Accretion and Magnetic Reconnection in the Classical T Tauri Binary DQ Tau

TL;DR

This study uses continuous multi-band photometry over multiple orbits of the DQ Tau binary to determine that accretion, rather than magnetic reconnection flares, dominates the observed periodic brightening events, aligning with recent models.

Contribution

It provides the first detailed, multi-epoch observational evidence that accretion dominates over flares in DQ Tau's variability, supporting pulsed accretion theory in eccentric binaries.

Findings

Accretion rate increases by a factor of 5 near periastron.

Brightening events are primarily due to accretion, not flares.

Observation of quasi-periodic apastron accretion events.

Abstract

Binary star-formation theory predicts that close binaries (a<100 AU) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk, cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed-accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (~daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominate physical mechanism seen in DQ Tau's low-cadence observations, we have obtained continuous, moderate-cadence, multi-band photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on 4 separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of 5 near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit-to-orbit. We argue this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the circumbinary disk and within the accretion streams themselves. Quasi-periodic apastron accretion events are also observed, which are not predicted by binary accretion theory.