Permitted Iron Emission Lines in the Classical T Tauri Star DR Tauri

Abstract

We present a study of permitted emission lines of Fe I and Fe II in the spectrum of the high-accretion rate classical T Tauri star DR Tau. Echelle spectra collected at the 4 m Mayall telescope at Kitt Peak National Observatory between 1988 and 1992 include four epochs with red spectral coverage (~5000-6800 A) and three with blue spectral coverage (~4000-4950 A). A total of 62 unblended Fe I and Fe II lines are identified, their profiles are examined, and ratios of line pairs that are sensitive to column density or temperature are analyzed. The unblended Fe profiles exhibit a systematic behavior, with FWHM increasing from 20 to between 70 and 90 km s-1 as the equivalent width increases from 0.05 to several A. Two-component fits to the profiles suggest a composite origin, comprising a narrow component (NC), with FWHM ~ 20 km s-1 and a radial velocity at rest with respect to the photosphere, and a broad component (BC), with FWHM ~ 100 km s-1 and a tendency to be blueshifted by ~ 1017-1018 cm-2 and NFe II >~ 1018-1019 cm-2 for kinetic temperatures in the range 4000-10,000 K. Ratios of NC-to-BC emission from a pair of Fe I lines that are insensitive to opacity suggest that the kinetic temperature in the NC exceeds that in the BC by several thousand degrees. The centroid velocity and width of the NC in Fe I and Fe II are comparable to those from photospheric lines, suggesting a thermal or turbulent origin further broadened by stellar rotation. In the context of a magnetospheric accretion model, the NC is consistent with an origin in the postshock gas close to the stellar surface. In contrast, the BC is likely to be broadened by bulk motion, such as infalling gas in the accretion funnel or rotation in the region coupling the inner disk to the stellar magnetic field.