Module Grainsize

Calculate the MRN grain size distribution for an AGB wind.

This assumes an average drift velocity independent of grain size, and a constant dust-to-gas ratio throughout the wind.

The MRN distribution is taken from Mathis, Rumpl and Nordsieck (1977), and reads: n_d(a,r) = A(r) n_Htot(r) a^-3.5

A(r) is a scaling factor that is derived from the dust mass-loss rate. This causes the H-number density to drop out of the equation, so this information is not needed: A(r) = md / [nHtot 16/3 pi^2 sd r^2 Int((vg + w) a^-0.5 da)]

The MRN distribution assumes minimum grain size of .005 micron and maximum grain size of 0.25 micron. While the method allows going outside this range (e.g. such as in the case of calculating a local value just outside the range when solving a differential equation), it is recommended changing them if the grain size distribution extends beyond it. These are the values used to derive A(r).

Parameters:

• r (array) - The radial grid (cm)
• a (array) - The grain size grid (cm)
• w (Drift()) - The drift velocity profile (cm/s) as a function of r and a.
• v (float/Velocity()) - The gas velocity (cm/s). Either as a profile, or as a cst
• mdot_dust (float/Mdot()) - The dust mass-loss rate (msun/yr). Either as a profile, or as a cst
• sd (float) - The average specific density of the dust grains in g/cm3.
• a_min (float) - The minimum grain size in cm for MRN

  (default: 0.005e-4)

• a_max (float) - The maximum grain size in cm for MRN

  (default: 0.25e-4)

• returnA (bool) - Return the scale factor separately as well. This is the second element of the tuple. Note that this does not contain Htot, ie A = A_local/nhtot

  (default: 0)

Returns: array

The grain size distribution as a function of a