Extremely precise measurements challenge our knowledge of Cepheids | Urania

Scientists have measured the speed of Cepheid stars with unprecedented accuracy, providing new information about them.

I will save you They are pulsating stars that change their brightness rhythmically. This allows astronomers to use them as “Standard candlesTo measure vast distances in space. Cepheids help us determine 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 produced inconsistent data, complicating our understanding of these stars.

Tracking Cepheid pulses with a high-precision velocimeter gives us insight into the structure of these stars and how they evolved. “This is a very exciting time for us,” said Richard I. Anderson, an astrophysicist at EPFL. Specifically, measurements of the speed at which stars expand and contract along the line of sight—so-called radial velocities – They are a prime analogue for accurate brightness measurements from space. However, high-quality radial velocity measurements are urgently needed because they are expensive to collect and few instruments are capable of examining them.

Philos Project
Anderson is leading a team of scientists doing exactly that as part of the VELOcities of Cepheids (VELOCE) project, a large collaboration that over 12 years has collected more than 18,000 high-precision measurements of the radial velocities of 258 Cepheids using advanced spectrometers between 2010 and 2022. This dataset will serve as an anchor to link Cepheid observations from different telescopes over time, and hopefully inspire further research in the community.

VELOCE is a collaboration between EPFL, the University of Geneva and KU Leuven. It is based on feedback from Swiss Euler’s telescope In Chile and Flemish Mercator telescope In La Palma. Anderson started the VELOCE project while earning his PhD at the University of Geneva, continued it as a postdoc in the US and Germany, and now completes it at EPFL. Anderson’s graduate student Giordano Viviani was instrumental in making it possible. Publications Speed ​​data.

Uncovering the secrets of the Cepheids with extreme precision
The excellent accuracy and long-term stability of the measurements have provided interesting new insights into how Cepheids pulsate. – Viviani said. The pulses cause changes in line-of-sight velocity of up to 70 km/s. We measured these changes with a typical accuracy of 130 km/h, and in some cases even 7 km/h, which is roughly the speed of a fast human walk.

To obtain such precise measurements, VELOCE scientists used two high-resolution spectrometers that separate and measure wavelengths in electromagnetic radiation: Hermes in the Northern Hemisphere and CORALIE in the Southern Hemisphere. Apart from VELOCE, CORALIE is known for its exploration Planet EgzoHermes serves stellar astrophysics.

The spectrographs detected tiny changes in the light of Cepheid stars, indicating their movements. Scientists used advanced techniques to ensure the stability and accuracy of the measurements, correcting for any instrumental drifts and atmospheric variations. We measure radial velocities using Doppler effect Anderson explained. This is the same effect that police use to measure speed, and the effect we know from the change in tone when an ambulance approaches or moves away from us.

Sefid’s strange dance
The VELOCE project has revealed some fascinating details about Cepheids. For example, the VELOCE data provide the most detailed look yet at the Hertzsprung progression – a pattern in stellar pulsations – which shows previously unknown double peaks and will provide clues to better understand the structures of Cepheids compared to theoretical models of pulsars.

The team found that many Cepheids exhibit complex, systematic variation in their motions. This means that the radial velocities of the stars vary 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 changes in these motions.

These changes are not consistent with the theoretical pulse models traditionally used to describe Cepheids. This suggests that more complex processes are occurring in these stars, such as interactions between different stellar layers or additional (non-radial) pulsation signals, which could provide an opportunity to determine the structure of Cepheids using astro-seismology – said Henrika Neitzel. The first detections of such signals based on VELOCE are described in an accompanying article.

Binary systems
The study also identified 77 Cepheids that are part of it. Binary systems Fourteen other candidates have been found. An accompanying paper led by Shreya Shetty, a former Anderson graduate student, describes these systems in detail, expanding our knowledge of the evolution of these stars and their interactions with each other. We see that about one in three Cepheids has an unseen companion whose presence can be determined using the Doppler effect. Shetty 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 to determine the distance and rate of expansion of the universe. Anderson said. Additionally, VELOCE provides the best cross-checking available for similar but less accurate z-measures. Gaia’s missionwhich will eventually conduct the largest Cepheid radial velocity survey.

Agnieszka Novak

more information:

Source: EPFL

Pictured: RS Puppis, one of the brightest Cepheid stars. Credits: NASA, ESA, Hubble Heritage Team (STScI/AURA) – Hubble Collaboration with Europe

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