The absence of sub-minute periodicity in classical T Tauri stars

TL;DR

This study searches for sub-minute variability in classical T Tauri stars TW Hya and AA Tau using high time-resolution observations, finding no significant periodicity and setting upper limits on possible pulsations.

Contribution

It provides the first high time-resolution optical and UV constraints on rapid variability in CTTS, testing predictions of accretion shock instabilities.

Findings

No significant periodicity detected in TW Hya and AA Tau.

Placed upper limits on pulsed fractions below 0.001 for TW Hya.

Observed variability timescales suggest shock instabilities are not directly visible.

Abstract

Classical T Tauri stars (CTTS) are young, late-type objects, that still accrete matter from a circumstellar disk. Analytical treatments and numerical simulations predict instabilities of the accretion shock on the stellar surface. We search for variability on timescales below a few minutes in the CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern African Large Telescope (SALT) in narrow-band filters around the Balmer jump. The observations were performed in slit mode, which provides a time resolution of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single photon-counting device with even better time resolution. Small-scale variability typically lasts a few seconds, however, no significant periodicity is detected. We place a 99 % confidence upper limit on the pulsed fraction of the lightcurves. The relative amplitude is below 0.001 for TW Hya in the frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50 Hz. The relevant timescales indicate that shock instabilites should not be seen directly in our optical and UV observations, but the predicted oscialltions would induce observable variations in the reddening. We discuss how the magnetic field could stabilise the accretion shock.