Testing the reality of strong magnetic fields on T Tauri stars: The naked T Tauri star Hubble 4

High-resolution optical and infrared (IR) echelle spectra of the naked (diskless) T Tauri star Hubble 4 are presented. The K-band IR spectra include four Zeeman-sensitive Ti I lines along with several magnetically insensitive CO lines. Detailed spectrum synthesis combined with modern atmospheric models is used to fit the optical spectra of Hubble 4 in order to determine its key stellar parameters: T-eff=4158 +/- 56 K; log g = 3.61 +/- 0.50; [M/H] = -0.08 +/- 0.05; v=sin i 14.6 +/- 1.7 km s(-1). These stellar parameters are used to synthesize K-band spectra to compare with the observations. The magnetically sensitive Ti I lines are all significantly broadened relative to the lines produced in the nonmagnetic model, while the magnetically insensitive CO lines are well matched by the basic nonmagnetic model. Models with magnetic fields are synthesized and fitted to the Ti I lines. The best-fit models indicate a distribution of magnetic field strengths on the stellar surface characterized by a mean magnetic field strength of 2.51 +/- 0.18 kG. The mean field is a factor of 2.0 greater than the maximum field strength predicted by pressure equipartition arguments. To confirm the reality of such strong fields, we attempt to refit the observed profiles using a two-component magnetic model in which the field strength is confined to the equipartition value representing plagelike regions in one component and the field is allowed to vary in a cooler component representing spots. It is shown that such a model is inconsistent with the optical spectrum of the TiO bandhead at 7055 Angstrom.