X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae

TL;DR

This paper presents a novel X-ray spectroscopic method to measure the variable mass accretion rate onto TW Hydrae by analyzing Ne IX line absorption, revealing significant short-term fluctuations.

Contribution

It introduces a new technique using Ne IX line absorption to directly measure accretion rates, improving understanding of accretion variability in young stars.

Findings

Average accretion rate is 1.5 x 10^{-9} M_sun/yr.

Detected a five-fold decrease in accretion rate over 2.7 days.

Observed significant changes in N_H, T_e, and n_e between exposures.

Abstract

Diagnostics of electron temperature (T_e), electron density (n_e), and hydrogen column density (N_H) from the Chandra High Energy Transmission Grating spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a classical accretion model, allow us to infer the accretion rate onto the star directly from measurements of the accreting material. The new method introduces the use of the absorption of Ne IX lines as a measure of the column density of the intervening, accreting material. On average, the derived mass accretion rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic field strength of 600 Gauss and a filling factor of 3.5%. Three individual Chandra exposures show statistically significant differences in the Ne IX line ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and 1.3, respectively. In exposures separated by 2.7 days, the observations reported here suggest a five-fold reduction in the accretion rate. This powerful new technique promises to substantially improve our understanding of the accretion process in young stars.