Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Blöcker, K.-H. Hofmann, J.
Lichtenthäler,
R. Osterbart, G. Weigelt and Y. Balega:
The hypergiant IRC+10420:
High-resolution speckle masking interferometry and dust-shell modelling
Astronomische Gesellschaft Meeting Abstracts
(AGM 14, P34)
Poster presented at the Annual Scientific Meeting of the Astronomische
Gesellschaft
at Heidelberg, September 14-19, 1998
Abstract.
The peculiar star IRC+10420 is an outstanding object for the study of
stellar evolution. It is one of the brightest IRAS objects due its very
strong infrared excess by circumstellar dust and one of the warmest
stellar
OH maser sources known.
It exhibits large mass-loss rates, typically of the order of
several 10^-4 Msol/yr, and, even more importantly, its spectral type
changed from F8 Ia in 1973 (Humphreys et al. 1973, ApJ 179, L49)
to A-type today (Oudmaijer et al. 1996, MNRAS 280, 1062) corresponding
to an
effective temperature increase of 1000-2000 K within only 20 yr.
Due to its distance, large wind velocity and photometric history
IRC+10420
is most likely a luminous hypergiant evolving off the RSG branch,
therefore being the only massive object observed up to now in its
transition
to the Wolf-Rayet phase.
We present the first diffraction-limited K-band observations of
IRC+10420
with 76 mas resolution. The speckle interferograms were obtained with
the
6 m telescope at the Special Astrophysical Observatory. The
visibility shows an unresolved central object and an extended dust
shell.
The dust shell contributes 40% to the total flux.
We performed radiative transfer calculations to model the
spectral energy distribution and visibility. The observed dust shell
properties cannot be fitted by single-shell models but require the
introduction of different temperature and density components.