Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

TL;DR

This study measures accretion rates of 21 T Tauri stars using nearly simultaneous UV and optical spectra, introducing multiple accretion components to better model the accretion shock and explain infrared excesses.

Contribution

It presents a novel method of fitting multiple accretion components based on magnetospheric models, improving the understanding of accretion processes in T Tauri stars.

Findings

Accretion rates generally agree with previous estimates after accounting for extinction and variability.

Multiple accretion components better explain near-infrared excesses.

Correlations between emission line luminosities and accretion rates are established.

Abstract

We analyze the accretion properties of 21 low mass T Tauri stars using a dataset of contemporaneous near ultraviolet (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph (STIS) and the ground based Small and Medium Aperture Research Telescope System (SMARTS), a unique dataset because of the nearly simultaneous broad wavelength coverage. Our dataset includes accreting T Tauri stars (CTTS) in Taurus, Chamaeleon I, $\eta$ Chamaeleon and the TW Hydra Association. For each source we calculate the accretion rate by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high density, low filling factor accretion spots co-exist with low density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near infrared. Comparing our estimates of the accretion rate to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the accretion rate estimates, to produce correlations between accretion indicators (H$\beta$, Ca II K, C II] and Mg II) and accretion properties obtained simultaneously.