2025A&A...696A..18C

Context. Compared to Class 0 protostars, the higher densities and lower temperatures of the disk midplanes of Class I young stellar objects (YSOs) limit the detectability of complex organic molecules (COMs). The elevated luminosities of eruptive YSOs increase disk temperatures causing frozen molecules to sublimate, thus making them easier to detect.
Aims. Our aim is to investigate the chemical composition of four FUor-like Class I YSOs (L1551 IRS 5, Haro 5a IRS, V346 Nor, and OO Ser) and to compare their abundances of COMs with other YSOs in the literature.
Methods. We searched for COMs line emission in ALMA Band 6 observations. We used the CASSIS software to determine their column densities (N) and excitation temperatures (T_ex) assuming local thermodynamical equilibrium.
Results. We detected 249 transitions from 12 COMs. In L1551 IRS 5 we identified CH3OH, ¹³CH₃OH, CH₃¹⁸OH, CH₂DOH, CH₃CHO, CH₃OCH₃, CH₃OCHO, CH₃COCH₃, C₂H₅OH, C₂H₅CN, ¹³CH₃CN, and CH₃C¹⁵N. Haro 5a IRS and OO Ser have emission from CH₃OH, CH₃CHO, CH₃OCH₃ , and CH₃ OCHO. CH₃COCH₃ is also detected in OO Ser. In V346 Nor we found CH₃OH, CH₂DOH, CH₃CHO, CH₃OCH₃ , CH₃OCHO, and C₂H₅CN. The emission of COMs is compact in all targets. The analysis indicates that their temperatures are above 100 K. The abundance ratios of COMs derived for these eruptive YSOs, and for other protostars in the literature, span several orders of magnitude without any clear differentiation between the eruptive and quiescent YSOs. The column density of the main isotopolog of CH₃ OH should not be used as a reference, as most of the lines are optically thick.
Conclusions. The hot and compact emission of COMs indicates that the four FUor-like targets are hot corino-like. Spectral studies of these objects can be useful for investigating the complex organic chemistry at later evolutionary stages than the usual Class 0 stage.