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“Classical Cepheids” are a type of pulsating star that rhythmically brightens and dims over time. These pulsations help astronomers measure vast distances in space, making Cepheids crucial “standard candles” that 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 accurately. The different instruments and methods used over the years have produced inconsistent data, complicating our understanding of these stars.

“Following the pulsations of Cepheids with high-resolution velocimetry gives us insight into the structure of these stars and how they evolve,” says Richard I. Anderson, an astrophysicist at EPFL. “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 precise brightness measurements from space. However, there is an urgent need for high-quality radial velocities because they are expensive to collect and because few tools are capable of collecting them.”

The VELOCE project

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

“This data set will serve as a backbone for linking Cepheid observations from different telescopes over time and will hopefully inspire further research by the community,” says Anderson.

VELOCE is a collaboration between EPFL, the University of Geneva and KU Leuven. It is based on observations from the Swiss Euler telescope in Chile and the Flemish Mercator telescope at 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 PhD Student Giordano Viviani contributed to making the VELOCE data release possible.

Unraveling the mysteries of the Cepheids with state-of-the-art precision

“The excellent precision and long-term stability of the measurements allowed interesting new insights into how Cepheids pulsate,” says Viviani. “The ripples cause line-of-sight velocity changes of up to 70 km/s, or about 250,000 km/h. We measured these variations with a typical accuracy of 130 km/h (37 m/s), and in some cases up to 7 km/h (2 m/s), which is roughly the speed of a fast walking human.”

To obtain such precise measurements, VELOCE researchers 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 finding exoplanets, and HERMES is a workhorse of stellar astrophysics.

The two spectrographs detected small changes in the light of the Cepheids, indicating their motion. The researchers used advanced techniques to ensure their measurements were stable and accurate, correcting for any instrumental drift and atmospheric changes.

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

The strange dance of the Cepheids

The VELOCE project has revealed some fascinating details about Cepheid stars. For example, the VELOCE data provide the most detailed look yet at the Hertzsprung progression – a pattern in the pulsations of stars – showing double-peaked bumps that were not known before and will provide clues to better understanding the structure of Cepheids compared to the theoretical models of 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 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 additional (non-radial) pulsation signals, which may provide an opportunity to determine the structure of Cepheid stars through asteroseismology,” says Postdoc of Anderson Henrika Netzel. The first VELOCE-based detections of such signals are reported in a companion paper (Netzel et al., in press).

Binary systems

The survey also identified 77 Cepheid stars that are part of binary systems (two stars orbiting each other) and found 14 more candidates. A companion paper, led by former Anderson postdoc Shriya Shetty, describes these systems in detail, adding to our understanding of how these stars evolve and interact with each other.

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

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