We identify the first representative of the long predicted, but never confirmed population of intermediate-mass stripped stars. “Stripped stars” are stars that have lost most of their outer layers, revealing the dense helium core. Most are formed in binary star systems in which one star’s strong gravitational pull peels off and absorbs matter from the companion. For a long time, we have known of low-mass stripped stars (known as subdwarfs) as well as their massive cousins (known as Wolf-Rayet stars) but until now, we have never been able to find any of the so-called “intermediate-mass stripped stars” that had long been predicted to lie in between, raising questions whether our basic theoretical picture needs a major revision.
By surveying hot and luminous stars with high-resolution spectroscopy of the VLT, we finally discover an intermediate-mass stripped star. It lives in a binary system, orbited by a very fastly rotating companion (a so-called Be star) that had been spun-up by accreting mass from the stripped-star progenitor. With our discovery, we demonstrate that the long-missed population of such stars is there! But our findings also indicate that they might look very different from what we had expected. It turns out that instead of having completely lost their outer layers, such stars may retain a small but sufficient amount of hydrogen on top of their helium cores, such that they appear much bigger and cooler than previously thought. We thus call them “partially stripped stars’’. Their mantle of remaining hydrogen is a form of disguise: partially stripped stars resemble most of the normal stellar population, hiding in plain sight, and only high-resolution data combined with careful spectral analysis and detailed computer models can reveal their true nature. No surprise they have evaded detection for so long! The particular giveaway of this star was its mass: a few times more massive than the Sun may seem like a lot, but that is extraordinarily light for its blue supergiant appearance.
The newfound system serves as the critical link in the evolutionary chain connecting several different “species” of exotic objects. Our stellar models predict that in about a million years from now, the stripped star will explode as the so-called stripped supernova, leaving behind a neutron star remnant. This is the first such stripped star found to date. If the binary survives the explosion, the roles will reverse: this time it is the Be-star companion that will donate mass to the neutron star accretor, becoming one of the so-called Be X-ray binaries. These fascinating systems are thought to be the progenitors of double neutron star merger events: perhaps the greatest cosmic spectacles observed to date. Understanding their formation path is one of the main challenges of modern astrophysics, and observation of intermediate evolutionary stages is the way we hope to achieve it. Our discovery adds a new piece to the puzzle, yielding the first direct constraints on how mass transfer evolution proceeds in such massive star systems.
Paper link https://www.aanda.org/articles/aa/full_html/2023/06/aa46818-23/aa46818-23.html Press release by Heidelberg University: https://zah.uni-heidelberg.de/news/detail/light-supergiant-reveals-a-missing-evolutionary-stage