🔭 astrophysics

A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: II. Diversity in C/O Ratios among Host Stars

This study presents high-resolution spectroscopic measurements of 16 elements for five directly imaged companion host stars, revealing diverse stellar C/O ratios and super-solar carbon and oxygen abundances that suggest volatile-rich protoplanetary disks, while finding no statistically significant difference in C/O distributions between these hosts and other planet-hosting stars.

Original authors: Aneesh Baburaj, Quinn M. Konopacky, Christopher A. Theissen, Roman Gerasimov, Kielan K. W. Hoch

Published 2026-06-24

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Aneesh Baburaj, Quinn M. Konopacky, Christopher A. Theissen, Roman Gerasimov, Kielan K. W. Hoch

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 you are trying to solve a mystery: How did these giant planets form?

For decades, astronomers have been finding massive planets far away from their stars, a place where the standard "recipe books" for planet formation say they shouldn't exist. To figure out how they got there, scientists look at the planets' atmospheres. Specifically, they look at the ratio of Carbon to Oxygen (C/O). Think of this ratio like a chemical fingerprint.

If a planet has the same fingerprint as its star, it likely formed by collapsing directly from the gas cloud (like a mini-star).
If a planet has a different fingerprint (more carbon or oxygen than the star), it likely formed by building up a rocky core first and then sucking up gas (like our own Jupiter).

The Problem:
To read the planet's fingerprint accurately, you need to know the star's fingerprint perfectly. But many of these stars are young, spinning very fast, and covered in "blurry" spectral lines, making them hard to read. It's like trying to read a book written in a language you don't know, while someone is shaking the book violently.

The Solution:
This paper is the second chapter in a detective story led by Aneesh Baburaj and colleagues. They used a powerful new telescope instrument called GHOST (on the Gemini South telescope) to take incredibly sharp, high-resolution "photos" (spectra) of five star systems hosting directly imaged planets. They treated the star's light like a complex puzzle, using two different methods to solve it:

Spectral Fitting: Matching the star's light against a library of computer-generated models.
Equivalent Widths: Measuring the "depth" of specific dark lines in the star's light, which act like barcodes for different elements.

What They Found:
They measured the chemical makeup of 16 different elements (including Carbon, Oxygen, Sodium, Iron, etc.) for five stars: HR 2562, AB Pic, PZ Tel, β Pic, and YSES 1.

Here is the breakdown of their discoveries:

The "Normal" Stars: Three stars (HR 2562, AB Pic, and YSES 1) have Carbon-to-Oxygen ratios that are solar (just like our Sun). This means their planets likely formed in a way that kept the chemical balance similar to the star.
The "Odd" Stars: Two stars (PZ Tel and β Pic) have sub-solar C/O ratios. This means they have significantly less Carbon relative to Oxygen than the Sun. This is a big deal because it changes the "target" for what their planets' atmospheres should look like.
The Population Trend: When they combined these new stars with five from their previous study, they found something interesting: The group of stars hosting these giant planets seems to have extra Carbon and Oxygen compared to the average star in the universe.
- The Analogy: Imagine a neighborhood where most houses have a standard amount of flour and sugar. These specific planet-hosting stars are like a bakery that bought a bulk sale of flour and sugar. This suggests that the "dough" (the protoplanetary disk) around these stars was extra rich in volatile ingredients (ices and gases), which might have helped giant planets form more easily.

The Big Picture:
The team compared the chemical fingerprints of the stars to the fingerprints of their planetary companions.

In some cases, the planet's C/O ratio was higher than the star's. This suggests the planet formed far out in the cold disk, gathering icy pebbles before becoming a gas giant (a process called core accretion).
In other cases, the ratios matched, suggesting a different formation story.

The Conclusion:
This paper proves that to understand how giant planets are born, we must first be able to read the chemistry of their parents (the stars) with extreme precision. The authors successfully measured these "parent" fingerprints for five difficult-to-study stars, revealing that the environment around these stars was likely rich in the building blocks of planets. They also found that while these stars and their planets have interesting chemical differences, there isn't yet enough evidence to say that all planet-hosting stars are chemically different from other stars in the galaxy.

In short: We finally have a clear recipe for the "parents" of these giant planets, and it turns out they were cooking in a kitchen that was extra well-stocked with carbon and oxygen.

Technical Summary: A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: II. Diversity in C/O Ratios among Host Stars

Problem Statement
The era of JWST has enabled high-precision measurements (∼0.1 dex) of elemental abundances (C, O, Na, S, K, Fe, etc.) in exoplanet atmospheres. These measurements serve as tracers for planet formation mechanisms (e.g., core accretion vs. gravitational instability) and formation locations relative to snowlines. However, accurate inference of these formation pathways requires corresponding, high-precision abundance measurements for the host stars. Direct imaging surveys typically target young, high-mass stars ( $M_* > 1.2 M_\odot$ ) with high rotational velocities ( $v \sin i > 30 \text{ km s}^{-1}$ ), which cause extensive spectral line blending and make accurate abundance determination difficult. Furthermore, existing literature often lacks uncertainty estimates or the full suite of elements necessary for modern formation studies. This work addresses the need for a uniformly analyzed sample of host stars with precise abundances of 16 elements to constrain planet formation models.

Methodology
This study presents the second set of results from a high-resolution spectroscopic survey, expanding the sample to five southern hemisphere targets: HR 2562, AB Pic, PZ Tel, $\beta$ Pic, and YSES 1.

Observations: Targets were observed using the Gemini High-resolution Optical SpecTrograph (GHOST) at Gemini South in high-resolution mode ( $R \sim 76,000$ ) across blue (347–542 nm) and red (520–1060 nm) arms. Total integration times were optimized to achieve Signal-to-Noise Ratios (SNR) $\ge 200$ per pixel.
Data Reduction: The DRAGONS pipeline was used for reduction, including bias correction, flat-fielding, and wavelength calibration using arc lamps. Spectrophotometric flux calibration and telluric correction were not applied, as telluric lines were utilized for precise wavelength solutions, and order-by-order continuum normalization was performed for abundance analysis.
Stellar Parameter Determination: Effective temperature ( $T_{\text{eff}}$ ), surface gravity ( $\log g$ ), and metallicity ([M/H]) were derived using a combination of single-order and multi-order Markov Chain Monte Carlo (MCMC) fits. The analysis utilized spectral regions sensitive to specific parameters (e.g., H $\alpha$ for $T_{\text{eff}}$ , Mg I b triplet for $\log g$ ) and adopted a noise estimate of 2% of the median flux to account for model systematics.
Abundance Analysis:
- Spectral Fitting: Carbon and oxygen abundances were determined using custom synthetic grids generated with PySME (SME) and MARCS (or ATLAS9 for $\beta$ Pic) atmospheric models. Non-Local Thermodynamic Equilibrium (NLTE) corrections were applied to oxygen triplet lines.
- Equivalent Width (EW): Abundances for 16 elements (C, O, Na, Mg, Si, S, K, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y) were measured using the MOOG code. For blended lines, the 'blends' driver was used; for unblended lines, Gaussian fitting followed by trapezoidal integration was employed.
- Uncertainty: Final uncertainties combined line-to-line scatter, and errors in $T_{\text{eff}}$ and $\log g$ .

Key Contributions

New Measurements: This work provides the first detailed elemental abundance measurements for HR 2562, PZ Tel, and YSES 1, and the first oxygen abundance measurement for $\beta$ Pic.
Uniform Analysis: It updates previous measurements for AB Pic and $\beta$ Pic using the same rigorous methodology as the first paper in this series (Baburaj et al. 2025), creating a uniformly analyzed sample of 10 directly imaged companion host stars.
Methodological Refinement: The study refines the MCMC fitting procedure to be more computationally efficient while maintaining precision, specifically for stars with high rotational velocities.

Results

Stellar Parameters: The study derived atmospheric parameters for all five targets with precisions of $\sim 0.1$ –$0.2$ dex.
C/O Ratios:
- Solar C/O: HR 2562 ( $0.58 \pm 0.09$ ), AB Pic ( $0.50 \pm 0.14$ ), and YSES 1 ( $0.45 \pm 0.05$ ) exhibit solar C/O ratios.
- Sub-solar C/O: PZ Tel ( $0.28 \pm 0.05$ ) and $\beta$ Pic ( $0.22 \pm 0.06$ ) show sub-solar C/O ratios with $4\sigma$ significance.
Elemental Abundances:
- HR 2562: Shows super-solar abundances for all measured elements, consistent with its super-solar metallicity.
- $\beta$ Pic: Exhibits a peculiar abundance pattern with sub-solar C, Mg, Si, Sc, Cr, Mn, Fe, and Ni, but solar/super-solar Na, S, K, Ca, Ti, Zn, and Y.
- Population Level: Combining this work with Paper I, the population average for the 10 directly imaged host stars shows super-solar carbon and oxygen, while other elements remain solar.
Stellar vs. Companion C/O:
- Several companions (e.g., YSES 1 b/c, GJ 504 b) display super-stellar C/O ratios, suggesting formation via core/pebble accretion beyond the water snowline.
- $\beta$ Pic b shows a super-stellar C/O ratio. However, due to the host star's peculiar chemical profile, the authors suggest using the abundance of the co-moving star HD 181327 as a proxy, which would imply a sub-stellar C/O for the planet, consistent with accretion of oxygen-rich ices.
- The Kolmogorov-Smirnov test indicates insufficient evidence to statistically differentiate the C/O distributions of directly imaged host stars from other planet-host populations.

Significance
The paper emphasizes that the detection of sub-solar C/O ratios in PZ Tel and $\beta$ Pic at $4\sigma$ significance highlights the critical importance of accurate stellar C/O estimates for constraining planet formation scenarios. The finding of super-solar carbon and oxygen at the population level suggests that protoplanetary disks around these stars were potentially rich in volatiles. By providing a uniformly analyzed dataset with high-precision abundances, this work enables more robust comparisons between stellar and planetary compositions, thereby refining the constraints on whether giant planets form via gravitational instability or core/pebble accretion. The authors modestly note that while trends are observed, larger samples and measurements of nitrogen abundances are required to fully investigate the implications of volatile-rich disks.

More like this