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<!DOCTYPE html>
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<title>Binary Stars - Beryl Hovis-Afflerbach</title>
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<h1>Stripped Stars and Binary Stellar Evolution</h1>
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<h2>High-mass stars stripped in binaries missing at low metallicity: tests of stellar astrophysics and gravitational wave progenitors</h2>
<p><i>Research conducted with Dr. Ylva Götberg at Carnegie Observatories.</i></p>
<a class='button' href='http://aas237-aas.ipostersessions.com/Default.aspx?s=ED-DE-4E-48-ED-82-A2-7E-3F-78-F3-09-0C-62-29-C9' style='margin: 12px;'>View poster</a>
<figure style='width: 33%; float: right; margin-top: 0; margin-right: 0;'>
<img src='assets/images/binary_stripping.png' style='width: 100%'>
<figcaption><b>Figure 1:</b> Creation of a stripped star in a binary. The massive star swells, overfilling its Roche lobe, and its outer material is stripped by its binary companion, leaving behind a hot, bright helium core - the stripped star.</figcaption>
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<p>Gravitational waves from binary black hole (BBH) mergers have almost only been detected at the very high mass end, likely because lower mass mergers have weaker signals. <b>But what if there is another reason - mergers within a certain mass range are actually less common?</b></p>
<p>Radial expansion of a star in a binary is thought to be crucial for the creation of a black hole binary that merges in a gravitational wave event. During expansion, the outer layers of a star can be stripped off by a binary companion star, leaving the hot helium core exposed - a stripped star (Figure 1). If the envelope-stripping is unstable, the companion is engulfed in the envelope and the stars spiral so close to each other that a gravitational wave merger can occur.</p>
<p>However, in low-metallicity environments such as the Small Magellanic Cloud (SMC), expansion is predicted to occur very late in a star’s lifetime, which directly impacts the future envelope-stripping and its outcome. Since stripped stars are also a product of the radial expansion of one star in a binary, <b>we predict that late expansion should result in noticeable features in the mass distribution of stripped stars - something that could be testable in the nearby SMC.</b></p>
<p>Using the stellar evolution code MESA and a binary stellar population synthesis code, we model the mass distribution of stripped stars in the SMC (Figure 2). We expect a drop in the number of stripped stars with masses above 7 M<sub>☉</sub> due to the late expansion of their progenitors.</p>
<p>If the lack of stripped stars we predict does affect the creation of black holes, this should leave a feature in the low-mass end of the black hole mass distribution as well - a distribution that will be revealed in the coming years.</p>
<figure style='max-width: 960px'>
<img src='assets/images/binary_mdist.png' style='width: 100%'>
<figcaption><b>Figure 2:</b> Modeled mass distribution of stripped stars in the SMC, on a log scale (left) and a linear scale (right). If, as expected, massive stars expand late at low metallicity (pink distribution), the number of stripped stars we expect to observe between 7-22 M<sub>☉</sub> is nearly half what we would expect otherwise (blue distribution).</figcaption>
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<a href='https://users.obs.carnegiescience.edu/ygoetberg/'><i>Header image: Ylva Götberg</i></a>
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