Constraints to the magnetospheric properties of T Tauri stars. I. The C II], Fe II] and Si II] ultraviolet features

TL;DR

This study analyzes ultraviolet spectral features of T Tauri stars to determine plasma properties in their magnetospheres, revealing correlations with accretion rates and identifying emission origins and outflows.

Contribution

It provides the first detailed measurements of electron densities and temperatures in T Tauri star magnetospheres using C II], Si II], and Fe II] lines from HST spectra.

Findings

Most stars have electron temperatures 10,000-30,000 K and densities 10^8-10^12 cm^-3.

Line broadening correlates with accretion rate.

Emission is mainly from magnetospheric accretion flows, with some outflow signatures.

Abstract

The C II] feature at ~2325 Angstrom is very prominent in the spectra of T Tauri stars (TTSs). This feature is a quintuplet of semiforbidden transitions excited at electron temperatures around 10,000 K that, together with the nearby Si II] and Fe II] features, provides a reliable optically thin tracer for accurate measurement of the plasma properties in the magnetospheres of TTSs. The spectra of 20 (out of 27) TTSs observed with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) have good enough signal to noise ratio (S/N) at the C II] wavelength. For these stars we have determined electron densities (ne) and temperatures (Te) in the line emission region as well as the profile broadening (sigma). For most of the stars in the sample (17) we obtain 10^{4.1} < Te < 10^{4.5} K and 10^{8} < ne < 10^{12} cm^{-3}. These stars have suprathermal line broadening (35 < sigma < 165 km s^{-1}), except TW Hya and CY Tau with thermal line broadening. Both C II] line luminosity and broadening are found to correlate with the accretion rate. Line emission seems to be produced in the magnetospheric accretion flow, close to the disk. There are three exceptions: DG Tau, RY Tau and FU Ori. The line centroids are blueshifted indicating that the line emission in these three stars is dominated by the outflow.