New Insight into the FS CMa System MWC 645 from Near-Infrared and Optical Spectroscopy

TL;DR

This study investigates the binary FS CMa system MWC 645 using near-infrared and optical spectroscopy, revealing molecular features, variability, and potential mass-transfer activity, advancing understanding of B[e] phenomena in such systems.

Contribution

It provides new spectroscopic data, detects CO molecular bands for the first time, and analyzes variability and envelope properties of MWC 645, contributing novel insights into FS CMa-type binaries.

Findings

Detection of CO absorption bands at 1.62 μm and 2.3 μm.

Correlation between Hα emission enhancement and optical brightness decrease.

Identification of a variable circumstellar envelope affecting brightness.

Abstract

The B[e] phenomenon is manifested by a heterogeneous group of stars surrounded by gaseous and dusty circumstellar envelopes with similar physical conditions. Among these stars, the FS CMa-type objects are suspected to be binary systems, which could be experiencing or have undergone a mass-transfer process that could explain the large amount of material surrounding them. We aim to contribute to the knowledge of a recently confirmed binary, MWC 645, which could be undergoing an active mass-transfer process. We present near-infrared and optical spectra, identify atomic and molecular spectral features, and derive different quantitative properties of line profiles. Based on publicly available photometric data, we search for periodicity in the light curve and model the spectral energy distribution. We have detected molecular bands of CO in absorption at 1.62 $\mu$m and 2.3 $\mu$m for the first time. We derive an upper limit for the effective temperature of the cool binary component. We found a correlation between the enhancement of the H$\alpha$ emission and the decrease in optical brightness that could be associated with mass-ejection events or an increase in mass loss. We outline the global properties of the envelope, possibly responsible for brightness variations due to a variable extinction, and briefly speculate on different possible scenarios.