Results of Selected Stellar Spectroscopy Programs at the 6-m Telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences Performed with the NES Echelle Spectrograph
Over the past two decades NES spectrograph attached to the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences was used to perform spectroscopy of stars at advanced stages of evolution spanning a wide range of masses. The studies cover a variety of star types with high mass-loss rates during the preceding and current stages of evolution. All these stars have extended atmospheres and structured circumstellar envelopes produced by strong stellar winds. We have studied luminous blue variables (LBVs) near the Eddington limit; hot supergiants exhibiting B[e] phenomenon, which are very likely intermediate-mass binary systems soon after the fast mass exchange stage; a group of yellow hypergiants, as well as an extensive sample of low-mass post-AGB supergiants. The diverse nature of the types of these stars whose common feature is the presence of a envelope makes the spectroscopy of such objects a comprehensive task. Such studies consist of many etapes, which include not only determining the specifics of the chemical composition and understanding the role of supergiants in the enrichment of the interstellar medium with freshly synthesized elements, but also the determination of the evolutionary stage of the objects considered, as well as search for and analysis of spectroscopic manifestations of kinematic processes in their extended and unstable atmospheres and gas-dust envelopes. We spectroscopically monitored selected objects to study in detail the instability of the kinematic state of the atmospheres of the stars considered. Studies of stars at neighboring evolutionary stages have been initiated. This review reports the most significant observational obtained within the framework of the 1998-2021 programs.