Structure and Dynamics of the Accretion Process and Wind in TW Hya

TL;DR

This study uses decade-long time-domain spectroscopy across multiple wavelengths to investigate the complex accretion and wind processes in the T Tauri star TW Hya, revealing dynamic emission features and wind properties.

Contribution

It provides new insights into the structure and variability of accretion shocks and stellar winds in TW Hya, highlighting the role of accretion-driven winds and challenging existing hydrogen line models.

Findings

Emission lines originate in turbulent post-shock regions.

Stellar wind velocities vary from 200 to 400 km/s over time.

Evidence for an accretion-driven stellar wind affecting outflow velocities.

Abstract

Time-domain spectroscopy of the classical accreting T Tauri star, TW Hya, covering a decade and spanning the far UV to the near-infrared spectral regions can identify the radiation sources, the atmospheric structure produced by accretion,and properties of the stellar wind. On time scales from days to years, substantial changes occur in emission line profiles and line strengths. Our extensive time-domain spectroscopy suggests that the broad near-IR, optical, and far-uv emission lines, centered on the star, originate in a turbulent post-shock region and can undergo scattering by the overlying stellar wind as well as some absorption from infalling material. Stable absorption features appear in H-alpha, apparently caused by an accreting column silhouetted in the stellar wind. Inflow of material onto the star is revealed by the near-IR He I 10830A line, and its free-fall velocity correlates inversely with the strength of the post-shock emission, consistent with a dipole accretion model. However, the predictions of hydrogen line profiles based on accretion stream models are not well-matched by these observations. Evidence of an accelerating warm to hot stellar wind is shown by the near-IR He I line, and emission profiles of C II, C III, C IV, N V, and O VI. The outflow of material changes substantially in both speed and opacity in the yearly sampling of the near-IR He I line over a decade. Terminal outflow velocities that range from 200 km/s to almost 400 km/s in He I appear to be directly related to the amount of post- shock emission, giving evidence for an accretion-driven stellar wind. Calculations of the emission from realistic post- shock regions are needed.