Coordinated Space- and Ground-based Monitoring of Accretion Bursts in a Protoplanetary Disk: The Orbital and Accretion Properties of DQ Tau

TL;DR

This work investigates the eccentric PMS binary DQ Tau by combining VLT X-Shooter and UVES spectroscopy with JWST campaign context to refine its orbital parameters and to trace accretion onto each star. The authors derive a near-circularized Keplerian solution with $P=15.71\pm0.07$ days and $e\approx0.55$–$0.59$, and detect prograde apsidal motion with $\Delta\omega\approx30^{\circ}$ over ~a decade, which could arise from disk–binary interactions or a possible external companion with $M_{\rm out}\gtrsim15\,M_{\rm J}$ at $a_{\rm out}\approx3a_{\rm bin}$ or up to $\sim1\,M_{\odot}$ at $a_{\rm out}\approx12a_{\rm bin}$. Using the Ca II $849.8$ nm line and Li veiling to disentangle the two stars, they find that the primary dominates accretion near periastron while the secondary takes over post-periastron, with elevated $L_{\rm acc}$ at apastron likely tied to irregular inner-disk structures. The sum of the disentangled $L_{\rm acc}$ agrees with UV-excess estimates for the unresolved system, making these measurements valuable inputs for inner-disk chemical models and the interpretation of JWST spectra. The results underscore the importance of coordinated, time-resolved ground- and space-based monitoring for time-variable disk chemistry and planet-formation studies in binary systems.

Abstract

Multiplicity in pre-main-sequence (PMS) systems shapes circumstellar and circumbinary disks, often producing features such as inner cavities, spiral arms, and gas streamers that facilitate mass transfer between the disk and stars. Consequently, accretion in eccentric close binaries is highly variable and synchronized with their orbits, producing bursts near periastron passages. In this study, we examine the orbital and accretion properties of the eccentric Classical T-Tauri binary DQ Tau using medium- to high-resolution spectroscopy from the Very Large Telescope (VLT) X-Shooter and UVES instruments. The data were taken during a monitoring of inner disk chemistry with JWST, and our analysis is needed for correct interpretation of JWST data. We refine the orbital parameters and report an increment in the argument of periastron of 30 degrees. This apsidal motion may be caused by the massive disk acting as a third body. We also explore the possibility that it is due to a still undetected additional (sub-)stellar companion, estimating a lower limit of 15 MJ for its mass at the cavity edge (a=3 abin). We investigate accretion of the primary and secondary using the Ca II 849.8 nm emission line. The primary accretes more at periastron than in previous quiescent phases, while the secondary dominates post-periastron. Additionally, we report elevated Lacc at apastron, possibly due to interaction with irregularly shaped structures near the closest approach to the circumbinary disk. Finally, we derive each star's accretion luminosity across disentangled epochs and compare it to UV-excess-based results, finding good agreement. The individual Lacc values can be used as input for chemical models.

Figures (19)

• Figure 1: Accretion variability of DQ Tau shown by observations with LCO u$'$ photometry, X-Shooter, and UVES taken across 5 months between August 13th, 2024 to January 22nd, 2025. The accretion luminosity (left axis) and mass accretion rate (right axis) are measured from X-Shooter spectra and LCO u$'$-band photometry for the whole system 2025arXiv250408029T. Time is reported in barycentric Julian days on the bottom axis and the binary orbital cycles are labelled on the top axis. Vertical dotted lines mark the periastron passages of DQ Tau with the JWST MIR observations indicated as black arrows.
• Figure 2: Spectra of DQ Tau taken with the visual arm of X-Shooter and with UVES. The wavelength ranges in black (590-600, 600-610, 610-620, 640-650, 658-667 nm) correspond to the five regions where the broadening function was calculated for each epoch. The wavelength ranges were chosen to be free of strong emission lines and telluric contamination.
• Figure 3: Lomb-Scargle Periodogram of RV1 data. The data shows a significant peak reported at 15.698 day. The $1\%$ false alarm probability (FAP) level is indicated by the magenta dotted line. In the background, the bootstrapped periodograms are shown in purple.
• Figure 4: The orbital solution for DQ Tau in phase. The magenta and black lines fit the primary and secondary data taken with X-Shooter and UVES, respectively. The RVs shown in open symbols at $\sim 0.3-0.6$ in phase are discarded from the fitting procedure. Below, the residuals of the fitted model in phase.
• Figure 5: Orbital solution for DQ Tau across multiple orbits. The magenta and black lines fit the primary and secondary data taken with X-Shooter and UVES, respectively. The RVs shown in open symbols are discarded from the fitting procedure. Below the residual plot. The number of DQ Tau orbital cycles is marked in vertical grey dashed lines.