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Disks around young stars are known to evolve from optically thick,gas-dominated protoplanetary disks to optically thin, almost gas-free debrisdisks. It is thought that the primordial gas is largely removed at ages of ~10Myr, but it is difficult to discern the true gas densities from gasobservations. This suggests using observations of dust: it has been argued thatgas, if present with higher densities, would lead to flatter radial profiles ofthe dust density and surface brightness than those actually observed. However,here we show that these profiles are surprisingly insensitive to variation ofthe parameters of a central star, location of the dust-producing planetesimalbelt, dustiness of the disk and - most importantly - the parameters of theambient gas. This result holds for a wide range of gas densities (three ordersof magnitude), for different radial distributions of the gas temperature, anddifferent gas compositions. The brightness profile slopes of -3...-4 we findare the same that were theoretically found for gas-free debris disks, and theyare the same as actually retrieved from observations of many debris disks. Ourspecific results for three young (10-30 Myr old), spatially resolved, edge-ondebris disks (beta Pic, HD 32297, and AU Mic) show that the observed radialprofiles of the surface brightness do not pose any stringent constraints on thegas component of the disk. We cannot exclude that outer parts of the systemsmay have retained substantial amounts of primordial gas which is not evident inthe gas observations (e.g. as much as 50 Earth masses for beta Pic). However,the possibility that gas, most likely secondary, is only present in little tomoderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses inthe beta Pic disk), remains open, too.

Can gas in young debris disks be constrained by their radial brightness profiles?