1996A&A...309..777G

Although magnetic fields are highly important for understanding the structure and evolution of T Tauri stars (TTS), very few attempts measure these fields have actually been reported, because of the high dispersion, and long integration times that are usually needed in the optical regime. In this paper we point out that the 1.5630-1.5695µm regime in the near infrared contains a number of suitable lines for this purpose, and present spectra which confirm the presence and strengths of these lines in the weak line T Tauri stars Tap 35 (=NTTS 042417+1744) and V410 Tau. We demonstrate that, using these lines and modern IR grating spectrometers such as the UKIRT's CGS4, the product fB of the filling factor (f) and the magnetic field strength (B) can be determined with an accuracy better than 500G in relatively short integration times if vsini<20km/s. In a quick exploratory observation we find that the lines in V410 Tau are too rotationally broadened to allow determination of a magnetic field but find an upper limit to fB of 2000G for the more slowly rotating weak line T Tauri star Tap35 using a spectrum with a total integration time of only 16 minutes.