Complex Organic Materials in the Circumstellar Disk of HR 4796A

We combine HST NICMOS imaging photometry of the HR 4796A disk at previously unobserved wavelengths between 1.71-2.22 μm with reprocessed archival observations to produce a measure of the dust's scattering efficiency as a function of wavelength. The spectrum of the dust, synthesized from the seven photometric measures, is characterized by a steep red slope increasing from 0.5 to 1.6 μm followed by a flattening of the spectrum at wavelengths >1.6 μm. We fit the spectrum with a model population of dust grains made of tholins, materials composed of complex organic materials seen throughout the outer parts of our solar system. The presence of organic material around a star that may be in the later stages of giant planet formation implies that the basic building blocks for life may be common in planetary systems.