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We report on multi-epoch optical and near-infrared spectroscopy around thefirst overtone ro-vibrational band of CO in the pulsating yellow hypergiant RhoCas, one of the most massive stars in the Galaxy and a candidate SN IIprogenitor. We argue that the double cores of the CO absorption lines, thathave previously been attributed to separate circumstellar shells expelledduring its recurrent outbursts, result in fact from a superposition of a wideabsorption line and a narrow central emission line. The CO line doublingreturns over subsequent pulsation cycles, where the superposed line emissionassumes its largest intensity near phases of maximum light. We find that themorphology and behavior of the CO band closely resemble the remarkable"line-splitting phenomenon" also observed in optical low-excitation atomiclines. Based on radiative transport calculations we present a simplified modelof the near-IR CO emission emerging from cooler atmospheric layers in theimmediate vicinity of the photosphere. We speculate that the kinetictemperature minimum in our model results from a periodical pulsation-drivenshock wave. We further discuss a number of alternative explanations for theorigin of the ubiquitous emission line spectrum, possibly due to aquasi-chromosphere or a steady shock wave at the interface of a fast expandingwind and the ISM. We present a number of interesting spectroscopic similaritiesbetween Rho Cas and other types of cool variable supergiants such as the RV Tauand R CrB stars. We further propose a possibly common mechanism for theenigmatic outburst behavior of these luminous pulsating cool stars.Comment: accepted to ApJ; 3 color fig

On the CO Near‐Infrared Band and the Line‐splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae