2017ApJ...843...45K

A long-standing open issue of the paradigm of low-mass star formation is the luminosity problem: most protostars are less luminous than theoretically predicted. One possible solution is that the accretion process is episodic. FU Ori-type stars (FUors) are thought to be the visible examples for objects in the high accretion state. FUors are often surrounded by massive envelopes, which replenish the disk material and enable the disk to produce accretion outbursts. However, we have insufficient information on the envelope dynamics in FUors, about where and how mass transfer from the envelope to the disk happens. Here we present ALMA observations of the FUor-type star V346 Nor at 1.3 mm continuum and in different CO rotational lines. We mapped the density and velocity structure of its envelope and analyze the results using channel maps, position-velocity diagrams, and spectro-astrometric methods. We found that V346 Nor is surrounded by gaseous material on a 10,000 au scale in which a prominent outflow cavity is carved. Within the central ∼700 au, the circumstellar matter forms a flattened pseudo-disk where material is infalling with conserved angular momentum. Within ∼350 au, the velocity profile is more consistent with a disk in Keplerian rotation around a central star of 0.1 M_☉. We determined an infall rate from the envelope onto the disk of 6×10^–6 M_☉ yr^–1, a factor of a few higher than the quiescent accretion rate from the disk onto the star, hinting at a mismatch between the infall and accretion rates as the cause of the eruption.