Measuring the Magnetic Field on the Classical T Tauri Star TW Hydrae

We present infrared (IR) and optical echelle spectra of the Classical T Tauri star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis to fit atomic and molecular absorption lines and determine key stellar parameters: Teff = 4126± 24 K, log g = 4.84± 0.16, [M/H]= −0.10± 0.12, v sin i = 5.8± 0.6 km s. The IR spectrum is used to look for Zeeman broadening of photospheric absorption lines. We fit four Zeeman sensitive Ti I lines near 2.2 microns and find the average value of the magnetic field over the entire surface is B̄ = 2.61± 0.23 kG. In addition, several nearby magnetically insensitive CO lines show no excess broadening above that produced by stellar rotation and instrumental broadening, Visiting Astronomer, Infrared Telescope Facility, operated for NASA by the University of Hawaii Visiting Astronomer, McDonald Observatory, operated by The University of Texas at Austin