Discovery of an accretion streamer and a slow wide-angle outflow around FU Orionis

TL;DR

This study uses ALMA observations to identify an accretion streamer and a slow wide-angle outflow around FU Orionis, providing insights into the system's accretion processes and outflow characteristics.

Contribution

First detection of a large-scale accretion streamer and a slow molecular outflow around FU Orionis, revealing complex gas dynamics in this outbursting system.

Findings

Accretion streamer feeding the binary system was identified.

The streamer’s mass infall rate is too low to sustain the outburst.

A slow, large-scale CO outflow was detected, unrelated to the current outburst.

Abstract

We present ALMA 12-m, 7-m & Total Power (TP) Array observations of the FU Orionis outbursting system, covering spatial scales ranging from 160 to 25,000 au. The high-resolution interferometric data reveals an elongated $^{12}$CO(2-1) feature previously observed at lower resolution in $^{12}$CO(3-2). Kinematic modeling indicates that this feature can be interpreted as an accretion streamer feeding the binary system. The mass infall rate provided by the streamer is significantly lower than the typical stellar accretion rates (even in quiescent states), suggesting that this streamer alone is not massive enough to sustain the enhanced accretion rates characteristic of the outbursting class prototype. The observed streamer may not be directly linked to the current outburst but rather a remnant of a previous, more massive streamer that may have contributed enough to the disk mass to render it unstable and trigger FU Ori's outburst. The new data detects, for the first time, a vast, slow-moving carbon monoxide molecular outflow emerging from this object. To accurately assess the outflow properties (mass, momentum, kinetic energy), we employed $^{13}$CO(2-1) data to correct for optical depth effects. The analysis indicates that the outflow corresponds to swept-up material not associated with the current outburst, similar to slow-molecular outflows observed around other FUor and Class I protostellar objects.