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Diameters and Temperatures VII: High-angular resolution measurements of Solar-type stars with the CHARA Array

This paper presents high-precision interferometric measurements of angular diameters for 27 nearby solar-type stars using the CHARA Array, enabling model-independent determinations of their radii, effective temperatures, and luminosities to provide empirical benchmarks for testing stellar evolutionary theories and refining exoplanet host star characterizations.

Original authors: Tabetha S. Boyajian, Ashley A. Elliott, Emelly D. Tiburcio, Kaspar von Braun, Gail Schaefer, Timothy R. White, Daniel Huber, Michael Ireland, Jeremy Jones, Tyler Ellis, Eliot Hiegel

Published 2026-08-13

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CC BY 4.0

Original authors: Tabetha S. Boyajian, Ashley A. Elliott, Emelly D. Tiburcio, Kaspar von Braun, Gail Schaefer, Timothy R. White, Daniel Huber, Michael Ireland, Jeremy Jones, Tyler Ellis, Eliot Hiegel

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the night sky as a giant, bustling city where every star is a unique building. For over a century, astronomers have been trying to measure the size of these buildings and figure out how old they are. But stars are incredibly far away, making them look like tiny, unblinking dots of light. To measure them, scientists use a clever trick called interferometry. Think of it like this: if you try to see the details of a distant lighthouse with just one eye, it's hard to tell how wide the beam is. But if you use two eyes (or in this case, two telescopes) spaced far apart, your brain can combine the images to see the shape much more clearly. This paper is about a team of astronomers who used a super-powered "stereo vision" system made of six telescopes to get a crystal-clear look at 27 stars that are our cosmic neighbors. They wanted to know exactly how big these stars are, how hot they feel, and how much light they give off, so they could test if our current theories about how stars grow up and age are actually correct.

The team, led by Tabetha S. Boyajian and colleagues, used a massive instrument called the CHARA Array, located on Mount Wilson. It's like a giant Y-shaped ruler stretching hundreds of meters across the mountain, connecting six telescopes to act as one giant eye. They pointed this super-eye at 27 stars that are similar to our Sun—some are just starting their lives, while others are middle-aged and slightly evolved. By measuring the tiny angle these stars take up in the sky and combining that with how bright they look from Earth and their precise distance (thanks to the Gaia space mission), the team calculated the stars' actual physical sizes and temperatures. They didn't just guess; they measured. The results are incredibly precise, with the team figuring out the stars' radii to within about 1% and their temperatures to within 1.5%. It's like measuring a basketball and knowing its size down to the width of a human hair.

Once they had these real-world measurements, the astronomers played a game of "match the model." They took their hard-earned data and compared it against four different computer simulations of how stars are supposed to evolve. These simulations are like different recipe books for baking a star, each with slightly different instructions on how ingredients mix and heat up. For most of the stars, the recipe books agreed pretty well on how heavy the stars are (their mass), usually within about 6% of each other. This suggests that for normal, Sun-like stars, our theories are on the right track. However, when it came to guessing the stars' ages, the recipe books started to disagree, sometimes wildly.

The paper highlights a few specific trouble spots where the models struggle. First, for the metal-poor stars (stars that are missing some of the heavy elements found in our Sun), the models often predicted ages that were older than the universe itself! This suggests the recipes are missing a key ingredient, likely related to how these stars handle different types of elements. Second, for very young stars just starting their lives, the models couldn't agree on their ages because they are growing so slowly that the computer simulations get confused. Finally, for stars that have started to evolve into subgiants (the "middle-aged" phase), the models worked great for one star but failed to distinguish between different evolutionary paths for another, showing that some stages of a star's life are just harder to predict than others.

Ultimately, this paper doesn't claim to have solved the mystery of stellar evolution, but it provides a set of "gold standard" measurements to test against. It shows that while our current models are good at telling us how heavy a star is, they still have a lot of work to do to accurately tell us how old it is, especially for stars that are metal-poor or in tricky stages of their lives. The authors suggest that future updates to these models, which include more complex chemistry, might fix these age discrepancies. For now, these 27 stars serve as a solid, measured foundation, proving that when we look closely enough, the universe reveals its secrets one tiny angle at a time.

Technical Summary: Diameters and Temperatures VII: High-Angular Resolution Measurements of Solar-Type Stars with the CHARA Array

Problem Statement
While interferometric measurements of stellar angular diameters have advanced stellar physics for over a century, there remains a critical need for precise, model-independent fundamental parameters for nearby solar-type stars. Indirect methods relying on photometric color-temperature relations or spectral modeling often suffer from systematic uncertainties. Furthermore, characterizing exoplanet host stars requires high-precision stellar radii, as errors in stellar radius propagate directly into estimates of planet radius and density. Existing surveys have largely focused on bright giants; there is a need for homogeneous, high-accuracy measurements of main-sequence and subgiant stars across a range of metallicities to serve as empirical benchmarks for testing stellar evolutionary theory and refining surface brightness-color relations.

Methodology
The authors present interferometric measurements of 27 nearby solar-type stars (primarily late-F and G types) observed between 2010 and 2018 using the Precision Astronomical Visible Observations (PAVO) beam combiner at the CHARA Array. The sample spans distances from 14 to 67 pc and includes stars with varying metallicities, several known exoplanet hosts, and evolutionary stages ranging from the zero-age main sequence (ZAMS) to mildly evolved subgiants.

The methodology proceeds as follows:

Observation and Calibration: Data were collected using baselines >278 meters to maximize resolution. Calibrator stars were selected as unresolved point sources with similar brightness and proximity to the science targets. Bad calibrators were screened out based on multiplicity and rapid rotation.
Angular Diameter Determination: Calibrated visibilities were fitted using the RADPy software to derive both uniform-disk ( $\theta_{UD}$ ) and limb-darkened ( $\theta_{LD}$ ) angular diameters. Limb-darkening coefficients were derived from PHOENIX model atmospheres based on R-band flux conservation. Uncertainties were computed via Monte Carlo resampling with bracket bootstrapping to account for both random noise and calibration variability.
Fundamental Parameter Derivation:
- Bolometric Flux ( $F_{bol}$ ): Determined by fitting PHOENIX model spectra to broadband photometry (Johnson, Tycho, 2MASS, TESS, Gaia) using the SEDFit framework. Interstellar extinction was fixed at zero due to the proximity of the targets.
- Radius and Temperature: Linear radii ( $R_*$ ) were calculated using the measured $\theta_{LD}$ and zero-point corrected Gaia parallaxes. Effective temperatures ( $T_{eff}$ ) were derived model-independently using the Stefan-Boltzmann law: $T_{eff} = 2341(F_{bol}/\theta_{LD}^2)^{0.25}$ .
- Luminosity: Calculated from $F_{bol}$ and the Gaia parallax.
Model Comparison: The derived empirical properties ( $T_{eff}$ , $L$ , [Fe/H]) were input into the Kiauhoku interpolation toolkit to estimate stellar masses and ages using four independent evolutionary model grids: YREC, MIST, Dartmouth, and Garstec.

Key Results

Precision Measurements: The study achieved typical uncertainties of $\sim1\%$ in stellar radius and $\sim1.5\%$ in effective temperature. These results provide a new set of empirical benchmarks for 27 solar-type stars.
Model-to-Model Systematics: Comparisons across the four evolutionary grids revealed:
- Mass: Inferred masses generally agree within $\sim6\%$ across grids for the majority of stars, particularly those more massive than $0.8 M_\odot$ . This convergence supports the use of 1-D models calibrated on solar analogs for mass estimation.
- Age: Inferred ages exhibit significantly larger dispersion. For stars more massive than $0.8 M_\odot$ , fractional age offsets decrease from $\sim35\%$ to $\sim10\%$ as stellar mass increases.
Specific Stellar Regimes:
- Metal-Poor Stars: Four metal-poor stars (HD 22879, HD 76932, HD 144579, HD 184499) yielded ages exceeding the age of the Universe in all grids. The authors attribute this to the models' inability to handle moderate metallicity reductions without accounting for $\alpha$ -element enhancement (which these stars possess) and differences in convective efficiency or diffusion treatments.
- Low-Mass and Young Stars: For stars near the ZAMS (e.g., HD 42807, HD 154345) or below $0.8 M_\odot$ , models showed high sensitivity to boundary conditions and edge effects, leading to large fractional age offsets (up to 85%) and convergence failures in some grids.
- Subgiants: Two subgiants (HD 190228 and HD 202568) showed divergent model behaviors. HD 202568, having reached the bottom of the giant branch, showed high mass and age offsets due to overlapping model tracks, whereas HD 190228, on the subgiant branch, showed excellent agreement.

Significance and Claims
The paper claims that these empirical measurements provide robust, model-independent anchors for stellar evolutionary theory. The results highlight that while current model grids are generally consistent for estimating masses of solar-type stars, they struggle with age determination, particularly for metal-poor, $\alpha$ -enhanced stars and those at specific evolutionary boundaries (ZAMS and giant branch).

The authors emphasize that their work:

Provides essential calibration points for surface brightness-color relations and mass-radius-temperature relations.
Directly improves the characterization of exoplanet systems by reducing uncertainties in host star radii and masses.
Identifies specific limitations in current evolutionary grids, specifically the need for grids that self-consistently include $\alpha$ -element enhancement and better treatments of boundary conditions.

The authors modestly conclude that while their results validate the general utility of current models for mass estimation, the discrepancies in age and metal-poor regimes necessitate future work with updated models (such as the $\alpha$ -enhanced MIST models of Dotter et al. 2026) to fully resolve these systematics.

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