Linear polarization in forbidden lines of the T Tauri star RY Tauri

Abstract

We performed high-dispersion spectropolarimetry for the T Tauri star RY Tauri and measured linear polarization in seven forbidden lines of [OI]λλ5577, 6300, 6364, [NII]λ6583, [SII]λλ6716, 6731, and [FeII]λ7155. This is the first high-dispersion spectropolarimetry for forbidden lines in T Tauri stars. We successfully detected intrinsic polarization in the [OI]λ6300 line. The intrinsic [OI]λ6300 polarization corrected for the foreground polarization was derived as p[OI] = 1.17 ± 0.54% and θ[OI] = 107 ◦ ± 13 ◦ , whereas the polarization of the nearby continuum was pcont = 2.4−3.1% and θcont = 2 ◦ −11 ◦ . The position angle of [OI]λ6300 polarization is compared with the circumstellar structures found by previous studies. It is nearly perpendicular to the disk long-axis and parallel to the optical Hα jet. Both the perpendicularity and parallelism potentially suggest relatively axisymmetric distribution of the [OI]λ6300 emitting region and surrounding scattering medium. We constructed a simplified scattering model composed of a point source on a jet axis as an [OI]λ6300 emitter and a flat disk with an inner hole as a scatterer. Applying the observed polarization to the model suggests that the [OI]λ6300 emission emerges close to the central star with a possible separation of less than a few ×0.1 AU.