Astron. Astrophys. 319, 694-698 (1997)

3. Results and discussions

The observed values of the percent polarization (%P) and position angle () alongwith the errors are listed in Table 1 and are also plotted in Fig. 1. The polarimetric data in continuum bands are already published in IAU Circular 6371 (Ganesh et al. 1996). Table 1 also lists the Geocentric distance (), heliocentric distance (r), and the phase angle () at the time of observations. These values are taken from comet Ephemeris based on orbital element given in IAU Circular 6329. As stated earlier, the observations have been made using an aperture of diameter of 26.5 arcsec, which corresponds to a linear scale of km and km respectively on March 13.97 and March 18.88.

Fig. 1. Wavelength dependence of observed degree of percent polarization (% P) and position angle () for comet Hyakutake. Present observations are connected with solid line. The symbols represent Kikuchi et al. (1987) data for comet P/Halley.

Within the error of observations, the angle of polarization is found independent of wavelength and also perpendicular to the scattering plane. In other words, it is positive. The position angle in U-band (0.365 ) shows some deviation which, within the error of observations, appears to be insignificant. However, the possibility of small deviation of the position angle from the normal to scattering plane cannot be ruled out, especially for short wavelengths, since for certain particles the polarization may be negative (i.e. parallel to the scattering plane) for the small phase angles (Krishnaswamy 1978; Weinberg & Beeson 1976). In such situation the polarization vector may not be exactly perpendicular to the scattering plane. For the present purpose we consider the position angle, for all the three bands, to be normal to the scattering plane.

On March 13.97 (phase angle 38.1 degrees) the degree of polarization showed marginal decrease at wavelength 0.4845 compared to the value at 0.3650 and 0.6840 . On the other hand, on March 18.88 (phase angle 36.9) there is a marginal increase in wavelength dependence of polarization from 0.3650  to 0.6840 .

Though the Sun-Comet-Earth phase angle has changed only by from March 13.97 to March 18.88 there is a noticeable change in the wave length dependence of polarization on these dates as is clear from Fig. 1. It appears that the wavelength dependence of polarization is very sensitive to change in the phase near . This point is further discussed in Sect. 4. Kikuchi et al.(1987) have made polarization observations on comet P/Halley using the similar IHW continuum filters covering a wide phase angle range. It is just a coincidence that two sets of observations from Kikuchi et al. (1987) on comet P/Halley are phase wise very close to the present observations on comet Hyakutake. Their polarimetric data at wavelengths 0.365 and 0.485 are available at phase angles 36.7 and 38.5 degrees which are very close to our observations on comet Hyakutake at the phase angles of 36.9 and 38.1 degrees. They also report observations at which is slightly different than the waveband used by us i.e. 0.684 . As the change in the degree of polarization with wavelength in this phase angle region is not much, the two data sets can be used for comparison. Comet P/Halley data (Kikuchi et al. 1987) is also plotted in Fig. 1 for comparison.

The wavelength dependence of polarization for the two comets appears quite similar for the above phase angles. In principle this is expected if the comets have the same origin and the dust particles responsible for producing polarized light are similar in nature. In fact the polarization data at phase angle 36.7 and 38.5 for comet P/Halley compare better with the data at phase angle at 38.1 and 36.9 degrees respectively for comet Hyakutake. The small difference in the wavelength dependence of % P in case of the two comets at nearly the same phase angle may be due to various reasons. Firstly, the aperture sizes used in the observations, which correspond to the different linear scales on the comets, are different in the present observations from those used by Kikuchi et al. (1987) for comet P/Halley. Small but systematic differences are noticed in the polarization values for the same phase angle when observed through different aperture sizes which map different linear scales at the comet location (Bastein et al. 1986). Secondly, comets are active objects in the sense that ejection of dust and gas is a continuous process and some strong ejection episodes may change the environment considerably but temporarily. Strong ejections, not uncommon in comets, may change the dust size distribution which in turn changes the polarization behaviour. Also, the possibility of radial dependence of the dust size distribution can not be ruled out.

Within these limitations, the observed wavelength dependence of polarization for comets P/Halley and Hyakutake appears similar which can be taken to be indicative of the similar nature of the grains in the two comets. In the following section we have attempted to compare the observed polarization behaviour with the theoretically calculated ones as well as with the data on other comets, especially the comet P/Halley.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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