High-resolution Linear Spectropolarimetry of Red Supergiant VY CMa: Identification of Atomic and Molecular Features

Abstract

We present a high-resolution optical spectropolarimetric observation of the red supergiant VY CMa. The optical spectrum of VY CMa contains numerous emission features that are mainly due to molecular bands of TiO, ScO, and VO. In addition, emission lines of Ba ii, Ca i, Na i, K i, and Rb i are identified in the spectrum. Our observation shows that all the molecular bands exhibit a very large and abrupt linear polarization reduction, i.e., depolarization, across the band heads of up to 10% or higher with respect to the continuum polarization level. The high-resolution observations demonstrate clearly that the molecular band heads are associated with the largest depolarization. Using this property together with a simulated emission spectrum of TiO we are able to identify most of the broad emission features in the optical spectrum of VY CMa as the TiO band heads in emission. Our observations also reveal a previously unknown phenomenon that the atomic lines either in emission or absoprtion also exhibit large depolarization across the line profile, with the K i doublet showing almost complete depolarization. The observed polarization pattern is very similar among either absoprtion or emission lines, even though the spectral lines belong to different atomic species. Together with recent high spectral resolution observations, we discuss the origin of this phenomenon and suggest that the observed large depolarization across molecular bands and atomic lines is due to absorption and scattering in the complex circumstellar environment of VY CMa, especially in the dense dusty clumps located almost on the plane of the sky.