On the nature of WO stars: a quantitative analysis of the WO3 star DR1 in IC 1613

Abstract

Context. Oxygen sequence Wolf-Rayet (WO) stars are thought to represent the final evolutionary stage of the most massive stars. The characteristic strong O  emission possibly originates from an enhanced oxygen abundance in the stellar wind. Alternatively, the O  emission can be caused by the high temperature of these stars, in which case the WO stars are the high-temperature extension of the more common carbon sequence Wolf-Rayet (WC) stars. Aims. By constraining the physical properties and evolutionary status of DR1, a WO star in the low-metallicity Local Group dwarf galaxy IC 1613 and one of only two objects of its class known in a SMC-like metallicity environment, we aim to investigate the nature of WO stars and their evolutionary connection with WC stars. Methods. We use the non-local thermodynamic equilibrium atmosphere code  to model the observed spectrum of DR1 and to derive its stellar and wind parameters. We compare our values with other studies of WC and WO stars, as well as with the predictions of evolutionary models. We also model the surrounding nebula using the photo-ionization code . Results. The oxygen and carbon abundances that we obtain are comparable to values found for WC stars. The temperature and luminosity are, however, higher than those of WC stars. DR1 is embedded in the hottest known H  region in the Local Group. The nebular properties can be consistently reproduced by  models adopting DR1 as central ionizing source. Conclusions. Comparison of the abundances and temperature of DR1 with core helium-burning models show that DR1 is currently well into the second half of helium burning. If the properties of DR1 are representative for the WO class, it would imply that WO stars are the high-temperature and high-luminosity extension of the WC stars, and do not necessarily represent a later evolutionary stage.

The evolution of massive stars and their spectra

“…This result is in line with suggestions from observational studies of WO stars (Crowther 1999;Crowther et al 2000;Tramper et al 2013). …”

“…These are based on the relative strength between Si  λ4117 and He  4122, Si  λ4554 to He  λ4389, and Si  λ4554 to Si  λ4090. As is common in the literature, we associate the Ia + luminosity class to B-type hypergiants (BHG; van Genderen et al 1982). BHGs are differentiated from BSGs based on the strength of the H Balmer line emission.…”

Wolf-Rayet stars in the Small Magellanic Cloud

“…Note that despite the very large uncertainty in T * , we can constrain T 2/3 fairly well, since all well-fitting models reach a similar temperature at their "photospheres". Tramper et al (2013), who were also unable to simultaneously fit both O iv and O vi lines at their observed strengths, suggest that this discrepancy may be related to soft X-ray K-shell ionization. However, we are unable to verify this claim.…”

“…For the O companions, we adopt H, C, N, O, Mg, Si, and Fe mass fractions as derived for B stars in the SMC by Korn et al (2000), Trundle et al (2007), and Hunter et al (2007):…”

“…We scale the mass fractions of the remaining metals to 1/7 solar, in accordance with the ratio of the solar metallicity (Asplund et al 2009) to the average SMC metallicity (Trundle et al 2007): X Ne = 1.8 × 10 . The same mass fractions are adopted for WR models, except for X H , which is derived in each case, and the CNO mass fractions, which are adopted as in Paper I: X C = 2.5 × 10…”