| Abstract
| - Context. FU Orionis-type objects (FUors) are low-mass young eruptive stars that probably represent an evolutionary phase characterized by episodic periods of increased accretion rate from the circumstellar disk to the star. Theory predicts that a circumstellar envelope, the source of continuous mass infall onto the disk, is necessary for triggering such accretion bursts. Aims. We intend to study the spatial and velocity structure of circumstellar envelopes around FUors by means of molecular line observations at millimeter wavelengths. We target three prototypical FUors, as well as an object possibly in a pre-outburst state. Methods. We present archival PdBI interferometric observations of the J = 1-0 line of 13CO at 110.2 GHz. For three of our targets, these represent the first millimeter interferometric observations. The data allow study of the molecular environment of the objects with a spatial resolution of a thousand AU and a velocity resolution of 0.2 km s -1. Results. Strong, narrow 13CO(1-0) line emission is detected from all sources. The emission is spatially resolved in all cases, with deconvolved sizes of a few thousand AUs. For V1057 Cyg and V1331 Cyg, the emitting area is rather compact, suggesting that the origin of the emission is a circumstellar envelope surrounding the central star. For V1735 Cyg, the 13CO emission is offset from the stellar position, indicating that the source of this emission may be a small foreground cloud, also responsible for the high reddening of the central star. The 13CO emission towards V1515 Cyg is the most extended in the sample, and it apparently coincides with the ring-like optical reflection nebula associated with V1515 Cyg. Conclusions. We suggest that millimeter interferometric observations are indispensable for a complete understanding of the circumstellar environment of FUors. Any theory of the FUor phenomenon that interprets the geometry of the circumstellar structure and its evolution using single-beam measurements must be checked and compared to interferometric observations in the future.
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