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Ecliptic latitude [deg]

-14 -12 -10 -8 Ecliptic latitude [deg]

Figure 4. ISOPHOT brightness profiles of the —10° asteroidal band. At 25/xm the profile extracted from the lower spatial resolution scan, observed on Jan. 8, 1998, is plotted with triangles. The band profiles are fitted with Gaussians, whose parameters are given in Tab. 1.

Figure 4. ISOPHOT brightness profiles of the —10° asteroidal band. At 25/xm the profile extracted from the lower spatial resolution scan, observed on Jan. 8, 1998, is plotted with triangles. The band profiles are fitted with Gaussians, whose parameters are given in Tab. 1.

The purpose of the high resolution was to take advantage of ISO's small beam and search for substructures within the bands, similar to those indicated by the IRAS data within the ±1.4° bands (see Fig.9 in [19]). The 25/xm scan was repeated one year later with a lower spatial resolution of 30'. Figure 3 shows the observed brightness profiles (upper panels). The /?=[-5,+5] and /5= [-13,5] scans were performed with a difference of 2 days, introducing an offset in the absolute brightness. Since the observed ecliptic latitudes of the bands depend on the annual motion of the observer as well as on the distance from the sun, we predict and mark in Fig. 3 the expected positions of the ±1.4° and —10° bands, using Figs. 4 and 5 of Reach [19].

The plots demonstrate that at 25 and 60/mi ISOPHOT detected both the ±1.4° and the -10° bands in the form of shoulders in the smooth profile. At 12/xm the -10° band is marginally detected, but the presence of the ±1.4° pair of bands is not obvious. In the lower panel of Figure 3 we plotted the residuals of the high resolution scans after subtracting a smoothed baseline. Though peaks and dips are present in the individual 12,25, and 60/xm scans, there are no significant features which are present at all three wavelengths (see the median of the 3 individual scans at the bottom). These results seem to exclude the presence of any arcminute-scale structure of the asteroidal bands at the la level of 0.067, 0.157, and 0.180 MJy/sr at 12, 25, and 60/xm, corresponding to 0.25%, 0.18%, and 0.82% of the total sky brightness, respectively.

In order to study the bands in details, we extracted the brightness profile of the —10° band by fitting a 4th order polynomial to the /3=[-13.5,-13.0] and f3=[-7.0,-4.0] sections of the scans which are not contaminated by asteroidal bands. The profiles are shown in Fig. 4, details of the scans as well as parameters of Gaussian fits to the brightness profiles are given in Tab. 1.

At 12/mi both the location and the width of the band differ from the corresponding values at longer wavelengths, and it is not clear if we see the same physical structure or

Table 1

ISOPHOT scans across the -10° asteroidal band, and parameters of the band derived from Gaussian fits to the observed profiles. The peak intensity of the band is colour corrected assuming a blackbody of 175 K. At 60/xm the 3x3 pixels of the C100 camera were averaged, thus as aperture the full size of the array is given._

Table 1

ISOPHOT scans across the -10° asteroidal band, and parameters of the band derived from Gaussian fits to the observed profiles. The peak intensity of the band is colour corrected assuming a blackbody of 175 K. At 60/xm the 3x3 pixels of the C100 camera were averaged, thus as aperture the full size of the array is given._

Wavelength

Aperture

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