The appearence of debris disks is an important stage in the evolution of young planetary systems, occurring after the dissipation of the gas-rich protoplanetary disk has left an optically thin disk of planetesimals that are the leftovers from the planetary formation. This disk in turn depletes with time, as the number of planitesimals declines, which 75% of young stars in the 18-million-year-old β Pictoris moving group having a detectable debris disk, compared to 20% of billion-year-old stars. Debris disks can be detected through their infrared emmissions, which are in excess of those produced by the stellar photosphere.
The A-type star HR 4796 is 231 light years from our Solar System in the constelation of Centurus. It is estimated to be 10 million years old, and has one of the best known debris disks, which has been studied with numerous different instruments. The ring is about 80 AU from the star (i.e. 80 times as far from the star as the Earth is from the Sun) and extends for about 10 AU.
In a paper posted on the Arxiv database at Cornell University on 4 December 2023, Julien Milli and Olivier Poch of the Université Grenoble Alpes, Jean-Baptiste Renard of the Université d'Orléans, Jean-Charles Augereau and Pierre Beck, also of the Université Grenoble Alpes, Elodie Choquet and Jean-Michel Geffrin of Aix Marseille Université, Edith Hadamcik of Sorbonne Université, Jérémie Lasue of the Université de Toulouse, François Ménard and Arthur Péronne, again of the Université Grenoble Alpes, Clément Baruteau, also of the Université de Toulouse, and Ryo Tazaki and Vanesa Tobon Valencia, once again of the Université Grenoble Alpes, present the results of a laboratory experiment intended to use dust analogues, to determine the nature of the material in the debris disk of HR 4796.
Examination of the HR 4796 system with the SPHERE instrument at the European Southern Observatory and the GPI instrument at the International Gemini Observatory has enabled polemetric analysis of the brightness of the ring at almost all azimuth angles. Every point on the ring forms a distinct angle between the light source (i.e. the star at the centre of the system) and the viewer, giving a different light scattering angle. The ring is angles at 76.5° to the star from our perspective, giving it minor axes (shortest apparent circumferances) with scattering angles of 13.5° to the northwest (brightest point) and 166.5° to the southeast (faintest point).
The HR 4796 disc at various wavelengths. The semi-major axis of the disc is roughly 1”. North is up, East to the left. Milli et al. (2023).
Most models of debris disks have assumed that they are made up of compact spheres of material similar to that found in comets, i.e. silicates, amorphous carbon, water ice and pore spaces. However, such models cannot reproduce the refractive properties of the HR 4796 disk. Changing the model to assume that the disk contained a high volume of metalic iron, as well as amorphous carbon and silicates, can reproduce the light scattering of the debris disk, but not the degree of polarisation. This suggests that either the particles in the disk are not spheres, or that the presumed size distribution of those spheres is very wrong. For most possible compositions of material, the degree of polarisation could be achieved only with milimetre sized particles, while the spectral energy distribution requires micron-scale particles.
The PROGRA² instrument is dedicated to the study of light scattered by solid particles deposited on a surface or lifted in clouds in microgravity conditions during parabolic flights that took place between 2018 and 2021. Samples were contained in a vial, and targetted with laser light; the scattered light produced by the particles was then split using a polarised beam splitter in two channels, and measured by two different detectors.
Examination of the database of results produced by this experiment revealed that the iron sulphide mineral pyrrhotite produced a degree of polarisation similar to that seen in the HR 4796 debris disk with particle sizes in the 1-200 µm range. The experiment did not record the degree of light scattering in the near infra-red, which would be necessary for a rigorous comparison, but the scattering at visible wavelengths was a good match for the HR 4796 disk.
Milli et al. postulate that pyrrhotite is an interesting candidate for the material of the HR 4796 debris disk. Because the data comes from an archived result from an experiment not designed to model the disk, it was not adjusted to try to gat a better fit. Nevertheless, it does appear that a cloud of pyrrhotite particles dominated by particles smaller than 100 µm, would produce a good match for the optical properties of the HR 4796 disk. Iron sulphide minerals are an unsurprising component for a circumstellar dust ring. Stratospheric dust particles, Antarctic Micro-Meteorites, and sampled material from Comet Wild 2 have all produced iron sulphide minerals, and comet 67P/Churyumov-Gerasimenko has been shown to contain about 7.5% iron, probably in the form of iron sulphides or iron-nickel alloys. These are dark minerals, responsible for the dark reflectance at optical and near infrared wavelengths from cometary and primitive asteroids surfaces, and would therefore be plausible components of the HR 4796 debris disk.
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