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Astron. Astrophys. 324, 357-365 (1997)

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1. Introduction

So far, near-infrared lightcurves (mainly at 2.3 µm) have been presented by many groups (see e.g., Herbst et al., 1995, Hamilton et al., 1995, Orton et al., 1995, Graham et al., 1995, Nicholson et al., 1995, Takeuchi et al., 1995) and a clear picture has emerged in which at least three different kinds of phenomena are identified by the three different features common to all near infrared lightcurves: first precursor, second precursor and main peak. The associated phenomena are bolide, fireball and fall-back phase, respectively. In addition, after the main peak, some lightcurves show a "shoulder" whose origin is still unclear, but may be due to re-entering material "bouncing" of the top of the atmosphere and re-entering a second time (Nicholson, 1996).

However, lightcurves at wavelengths below 1 µm have not been compared with them, to present at least a phenomenology consistent with all of the major impacts, as it is the case for the 2.3-µm lightcurves. Unfortunately, Hubble Space Telescope (HST) did not observe the time evolution of any of the plumes using the same filter for all the exposures (alternating between filters instead, in order to maximize the probability of plume detection (Hammel et al., 1995)). Hence, HST cannot provide the data set needed to study the evolution of the flux.

Only a few impacts generated large enough plumes to be detected clearly in the visible by observing sites with good seeing. Hence, the data sets showing impact phenomena are very scarce. From those few known to us, there is a minimum between two maxima in the H lightcurve (which we present here) from the 4.2-m WHT telescope (La Palma, Spain) at 948 nm. An analog minimum between two maxima was observed for L impact by Schleicher et al. (1994) at 892 nm, and by Fitzsimmons et al. (1996) at 907 nm. Hence, there seems to be a common pattern in all "visible" lightcurves.

In this paper, we present the WHT data on H in the section labeled "Observational results". These observations were made as part of the Comet Impact Network Experiment (CINE). In the section "Definitions and geometric analysis", we offer an explanation for the phenomenology seen in all lightcurves based on geometrical computations and timing arguments for a ballistic plume. In the section "Discussion and comparison with near infrared lightcurves", we discuss the results obtained for H and use some data on L impact by Schleicher et al. (1994) and Fitzsimmons et al. (1996) to derive some physical properties. We also compare some predictions on near-infrared features from our analysis with measured near-infrared lightcurves.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998