Line Structure in the Spectrum of FU Orionis

TL;DR

High-resolution optical spectra of FU Orionis reveal that its lines originate from a rapidly rotating G-type star with a dark polar spot, challenging the accretion disk model predictions and indicating a stellar origin for optical lines.

Contribution

The study provides direct spectral evidence that optical lines in FU Ori come from a rotating star rather than an accretion disk, contradicting previous models.

Findings

Optical lines do not show expected disk-related dependencies.

Lines originate from a rapidly rotating G-type star.

Radial velocity remains constant, indicating no binary companion.

Abstract

New high-resolution spectra of FU Ori, obtained with the HIRES spectrograph at the Keck I telescope in 2003-2006, make it possible to compare the optical line profiles with those predicted by the self-luminous accretion disk model. A dependence of line width on excitation potential and on wavelength, expected for a Keplerian disk, is definitely not present in the optical region, nor is the line duplicity due to velocity splitting. The absorption lines observed in the optical region of FU Ori must originate in or near the central object, and here their profiles are shown to be those expected of a rigidly rotating object. They can be fitted by a rapidly rotating (v sin i = 70 km/s) high-luminosity G-type star having a large dark polar spot, with axis inclined toward the line of sight. Over these years, the radial velocity of FU Ori has remained constant to within +/-0.3 km/s, so there is no indication that the star is a spectroscopic binary. These results apply to the optical region ($\lambda< 8800$ \AA); more distant, cooler regions of the disk contribute in the infrared.