Structure of the Barnard Loop Nebula as Determined from Gemini 11 Photographs

Abstract

Ultraviolet objective-prism photographs in the 2200-4900 Â wavelength region made from the Gemini 11 spacecraft have revealed an unexpected brightness of the Barnard Loop Nebula. Micropho-tometer tracings of the film have been reduced to absolute energy units by reference to stars of known flux. The form of the nebula differs significantly in the ultraviolet from that surface brightness distribution seen in the recombination line Ha By comparison with optical wavelength measurements it is seen that in the optically brightest region the observed surface brightness is slightly greater than that predicted for pure atomic emission and that the discrepancy is usually even larger for other regions. It is hypothesized that the excess radiation is due to starlight scattered by interstellar grains in the nebula. Owing to its low optical depth, this radiation should clearly outline the structure of the nebula in both the ionized and un-ionized regions. It is shown that the surface brightness distribution requires a non-homogeneous density distribution, with the total atomic density increasing outward. Such a density distribution may be the result of a dynamical radial force due to stellar radiation pressure acting on the grains mixed with the gas. I. INTRODUCTION Gemini spacecraft objective-prism photographs in the wavelength range of 2200-4900 Â have revealed for the first time the appearance of many faint objects in this spectral region. Although not the direct purpose of that photographic series, we have found that considerable information about the appearance of the nebulosity in Orion was contained on one exposure. This information has enabled us to make a thorough investigation of the structure of the Barnard Loop Nebula that would not have been possible from the terrestrial surface. Although the presence of the Barnard Loop Nebula has been known for some time (Barnard 1895), it has been studied only little in the intervening years because of its low optical surface brightness. On the Palomar Sky Survey it is possible to trace the loop almost completely around the lower constellation, where the greatest concentration of early-type stars appears. The brightest red regions are brighter in the Ha photograph than in the blue, thus suggesting that these areas are emission-line regions; but the low ratio of red to blue brightness would indicate that strong blue-violet radiation is present and that the ratio of Ha to Hß is not extremely high. The ratio of the red and blue intensities changes strongly with position. As an extreme example, parts on the western side appear only in the blue photograph. On the Gemini 11 photograph the nebula is remarkable because of its unexpected brightness, its amorphous appearance, and the clearer delineation of the amorphous nebulosity in the western part of its extension. In the following paragraphs we shall discuss how this photograph enables us to build a realistic model of the distribution of material in this giant nebula. A dynamical interpretation of the inferred structure will also be applied. The shape of the nebula as determined on the red Palomar Atlas photographs is roughly that of an ellipse with axes of 14° and 10°, centered about midway between the Orion sword and belt regions. The major axis of this ellipse is aligned roughly north-south in the plane of the sky. At the generally adopted distance for this region of 450 pc, 835