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Yellow hypergiants are evolved massive stars that were suggested to be inpost-red supergiant stage. Post-red supergiants that evolve back to the blue,hot side of the Hertzsprung-Russell diagram can intersect a temperature domainin which their atmospheres become unstable against pulsations (the Yellow Voidor Yellow Wall), and the stars can experience outbursts with short, but violentmass eruptions. The yellow hypergiant Rho Cas is famous for its historical andrecent outbursts, during which the star develops a cool, optically thick windwith a very brief but high mass-loss rate, causing a sudden drop in the lightcurve. Here we report on a new outburst of Rho Cas which occurred in 2013,accompanied by a temperature decrease of ~3000 K and a brightness drop of 0.6mag. During the outburst TiO bands appear, together with many low excitationmetallic atmospheric lines characteristic for a later spectral type. With thisnew outburst, it appears that the time interval between individual eventsdecreases, which might indicate that Rho Cas is preparing for a major eruptionthat could help the star to pass through the Yellow Void. We also analysed theemission features that appear during phases of maximum brightness and find thatthey vary synchronous with the emission in the prominent [CaII] lines. Weconclude that the occasionally detected emission in the spectra of Rho Cas, aswell as certain asymmetries seen in the absorption lines of low tomedium-excitation potential, are circumstellar in nature, and we discuss thepossible origin of this material.

A new outburst of the yellow hypergiant star Rho Cas