Probing binary stars in the Small Magellanic Cloud with the JWST

The formation and evolution of individual stars in a galaxy shape the evolution of the galaxy itself. Stars and stellar remnants make up the vast majority of a galaxy’s mass, so it’s not surprising that the stellar population affects the evolution of galaxies. In the Milky Way’s disk and bulge, stars and dead stars make up about 85% of the baryonic mass.

Astronomers work with a galaxy’s initial mass function (IMF), which is a probability distribution of stars of different masses formed within a galaxy, to understand galaxies themselves. They want to understand how universal it is from galaxy to galaxy.

The IMF is important because when a star forms, its initial mass determines its lifetime trajectory. So the IMF is an important link between stellar physics and the evolution of galaxies. But understanding the IMF requires observing lots of individual stars in a galaxy.

To make progress on understanding the IMF, astronomers need to be able to accurately observe individual stars in other galaxies—not just the brightest or easiest to see, but a representative sample of the entire stellar population, including the dim ones. And that’s not easy. Astronomers also need a way to determine which stars are actually binary stars.

From this perspective, the best targets are the closest galaxies, and our best space telescope, the JWST, is the best tool for observing them. This is why the Small Magellanic Cloud (SMC) is such an important observing target.

New research in Astronomy & Astrophysics tackles the problem of discerning binary stars in the SMC. Titled “The Small Magellanic Cloud through the lens of the James Webb Space Telescope: Binaries and the mass function in the galaxy’s outskirts,” the lead author is Maria Legnardi, Ph.D. student at the University of Padua in Italy.

The Small Magellanic Cloud is a desirable target, but not just because it is close, only about 200,000 light-years (1.2 quintillion miles) away. It also has low metallicity, which creates an opportunity to test how a galaxy’s actual IMF compares to what simulations have shown us. The SMC also has diffuse star-forming regions, making it harder to confuse metallicity with density, which can cloud our understanding of the IMF.

“The stellar initial mass function (IMF) and the fraction of binary systems are fundamental ingredients that govern the formation and evolution of galaxies,” the authors write. One of the big questions in astrophysics is whether the IMF is universal or different from environment to environment. If it is different, what factors govern it?

“Dwarf galaxies such as the Small Magellanic Cloud (SMC), with their low metallicity and diffuse star-forming regions, offer critical laboratories to address this issue,” the authors explain.

“Its proximity, low foreground extinction, and relatively low crowding compared to more massive galaxies enable us to resolve individual stars across a broad range of stellar masses, from low-mass main sequence (MS) stars to evolved giants.”

The IMF and the fraction of binary stars in a galaxy influence how stellar systems evolve and how many supernovae explode. They also affect chemical enrichment and how many exotic objects like blue stragglers, X-ray binaries and gravitational-wave sources form. “Despite their importance, both the IMF and the binary fraction remain poorly constrained in external galaxies, particularly in the field populations of low-mass systems like the SMC,” the authors write.

The researchers used the JWST’s deep photometry of stars in the SMC to determine the fraction of unresolved binary systems.

Unresolved binaries can pollute the color-magnitude diagram (CMD) because they can look like single stars when both stars have similar masses. That, in turn, leads to a false understanding of the IMF. The JWST can’t resolve individual stars in a binary, but that information can be extracted from the JWST’s color-magnitude diagrams (CMDs).

The results show that about 14% of stars in the SMC field have a binary companion at least 60% as massive as themselves. This is a lower limit, but that’s the limitation of this method.

What’s really important is how this result compares with binary-fraction measurements in other galaxies. The 14% is statistically equivalent to what’s found in Milky Way open clusters of similar age and metallicity. It’s also similar to the binary fraction in the Milky Way’s field stars.

So despite the fact that the SMC’s metallicity is five times lower than the Milky Way’s, and despite the fact that the SMC’s stellar density is much lower and its stars have a different formation history, the binary fractions of the SMC and the Milky Way are the same.

“The binary fraction measured for the SMC field in this work is comparable to that observed in galactic open clusters of similar ages,” the authors write. “Furthermore, our result for the SMC field is consistent with the total binary fraction measured for Milky Way field stars of similar primary masses.”

These results agree with a general trend in binary fractions. They tend to be higher in lower-density environments because there are fewer dynamical stellar interactions that could separate binary pairs. Conversely, in globular clusters where stellar density is higher, binary disruptions are more frequent, generating lower binary fractions.

“This comparison suggests that the binary star formation and evolutionary processes in the low-density environment of the SMC resemble those in Galactic OCs and field populations, supporting the notion that environmental density and dynamical interactions play a key role in shaping binary fractions across different stellar systems,” the authors explain.

The results also indicate how environmental factors can shape the IMF and pose a challenge to the IMF’s universality.

“This suggests that environmental conditions such as metallicity, temperature, and gas density influence the fragmentation of molecular clouds and the resulting stellar mass distribution,” the authors write.

“Such variations challenge the universality of the IMF and have important consequences for the chemical evolution, feedback, and dynamical history of dwarf galaxies like the SMC,” the researchers conclude.

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Sadie Harley

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Andrew Zinin

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