How Variable is Accretion in Young Stars?

TL;DR

This study investigates the variability of accretion in young stars by analyzing multi-epoch spectra, finding that accretion rate changes are typically limited to 0.5 dex over days to months, with rotation likely driving observed variability.

Contribution

It provides a detailed comparison of accretion indicators, showing CaII fluxes are more reliable than H-alpha width, and quantifies the typical extent and timescale of accretion variability in young stars.

Findings

CaII fluxes are more robust indicators of accretion than H-alpha width.

Intrinsic accretion rate changes usually do not exceed 0.5 dex.

Variability is mainly driven by stellar rotation and object-to-object differences.

Abstract

We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Tau-Aur and Cha I. For all stars, we obtain typically four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the H-alpha 10% width and the CaII-8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the CaII fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while H-alpha 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that CaII fluxes are a more robust quantitative indicator of accretion than H-alpha 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates towards later spectral types, reflecting the dM/dt vs. M relationship. There is a gap between accretors and non-accretors, pointing to a rapid shutdown of accretion. We conclude that the ~2 orders of magnitude scatter in the dM/dt vs. M relationship is dominated by object-to-object scatter instead of intrinsic source variability.