Highly precise measurements challenge our understanding of Cepheids

“Classical Cepheids” are a type of pulsating star that brightens and darkens rhythmically 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 possible interactions with companion stars create complex patterns that are difficult to measure accurately. Different instruments and methods used over the years have led to inconsistent data, complicating our understanding of these stars.

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

The VELOCE project

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

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

VELOCE is the result of 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 on La Palma. Anderson started the VELOCE project during his Ph.D. at the University of Geneva, continued as a postdoc in the US and Germany and has now completed it at EPFL. Anderson’s Ph.D. student, Giordano Viviani, was instrumental in making the VELOCE data publication possible.

Unravel the Cepheid mysteries with cutting-edge precision

“The wonderful accuracy and stability of the long-term measurements have enabled interesting new insights into the way Cepheids pulsate,” says Viviani. “The pulsations lead to changes in line-of-sight speed of up to 70 km/s, or approximately 250,000 km/h. We have measured these variations with a typical accuracy of 130 km/h (37 m/s), and in some cases even up to 7 km/h (2 m/s), which is approximately the speed of a fast-running human.”

To obtain such precise measurements, the VELOCE researchers used two high-resolution spectrographs, which 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 in stellar astrophysics.

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

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

The strange dance of Cepheids

The VELOCE project has uncovered several fascinating details about Cepheid stars. For example, VELOCE data provides the most detailed look yet at the Hertzsprung progression – a pattern in the stars’ pulsations – and reveals double-peaked bumps not previously known, and will provide clues to better understanding the structure of Cepheids compared to theoretical models. of pulsating stars.

The team found that several Cepheids exhibit complex, modulated variability in their movements. This means that the stars’ radial velocities 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 movements.

These variations are not consistent with the theoretical pulsation models traditionally used to describe Cepheids. ‘This suggests that more complicated processes are taking place within these stars, such as interactions between different layers of the star, or additional (non-radial) pulsation signals that could provide an opportunity to determine the structure of Cepheid stars through asteroseismology,’ says Anderson’s postdoc Henryka Netzel. The first detections of such signals based on VELOCE 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 more candidates. A companion paper led by Anderson’s former postdoc, Shreeya Shetye, 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 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 how stars evolve in general, and because we rely on them to determine distances and the expansion rate of the universe,” says Anderson. “In addition, VELOCE provides the best available cross-checks for similar but less precise measurements from the ESA mission Gaia, which will ultimately conduct Cepheid’s largest radial velocity survey.”