Speckle masking imaging and radiative transfer modeling of the oxygen-rich dust shells of AFGL 2290 and CIT 3
Abstract
The extreme mass loss suffered by stars on the asymptotic giant branch (AGB) substantially affects the appearance of these objects, which is dominated by the development of opaque, massive, circumstellar dust shells (CDS), and even more important, it determines the final evolution to the planetary nebula stage. High spatial resolution observations directly provide information on important properties of CDS around AGB stars, such as the dimensions and geometry of the shell, and thereby contribute strong constraints for the modeling of the mechanisms and processes determining these circumstellar environments. We present diffraction-limited speckle masking observations of the oxygen-rich AGB stars AFGL 2290 (OH 39.7+1.5) at 2.2 microns and of CIT 3 (OH 128.6-50.1) at 1.65 microns and 2.2 microns. The speckle interferograms were obtained with the SAO 6 m telescope, and we achieved the diffraction-limited resolutions of 56 mas and 76 mas at 1.65 microns and 2.2 microns, respectively. The CDS of AFGL 2290 is partially resolved and found to be slightly asymmetric with a mean Gaussian FWHM diameter of ~42 mas. The azimuthally averaged visibility yields an upper limit of ~25% for the stellar contribution to the 2.2 microns flux, suggesting a rather high optical depth. We have performed detailed radiative transfer calculations assuming a spherically symmetric dust shell, and found that such models reproduce either the observed spectral energy distribution, or the measured visibility of AFGL 2290, but not both simultaneously. We interpret this behaviour as being due to a non-spherical dust distribution supporting the evidence from the speckle masking image reconstruction. The speckle imaging results for CIT 3 indicate a more structured CDS compared to AFGL 2290. The azimuthally averaged visibilities can only be well fitted with a two-component model consisting of a partially resolved smaller component and a fully resolved nebulosity, which is several times more extended. The smaller component represents the hot innermost region of the CDS, whereas the presence of the extended component might point to a change of physical properties in the outflow.
- Publication:
-
IAU Symposium
- Pub Date:
- 1998
- Bibcode:
- 1998IAUS..191P.408I