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

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 $\sim 2$, while changes of up to a factor of $\sim 5$ 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.

27 pages, 10 figures, 7 tables

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• The UV Perspective of Low-Mass Star Formation
• Revealing the Star-Disk-Jet Connection in GM Aur using Multiwavelength Variability
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• Monitoring accretion rate variability in the Orion Nebula Cluster with the Wendelstein Wide Field Imager
• Testing the Potential for Radio Variability in Disks around T Tauri Stars with Observations and Chemical Modeling
• Correlation between the optical veiling and accretion properties: A case study of the classical T Tauri star DK Tau
• Probing accretion and stellar properties in the Orion Nebula with VLT/X-Shooter
• Molecules with ALMA at Planet-forming Scales (MAPS) XIX. Spiral Arms, a Tail, and Diffuse Structures Traced by CO around the GM Aur Disk
• Understanding Accretion Variability Through TESS Observations of Taurus
• Continuum And Line Emission From Accretion Shocks at T Tauri Stars I. Correlations With Shock Parameters