Atomic and Molecular Emission Lines from the Red Rectangle

Abstract

HD 44179 is the binary, post-asymptotic giant branch, central star of the Red Rectangle Nebula. Echelle spectra of the star have been obtained over a wavelength range from about 3800 to 10000 A at a resolving power R = 38,000. A maximum S/N of 850 was achieved near 6800 A. Fifty-seven identified atomic or ionic emission lines of 12 elements are detected in the star's spectrum, along with 76 emission lines of CH, CH+, or CN. Three other CN lines are also present in absorption. Fewer than 30 of these 136 lines apparently have been previously reported, and the newly detected species include N II, Mg I, S II, K I, Fe I, Fe II, Rb I, Ba II, CH, and CN. On the basis of their shapes and widths, the line profiles of the various species can be classified into three groups: narrow, broad, or double-peaked. The emission apparently originates in an unusual, compact H II region and in associated neutral gas, both concentrated primarily within the small, dusty torus that optically obscures the central star. The detection of the Ba II emission from the gas may support the hypothesis of Waelkens et al. that HD 44179 is destined to become a barium star.