Measuring the temperature of stars to the nearest 0.1°C

Astronomers study stars with spectroscopy, which allows us to analyze the light they emit in all colors. A team led by Étienne Artigau, researcher at the Trottier Institute for Research on Exoplanets (iREx), has developed a method which makes it possible to extract from the spectrum of a star the variation in its temperatureto the tenth of degree Celsius closely, on various time scales.

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

In an article which will soon appear in theAstronomical Journalthe effectiveness and great versatility of the technique are demonstrated thanks to 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 star spectra 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 identify low-mass planets. Étienne Artigau and his team have developed a velocimetry technique which consists of exploiting the entire spectrum of the star, and not just a few portions, as was usual, in order to be able to detect such low-mass planets. as the Earth around small stars.

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

This measurement turns out to be just as crucial for the study of exoplanets, which we observe most of the time indirectly, by closely following their star. In recent years, astronomers have encountered the difficulty of distinguishing in their observations what relates to the star and what relates to its planets. This proves to be 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 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 that we acquire on exoplanets is solid and to go further in our characterization of the latter”, indicates Charles Cadieux, doctoral student at iREx who contributed to the study.

Unrivaled precision

The surface temperature of stars is a property fundamental that astronomers want to measure, because it allows them to deduce the brightness of the latter and their chemical composition. In the best case, the exact temperature of a star can be known to 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 achieved this. Determining this kind of change in temperature is already quite a challenge for the human body, so imagine for a ball of gas at thousands of degrees that is tens of light years away! enthuses Étienne Artigau.

A new efficient 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 meter telescope of the European Southern Observatory).

In the data obtained by these two telescopes for four small stars in the solar neighborhood, the team is able to 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 makes it possible to measure large temperature variations. For the star AU Microscopii, known to have very high stellar activity, the team recorded variations of nearly 40°C. Thanks to this technique, we can identify both very rapid changes, such as those caused by the rotation on themselves in a few days of AU Microscopii or Epsilon Eridani, as well as those which occur on a much longer scale, a a very difficult feat to achieve with ground-based telescopes.

“We are able to measure changes of a few degrees or less which occur over very long periods, for example those associated with the rotation of Barnard’s star, a very calm star which rotates on its axis in five months, mentions Étienne Artigau To measure this variation, subtle and very slow, we had to resort to Hubble at the time!”