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

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High resolution near-infrared polarimetry of [FORMULA] Carinae and the Homunculus Nebula

J.R. Walsh 1 and N. Ageorges 1,2

1 Space Telescope European Co-ordinating Facility, European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany (jwalsh@eso.org)
2 Physics Department, National University of Ireland - Galway, Galway, Ireland (nancy@epona.physics.ucg.ie)

Received 5 March 1999 / Accepted 7 March 2000

Abstract

High resolution near-infrared adaptive optics imaging and polarimetry have been obtained of the Homunculus Nebula and its central illuminating star [FORMULA] Carinae (HD 93308). Images and maps of the linear polarization at a resolution of [FORMULA]0.2" have been made in the J, H and K bands and in a narrow 2.15 µm continuum band (Kc). The overall appearance of the nebula in the near-IR is similar in all bands and to that at V, with specific features less sharp to longer wavelengths. A comparison between the published HST WFPC2 1.042 µm map and the AO J band image demonstrates that ground-based AO resolution can approach that of HST. The large-scale pattern of polarization vectors is centro-symmetric demonstrating that single scattering dominates everywhere except perhaps in the central bright core. The only difference between the near-IR and optical appearance is a narrow linear feature at position angle 320o extending across the NW lobe of the Homunculus. Polarization maps at Kc in the near vicinity of [FORMULA] Carinae were restored using a PSF derived from blind deconvolution. There is a definitely detected extension in the direction of the optically visible speckle knots and an estimate of 18% for the polarization of one of the knots was made. This level of polarization suggests that the knots arise in dust+gas clouds in the near environment of [FORMULA] Carinae, perhaps in an equatorial disc.

The most remarkable result of the linear polarization mapping is the level of similarity in the spatial structure, and also in the degree of linear polarization, between the near-IR and optical data. Comparison of the polarization along the projected major axis of the Homunculus shows values in the SE lobe to within a few percent from V band to 2.2 µm. In the NW lobe the near-IR linear polarization values agree to within a few percent over the 1.2-2.2 µm range, but are upto 10% lower than at V. Such a polarization pattern cannot arise in Mie scattering from a single power law distribution of grain sizes unless the particles are very small compared with the wavelength, in disagreement with mid-IR observations. In addition the colour dependence of the extinction was found to be shallower than the typical ISM, indicating the presence of large grains. Several possibilities are explored to try to explain these contradictory results. Optical depth effects and a broad distribution in grain sizes are favoured, perhaps with a dependence of the grain size with depth into the small obscuring clouds in the lobes of the Homunculus. However the presence of aligned grains, previously inferred from mid-IR polarization, may also affect the scattered radiation from the dust.

Key words: ISM: reflection nebulae – stars: individual: j Car – techniques: polarimetric

Send offprint requests to: J.R. Walsh

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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