The nature of the companion in the Wolf-Rayet system EZ Canis Majoris

¹ Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Ave. Universidad S/N, Cuernavaca, 62210 Morelos, México
e-mail: gloria@icf.unam.mx, gloria@astro.unam.mx
² Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland
e-mail: werner.schmutz@pmodwrc.ch

Received: 12 December 2019
Accepted: 29 April 2020

Abstract

Context. EZ Canis Majoris is a classical Wolf-Rayet star whose binary nature has been debated for decades. It was recently modeled as an eccentric binary with a periodic brightening at periastron of the emission originating in a shock heated zone near the companion.

Aims. The focus of this paper is to further test the binary model and to constrain the nature of the unseen close companion by searching for emission arising in the shock-heated region.

Methods. We analyze over 400 high resolution International Ultraviolet Explorer spectra obtained between 1983 and 1995 and XMM-Newton observations obtained in 2010. The light curve and radial velocity (RV) variations were fit with the eccentric binary model and the orbital elements were constrained.

Results. We find RV variations in the primary emission lines with a semi-amplitude K₁ ∼ 30 km s⁻¹ in 1992 and 1995, and a second set of emissions with an anti-phase RV curve with K₂ ∼ 150 km s⁻¹. The simultaneous model fit to the RVs and the light curve yields the orbital elements for each epoch. Adopting a Wolf-Rayet mass M₁ ∼ 20 M_⊙ leads to M₂ ∼ 3−5 M_⊙, which implies that the companion could be a late B-type star. The eccentric (e = 0.1) binary model also explains the hard X-ray light curve obtained by XMM-Newton and the fit to these data indicates that the duration of maximum is shorter than the typical exposure times.

Conclusions: The anti-phase RV variations of two emission components and the simultaneous fit to the RVs and the light curve are concrete evidence in favor of the binary nature of EZ Canis Majoris. The assumption that the emission from the shock-heated region closely traces the orbit of the companion is less certain, although it is feasible because the companion is significantly heated by the WR radiation field and impacted by the WR wind.