Photometry of NGC 3587, "the Owl nebula", in Halpha light, and its structure.

Summary. Isophotic contours of NGC 3587 in H light are derived from measurements on direct photographs secured with the 120 cm reflector of the Observatory of Haute Provence. The variations of brightness show no radial symmetry, but a kind of symmetry along two perpendicular lines - one of them passing through the two bright observed condensations. Neglecting some smooth features without any symmetry, we obtain a schematic picture of the brightness distribution on the surface of the nebula. Comparing this result with the distribution that w6uld be given by a shell uniformly filled with a luminous gas without absorption, and taking the observed brightness distributions along the two lines of symmetry into account, the nebula appears like a shell whose thickness varies roughly from one half of the radius of the nebula, in a plane passing through the bright condensations, to one quarter of this radius along the diameter perpendicular to that plane: roughly a spherical shell with an inner belt twice as thick as the shell, with an emission density higher in the belt as in the shell in a ratio that does not exceed 1.65. The thickness and the emission density of the belt, of course, are not at all equable and their variations give rise to the observed departures of the brightness from the theoretical distribution: after our data, the belt may be somewhat thicker and the emission density higher in the vicinity of the center than on the observed condensations. Key words: planetary nebulae - photographic photometry - brightness distribution on gaseous shells - emission density