Surface brightness [MJy/sr]

0 10 20 JO Separation in orcminutes

Figure 6. Map of 45'x45' obtained at the North Galactic Pole at 25 fim (left), the histogram of the brightnesses measured in the central 13x13 pixels (middle), and the structure function (right).

values measured towards the orbit of the comet gives 215 K (or 225 K if the surface brightness values were transformed into the DIRBE photometric system, see Sect. 2). Since during these observations P/Kopff was located at about 2 AU from the sun, the derived temperature is somewhat higher than the expected « 200 K at this heliocentric distance. A similar temperature increase was also found from IRAS measurements.

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