On the presence and absence of disks around O-type stars

Context. As the favoured progenitors of long-duration gamma-ray bursts, massive stars may represent our best signposts of individual objects in the early Universe, but special conditions seem required to make these bursters. These are thought to originate from the progenitor's rapid rotation and associated asymmetry. Aims. To obtain empirical constraints on the interplay between stellar rotation and wind asymmetry, we perform linear H alpha spectropolarimetry on a sample of 18 spectroscopically peculiar massive O stars, including OVz, Of?p, Oe, and Onfp stars, supplemented by an earlier sample of 20 O supergiants of Harries et al. (2002, MNRAS, 337, 341), yielding a total number of 38 O-type stars. Our study's global aim is to characterize the differences between, and similarities amongst, different classes of peculiar O stars and to establish in how far they di. er from garden-variety O stars. Methods. Our linear (Stokes QU) spectropolarimetry data should be regarded a geometric counterpart to (Stokes I) spectral classification, setting the stage for circular (Stokes V) polarimetric searches for magnetic fields. Results. Despite their rapid rotation (with v sin i up to similar to 400 km s(-1)) most O-type stars are found to be spherically symmetric, but with notable exceptions amongst specific object classes. We divide the peculiar O stars into four distinct categories: Group I includes the suspected young zero-age main sequence OVz stars and related weak-winds objects, of which the magnetic star Theta(1) Ori C is the most famous member. These objects show no evidence for significant linear polarization. Group II includes the Of? p stars, in which one of its members, HD 191612, was also found to be magnetic. These objects show more linear polarization activity than those in Group I. Group III includes the Oe stars, which have been suggested to be the more massive counterparts to classical Be stars, and Group IV concerns the Onfp stars. Objects from the latter two groups are on the high-end tail of the O-star rotation distribution and have in the past been claimed to be embedded in disks. Here we report the detection of a classical depolarization line effect in the Oe star HD45314, but the overall incidence of line effects amongst Oe stars is significantly lower (1 out of 6) than amongst Be stars. The chance that the Oe and Be datasets are drawn from the same parent population is negligible (with 95% confidence). This implies there is as yet no evidence for a disk hypothesis in Oe stars, providing relevant constraints on the physical mechanism that is responsible for the Be phenomenon. Finally, we find that 3 out of 4 of the Group IV Onfp stars show evidence for complex polarization effects, which are likely related to rapid rotation, and we speculate on the evolutionary links to B[e] stars.