An in-depth reanalysis of the alleged type Ia supernova progenitor Henize 2−428

Abstract

Context. The nucleus of the planetary nebula Hen 2-428 is a short orbital-period (4.2 h), double-lined spectroscopic binary, whose status as a potential supernova type Ia progenitor has raised some controversy in the literature. Aims. With the aim of resolving this debate, we carried out an in-depth reanalysis of the system. Methods. Our approach combines a refined wavelength calibration, thorough line-identifications, improved radial-velocity measurements, non-LTE spectral modeling, as well as multi-band light-curve fitting. Our results are then discussed in view of state-of-the-art stellar evolutionary models. Results. Besides systematic zero-point shifts in the wavelength calibration of the OSIRIS spectra which were also used in the previous analysis of the system, we found that the spectra are contaminated with diffuse interstellar bands. Our Voigt-profile radial velocity fitting method, which considers the additional absorption of these diffuse interstellar bands, reveals significantly lower masses (M1 = 0.66 ± 0.11 M⊙ and M2 = 0.42 ± 0.07 M⊙) than previously reported and a mass ratio that is clearly below unity. Our spectral and light curve analyses lead to consistent results, however, we find higher effective temperatures and smaller radii than previously reported. Moreover, we find that the red-excess that was reported before to prove to be a mere artifact of an outdated reddening law that was applied. Conclusions. Our work shows that blends of He II λ 5412 Å with diffuse interstellar bands have led to an overestimation of the previously reported dynamical masses of Hen 2−428. The merging event of Hen 2−428 will not be recognised as a supernova type Ia, but most likely leads to the formation of a H-deficient star. We suggest that the system was formed via a first stable mass transfer episode, followed by common envelope evolution, and it is now composed of a post-early asymptotic giant branch star and a reheated He-core white dwarf.