Testing the Binary Trigger Hypothesis in FUors

TL;DR

This study used nonredundant aperture-mask interferometry to search for binary companions to FU Orionis objects, finding a close binary in one case and suggesting FUor outbursts may not be directly caused by binary orbital periods.

Contribution

First high-resolution interferometric observations of FUors that constrain the presence of close binary companions and their potential role in outburst mechanisms.

Findings

Detected a close binary companion to V1057 Cyg at 30 AU.

No companions found around V1515 Cyg or HBC 722.

Outbursts are unlikely to be directly triggered by binary orbital periods.

Abstract

We present observations of three FU Orionis objects (hereafter, FUors) with nonredundant aperture-mask interferometry (NRM) at 1.59 um and 2.12 um that probe for binary companions on the scale of the protoplanetary disk that feeds their accretion outbursts. We do not identify any companions to V1515 Cyg or HBC 722, but we do resolve a close binary companion to V1057 Cyg that is at the diffraction limit (rho = 58.3 +/- 1.4 mas or 30 +/- 5 AU) and currently much fainter than the outbursting star (delta(K') = 3.34 +/- 0.10 mag). Given the flux excess of the outbursting star, we estimate that the mass of the companion (M ~ 0.25 Msun) is similar to or slightly below that of the FUor itself, and therefore it resembles a typical T Tauri binary system. Our observations only achieve contrast limits of delta(K') ~ 4 mag, and hence we are only sensitive to companions that were near or above the pre-outburst luminosity of the FUors. It remains plausible that FUor outbursts could be tied to the presence of a close binary companion. However, we argue from the system geometry and mass reservoir considerations that these outbursts are not directly tied to the orbital period (i.e., occurring at periastron passage), but instead must only occur infrequently.