Radius Tracing from Mid-IR Light Curve Derivatives: A New Distance Estimation Technique for Cepheids

NEOWISE, the reactivated phase of the WISE mission, has been conducting full-sky surveys in the 3.4 and 4.6 μm bands since December 2013, following the exhaustion of its cryogenic coolant. The mission’s regular six-month cadence and wide-field coverage yield an extensive temporal baseline of mid-infrared photometry, rendering it highly suitable for time-domain analyses of variable stars such as Cepheids. The mid-infrared wavelength regime offers significant advantages for studying the period–luminosity relation and implementing distance determination methodologies, including the Baade–Wesselink (BW) technique. In this work, we develop a novel method that utilizes the first temporal derivative of the 3.6 μm light curve, which predominantly traces stellar radius variations with minimal temperature dependence, and directly correlates this derivative with spectroscopically measured radial velocities (expressed in km s⁻¹) to infer absolute magnitudes. This approach facilitates distance estimation using solely single-band photometry and radial velocity data, thereby providing a streamlined alternative to the classical BW method that conventionally necessitates multi-band photometry to disentangle radius and temperature effects. By effectively isolating radius variability, the proposed method circumvents one of the principal challenges inherent to traditional BW analyses, namely the separation of radius and temperature contributions.