We present high resolution ( R \approx 60 , 000 ) circular spectropolarimetry of the classical T Tauri star TW Hydrae .
We analyze 12 photospheric absorption lines and measure the net longitudinal magnetic field for 6 consecutive nights .
While no net polarization is detected the first five nights , a significant photospheric field of B _ { z } = 149 \pm 33 G is found on the sixth night .
To rule out spurious instrumental polarization , we apply the same analysis technique to several non-magnetic telluric lines , detecting no significant polarization .
We further demonstrate the reality of this field detection by showing that the splitting between right and left polarized components in these 12 photospheric lines shows a linear trend with Landé g -factor times wavelength squared , as predicted by the Zeeman effect .
However , this longitudinal field detection is still much lower than that which would result if a pure dipole magnetic geometry is responsible for the mean magnetic field strength of 2.6 kG previously reported for TW Hya .
We also detect strong circular polarization in the He i 5876 Å and the Ca ii 8498 Å emission lines , indicating a strong field in the line formation region of these features .
The polarization of the Ca ii line is substantially weaker than that of the He i line , which we interpret as due to a larger contribution to the Ca ii line from chromospheric emission in which the polarization signals cancel .
However , the presence of polarization in the Ca ii line indicates that accretion shocks on Classical T Tauri stars do produce narrow emission features in the infrared triplet lines of Calcium .