A spectroscopic study of three FU Orionis objects: Accretion disks and wind signatures

It has been suggested that in outburst, FU Ori objects (FUors) are luminous accretion disks surrounding young, low luminosity stars. We obtained high resolution, high signal-to-noise, 3985-4920 A, 4940-6830 A, and 8030-9510 A spectra of V1057 Cyg, Z CMa, and FU Ori at the Mayall telescope at KPNO to examine predictions of the accretion disk model of Kenyon, Hartmann, and Hewett (1988). Synthetic disk spectra are calculated assuming the temperature gradient, $T(R)$, of a classical accretion disk, and assuming that disk photospheres can be modeled by adding contributions from concentric disk annuli represented by spectra of supergiant stars having $T\sb{eff}$ $\approx$ $T(R)$. Given the simplicity of the model, we find surprising agreement between observed and synthetic spectra. We examine variation of absorption linewidth with wavelength of synthetic and FUor spectra, and find good correlations for the model and two FUors. We also investigate variation of linewidth with lower excitation potential, and find a subtle effect, less useful as a disk diagnostic. We subtract synthetic spectra from object spectra to investigate overall goodness of fit. For V1057 Cyg and Z CMa, the percentage of lines fit to $\u3c$10 mA accuracy rises from $\sim$50% at 4000 A to $\u3e$80% at 5000-6800 A. At 8000-9000 A, the goodness of fit is $\sim$50%; the decline attributed to disk flaring. For FU Ori, the goodness of fit is $\sim$10% at 4000 A, rising to $\sim$80% longward of 5700 A. We examine residual features in V1057 Cyg and Z CMa. Most of the strongest features are either emission or P Cygni lines, suggesting origin in powerful winds driven by these objects. For FU Ori, most residuals are absorption features, as expected if FU Ori has lower surface gravity than do supergiants, or if wind absorption is particularly strong. The signal-to-noise ratio in Z CMa\u27s residual spectrum is sufficient to examine the P Cygni structure in lines of a large range of optical depths. We leave detailed analysis to a subsequent paper, but point here to unusual line structure that may arise in a rotating wind, suggesting that FUor winds carry large amounts of angular momentum