2025A&A...694A.101H

Context. The circumstellar envelopes (CSE) of Cepheids are still only poorly characterized despite their potential impact on the distance determination via both the period-luminosity relation and the parallax-of-pulsation method.
Aims. This paper aims to investigate Galactic Cepheids across the instability strip in the mid-infrared with MATISSE/VLTI in order to constrain the geometry and physical nature (gas and/or dust) of their CSEs.
Methods. We secured observations of eight Galactic Cepheids with short- and up to long-period pulsations with MATISSE/VLTI in the L, M, and N bands. For each star, we calibrated the flux measurements to potentially detect the spectral dust signature in the spectral energy distribution (SED). We then analyzed the closure phase and the visibilities in L, M, and N bands. The parallax-of-pulsation code SPIPS was used in order to derive the infrared excess and the expected angular stellar diameter at the date of the MATISSE observations. We also computed test cases of a radiative transfer model of dusty envelopes with DUSTY to compare them with the visibilities in the N band.
Results. The SED analysis in the mid-IR confirmed the absence of a spectral dust signature for the entire star sample. For each star, we observed closure phases in the L, M, and N bands that are consistent with a centro-symmetric geometry for the different targets. Finally, the visibilities in the L, M, and N bands agree with the expected angular stellar diameter. Although we did not resolve any circumstellar emission, the observations are compatible with the presence of compact CSEs within the uncertainties. We provide 2 σ upper limits on the CSE flux contribution based on model residuals for several CSE radii, which exclude models that are simultaneously large and bright (R_CSE ≈ 10 R_⋆ and f_CSE ≈ 10%) for all the stars of the sample. Last, the visibilities in the N band rule out CSE models with optical depth τ_V ≳ 0.001 for different types of dust.
Conclusions. The MATISSE observations of eight Cepheids with different pulsation periods (from 7 to 38 days) and evolution stages provide a comprehensive picture of Cepheids from mid-infrared interferometry for the first time. We present additional evidence that circumstellar dust emission is negligible or absent around Cepheids for a wide range of stellar parameters in the instability strip. Further interferometric observations in the visible and near-infrared are required to separate the star and CSE, which is crucial for constraining the CSE contribution and its possible gaseous nature.