The magnetic field and circumstellar environment of the helium-strong star HD 36485=δ Ori C

Extensive H alpha spectroscopy of the cool, helium-strong star delta Ori C obtained over two decades has enabled a detailed analysis of the variable H alpha emission observed in this sharp-lined and strongly magnetic B3V star. Radial velocity measurements from these and other spectra have also confirmed the binary nature of the star, the first detection of the secondary being provided, and permitted an improved determination of the system's orbital parameters. The amplitude of the radial velocities and a preliminary spectroscopic analysis indicate that the secondary component is an early A-type star. Disentangling the contribution of the two components to the Balmer line profiles, we determine an effective temperature for delta Ori C slightly larger than previously adopted in the literature. The H alpha emission is variable with a period of 1.477 75 +/- 0.000 04 d, assumed to be the rotation period of the primary component. After removal of the binary system's photospheric contribution to the H alpha line, we find that the emission arises from two distinct, but asymmetrical, optically translucent circumstellar clouds. When strongest the emission peaks have intensities of approximately 15 per cent of the continuum level at about 150 km s(-1) on either side of the line centre, and emission is apparent to +/- 225 km s(-1), or 6 R(*) above the photosphere if we assume that the magnetic field forces the circumstellar material into rigid rotation about the star. New 6 cm Very Large Array radio measurements, when combined with archival data, suggest that the radio flux of the star is also variable. An analysis of new and previously published magnetic field observations enabled us to derive a value for the inclination of the rotation axis of i = 12 degrees +/- 3 degrees and an obliquity of the magnetic axis of beta <= 52 degrees. The nature of the emission variability is similar to that of the prototypical helium-strong star sigma Ori E, but since the latter object has an inclination of > 75 degrees, delta Ori C provides us with an opportunity to investigate the cool components of the winds and magnetospheres of the helium-strong stars from a different vantage point.