A radical transition in the post-main-sequence system U Equulei

Context. U Equ is an unusual maser-hosting infrared source discovered in the 1990s. It was tentatively classified as a post-asymptotic giant branch (post-AGB) star with a unique optical spectrum displaying rare emission and absorption features from molecular gas at a temperature of about 500 K. In 2022, we serendipitously discovered that its optical spectrum had drastically changed since the last observations in the 1990s. Aims. We aim to characterize the drastic change in the spectrum and analyze the photometric behavior of the object since 1989. Methods. Optical high-resolution spectra of U Equ from the Southern African Large Telescope were supplemented by archival data and near-infrared photometry from the Nordic Optical Telescope. New spectral line observations with the Effelsberg 100 m radio telescope and Atacam Large Millimeter Array are presented. Radiative transfer modeling of multiple epoch spectral energy distributions was performed. Results. No circumstellar molecular features are present in the contemporary optical spectra of U Equ. Nonphotospheric absorption and emission from neutral and ionized species dominate the current spectrum. Some of the observed features indicate an outflow with a projected terminal velocity of 215 km s(-1). Broad H&K lines of [Ca II] indicate a photosphere of spectral type F or similar. For the first time, we find SiO J = 1-0 upsilon=1 maser emission in U Equ. Our collected photometric measurements show that the source has monotonically increased its optical and near-infrared fluxes since about the beginning of this century and continues to do so. The current rise in the optical regime is about 1 mag. Spectral energy distributions at different epochs show dusty circumstellar material that is very likely arranged in a highly inclined disk. Adopting a distance of 4 kpc, informed by the Gaia parallax of U Equ, we find that the source luminosity is about 10(4)L(circle dot). This luminosity has likely increased by a factor of a few in the last decades, which is most probably related to the drastic change in the optical circumstellar spectrum of the object. Conclusions. The object has changed considerably in the past three decades, either due to geometrical reconfiguration of the circumstellar medium, evolutionary changes in the central star, or owing to an accretion event that started very recently in the system. Observationally, U Equ appears to resemble category 0 of disk-hosting post-AGB stars reported previously, especially the post-common-envelope binary HD 101584. It is uncertain whether the drastic spectral change and the associated optical and mid-infrared rise in brightness witnessed in U Equ are common in post-AGB stars, but this radical change may be related to the real-time onset of the evolution of the system into a planetary nebula. We find that the post-AGB star V576 Car has undergone a similar transformation as U Equ in the past few decades, which means that the phenomenon is not extremely rare.