A Census of the Low Accretors. II: Accretion Properties

TL;DR

This study analyzes accretion properties of low accretors in pre-main-sequence stars, revealing a power-law relation between disk truncation and accretion rate, and identifying a lower limit consistent with photoevaporation effects.

Contribution

It provides detailed modeling of accretion in low accretors, establishing a relation between truncation radius and accretion rate, and determining the physical lower limit of detectable accretion rates.

Findings

Power-law relation between disk truncation radius and accretion rate.

Most targets are in the unstable accretion regime.

Lower detection limit of accretion rates is around 10^{-10} M_sun/yr.

Abstract

Much is known about the processes driving accretion from protoplanetary disks onto low-mass pre-main-sequence stars (T Tauri stars). Nevertheless, it is unclear how accretion stops. To determine the accretion properties and their relation to stellar properties and gain insight into the last stages of accretion, we present a detailed analysis of 24 low and possible accretors, previously identified using the He I $\lambda$10830 line. We model moderate-resolution H$\alpha$ profiles of these stars using magnetospheric accretion flow models that account for the chromospheric contribution at the line center. Based on parameters derived from the fits of 20 stars that can be reproduced with the models, we find a power-law relation between the disk truncation radius and the mass accretion rate consistent with predictions from theory and simulations. Comparing the corotation and truncation radii, we find that most of our targets are accreting in the unstable regime and rule out the propeller as the main process stopping accretion. For the truncation radius to be the same as the magnetic radius, the dipole magnetic field and/or the efficiency parameter $\xi$ need to be smaller than previously determined, suggesting that higher-order fields dominate in low accretion rates. Lastly, we determine that the lowest accretion rates that can be detected by H$\alpha$ line modeling are $1-3\times10^{-11}M_{\odot}yr^{-1}$ for M3 stars and $3-5\times10^{-11}M_{\odot}yr^{-1}$ for K5 stars. These limits are lower than the observed accretion rates in our sample, suggesting that we have reached a physical lower limit. This limit, $\dot{M}\sim10^{-10}M_{\odot}yr^{-1}$, is consistent with EUV-dominated photoevaporation.