Документ взят из кэша поисковой машины. Адрес оригинального документа : http://star.arm.ac.uk/preprints/2007/506.pdf Дата изменения: Mon Sep 24 13:05:31 2007 Дата индексирования: Tue Oct 2 02:54:10 2012 Кодировка: Поисковые слова: comet linear

Astronomy & Astrophysics manuscript no. DIDustESO7 September 17, 2007

c ESO 2007

Dust observations of Comet 9P/Tempel 1 at the time of the Deep Impact
G.P. Tozzi1 , H. Boehnhardt2 , L. Kolokolova3 , T. Bonev4 , E. Pompei5 , S. Bagnulo6 , N. Ageorges5 , L. Barrera7 , O. Ё Hainaut5 , H.U. Kaufl8 , F. Kerber8 , G. LoCurto5 , O. Marco5 , E. Pantin9 , H. Rauer10 , I. Saviane5 , C. Sterken11 , and M. Weiler10
1 2 3 4 5 6 7 8 9 10 11

INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy (tozzi@arcetri.astro.it) Max-Planck Institute for Solar System Research, Max-Planck-Str. 2, D-37191 Katlenburg-Lindau, Germany, University of Maryland, Department of Astronomy, College Park, MD 20742, USA Institute of Astronomy, Bulgarian Academy of Sciences, Tsarigradsko chaussee 72, 1784 Sofia, Bulgaria European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland Universidad Metropolitana de Ciencias de la Educacion, Av. J.P. Alessandri 774, Nunoa, Santiago, Chile European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany Commissariat Energie Atomique F-91191 Gif-sur-Yvette, France Deutsche Luft- und Raumfahrt Agentur, Rutherfordstr. 2, D-12489 Berlin-Adlershof, Germany Vrije Universitet Brussel Pleinlaan 2, B-1050 Brussels, Belgium

Received: today; accepted: tomorrow
ABSTRACT Context. On 4 July 2005 at 05:52 UT, the impactor of NASA's Deep Impact (DI) mission crashed into comet 9P/Tempel 1 with a velocity of about 10 km s-1 . The material ejected by the impact expanded into the normal coma, produced by ordinary cometary activity. Aims. Based on visible and near-IR observations, the characteristics and the evolution with time of the cloud of solid particles released by the impact is studied in order to gain insight into the composition of the nucleus of the comet. An analysis of solid particles in the coma not related to the impact was also performed. Methods. The characteristics of the non-impact coma and cloud produced by the impact were studied by observations in the visible wavelengths and in the near-IR. The scattering characteristics of the "normal" coma of solid particles were studied by comparing images in various spectral regions, from the UV to the near-IR. For each filter, an image of the "normal" coma was then subtracted from images obtained in the period after the impact, revealing the contribution of the particles released by the impact. Results. For the non-impact coma the Af, a proxy of the dust production, has been measured in various spectral regions. The presence of sublimating grains has been detected. Their lifetime was found to be 11 hours. Regarding the cloud produced by the impact, the total geometric cross section multiplied by the albedo, SA, was measured as a function of the color and time. The projected velocity appeared to obey a Gaussian distribution with the average velocity of the order of 115 m s-1 . By comparing the observations taken about 3 hours after the impact, we have found a strong decrease in the cross section in J filter, while that in Ks remained almost constant. This is interpreted as the result of sublimation of grains dominated by particles of sizes of the order of some microns.

Key words. Comet 9P/Tempel 1 ­ Deep Impact event ­ organic grains ­ dust ejecta cloud

1. Introduction
The Deep Impact mission (hereafter DI) to the Jupiter family comet 9P/Tempel 1 (hereafter 9P) was aimed at studying the cratering physics in minor bodies in the solar system and the primordial material preserved inside cometary nuclei. On July 4, 2005 the impactor of the DI experiment produced a highSend offprint requests to: G.P. Tozzi (tozzi@arcetri.astro.it) Based on observations performed at the ESO La Silla and Paranal Observatories in Chile (program ID 075.C-0583)

speed (about 10 km s-1 ) impact in the nucleus of 9P excavating a considerable amount of cometary material that was observed and measured both in-situ by the DI fly-by spacecraft and remotely by Earth-based instrumentation. First results of the mission are described in A'Hearn et al. (2005) and Sunshine et al. (2006). Early earth-based and other space-based measurements of the event have been published by Meech et al. (2005), Sugita et al. (2005), Harker et al. (2005), Lisse et al. (2006), and Schleicher et al. (2005).


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G.P. Tozzi et al.: DI Continuum Imaging at ESO

At the European Southern Observatory (ESO) DI received considerable observing time allocated to observe the event at their Chilean observatory, at Cerro La Silla and at Cerro Paranal sites (Kaeufl et al. , 2005a). Here, we summarize results from the visible and near-IR measurements of the dust in the cometary coma obtained both shortly before and after the DI event. We focus on the dust ejecta properties such as scattering properties, projected velocity, and spatial distribution and their evolution with time. Complementary data from the ESO DI campaign on polarimetric and mid-IR observations as well as on the cometary gas emission and the large-scale coma activity of the comet are described elsewhere (see e. g., Boehnhardt et al., 2007). Pre-impact monitoring of the cometary activity is described by Kaeufl et al. (2005b) and Lara et al. (2006).

2.2. Calibrations on the sky
For calibration purposes, some photometric standards were also observed before and after the comet observations on each clear night. In the near-IR, the normal J H Ks photometric standards were used (Persson et al. , 1998), while for the calibration of the narrow band filters in the visible, well known spectrophotometric standards from the list by Hamuy et al. (1994) were measured. The required calibration frames (bias and sky flatfield exposures for the visible imaging and screen and/or lamp flatfields with lamp illumination on and off for the nearIR) were obtained during daytime and/or twilight periods.

2.3. Observing techniques
The comet imaging was performed with the telescope tracking at the speed of the moving target. Jitter offsets of small amplitude (order of 10-30 arcsec) were applied between individual exposures through a single filter. As usual for extended objects, the observations of the comet in the near-IR spectral region were interlaced by observations of the sky at an offset of 8 in a different region of the sky. A sequence of 5 comet and 5 sky images were usually taken in each near-IR filter. The jitter sequence typically lasted for 11-12 minutes per filter. Due to the mentioned shortage of time, observations on night July 5-6 with ISAAC consisted of only 2 comet and 2 sky images per filter. Observations in the visible were also repeated 5 times for each filter, offsetting the telescope by 10-30 arcsec. Calibration observations (standards, sky flatfields) were performed with the telescope tracking at the sidereal rate. Daytime calibration images (bias, dome flats) used fixed telescope pointing. Since EMMI and SOFI focal plane instruments were mounted on the two Nasmyth foci of the NTT telescope, fully simultaneous observations in the near-IR and visible were not possible. However, the switching time between the two instruments was short (less than 15 min) and allowed us to use both instruments sequentially during the nightly visibility window of the comet. The summary log of observations is given in Table 1. During the observing period the Sun (rh ) and Earth () distances of the comet were 1.51 AU and 0.89­0.91 AU, respectively. The phase (Sun-Comet-Observer) angle was 41 , and the position angle of the Sun projected on the sky at the position of the comet, was 290 .

2. Observations 2.1. Telescopes, instruments, filters
The majority of the observations, described here, were performed at the European Southern Observatory (ESO) in La Silla/Chile using the 3.5 m New Technology Telescope (NTT) by switching between two focal plane instruments: EMMI (ESO Multi-Mode Instrument), for the visible spectral region, and SOFI (Son of ISAAC), for the near-IR (JHK). Both instruments are of focal reducer type for imaging and spectroscopic observations. EMMI provides a field of view of 9.1 в 9.9 arcmin with a two-detector array in the red (400 - 1 000 nm) and of 6.2 в 6.2 arcmin with a single detector in the blue arm (300500nm) at 0.32 and 0.37 arcsec/pixel resolution (using the 2 в 2 and 1 в 1 binning options), respectively. In its large field option used for these observations, SOFI has a single detector of 4.9 в 4.9 arcmin field of view at 0.288 arcsec/pixel resolution. In the visible, narrow band filters, with bandpasses within selected wavelength regions of interest for cometary science, were used. In particular, for the study of the cometary dust, the following filters with no or negligible gas emission in their passband were used: one in the ultraviolet (Uc ), one in the blue ( Bc ) and one in the red (Rc ) spectral region. The near-IR observations were performed with the regular J , H , and Ks broad band filters since in this region the gas contamination is negligible. Table 1 gives the log of observations together with the list of filters used including the respective central wavelength and full width at half maximum (FWHM) of the wavelength passband. Technical information on the La Silla telescope and instruments can be found at http://www.ls.eso.org/lasilla/sciops. Since La Silla was clouded over during night 5-6 July 2006, near-IR imaging of the comet was shifted to the on-going DI campaign at the Cerro Paranal Observatory using ISAAC (Infrared Spectrometer And Array Camera) at the 8.2 m unit telescope Antu of the ESO's Very Large Telescope (VLT). Due to the shortage of time at the end of the nightly visibility window, only part of the J , H and Ks filter imaging sequence was performed. ISAAC is a focal reducer-type instrument providing a field of view of 2.5 в 2.5 arcmin at a pixel resolution of 0.148 arcsec. Technical information on the VLT and the ISAAC instrument can be found at http://www.eso.org/paranal/sciops.

3. Data reduction 3.1. Frame pre-processing
For the visible imaging, all comet and standard star images were corrected for the bias and the flatfield. Both bias and flatfield maps were computed as the average of a series of bias and sky flatfield exposures taken during the observing interval and through the corresponding filters (for the flatfield). Subsequently, first-order sky background correction was applied by subtracting the average sky flux value measured at the four edges of the individual flatfielded images. For the near-IR data the flatfield maps were computed from the screen flat images in each filter with the lamp illumina-


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Table 1. Filter characteristics and summary log for the comet observation. The table lists on the top the filter name, central wavelength, bandpass FWHM and the instrument, as well as, in the bottom, the starting exposure time for the respective filter imaging performed before (- hours) or after (+ hours) the impact. The sky conditions are indicated in the last line of the table, using the following abbreviations: CLR = clear sky, THN = thin cirrus , COUT = clouded out. On night 5-6/07 the table lists J H Ks imaging from the VLT using ISAAC. t - t0 is the time difference between the observation epoch and the DI impact time at the comet. It is given in hours and minutes, with negative values before the impact and positive ones after the event. BAND Rc J 683.8 1247 8.1 290 EMMI SOFI t - t0 t - t0 (hh:mm) (hh:mm) -28:45 +22:32 +90:14 +92:33 +118:21 +138:25 +92:21 +118:40 +138:12 -26:08 -04:39 +17:29 +20:29 +45:32 +65:39 +94:13 +94:26 +113:55 +114:37

0 (nm): (nm): Instrument:

Uc 372.5 6.9 EMMI t - t0 (hh:mm) -29:42

Bc 442.2 3.7 EMMI t - t0 (hh:mm) -28:13

H 1653 297 SOFI t - t0 (hh:mm) -26:21 -04:52 +17:16 +17:16 +45:44 +65:26 +94:00 +113:42 +114:29

Ks 2162 275 SOFI t - t0 (hh:mm) -26:34 + + + + + 17:03 20:13 45:55 65:13 93:47

Night (YYYY-MM-DD) 2007-07-02/03 2007-07-03/04 2007-07/04/05 2007-07-05/06 2007-07-06/07 2007-07-07/08 2007-07-08/09 2007-07-09/10

Weather CLR-THN THN CLR CLR COUT THN THN THN CLR CLR CLR

+141:15

+113:28 +114:09

tion on and off. Then, for each sequence, a median average sky+bias was computed from the sequence of five sky observations. The comet images were then reduced by subtracting the median averaged sky frame and by dividing the result by the flatfield for the corresponding filter. Finally, comet images for each filter/sequence in the visible and near-IR were obtained as the median average of the single 5 images, after their recentering on the photometric nucleus. With the median average of 5 images, all the possible background stars and detector defects (hot or dead pixels) were almost completely erased. For the night of July 5-6, this was not possible, since only 2 comet and 2 sky images were recorded. In this case, the background stars and detector defects were erased manually. Although for morphology studies this was acceptable, it prevented any precise quantitative measurements for this particular night. The same procedure was applied to the standards stars. From the reduced standard star images, photometric zero points were derived for clear nights (using aperture photometry and the procedure described in Boehnhardt et al. (2007) for the EMMI images).

3.2. Residual background flux removal
The presence of a constant residual background was checked and corrected by measuring the function Af at large projected nucleocentric distances, . Af, derived from the Af introduced by A'Hearn et al. (1984), describes the dust albedo (A) multiplied by the total area covered by the solid particles in an annulus of radius and unitary thickness. It is equal to 2Af, where f is the average filling factor of the grains at the projected distance . Note that the definition given here is slightly

different from the the original one given in Tozzi and Licandro (2002) and Tozzi et al. (2004), even though the physical meaning is the same. Assuming a simple outflow pattern, i.e. geometric attenuation and expansion at constant outflow velocity of the cometary dust, the Af function should be independent of . In this case, a small residual background (for instance, from incomplete sky subtraction) would introduce a linear dependence of the function with . Hence, by applying a trial and error procedure, the residual background can be removed from the reduced images such that the Af function becomes constant at large . This procedure does not affect the detection of changes in the cometary activity, since the latter introduces an "expanding bump" in the profile (see section 4.2.2), a very different behavior from the linear dependence with introduced by uncorrected background subtraction. This approach for residual background removal is still applicable for the observations taken within about a day after the impact, since the dust produced by the impact was confined to cometary distances shorter than 20 000 km and the coma flux measured in the SOFI images at larger distances could still be used for the above-mentioned calculations. This method of the residual background removal is not easily applicable to the coma images taken with ISAAC during the night July 5-6, 2005 since the DI ejecta had already expanded to the edge of the field of view. For those observations, we assumed that the integral over the position angle (PA) of the function Af, obtained on the opposite side of the ejecta cloud (over PA between 0 and 90 deg), remained unchanged from night to night. By changing the background level in ISAAC images, the flux profiles measured in this quadrant before and after DI were forced to be constant with . Indications for the assumption of unchanged


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G.P. Tozzi et al.: DI Continuum Imaging at ESO Table 2. Measured Af for the "quiet" comet (see text) Filter Uc Bc Rc J H Ks Af(cm) 111 ± 11 125 ± 12 191 ± 19 228 ± 23 253 ± 25 269 ± 27

appearance of the normal coma comes from the analysis of pre-impact observations (see Lara et al., 2006) and from the fact that the coma signal disappears in the respective PA range when subtracting a pre-impact image from a post-impact one (both taken through the same filter).

3.3. Flux calibration
All the images taken in clear conditions were then calibrated in Af using the following formula, derived from A'Hearn et al. (1984) Af = 5.34 в 10
11

4. Results 4.1. The "quiet" comet

rh dx

2

C s в 10-

0.4(Z p - M s )

(1)

where rh is the heliocentric distance in AU, d x is the detector pixel size in arcsec, C s is the pixel signal in e- /s, Z p and M s are the zero points and the solar magnitude in the used filter, respectively. Images taken at non-photometric conditions are flux calibrated assuming that the Af profiles at large are coincident with those of the day before and/or day after. This was justified by the fact that, due to the low dust expansion velocity, any change in the dust production would not affect regions at larger than 20 000­30 000 km in 24 hours. Moreover, the coma analysis by Boehnhardt et al. (2007) suggests that no significant changes in the flux distribution (except for the DI ejecta cloud) took place between July 3 and 10, 2005. The relative calibration from consecutive good nights was checked by comparing the Af values of the comet at large nucleocentric distances. By a careful examination of all comet exposures, we noticed that the images recorded before and 90 hours after the impact were very similar and no or negligible traces of DI ejecta were found. Hence, in order to increase the signal-tonoise (SN) ratio, we computed images (one per filter) of the "undisturbed" comet (hereafter called the "quiet" comet) as the median average of the comet images taken before and after 90 hours from the impact. The standard deviation of the median average is within 2 - 6 % for the most part of the comet, i.e. , for regions at nucleocentric distances between 2 000 and 50 000 km. In regions closer to the nucleus, this standard deviation increases slightly because of the effect of different seeing in the various nights. It also increases at distances larger than 50 000 km due to the low coma signal. The subsequent scientific analysis of the calibrated images is based mainly on Af profiles and Af, both easily obtained by numerical integration of the flux in the comet images in concentric apertures centered on the nucleus. The physical meaning of the Af profiles is described above. Following the original definition by A'Hearn et al. (1984), Af is proportional to the average comet flux in the aperture multiplied by its equivalent cometocentric projected distance . This function does not depend on for constant outflow velocity. Thus, when using filter images taken in the dust continuum bands, Af is a proxy of the dust production rate, Qdust , of the cometary nucleus. However, due to unknown dust properties such as the dust size distribution and the dust albedo A, it is not straightforward to quantify Qdust using Af measurements of comets.

4.1.1. Af and Af as a measure of cometary dust production
The Af and Af profiles of the "quiet" comet, derived from the observations of 9P in the six continuum bands Uc , Bc , Rc , J , H , Ks during the nights on July 2/3, 3/4, 7/8, 8/9 and 9/10, 2005, are plotted in Fig. 1. Various pieces of information on the comet dust production can be derived from these profiles. The horizontal profiles at distances beyond 10 000 km from the nucleus suggest a steady-state level in the dust production that resembles homogeneous and isotropic dust expansion in the coma at a constant speed. From the existence of jet and fan structures in the 9P coma and since the radiation pressure modifies the dust distribution in the coma, it is clear that these ideal conditions are not fulfilled. However, as long as the jets and fans are stable, i.e. they don't change the dust production, the Af and Af functions are constant. The solar radiation pressure may introduce a linear dependence of these functions with , but it becomes noticeable only at large scales. The Af values as a measure of the dust production of the "quiet" comet are determined at projected distances larger than 40 000 km. where the radial profiles in Fig. 1 have reached constant values. Results are presented in Table 2. The error in the Af measurements is mainly due to the relative photometric calibration error, which is estimated to be of the order of 10 %. The Af values given here are slightly higher than the value of 112 cm given by Schleicher et al. (2005) for observations in the green wavelengths (445­526 nm). They are also higher than the value of 102 cm, later revised to 99 cm, derived from Rosetta/Osiris observations, using the NAC (Near Angle Camera) broad-band filters ((Keller et al. , 2005) and (Keller et al. , 2007)). However, as already pointed out by Schleicher et al., this may be due to the larger phase angle of the spacecraft observations (69 ) compared to the measurements from the Earth (41 ).

4.1.2. Signatures of dust sublimation:
It is evident from Fig. 1 that the near-IR Af profiles are not constant. They increase significantly for distances smaller than 15 000 km, showing also a little spike very close to the photometric nucleus. The spike is probably the signature of the nucleus convolved with the seeing. However, the SN ratio of this signature is too low to derive any useful information. Instead, it can be evaluated through the spatial resolution imaging of


G.P. Tozzi et al.: DI Continuum Imaging at ESO
400 1200

5

300 !Af(cm) Af!(cm)

900

200

600

100

300

0 0 10000 20000 30000 !(Km) 40000 50000 60000

0 0 10000 20000 30000 "(Km) 40000 50000 60000

Fig. 1. Af and Af profiles determined from the "quiet" comet (see text): Uc (solid line), Bc (long dashed line), Rc (short dashed line), J (dotted line), H (Long dashed - dotted line) and Ks (short dashed - dotted line)

Table 3. Af 0 and Af 1 best fit results (see text)
300

Band Uc Bc Rc J H Ks

Af0 (cm) 345.3 ± 1.3 377.9 ± 1.3 603.5 ± 0.6 682.0 ± 0.6 758.4 ± 0.9 792.9 ± 1.6

Af1 (cm) 10.7 ± 4.1 62.8 ± 2.8 2.5 ± 2.8 251.0 ± 1.9 313.8 ± 3.1 392.4 ± 6.3

250 200 !Af(cm) 150 100 50

the coma using adaptive optics systems, such as those collected during the impact week using the NACO instrument at the VLT (not described here). The slow increase of Af cannot be due to dynamical phenomena (for example, increased cometary activity), since the near-IR profiles derived from different observing nights look very much the same. Instead the Af profiles in the visible do not show any evident increase at small nucleocentric distances. Similar Af profiles have been found in comet C/2000 WM1 (LINEAR) (hereafter WM1 ) (Tozzi et al. , 2004). At that time this phenomenon was interpreted as a result of sublimation of two kinds of organic grains: one with a lifetime of 1.3 h and the other of 17 h. Following the analysis of the WM1 data, the near-IR Af profiles of 9P for > 1 000 km were fit by a function of the kind Af () = Af 0 + Af 1 в e
-(
L1

300

800

1300 Central "(nm)

1800

2300

Fig. 2. Wavelength dependence of the permanent (solid line) and sublimating (dashed line) dust components in the coma of comet 9P before the impact. Error bars of the fit parameters Af0 and Af1 in eq. 2 are also plotted, but are too small to be easily seen at the plotted data points.

)

(2)

which contains a constant term Af 0 representing the nonsublimating (permanent) dust component, and just one decaying term Af 1 representing sublimating grains and characterized by the length-scale L1 . The fit achieved for 9P is very good and shows length-scales L1 of similar value for all three near-IR bands, i.e. 6300 ± 160 km. Using the fixed length-scale 6 300 km, the fitting procedure was done one more time, now for all profiles including those derived from visible data. The best fit parameters Af 0 and Af 1 , including standard deviations, are listed in Table 3. Again, the fit gives very good results, with the exceptions of Uc and Rc filters, where Af 1 has values close to zero. Interesting trends appear when plotting the wavelength dependence of the decaying (Af1 ) and the constant (Af0 ) terms of the fits (see Fig. 2). In the near-IR the constant term varies

only by a factor of 1.16 going from J to Ks , while the decaying term changes by a factor of 1.55. This finding may indicate that the two solid components are of a very different nature: one (the permanent one) composed of refractory grains and the other (the decaying one) made of sublimating grains (or dust covered by sublimating material) that scatter very efficiently in the near-IR, but are inefficient scatterers in the visible light. As for the comet WM1 , taking into account that the length-scale for the density was about 25 % longer than that for the column density, and assuming an outflow velocity of about 0.2 km s-1 , the lifetime of the sublimating material was of the order of 40000 s (about 11 hours). Assuming that the sublimation is driven directly by the radiation, the scalelength and lifetime scale as the square of the heliocentric distance, rh . Then the density length-scale and the lifetime at rh = 1 AU should be 3 500 km and 17 800 s ( 5 h). That lifetime differs from those found for the volatile dust grains in WM1 , which, assuming as well an outflow velocity equal to 0.2 km s-1 , were 61 000 s ( 17 h) and 4700 s ( 1.3 h) at the heliocentric distance of that comet (1.2 AU). They scaled to 42000 s ( 12 h) and 3300 s (54 min) at 1 AU. The grain sublimation may not depend directly on the solar irradiation, but may depend indirectly on it, through the grain temperature. In this case the


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4.2. The ejecta cloud 4.2.1. Geometry of the ejecta cloud
For the study of the various effects produced by the DI event, the signal of normal activity was removed from the post-impact images of 9P by simple subtraction of the image of the "quiet" comet, taken through the same filter. This processing should remove the non-impact comet coma without introducing new unwanted features from day-to-day variability, since the normal activity of 9P displayed a rather steady-state appearance. The expanding cloud of solid particles is clearly noticeable in the visible and near-IR images until at least July 6-7, 2005 (three days after DI). Fig. 4 shows the ejecta cloud in J band as seen 17:29 and 20:29 hours after the impact. It can be seen in the figure that the cloud is initially expanding into the coma sector between PA of 120 to 345 . The time evolution of the cloud expansion can be characterized by using visible broadband imaging (Boehnhardt et al. , 2007).

Fig. 3. Difference image of the "quiet" comet minus a synthetic one, as computed from a Ks filter exposure of the comet on July 2-3, 2005 following the procedure described in the text. North is up, East to the left. The Sun direction is indicated. The FoV of the image is 40 000 в 40 000 km2 and intensity in Af (0-15 в10-7 ), with a logarithmic look-up-table. The image shows an enhanced flux level in the southern coma hemisphere due to the sublimating dust and above average activity from isolated regions on the nucleus (jets&fans). It appears as if the sublimating grains are emitted between PA 100 and 200 .

4.2.2. Ejecta dust production
Integrating the Af in the images difference over the position angle range of the initial ejecta cloud (PA=120­345 ), we obtain the Af profile of the cloud vs . The Af profiles determined in the Rc , J , H and Ks filters for the night just after the impact are shown in Fig. 5. Note that the different extension of the cloud profiles in the figure is due to the different observing epochs during which the cloud was expanding in the field of view. By integrating these profiles over , we can obtain the total scattering cross section (SA) of the dust ejecta, SA = (Af )d, i.e., the albedo at the phase angle of the comet multiplied by the total geometric grain cross section. SA provides useful information to evaluate the number and the intrinsic color of the particles produced by the impact and their evolution with time. For the first observations of the first night (July 4-5, 2005) after the impact, we measured the SA to be 27.6, 27.3 and 34.6 km2 in J , H and Ks respectively. Assuming that the scattering properties of the ejecta grains are the same as those of the refractory component in the pre-impact coma, it is possible to estimate the time interval TI necessary to produce SA the same amount of dust by the normal activity: TI = vAf 0 , where v is the mean outflow velocity, and Af0 are the values obtained for the normal activity. To estimate the order of magnitude, any possible diffe