Infrared Spectroscopy of U Equulei's Warm Circumstellar Gas

Medium- and high-resolution spectroscopy of U Equulei from 1 to 4 μm during 1997-2003 has revealed information about its unusual circumstellar envelope, observed previously at optical and radio wavelengths. Strong absorption bands of H₂O and of CO dominate the 1-4 μm spectrum. The gas has a mean temperature of 600 K, and ¹²C/¹³C<=10. The CO 2-0 line profiles and velocities imply no net ejection or infall, and indicate either rapid radial gas motions being seen along a narrow continuum beam or absorption by orbiting gas that is nearly coincident with a highly extended continuum source. The gas could be located in a disklike structure. The observed high column densities of warm CO and H₂ normally would be associated with sufficient dust to completely obscure the star at optical wavelengths. The observations thus indicate either a highly abnormal gas-to-dust ratio, consistent with the earlier optical observation of abundant refractory metal oxides in the circumstellar gas, or peculiar geometry and/or illumination.