Circular polarization of Zodiacal Light and stellar radiation scattered on circumstellar clouds.

Summary. The circular polarization degree is estimated for radiation scattered on different kinds of dust grains. Their orientation due to the action of gaseous and radiative fluxes and the magnetic field is examined. The circular polarization of the VY CMa and NML Cyg infrared radiation is shown to arise due to single scattering on small oriented graphite dust grains, a dipole magnetic field > 300 Gauss on the stellar surface being required. The circular polarization of the ZEri visible light is explained in a similar manner. The variable circular polarization of the Vt057 Cyg radiation in the blue filter seems to arise due to single scattering on dust grains in the H ii region of the circumstellar gaseous and dust cloud. Grains are oriented by the action of both the outfiowing gas and magnetic field which is frozen into the plasma. This field has the geometry of an Archimedes spiral. If the angle between the flux of plasma and magnetic lines is smaller (larger) than 550, strongly prolate (oblate) grains are oriented. The signs of the polarization degree of radiation scattered on oblate and prolate grains are opposite. The polarization of the V1057 Cyg radiation seems to change its sign at variations of the velocity of the stellar wind. A magnetic field 200 Gauss on the stellar surface is necessary. The circular polarization of the Zodiacal Light may be explained in a similar way but in this case an observer is inside the dust cloud. The analysis of the polarization dependence on the elongation is shown to confirm the existence of the regular component of the interplanetary magnetic field perpendicular to the ecliptic plane. Key words: circumstellar clouds, zodiacal cloud - circumstellar grains orientation - circular polarization - stellar magnetic fields.