Multi-chromatic observations of classical Cepheids using the CHARA Array interferometer: Surface brightness-colour relation, projection factor, and limb-darkening

@misc{bailleul2026multichromatic,
  title = {{Multi-chromatic observations of classical Cepheids using the CHARA Array interferometer: Surface brightness-colour relation, projection factor, and limb-darkening}},
  author = {M. C. Bailleul and N. Nardetto and D. Mourard and A. Mérand and P. Kervella and A. Gallenne and G. Bras and J. Monnier and K. Sivkova and B. Apostolova and L. Breuval and W. Kiviaho and W. Gieren and V. Hocdé and R. S. Rathour and G. Pietrzyński and P. Bério and F. Morand and D. Salabert and J. Jones and C. Lanthermann and N. Ebrahimkutty and J. Jonàk and H. Nowacki and R. V. Ibañez Bustos and M. Vrard and S. Kraus and N. Anugu and M. Gutierrez and N. Ibrahim and T. Gardner},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.15804},
  eprint = {2609.15804},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.15804},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Multi-chromatic observations of classical Cepheids using the CHARA Array interferometer: Surface brightness-colour relation, projection factor, and limb-darkening
AU  - M. C. Bailleul
AU  - N. Nardetto
AU  - D. Mourard
AU  - A. Mérand
AU  - P. Kervella
AU  - A. Gallenne
AU  - G. Bras
AU  - J. Monnier
AU  - K. Sivkova
AU  - B. Apostolova
AU  - L. Breuval
AU  - W. Kiviaho
AU  - W. Gieren
AU  - V. Hocdé
AU  - R. S. Rathour
AU  - G. Pietrzyński
AU  - P. Bério
AU  - F. Morand
AU  - D. Salabert
AU  - J. Jones
AU  - C. Lanthermann
AU  - N. Ebrahimkutty
AU  - J. Jonàk
AU  - H. Nowacki
AU  - R. V. Ibañez Bustos
AU  - M. Vrard
AU  - S. Kraus
AU  - N. Anugu
AU  - M. Gutierrez
AU  - N. Ibrahim
AU  - T. Gardner
PY  - 2026
DA  - 2026/09/14
PB  - arXiv
DO  - 10.48550/arxiv.2609.15804
UR  - https://arxiv.org/abs/2609.15804
AB  - The Baade-Wesselink (BW) method compares the linear and angular variations of Cepheids to derive their distance. This method is limited, however, by the projection factor, which relates the observed radial velocity to the true pulsation velocity of the star. Using simultaneous observations from the CHARA Array interferometer in the K, H, and R bands with the MYSTIC, MIRC-X, and SPICA combiners, respectively, we aim to understand the physics of Cepheid atmospheres better. Applying a specific method to multi-chromatic simultaneous interferometric observations of Cepheids, we derived robust limb-darkened angular diameters that were then used to calibrate the SBCR and study the projection factor. We also developed a strategy to measure the limb-darkening of Cepheids in R, H, and K bands. These measurements were then used to constrain the geometrical component of the projection factor. From the limb-darkened angular diameter curves of three Cepheids, we decreased the scatter of the SBCR in V−K colour to 0.0011 magnitude and to 0.0040 for the SBCR in G_BP−G_RP. These SBCRs are particularly robust because they are based on multi-chromatic diameters and are homogeneous, which previous calibrations for Cepheids were not. For the very first time, we derived the limb-darkening of Cepheids in R, H, and K bands. For δ Cep, we derived an R-based projection factor of 1.275 ± 0.051, and the geometrical part obtained from the measured limb-darkening coefficient in R band was estimated to 1.420 ± 0.016 (on average), as expected from stellar static and hydrodynamical atmosphere models. The limb-darkening coefficients obtained in H and K band are consistent with models. These results demonstrate that multi-chromatic interferometry can improve the accuracy of the BW method. It is therefore essential to continue the CHARA survey of Cepheids in the coming years.
SN  - 0004-6361
SN  - 1432-0746
ER  -