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

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.