Summary of the constraints on the hot ring, assuming it consists of 0.01 μm carbon grains located at 0.23 AU. Panel a) the diagonal line shows the relation between the mass flux and the dust grain survival timescale, as constrained by the observed fractional luminosity (black, with error margins in grey, Eq. (29)). The horizontal lines indicate the typical timescales (in a gas free environment) for destruction by sublimation (red, dash-dotted, Eq. (30)), and removal by blowout (blue, dashed, Eq. (31)). The vertical line with the arrow indicates the maximum mass flux due to P-R drag from a very dense source region located at 2 AU (green, Eq. (28)). Panel b) the dependence on gas density of the sublimation timescale (red, dash-dotted, Eq. (30)), and the blowout timescale (blue, dashed, the sum in quadrature of Eqs. (31) and (32)) The top axis gives the total gas mass corresponding to the midplane gas densities on the bottom axis, assuming the gas is located in a vertically isothermal ring of width Δr = r (Eq. (34)). The horizontal, black, dotted line marks the minimum survival time required if the observed material is to be provided by PR drag. It extends across both panels to show the gas density and total gas mass this would imply.