By measuring a star’s temperature, we can learn important details about its rotation, activity, and magnetic field, which helps us find and study its orbiting planets. A team led by Étienne Artigau from Université de Montréal has developed a technique that uses a star’s spectrum to detect temperature variations to the nearest tenth of a degree Celsius over different time scales.
They applied this method to four diverse stars observed with the European Southern Observatory’s 3.6-m telescope in La Silla, Chile.
Scientists studied stellar spectra to enhance exoplanet detection using the radial velocity method, which identifies tiny star movements caused by orbiting planets. Larger movements indicate more giant planets, but small movements can be hard to detect, especially for low-mass planets. To address this, Artigau and his team developed a technique that analyzes a star’s full spectrum instead of just parts.
This technique enables the identification of Earth-sized planets around smaller stars and detects changes in a star’s temperature. Measuring temperature is crucial in the search for exoplanets, as we mainly observe them by studying their stars.
However, astronomers face a significant challenge in distinguishing between a star’s effects and those of its planets. This issue impacts the radial velocity method for finding exoplanets and analyzing their atmospheres through transit spectroscopy.
Charles Cadieux, a doctoral student at IREx who contributed to the study, said, “It’s very difficult to confirm the existence of an exoplanet or to study its atmosphere without precise knowledge of the host star’s properties and how they vary over time.”
“This new technique gives us an invaluable tool for ensuring that our knowledge of exoplanets is solid and for advancing our characterization of their properties.”
Credit: NASA, ESA, Joseph Olmsted (STScI)
A star’s surface temperature can be used to determine the star’s luminosity and chemical composition. At best, a star’s exact temperature can be known to an accuracy of about 20°C.
The new technique measures not just exact temperatures but also how temperatures change over time, and it does this with impressive accuracy.
Artigau said, “We can’t tell whether a star is 5,000°C or 5,020°C, but we can determine if it has increased or decreased by a degree, even a fraction of a degree—no one’s ever done this before.”
“It’s a challenge to detect such minute temperature changes in the human body, so imagine what it’s like for a gaseous ball with a temperature in the thousands located dozens of light-years away.”
Scientists tested their new technique using observations from the SPIRou spectrograph at the Canada-France-Hawaii Telescope and the HARPS spectrograph at ESO’s 3.6-m telescope. They observed four small stars and detected clear temperature variations linked to the stars’ rotation or surface events.
The technique allowed them to measure significant temperature changes, including nearly 40°C for AU Microscopii, a star with high activity. They could capture both rapid temperature changes tied to short rotation periods, like those of AU Microscopii and Epsilon Eridani, and longer-term changes, which is challenging for ground-based telescopes.
Artigau said, “We were able to measure changes of a few degrees or less occurring over very long periods, such as those associated with the rotation of Barnard’s star, a very quiet star that takes five months to complete a full rotation. Before, we would have had to use the Hubble Space Telescope to measure such a subtle and slow variation.”
The new technique also allowed researchers to spot very slight temperature changes on the surfaces of stars. For instance, they observed subtle temperature shifts in star HD 189733 that matched the orbit of its exoplanet, HD 189733 b, a giant hot Jupiter.
Scientists noted, “The technique works not only with SPIRou and HARPS but with any spectrograph operating in the visible or infrared range.”
“The innovative technique will be directly applicable to observations from NIRPS, a spectrograph installed last year in the ESO telescope in Chili. According to the researchers, it would also be possible to use this technique with space-based instruments, such as the James Webb Space Telescope.”
Artigau said, “The power and versatility of this technique means we can exploit existing data from numerous observatories to detect variations that were previously far too small to be perceived, even on very long timescales.”
“This opens up new horizons in our study of the stars, their activity, and their planets.”
Journal Reference:
- Étienne Artigau et al, Measuring Sub-Kelvin Variations in Stellar Temperature with High-Resolution Spectroscopy, arXiv (2024). DOI: 10.48550/arxiv.2409.07260