Arcetri Astrophysical Observatory

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The mechanisms responsible for the formation of planets in our Solar System or around other host stars are still not fully understood, in spite of the large laboratory, theoretical, and observational e fforts.

A critical step is the growth of small dust particlesfrom the submicron sizes of interstellar medium to pebbles and kilometer-sized bodies. This process is thought to occur in circumstellar disks of gas and dust around young stars on rather short timescales: infrared surveys show that the number of low mass stars with disks drops dramatically for stars older than 10 Myr, suggesting that in order to form large solids before the disk disperses, the growth process has to take place within a few million years.
Due to vertical settling the bulk of the solids lies in the inner and colder region, or midplane, of the disk. This is not heated directly by the central star, but receives the reprocessed radiation of the surface layers, and re-emits in the submillimeter and millimeter range. This emission can be directly observed with radio telescopes, and the great sensitivity and angular resolution achieved by modern interferometers allows us to resolve the emission throughout the disk.
A group of astronomers, led by Greta Guidi (PhD student at the Osservatorio Astrofisico di Arcetri) recently obtained observations with ALMA (Atacama Large Millimeter/submillimeter Array, in Atakama desert -Chile) and VLA (Karl G. Jansky Very Large Array, in New Mexico - USA) of HD 163296, an Herbig Ae star of 2.3 M at 122 parsec from our Sun.

In the paper (Guidi et al. 2016), they showed that grain growth is taking place in the disk.

The ratio of the fluxes at the diff erent wavelengths gives an indication of the maximum grain size at di fferent radii, through the measurement of the opacity spectral index : its increasing values with the distance from the star are consistent with the di fferential action of radial drift and suggest the presence of large grains and pebbles (1 cm) in the inner region of the disk (inside 50 AU).
The analysis of the radial profi les of the flux density at 850 μm also reveals an excess emission near the location of the CO snowline at 90 AU(see Figure 1): this feature could indicate a local change in the dust properties. The possibility of large grains at the snow-line may also be connected with a local increase in the surface density (or temperature), which could also explain the observed excess. A similar feature was found in scattered lightin the Ks band by Garu fi et al. (2014), who saw a ring in the polarized diff erential images centered at a radius of 100 AU (Figure 1, bottom panel).
The detection of this excess at both infrared and submillimeter wavelengths, tracing two di fferent regions of the disk (the surface and the midplane respectively), shows that it is not a purely surface eff ect and suggests the presence of a structure covering the whole vertical extent of the disk.Higher resolution observations are required to see what is really happening in what looks like a "smooth" disk, but that could be already hosting larger solids or even planets.

greta fig1

Figure 1

Top panel:  Flux density at 850 μm along the disk major axis from SE (left) to NW (right). The solid line represents the fit performed excluding the inner 0.2 arcsec of the disk (gray shaded area). The vertical dashed line corresponds to the CO snowline at 90  10 AU (from Qi et al. 2015).

Bottom panel: residuals obtained by subtracting the polynomial fi t from the data are shown with black dots binned by 0.15 arcseconds. The blue solid line represents the Gaussian that best fits the excess, with the shaded area showing the 1 sigma uncertainty. The red triangles are the polarized light contrast in the Ks band (from Garufi et al. 2014) the scale is on the right side vertical axis.