Détails

Mis à jour : 31 Janvier 2025

MEETINGS

• 2022/05/05 : Journée Lagrange sur l'ELT
• 2023/12/15 : première réunion avec le consortium MOSAIC
• 2024/02/21 : Première réunion en interne MOSAIC@OCA
• 2024/03/27 : présentation de 3 designs par Nick et Julien
• 2024/05/29 : debriefing de la visite de Kacem El Hadi (PM MOSAIC)
• 2024/06/26 : point instrumental et discussion sur les cas scientifique à l'OCA
• 2024/09/18 : point instrumental et discussion sur les cas scientifiques
• 2024/10/02 : Journée MOSAIC@OCA (Instrument et Science)
• 2024/11/20 : point instrumental et discussion sur les cas scientifiques
• 2024/12/18 : point instrumental et finalisation des cas scientifiques
• 2025/01/29 : Kick-Off meeting de l'ANR-FR à Marseilles (Nick, Julien & Nicolas en présentiel)

Détails

Mis à jour : 31 Janvier 2025

PROJECT OVERVIEW

Implication instrumentale à l'OCA

Le laboratoire Lagrange et l'Observatoire de la Côte d'Azur ont pour responsabilité de concevoir et de développer une optique de relais dont le but est de réimager à partir du plan focal, le champ de vue des IFU (Integral Field Unit) vers les faisceaux de fibres.

Trois étapes importantes sont à franchir dans ce développement. La première est de fournir un premier design des optiques de relais (fin 2025). L’objectif est ensuite pour les deux années suivantes d’étudier la faisabilité et de consolider le design optique, tâche qui sera achevée fin 2027. La phase de design final sera atteinte fin 2028, de manière à ce que les relais optiques puissent être fabriquées à partir de cette date. L’étude des contraintes mécaniques est cruciale car elles imposent des exigences fortes aux sous-traitants, et impactent la faisabilité.

L'implication scientifique à l'OCA

Le consortium MOSAIC est consisté de 5 groupes de travail (SWG):

• SWG1 - First Light : MOSAIC détectera et étudiera les toutes premières galaxies. Leur lumière, qui a mis plus de 13 milliards d’années à nous parvenir, nous fournira des indices essentiels pour comprendre l’époque primitive de la « réionisation » de l’Univers, au cours de laquelle son gaz est passé d’un état universellement neutre à un état ionisé.
• SWG2 - Inventory of matter : MOSAIC combinera des observations visibles et proches de l'infrarouge pour réaliser le premier inventaire direct de la matière dans les galaxies lointaines à z~3, y compris la caractérisation des profils de matière noire dans les galaxies à disque.
• SWG3 - Mass assembly of galaxies through cosmic time : MOSAIC réalisera une étude à grande échelle (des centaines à des milliers de galaxies) et représentative de galaxies spatialement résolues sélectionnées de manière homogène sur la plage de décalage vers le rouge z=2-4.
• SWG4&5 - Revolved Stellar population beyond the local group and galactic Archeology: Les observations de populations stellaires résolues avec MOSAIC nous permettront de retracer et d'explorer l'histoire de la formation des étoiles et de l'enrichissement chimique de grands échantillons de galaxies au delà du groupe local.
• SWG6 - Transients and multimessengers: Un événement astronomique transitoire est un objet ou un phénomène astronomique dont la durée peut aller de quelques millisecondes à quelques jours, semaines ou même plusieurs années. Cela contraste avec l'échelle de temps des millions ou milliards d'années au cours desquelles les galaxies et leurs étoiles composantes dans notre univers ont évolué. Au singulier, le terme est utilisé pour les événements violents du ciel profond, tels que les supernovae, les novae, les explosions de nova naine, les sursauts gamma et les événements de perturbation par marée, ainsi que les microlentilles gravitationnelles, les transits, les éclipses et les comètes. Ces événements font partie du sujet plus large de l'astronomie du domaine temporel.

Les 15 chercheurs de l'Observatoire de la Côte d'Azur intéressés par MOSAIC se répartissent sur l'ensemble des SWGs:  SWGs 123 (1), SWG3 (4), SWG4&5 (9), SWG6 (1)

L'Observatoire est par ailleurs leader de 3 Science Reference Programs (SRPs)

• Détermination de la distance de Baade-Wesselink des Céphéides dans le groupe local : une nouvelle voie vers la constante de Hubble (N. Nardetto)
• Sondage des environnements stellaires à haute densité avec MOSAIC : tester les limites chimiques entre l'amas stellaire nucléaire et les amas globulaires (Felipe Gran, Camilla Navarete, Mathias Schultheis, Orlagh Creevey)
• Proto-clusters et développement de structures à grande échelle (N. Nesvadba)

Page réalisée par N. Nardetto : last update 31/01/2025

Détails

Mis à jour : 7 Mars 2025

TEAM

Instrumental Team

• BOEBION Olivier
• CAUJOLLE Yan
• CVETOJEVIC Nick (Project Manager)
• DEJONGHE Julien (System ingenior)
• GIRARD Paul
• LAGARDE Stéphane
• LECRON Daniel
• MARCOTTO Aurélie
• MAUCLERT Nicolas
• OTTOGALLI Sébastien

Science Team

• ANTIER Sarah
• BENOIST Christophe
• BOQUIEM Médéric
• CHIAVASSA Andrea
• CREEVEY Orlagh
• GRAN Felipe
• HILL Vanessa
• KORDOPATIS Georges
• MAUROGORDATO Sophie
• NARDETTO Nicolas (coordination MOSAIC@OCA, MOSAIC-FRANCE)
• NAVARRETE Camila
• PRUNET Simon
• RECIO-BLANCO Alejandra
• ROBBE-DUBOIS Sylvie
• SCHULTHEIS Mathias
• SLEZAK Éric

Page réalisée par N. Nardetto : last update 31/01/2025

Détails

Mis à jour : 13 Février 2025

AVAILABLE JOBS

Postdoctoral position on the “Tomography of the atmosphere of Cepheids: The Baade-Wesselink distance of the Cepheids of the Local Group”

Job Summary

Category: Post-doctoral Positions and Fellowships

Institution : Observatoire de la Côte d’Azur

Department : Lagrange

Number of Positions Available: 1 (2 years + 1 year)

Work Arrangement : In-Person

Job Description summary

Postdoctoral researcher position at Observatoire de la Côte d’Azur (Lagrange) on the calibration of the extragalactic distance scale: The Baade-Wesselink distance of the Cepheids of the Local Group

We advertise a postdoctoral research position to work on improving the calibration of the extragalactic distance scale within the framework of the ANR Grant Unlockpfactor.

The researcher will revisit the Baade-Wesselink method of distance determination of Cepheids by building a tomographic view of their atmosphere using high spectral resolution observations and stellar atmosphere models. The principal objective is to improve the robustness of the BW distance of Cepheids.

This position is funded for two years, with a possible extension of one year, to start the 1st October 2025. The successful applicant will join the Lagrange laboratory, located in Nice.

A PhD in Astrophysics is required at the time of the contract signature. The ideal candidate should be familiar with the analysis of different types of observational data sets in particular spectroscopy. Previous experience with variable stars or stellar atmosphere modeling is desirable. Skills in programming (python) are also required.

The applicants are invited to send via email a cover letter, a curriculum vitae, a brief statement of research interests (maximum 4 pages) with links to recent publications, and two letters of reference to Prof. Nicolas Nardetto. The applications should ideally be received by 15^th of June 2025. After this date, the applications will be considered until the position is filled.

Compensation and benefits

Compensation range : 33500 to 47500

Included benefits: The benefits include 47 days of vacation per year, as well as the legal benefits in effect in France (maternity and parental leave, health insurance...).

Applicant Details

Publication Start Date : 2025 May 15th

Application Deadline : 2025 June 15th

Inquiries

Name : Prof. Nicolas Nardetto

Email: Nicolas.Nardetto@oca.eu

Detailed of the post-doc position

Tomography of the atmosphere of Cepheids: The Baade-Wesselink distance of the Cepheids of the Local Group

To understand the nature of dark energy, one must measure the rate of expansion of the universe, i.e. the Hubble constant (Ho), with a precision and accuracy of 1%, which is the objective of several groups at the international level, including the group of Adam Riess (corecipient of the 2011 Nobel Prize) which is piloting the SHOES project (Riess+16, Riess+21). There are mainly two methods to measure Ho with an accuracy better than 1-2%: on the one hand, the cosmic background radiation, and on the other hand, the determination of distances in the universe. These two approaches currently present significant disagreements: one speaks of 'tension'. The value of Riess+21 is 5 sigma higher than that deduced from the cosmic background radiation established by the Planck satellite. Cepheids, thanks to their period-luminosity relation (PL, Leavitt & Pickering 1912), are currently used to calibrate the distance scales in the universe and to constrain the Hubble constant (Riess et al. 1998; 2011 Nobel prize). A first way to calibrate the PL relation is to use the trigonometric parallaxes of Galactic Cepheids (HST, Gaia). A second approach is to apply the Baade-Wesselink method of determining the distance of Cepheids.

The principle of the method is simple: it consists in comparing the linear and angular dimensions of the Cepheid in order to determine its distance by means of a simple division. Photometric measurements (associated with a surface brightness – color relation) and/or interferometric measurements provide the variation of the photospheric angular diameter of the star over the whole pulsation cycle, while the variation of the linear diameter is determined by a temporal integration of the pulsating velocity (Vpuls) of the star. The determination of the latter, from the Doppler shift of the spectral line (Vrad) is extremely delicate and involves what is called the projection factor, p, defined by Vpuls=p x Vrad. This number alone summarizes all the physics of the Cepheid atmosphere (limb darkening, velocity gradient and atmospheric dynamics). The projection factor has been studied from different angles using hydrodynamic models or via spectroscopy in the visible range (Nardetto et al. 2004, 2006, 2009, 2017). To date, the Baade-Wesselink method is limited by several uncertainties (Nardetto et al. 2023) and a piece of the puzzle is missing: what is the dynamical structure of the atmosphere of Cepheids? Indeed, it has been shown that the atmosphere of Cepheids is not pulsating in one block: there is a velocity gradient. Thus, the key-point of the BW method is to extrapolate the radial velocity curve associated to the line-forming region(s) to the photosphere, i.e., to the layer corresponding to photometric and interferometric measurements. This is not trivial and certainly corresponds to the largest source of uncertainty in the projection factor. One way to proceed, and this is the current procedure adopted by the community, is to estimate where the lines are forming in the atmosphere by using their depth: a deep line forms in the upper part of the atmosphere while a “zero-depth” line corresponds by definition to the photosphere. The key question we want to answer is "Is the atmosphere dynamic of Cepheids of similar periods the same or not? In particular, how the pulsating atmosphere of Cepheids is moving as a function of time; and ultimately determine what is the impact of the moving atmosphere on the p-factor, and thus, on the distance of Cepheids?

The objective of this post-doctoral fellowship is to perform for the first time a tomography of the pulsating atmosphere of Cepheids (i.e. by determining exactly where the spectral line are forming within the atmosphere) in order to better extrapolate the radial velocity of line-forming regions toward the pulsation velocity of the photosphere. Performing the tomography of the atmosphere of Cepheids has the potential to derive precisely their atmospheric velocity gradient for a given period and reduce the systematics on the BW method from ~7% to 1-3% (see error budget in Nardetto et al. 2023).

This work is part of the ANR Unlockpfactor project as well as the Araucaria project for determining distances in the local group (https://araucaria.camk.edu.pl/). The thesis will be carried out at the Côte d'Azur Observatory on the Mont Gros site. The overall goal is to improve the robustness of the BW method and apply it to Cepheids in the local group with the ELT/MOSAIC instrument.

Context

Distances in the universe. Hubble constant. Cepheids and the Baade-Wesselink method.

Today, there are 2 methods for estimating the Hubble constant (Ho) to better than 2%: by analyzing the cosmic microwave background radiation (PLANCK) on the one hand, and by determining distances (Cepheids, eclipsing binaries, supernova 1a). The 2 methods are incompatible up to 5.5 sigma. The aim is to verify the absence of bias in the 'distances' method by studying the physics of Cepheids, and also, to open a new route toward Ho based on the BW method in the context of MOSAIC/ELT.

Objectives

1/ perform for the first time the tomography of the pulsating atmosphere of Cepheids using high-resolution spectroscopic data and stellar atmosphere models. Determine the atmospheric velocity gradient of Cepheids of different periods.

2/ Initiate a High-Resolution Survey with Armazones and others facilities in collaboration with the Araucaria Group

Method

Our methodology from the HARPS+NIRPS data toward a better understanding of the projection factor will be the following:

• Reduce and analyze high resolution spectroscopic data of Cepheids already in hands
• Initiate a High Resolution Survey with Armazones and others facilities in collaboration with Araucaria Group
• Fit quasi-static atmosphere models to spectral lines profiles using different approaches
• For each fitted line, we will implement the flux contribution function into the quasi-static atmosphere models. This will provide the position of the line forming region within the atmosphere. Including this tool into Cepheids models (that are otherwise extremely well-constrainted by observations) will allow to better understand the link between the dynamical structure of the pulsating atmosphere (velocity, temperature, pressure profiles) and the spectral lines forming regions (tomography).
• Produce figure 4 of Nardetto et al. 2023 but with a real spatial dimension (for e.g. in km) in X-axis instead of the basic line depth estimator and calculate the atmospheric velocity gradient
• Colleagues from the Unlockpfactor team will use 5) to perform an extrapolation to the photosphere using either the methodology described in Nardetto et al. (2009) or by defining a new methodology based on the result of the different WPs of the ANR. A new methodology will be provided to the community to derive the p-factors in the context of the BW method.

Expected results

A better understanding of Cepheid physics (photosphere, atmosphere, environment).

Strengthening the Baade-Wesselink method to open up extragalactic applications with the ELT

Bibliographie references

Habilitation Thesis of N. Nardetto

https://ui.adsabs.harvard.edu/abs/2018arXiv180104158N/abstract

https://www.aanda.org/articles/aa/pdf/2017/01/aa29400-16.pdf

https://ui.adsabs.harvard.edu/abs/2023A%26A...671A..14N/abstract

Precision on the post-doc position

The thesis will take place at Mt Gros at the Côte d'Azur Observatory. Missions abroad (Poland, Mt Wilson in California, Chile) are possible and to be planned. It is also possible to observe in remote control mode the CHARA interferometer (Mt Wilson) from the Calern plateau in the Grasse hinterland.

Conditions scientifiques matérielles et financières du projet de recherche

This post-doctoral fellowship is funded within the framework of the ANR Unlockpfactor Project. This position is funded for two years, with a possible extension of one year, to start the  1st of October 2025. The successful applicant will join the Lagrange laboratory, located in Observatoire de la Côte d’Azur in Nice/France.

International opening

The post-doc is at the heart of a large network of collaboration and in particular is part of the Araucaria project for determining distances in the local group. An ERC Synergy (UniverScale) was obtained within the framework of the Araucaria project (PIs: G. Pietrzynski, W. Gieren, P. Kervella, Bożena Czerny): https://araucaria.camk.edu.pl/

This thesis project also aims to prepare the scientific case "Cepheids" of the ELT/MOSAIC. The Lagrange laboratory and the Côte d'Azur Observatory are responsible for WP11 (relay optics) of the instrument.

Objectives for promoting the doctoral student's research work: dissemination, publication and confidentiality, intellectual property rights, etc.

3 A-rank publications as first author and several co-author publications are planned

Collaborations forseen

With colleagues from the Araucaria project mainly for the distances in the universe and the spectroscopy aspects. For the stellar atmosphere modeling, several colleagues will help us: P. Kervella (LESIA, Paris) and A. Mérand (ESO). Also, for the implementation of the contribution functions in the stellar atmosphere model, we will be in contact with A. Chiavassa (at the Lagrange Laboratory).

Profil et compétences recherchées - Profile and skills required

A PhD in Astrophysics is required at the time of the contract signature. The ideal candidate should be familiar with the analysis of different types of observational data sets in particular spectroscopy. Previous experience with variable stars or stellar atmosphere modeling is desirable. Skills in programming (python) are also required.

How to apply for this position: 

The applicants are invited to send via email a cover letter, a curriculum vitae, a brief statement of research interests (maximum 4 pages) with links to recent publications, and two letters of reference to Prof. Nicolas Nardetto. The applications should ideally be received by 15^th of June 2025. After this date, the applications will be considered until the position is filled.