# Multi-epoch Ultraviolet HST Observations of Accreting Low-mass Stars

## TL;DR

This study presents the largest HST variability analysis of Classical T Tauri stars, revealing significant accretion rate fluctuations, potential accretion bursts, and correlations between accretion rates and line luminosities, advancing understanding of young star accretion processes.

## Contribution

It provides new insights into accretion variability in low-mass stars through multi-epoch UV observations and Bayesian modeling, including detection of accretion bursts and improved secondary accretion tracers.

## Key findings

- Accretion rates vary up to a factor of 2 over a week.
- Filling factors change by up to a factor of 5.
- Detected a possible accretion burst in GM Aur.

## Abstract

Variability is a defining characteristic of young low-mass stars that are still accreting material from their primordial protoplanetary disk. Here we present the largest \textit{HST} variability study of Classical T Tauri stars (CTTS) to date. For 5 of these objects, we obtained a total of 25 spectra with the Space Telescope Imaging Spectrograph (STIS). Mass accretion rates and the fraction of the star covered by accretion columns (i.e., filling factors) were inferred using 1D NLTE physical models whose parameters were fit within a Bayesian framework. On week long timescales, typical changes in the mass accretion rates range up to a factor of $\sim2$, while changes of up to a factor of $\sim5$ are inferred for the filling factors. In addition to this, we observed a possible accretion burst in the transitional disk system GM Aur, and an incident we interpret as a chance alignment of an accretion column and the undisturbed photosphere along our line of sight in the full disk system VW Cha. We also measure correlations between mass accretion rate and line luminosities for use as secondary tracers of accretion. We place our objects in context with recent high-cadence photometric surveys of low-mass star formation regions and highlight the need for more broad-wavelength, contemporaneous data to better understand the physical mechanisms behind accretion variability in CTTS.