Protostars and Planets VI, Heidelberg, July 15-20, 2013
Dust grain sizes in Herschel-resolved debris discs
Pawellek, Nicole (Astrophysikalisches Institut und Universitätssternwarte Jena)
Krivov, Alexander (Astrophysikalisches Institut und Universitätssternwarte Jena)
Marshall, Jonathan (Departamento de Física Teórica, Faciltad de Ciencias, Universidad Autónoma de Madrid)
Montesinos, Benjamin (Departamento de Astrofísica, Centro de Astrobiología, Madrid)
Eiroa, Carlos (Departamento de Física Teórica, Faciltad de Ciencias, Universidad Autónoma de Madrid)
Collisional models of debris discs robustly predict that the effective minimum size of their dust grains, smin, should be set by stellar radiation pressure. Under reasonable assumptions for the composition and density of dust, the blowout radius sblow – the radius of the smallest grains that can stay in bound orbits around the star against radiation pressure – is expected to vary from several tenths of a micron for G-type stars to several microns for A-type stars. For most of the discs, smin is predicted to be a few times sblow, and the size distribution index q to lie approximately between 3 and 4. As a result, the grains with s0=smin*sqrt((1-q)/(3-q)) should dominate the cross section of material and thus contribute the most into the observed emission at mid- to far-infrared wavelengths. The exact smin/sblow ratio depends on several factors, such as the optical depth of the disc or the degree of dynamical excitation of dust-producing planetesimals.
At a first glance, this seems to be supported by observations. Typical grain sizes inferred from thermal emission and scattered light images of many individual discs lie at a few microns. However, the expected correlation between s0 (or smin) and sblow across a broad luminosity range have not been checked in depth.
Here we use a uniform sample of 22 discs well resolved in several Herschel programmes to re-address the question. Setting the disc radii rdisc to the values inferred from the resolved images, we perform the SED modelling to find best-fit smin and q. We confirm that rdisc does not correlate with the stellar type, as found before. We also confirm that the ratio of the dust temperature to the blackbody temperature at the dust location goes from ~2 - 2.5 for G-type stars to about unity for the most luminous A-type stars, as pointed out by Booth et al. (2013). However, we find that the data are more consistent with a constant s0 (~7 microns) or smin (~3 microns) across the luminosity range, rather than with a constant smin/sblow. This finding needs further verification with a larger sample of discs. If confirmed, it might indicate systematic differences in disc properties with the stellar type, such as those in the chemical composition of dust, degree of stirring or width of the underlying planetesimal belts.
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