GIARPS simultaneous optical and infrared high-spectral-resolution observations of Cepheids I. Limb darkening from cross-correlated radial velocities

N. Nardetto; R. S. Rathour; M. C. Bailleul; V. Hocdé; P. Kervella; G. Bras; W. Gieren; J. Storm; G. Pietrzynski; B. Zgirski; P. Wielgorski; A. Merand; A. Gallenne; E. Poretti; M. Rainer; L. Breuval; M. Duparc; B. Apostolova; W. Kiviaho; K. Sivkova

arXiv · 2609.21855

GIARPS simultaneous optical and infrared high-spectral-resolution observations of Cepheids I. Limb darkening from cross-correlated radial velocities

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Abstract

The Baade-Wesselink method, while theoretically applicable for calibrating the distance scale via Cepheids, is not currently employed in the cosmic distance ladder. There are various versions of this method. In all these approaches, the projection factor used to convert the radial into the pulsation velocity remains the limiting quantity. By comparing for the first time simultaneous cross-correlated optical and H-band radial velocities of five Cepheids, we aim to better understand the geometric part of the projection factor that is directly linked to the limb darkening of the stars. The five Cepheids were observed with the HARPS-N and GIANO-B instruments mounted at the Telescopio Nazionale Galileo in the GIARPS configuration. A two-parameter linear fit was applied to the simultaneous radial velocity measurements of each star in order to extract the amplitude ratio of both curves (fVH) as well as the difference in mean velocities. We find a mean value of fVH of 1.056 with a root mean square over the five stars of 0.016, which means that the V-band radial velocity amplitudes are around 5% larger than the ones obtained in the H band. In addition, we find that the H-band radial velocity curves have an average (i.e., a gamma-velocity) of about 1 km/s blueshifted compared to the visible ones, except for one star, X Cyg, which has a blueshift of 0.4 km/s. At first order, the fVH values are directly linked to the ratio of limb darkening coefficients in both bands (or equivalently the ratio of geometrical projection factors). The fVH values are found to be consistent with expectations from static stellar atmosphere models. This work shows that the differential limb darkening of Cepheids in visible and H bands, and consequently the geometrical part of the projection factor, follows what is expected from stellar atmosphere models.

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N. Nardetto, R. S. Rathour, M. C. Bailleul, V. Hocdé, P. Kervella, G. Bras, W. Gieren, J. Storm, G. Pietrzynski, B. Zgirski, P. Wielgorski, A. Merand, A. Gallenne, E. Poretti, M. Rainer, L. Breuval, M. Duparc, B. Apostolova, W. Kiviaho, K. Sivkova. 2026-09-18. GIARPS simultaneous optical and infrared high-spectral-resolution observations of Cepheids I. Limb darkening from cross-correlated radial velocities. https://arxiv.org/abs/2609.21855

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