Spectroscopic observations of the recurrent nova CI Aquilae during the 2000 outburst

High- and low-resolution optical spectra of CI Aql were obtained during the outburst in 2000. Multiple absorption components of H I and Fe II lines were detected at the early decline stage. Their radial velocities were roughly -2500 km s(-1), -2200 km s(-1), -1700 km s(-1), and -1400 km s(-1), among which only the last components were likely still accelerating during the early decline stage. Prominent emission lines of [O III] and [N II] appeared about one month after light maximum. The duration of the nebular stage, however, was only one month and a few weeks. The ejected gas shells seem to have started to shrink about 70 days after light maximum. The amount of interstellar extinction is estimated to be E(B - V) = 0.92 +/- 0.15 from the equivalent widths of the diffuse interstellar absorption bands. The helium abundance in the ejecta is estimated to be N(He) = 0.19 +/- 0.05 and the mass of the ejecta to be about 2 x 10(-6) M-circle dot. This object has been classified as a U Sco type recurrent nova, but its spectral evolution during the outburst resembled those of T Pyx type recurrent novae. It is doubtful whether the peak of m(V) congruent to 9 mag on 2000 May 5 was the true light maximum or an earlier brighter peak had been overlooked, because the spectral data suggest that the ejections of gas shells occurred prior to the discovery of the outburst on 2000 April 28.