The search for magnetic fields in two Wolf–Rayet stars and the discovery of a variable magnetic field in WR 55

ABSTRACT Magnetic fields in Wolf–Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR 46 and WR 55, for the search of the presence of magnetic fields using FORS 2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in WR 55. The field detection in this star, which is associated with the ring nebula RCW 78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3σ was achieved for WR 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by FUSE(Far Ultraviolet Spectroscopic Explorer) observations.

Blue supergiant progenitors from binary mergers for SN 1987A and other Type II-peculiar supernovae

We present results of a systematic and detailed stellar evolution study of binary mergers for blue supergiant (BSG) progenitors of Type II supernovae, particularly for SN 1987A. We are able to reproduce nearly all observational aspects of the progenitor of SN 1987A, Sk –69 °202, such as its position in the HR diagram, the enrichment of helium and nitrogen in the triple-ring nebula and its lifetime before its explosion. We build our evolutionary model based on the merger model of Podsiadlowski et al. (1992), Podsiadlowski et al. (2007) and empirically explore an initial parameter consisting of primary masses, secondary masses and different depths up to which the secondary penetrates the He core during the merger. The evolution of the post-merger star is continued until just before iron-core collapse. Of the 84 pre-supernova models (16 M⊙ − 23 M⊙) computed, the majority of the pre-supernova models are compact, hot BSGs with effective temperature >12 kK and 30 R⊙ − 70 R⊙ of which six match nearly all the observational properties of Sk –69 °202.

Mapping the Physical and Chemical Conditions of the Ring Nebula

We observed the Planetary Nebula NGC 6720 with the Gemini Telescope and the Gemini Multi-Object Spectrographs. We obtained spatial maps of 36 emission-lines in the wavelength range between 3600 Å and 9400 Å. We derived maps of c(Hβ), electronic densities, electronic temperatures, ionic and elemental abundances, and Ionization Correction Factors (ICFs) in the source and investigated the mass-loss history of the progenitor. The elemental abundance results indicate the need for ICFs based on three-dimensional photoionization models.