# Extended atmosphere of the yellow hypergiant V509 Cas in 1996-2018

## TL;DR

This study analyzes 22 years of spectral data of the yellow hypergiant V509 Cas, revealing a stable extended atmosphere with specific velocity patterns, and finds no evidence of a companion star.

## Contribution

It provides a detailed long-term spectral monitoring analysis of V509 Cas, highlighting atmospheric stability and velocity characteristics over two decades.

## Key findings

- Stable systemic velocity of -63 km/s.
- Forbidden [NII] emissions are stable and systematically shifted.
- Atmospheric layers near the photosphere show variability likely due to pulsations.

## Abstract

Based on the data of spectral monitoring of the yellow hypergiant V509 Cas performed in 1996-2018 at the 6-m telescope with the spectral resolution of R$\ge$60 000, we studied in detail its kinematic state at various levels of extended atmosphere. No signs of presence of a companion were found. An agreement of radial velocities measured on the permitted and forbidden emissions of metal ions, as well as their strict temporal stability led us to the choice of the systemic velocity of the star Vsys=$-63$ km/s. The position of forbidden [NII] emissions forming in the circumstellar medium is strictly stable and is systematically shifted by $-6$ km/s relative to the metal ion emissions. A conclusion on the variation of the [NII] emission halfwidths and intensities (the lines have become narrower and more intense) is made after the observations in 1996 and these parameters did not vary over the next 22 years of observations. The velocities measured from the shortwave FeII(42) absorption components are located in a narrow interval of Vr=$-$(84$\div$87) km/s, which indicates the stability of expansion of the upper layers of the atmosphere. The overall atmosphere of the hypergiant is stable, excluding the layers close to the photosphere. The velocity variability in range of Vr=$-$(52$\div$71) km/s, identified by the positions of strong metal ion absorption cores, may be a manifestation of pulsations in deep atmospheric layers, where this type of lines are formed.