Program: GN-2018B-Q-410

Yellow hypergiants (YHGs) are evolved massive stars that were suggested to be in post-red supergiant stage. Post-red supergiants that evolve back to the blue, hot side of the Hertzsprung-Russell diagram can intersect a temperature domain in which their atmospheres become unstable against pulsations (the Yellow Void), and the stars can experience outbursts with short, but violent mass eruptions. The YHG Rho Cas is famous for its historical and recent outbursts, during which the star develops a cool, optically thick wind with a very brief but high mass-loss rate. Atomic and molecular emission features in the optical and near-infrared regime appear during phases of maximum brightness and have been suggested to be circumstellar in nature, whereas absorption features seem to vary in phase with the pulsation cycle. We wish to test this hypothesis by a detailed study of the CO bands seen in the near-infrared regime. For this, we request a series of high-resolution (R ~ 18 000) K-band spectra with GNIRS. Modeling these features in combination with supplementary data (photometric light-curve and optical spectroscopy taken by other facilities) will provide unambiguous evidence for the origin of the emission as well as detailed knowledge on the global dynamics of the atmosphere of Rho Cas and its pulsation and mass-loss activity. This information is vital for the fate of this object as an important representative of the class of YHGs.