In the vast cosmos, stars often evolve in pairs called binary systems. In these systems, one star can strip the other of its hydrogen-rich outer layers. This process creates "stripped stars," which are compact and shine brightly, contributing to supernovae and ionizing radiation. This radiation is crucial in distant, metal-poor galaxies, potentially explaining their unique features. However, scientists are just beginning to understand how common stripped stars are and how their properties change with metallicity—the abundance of elements heavier than hydrogen and helium in a star.

Building the Models

To study stripped stars, the authors simulated star populations using detailed computer models. They relied on data from single and binary star evolution and tested different metallicity levels. Metallicity significantly affects a star’s behavior, with low-metallicity stars tending to be hotter and having weaker stellar winds. These winds often play a critical role in removing outer layers, and without them, stars might not become stripped as expected.

Findings: Mass and Metallicity Matter

The study revealed that most stripped stars have masses below five times that of the Sun, and this distribution is largely unaffected by metallicity. However, for more massive stars, the story changes. At low metallicity, massive stars don't expand as much, making it harder for binary interactions or winds to strip them effectively. This results in a "helium-star desert," a scarcity of massive, stripped stars, particularly in environments with very low metallicity.

Galactic Implications

The authors estimated the number of stripped stars in galaxies like the Milky Way and its neighbors, the Large and Small Magellanic Clouds. The Milky Way, for example, likely has tens of thousands of stripped stars, while the Magellanic Clouds host fewer due to their lower star formation rates. These stars also influence the rate of supernova explosions, as those with masses above 2.6 times that of the Sun are expected to end their lives in dramatic explosions.

The High-Redshift Connection

The findings have implications for early galaxies, where metallicity was much lower than it is today. Stripped stars contribute to the intense radiation in these galaxies, potentially aiding in cosmic reionization, the process that made the universe transparent. However, the helium-star desert suggests that such contributions might be limited for massive stars in these metal-poor environments.

What’s Next?

The paper emphasizes the need for further observations to confirm the helium-star desert and better understand binary star interactions. Future missions and telescopes will help uncover more stripped stars and clarify their role in galaxy evolution.

Source: Hovis-Afflerbach