2025A&A...694A.329S

Context. Studying accretion-driven episodic outbursts in young stellar objects (YSOs) is crucial for understanding the later stages of star and planet formation. FU Orionis-type objects (briefly, FUors) represent a small but rather central class of YSOs, whose outbursts are characterized by a rapid multi-magnitude increase in brightness at optical and near-infrared wavelengths. These outbursts may have a long-lasting influence on the chemistry and molecular inventory around eruptive young stars. However, no complete line survey in the millimeter wavelength range exists in the literature for more evolved (i.e., Class II) sources, in contrast to wide-band coverages at optical and near-infrared wavelengths.
Aims. We carried out the first dedicated wide-band millimeter line survey toward the low-mass young eruptive star and classical FUor V1057 Cyg, which has the highest observed peak accretion rate among FUors. This source is known to have a molecular outflow, and it is associated with dense material. This makes it a good candidate for a search for molecular species.
Methods. We performed a wide-band spectral line survey of V1057 Cyg with the IRAM 30 m telescope from ∼72 to ∼263 GHz (with a spatial resolution between ∼36″ and ∼10″), complemented by on-the-fly maps of selected molecules. We also recorded additional spectra around 219, 227, 291, and 344 GHz (with a spatial resolution between ∼30″ and ∼19″) with the APEX 12 m telescope. We conducted simple radiative transfer and population diagram analyses to derive the column densities and excitation temperatures. We constructed integrated-intensity maps of the emission from several molecular species, including those that reveal outflows. These maps and a ¹²CO (3-2) position-velocity diagram provide insight into the past outburst activity of the source.
Results. We identified mainly simple C-, N-, O-, and S-bearing molecules, deuterated species, molecular ions, and complex organic molecules. Several molecular species (HCN, HC₃N, and HNC) trace large-scale (∼2') structures in the environment of V1057 Cyg with indications of small-scale fragmentation that remains unresolved by the single-dish data. The position-velocity diagram of ¹²CO shows concentrated knots, which may indicate past episodic outburst activity. We calculated the dynamical timescale of the outflow and found it to be on the order of a few ten thousand years (between 15 000 and 22 000 years), similar to other eruptive stars. This suggests that the outflow cannot result from the ongoing outburst alone, since the source has been in the current outburst for less than a century. The population diagrams for species such as CH₃OH, H₂CO, and HC₃N indicate rotational temperatures that range from 8 K to 15 K and column densities that range from 1.4×10¹² cm^–2 to 2.8×10¹³ cm^–2.
Conclusions. With over 30 detected molecular species (including isotopologs), V1057 Cyg and its environment display a rich chemistry considering the more evolved state of this source compared to well-studied but younger (i.e., Class 0/I) FUors, e.g., V883 Ori. The results of our line survey show that V1057 Cyg is a good candidate for future interferometric observations aimed at resolving emission extents to constrain molecular freeze-out and to search for emission lines of water and additional complex organic molecules. Our observations highlight the potential of millimeter line surveys to characterize the chemistry of eruptive stars and their environments, including more evolved sources, and to complement optical and near-infrared studies in this way to improve current statistics of the molecular inventories of these objects.