Revealing the dusty shell of the semi-regular variable V CVn

Abstract

The semi-regular variable (SRV) star V CVn exhibits a highly variable optical linear polarization, yet its position angle (PA) has remained remarkably stable at approximately 100 +/- 20 deg. over several decades. We present a comprehensive multi-technique study of the system covering six pulsation cycles from 2020 to 2025. This campaign includes high-cadence photopolarimetry, spectropolarimetry, and the first interferometric observations of V CVn using the CHARA Array (MIRC-X and MYSTIC) in 2024 and 2025. Our photopolarimetric data reveal a significant increase in polarization, reaching peaks of up to 6% during the 2023-2025 cycles, while the characteristic anticorrelation with brightness and the stability of the PA persisted. The interferometric observations resolve the stellar photosphere but show no evidence of large-scale asymmetric structures, such as a bow shock or isolated scattering arcs, at the scales and sensitivities probed. Based on these results, we exclude previous models based on interstellar medium (ISM) interactions or static reflection clouds as the primary drivers of the observed polarization. Instead, we propose a circumstellar disk or flattened envelope, analogous to the L2 Puppis system, as the most plausible explanation. We hypothesize that this stable geometry may be shaped or maintained by the star's high proper motion, where ram pressure from the ISM interaction favors a flattened distribution of circumstellar material. Within this stable framework, the polarization peaks are interpreted as the result of stochastic dust clumps, likely triggered by the star's long secondary period (LSP), which causes episodic increases in mass loss. This combination of an ISM-influenced disk geometry and transient, LSP-driven dust clumps naturally accounts for both the decadal stability of the position angle and the highly variable polarization amplitude of V CVn.