2007PASJ...59..973Y

begin{HTML}We made spectroscopic monitoring observations of the three yellow hypergiants (ρ Cassiopeia, HR8752, and IRC+10420) in the 0.9-2.4 µm wavelength range with a resolution of ∼2600 over a fifteen month period. The spectra of HR8752 and IRC+10420 showed no time variations. On the other hand, those of ρ Cas showed just a series of time variation in 2.3 µm CO features from nondetection to absorption through emission during 2002-2004. We interpret this variation as being a result of an expanding gas shell whose excitation temperature was being cooled down to ∼800K. The CO column densities were estimated from the CO absorption depths. Moreover, assuming spherical symmetry of the expanding shell with a constant expansion velocity of 35 km s^–1, a stellar effective temperature of 7000K, a stellar radius of 400 R_☉, solar abundance, and all carbon atoms forming CO molecules, we estimated the mass of the shell to be ∼2 x 10^–3M_☉ for a model with a single shell expanding from the photosphere, or ∼5 x 10^–4M_☉ for another model with an expanding shell and a static envelope. These estimates of the released mass strongly depend on assumptions of the expansion velocity, the abundance, and the dissociation of the CO molecules.