Massive compact disks around FU Orionis-type young eruptive stars revealed by ALMA

TL;DR

This study used ALMA to survey FU Orionis-type stars, revealing that their disks are more massive, smaller, and often gravitationally unstable compared to typical young stellar objects, providing insights into their accretion bursts.

Contribution

First ALMA survey of FUors' disks showing they are more massive, smaller, and often gravitationally unstable than non-bursting young stars.

Findings

Disks are significantly more massive than optically thin estimates suggest.

FUor disks are smaller in size compared to regular Class I and II disks.

A majority of FUor disks may be gravitationally unstable.

Abstract

FU Orionis-type objects (FUors) are low-mass pre-main sequence stars undergoing a temporary, but significant increase of mass accretion rate from the circumstellar disk onto the protostar. It is not yet clear what triggers the accretion bursts and whether the disks of FUors are in any way different from disks of non-bursting young stellar objects. Motivated by this, we conducted a 1.3 mm continuum survey of ten FUors and FUor-like objects with ALMA, using both the 7 m array and the 12 m array in two different configurations to recover emission at the widest possible range of spatial scales. We detected all targeted sources and several nearby objects as well. To constrain the disk structure, we fit the data with models of increasing complexity from 2D Gaussian to radiative transfer, enabling comparison with other samples modeled in a similar way. The radiative transfer modeling gives disk masses that are significantly larger than what is obtained from the measured millimeter fluxes assuming optically thin emission, suggesting that the FUor disks are optically thick at this wavelength. In comparison with samples of regular Class II and Class I objects, the disks of FUors are typically a factor of 2.9-4.4 more massive and a factor of 1.5-4.7 smaller in size. A significant fraction of them (65-70%) may be gravitationally unstable.