Toward Understanding the B[e] Phenomenon. VII. AS 386, a Single-lined Binary with a Candidate Black Hole Component

We report the results of spectroscopic and photometric observations of the emission-line object AS 386. For the first time we found that it exhibits the B[e] phenomenon and fits the definition of an FS CMa type object. The optical spectrum shows the presence of a B-type star with the following properties: T-eff = 11,000 +/- 500 K, log L/L-circle dot = 3.7 +/- 0.3, a mass of 7 +/- 1 M-circle dot, and a distance D = 2.4 +/- 0.3 kpc from the Sun. We detected regular radial velocity variations of both absorption and emission lines with the following orbital parameters: P-orb = 131.27 +/- 0.09 days, semiamplitude K-1 = 51.7 +/- 3.0 km s(-1), systemic radial velocity gamma = -31.8 +/- 2.6 km s(-1), and a mass function of f (m) = 1.9 +/- 0.3 M-circle dot. AS 386 exhibits irregular variations of the optical brightness (V = 10.92 +/- 0.05 mag), while the near-IR brightness varies up to similar to 0.3 mag following the spectroscopic period. We explain this behavior by a variable illumination of the dusty disk inner rim by the B-type component. Doppler tomography based on the orbital variations of emission-line profiles shows that the material is distributed near the B-type component and in a circumbinary disk. We conclude that the system has undergone a strong mass transfer that created the circumstellar material and increased the B-type component mass. The absence of any traces of a secondary component, whose mass should be >= 7 M-circle dot, suggests that it is most likely a black hole.