PREPRINT

γ Columbae: the recently stripped, pulsating core of a massive star

Andreas Irrgang, Norbert Przybilla, Georges Meynet

Submitted on 1 November 2022

Abstract

A vital condition for life on Earth is the steady supply of radiative heat by the Sun. Like all other stars, the Sun generates its emitted energy in its central regions where densities and temperatures are high enough for nuclear fusion processes to take place. Because stellar cores are usually covered by an opaque envelope, most of our knowledge about them and their life-giving nuclear processes comes from theoretical modelling or from indirect observations such as the detection of solar neutrinos and the study of stellar pulsations, respectively. Only in very rare cases, stars may expose their cores, e.g., when a tiny fraction of them evolves into Wolf-Rayet or helium hot subdwarf stars. However, for the vast majority of stars, namely unevolved stars that burn hydrogen to helium in their centres, direct observational clues on the cores are still missing. Based on a comprehensive spectroscopic and asteroseismic analysis, we show here that the bright B-type star γ Columbae is the stripped pulsating core (with a mass of 4-5M, where M is the mass of the Sun) of a previously much more massive star of roughly 12M that just finished central hydrogen fusion. The star's inferred parameters indicate that it is still in a short-lived post-stripping structural readjustment phase, making it an extremely rare object. The discovery of this unique star paves the way to obtain invaluable insights into the physics of both single and binary stars with respect to nuclear astrophysics and common-envelope evolution. In particular, it provides first observational constraints on the structure and evolution of stripped envelope stars.

Preprint

Comment: This preprint has not undergone peer review or any post-submission improvements or corrections. The Version of Record of this article is published in Nature Astronomy, and is available online at https://doi.org/10.1038/s41550-022-01809-6

Subjects: Astrophysics - Solar and Stellar Astrophysics; Astrophysics - High Energy Astrophysical Phenomena

URL: http://arxiv.org/abs/2211.00358

\small {\bf Observational signature of the peculiar abundances of carbon and nitrogen.} A small portion of an observed spectrum of $\gamma$\,Columbae (black line) is compared to one of the solar-neighbourhood reference star 18\,Pegasi (red line; smoothed in such a way that the broadening of the spectral lines is comparable), which has similar atmospheric parameters ($T_{\mathrm{eff}}=15,800$\,K, $\log(g\,\mathrm{(cm\,s^{-2})})=3.75$) but a standard, solar-like chemical composition\cite{2012A&A...539A.143N}. Singly and doubly ionized lines of nitrogen, iron, sulphur, and carbon are labelled. Already a visual inspection of the two spectra shows that the abundances, i.e., the strengths of the absorption lines, of iron and sulphur (and of other chemical species whose spectral lines are not exhibited here) are very similar while carbon is severely depleted and nitrogen considerably enriched in the atmosphere of $\gamma$\,Columbae.