Comet West 1976 VI: Photopolarimetry by the Helios 2 Zodiacal Light Experiment

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Space

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Vol.2, No.12, pp. 163—166, 1983

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COMET WEST 1976 VI: PHOTOPOLARIMETRY BY THE HELLOS 2 ZODIACAL LIGHT EXPERIMENT B. Kneissel*, G. H. Schwehm*, C. Leinert~’4’,I. Richter~’ and B. Planck** *Berejch Extraterrestrische Physik, Ruhr-Universitat Bochum, F. R. G. **Mg~..planck..JnstjtutfurA5troflQmje,Heidelberg, F.R.G.

ABSTRACT Shortly after it passed through perihelion on February 25, 1976 Comet West was observed by the Helios Zodiacal Light Experiment Broadband photopolarimetric measurements of the head and of the tail were performed ‘8 three wavelength bands (U B v) The phase angles encountered range from 670 to 85 . The polarization measurements indicated wavelength independent linear polarization. The maximu 8 degree of polarization in the visual channel was found to be .28 at a phase angle of 83 INTRODUCTION Optical observations of the continuum radiation from comets can be used to infer the physical properties of the dust grains. Measurements of the brightness and polarization of the sunlight scattered bythe dust particles as a function of scattering angle and wavelength together with the infrared thermal emission from the dust provide information on the composition and size distribution of the dust (for a review c.f. Hanner 1980). Accidentally the photopolarimeters Onboard the Helios 2 spaceprobe provided an opportunity for space—borne UBV photometry arid polarization measurements of t1~ehead and to our knowledge unique measurements of the tail. 8la and 1981b) He! tos 2hasitalready scans the The Helios Zodiacal light Experiment, its calibration data On reduction beensky described at constantinecliptic great detail latitudes (Leinert of 8et= al 16 and l~75 13 =1~31 Measurements are performed using three different broad bandpass filters (±50 nm) centered at 360 nm, 1420 nm, and 530 nm, respectively Polarization is ~eing 0measured by consecutive measurements through three polirizing foils oriented at 0 145 900 with respect to the ecliptic Helios 2 was launchedon January 15, 1976. Its aphelion is at 0.98 AU, its perihelion near 0.30 AU. As the photometers are fixed into the space probe unfortunately no pre-determined scans can be performed across the cometary coma and along the tail So we depend on the observed signals from the sectors where the comet happens to be in the field-of—view. The geometry of the observations is given in Figure 1.

Fig. 1

Observing Geometry.

B~k’neissel et ci.

164

Shortly after its perihelion passage on February 25, 1976 Comet West was in the field—of— view of the 15 photometer The dramatic Increase in brightness in sector I of the 150 photometer (Figure 2) occured on February 29 1976 and from the geometry we derived that the head of the comet must have entered in the field—of—view.

0~

~

~

tAGE

Fig. 2

Intensities (S from sector 1 of the 150 photometer as a function of heliocentric distance in U (o), B (~)and V (o).

OBSERVATIONS The observation report coveisthetime interval February 29, 1976 — when the cornet was observed by the 150 photometer till March 5 when the tail of the cornet could be observed with the 3Q0 photometer The head of the comet must have been intercepted by the 30° photometer on March 3rd. This provides u.s with the only simultaneous photopolarimetric measurements of both the head and the tail at nearly the same phase angle Phase angles covered are in the range from 67°to 85°. ~~g~tness An example in Figure 2.

of

the reduced data available from the Helios 2 experiment is shown

In Table I ~ have compiled the observed intensities for the cometary tail for the 150 photometer at different dates corresponding to different phase angles. TABLE 1

Tail Brightness ~

Day

Phase angle

February 29, 1976

U

March 2.

for the visual channel

1976

~

~10

112°

20 000

113°

23 000

%

114°

25000

‘~.

104°

28000

i..

102°

31 000

Unfortunately no direct comparison of the run of brightness can be made with the data given by Ney and Merrill (1976) as their measurements refer to the coma but there seems to exist no significant deviation. Polarization. Figure 3 gives an example of the degree of polarization as a function of heliocentric distance. The smooth curves are those for the zodiacal light brightness. The peak on top of that curve around a heliocentric distance of Helios 2 R = 0.8040 AU gives the degree of polarization of Cornet West.

Comet West 1976 VI

‘I

Fig.

3

Helios 2 Zodiacal Light Experiment

h

6.1

165

~

Degree and direction of polarization as a function of heliocentric distance. The direction of polarization is given with respect to the normal to the scattering plane, counterclockwise on the sky.

The data of the 150 photometer, where polarization measurements have been performed in all three wavelength bands indicate no wavelength dependence of the linear polarization For the 300 photometer polarization measurements have only been performed in the visual channel. For phase angles a 67° the degree of polarization corresponds to 22~and increases to 28~ for phase angles a 830. Here we have selected observations close to the cometary coma to allow comparison with other polarimetric data. A more thorough analysis of the polarization data will be discussed in a forthcoming paper including the reversal of the orientation of the polarization as qualitatively indicated in the data (cf. Figure 3 lower curve). Colour. The colour indices for the three wavelength bands show a rather complicated dependence (Fig 4,5) The straight lines show the slight reddening of the zodiacal light (leinert et al. l981b), whereas for the comet it passes from red to slightly bluer colour than the sun (Figure 4). For fainter regions the colour indices tend to indicate a nearly solar type spectrum. 66 It *06

s

II.

-

0~?C0h

2~. ~

or

L.~ 111111

Fig. 4

Colour of zodiacal light and Comet West relative to solar colour observed with Helios 2 at B = 16°(Fig. 4) and 13 = 310 (Fig. 5) during ingoing part of the first orbit. Numbers smaller than 1.0 signify a reddening.

The dependence of the colour indices on the time of observation seem to be due to different contributions of the dust and plasma tail, respectively, in the data for consecutive observations.

B. Kneissel et ci.

166

~ *

‘.4

.

~--~

~

~-

..

0’

~ ~

‘I/V

~

-~

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4

TACE

Fig.

5

See caption

Figure 4.

As Comet West is an example ofa very dusty comet (A’Hearn et al. 1977) the contamination by the C 2 C~ and CN bands in the U and B data should be only a minor effect so that the similar run 6-f the colour indices for the different bandpasses as plotted in Fig. 4 and 5 should be real. COMPARISON WITH OTHER OBSERVATIONS Comet West was one of the most carefully studied of recent comets and there have been a few photopolarimetric studies published which allow comparison with our data As mentioned earlier Helios 2 provided the only photopolarimetric observations of the tail therefore we will concentrate on the observations of or close to the cometary atmosphere. Another disadvantage for direct comparison with observations were made to a much earlier date and Kiselev and Chernova (1981) and concentrate scattering angles 0 our data are-sI ightly above

other data is that most of the Helios 2 We refer to the papers by M,chalsky (1981) on the visual polarization For the same those given by llichalsky ‘(1981):

0

28~compared to 25~at 0 22~compared to 2O?~at 0

= =

0

and e 112° and 0

93

= =

92

, respectively 108°, respectively.

Both measurements are above the values reported by Kiselev and Chernova (1981). All three studies are in good agreement on the increase of the polarization with increasing phase angle in the range covered. Acknowledgements This work was supported by the Bundesministerium fDr Forschung und Technolog~’e with grant WRS 0108 and 01OF072 6 The computations and data plots were performed at the computing facility of the ?lax—Planck—Institut fOr Kernphysik. References M.F. A’Hearn, C.H. Thurber, and R.L. Millis, Astron. J., 82, 518 (1977) N N N

Hanner

in Solid Particles in the Solar System (I 223 (1980)

Kiselev and C P

Chernova

Icarus

48

1473

Halliday and B A

McIntosh

eds

(1981)

C. Leinert, E. Pitz, H. Link, N. Salm, Raurnfahrtforschung, 19, 264 (1975) 81a) CC Leinert E Pitz H Link N Salm J Space Sci Instr , 5 257 (19 Leinert I Richter E Pitz and B Planck Astron Astrophys 103 177 (1981b) J.J. Michalsky, Icarus, 1,7, 388 (1981) E P

Ney and K N

Merrill

Science

19~+ 1051 (1976)