The icy side of Frosty Leo

An imaging study of the bipolar nebula Frosty Leo is presented. In particular, we have studied the effects of spatial distribution of H2O ice on the circumstellar environment. Using the Keck telescope with the NIRC infrared camera, we have recovered images at a wavelength of 3.08 μm within the prominent (attenuation of about 5 mag) water-ice absorption band, and in the adjacent quasi-continuum at 1.6 μm, 2.2 μm, and 3.3 μm. While the well-known bipolar structure appears quite symmetrical in all quasi-continuum images, the Northern lobe seems to be almost totally extinguished (by a factor of > ∼10) compared to the Southern one in images at 3.08 μm. This suggests a much greater concentration of ice between the observer and the Northern lobe. The question of the physical structure of the nebula has been addressed with the use of numerical radiative transfer simulations which have been able to reproduce the general behavior of the images and of earlier spectral data. From this, we deduce and/or confirm several parameters of interest: the typical dust grain size, the dust density distribution in the envelope, and the abundance of water ice. Models were produced with physically symmetrical lobes in which some one-sided extinction was produced by an effect related to the observer’s viewing angle. However, in order to reproduce the extreme extinction observed, more complicated scenarios, involving asymmetry with respect to the plane of the disk, were needed. Additional imaging observations were conducted at 2.26 μm (where the system is bright) with the intent to recover high-angular-resolution information. Data sets consisted of one hundred of rapid-exposure data frames from which images were recovered using a shift-and-add algorithm. Although these images did contain structures at the diffraction limit of the telescope (∼50 milliarcsec), no evidence for binarity as reported by Roddier et al. (1995) from adaptive optical studies was found within the dynamical range obtained.