The Periastron Passage of T Tauri South B as Viewed by ALMA: Millimeter Flux Variations and Dust Heating Triggered by Orbital Motion

Abstract

We present 225 and 350 GHz imaging of the iconic T Tauri system using the Atacama Large Millimeter submillimeter Array (ALMA). T Tauri is a hierarchical triple system, and the close binary T Tau Sa/Sb underwent periastron passage in March 2023. The ALMA images were obtained in epochs spanning November 2019 through June 2023, and therefore covered the time frame of the recent periastron passage. We clearly resolve the Sa-Sb binary in two epochs of high-resolution measurements with ALMA. We find increases in millimeter flux from heating of the Sa disk and the wider distribution of dust in the environment of the binary. This heating is likely in response to increased stellar accretion activity triggered by orbital motion during the dynamic periastron passage of T Tau Sb around Sa. Resolved, extended millimeter emission is also found to change morphology and increase in flux in the immediate environment of the Sa-Sb binary after periastron passage. This may suggest an increase in nonthermal emission from magnetic interaction, gravitational disruption of the circumstellar disks as the stars passed through periastron, or both of these phenomena. We also detected structures in the compact (24 au radius), thermal dust disk around T Tau N. In particular, we identify a crescent-shaped emission excess just outside a shallow gap at 12 au radius that appears to move at Keplerian speed. Future measurement of dust spectral indices can clarify the origin of increased and variable millimeter emission in the environment of the T Tau S binary.

Figures (8)

• Figure 1: (Left) Low resolution image of T Tau at 225 GHz in 2021, showing the low level extended emission. Noise level is 45 $\mu$Jy/beam, 4.2 mK at this resolution of $0.55 \times 0.47"$ at PA $32^\circ$. Contour levels are -12, 12, 24, and 36 mK and then in steps of $\sqrt{2}$ up to 13 K. 12 mK is about 128 $\mu$Jy/beam or $2.8 \sigma$. (Right): High resolution image of T Tau at 225 GHz in 2021. Noise level is 20 $\mu$Jy/beam, 0.7 K at at this resolution of $32.7 \times 20.9$ mas at PA $160^\circ$. Contour levels are -2, 2, 4, and 6 K and then in steps of $\sqrt{2}$ up to 256 K. 2 K is about 57 $\mu$Jy/beam or $2.8 \sigma$. The cross marks the GAIA position at the epoch of observation (Nov 2021). The blue ellipse presents the best fit orbital model for Sa, Sb, using orbital parameters from scha20. Beam sizes are presented at lower left for clarity. The color scale indicates the flux density in mJy/beam.
• Figure 2: As in Figure 1, but for the 2022-2023 data. Left: low resolution image combining Oct 2022 and June 2023 data. Noise level is 59 mJy/beam, 5.5 mK at the angular resolution of $0.55 \times 0.47"$ at PA $56^\circ$. Contour spacings are 16.5, 33, 48.5 mK and then in steps of $\sqrt{2}$ up to 12.7 K. Right: High resolution image using June 2023 data only. Noise level is 19 mJy/beam, 0.33 K at the angular resolution of $0.043\times 0.033"$ at PA $174^\circ$. Contour levels are -1, 1, 2, and 3 K and then in steps of $\sqrt{2}$ up to 128 K. A 25 mas shift in Declination was required to register both data sets. The blue ellipse presents the best fit orbital model for Sa, Sb, using orbital parameters from scha20. In the high resolution image (right), the position of T Tau Sb has changed because of orbital motion from East-Northeast in 2021 (Figure 1) to South-Southeast here in 2023. Beam sizes are presented at lower left for clarity.
• Figure 3: A comparison of the 225 GHz continuum emission at the two epochs (09-Nov-2021, 23-Jun-2023), at the same 42.5 mas spatial resolution. Contour levels are in steps of 0.666 K (about 50 $\mu$Jy/beam, approximately 4.7 $\sigma$ in Nov-2021 and 3.1 $\sigma$ in Jun-2023 respectively), up to 2 K and then increase by a factor $\sqrt{2}$ up to 128 K (peak value 225 K). The red and cyan X's mark the measured Sa and Sb positions in the two epochs, and the black cross is the Sa/Sb center of mass. Beam sizes are presented at lower left for clarity.
• Figure 4: Difference between the two epochs (09-Nov-2021 subtracted from 23-Jun-2023), at 217 GHz (left) and 234 GHz (right). The positive regions in these images reveal where the emission is enhanced in 2023 with respect to 2021. Red crosses show the positions of T Tau Sa, and cyan show the positions of Sb; both are for the 2023 epoch. A small white star shows the position of Sb in 2021 and is coincident with a negative subtraction residual. The black plus is the position of the Sa/Sb center of mass. Beam sizes are presented at lower left for clarity.
• Figure 5: From low resolution 356 GHz measurements ($\sim$0.7$.\!\!^{\prime\prime}$; Table 1), the residual emission around T Tau in Dec-2019 (left) and Oct-2022 (right) after removal of N, Sa and Sb source models. Spatial resolution is 0.85$"$. Contour levels are 7 mK, or approximately 0.5 mJy/beam, corresponding to 2.2 and 5.8 $\sigma$ resp in Dec-2019 (left) and Oct-2022 (right). The large black cross is the T Tau N position. Beam sizes are presented at lower left for clarity.