LIMITS ON THE MAGNETIC-FLUX OF PRE-MAIN-SEQUENCE STARS

We attempt to detect magnetic fields on two weak-emission-line T Tauri stars by searching for the enhancement of the equivalent widths of Zeeman-sensitive absorption lines. For both stars, we synthesize 26 observed Fe I lines, which represent a wide range of Zeeman sensitivities, using an LTE Stokes line-transfer calculation. The theoretical growth of equivalent width with magnetic field is discussed for lines with various Zeeman patterns. Oscillator strengths for all lines are empirically determined a priori using a magnetically quiet star of similar spectral type. The iron abundances of both T Tauri stars are established by synthesizing lines that are insensitive to Zeeman splitting. We find that the observed equivalent widths of Zeeman-sensitive lines in TA P 35 are systematically enhanced relative to those calculated for no magnetic field, while such an enhancement is not seen in Zeeman-insensitive lines. The excess W(eq) of Zeeman lines can be explained by a field of approximately 1000 G covering the entire surface, or by stronger fields covering proportionately less of the surface. With greater certainty, an upper limit can be placed on the product: Bf < 1500 G for TAP 35. If fields do cover a significant fraction of the surface, the strength is likely less than 3000 G, based on the lack of Zeeman broadening in the most Zeeman sensitive lines. Our method is not sensitive to the presence of fields in dark starspots. No Zeeman enhancement of line equivalent widths is seen in TAP 10, which provides an upper limit to its magnetic flux of Bf < 700 G. This magnetic nondetection is consistent with the apparently weaker chromospheric activity on that star as measured by the Ca II infrared triplet emission.