The mass distribution of clumpy accretion onto the nearby young star TW Hya

TL;DR

This study analyzes accretion bursts onto the young star TW Hya using high-cadence photometry calibrated with spectroscopy, revealing clumpy accretion events and their timescales over hours to years.

Contribution

It provides a detailed calibration of photometric variability to accretion rates and links burst properties to stellar and disk dynamics, advancing understanding of star-disk interactions.

Findings

Accretion bursts have masses from 10^{-13} to 3×10^{-11} M_sun.

Average burst duration is 1.8 days, longer than thermal response times.

Burst reset timescales are 1.2–2 days, related to stellar rotation or disk asymmetries.

Abstract

The proliferation of high time-resolution and decades-long monitoring of classical T Tauri stars provides a vast opportunity to test the variability of the star-disk connections. However, most monitoring surveys use single broad-band filters, which makes the conversion of photometric variability into accretion rate difficult. In this study, we analyze accretion bursts onto the nearby young star TW Hya over short (hours, days) and long (months, years) timescales by calibrating TESS and ASAS-SN $g$-band photometry to accretion rates with simultaneous spectroscopy. The high cadence TESS light curve shows bursts of accretion in clumps with masses from a sensitivity limit of $\sim10^{-13}$~M$_\odot$ up to $3\times 10^{-11}$\,M$_\odot$. The average burst duration of 1.8 days is longer than a simple estimate of the thermal response timescale, supporting the interpretation that the photometric variability probes the instantaneous accretion rate. The reset timescale of 1.2--2 days derived from the structure function and previously reported quasi-periods of 3.5--4 days are consistent with bursts that may be related to the different rotation between the stellar magnetosphere and inner disk or with azimuthal asymmetries in the inner disk. The near-daily ASAS-SN light curve across 8 years reveals some seasonal changes in brightness with a standard deviation of $\sim 0.13$ mag, about half of the scatter seen on short timescales. This study demonstrates the importance of coordinating contemporaneous multi-epoch spectroscopy with time domain surveys to interpret light curves of young stars.