Spectropolarimetry of the Classical T Tauri Star TW Hydrae

We present high resolution (R ~ 60,000) circular spectropolarimetry of theclassical T Tauri star TW Hydrae. We analyze 12 photospheric absorption linesand measure the net longitudinal magnetic field for 6 consecutive nights. Whileno net polarization is detected the first five nights, a significantphotospheric field of Bz = 149 \pm 33 G is found on the sixth night. To ruleout spurious instrumental polarization, we apply the same analysis technique toseveral non-magnetic telluric lines, detecting no significant polarization. Wefurther demonstrate the reality of this field detection by showing that thesplitting between right and left polarized components in these 12 photosphericlines shows a linear trend with Lande g-factor times wavelength squared, aspredicted by the Zeeman effect. However, this longitudinal field detection isstill much lower than that which would result if a pure dipole magneticgeometry is responsible for the mean magnetic field strength of 2.6 kGpreviously reported for TW Hya. We also detect strong circular polarization inthe He I 5876 and the Ca II 8498 emission lines, indicating a strong field inthe line formation region of these features. The polarization of the Ca II lineis substantially weaker than that of the He I line, which we interpret as dueto a larger contribution to the Ca II line from chromospheric emission in whichthe polarization signals cancel. However, the presence of polarization in theCa II line indicates that accretion shocks on Classical T Tauri stars doproduce narrow emission features in the infrared triplet lines of Calcium.Comment: One tar file. The paper has 22 pages, 5 figures. Accepted by AJ on Sep 10, 200