Gas lines in outbursting young stars as signatures of episodic accretion

The formation of young stars undergoes episodic, albeit quite common, accretion events in the early life of a young star. The separation of the classical types of outbursting sources, FU Orionis objects and EXor-type objects, has now become blurred thanks to new outbursting sources discovered in the recent past. The physical mechanism (if a common one exists) at the origin of the outbursts still remains unclear. Massive mass transfer through the disk occurs due to episodic accretion, with consequences for planet formation; in addition the strong increase in luminosity can evaporate ices present in the disk. We present our analysis of 12 erupting young stars observed with Herschel and Spitzer (Postel et al. 2019, A&A, 631, A30). The Herschel spectra show many rotational lines of CO, ranging from J=38-37 to J=4-3. Rotational diagrams indicate several excitation temperatures in the low temperature regime (<100K), mid-temperature regime (400-500K) and evidence for a higher temperature component in some cases. Additional lines are detected such as atomic lines (O I, C I, CII, N II), and possibly faint OH and H2O lines. Spitzer data further reveal several ices in the embedded objects (such as CO2, CH3OH, NH4+) with silicate in absorption, while at least one target shows silicate in emission, similar to the prototype FU Ori. We present the data in the context of episodic accretion and its signature in emission gas lines, and an analysis of the FUor Re 50 N IRS 1 with the thermochemical code ProDiMo (Postel et al. 2021, submitted).