Can the Red Rectangle Molecular Emission Bands Be Assigned to C

Abstract

We review the current information on the unique group of visibleemission bands known as the Red Rectangle bands (RRBs). A laboratoryspectrum exactly matching these bands is not yet available; however, thereis sufficient reason to hypothesize phosphorescence from C3 asthe source. The points in favor of the C3/RRB hypothesis canbe summarized as follows: (1) The RRBs are found in the spatial locationwhere carbonaceous grains are being impinged upon by a bipolar outflow. (2)C3 is known to accompany circumstellar carbon grains and mayconstitute a ‘magic’ fragment in the UV photofragmentation ofCn clusters. (3) Our recent observation of the spin-forbiddenCO Cameron emission bands in this object suggests that charged particleimpact excites triplet states. (4) The energy of the lowest triplet state ofC3 is known to be very close to that of the RRBs. (5) The RRBsdisplay a vibrational spacing that is quantitatively consistent withC3, including a rare negative anharmonicity.