2012A&A...546A.105N

We study the time history of the yellow hypergiant HR 8752 based on high-resolution spectra (1973-2005), the observed MK spectral classification data, B-V- and V-observations (1918-1996) and yet earlier V-observations (1840-1918). Our local thermal equilibrium analysis of the spectra yields accurate values of the effective temperature (T_eff), the acceleration of gravity (g), and the turbulent velocity (v_t) for 26 spectra. The standard deviations average are 82K for T_eff, 0.23 for logg, and 1.1km/s for v_t. A comparison of B-V observations, MK spectral types, and T_eff-data yields E(B-V), ``intrinsic'' B-V, T<sub>eff</sub>, absorption A<sub>V</sub>, and the bolometric correction BC. With the additional information from simultaneous values of B-V, V, and an estimated value of R, the ratio of specific absorption to the interstellar absorption parameter E(B-V), the ``unreddened'' bolometric magnitude m_bol,0 can be determined. With Hipparcos distance measurements of HR 8752, the absolute bolometric magnitude M_bol,0 can be determined. Over the period of our study, the value of T_eff gradually increased during a number of downward excursions that were observable over the period of sufficient time coverage. These observations, together with those of the effective acceleration g and the turbulent velocity v_t, suggest that the star underwent a number of successive gas ejections. During each ejection, a pseudo photosphere was produced of increasingly smaller g and higher v_t values. After the dispersion into space of the ejected shells and after the restructuring of the star's atmosphere, a hotter and more compact photosphere became visible. From the B-V and V observations, the basic stellar parameters, T_eff, logM/M_☉, logL/L_☉, and logR/R_☉ are determined for each of the observational points. The results show the variation in these basic stellar parameters over the past near-century. We show that the atmospheric instability region in the HR-diagram that we baptize the yellow evolutionary void actually consists of two parts. We claim that the present observations show that HR 8752 is presently climbing out of the ``first'' instability region and that it is on its way to stability, but in the course of its future evolution it still has to go through the second potential unstable region.