Identification of the Mass Donor Star's Spectrum in SS 433

We present spectroscopy of the microquasar SS 433 obtained near primary eclipse and disk precessional phase Psi=0.0, when the accretion disk is expected to be most ``face-on.'' The likelihood of observing the spectrum of the mass donor is maximized at this combination of orbital and precessional phases, since the donor is in the foreground and above the extended disk believed to be present in the system. The spectra were obtained over four different runs centered on these special phases. The blue spectra show clear evidence of absorption features consistent with a classification of A3-7 I. The behavior of the observed lines indicates an origin in the mass donor. The observed radial velocity variations are in antiphase to the disk, the absorption lines strengthen at mideclipse when the donor star is expected to contribute its maximum percentage of the total flux, and the line widths are consistent with lines created in an A supergiant photosphere. We discuss and cast doubt on the possibility that these lines represent a circumstellar shell spectrum rather than the mass donor itself. We reevaluate the mass ratio of the system and derive masses of 10.9+/-3.1 and 2.9+/-0.7M_solar for the mass donor and compact object plus disk, respectively. We suggest that the compact object is a low-mass black hole. In addition, we review the behavior of the observed emission lines from both the disk/wind and high-velocity jets and show that the current orbital ephemeris and disk precession/nodding model parameters are still valid.