Radio and millimeter observations of the environments of young stars undergoing accretion bursts

Studying the formation and evolution of Sun-like stars is crucial for understanding the origins of the Solar System. Young stellar objects (YSOs) undergo episodic outbursts that may impact disk chemistry and planet formation. The FU Orionis- and EX Lupi-type objects (FUors and EXors) experience dramatic brightness increases due to enhanced accretion, and while these outbursts may have lasting effects on the disk chemistry and surrounding environments, their impact remains poorly explored, especially in the radio and millimeter wavelengths. In this PhD thesis, I conducted the first dedicated radio surveys of FUors and EXors, using ammonia (NH₃) with the Effelsberg 100-m telescope to trace their dense cores and water (H₂O) maser emission as an accretion burst indicator. Additionally, I performed the first wideband millimeter line survey of the evolved FUor V1057 Cyg with the IRAM 30-m telescope, complemented with additional data from the APEX 12-m telescope. The NH3 survey detected (1,1) emission in over half of the sampled objects (28/51), revealing cold regions (below 25 K) with column densities similar to those found in infrared dark clouds. Archival Herschel SPIRE data provided H₂ column densities and dust temperatures, identifying dense material in at least seven sources. The detection of the (2,2) emission suggests some FUors and EXors may be younger than previously classified. The 22.2 GHz H₂O maser survey detected emission in three sources (6%), including two FUors and one EXor. Notably, masers were observed in the FUor HH 354 IRS for the very first time, and multiple flares were identified in archival data for other sources, suggesting episodic variability. The millimeter survey of V1057 Cyg identified over 35 molecular species, including complex organic molecules. Position-velocity diagrams of ¹²CO suggest past episodic outbursts, with a dynamical timescale of the outflow on the orders of tens of thousands of years, indicating that it cannot represent the ongoing outburst alone. This work demonstrates the potential of molecular line studies to refine the classification and chemical characterization of eruptive stars.