High-precision measurements challenge our understanding of Cepheids

“Classical Cepheids” are a type of pulsating star that brightens and dims rhythmically over time. These pulsations help astronomers measure vast distances across 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 precisely. The different instruments and methods used over the years have led to inconsistent data, complicating our understanding of these stars.

“Monitoring the pulsations of Cepheids with high-definition velocimetry gives us insight into the structure of these stars and their evolution,” explains Richard I. Anderson, astrophysicist at EPFL. “In particular, measurements of the rate at which stars expand and contract along the line of sight, called radial velocities, provide a crucial counterpart to precise brightness measurements from space. However, there is a need There is an urgent need for high-quality radial velocity data because they are expensive to collect and because few instruments are capable of collecting them.

The 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, over 12 years, has collected more than 18,000 high-precision measurements of 258 Cepheid radial velocities at the using advanced spectrographs between 2010 and 2022. Their research is published in the journal. Astronomy and astrophysics.

“This dataset will serve as an anchor to connect observations of Cepheids from different telescopes over time and will hopefully inspire further studies 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 in La Palma. Anderson started the VELOCE project during his Ph.D. at the University of Geneva, he continued it as a postdoc in the United States and Germany and has now completed it at EPFL. Anderson’s Ph.D. Student Giordano Viviani was instrumental in publishing the VELOCE data.

Revealing the mysteries of the Cepheids with cutting-edge precision

“The wonderful precision and long-term stability of the measurements have enabled exciting new insights into how Cepheids pulsate,” says Viviani. “Pulsations cause changes in line-of-sight velocity of up to 70 km/s, or about 250,000 km/h. We measured these variations with a typical precision 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, which separate and measure the wavelengths of electromagnetic radiation: HERMES in the northern hemisphere and CORALIE in the southern hemisphere. Apart from VELOCE, CORALIE is famous for the discovery of exoplanets and HERMES is a workhorse of stellar astrophysics.

Both spectrographs detected tiny changes in the Cepheids’ light, indicating their movements. The researchers used advanced techniques to ensure the stability and precision of their measurements, correcting for instrumental drifts and atmospheric changes.

“We measure radial velocities using the Doppler effect,” explains Anderson. “It’s the same effect the police use to measure your speed, and also the effect you know of changing your tone as an ambulance approaches or moves away from you.”

The strange dance of the Cepheids

The VELOCE project has discovered several fascinating details about Cepheid stars. For example, the VELOCE data provides the most detailed examination to date of the Hertzsprung progression (a pattern in the pulsations of stars) showing double-peaked bumps that were not previously known and will provide clues to better understand the structure of Cepheids compared to theoretical models. of pulsating stars.

The team discovered that several Cepheids exhibit complex and modulated variability in their movements. This means that the radial velocities of stars change in ways that cannot be explained by simple, regular pulsation patterns. In other words, while one would expect the Cepheids to pulsate at a predictable rate, the VELOCE data reveals additional, unexpected variations in these movements.

These variations are not consistent with the theoretical models of pulsations traditionally used to describe Cepheids. “This suggests that there are more complex processes occurring within these stars, such as interactions between different layers of the star, or additional (non-radial) pulsation signals that could present an opportunity to determine the structure of Cepheid stars by asteroseismology,” says Henryka Netzel, Anderson’s postdoctoral fellow. The first detections of such signals based on VELOCE are reported in a companion article (Netzel et al., in press).

Binary systems

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

“We find that about one in three Cepheids has an invisible companion whose presence we can determine by Doppler,” explains 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 expansion rate of the universe,” says Anderson. “In addition, VELOCE provides the best cross-checks available for similar but less precise measurements from ESA’s Gaia mission, which will complete the largest study of Cepheid radial velocity measurements.”