VELOCE Discovers Double-Peaked Pattern in Cepheid Pulsation

In a recent study in the journal Astronomy & Astrophysics, researchers from EPFL used Doppler effects to study the Cepheids stars. The VELOCE project, led by Richard I. Anderson, collected over 18,000 high-precision radial velocity measurements, uncovering new insights into Cepheid behavior and providing a detailed dataset to support further astronomical research.

RS Puppis, one of the most luminous Cepheid variable stars, rhythmically brightens and dims over a six-week cycle. Image Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration.

“Classical Cepheids” are a type of pulsating star that brightens and darkens in a rhythmic pattern over time. Cepheids are essential “standard candles” for measuring enormous distances throughout space. They help researchers in understanding the size and scale of the universe.

Cepheids are important, yet studying them is difficult. Their pulsations and potential interactions with companion stars create complex patterns that are difficult to measure accurately. Various tools and techniques have been employed over the years, leading to inconsistent data, thereby complicating our understanding of these stars.

Tracing Cepheid pulsations with high-definition velocimetry gives us insights into the structure of these stars and how they evolve. In particular, measurements of the speed at which the 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 has been an urgent need for high-quality radial velocities because they are expensive to collect and because few instruments are capable of collecting them. Richard I. Anderson, Astro Physicist, Swiss Federal Institute of Technology Lausanne

The VELOCE Project

With the VELOcities of CEpheids (VELOCE) project, Anderson has now led a group of scientists to do just that. Using cutting-edge spectrographs, the vast cooperation has amassed over 18,000 high-precision measurements of 258 Cepheid radial velocities over 12 years, between 2010 and 2022.

“This dataset will serve as an anchor to link Cepheid observations from different telescopes across time and hopefully inspire further study by the community.”

VELOCE is the result of an alliance between KU Leuven, the University of Geneva, and EPFL. It is based on observations made with the Flemish Mercator telescope in La Palma and the Swiss Euler telescope in Chile.

After starting the VELOCE project during his Ph.D. at the University of Geneva, Anderson carried it out as a postdoc in the US and Germany before finishing it at EPFL. Giordano Viviani, Anderson's Ph.D. student, played a crucial role in facilitating the release of the VELOCE data.

Unraveling Cepheid Mysteries with Cutting-Edge Precision

The wonderful precision and long-term stability of the measurements have enabled interesting new insights into how Cepheids pulsate. The pulsations lead to changes in the line-of-sight velocity of up to 70 km/s, or about 250,000 km/h. We have measured these variations with a typical precision of 130 km/h (37 m/s) and, in some cases, as good as 7 km/h (2 m/s), which is roughly the speed of a fast-walking human. Giordano Viviani, Ph.D. Student, Swiss Federal Institute of Technology Lausanne

The VELOCE researchers employed two high-resolution spectrographs, CORALIE in the southern hemisphere and HERMES in the northern hemisphere, to isolate and measure electromagnetic radiation wavelengths to obtain such accurate data. Apart from VELOCE, CORALIE is well-known for its exoplanet discoveries, while HERMES is a mainstay of stellar astrophysics.

The two spectrographs discerned the movements of the Cepheids by minute changes in their luminosity. To account for atmospheric variations and sensor drift, the researchers employed sophisticated algorithms to guarantee the stability and accuracy of their measurements.

We measure radial velocities using the Doppler effect. That is 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 or recedes from you. Richard I. Anderson, Astro Physicist, Swiss Federal Institute of Technology Lausanne

The Strange Dance of Cepheids

The VELOCE study has revealed a number of intriguing facts concerning Cepheid stars. In comparison to theoretical models of pulsating stars, the VELOCE data, for instance, offer the most detailed look yet at the Hertzsprung progression, a pattern in the stars' pulsations. They reveal double-peaked bumps that were previously unknown and offer hints to a better understanding of the structure of Cepheids.

The group discovered that the motions of several Cepheids show intricate, regulated variability. This indicates that simple, regular pulsation patterns cannot account for the variations in the stars' radial velocities. In other words, the VELOCE data show additional, unanticipated variations in the pulsating movements of Cepheids, even though we would expect them to have a regular rhythm.

The theoretical pulsation models typically used to describe Cepheids are inconsistent with these changes.

Henryka Netzel, a Postdoc under Anderson, said, “This suggests that there are more intricate processes occurring within these stars, such as interactions between different layers of the star, or additional (non-radial) pulsation signals that may present an opportunity to determine the structure of Cepheid stars by asteroseismology.”

A companion work reports the first detections of such signals based on VELOCE (Netzel et al. in press).

Binary Systems

In addition, 14 more Cepheid stars were discovered, and 77 Cepheid stars that are part of binary systems, two stars orbiting one another, were recognized. A companion work led by Shreeya Shetye, Anderson's former postdoc, detailed these systems and advanced knowledge of the evolution and interplay of these stars.

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

Anderson said, “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 for determining distances and the expansion rate of the Universe. Additionally, VELOCE provides the best available cross-checks for similar, but less precise, measurements from the ESA mission Gaia, which will eventually conduct the largest survey of Cepheid radial velocity measurements.”

Journal Reference:

Anderson, R. I., et al. (2024) VELOcities of CEpheids (VELOCE). I. High-precision radial velocities of Cepheids. Astronomy & Astrophysics. doi.org/10.1051/0004-6361/202348400