The Birth of Be Star Disks. II. A High-resolution Spectroscopic Campaign and TESS Observations of an Outburst of the Classical Be Star <i>λ</i> Pavonis

Abstract Be stars are rapidly rotating B stars that have shown emission lines originating in a circumstellar disk. The mechanisms that lead to disk formation and dissipation are not known although progress has been made with some systems. We present a study of a disk outburst of the Be star λ Pavonis. Our data set comprises 698 high-resolution spectra contemporaneous with TESS photometry obtained in 2023. Near the end of the TESS monitoring, the star began disk building from a pristine diskless state. We find the disk was built within 5 days via the optical H i and He i lines, while the disk circularized in about 12 days. The disk began to decay in higher-excitation He i first, then at lower-excitation transitions, with the decay ending last for H α . We examine nonradial pulsations both through the TESS photometry and the line profile variations (LPVs) in the spectroscopy. Our analysis indicates that two periodicities seen in the TESS photometry (at 1.644 and 1.485 cycles day −1 ) are not seen in the spectral lines before, during, or after the outburst. The strongest photometric signal is a periodicity of 0.163 cycles day −1 , which appears as a difference between the two weaker signals and is visible in the spectra without any apparent changes in amplitude or phase. We additionally find evidence for fast nonphotometric pulsational variations over the course of spectroscopy obtained before, during, and after the outburst. These fast LPVs are strong, and interfere with the two weaker signals, hampering our ability to detect them via spectroscopy.