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Astron. Astrophys. 348, 805-814 (1999)

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1. Introduction

The star IRC +10 420 (= V 1302 Aql = IRAS 19244+1115) is an outstanding object for the study of stellar evolution. Its spectral type changed from [FORMULA] in 1973 (Humphreys et al. 1973) to mid-A today (Oudmaijer et al. 1996, Klochkova et al. 1997) corresponding to an effective temperature increase of 1000-2000 K within only 20 yr. It is one of the brightest IRAS objects and one of the warmest stellar OH maser sources known (Giguere et al. 1976, see also Mutel et al. 1979, Diamond et al. 1983, Bowers 1984, Nedoluha & Bowers 1992). Ammonia emission has been reported by McLaren & Betz (1980) and Menten & Alcolea (1995). Large mass-loss rates, typically of the order of several [FORMULA] [FORMULA]/yr (Knapp & Morris 1985, Oudmaijer et al. 1996) were determined by CO observations. Two evolutionary scenarios have been suggested for IRC +10 420: It is either a post-AGB (AGB: Asymptotic Giant Branch) star evolving through the proto-planetary nebula stage (e.g. Fix & Cobb 1987, Hrivnak et al. 1989, Bowers & Knapp 1989), or it is a massive hypergiant evolving from the RSG (Red Supergiant Branch) branch towards the Wolf-Rayet phase (e.g. Mutel et al. 1979, Nedoluha & Bowers 1992, Jones et al. 1993, Oudmaijer et al. 1996, Klochkova et al. 1997). However, due to its distance (d = 3-5 kpc), large wind velocity (40 km s-1) and photometric history, IRC +10 420 is most likely a luminous massive star (see Jones et al. 1993and Oudmaijer et al. 1996), therefore being the only massive object observed until now in its transition to the Wolf-Rayet phase. The structure of the circumstellar environment of IRC +10 420 appears to be very complex (Humphreys et al. 1997), and scenarios proposed to explain the observed spectral features of IRC +10 420 include a rotating equatorial disk (Jones et al. 1993), bipolar outflows (Oudmaijer et al. 1994), and the infall of circumstellar material (Oudmaijer 1998).

Previous infrared speckle and coronographic observations were reported by Dyck et al. (1984), Ridgway et al. (1986), Cobb & Fix (1987), Christou et al. (1990) and Kastner & Weintraub (1995). In this paper we present diffraction-limited 73 mas bispectrum speckle-interferometry observations of the dust shell of IRC +10 420 as well as radiative transfer calculations to model its spectral energy distribution and visibility.

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199