A rare molecule, chromium hydride, has been discovered in the atmosphere of the hot Jupiter WASP-31b.
An extraordinary molecule, chromium hydride (CrH), rare and highly responsive to temperature fluctuations, has been dubbed the “stellar thermometer.” Renowned astronomer, Dr. Laura Flagg, suggests its concentration peaks between a specific range of 1,200-2,000 degrees Kelvin, which becomes pivotal when exploring the temperatures of cool stars, brown dwarfs, and potentially hot Jupiter exoplanets.
Dr. Flagg, an esteemed research associate in astronomy at the College of Arts and Sciences (A&S), recently made a remarkable discovery with her research team from Cornell. They have definitively detected the presence of chromium hydride in the atmosphere of the hot Jupiter WASP-31b. This discovery, detailed in the paper titled “ExoGemS Detection of a Metal Hydride in an Exoplanet Atmosphere at High Spectral Resolution,” published in ApJ Letters on August 16, is monumental. Notably, it’s the first-ever detection of a metal hydride using a high-resolution exoplanet spectrum.
Understanding the significance of this, Dr. Flagg explains, “Chromium hydride molecules exhibit a unique sensitivity to temperature. At higher temperatures, only isolated chromium is observable, whereas cooler temperatures result in different compositions. The sweet spot where chromium hydride is abundant lies between 1,200 and 2,200 Kelvin.”
In our solar system
Sunspots are the only places in our solar system where this molecule has been identified. Given the sun’s surface temperature of approximately 6,000 K, this makes sense, while other celestial objects are cooler than the molecule’s detectable range.
Delving deep into her innovative research methods, Dr. Flagg employs high-resolution spectroscopy to discern exoplanet atmospheres. This involves a detailed comparison of system light in different positions of the planet relative to its star. By recognizing which elements obstruct more light at specific wavelengths, researchers can unveil the planet’s elemental composition.
High spectral resolution
Utilizing high spectral resolution offers incredibly precise wavelength data. “With this, thousands of distinct lines emerge. Through statistical methods and a template—a basic spectrum representation—we juxtapose this against our data,” Flagg elaborates. In the context of WASP-31b, the chromium hydride template was perfectly aligned with the observed data.
To investigate WASP-31b, the team utilized high-resolution spectra from an observation in March 2022, leveraging Hawaii’s Maunakea’s Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES). This data was enhanced with archives from 2017, despite its original purpose being unrelated to metal hydrides.
Flagg’s anticipations are high: “I urge other researchers to seek chromium hydride and similar metal hydrides in their data.” With this paper, she hopes to spark more investigations, possibly unveiling other exoplanets bearing this fascinating molecular thermometer.
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