Possible variability of the magnetic field of T Tau

Abstract

Using the Main Stellar Spectrograph of the 6-m Special Astrophysical Observatory telescope equipped with a polametric analyzer, we measured the longitudinal magnetic field component B ∥ for the T Tauri stars T Tau and AS 507 on January 16 and 18 and February 15, 2003. For both stars, we determined only the upper limits on B ∥ from photospheric lines: +15±30 G for T Tau and −70±90 G for AS 507. The magnetic field of AS 507 was not measured previously, while B ∥ for T Tau is lower than its values that we obtained in 1996 and 2002 (B ∥≂150±50G), suggesting that the longitudinal magnetic field component in the photosphere of T Tau is variable. We also measured the longitudinal magnetic field component for T Tau in the formation region of the He I 5876 A emission line. We found B ∥ in this region to be ⋍+650, ⋍+350, and ⋍+1100 G on January 16, 18, and February 15, 2003, respectively. Our observations on January 18 and February 15 correspond to virtually the same phase of the star's rotation period, but the profiles of the He I 5876 A line differ markedly on these two nights. Therefore, we believe that the threefold difference between the B ∥ values on these nights does not result from observational errors. We discuss the possible causes of the B ∥ variability in the photosphere and the magnetosphere of T Tau.