Spectroscopic study of the extremely young O-type triple system Herschel 36 A in the Hourglass nebula - I. Orbital properties

We present a detailed spectroscopic study of Herschel 36 A (H36A), the main stellar component of the massive multiple system Herschel 36 in the Hourglass nebula, based on high-resolution optical spectra obtained along an 11 yr span. The three stellar components present in the spectrum of H36A are separated by means of a spectral disentangling technique. Individual spectral classifications are improved, and high-precision orbital solutions for the inner and outer orbits are calculated. H36A is confirmed to be a hierarchical triple system composed of a close massive binary (Ab1 + Ab2, O9.5 V + B0.7 V) in wide orbit around a third O-type star (Aa, 07.5 Vz). The inner-pair orbit is characterized by a period of 1.54157 +/- 0.00006 d, and semi-amplitudes of 181.2 +/- 0.7 and 295.4 +/- 1.7 km s(-1). The outer orbit has a period of 492.81 +/- 0.69 d, and semi-amplitudes of 62.0 +/- 0.6 and 42.4 +/- 0.8 km s(-1). The inner and outer orbits are not coplanar, having a relative inclination of at least 20 degrees. Dynamical minimum masses of 20.6 +/- 0.8 M-circle dot, 18.7 +/- 1.1 M-circle dot, and 11.5 +/- 1.1 M(circle dot )are derived for the Aa, Ab1, and Ab2 components, respectively, in reasonable agreement with the theoretical calibrations.