Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.astro.spbu.ru/staff/ilin2/INTAS2/PAP/P6_1.pdf Äàòà èçìåíåíèÿ: Fri Nov 19 16:18:17 2010 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 03:04:02 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: mercury surface

ICARUS

ARTICLE NO

133, 286 ­ 292 (1998) . IS985932

Polarimetry and Photometry of Comet C / 1996 B2 Hyakutake
N. N. Kiselev and F. P. Velichko
Astronomical Observatory of Kharkov State University, Kharkov, Ukraine E-mail: kiselev@astron.kharkov.ua Received July 24, 1997; revised March 12, 1998

2. OBSERVATIONS AND DATA REDUCTION Polarimetric and photometric observations of Comet C / 1996 B2 Hyakutake were carried out at the 70-cm telescope of Kharkov State University Observatory from March 25 to April 7, 1996. The goal of the observations was to determine the polarization maximum of the cometary continuum. We have found ° that the maximal degrees of polarization in the blue (4845 A) ° and red (6840 A) continuum domains are 24.0% and 26.1%, respectively, and lie at a phase angle close to 94 , slightly dependent on wavelength. According to the gas-to-dust ratio and color, Comet Hyakutake belongs to the group of dusty comets such as West 1976 VI, P / Churyumov ­ Gerasimenko 1982 VIII, P / Kopff 1983 XIII, P / Hartley ­ IRAS 1984 III, P / Giacombini ­ Zinner 1985 XIII, P / Halley 1986 III, Bradfield 1987 XXIX, Liller 1988 V, Levy 1990 XX, and P / Faye 1991 XXI. The polarization data for comet Hyakutake are in good agreement with the data for these comets in the 37 ­73 overlap region of phase angles. Consequently, the maximum of Hyakutake's polarization may be taken as the maximum of polarization at least of this group of dusty comets. The fluxes for the C2 and C3 emission bands and dust continuum and the corresponding production rates for Comet Hyakutake are given. © 1998 Academic Press Key Words: Comets; Hyakutake; Polarimetry; Photometry

1. INTRODUCTION

This investigation continues the study of the polarization phase dependence of the cometary continuum initiated by Kiselev and Chernova (1978, 1981) and Chernova et al. (1993). Narrow-band observations of comets at phase angles larger than 90 are still not numerous. The goal of these observations is the determination of the polarization maximum of the cometary continuum. They are extremely important for the construction of the whole synthetic polarization phase dependence of the cometary continuum and the development of an adequate theory of scattering. On the other hand, a search for similarities and differences in polarization of dust among comets is also very important for the taxonomy of comets. Many authors (Chernova et al. 1993; Hadamcik et al. 1996; Levasseur-Regourd et al. 1996) pay close attention to these problems.
0019-1035/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.

Comet C / 1996 B2 Hyakutake was observed with the 70-cm reflector with a Cassegrain configuration (f / 16) at the Chuguev Observation Station of Kharkov State University from March 25 to April 7, 1996. This station is located about 70- km to the southeast of Kharkov at 156 m altitude. The observations were made with a set of narrowband comet filters. The ultraviolet, blue, and red contin° uum filters centered at 3650 (UC), 4845 (BC), and 6840 A (RC) and the molecular band filters for C3 at 4060 and C2 ° at 5140 A were used. The diaphragm was 88 arcsec. The comet was placed visually near the center of the diaphragm. Due to the high velocity of the comet the single integration time was chosen in the range from 3 to 10 sec. This allowed the brightest part of the coma to be kept near the center of the diaphragm during the integration. The night sky measurements were conducted at the 2 position from the coma in the anti-tail direction. A photometer ­ polarimeter and a polarimetric observation technique similar to the apparatus and technique which were developed at the Sanglok Observatory, Tadjikistan (Kayumov et al. 1989), were used. A photon counting detector was controlled by the PC 386 computer which was also used for the preliminary calculation of the polarization parameters. The instrumental polarization was obtained from observations of nonpolarized standard stars and was subtracted from the data set. Only two nights were reasonably suited for photometric observations which were made in the polarimetric mode. This permitted us to obtain the photometric and polarimetric data simultaneously. The extinction coefficients were determined in each night using Bouguer's method. The extinction and standard stars were taken from the list of Osborn et al. (1990). The reduction of photometric observations was carried out by the method described by A'Hearn (1991). We used the new expression (A'Hearn et al. 1995) for correction of the C2 emission in the blue continuum. For the determination of molecular column densities, we used the fluorescence efficiencies g (C3) 4.4 10 13 erg mol 1 given 10 12 erg mol 1 and g (C2)

286


POLARIMETRY AND PHOTOMETRY OF COMET HYAKUTAKE

287

TABLE I Observational Data and Production Rates for Comet Hyakutake 1996 B2
log(F ) (erg cm 2 s 1) Date, UT (1996) Mar 25.959 Apr 5.743 log (km) 3.521 4.095 r (AU) 1.028 0.790 (AU) 0.104 0.390 () 70.11 111.36 C
2

log Q (s 1) C
2

C

3

C

3

W4845 ° (A) 134 274

8.556 0.029 8.106 0.020

-- -- 8.701 0.038

26.220 26.612

-- 25.385

by Newburn and Spinrad (1989). They were transformed into the production rates (Q) using Haser's model. According to Festou et al. (1990), the following characteristic scale lengths of the parent molecules (lp) and of the daughter molecules (ld) were taken; for C3 lp 4 103 r 2, ld 2.5 104 r 2, ld 1.2 105 r 2, 1.4 105 r 2; for C2 lp where r is the heliocentric distance in AU. The velocity of the outflow from the comet nucleus was determined by Delsemme's formula v 0.58 r 0.5 (km s 1) (Delsemme, 1982).
3. RESULTS

The photometric results are listed in Table I. The time of observations, the projected diaphragm radius at the comet, , the heliocentric r and geocentric distance , the phase angle , the fluxes in the C2 and C3 emission bands F , and the corresponding production rates Q are given there. In the last column the equivalent width W4845 (Krishna Swamy, 1986) describing the gas-to-dust ratio is given. This parameter is rather susceptible to geometric effects because the dust is strongly concentrated near the nucleus, while the molecular species have a much flatter spatial distribution. Nevertheless, the degrees of polarization of different comets may be compared according to their gas-to-dust ratios if widths W4845 , as well as polarization, are observed with the same diaphragms. To describe that dust production, we use the product p ( ) f (Jockers et al. 1993), where p is the geometric albedo, ( ) is the phase function, f is the filling factor (A'Hearn et al. 1984), and is the projected diaphragm radius at the comet. It is worth mentioning that our product p ( ) f is a factor of 4 smaller than the parameter Af introduced by A'Hearn et al. (1984). The observed dust ° fluxes, the value p ( ) f for the 3650, 4845, and 6840 A continuum bands and colors UC-BC and BC-RC are given in Table II. A comparison of our data for the production rates of species C2 and C3 shows a good agreement with the data obtained by Osip et al. (1996). The value p ( ) f was about 10 times larger than that for comet P / Giacobini ­

Zinner 1985 XIII (Storrs et al. 1992) at the same heliocentric distance. The results of the polarimetric observations are presented in Table III. We list the date of observations, the heliocentric distance, the projected aperture radius at the comet, the filter, the linear polarization degree P, the position angle of the polarization vector, their statistical mean-square errors, the phase angle , the position angle of the scattering plane , and the angle r as defined by Chernova et al. (1993). The approximation of polarimetric data for the comet Hyakutake by the second degree polynomial provides Pmax and max of 24.0%, 93.4 ; 26.1%, 94.1 ; and 15.2%, 88.3 in ° ° ° filters 4845 A, 6840 A, and 5140 A, respectively. The gasto-dust ratio, W4845 , was sufficiently small according to Table I, indicating that the comet had a strong continuum. This was also suggested by Schleicher (private communication) who found that the log Q(OH) / Af ratio was 25.2 on March 25, similar to the value found for P / Halley (A'Hearn et al. 1995). Because of this, the depolarizing influence of the molecular emissions on polarimetric data in the continuum filters was negligibly small and hence the position of polarization maximum depended only upon the properties of the cometary dust particles. On the contrary, ° the leak of the cometary continuum in the 4060 and 5140 A filters was rather large; therefore the observed polarization degree for the C2 emission was almost two times larger than the polarization degree Pmax 7.7% expected from the resonance-fluorescent mechanism of scattering. The degrees of polarization of Comet Hyakutake in the ultraviolet, blue, and red continuum bands are presented in Figs. 1 ­ 3, respectively. In the same figures the corresponding polarimetric data for several comets (Chernova et al. 1993) are given, as well as the data for P / Halley from the IHW CD-ROM digital archive in the UC filter and the data for Hyakutake (Joshi et al. 1997). Figure 2 was also supplemented by the data for Comet West 1976 VI ° in the filter 5300 / 50-A (Kiselev and Chernova, 1978) and data by Michalsky (1981) interpolated for wavelength 4850 ° ° A. It should be noted that the 5300 / 50-A filter had rather


288

KISELEV AND VELICHKO

TABLE II Dust Data for Comet Hyakutake 1996 B2
log(F )(erg cm Date, UT (1996) Mar 25.961 Apr 5.744 3650 -- -- 10.873 0.021 4845 10.684 0.028 10.544 0.028
2

s

1

° A 1) 6840 10.723 0.013 10.647 0.021

log [ p ( ) f ] (cm) 3650 -- 3.297 4845 2.910 3.396 6840 2.992 3.413

Color (mag) UC-BC -- 0.17 0.06 BC-RC 0.26 0.06 0.10 0.06

broad wings. Because of this, gas contamination might occur in this filter. The dependence of the polarization of Comet West on diaphragm size and the discrepancy between the data of Kiselev and Chernova (1978) and of Michalsky (1981) offer evidence for gas contamination. In

the range of small phase angles 1.6 ­ 18.1 (see Fig. 3) we used the polarization data for P / Halley obtained with the ° wide-band R filter (6855 / 1080 A). Their accuracy is the best available for all measurements. The ``flat'' wavelength dependence of the cometary polarization in this range of

TABLE III Polarimetric Data for Comet Hyakutake 1996 B2
Date, UT 1996 Mar 25.006 25.026 25.914 25.924 25.937 25.950 25.957 25.960 25.962 25.964 27.747 27.769 27.787 27.799 27.801 27.802 27.803 27.804 28.035 28.053 28.067 28.074 31.734 31.836 31.861 Apr 4.740 4.757 4.772 5.736 5.737 5.738 5.739 5.743 7.774 r AU 1.048 1.048 1.029 1.029 1.029 1.028 1.028 1.028 1.028 1.028 0.990 0.990 0.989 0.989 0.989 0.989 0.989 0.989 0.984 0.984 0.983 0.983 0.903 0.901 0.901 0.814 0.813 0.813 0.791 0.791 0.791 0.791 0.790 0.743 log km 3.513 3.513 3.521 3.521 3.521 3.521 3.521 3.521 3.521 3.521 3.618 3.618 3.621 3.621 3.621 3.621 3.621 3.621 3.638 3.638 3.641 3.641 3.875 3.881 3.882 4.058 4.059 4.059 4.095 4.095 4.095 4.095 4.095 4.161 Filter ° A 6840 4845 4845 5140 6840 3650 4060 4845 5140 6840 6840 4845 5140 3650 4060 4845 5140 6840 6840 4845 5140 6840 6840 4845 5140 4845 6840 5140 5140 6840 4845 4060 3650 6840 15.94 13.93 18.61 12.18 21.27 17.96 14.62 19.81 12.33 21.74 26.30 23.92 14.75 18.61 18.12 24.36 15.24 24.66 26.80 24.32 15.27 26.89 24.71 20.75 12.22 21.25 24.03 11.19 9.76 23.48 22.64 16.26 15.86 24.04 P % 0.26 0.27 0.19 0.15 0.18 1.22 0.46 0.39 0.30 0.42 0.18 0.18 0.16 2.53 0.76 0.64 0.36 0.59 0.19 0.20 0.16 0.30 0.18 0.45 0.43 0.23 0.19 0.14 0.25 0.44 0.46 0.84 1.70 0.26 129.5 129.3 119.4 119.3 119.9 118.5 119.3 119.3 119.5 119.7 150.4 149.5 149.2 154.9 149.1 150.7 148.4 148.1 145.0 144.2 144.1 143.9 135.7 136.9 136.6 135.1 135.3 135.3 134.1 135.0 135.2 135.1 142.6 134.2
P r

deg 0.5 0.6 0.4 0.4 0.3 1.8 0.9 1.0 0.8 0.6 0.3 0.3 0.4 3.9 1.2 0.8 0.7 0.7 0.3 0.3 0.4 0.4 0.3 0.6 1.0 0.4 0.4 0.4 0.7 0.5 0.6 1.5 3.1 0.4

deg 57.70 57.96 69.56 69.68 69.85 70.02 70.11 70.15 70.17 70.20 89.78 89.97 90.12 90.22 90.24 90.25 90.25 90.26 92.09 92.22 92.32 92.38 107.41 107.60 107.65 111.21 111.21 111.22 111.36 111.36 111.36 111.36 111.36 111.13

deg 219.4 219.2 209.6 209.4 209.3 209.1 209.0 208.9 208.9 208.9 60.5 59.8 59.4 59.0 59.0 58.9 58.9 58.9 54.5 54.2 54.1 54.0 45.4 45.4 45.4 45.2 45.2 45.2 45.0 45.0 45.0 45.0 45.0 44.4

deg 0.1 0.1 0.2 0.1 0.6 0.6 0.3 0.4 0.6 0.8 0.1 0.3 0.2 5.9 0.1 1.8 0.5 0.8 0.5 0.0 0.0 0.1 0.3 1.5 1.2 0.1 0.1 0.1 0.9 0.0 0.2 0.1 7.6 0.2


POLARIMETRY AND PHOTOMETRY OF COMET HYAKUTAKE

289

FIG. 1. Phase dependence of polarization for dusty comets measured in the UC filter. Open circles: data from LHW CD-ROM digital Archive; filled circles: present work; filled rhomb: data from Joshi et al. (1997).

FIG. 3. Phase dependence of polarization for dusty comets measured in the RC filter. All data are from Chernova et al. (1993).

FIG. 2. Phase dependence of polarization for dusty comets measured in the RC filter. Most data are from Chernova et al. (1993). Data for ° Comet West 1976 VI in the filter 5300 A (Kiselev and Chernova, 1978) ° and in the filter 4850 A (Michalsky, 1981) are added. Crosses: data for Hyakutake from Joshi et al. (1997); filled circles present data for Hyakutake.

angles and the low values of W4845 for comet P / Halley at the same time permit us to do this. As can be seen from Figs. 2 ­ 3, the polarization data for Comet Hyakutake are in good agreement with data for the dusty comets in the 37 ­73 overlap region and these data complement each other very well. The UC-BC and BC-RC colors of Comet Hyakutake are close to those of comets P / Giacobini ­ Zinner 1985 XIII and Levy 1990 XX (Kolokolova et al. 1997). The dust-poor comets usually show blue colors (see, e.g., Sanzovo et al. 1996). Consequently, by the polarimetric as well as colorimetric properties of dusty particles, and according to the gas-to-dust ratio, Comet Hyakutake belongs to the group of dusty comets consisting of West 1976 VI, P / Churyumov ­ Gerasimenko 1982 VIII, P / Kopff 1983 XIII, P / Hartley ­ IRAS 1984 III, P / Giacobini ­ Zinner 1985 XIII, P / Halley 1986 III, Bradfield 1987 XXIX, Liller 1988 V, Levy 1990 XX, and P / Faye 1991 XXI. In such a case, taking into consideration the results from Chernova et al. (1993), the complete phase dependences of polarization of the dusty comets can be obtained for two continuum domains in the phase angle range 0 ­ 110 . Their parameters are Pmin 1.5% and 0 22.1 , almost independent of wavelength; h 0.34 per cent per degree; Pmax 24.0%, and max 94 for the blue filter; and h 0.41 per cent per degree, Pmax 26%, and max 94 for the red one. As for the ultraviolet continuum, one can only say that the polarization maximum is in the vicinity of 19%. It is common knowledge that the wavelength dependence of polarization changes with the phase angle. For


290

KISELEV AND VELICHKO

phase angles 30 it is nearly constant but for 30 P ( ) increases with phase angle increasing. Using the current observations of Comet Hyakutake and the available data for Comets P / Halley 1986 III, Bradfield 1987 XXIX, and Liller 1988 V (Chernova et al. 1993), we have found that the increase of dP ( )/ d in the 30 ­94 phase ° angle range and between 4845 and 6840 A is 0.023 ° 0.002% per 1 within 1000 A.
4. DISCUSSION

The active regions (jets) of the inner coma of Comet Hyakutake were observed by Tozzi et al. (1997). As is evident from the observations of Comet Okazaki ­ Levy ­ Rudenko (1989r) (Eaton et al. 1991), Comet Levy 1990 XX (Renard et al. 1992), and Comet P / Swift ­ Tuttle 1992 XXVIII (Goldberg and Brosch, 1995), the polarization degree of jets tends to be higher than that of the surrounding parts of the coma. As a rule, the areas with higher polarization degree are rather small and do not perceptibly affect the aperture polarimetry. The assurance that the aperture polarimetry can rather well provide the information on the mean polarimetric properties of cometary dust particles is based on the similarity of the polarization phase dependences of many dusty comets. Indeed, the scatter of data in the composite polarization phase dependence of the comets is not too significant, despite the different accuracies of different observations the diversity of cometary gas-to-dust ratios and the appearance of jets in the comae. Up to the present work, the position and value of the maximum of the polarization in the cometary continuum were poorly studied. There were only three dusty comets with polarimetric observations at phase angles greater than 90 . They are Comet Bennett 1970 II (Kharitonov and Rebristyi, 1973), Comet West 1976 VI (Kiselev and Chernova, 1978; Oishi et al. 1978; Michalsky, 1981) and Comet Austin 1982 VI (Myers and Nordsieck, 1984). The spectropolarimetric observations by Kharitonov and Rebristyi were not very accurate, while observations by Oishi et al. and Myers and Nordsieck were carried out in broad wavelength intervals. It is particularly important that Comet Austin 1982 VI had a low continuum flux and a large decrease in the dust production rate at that time when its phase angles were large (see Myers and Nordsieck, 1984). Because of this fact, only data for Comet West could be used for the combined polarization phase dependence. That is why the values of max obtained by LevasseurRegourd et al. (1996) (see Figs. 3a and 3b) are essentially extrapolated. Previously, it was assumed that the maximum of cometary polarization is close to 90 (Dobrovolsky et al. 1986) or corresponds to the maximum of polarization of atmosphereless bodies (Dollfus, private communication). Levasseur-Regourd et al. (1996) have found that max is equal

to 103 and 95 in the green and red domains, respectively. We have shown that the maximum of polarization of dusty comets occurs at about 94 . Possibly, max slightly depends on wavelength in the visible region of spectra, and the value of Pmax increases in the red. A knowledge of the Pmax and max for comets is very important at least from two points of view. First, the calculated models may be limited in number because the real models should fit the observed maximum of polarization. Second, as was shown by Kolokolova et al. (1997), the maximum of polarization indicates the composition of dust material (complex refractive index). Discussion of the parameters Pmax and max for dustpoor comets is not a subject of this paper. Nevertheless, it should be emphasized that both the differences of gasto-dust ratios among the comets and the problem of gas contamination in the continuum filters complicate the study of the polarization phase dependence of dust-poor comets (Kiselev and Chernova, 1996). It is interesting to note that the gas-to-dust ratio for comet Austin 1990 V changed in large ranges with the heliocentric distance. During our observations (Chernova et al. 1993) the comet was quite dusty. But a low maximum of polarization, observed by Kikuchi et al. (1990), occurred for large gas-to-dust ratios. Therefore, the intermediate-band observations by Kikuchi et al. (1990) could be explained more naturally by the changing of the gas-to-dust ratio rather than by the changing of the dust properties in the comet with heliocentric distance, as speculated by Mukai et al. (1991). It might be good to compare the polarimetric properties of the dust particles of the comets with those of the atmosphereless bodies. In contrast to comets, the polarization maxima for the S-type atmosphereless bodies (NEAs) and for the whole Moon and Mercury as well as are close to 9% at around 100 ­ 110 (Dollfus and Bowell 1971; Dollfus and Auriere 1974; Kiselev et al. 1990). Pmax is greater in the blue than in the red domain, while max is wavelengthindependent at least for the whole Moon (Dollfus and Bowell, 1971). Dollfus (1989) has indicated that polarimetric properties of comets are close to those of the C-type asteroids. They have at least similar values of Pmin . Unfortunately, the position and degree of the maximum polarization of the C-type asteroids are still unknown. There are no observations at phase angles larger than 40 . A weak increase of P ( ) is observed for the positive branch of polarization for the C-type asteroids (Myers et al. 1985). Simultaneously, the slope and the inversion angle decrease slightly with increasing wavelength (Chernova et al. 1993). Thus one cannot say what the wavelength polarization dependence of C-type asteroids will be at large phase angles. It is well known that the polarimetric properties of the asteroids at large phase angles are determined by Umov's law. This indicates a combination of Fresnel single scatter-


POLARIMETRY AND PHOTOMETRY OF COMET HYAKUTAKE

291

ing and multiple scattering. As a result, the polarization degree is inversely proportional to albedo. The albedo of the S-type asteroids increases with wavelength, while the albedo of the C-type asteroids is almost constant in the visible range of spectra (Tedesco et al. 1989). The polarization maximum of the S-type asteroids lies approximately at twice Bruster's angle. Reflectivity of dusty comets is slightly increasing with wavelength (Lamy, 1985) and polarization is increasing also. Does this mean that Umov's law does not hold for cometary dust particles? As a first approximation cometary dust particles may be considered independent Mie scatterers, while asteroid dust particles form a surface. Therefore the unpolarized component of light arising under multiple scattering by a rough surface is absent for comets. The higher maximum polarization degree and the observed wavelength polarization dependence of comets can thus be explained. A direct comparison of the scattering processes for the dust particles of cometary atmospheres with those for the asteroid dust particles is hardly possible. Cometary particles are actually aggregated structures involving many submicrometer grains. The scattering process for these particles is more complicated. Multiple scattering within aggregate-structured dust particles produces negative polarization, the opposition effect, and a maximum polarization around 90 (see, e.g., Tishkovets 1994; Vaida and Desai 1996; Xing and Hanner 1996; Tishkovets 1997).

ACKNOWLEDGMENTS
The authors thank L. Kolokolova for help in calculation of the production rates of species, V. Tsvetkova and V. Rozenbush for their help in preparation of the paper, and reviewers T. Mukai and D. Schleicher for useful comments and suggestions.

REFERENCES
A'Hearn, M. F. 1991. Photometry and polarimetry network. In The Comet Halley Archive Summary Volume (Z. Sekanina, Ed.), pp. 193 ­ 235. Jet Propulsion Laboratory, Pasadena, CA. A'Hearn, M. F., R. L. Millis, D. G. Schleicher, D. J. Osip, and P. V. Birch 1995. The ensemble properties of comets. Results from narrowband photometry of 85 comets, 1976 ­ 1992. Icarus 118, 223 ­ 270. A'Hearn, M. F., D. G. Schleicher, P. D. Feldman, R. L. Millis, and D. T. Thompson 1984. Comet Bowell 1980b. Astron. J. 89, 579 ­ 591. Chernova, G. P., N. N. Kiselev, and K. Jockers 1993. Polarimetric characteristics of dust particles as observed in 13 comets: Comparison with asteroids. Icarus 103, 144 ­ 158. Delsemme, A. H. 1982. Chemical composition of cometary nuclei. In Comets (L. L. Wilkening, Ed.), pp. 85 ­ 130. Univ. of Arizona Press, Tucson. Dobrovolsky, O. V., N. N. Kiselev, and G. P. Chernova 1986. Polarimetry of comets: A review. Earth Moon Planets 34, 189 ­ 200. Dollfus, A. 1989. Polarimetry of grains in the coma of P / Halley. II. Interpretation. Astron. Astrophys. 213, 469 ­ 478. Dollfus, A., and M. Auriere 1974. Optical polarimetry of planet Mercury. Icarus 23, 465 ­ 482. Dollfus, A., and E. Bowell 1971. Polarimetric properties of lunar surface and its interpretation. I. Telescopic observations. Astron. Astrophys. 10, 29 ­ 53. Eaton, N., S. M. Scarrott, and R. D. Wolstencroft 1991. Polarization studies of Comet Okazaki ­ Levy ­ Rudenko. Mon. Not. R. Astron. Soc. 250, 654 ­ 656. Festou, M. G., G. P. Tozzi, L. A. Smaldone, P. Felenbok, R. Falciani, and J.-M. Zucconi 1990. Did an outburst occur on 4 December 1985 in Halley's comet? Astron. Astrophys. 227, 609 ­ 618. Goldberg, Y., and N. Brosch 1995. Imaging polarimetry of P / Swift ­ Tuttle. Mon. Not. R. Astron. Soc. 273, 431 ­ 442. Hadamcik, E., A. C. Levasseur-Regourd, J. C. Worms, and J. B. Renard 1996. Polarimetric measurements of scattered light by dust grains in Earth and microgravity conditions. Tentative interpretation of the dichotomy between comets. Physics, Chemistry, and Dynamics of Interplanetary Dust (B. Gustafson and M. Hanner, Eds.), ASP Conference Series, Vol. 104, pp. 391 ­ 394. Jockers, K., N. N. Kiselev, H. Boehnhardt, and N. Thomas 1993. CN, C2 , and dust observed in Comet P / Grigg-Skjellerup from the ground eight hours after the Giotto encounter. Astron. Astrophys. 268, L9 ­ L12. Joshi, U. C., K. S. Baliyan, S. Ganesh, A. Chitre, H. O. Vats, and M. R. Desphande 1997. A polarization study of Comet Hyakutake (C / 1996 B2). Astron. Astrophys. 319, 694 ­ 698. Kayumov, V. V., N. N. Kiselev, P. A. Pushnin, V. J. Rakhimov, V. I. Siklitskij, K. V. Tarasov, G. P. Chernova, and V. N. Yakutovich 1989. Investigation of the photometer ­ polarimeter of the 1-m RCC telescope. Bull. Inst. Astrophys. Tadjik. Acad. Sci. 78, 10 ­ 16. Kharitonov, A. V., V. T. Rebristyi 1973. Spectropolarimetry and absolute spectrophotometry of the nuclear region of Comet Bennett. Astron. Zh. 50, 1062 ­ 1070. Kikuchi, S., A. Okazaki, M. Kondo, and R. Harita 1990. Continuum

5. CONCLUSIONS

The results of photometric and polarimetric observations of Comet 1996 B2 hyakutake were presented. 1. It is shown that Comet Hyakutake belongs to the group of dusty comets involving West 1976 VI, P / Churyumov ­ Gerasimienko 1982 VIII, P / Kopff 1983 XIII, P / Hartley ­ IRAS 1984 III, P / Giacobini ­ Zinner 1985 XIII, P / Halley 1986 III, Bradfield 1987 XXIX, Liller 1988 V, Levy 1990 XX, and P / Faye 1991 XXI. 2. The degree and position of the polarization maximum for Comet Hyakutake and hence for the group of dusty comets were found for ultraviolet ( 19%), blue (24%), and red (26%) domains of the continuum at a phase angle close to 94 . Consequently, at present the complete synthetic phase dependence of the polarization of dusty comets is known in the range 0 ­ 110 . 3. Pmax increases with wavelength and max is slightly wavelength-dependent. Polarimetric observation of the C-type NEAs 1580 Betulia and 2100 Ra-Shalom is particularly important in the future. So far only these C-asteroids give a rare possibility of ground-based observations at large phase angles.


292

KISELEV AND VELICHKO Oishi, M., K. Kawara, Y. Kobayashi, T. Maihara, K. Noguchi, H. Okuda, S. Sato, T. Iljima, and T. Ono 1978. Infrared observations of Comet West (1975n). I. Observational results. Publ. Astron. Soc. Jpn. 30, 149 ­ 159. Osborn, W. H., M. F. A'Hearn, U. Carsenty, R. L. Millis, D. G. Schleicher, P. V. Birch, H. Moreno, and A. Gutierrez-Moreno 1990. Standard stars for photometry of comets. Icarus 88, 228 ­ 245. Osip, D. J., D. G. Schleicher, R. L. Millis, and S. Lederer 1996. Narrowband photometry and imaging of comet Hyakutake (1996 B2). In Asteroids Comets Meteors 96, Versailles, 8 ­ 12 July, 1996 [Abstracts], p. 28. Renard, J. B., A. C. Levasseur-Regourd, and A. Dollfus 1992. Polarimetric CCD imaging of Comet Levy (1990c). Ann. Geophys. 10, 288 ­ 292. Sanzovo, G. C., P. D. Singh, and W. F. Huebner 1996. Dust colors, dust release rates, and dust-to-gas ratios in the comae of six comets. Astron. Astrophys. Suppl. 120, 310 ­ 311. Storrs, A. D., A. L. Cochran, and E. S. Barker 1992. Spectrophotometry of the continuum in 18 comets. Icarus 98, 163 ­ 178. Tedesco, E. F., J. G. Williams, D. L. Matson, G. J. Veeder, J. C. Gradie, L. A. Lebofsky 1989. A three-parameter asteroid taxonomy. Astron. J. 97, No. 2, 580 ­ 606. Tishkovets, V. P. 1994. Light scattering by clusters of spherical particles, cooperative effects under chaotic orientation. Kinemat. Fiz. Nebesn. Tel 10 (No. 2), 58 ­ 63. Tishkovets, V. P. 1997. Negative polarization of light scattered by closely packed disperse media. In Proceedings of the XXVIII Lunar and Planetary Science Conference, pp. 1437 ­ 1438. Tozzi, G. P., A. Cimatti, S. di Serego Alighieri, and A. Cellino 1997. Imaging polarimetry of Comet C / 1996 B2 (Hyakutake) at the perigee. Planet. Space. Sci. 45, 535 ­ 540. Vaida, D. B., and J. H. Desai 1996. Scattering properties of cometary grains. In Physics, Chemistry, and Dynamics of Interplanetary Dust. ASP Conference Series (A. S. Gustafson and M. Hanner, Eds.), Vol. 104, pp. 433 ­ 436. Xing, Z., and M. Hanner 1996. Light scattering by aggregate particles. Astron. Astrophys. [Cometary Science Team Preprint Series No. 167, JPL]

polarization in Comet Austin (1989c1). In Proceedings of the 23 ISAS Lunar and Planetary Symposium, pp. 39 ­ 42. Kiselev, N. N., and G. P. Chernova 1978. Polarization of the radiation of Comet West 1975n. Astron. Zh. 55, 1064 ­ 1071. Kiselev, N. N., and G. P. Chernova 1981. Phase functions of polarization and brightness and the nature of cometary atmosphere particles. Icarus 48, 473 ­ 481. Kiselev, N. N., and G. P. Chernova 1996. On scattering of cometary continuum polarization at phase angles close to 90 degrees. In Asteroids Comets Meteors 96, Versailles, 8 ­ 12 July, 1996 [Abstracts], p. 95. Kiselev, N. N., D. F. Lupishko, G. P. Chernova, and Yu. G. Shkuratov 1990. Polarimetry of asteroid 1685 Toro. Kinemat. Fiz. Nebesn. Tel 6, 77 ­ 82. Kolokolova, L., K. Jockers, G. Chernova, and N. Kiselev 1997. Properties of cometary dust from color and polarization. Icarus 126, 351 ­ 361. Krishna Swamy, K. S. 1986. Physics of Comets. World Scientific, Singapore. Lamy, P. L. 1985. Cometary dust: Observational evidences and properties. In Proceedings of a Meeting Held in the Astronomical Observatory of the Uppsala University, June 3 ­ 6, pp. 373 ­ 388. Levasseur-Regourd, A. C., E. Hadamcik, and J. B. Renard 1996. Evidence for two classes of comets from their polarimetric properties at large phase angles. Astron. Astrophys. 313, 327 ­ 333. Michalsky, J. J. 1981. Optical polarimetry of Comet West 1976 VI. Icarus 47, 388 ­ 396. Mukai, S., T. Mukai, and S. Kikuchi 1991. Scattering properties of cometary dust based on polarimetric data. In Origin and Evolution of Interplanetary Dust (A. C. Levasseur-Regourd and H. Hasegawa, Eds.), pp. 249 ­ 252. Kluwer Academic, Publish Dordrecht. Myers, R. V., and K. H. Nordsieck 1984. Spectropolarimetry of Comets Austin and Churyumov ­ Gerasimenko. Icarus 58, 431 ­ 439. Myers, R. V., B. A. Whytney, and K. H. Nordsieck 1985. Wavelength dependence of polarization of asteroids. Bull. Am. Astron. Soc. 17, (1), 515. Newburn, R. L., and H. Spinrad 1989. Spectrophotometry of 25 comets: Post-Halley updates for 17 comets plus new observations for eight additional comets. Astron. J. 97, 552 ­ 569.