduced to 4 straight jets. On June 14, observations from Calar Alto were the first ones in detecting an outburst of the comet, that was later on also detected by the Hubble Space Telescope and by the imaging system on the Deep Impact spacecraft. This outburst is manifested as an expanding arclet of material spanning about 120º in the NW direction.

 

 

 

Four days later, when the comet was successfully imaged again, this ejection of material was not seen anymore. Typical dust velocities, projected on the plane of the sky, of 50-100 m/s,  produce an expansion and dilution of the shell into space.

 

During the impact week, observations were carried out "in situ" and the comet was regularly imaged in B,V,R,I Johnson. Spectrophotometric observations were also acquired to monitor the CN, C2, C3, NH2, CH, etc. production rates pre-impact, hours after the impact and days after the impact until July 12, 2005. A cometary spectrum typically looks like this

where beside the skylines, gas emissions as CN, C3, C2 (Dn=0,-1) and solar light scattered by the dust particles in the coma can be seen.  This particular spectrum was acquired on May 11, when the comet was 1.601 AU from the Sun and 0.716 AU from the Earth (still in the pre-perihelion branch). Spectra taken during the impact phase show the same pattern although the gas emissions are stronger and the dust ones show a remarkably asymmetry in the North-South direction, where the slit was positioned.

 

From July 1^st to the 3^rd, the comet did not show any noticeable differences from previous weeks and months. On July 04, at 5:52UT, it was impacted by the projectile that the Deep Impact spacecraft released 24 hours before. This event was not visible from Calar Alto (it occurred during daytime). However, 16.5 hours later, when the comet became visible to CA telescopes again, the coma –before doing any processing to enhance previously existing or new structures- did show an outstanding asymmetry. After laplace filtering, the cloud of dust produced by the impact became clear and its width and expansion velocity could be determined.

 

A few days later, this dust cloud moving at a velocity of 100-300 m/s had expanded and diluted in such a way that it was not clearly detected in our frames anymore.

 

All of the data acquired during this 6 and a half months are already reduced and partially analysed. For the pre-impact phase, a paper has been submitted for publication to A&A. The impact and post-impact phase data, further results and analysis will be presented in the Asteroids, Comets and Meteors 2005 Conference,. They will be also submitted for several publications dealing with dust and gas coma: its morphology, production rates and implications on the comet nuclei derived from the Deep Impact experiment.

 

Although the comet has been exhaustively studied from the 1^st to the 7^th of July from most observatories on ground, data acquired at Calar Alto Observatory are unique in the sense that they cover more than 6 months of thorough study of the comet. The comprehension of the impact experiment itself, the short and long term effects this might have produced on the nucleus and a better understanding of the structure and composition of the comet nuclei require to have the most complete picture of the pre-impact state. The most updated baseline for pre- post- impact comparison of the comet state has been acquired from Calar Alto Observatory, and it is already having a great repercussion within the Deep Impact Project.