Astron. Astrophys. 357, 255-267 (2000)

1. Introduction

Carinae, situated in the Carina complex at about 2.3kpc (Davidson & Humphreys 1997), is one of the most massive stars known in the Galaxy and is going through the Luminous Blue Variable phase (Humphreys & Davidson 1994) of unsteady mass loss (Davidson et al. 1986). During the 1840's it underwent an outburst and reached visual magnitude -1; since then is has been emerging from the dust which condensed after this ejection (Walborn & Liller, 1977). Long term monitoring of optical, IR, radio and X-ray spectra has revealed evidence of periodicity perhaps related to a binary or multiple star at the core of the nebula (Daminelli et al. 1997).

The compact nebula around Carinae (HD 93308), called the Homunculus, was first shown by Thackeray (1956) to be highly polarized. The initial polarimetry was confirmed by Wesselink (1960) who measured linear polarization of around 40%. Visvanathan (1967) observed that the polarization centred on Carinae was almost constant with wavelength from U to R and increased with increasing aperture size. In a small aperture, higher polarization was observed on the NW side of the nebula than on the SE. The first systematic polarization maps were made by Warren-Smith et al. (1979) in the V band and demonstrated a centro-symmetric pattern of polarization vectors with a marked asymmetry in the polarization values along the major axis (position angle 130^o) with values upto 40% in the NW lobe. To produce such high values of polarization in a reflection nebula, Mie scattering by silicate particles with a size distribution weighted to smaller particles was invoked and modelled by Carty et al. (1979). In the near (0.5") vicinity of Car itself, speckle masking polarimetry in the H line and local continuum has revealed evidence for a compact equatorial disc aligned with the minor axis of the Homunculus (Falcke et al. 1996). Within 1" of Car the polarization vector pattern does not remain centrosymmetric in the R band, suggesting that local structures and perhaps intrinsic emission may contribute to the morphology and scattered light (Falcke et al. 1996). Polarimetry in the mid-infrared, where the dust emits rather than scatters radiation, shows an entirely different pattern of polarization vectors with a trend to be oriented radially, particularly near the boundary of the emission (Aitken et al. 1995). Such a pattern can be interpreted in terms of emission from aligned grains; Aitken et al. (1995) suggest that the alignment mechanism may be gas-grain streaming, driven by the high outflow velocity, or a remnant magnetic field from a dense magnetized disc.

There is a wealth of IR observations of Car and the Homunculus on account of its intrinsic IR brightness, first observed by Westphal & Neugebauer (1969), and astrophysical interest. The IR spectrum is characterized by a peak around 10 µm, indicative of silicate grains (Mitchell & Robinson, 1978). There is a central IR point source together with a second peak on the minor axis of the nebula, whose separation increases from 1.1 to 2.2" from 3.6 to 11.2 µm (Hyland et al. 1979). The near-IR spectrum of Car shows a steep increase with wavelength and prominent hydrogen lines of the Paschen and Brackett series as well as He I lines (Whitelock et al. 1983) and weaker Fe II and [Fe II] lines (Altamore et al. 1994). Maps in the J, H and K bands show that the structure is dominated by scattering, but beyond about 3 µm dust emission dominates (Allen 1989), with many clumps present. High spatial resolution observations have reported an unresolved central source (at L and M band, Bensammar et al. 1985), with filaments and unresolved knots within 1" detected in many IR bands (Gehring 1992). Maps in the mid-IR show a similar structure and the compact central source has a dust temperature 650 K and dust mass of 10^-4 with nearby dusty clouds associated into loop features (Smith et al. 1995). This source has been so prodigiously studied at so many wavelengths that it possesses its own review article in Annual Reviews of Astronomy and Astrophysics (Davidson & Humphries 1997).

Car can be considered an ideal source for adaptive optics on account of its very bright central, almost point, source (V7mag. - van Genderen et al. 1994) and the limited radial extent (10") of the Homunculus, which means that the source itself can be used as a reference star for the wavefront sensor. As a consequence, off-axis anisoplaniticity does not significantly affect the adaptive optics (AO) correction out to the edges of the nebula. Previous near-infrared AO imaging of Car was obtained (Rigaut & Gehring 1995), including some limited polarimetry (Gehring 1992) using the COME-ON AO instrument. Here we report on dedicated high resolution near-IR AO imaging polarimetry conducted at J, H, K, and in a continuum band at 2.15 µm, using the ADONIS system and SHARP II camera with the aim of studying the small-scale polarization structure of the Homunculus. The observations are described in Sect. 2; the reductions and polarization data are presented in Sect. 3 and the relevance of the results for the structure and dust properties of this remarkable reflection nebula are discussed in Sect. 4.

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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