PROJECT OVERVIEW

The team :

  • N. Nardetto (OCA) : PI of the ANR
  • P Kervella (Observatoire de Paris) : PI of WP2
  • A. Merand (ESO, Germany)
  • V. Hocdé (post-doc in Araucaria team, Poland)
  • A. Gallenne (post-doc in Araucaria team, Chile)
  • G. Pietrzynski (PI of Araucaria Project, Poland)
  • W. Gieren (PI of Arauracaria Project, Chile)

The ANR grants over 5 years (2024-2028) :

  • 1 PhD in Nice : Brankisa Apostolova (2025-2028)
  • 1 PhD in Paris
  • 1 post-doc in Nice
  • and also 1 PhD granted by UniCA is officially part of the ANR Unlockpfactor project : Manon Bailleul (2024-2027)

Short description of the 5 Working Package of the ANR :

  • WP1 : Calibrating a dedicated SBCR for Cepheids
  • WP2 : Deriving the empirical projection factor of Cepheids (p)
  • WP3 : The Limb-darkening variation of Cepheids (po)
  • WP4 : Tomography of the atmosphere of Cepheids (fgrad)
  • WP5 : Unlocking the projection factor (p=poxfgradxfog)

At the moment, the BW method of distance determination is locked because of the projection factor issue. This is true whatever the BW version used: interferometry, SBCR or even a combination of both (SPIPS). In 5 years (60 months), our immediate objectives are to use a homogeneous set of new CHARA/SPICA data of about 50 Cepheids in order to calibrate a robust SBCR dedicated to Cepheids. The CSEs of Cepheids will be also characterized in the visible domain (WP1). In parallel, we will apply the inverse BW method using the three different approaches (interferometry, SBCR and SPIPS) together with the Gaia DR4 in order to derive the projection factor of about 100 Cepheids (WP2). For the first time, we plan to measure directly the limb-darkening of few Cepheids with the CHARA/SPICA instrument in the visible domain (WP3). Besides, we will improve our models of the atmosphere of Cepheids (hydrodynamical and quasi static) in order to study the line forming regions as a function of the pulsation and the period.  Such very first tomography of Cepheids is indeed crucial to infer the real pulsation velocity of Cepheids (WP4). By injecting these new constraints (SBCR including CSEs properties, limb-darkening and atmospheric velocity gradients) into the projection factor decomposition already established by our team, we will develop a new methodology to infer the projection factor for any Cepheids (WP5). If achieved, these ambitious objectives have the potential to unlock the projection factor of Cepheids and open a new road toward Ho.