2024/01/16 by Garance Bras, Bras, Garance, P. Kervella +23
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Gamma-ray bursts and supernovae #Solar and Stellar Astrophysics (astro-ph.SR) #Stellar, planetary, and galactic studies
paper · pdf · doi:10.48550/arxiv.2401.08566
openalex publication_date 2024/01/16 · openalex created_date 2024/01/18 · openalex updated_date 2026/07/28
The application of the parallax of pulsation (PoP) technique to determine distances of pulsating stars implies the use of a scaling parameter, the projection factor (p-factor), required to transform disc-integrated radial velocities (RVs) into photospheric expansion velocities. The value of the p-factor is poorly known and debated. Most PoP applications assume a constant p-factor. However, it may actually depend on the physical parameters of each star. We aim to calibrate p-factors for RR Lyrae stars (RRLs) and compare them with classical Cepheids (CCs). Due to their higher surface gravity, RRLs have more compact atmospheres, and provide a valuable comparison with their supergiant siblings. We determined the p-factor of 17 RRLs using the SPIPS code, constrained by Gaia DR3 parallaxes, photometry, and new RVs from the OHP/SOPHIE spectrograph. We carefully examine the different steps of the PoP technique, particularly the method to determine RV from spectra using the classical cross-correlation function (CCF) approach. The method employed for RV extraction from the CCF has a strong impact on the p-factor, of up to 10%. However, this choice of method results in a global scaling of the p-factor, marginally affecting the scatter within the sample for a given method. Over our RRL sample, we find a mean value of p = 1.248 ± 0.022 for RVs derived using a Gaussian fit of the CCF. There is no evidence for a different value of the p-factor of RRLs, although its distribution for RRLs appears significantly less scattered than that for CCs. The p-factor does not appear to depend in a simple way on fundamental stellar parameters. We argue that large-amplitude dynamical phenomena occurring in the atmospheres of RRLs and CCs during their pulsation affect the relative velocity of the spectral line-forming regions compared to the velocity of the photosphere.