Spectral energy distributions of classical cepheids in the Magellanic Clouds

Martin Groenewegen, Jan Lub

Published 2023-07-14Version 1

(abridged) In this study, we constructed spectral energy distributions (SEDs) for a sample of 142 LMC and 77 SMC fundamental-mode classical Cepheids (CCs) using photometric data from the literature. When possible, the data were taken to be representative of mean light or averaged over the light curve. The sample was built from stars that either have a metallicity determination from high-resolution (HR) spectroscopy or have been used in Baade-Wesselink types of analyses, or have a radial velocity curve published in Gaia DR3 or have Walraven photometry, or have their light- and radial-velocity curves modelled by pulsation codes. The SEDs were fitted with stellar photosphere models to derive the best-fitting luminosity and effective temperature. Only one star with a significant infrared excess was found in the LMC and none in the SMC, suggesting that IR excess may be more prominent in MW cepheids than in the Magellanic Clouds. For the large majority of stars, the position in the Hertzsprung-Russell diagram is consistent with theoretical instability strips. Period-luminosity (PL) and period-radius relations were derived and compared to these relations in the MW. For a fixed slope, the zero point of the bolometric PL relation does not depend on metallicity, contrary to recent findings of a significant metallicity term when considering the PL relation in different photometric bands. An intriguing result concerns the flux-weighted gravity (FWG, a quantity derived from gravity and Teff) and its relation to period and luminosity. Both relations agree with theory, with the results for the MW, and with the independent estimates from the six known LMC eclipsing binaries that contain CCs. However, the FWG (as determined from dedicated HR spectroscopy for the sample) is too low by about 0.8 dex in 90 percent of the cases.