High-precision measurements challenge our understanding of Cepheids

“Classical Cepheids” are a type of pulsating star that rhythmically brightens and dims over time. These pulsations help astronomers measure vast distances across the universe, making Cepheids key “standard candles” to help us understand the size and scale of our universe.

Despite their importance, studying Cepheids is challenging. Their pulsations and potential interactions with companion stars create complex patterns that are difficult to measure precisely. Different instruments and methods used over the years have resulted in inconsistent data, complicating our understanding of these stars.

“Following Cepheid pulsations with high-resolution velocimetry gives us insight into the structure of these stars and how they evolve,” says EPFL astrophysicist Richard I. Anderson. “In particular, measurements of the rate at which stars expand and contract along the line of sight—so-called radial velocities—provide a crucial counterpart to accurate brightness measurements from space. However, there is an urgent need for high-quality radial velocities because they are expensive to collect and because few instruments can capture.”

VELOCE project

Anderson has now led a team of scientists to do just that with the VELOCities of CEpheids (VELOCE) project, a large collaboration that has collected more than 18,000 high-precision measurements of the radial velocities of 258 Cepheids using advanced spectrographs over 12 years between 2010 and 2022. Their research is published in a journal Astronomy and astrophysics.

“This data set will serve as an anchor to connect Cepheid observations from different telescopes over time and hopefully inspire further study by the community,” says Anderson.

VELOCE is a collaboration between EPFL, the University of Geneva and KU Leuven. It is based on the observations of the Swiss Euler telescope in Chile and the Flemish Mercator telescope in La Palma. Anderson started the VELOCE project during his Ph.D. at the University of Geneva, continued it as a postdoctoral fellow in the USA and Germany, and now completed it at EPFL. Anderson’s Ph.D. student Giordano Viviani was instrumental in making it possible to publish the VELOCE data.

Unravel the mysteries of Cepheids with superior precision

“The amazing precision and long-term stability of the measurements have enabled interesting new insights into how Cepheids pulsate,” says Viviani. “The pulsations lead to changes in visibility speed of up to 70 km/s, or about 250,000 km/h. We measured these changes with a typical accuracy of 130 km/h (37 m/s), and in some cases as much as 7 km/h (2 m/s), which is roughly the speed of a fast walking person.”

To obtain such precise measurements, VELOCE scientists used two high-resolution spectrographs that separate and measure wavelengths in electromagnetic radiation: HERMES in the Northern Hemisphere and CORALIE in the Southern Hemisphere. Outside of VELOCE, CORALIE is known for searching for exoplanets, and HERMES is a force in stellar astrophysics.

Two spectrographs recorded slight shifts in the Cepheid’s light, indicating their movement. The researchers used advanced techniques to ensure their measurements were stable and accurate, correcting for any instrumental drifts and atmospheric changes.

“We measure radial velocities using the Doppler effect,” explains Anderson. “That’s the same effect the police use to measure your speed, and also the effect you know from the change in tone when an ambulance is approaching or receding.

The strange dance of the Cepheid

The VELOCE project has revealed some fascinating details about Cepheid stars. For example, the VELOCE data provide the most detailed view yet of the Hertzsprung progression—the pattern in stellar pulsations—showing double peaks that were previously unknown, and will provide clues to better understanding the structure of Cepheids compared to theoretical models. pulsating stars.

The team found that several Cepheids show complex, modulated variability in their motions. This means that the radial velocities of the stars change in ways that cannot be explained by simple, regular pulsation patterns. In other words, while we would expect Cepheids to pulsate with a predictable rhythm, the VELOCE data reveal additional, unexpected variations in these motions.

These variations are inconsistent with the theoretical pulsation models traditionally used to describe Cepheids. “This suggests that there are more complex processes going on in these stars, such as interactions between different layers of the star or other (non-radial) pulsation signals, which may present an opportunity to determine the structure of Cepheid stars using asteroseismology,” he says. Anderson’s postdoc Henryka Netzel. The first VELOCE-based detections of such signals are reported in a companion paper (Netzel et al., in press).

Binary systems

The study also identified 77 Cepheid stars that are part of binary systems (two stars orbiting each other) and found 14 other candidates. A companion paper led by Anderson’s former postdoc, Shreya Shetye, describes these systems in detail and contributes to our understanding of how these stars evolve and interact with each other.

“We see that about one in three Cepheids has an invisible companion whose presence we can determine using the Doppler effect,” says Shetye.

“Understanding the nature and physics of Cepheids is important because they tell us about how stars evolve in general, and because we rely on them to determine the distances and rates of expansion of the universe,” says Anderson. “Additionally, VELOCE provides the best available cross-checks for similar but less precise measurements from ESA’s Gaia mission, which will ultimately perform the largest survey of Cepheid radial velocity measurements.”