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Astron. Astrophys. 343, L87-L90 (1999)

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2. Observations

Observations were performed by ISOPHOT-S (Lemke et al., 1996), which allows the simultaneous observation in the two spectral ranges 2.5-5 µm (hereafter SW) and 6-12 µm (hereafter LW) with two 64-element detector arrays. The spectral resolution achievable by this observing mode is not very high, [FORMULA] 85 in the SW and [FORMULA] 95 in the LW, but sufficient to identify typical features of gas and dust components. The field of view is a [FORMULA] x [FORMULA] square. The integration time of the observations was 4096 s. The observations were carried out on 1 Jan. 1998 at 22:58:04 UT (starting time of observation), at RA (J2000) = 23h 34m 16s and dec. (J2000) = [FORMULA] [FORMULA] [FORMULA], when the comet was at geocentric distance R = 1.043 AU and heliocentric distance D = 0.824. The spectrum of the background was taken on 5 Jan. 98 at 19:23:59 UT with the same instrument parameters and on the same sky position. The background spectrum is used to remove the background sky contribution. The data reported in the present paper are the output of the ISOPHOT auto-analysis data reduction (pipeline vers. 7.0, see Laureijs et al., 1996 for more details). Thus, they must be considered preliminary and need a further more careful analysis for intensity calibration.

In order to correctly interpret the comet observation, we have to consider the actual position of the comet with respect to the pointing position. In this respect, the parallel ISOCAM observations, which will be presented in a forthcoming paper, can help us. The spot of the ISOPHOT observation is well centred on the comet positions. The spectrum of 103P/Hartley 2 is shown in Figs. 1 and 2. The SW portion presents well pronounced features and first evidence of the thermal continuum emission at the longest wavelengths; the reflected sunlight contribution is negligible. The LW range is characterised by a featureless thermal emission, with no evident emission features.

[FIGURE] Fig. 1. The 2.5-5 µm spectrum of comet 103P/Hartley 2 obtained with ISOPHOT-S on 1 January 1998. The percent errors on the band peak intensity are of the order of 20%

[FIGURE] Fig. 2. The 6-12 µm spectrum of comet 103P/Hartley 2 obtained with ISOPHOT-S on 1 January 1998. The percent error on the flux is about 20%. The thick solid line is a 285 K blackbody fitted to the data

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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