CHARA Array survey finds stars dim toward their edges more than models predict

Astronomers using Georgia State University's Center for High Angular Resolution Astronomy (CHARA) Array have found that nearby stars darken toward their edges more strongly at near-infrared wavelengths than several widely used models of stellar atmospheres predict. The result provides a new benchmark for understanding how stars are structured.

The research team measured how the brightness of 31 nearby stars changes from the center of each stellar disk to its edge. This effect, known as limb darkening, makes a star appear fainter at its edge than at its center. The observations show that limb darkening changes more strongly with wavelength in the near-infrared than the models predict.

Limb darkening arises because stars are not uniformly bright disks. Light from near the center of a stellar disk comes from deeper, hotter layers of the atmosphere, while light from the edge comes from shallower, cooler layers and passes through more stellar material.

"We are not just measuring how large these stars are," said lead author Narsireddy Anugu, a staff scientist at Georgia State University's CHARA Array. "We are measuring how their light is distributed across the stellar disk, which directly tests stellar-atmosphere models."

By observing the same stars simultaneously through two different near-infrared filters, the team measured how the center-to-edge fading changes with wavelength. As expected, the researchers found that limb darkening is weaker at longer wavelengths. However, the measured change was larger than several commonly used stellar-atmosphere models predict.

"This study demonstrates the powerful capabilities of our facility," said Gail Schaefer, director of the CHARA Array. "By combining light from telescopes across the mountaintop, we can image stars with enough detail to see what their surfaces actually look like."

Located on Mount Wilson in California, the CHARA Array combines light from six telescopes positioned at different sites across the observatory grounds. By interfering the light together, CHARA achieves the resolving power of a much larger telescope. Rather than capturing direct images of stars, CHARA measures how the contrast of the interference pattern changes with the spacing between telescopes. These measurements reveal both the size of a star and how the brightness changes across its surface.

The survey focused on 31 bright stars in late stages of their lives, whose outer layers have expanded outward. These evolved subgiant, giant and supergiant stars are especially valuable tests of stellar-atmosphere models because the large convective motion in their extended atmospheres can produce complex brightness profiles.

Limb darkening affects measurements of stellar diameters. It also affects how astronomers characterize exoplanets that transit across the face of their parent star, because the amount of light the planet blocks during a transit depends on the brightness across the stellar disk.

Across the sample, limb-darkening strength decreased by about 38 percent going from near-infrared wavelengths of 1.6 micron to 2.2 micron. The atmosphere models tested in the study predicted the same overall trend - weaker limb darkening at longer infrared wavelengths - but a smaller decrease of about 17 to 22 percent. The results show that current models capture the broad behavior but do not fully reproduce the wavelength dependence.

The study offers a new empirical benchmark for testing stellar-atmosphere models in the near infrared. The team also found no evidence of surface features such as large starspots, or of hidden companion stars, in the sample.

Future work will extend the comparison to broader wavelength coverage, from visible to near-infrared light. The team also plans to image smaller main-sequence stars, which are harder to resolve but are especially important for understanding transiting exoplanets around Sun-like stars.

CONTACT: Noelle Reetz, Georgia State University, [email protected], 404-431-1390