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

TL;DR

The paper addresses how to quantify short-timescale accretion bursts in the nearby classical T Tauri star TW Hya by calibrating high-cadence optical photometry to instantaneous accretion rates using simultaneous spectroscopy. It establishes robust linear relations between TESS and ASAS-SN $g$-band fluxes and accretion, enabling the conversion of light curves into time-resolved $\dot{M}_{acc}$, and it systematically measures the properties of 112 bursts across four TESS epochs. Burst masses span $10^{-13}$–$3\times10^{-11}$ M$_\odot$ with average durations of $\\sim$1.8 days and peak accretion rates of $1$–$3\times10^{-9}$ M$_\odot$ yr$^{-1}$; structure-function analysis yields reset timescales of $\\sim1.2$–$2$ days, while long-term variability on ~100-day scales is modest yet significant. The authors present a practical framework to apply this calibration to other young stellar objects, highlighting the value of coordinated multi-epoch spectroscopy with time-domain surveys to interpret accretion-driven variability in YSOs.

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.

Figures (8)

• Figure 1: The 25-day TESS light curves of TW Hya from (top to bottom) 2019, 2021, 2023, and 2025, showing frequent bursts. Red crosses mark the data points with simultaneous accretion rates from high-resolution spectroscopy (see Section \ref{['section:correlation_tess']}). The TESS flux, normalized by the median ($22890 \ {\rm e^{-} / s}$, $9.41$ mag) of these four lightcurves is plotted on the right y-axis.
• Figure 2: The 2700-day $g$-band photometry of TW Hya from ASAS-SN, with approximately daily observations from 2017 Dec to 2025 May (gray points; right y-axis shows flux normalized by the median brightness of 11.52 mag), with simultaneous accretion rate measurements from high-resolution spectroscopy (red points) and shaded regions marking the four 25-day TESS observing windows. Short-term fluctuations on day timescales are larger than the variability of monthly averages (blue points).
• Figure 3: Linear relationships (dashed lines) between TESS photometry and simultaneous accretion rate measurements from high resolution spectroscopy (left), the g-band and TESS fluxes (middle), and accretion rate and g-band flux (right). The different epochs of TESS are identified with different colors and are fit separately.
• Figure 4: Fits of Gaussian profiles (dashed gray, with a combined red curve) to bursts in the TESS 2019 accretion time series (black points). The top panel shows major bursts, identified visually and measured as excesses of the baseline (dashed gray line) at the flux minimum during the epoch. The bottom panel shows the residual from the fits to the major bursts (blue points). The locations above $2\times$ RMS of the residual, identified by the orange crosses above the gray dashed line), termed minor bursts, are fit with Gaussian profiles to assess our sensitivity limits.
• Figure 5: Summary of the properties of major and minor bursts measured from model fits, including burst mass (upper left), peak mass accretion rate (upper right), duration (lower left), and mass versus duration (lower right), In each plot, the darker regions or points show the bursts while the lighter shades show our sensitivity limits from fits to the residual time series, termed minor bursts.