Tracing Planet Formation Through Stellar Fingerprints: A Spectroscopic Look at C/O Ratios in Directly Imaged Exoplanet Hosts

In their paper “A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: II. Diversity in C/O Ratios among Host Stars,” Aneesh Baburaj and collaborators present the second phase of a major survey investigating the chemical make-up of stars that host directly imaged exoplanets. Using new high-resolution observations, the authors explore how the elemental ratios of carbon and oxygen, key ingredients for life and planet formation, vary among five nearby stars. Their work aims to link stellar chemistry with the origins of planetary systems in the era of precision spectroscopy enabled by instruments like JWST.

Understanding Why Stellar Chemistry Matters

The study begins by setting the context for why carbon-to-oxygen (C/O) ratios are crucial. The C/O ratio in a planet’s atmosphere can hint at where and how that planet formed within its protoplanetary disk. For instance, planets forming closer to their stars may show different chemical signatures than those forming farther out beyond the “water snowline.” However, to use this ratio as a reliable tracer, astronomers must also know the same quantities in the planet’s host star. Measuring these abundances is challenging, especially for young, fast-spinning stars that blur spectral lines. Building on earlier work (Paper I), Baburaj et al. aim to fill this gap by analyzing stellar compositions for five southern-sky stars that host directly imaged companions: HR 2562, AB Pic, PZ Tel, β Pictoris, and YSES 1.

The Observations Behind the Study

All five stars were observed with the Gemini South telescope’s GHOST spectrograph, capable of spreading starlight into high-resolution spectra across visible wavelengths. The team used exposure times tailored to achieve high signal-to-noise, ensuring precise measurements of faint spectral lines. Data were reduced using the Gemini DRAGONS pipeline, which corrects for instrumental effects and calibrates wavelengths. Rather than relying on overall flux, the spectra were normalized order by order, focusing on relative line strengths that reveal elemental abundances.

Analyzing the Starlight

To extract chemical information, the authors used two complementary techniques. The spectral fitting method compares observed spectra to grids of synthetic models, adjusting parameters like temperature, gravity, and metallicity until the best match is found. The equivalent width method measures the total area of absorption lines to infer how much of an element is present. Both methods were applied to carbon and oxygen lines, with further elements (like Na, Mg, Si, S, and Fe) analyzed via equivalent widths. Special care was taken to correct for non-local thermodynamic effects that can bias oxygen measurements.

What the Researchers Found

The study reveals a striking diversity in C/O ratios among these stars. HR 2562, AB Pic, and YSES 1 exhibit solar-like C/O ratios (around 0.5–0.6), while PZ Tel and β Pictoris show significantly sub-solar values (0.22–0.28). These differences are statistically robust, with β Pic and PZ Tel standing out at over four standard deviations below solar values. When combining these results with their earlier sample, the authors find that this group of directly imaged companion hosts tends to have slightly super-solar carbon and oxygen abundances overall, suggesting that their birth disks may have been rich in volatile elements.

Comparing to Other Stellar Populations

To place their findings in a broader context, Baburaj et al. compare their sample’s chemistry with that of other known planet-hosting stars using statistical tests. They find no strong evidence that directly imaged host stars form a chemically distinct population. However, when comparing individual stars to their planets’ atmospheres, several companions exhibit super-stellar C/O ratios, supporting the idea that these planets formed by gas accretion beyond the water snowline, consistent with “planet-like” rather than “brown dwarf–like” formation pathways.

Implications and Future Directions

By precisely measuring stellar compositions, this work provides the necessary baseline for interpreting exoplanet atmospheric data from JWST and future telescopes. The finding that some host stars show sub-solar C/O while their planets exhibit enhanced values strengthens the case that planet formation leaves detectable chemical fingerprints. As Baburaj and colleagues continue their survey, expanding it to more targets will help clarify how universal these trends are and how closely the chemistry of a planet reflects that of its parent star.

Source: Baburaj