## The hypergiant HR 8752 evolving through the yellow evolutionary
void^{⋆,}^{⋆⋆}

^{1}
SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA
Utrecht, The
Netherlands

e-mail: h.nieuwenhuijzen@sron.nl;
h.nieuwenhuijzen@xs4all.nl

^{2}
NIOZ, Royal Netherlands Institute for Sea Research,
PO Box 59, Den Burg, The
Netherlands

e-mail: cdej@kpnplanet.nl

^{3}
Tartu Observatory, 61602
Tõravere,
Estonia

e-mail: indrek@aai.ee

^{4}
Instituto de Astrofisica de Canarias, via Lactea s/n, 38200 La Laguna, Tenerife, Spain

e-mail: gil@iac.es

^{5}
Royal Observatory of Belgium, Ringlaan 3, 1180
Brussels,
Belgium

e-mail: alobel@sdf.lonestar.org; Alex.Lobel@oma.be

^{6}
Konkoly Observatory, PO Box 67, 1525
Budapest,
Hungary

e-mail: zsoldos@konkoly.hu

^{7}
Observatoire de Genève, 1290
Sauverny,
Switzerland

e-mail: Andre.Maeder@unige.ch; Georges.Meynet@unige.ch

Received:
30
April
2011

Accepted:
13
March
2012

*Context.* 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).

*Aims.* 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 82 K for *T*_{eff}, 0.23 for log *g*,
and 1.1 km s^{-1} for *v*_{t}.

*Methods.* A comparison of *B* − *V*
observations, MK spectral types, and *T*_{eff}-data yields
*E*(*B* − *V*), “intrinsic”
*B* − *V*, *T*_{eff},
absorption *A*_{V}, 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.

*Results.* 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},
log *M*/*M*_{⊙},
log *L*/*L*_{⊙}, and
log *R*/*R*_{⊙} are
determined for each of the observational points. The results show the variation in these
basic stellar parameters over the past near-century.

*Conclusions.* 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.

Key words: stars: atmospheres / stars: evolution / supergiants / stars: mass-loss / stars: fundamental parameters / stars: variables: S Doradus

^{⋆⋆}

Tables A.x and B.x are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/546/A105, and at the external site http://www.aai.ee/HR~8752

*© ESO, 2012*