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Astron. Astrophys. 348, 1020-1034 (1999)

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4. Comet Hyakutake

The adverse weather during the scheduled observations was mentioned above. The close approach of the comet resulted in further problems: the cometary emission near perigee was partially resolved at all frequencies, making size corrections for the total flux density difficult and also uncertain.

4.1. Halo size

The first scans through Comet Hyakutake showed a symmetric Gaussian shape, which was obviously broadened in comparison to a point source. But only the first two observations, when the comet was further away, were sensitive enough for a size determination. An ON-THE-FLY map on the second day marginally showed the comet, but it did not allow an accurate size determination. As additional input the excellent map at 375 GHz, taken by Jewitt & Mathews (1997) with the JCMT near perigee of the Comet, was used. From both sets of data a deconvolved Gaussian half power width of 23" was derived for the geocentric distance [FORMULA] = 0.112 a.u., corresponding to a linear (Gaussian) diameter of 1870 km.

4.2. Light curve

Also for Comet Hyakutake the ON-OFF measurements were used to derive the light curve at 250 GHz. In Fig. 8 the observed flux densities per beam of the 30m telescope are shown as dots, the values of the HHT as triangles. Assuming again that the cometary signals are predictable by the helio- and geocentric distances, the observed flux densities per beam are reduced to total flux densities, normalized to d = 1.02 a.u. and [FORMULA] = 0.102 a.u. The resulting average flux density at 250 GHz becomes [FORMULA] = 496 mJy. This value was used to calculate the predicted flux densities per beam for both sites, similarly as for Comet Hale-Bopp; they are shown as solid lines in Fig. 8. The curves predict the general shape of the light curve and the ratio of the observed flux densities per beam for both telescopes reasonably well, the systematic deviations from the predictions at both ends of the time interval will be explained later. The deviations of the observed flux densities per beam from the predictions is attributed to unstable observing conditions, insufficient calibration, and some uncertainty of instrumental parameters (like [FORMULA]) rather then to intrinsic variability of the comet. Contrary to our light curve Jewitt & Mathews (1997) claim a constant emission of the comet, because their observations on two consecutive days near perigee did not show a "substantial change of the observed flux density"; this is no surprise, because the geocentric distance between their observations changed only by about 10 %!

[FIGURE] Fig. 8. The "light" curves for Comet Hyakutake at 250 GHz as function of Julian date. (Julian date = 2450000 + day.) Dots are data of Pico Veleta, triangles data of the Heinrich-Hertz-Telescope. Intensities are normalized to heliocentric distance 1.02 a.u.. The plotted curves give the predicted flux densities per beam for the given geocentric distance.

4.3. Nucleus

All our interferometric observations of the nucleus of Comet Hyakutake are listed in Table 6; they resulted only in upper limits to the size of the nucleus which are all consistent with the first estimate by Harmon et al. (1996), derived by radar observations. The low limit for March 24 is favored by the small geocentric distance. Attempts to detect the nucleus by properly combining either the two simultaneous maps at different frequencies or two maps at different days were not successful; the effective gain of sensitivity by these manipulations may be partially offset by systematic effects.


[TABLE]

Table 6. Size estimates for Comet Hyakutake


4.4. Spectral energy distribution

The averaged total flux densities, normalized for March 25, 1996 to [FORMULA] = 0.102 a.u. and d = 1.02 a.u. as the 250 GHz data above, are collected in Table 7. The quality of our data is, as noted above, quite poor. In addition to atmospheric problems the instrumental parameters were not known accurately enough to allow an accurate size correction by about one order of magnitude. Such problems are avoided at the JCMT by limiting the angular resolution at any frequency to about 18", allowing good spectral index determinations. Furthermore, the comet observations by Jewitt & Matthews (1997) are of high quality. By combination of our total flux density measurements at 250 GHz with their spectral index determination, improved physical parameters of Comet Hyakutake can be obtained.


[TABLE]

Table 7. Integrated flux densities, normalized to [FORMULA] = 0.102 a.u. and d = 1.02 a.u. for Comet Hyakutake


In Fig. 9 the SED of Comet Hyakutake is plotted as a continuous line fitted to our data (filled squares). The dashed line refers to the data of Jewitt & Matthews (1997) (small triangles) with a spectral index [FORMULA] = 2.8. After conversion to flux densities (by size correction) the JCMT data (big triangles) agree well with our SED. Also here the photometric diameter is used to get an order of magnitude estimate of the mass in the halo. At 250 GHz the photometric diameter is 11.6 km, yielding a rough mass estimate of [FORMULA]2.8 [FORMULA] g.

[FIGURE] Fig. 9. The SED for Comet Hyakutake, triangles come from Jewitt & Matthews (1997) squares from this paper; the broken line is a fit to flux densities per JCMT beam of [FORMULA]18 arcsec, the full line is a fit to flux densities in the Gaussian component. The lower thin line is an estimate of the nuclar emission.

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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