Taking the temperature of the stars

Astronomers study stars using spectroscopy, which allows them to analyze the light they emit in all colors. A team led by Étienne Artigau, a researcher at the Trottier Institute for Research on Exoplanets (iREx), has developed a method that allows them to extract the variation in a star’s temperature from its spectrum, to the nearest tenth of a degree Celsius, over various time scales.

“By monitoring the temperature of stars, we can learn a lot about them: their rotation period, their stellar activity, their magnetic field. This intimate knowledge of stars is also essential for finding and studying their planets,” explains the researcher.

In an article that will appear soon in theAstronomical Journalthe effectiveness and great versatility of the technique are demonstrated through observations of four very different stars made with the Canada-France-Hawaii telescopes and the 3.6 m La Silla telescope.

Know the stars to know their planets

The team first looked at the spectra of stars to improve the detection of exoplanets using the velocimetry method. This method consists of measuring the slight oscillation of a star caused by the gravitational attraction of a planet orbiting this star. The more we can measure small variations in the star’s speed, the more we can detect low-mass planets. Étienne Artigau and his team have developed a velocimetry technique that consists of using the entire spectrum of the star, and not just a few portions, as was customary, in order to detect planets as low in mass as the Earth around small stars.

Inspired by the success achieved with this technique, the researcher had the idea of ​​using a similar strategy to determine not the variations in the speed of stars, but rather those in their temperature.

This measurement is just as crucial for the study of exoplanets, which are mostly observed indirectly, by closely following their star. In recent years, astronomers have come up against the difficulty of distinguishing in their observations what is the star from what is its planets. This is a problem both for discovering exoplanets with the velocimetry method and for learning more about their atmosphere with the transit spectroscopy method.

“It is very difficult to confirm the presence of an exoplanet or to study its atmosphere without knowing precisely the properties of the host star and their variability over time. This new technique offers us an invaluable tool to ensure that the knowledge we acquire on exoplanets is solid and to go further in our characterization of the latter,” says Charles Cadieux, a doctoral student at iREx who contributed to the study.

Unparalleled precision

The surface temperature of stars is a fundamental property that astronomers are keen to measure, because it allows them to deduce the luminosity of the stars and their chemical composition. In the best case, the exact temperature of a star can be known with an accuracy of about 20 °C.

With this new technique, we are not interested in the exact temperatures, but in their variations in time. And we can measure them with remarkable precision.

“We don’t know if the star is at 5000 or 5020°C, but we can know if its temperature has increased or decreased by one degree or even less! No one has ever managed to do that. Determining this kind of temperature change is already quite a challenge for the human body, so imagine for a ball of gas at thousands of degrees that is dozens of light years away!” enthuses Étienne Artigau.

A new, effective and versatile technique

To demonstrate that their technique works, the astronomers used observations made with the SPIRou spectrograph (Canada-France-Hawaii telescope) and the HARPS spectrograph (3.6-metre telescope of the European Southern Observatory).

In the data obtained by these two telescopes for four small stars in the solar neighborhood, the team can clearly see temperature changes, which they attribute sometimes to the rotation of the stars, sometimes to what is happening on their surface or in the surroundings.

The new technique allows large temperature variations to be measured. For the star AU Microscopii, known to have very high stellar activity, the team recorded variations of almost 40 °C.

This technique makes it possible to detect both very rapid changes, such as those caused by the rotation on themselves in a few days of AU Microscopii or Epsilon Eridani, and those that occur on a much longer scale, a feat that is very difficult to achieve with ground-based telescopes.

“We can measure changes of a few degrees or less that occur over very long periods, for example those associated with the rotation of Barnard’s Star, a very calm star that rotates on itself every five months,” says Étienne Artigau. “To measure this variation, which is subtle and very slow, we had to use Hubble at the time!”

About this study

The article “Measuring Sub-Kelvin Variations in Stellar Temperature with High-Resolution Spectroscopy” will be published shortly in theAstronomical JournalIn addition to Étienne Artigau and Charles Cadieux, the team includes researchers Neil J. Cook and Romain Allart, Professor René Doyon, master’s student Laurie Dauplaise and intern Maya Cadieux, from iREx, as well as seven co-authors from the United States and France.