The role of Cepheids in the cosmic distance ladder

The late universe rate of cosmic expansion, the Hubble constant H₀, does not agree with the early universe measurements extrapolated using ΛCDM: this is known as the Hubble tension. The most precise H₀ measurements result from a construction called the cosmic distance ladder, which stacks different relative distance indicators to link anchors, which distances are known, to galaxies in the Hubble flow which movement compared to us is dominated by cosmological effects. Cepheids are a primary rung, and traditionally the most widely used to connect anchors to secondary distance indicators such as supernovae Ia. With my collaborators, we directly measure distances to closeby Cepheids, to better understand the precision and accuracy we can reach, in particular when using Cepheids as absolute distance indicators.

Some recent publications:
• "Inspecting the Cepheid parallax of pulsation using Gaia EDR3 parallaxes. Projection factor and period-luminosity and period-radius relations” Trahin, Breuval, Kervella, Mérand et al. 2021A&A...656A.102T
• “Extended envelopes around Galactic Cepheids. V. Multi-wavelength and time-dependent analysis of IR excess” Gallenne, Mérand, Kervella et al. 2021A&A...651A.113G
• "Inspecting the Cepheid Distance Ladder: the Hubble Space Telescope Distance to the SN Ia Host Galaxy NGC 5584” Javanmardi, Mérand, Kervella et al. 2021ApJ...911...12J

Data analysis in Optical Interferometry

Optical interferometry (OI) is an observing powerful technique which reaches unique angular resolutions. However, the data analysis is often not trivial. I am active in the field of data interpretation of OI from data theory, data analysis and even publish codes to share my knowledge (see also next section). My tools have been used in 80+ publications in the past 10 years.

Some recent publications:
• "Flexible spectro-interferometric modelling of OIFITS data with PMOIRED" Mérand 2022SPIE12183E..1NM
• "Increasing the achievable contrast of infrared interferometry with an error correlation model" Kammerer, Mérand, Ireland & Lacour 2020A&A...644A.110K
• "Robust high-contrast companion detection from interferometric observations. The CANDID algorithm and an application to six binary Cepheids" Gallenne, Mérand, Kervella et al. 2015A&A...579A..68G

High precision Optical Interferometry

Using my experience as an interferometrist, I collaborate with teams who want to derive parameters with high accuracy from interferometric data, such as distances, dynamical masses, etc. In recent years, I have been particularly interested in exoplanets and stellar mass black holes (via microlensing).

Some recent publications:
• "Observations of Microlensed Images with Dual-field Interferometry: On-sky Demonstration and Prospects" Mróz P., Dong S., Mérand A. et al 2025ApJ...980...47M
• "The cool brown dwarf Gliese 229 B is a close binary" Xuan J.W., Mérand A., Thompson W. et al. 2024Natur.634.1070X
• "Combining Gaia and GRAVITY: Characterising five new directly detected substellar companions" Winterhalder, Lacour, Mérand et al. 2024A&A...688A..44W
• "First resolution of Microlensing Images" Dong, Mérand et al. 2019ApJ...871...70D

My publications' list on the Astrophysics Data System, based on my ORCID 0000-0003-2125-0183.

PMOIRED Parametric modeling of astronomical spectro-interferometric data stored in the OIFITS format. It has flexible linear combination of components, modeling of spectral lines, as well as advanced features such as telluric correction, grid search, bootstrapping, etc. It has been used in a growing number of publications.

CANDID analyses optical interferometric data to detect or put upper limit on faint companion around stars. It has been used in many refereed publications

SPIPS is a parallax of pulsation code for Cepheids using synthetic observations. It has been used in many refereed publications.

CONFTIMER is a micropython code to implement a conference timer on a micro:bit microcontroller.

VLTI Programme Scientist

2017 - Present, European Southern Observatory's Headquarters (Germany)

Working under ESO Director for Science, I oversee the strategic developments of Very Large Telescope Interferometer (VLTI). I ensure the facility is best exploited, engage the community, and establish scientific priorities for the present and the future.

Operations Astronomer and System Scientist for the VLTI

2008 - 2017, European Southern Observatory's Paranal Observatory (Chile)

I supported observations at the Very Large Telescope Interferometer (VLTI). Instrument Scientist for AMBER (2008-2010) and PRIMA (2010-2013). Starting in 2012, I doubled as System Scientist for the VLTI and was responsible for the operational performances. I commissioned new systems, performed operators' training, and advised the observatory director and head of operations on VLTI matters.

Research Associate

2006 - 2008, CHARA Array Interferometer (California)

I participated in the development of the CHARA Array, operated by Georgia State University. I integrated a PICNIC infrared detector for the FLUOR instrument, and participated in the PAVO instrument design and construction. I also supported observations with the FLUOR instrument, which I integrated during my PhD.

PhD in Astrophysics and Instrumentation

2002-2005, Paris Observatory (France)

My thesis work was entitled High angular resolution studies of Cepheids, which I realised under the supervision of Vincent Coudé Du Foresto and Marie-José Goupil. I installed and commissioned the FLUOR interferometric beam combiner at the CHARA Array. I adapted the control, data reduction and calibration software for the new environment. I obtained the most accurate interferometric observations of Cepheids pulsating stars and fast rotating stars. Degree obtained with honours.

Master in Physics

1999-2002, Paris University (France)

I attended the physics classes at École Normale Supérieure de Paris, as a university student. My major was in Astrophysics. My master research project was on modeling interferometric data of Mira stars, at NOAO in Tucson (Arizona), with Steve Ridgway, collaborating with Guy Perrin (Paris Obs.) and Bertrand Mennesson (JPL).

Classe Préparatoire Scientifique

1997-1999, Lycée Clemenceau de Nantes (France)

I prepared for the entrance examination for engineering and high education schools. I majored in math, with minors in physics and computer science.