TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics

TL;DR

This study presents simultaneous spectroscopic, photometric, and X-ray observations of TW Hya, revealing the sequence of accretion-related events and their impact on stellar atmosphere and wind, providing new insights into T Tauri star accretion processes.

Contribution

First direct spectroscopic tracking of an accretion event in TW Hya linking X-ray, optical, and coronal responses, proposing a sequential model of accretion shock and heating.

Findings

Accretion event traced via soft X-rays and optical line profiles.

Optical veiling increases with a 2-hour delay after X-ray accretion.

Stellar corona heated in part by accretion, with subsequent wind re-establishment.

Abstract

The nearest accreting T Tauri star, TW Hya was observed with spectroscopic and photometric measurements simultaneous with a long se gmented exposure using the CHANDRA satellite. Contemporaneous optical photometry from WASP-S indicates a 4.74 day period was present during this time. Absence of a similar periodicity in the H-alpha flux and the total X-ray flux points to a different source of photometric variations. The H-alpha emission line appears intrinsically broad and symmetric, and both the profile and its variability suggest an origin in the post-shock cooling region. An accretion event, signaled by soft X-rays, is traced spectroscopically for the first time through the optical emission line profiles. After the accretion event, downflowing turbulent material observed in the H-alpha and H-beta lines is followed by He I (5876A) broadening. Optical veiling increases with a delay of about 2 hours after the X-ray accretion event. The response of the stellar coronal emission to an increase in the veiling follows about 2.4 hours later, giving direct evidence that the stellar corona is heated in part by accretion. Subsequently, the stellar wind becomes re-established. We suggest a model that incorporates this sequential series of events: an accretion shock, a cooling downflow in a supersonically turbulent region, followed by photospheric and later, coronal heating. This model naturally explains the presence of broad optical and ultraviolet lines, and affects the mass accretion rates determined from emission line profiles.