Among the low-mass pre-main sequence stars, a group called FU Orionis-type objects (FUors) are notable for undergoing powerful accretion outbursts, that significantly impact the planet-forming zone in their circumstellar disks. The group was defined based on a handful of sources decades ago, but new monitoring surveys, like Gaia alerts, keep discovering new members of this influental class. Here we report on an extensive project aiming to re-observe and re-investigate a sample of classical FUors several decades after their discovery. We present new optical/infrared photometric and spectroscopic observations of V1057 Cyg, V1515 Cyg, V1735 Cyg, and V733 Cep. We describe the general brightness evolution of the outburst, identify temporary changes in the fading rate, and investigate quasi-periodic modulations. We compare our spectra with ones from earlier phases of the outburst, and find that the main spectroscopic features which define the FUor class are still present in these objects. This is in accordance with them still being in the high accretion phase despite the (often) significant/substantial fading they have displayed in the past decades. P Cygni profiles and broad blueshifted absorption lines provide information on wind properties and their long-term changes. Some newly obtained near-infrared spectra show the strengthening of the CO bandhead and the FeH molecular band, indicating that the disk has become cooler. We carefully searched for signatures of jets, leading the first detection of [S II], [N II], and [O III] lines in V1057 Cyg. Finally, we applied a simple accretion disk modeling on the multi-epoch spectral energy distributions of each FUor in a homogeneous way, and could outline the evolution of both the accretion rate and the circumstellar extinction. The ultimate goal of our on-going project is to provide new constraints for present and future physical theories of the eruption phenomenon in young stellar objects.
[Poster PDF File]