6.17.24
Nik Papageorgiou
RS Puppis, one of the most luminous Cepheid variable stars, rhythmically brightens and dims over a six-week cycle. Credit: NASA/ESA Hubble Heritage Team (STScI/AURA).
EPFL scientists, through the VELOCE project, have clocked the speed of Cepheid stars – “standard candles” that help us measure the size of the universe – with unprecedented precision, offering exciting new insights about them.
“Classical Cepheids” are a type of pulsating star that rhythmically brightens and dims over time. These pulsations help astronomers measure vast distances across space, which makes 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. Different instruments and methods used over the years have led to inconsistent data, 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,” says Richard I. Anderson, an astrophysicist at EPFL. “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.”
Anderson has now led a team of scientists to do exactly 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. “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 fruit of a collaboration between EPFL, the University of Geneva, and the KU Leuven. It is based on observations from the Swiss Euler telescope in Chile and the Flemish Mercator telescope on La Palma.
ESO Swiss 1.2 meter Leonhard Euler Telescope at La Silla, using the CORALIE spectrograph.
DESY HERMES on the Flemish Mercator telescope.
Anderson began the VELOCE project during his PhD at the University of Geneva, continued it as a postdoc in the US and Germany, and has now completed it at EPFL. Anderson’s PhD student, Giordano Viviani, was instrumental in making the VELOCE data release possible.
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,” says Viviani. “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.”
To get 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 famous for finding exoplanets and HERMES is a workhorse of stellar astrophysics.
The two spectrographs detected tiny 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 drifts 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 or recedes from you.”
The strange dance of Cepheids
The VELOCE project uncovered several fascinating details about Cepheid stars. For example, VELOCE data provide the most detailed look yet at the Hertzsprung progression – a pattern in the stars’ pulsations – showing double-peaked bumps that were not previously known and will provide clues to better understanding the structure of Cepheids when 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 theoretical pulsation models traditionally used to describe Cepheids. “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,” says Anderson’s postdoc Henryka Netzel. First detections of such signals based on VELOCE are reported in a companion paper (Netzel et al. in press).
VELOCE observations trace the expansion and contraction of Cepheid stars with unprecedented precision. On the left: observed spectra of the Cepheid archetype Delta Cephei as they change in wavelength due to the pulsations. On the right: the radial velocity curve measured by VELOCE, with the star’s variable size shown (not to scale) using star-shaped symbols. Credit: R.I. Anderson (EPFL).
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 unseen companion whose presence we can determine by the Doppler effect,” says 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 for determining distances and the expansion rate of the Universe,” says Anderson. “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.”
Science paper:
Other contributors:
Ruder Bošković Institute
Harvard-Smithsonian Center for Astrophysics
University Hospital of Lausanne (CHUV)
University of Lausanne (UNIL)
Max Planck Institute for extraterrestrial Physics
Lund University
Eberhard Karls University
Université de Liège
European Southern Observatory
Tufts University
Stanford University
Universidade do Porto
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The Swiss Federal Institute of Technology in Lausanne [EPFL-École Polytechnique Fédérale de Lausanne] (CH) is a research institute and university in Lausanne, Switzerland, that specializes in natural sciences and engineering. It is one of the two Swiss Federal Institutes of Technology, and it has three main missions: education, research and technology transfer.
The QS World University Rankings ranks EPFL(CH) very high, whereas Times Higher Education World University Rankings ranks EPFL(CH) as one of the world’s best schools for Engineering and Technology.
EPFL(CH) is located in the French-speaking part of Switzerland; the sister institution in the German-speaking part of Switzerland is The Swiss Federal Institute of Technology ETH Zürich [Eidgenössische Technische Hochschule Zürich] (CH). Associated with several specialized research institutes, the two universities form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles Polytechniques Fédérales] (CH) which is directly dependent on the Federal Department of Economic Affairs, Education and Research. In connection with research and teaching activities, EPFL(CH) operates a nuclear reactor CROCUS; a Tokamak Fusion reactor; a Blue Gene/Q Supercomputer; and P3 bio-hazard facilities.
ETH Zürich, EPFL (Swiss Federal Institute of Technology in Lausanne) [École Polytechnique Fédérale de Lausanne](CH), and four associated research institutes form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales] (CH) with the aim of collaborating on scientific projects.
The roots of modern-day EPFL(CH) can be traced back to the foundation of a private school under the name École Spéciale de Lausanne in 1853 at the initiative of Lois Rivier, a graduate of the École Centrale Paris (FR) and John Gay the then professor and rector of the Académie de Lausanne. At its inception it had only 11 students and the offices were located at Rue du Valentin in Lausanne. In 1869, it became the technical department of the public Académie de Lausanne. When the Académie was reorganized and acquired the status of a university in 1890, the technical faculty changed its name to École d’Ingénieurs de l’Université de Lausanne. In 1946, it was renamed the École polytechnique de l’Université de Lausanne (EPUL). In 1969, the EPUL was separated from the rest of the University of Lausanne and became a federal institute under its current name. EPFL(CH), like ETH Zürich (CH), and it is thus directly controlled by the Swiss federal government. In contrast, all other universities in Switzerland are controlled by their respective cantonal governments. EPFL(CH) has started to develop into the field of life sciences. It absorbed the Swiss Institute for Experimental Cancer Research (ISREC) in 2008.
In 1946, there were 360 students. In 1969, EPFL(CH) had 1,400 students and 55 professors. In the past two decades the university has grown rapidly and over 14,000 people study or work on campus, about 10,000 of these being Bachelor, Master or PhD students. The environment at modern day EPFL(CH) is highly international with the school attracting students and researchers from all over the world. More than 125 countries are represented on the campus and the university has two official languages, French and English.
Organization
EPFL is organized into eight schools, themselves formed of institutes that group research units (laboratories or chairs) around common themes:
School of Basic Sciences
Institute of Mathematics
Institute of Chemical Sciences and Engineering
Institute of Physics
European Centre of Atomic and Molecular Computations
Bernoulli Center
Biomedical Imaging Research Center
Interdisciplinary Center for Electron Microscopy
MPG-EPFL Centre for Molecular Nanosciences and Technology
Swiss Plasma Center
Laboratory of Astrophysics
School of Engineering
Institute of Electrical Engineering
Institute of Mechanical Engineering
Institute of Materials
Institute of Microengineering
Institute of Bioengineering
School of Architecture, Civil and Environmental Engineering
Institute of Architecture
Civil Engineering Institute
Institute of Urban and Regional Sciences
Environmental Engineering Institute
School of Computer and Communication Sciences
Algorithms & Theoretical Computer Science
Artificial Intelligence & Machine Learning
Computational Biology
Computer Architecture & Integrated Systems
Data Management & Information Retrieval
Graphics & Vision
Human-Computer Interaction
Information & Communication Theory
Networking
Programming Languages & Formal Methods
Security & Cryptography
Signal & Image Processing
Systems
School of Life Sciences
Bachelor-Master Teaching Section in Life Sciences and Technologies
Brain Mind Institute
Institute of Bioengineering
Swiss Institute for Experimental Cancer Research
Global Health Institute
Ten Technology Platforms & Core Facilities (PTECH)
Center for Phenogenomics
NCCR Synaptic Bases of Mental Diseases
College of Management of Technology
Swiss Finance Institute at EPFL
Section of Management of Technology and Entrepreneurship
Institute of Technology and Public Policy
Institute of Management of Technology and Entrepreneurship
Section of Financial Engineering
College of Humanities
Human and social sciences teaching program
EPFL Middle East
Section of Energy Management and Sustainability
In addition to the eight schools there are seven closely related institutions
Swiss Cancer Centre
Center for Biomedical Imaging (CIBM)
Centre for Advanced Modelling Science (CADMOS)
École Cantonale d’art de Lausanne (ECAL)
Campus Biotech
Wyss Center for Bio- and Neuro-engineering
Swiss National Supercomputing Centre
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