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Subsections

   
5 Scientific results

The procedure from Sect. 3 resulted in a list of potential SSO candidates in the ISOSS. Mainly for the following reasons, not all of them were visible in the slew data:

1.)
The structured and bright cirrus background caused source confusion and limited the point source extraction[*]. It also affected strongly the analysis of extended structures, like from a cometary coma.
2.)
The sensitivity limit of approximately 1 Jy at 170 $\mu $m allowed only the detection of bright asteroids and comets, which are already well known through other observing programmes and techniques (IRAS, occultation measurements, radar, ...) and through dedicated ISO measurements.
3.)
The source extraction from a 4-pixel camera is difficult due to the high slewing speed and the variety of impact parameters. The resulting fluxes or flux limits had usually larger error bars than comparable pointed observations, where sources were usually either centred in the C200 array or on a single pixel.
4.)
For some faint sources the allowed maximal offset of 5$\arcmin$ was too large to produce a noticable signal increase.

In some cases the object was visible, but a reliable flux determination from the ISOSS was not possible. In these cases upper or lower limits are given.

Tables 4, 5 and 6 include all SSO predictions which are within 5$^{\prime}$ of the slew centre, fulfill the flux requirements and have not been used in Sect. 4. For the scientific comparison between ISOSS fluxes and model predictions we did the following calibration steps: We determined the ISOSS calibrated inband fluxes through the different methods and corrected them by an estimated factor based on the slopes visible in Figs. 4 and 5. As a last step we applied the colour correction to obtain monochromatic flux densities at 170 $\mu $m (ISOSS values in Tables 4, 5).

   
5.1 Planets

The inner planets were not visible for ISO due to pointing constraints. Mars, Jupiter and Saturn exceeded the saturation limits, Pluto was below the 1.0 Jy limit. Therefore, only Uranus and Neptune were seen, but were already used to extend the ISOSS calibration (Sects. 4.1, 4.2 and 4.3). However, the bright planets had to be included in the search programme with the objective to identify close-by slews. For the extremely IR bright planets, diffraction effects of the optical system in combination with certain satellite-planet constellations produced bright spots, spikes and ring like structures around the planets, which are visible in the slew data. Mars, Jupiter and Saturn, with 170 $\mu $m brightnesses between 10 000 and 400 000 Jy, influenced slews up to 1$^{\circ}$ distance, Uranus and Neptune (>200 Jy) up to 10$^{\prime}$ distance. These slews have been identified (with the above mentioned SSO extraction method in combination with a large search radius) and the planet influence can now be taken into account for further scientific catalogues based on ISOSS. Some planetary satellites are bright enough to be visible in principle. However, close to Jupiter (the maximal distance for the Galilean satellites is about 11$^{\prime}$) and Saturn (the maximal angular distance for the 8 largest satellites is about 10$^{\prime}$) no 170 $\mu $m fluxes can be derived, due to the strong planet influences.

The measured Uranus and Neptune flux values from Tables 1, 2 and 3 can also be used as input to future models of planetary atmospheres. Current models are based on Voyager IRIS data from 25 to 50 $\mu $m and sub-millimetre data beyond 350 $\mu $m (Griffin & Orton 1993) with an interpolation in between. The ISOSS observations make this wavelength range directly accessible for model tests in the far-IR.

   
5.2 Asteroids

After establishing new methods for the flux calibration, based now additionally on measurements of Uranus, Neptune, Ceres, Pallas, Juno and Vesta, the monochromatic flux densities at 170 $\mu $m of the remaining asteroids were derived. 23 out of the 56 asteroid "hits'' were already used in this calibration context in the previous Sect. 4. The remaining 33 hits can be split in 3 groups:

5.2.1 IRAS and poor ISOSS detection

18 asteroid predictions have reliable IRAS detections, but only low quality ISOSS detections. The reasons for the poor ISOSS fluxes are manifold: slew offsets, bright backgrounds, technical problems, low fluxes, etc. For these 18 asteroids Tedesco et al. (1992) calculated already diameters and albedos, based on IRAS observations. Upper flux limits from ISOSS would therefore not give any new information.

5.2.2 IRAS and good ISOSS detection


 

 
Table 4: ISOSS observational results for asteroids with IRAS detections. Note, that the ISOSS results were first flux corrected according to Sect. 4 and then colour corrected.The uncertainties in the table, given in brackets for method 3a, are statistical errors of weighted results from all 4 pixels.

TDT
Date/Time SSO \ensuremath{F_{\rm Obs}} \ensuremath{F_{\rm Model}} Method Slew Remarks
No.     (Jy) (Jy)   speed  
(1) (2) (3) (4) (5) (6) (7) (8)

11080300
06-Mar.-96 11:42:00 (5) Astraea <4 1.3 2 fast upper limit, cirrus bgd.
12780300 23-Mar.-96 02:50:17 (5) Astraea <4 1.6 2 fast upper limit, cirrus bgd.
85480200 18-Mar.-98 13:48:12 (7) Iris <3 2.1 2 fast in cirrus knot
25381400 27-Jul.-96 03:22:14 (15) Eunomia 2.9 2.5 1 fast ok
85480300 18-Mar.-98 17:22:43 (89) Julia <3 1.7 2 fast upper limit
18780100 22-May-96 05:40:00 (344) Desiderata 4.5 5.8 1 fast ok
18280800 17-May-96 03:50:00 (532) Herculina 5.3(1.0) 5.5 3a stop ok
21780900 21-Jun.-96 06:01:48 (532) Herculina 3.4(0.3) 3.8 3a stop ok
83380500 25-Feb.-98 11:36:26 (1036) Ganymed <4 0.1 2 fast upper limit


7 asteroids (9 hits) have reliable IRAS detections and also good quality ISOSS detections (see Table 4). For these asteroids we could successfully derive fluxes and upper limits with our newly established calibration, based on clear detections. The IRAS diameter and albedo values in the following are all taken from the Minor Planet Survey (MPS, Tedesco et al. 1992).

Astraea:

The main-belt asteroid Astraea has been well observed by different techniques, including IRAS, radar and lightcurve observation. The combination of all measurements led to the description of the object as a rotating ellipsoid with a well determined spin vector (Erikson 2000) and axis dimensions of $143 (\pm 12\%) \times 115 \times 100$ km (Magri et al. 1999). Using the TPM with default thermal parameters for main-belt asteroids (Müller et al. 1999) together with the shape, size and spin vector information gave fluxes of 1.3 $\pm$ 0.3 Jy and 1.6 $\pm$ 0.4 Jy at the 2 ISOSS epochs (see Table 4). The measured ISOSS upper limits are in agreement with the TPM predictions.

Iris:

As for Astraea, a shape model has been established for Iris based on a combination of radiometric, lightcurve and occultation data (Magri et al. 1999). The corresponding TPM prediction gives 2.1 $\pm$ 0.2 Jy at the ISOSS epoch, with an additional lightcurve variation of about 25% (min to max). The measured lower flux limit is in agreement with the calculations.

Eunomia:

Eunomia was one of the best observed asteroids by IRAS: 7 epochs distributed over almost one month, each time observed with high S/N in all 4 bands. The MPS diameter is 255.3 $\pm$ 15 km and the albedo 0.21 $\pm$ 0.03. Two single chord occultation measurements led to diameters of >309 $\pm$ 5 km (Overbeek 1982) and >232 km (Stamm 1991). The TPM prediction (based on MPS diameter and albedo and on shape and spin vector by Erikson 2000) gives 2.5 $\pm$ 0.5 Jy at the ISOSS epoch, with the main error contribution coming from the large lightcurve variation. The measured ISOSS flux of 2.9 Jy agrees within the errorbars.

Julia:

IRAS observed this asteroid 4 times within 2 weeks, each time with high S/N in all 4 bands. The MPS diameter is 151 $\pm$ 3.1 km and the albedo 0.18 $\pm$ 0.01. No shape or spin vector is available, but the possible lightcurve amplitudes range between 0.10 and 0.25 mag (Lagerkvist et al. 1989). The TPM prediction (based on MPS diameter and albedo together with a spherical shape) gives 1.7 $\pm$ 0.1 Jy at the ISOSS epoch, with an additional maximal lightcurve variation of approximately 25% (min to max). The measured upper limit agrees with this prediction.

Desiderata:

This asteroid was observed by IRAS extensively at 9 epochs during a period of 2 months with high S/N in all bands. The MPS diameter is given with 132.3 $\pm$ 5.5 km and the albedo 0.06 $\pm$ 0.01. No shape and spin vector exists currently for Desiderata, but a 0.17 mag lightcurve amplitude has been stated by Lagerkvist et al. (1989). The TPM prediction (based on MPS diameter and albedo together with a spherical shape) gives 5.8 $\pm$ 0.5 Jy at the ISOSS epoch, with an additional maximal lightcurve variation of approximately 17% (min to max). Assuming an ISOSS observation at lightcurve minimum and a diameter at the lower end of MPS diameter range would result in a TPM flux which is only a few percent above the measured ISOSS value, but well within the ISOSS measurement error bars.

Herculina:

7 IRAS observations with high S/N in either 3 or 4 bands were obtained between March and October 1983. The The MPS diameter is given with 222.2 $\pm$ 7.6 km and the albedo 0.17 $\pm$ 0.01. The occultation diameter of 217 $\pm$ 15 km is based on several chords in combination with information on the pole orientation and a lightcurve fit (Bowell et al. 1978). MPS and occultation diameters agree nicely. The complete shape and spin vector solutions derived from lightcurve observations are given in Erikson (2000). Using the full information for Herculina (see also Müller & Lagerros 1998 for details) led to 170 $\mu $m fluxes of 5.5 $\pm$ 0.4 Jy and 3.8 $\pm$ 0.3 Jy. These values have been calculated using the exact lightcurve phase and amplitude at the time of the ISOSS observations. The almost perfect agreement between predicted and measured fluxes (see Table 4) confirms in an independent way the reliable calibration of this new flux extraction method for the ISOSS.

Ganymed:

IRAS saw Ganymed only twice and in both cases only the 25 $\mu $m flux was useable for the radiometric calculations, resulting in a diameter of 31.7 $\pm$ 2.8 km and an albedo of 0.29 $\pm$ 0.06. A single chord occultation measurement gave a lower diameter limit of 16 km (Langans 1985[*]). The large lightcurve amplitude of 0.45 mag (Lagerkvist et al. 1989) adds more uncertainties to the model calculations. Purely based on the given diameter and albedo, the TPM predicts approximately 0.1 Jy for the time of the ISOSS observation, which is well below the detection limit of this observing mode.

5.2.3 No IRAS detection


 

 
Table 5: ISOSS observational results for asteroids without IRAS detection. $^\star $ Closest approach to edge of closest pixel.

TDT
Date/Time SSO \ensuremath{F_{\rm Obs}} \ensuremath{F_{\rm Model}} Method Slew Real source Remarks
No.     (Jy) (Jy)   speed offset$^\star $  
(1) (2) (3) (4) (5) (6) (7) (8) (9)

83081700
22-Feb.-98 19:07:18 (9) Metis 3.8 4.1 1 fast 0$\arcmin$ ok
84281000 06-Mar.-98 16:31:55 (9) Metis >1 3.4 2 fast 0.3$\arcmin$ $\sim$$3\sigma$ detection
63681300 13-Aug.-97 10:42:38 (27) Euterpe -- 1.5 -- fast 1.9$\arcmin$ no detection
25880900 01-Aug.-96 07:42:48 (391) Ingeborg -- 0.2 -- fast 2.5$\arcmin$ no detection
71682300 01-Nov.-97 06:03:56 (3753) Cruithne -- 0.1 -- fast 1.0$\arcmin$ no detection
27482100 17-Aug.-96 01:22:01 (7822) 1991 CS -- 0.1 -- fast 2.5$\arcmin$ no detection


5 asteroids (6 hits) have no IRAS detection, but fulfilled the conservative flux requirements for the ISOSS asteroid search (see Table 5). Unfortunately 4 sources (Euterpe, Ingeborg, Cruithne and 1991 CS) have only marginal ISOSS detections and establishing upper flux limits was difficult. The results of Table 5 per Object:

Metis:

Kristensen (1984) has determined a size of 190 $\pm$ 19 km for this asteroid from an occultation event. A second occultation a few years later gave a high quality 173.5 km diameter (Stamm 1989; Blow 1997). Recent HST images (Storrs et al. 1999) revealed an elongated disk with a long axis of 235 km and a short axis of 165 km, which corresponds to an effective diameter of 197 km. Given the uncertainties involved we adopt the occultation result which is perfectly consistent with both techniques (see also Lagerros et al. 1999). The full light curve and shape information has been taken from Erikson (2000). The TPM predictions gave 4.1 $\pm$ 0.8 Jy and 3.4 $\pm$ 0.7 Jy, respectively (see Table 5). Adopting the HST results instead led to about 5% and 10% higher fluxes.

Based on the ISOSS flux of 3.8 Jy, the TPM allowed the calculation of an effective projected diameter of 178 km and an albedo of pV=0.15 at the epoch of the ISOSS observation. A possible 20% ISOSS flux uncertainty would correspond to about 10% diameter uncertainty, resulting in a size of the rotating ellipsoid of $213 \times 164 \times 132$ km with 10% minimum uncertainties.

Within the different uncertainties and based on the shape and spin vector solutions, the results agree nicely. The 3 methods - occultation, HST direct imaging and ISOSS radiometric method - led to comparable diameters and albedos.

Euterpe:

No IRAS observations are available. We used instead the largest extension from an occultation measurement (Dunham 1998) together with a shape and spin-vector model (Erikson 2000), H, G values (Piironen et al. 1997) and an albedo of 0.13 related to the occultation cross section. The TPM prediction was 1.5 Jy with a large uncertainty due to the limited size knowledge. This is well within the detection limits, but the source was too far from the slew center to determine an upper flux limit.

Ingeborg:

There exists hardly any information about this asteroid. Based on its H-value of 10.1 mag, together with a typical S-type (Tholen 1989) albedo of 0.155 one can calculate an approximate diameter of 32 km. The corresponding flux calculation for the ISOSS epoch gave 0.2 Jy, which is clearly below the detection limit. Even under the assumption of an extreme albedo of 0.03 the asteroid flux at 170 $\mu $m would only be 1.3 Jy and therefore hardly detectable. Like in the case of Euterpe, Ingeborg had a slew center offset which was close to the maximal allowed 5$\arcmin$.

Cruithne:

Cruithne is currently the only known object on a horseshoe orbit around the Earth (Christou 2000). It was also part of a special near-Earth object observations programme (Erikson et al. 2000a). Based on an unweighted mean of typical C and S-type asteroids (pV=0.12) and an H-value of $H=15.13\pm0.05$, they calculated a diameter of 3.7 km and a slow rotation period of 27.4 hours. Although the observing geometry with only 0.37 AU from Earth was almost ideal, the 170 $\mu $m flux prediction was only 0.1 Jy. Even an extremely low albedo (leading to a diameter of about 8 km) would only give 0.3 Jy well below the detection limit. Therefore an upper limit from a background analysis would not give any new information.

1991 CS:

The case of 1991 CS is similar to Cruithne: a near-Earth asteroid, at only 0.14 AU from Earth at the time of the ISOSS slew and with an H-value of 17.4 mag. A radar campagne resulted in an estimated diameter of 1.1 km, an albedo of 0.14 and a rotation period of 2.39 hours (Pravec et al. 1998). The TPM predicts a 170 $\mu $m flux below 0.1 Jy and even for extreme albedo values the flux would be below 0.3 Jy and therefore not detectable for ISOSS.

5.2.4 Additional results

The Juno observations in Tables 1 and 3 have flux ratios systematically higher than ratios from comparable sources. Calibrating the ISOSS values with the corresponding newly established methods 1 and 3a resulted in an average observation over model ratio of 1.14, indicating that the model diameter of Juno is about 7% too low. Müller & Lagerros (2002a) analysed 11 independent ISO observations, taken with the long wavelengths ISOPHOT detectors. They find a similar mean ratio of 1.13 $\pm$ 0.10, which confirms the tendency to higher diameter values. Both investigations indicate that the effective diameter should be close to 260 km, compared to the published values of 241.4 km (Müller & Lagerros 1998) and 233.9 $\pm$ 11.2 km (Tedesco et al. 1992).

For Vesta the situation is not that clear. The values in Table 1 are not conclusive since the Vesta fluxes cover the difficult transition region between little flux loss and the more than 40% flux loss for sources brighter than 25 Jy (see Fig. 4). It seems that two of the measured fluxes (TDT 07881200, 79781500) are higher than one would expect from other sources with similar brightness. This contradicts the findings by Redman et al. (1998; 1992). They state for Vesta an extremely low emissivity of 0.6 in the submillimetre. Assuming that the emissivity is already lower in the far-IR would mean that the Vesta points in Fig. 4 should lie below the general trend and not above. The measurement from methods 3a (TDT 57581500) and 3b (TDT 61580800) agree within the errorbars with the model predictions. As in Müller & Lagerros (2002a), we see no clear indications of far-IR emissivities lower than the default values given in Müller & Lagerros (1998)

   
5.3 Comets

5.3.1 Observational results

The results of the positional search through the ISOSS pointing data, combined with the flux estimates are given in Table 6. The table columns are: (1-3) same as in Table 1, (4-5) ISO-centric coordinates (2000.0), (6-7) Sun and Earth distance at the time of the observation, (8-9) ISOSS and model flux.
   
Table 6: Observational geometry for the comets. No model values have been calculated for C/1995 O1 (Hale-Bopp). All hits are discussed in the text. One predicted hit was a "no detection'', one a "poor detection'' and in one case (--) the slew length was too short.

TDT
Date/Time SSO RA Dec. r $\Delta$ \ensuremath{F_{\rm Obs}} \ensuremath{F_{\rm Model}}
No.     (hms) (dms) (AU) (AU) (Jy) (Jy)
(1) (2) (3) (4) (5) (6) (7) (8) (9)

60780100
14-Jul.-97 22:57:49 2P/Encke 14 56 28.6 -63 36 09 1.164 0.264 5-10 >50

34881300
29-Oct.-96 22:27:01 22P/Kopff 21 23 56.6 -19 53 22 1.961 1.523 0.5-1 <1

23380800
07-Jul.-96 11:31:51 96P/Machholz 1 23 10 27.5 -68 19 46 2.052 1.328 no det. $\approx$1

77780200
31-Dec.-97 16:38:09 103P/Hartley 2 23 27 54.1 -07 29 09 1.041 0.825 poor det. $\approx$25

80280100
25-Jan.-98 15:07:48 104P/Kowal 2 00 43 46.0 +08 38 35 1.451 1.496 1 $\approx$2

33280100
13-Oct.-96 18:43:22 126P/IRAS 21 38 46.7 -29 54 48 1.712 1.028 -- >2
36280400 12-Nov.-96 13:51:16 126P/IRAS 21 45 50.3 -08 47 05 1.709 1.307 1.0 <2

13481800
30-Mar.-96 17:26:33 C/1995 O1 = 19 42 20.5 -19 43 10 4.867 5.004 see text  
16280600 27-Apr.-96 14:17:19 Hale-Bopp 19 44 35.1 -17 37 42 4.588 4.259 9.3$\pm$1.8  
31580500 27-Sep.-96 0:05:16 $\prime\prime$ 17 29 43.0 -05 11 32 2.934 2.965 30.9$\pm$7.3  
32081300 01-Oct.-96 17:21:44 $\prime\prime$ 17 29 42.8 -04 57 58 2.878 2.987 see text  
32280200 03-Oct.-96 15:16:59 $\prime\prime$ 17 29 50.7 -04 52 28 2.856 2.995 see text  
32580600 06-Oct.-96 23:28:20 $\prime\prime$ 17 30 13.8 -04 42 47 2.816 3.009 see text  
77081500 25-Dec.-97 0:18:51 $\prime\prime$ 06 32 55.7 -64 09 08 3.851 3.683 43.8$\pm$4.0  
86880300 01-Apr.-98 14:02:15 $\prime\prime$ 05 02 13.8 -53 09 12 4.855 4.945 see text  
87380400 06-Apr.-98 16:12:29 $\prime\prime$ 05 05 07.0 -52 34 21 4.905 5.009 15.0$\pm$2.9  

2P (Encke):

The comet has been detected at a slew end position on an extremely high background close to the galactic plane. The flux increase towards the comet nucleus corresponds to about 5-10 Jy. The coma extension and its brightness profile could not be determined due to the high background brightness. The model flux at this close encounter with Earth (0.26 AU) may have been strongly overestimated due to the large apparent size of the central coma which was assumed to be of constant brightness.

22P (Kopff):

The ISOSS slew ends again on the comet, but this time the source is located on a clean low background. The signal pattern is similar to that of a point-source with $\sim$ 0.5-1 Jy, which is close to the detection limit. An upper flux limit of 2 Jy can be given, which is in good agreement with the simple model calculations (Table 6).

96P (Machholz 1):

The comet has not been detected. The position calculation showed that the source was just outside the slew path, but within the specified 5$\arcmin$ search limit. The low model flux indicated already the difficulty to detect the coma or the comet nucleus.

103P (Hartley 2):

Only a poor detection of an extended source was found, although the comet was on a low background. ISOPHOT observations close to the ISOSS observing epoch show that Hartley 2 had a colour temperature of 285 K (Colangeli et al. 1999). This is 30 K colder than the calculated model temperature. A second reason for the discrepancy between a low ISOSS flux and the predicted 25 Jy is probably the too large apparent size of the central coma which was assumed to be of constant brightness. At an Earth distance of 0.82 AU the model comet core covers a significant part of the aperture. Both effects together might explain the model value.

104P (Kowal 2):

A source of approximately 1 Jy was detected by one pixel at the predicted position of the comet, but confusion with a close IRAS source could not be excluded.

126P (IRAS):

In the first case the slew length was only 1$\arcmin$ which was not sufficient for the data analysis. The second case was a clear detection by one pixel (Method 2). The derived flux of 1 Jy is in agreement with the calculated upper limit of 2 Jy.

C/1995 O1 (Hale-Bopp):

13481800: the slew passed over the coma with the nucleus only 30$\arcsec$ outside the closest pixel. A weak signal of $\sim$ 2  \ensuremath{{\rm MJy~sr^{-1}}} has been detected in this pixel. 16280600: the slew ended on Hale-Bopp and the integrated 4-pixel flux was determined to 9.3 $\pm$ 1.8 Jy (Method 3a). 31580500: method 3a was applicable and a 170 $\mu $m flux of 30.9 $\pm$ 7.3 Jy has been derived. 32081300: slew over the comet nucleus, with one pixel crossing centrally, two pixels in 1$\arcmin$ distance and one pixel in 2$\arcmin$ distance. The slew crossed the nucleus under an angle of 45$^{\circ}$ relative to the orientation of the dust tail (PsAng[*] $=87.8^{\circ}$, ISOSS PosAng[*] $=42.0^{\circ}$). The measured brightness profile clearly deviates from that of a point-source. The asymmetric profile is stronger towards the east, i.e. on the tail-side of the nucleus. Due to a nearby cirrus ridge, the dust tail extension is confirmed out to 2$\arcmin$ only (but would be probably larger on a flat background). 32280200: the slew crossed the dust tail of Hale-Bopp under an angle of approx. 30$^{\circ}$ in 8$\arcmin$ distance from the nucleus ( $PsAng=87.1^{\circ}$, $ISOSS PosAng=55.8^{\circ}$). A signal increase at the position of the dust tail can be seen, but the signal pattern is difficult to discriminate from the cirrus structures in the background, hence quantitatively not helpful. At the closest comet approach of 4$\arcmin$ a second signal increase can be seen which coincides with the position angle of the negative of the target's heliocentric velocity vector (PsAMV[*] $=151.2^{\circ}$). The signal increase is either related to the large cometary coma at a distance of only r=2.86 AU from the sun or a kind of trail formation in the direction of PsAMV similar to what Reach et al. (2000) found for comet Encke. 32580600: the detectors moved centrally along the dust tail and cross over the comet nucleus ( $PsAng=85.9^{\circ}$, $ISOSS PosAng=85.0^{\circ}$). The measured brightness profile clearly deviates from a point-source profile (see Fig. 6). A dust tail extension of more than 4$\arcmin$ can be seen where the satellite approaches the nucleus. The signal in anti-tail direction decreases more rapidly (see also Sect. 5.3.2). 77081500: method 3a was applicable again and a 170 $\mu $m flux of 43.8$\pm$4.0 Jy was derived. 86880300: the slew passes in 4$\arcmin$ distance ahead of the comet tail under an angle of approx. 70$^{\circ}$ with the sun direction ( $PsAng=113.3^{\circ}$, $ISOSS PosAng=183.5^{\circ}$). A signal change of 2  \ensuremath{{\rm MJy~sr^{-1}}} extended over 15$\arcmin$ can clearly be seen. Due to the viewing geometry (the phase angle is only 11$^{\circ}$) coma and tail are difficult to separate and the signal increase is most likely connected to the dust emission of the extended coma and tail structures of Hale-Bopp. Here, as in 32280200, the signal maximum coincides with the PsAMV angle of $1.2^{\circ}$. A connection might be possible between the 170 $\mu $m signal pattern and large particles forming an elongated structure behind the comet nucleus while it is moving away from perihelion. 87380400: method 3a was applicable again and a 170 $\mu $m flux of 15.0 $\pm$ 2.9 Jy was derived.

   
5.3.2 C/1995 O1 (Hale-Bopp)


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{tmueller_fig6.ps}
\end{figure} Figure 6: An ISOSS profile of Hale-Bopp in comparison with the signal pattern of a point source. The slewing speeds for these two hits were identical and already quite low (2$\arcmin$/s), resulting in a high sampling rate. The offsets are defined with respect to the positions from the ephemeris calculations and have not been shifted for the two sources relative to each other. Hale-Bopp shows an asymmetrically extended profile. Note that the profiles are not deconvolved.

Figure 6 illustrates a measured signal profile from a central slew over Hale-Bopp (TDT 32580600) in comparison with a slew over the point source Neptune (TDT 72081600). For these two detections, the geometrical configurations and the slewing speeds (2$\arcmin$/s) have been identical. The detectors moved first centrally over the dust tail (left side of the peak) and then over the nucleus of Hale-Bopp (peak). An asymmetric signal profile between $-6\arcmin$ and $+4\arcmin$ can be seen. In the case of Neptune the signal increase starts approximately 2$\arcmin$ ahead of the true position, which is related to a combination of the Airy disk with the slew speed and read out frequency (see also Hotzel et al. 2001). The slight shift between the two peaks is most probably related to the possible positional uncertainties of ISOSS data (see Sect. 4.4). This Hale-Bopp asymmetry has not been seen in dedicated 170 $\mu $m maps (Peschke et al. 1999) which were taken at r=3.904 AU (as compared to r=2.816 AU in Fig. 6).

The fluxes derived from Method 3a can be compared to results from Grün et al. 2001 through the following corrections: 1) Flux corrections according to Fig. 5; 2) Normalization to a standard aperture diameter of 23$\arcsec$  assuming that the coma brightness scales linearly with aperture diameter ( ca23=0.120); 3) Point-spread-function correction which takes into account the differences of a point source and a $1/\rho$-coma ( $c_{{\rm psf}}=1.092$); 4) Colour correction[*] which changes for different distances from the sun ( $c_{{\rm colour}}=f(r)$).

The first two measurements (16280600, 31580500) were obtained on the same days as the ones in Grün et al. (2001). The calibrated and reduced 23$\arcsec$ fluxes agree within the errorbars. The third observation (77081500), taken 5 days earlier than the dedicated Hale-Bopp observation, lead to a flux of 5.64 $\pm$ 0.56 Jy, as compared to 2.66 Jy. This large difference can not be explained by epoch or geometry differences. However, the dedicated measurement was mis-pointed by 24$\arcsec$ which caused large uncertainties in the applied corrections. The ISOSS flux provides therefore valuable information for the colour temperature determination and, through grain size models, might give clues whether icy grains were present in the coma in December 1997 at almost 4 AU post-perihelion. The last measurement of Method 3a (87380400) was obtained when Hale-Bopp was already 4.9 AU from the sun. The calibrated and reduced 23$\arcsec$ flux was 1.97 $\pm$ 0.39 Jy. This is the most distant thermal far-IR observation of Hale-Bopp post-perihelion. A comparison of the flux with a dedicated observation (Grün et al. 2001; $F_{\nu}=1.06$ Jy) at a similar distance from the sun, but pre-perihelion, shows that the dust emission post-perihelion was higher as the comet receeded from the sun. In fact, the higher far-IR fluxes post-perihelion are related to contributions from large particles which have been accumulated during the passage around the sun and which stay on similar orbits as the nucleus.

Two measurements (32280200, 86880300) show signal peaks a few arcminutes away from the nucleus in anti-orbital velocity (trail) direction. It seems that the emitting dust particles are not homogeneously distributed and are concentrated in a narrow region of the outer parts of the dust coma towards the trail direction. These features are not seen in slews over other parts of the outer coma. Reach et al. (2000) observed for the first time the dust trail formation in comet Encke in the mid-IR. The ISOSS data provide now evidence for this process in the far-IR where the emission is strongly connected to the largest particles.


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