UR: Are black hole progenitors missing in metal-poor environments?

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Beryl Hovis-Afflerbach

Caltech

Beryl Hovis-Afflerbach is an undergraduate studying astrophysics at Caltech. She completed this research as part of the Carnegie Astrophysics Summer Student Internship (CASSI) Program at Carnegie Observatories under the supervision of Dr. Ylva Götberg and presented these results in the poster session at AAS237.

Gravitational waves (ripples in the fabric of spacetime) 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. But what if there is another reason – that lower-mass mergers are actually less common?

Radial stellar expansion, which occurs throughout a massive star’s lifetime, is thought to be crucial for a BBH merger. 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. If the envelope-stripping becomes unstable, the companion is engulfed in the envelope and the stars spiral so close that they ultimately coalesce in a gravitational wave event.

However, in low-metallicity environments, where there are few elements other than hydrogen and helium, expansion is predicted to occur very late in a star’s lifetime. This could directly impact future envelope stripping and eventual BBH coalescence, as well as leave a gap in the mass distribution of stripped stars. Using the stellar evolution code MESA and a binary stellar population synthesis code, we model the mass distribution of stripped stars in the Small Magellanic Cloud (see figure below), a nearby satellite galaxy with metallicity similar to that of environments in which BBHs are thought to form. We expect a drop in the number of stripped stars with masses above 7 M☉ due to the late expansion of their progenitors, creating a gap between binary-stripped stars and Wolf-Rayet stars (high mass stars which have been stripped by their own strong stellar winds).

If true, this could impact the black hole mass distribution as well – a distribution that will be revealed in the coming years.

Number of stripped stars for binned star masses from 0 to 19 solar masses. Where there is no late expansion, there are 124 stripped stars between 7 and 22 solar masses. Where there is late expansion, this number drops to 63.
Modeled mass distribution of stripped stars in the Small Magellanic Cloud, on a logarithmic 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 above 7 M☉ is nearly half what we would expect otherwise (blue distribution). The purple distribution is the overlap between the pink and blue distributions. Image Credit

Astrobite edited by: Haley Wahl

Featured image credit: Northwestern Visualization / Carl Rodriguez

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