**A.1. Extent of the Ionized Region**

The size of the H II region has been calculated by considering
photoionization and recombination of hydrogen,
along with the absorption due to the dust grains.
The presence of the dust component reduces the size of the ionized
region, (R_{HII}), compared to
that of pure gas Stromgren sphere considerably,
depending on the density and the gas to dust ratio.
The dust grains can exist in principle, only beyond a radial distance,
say r_{subl},
depending on its sublimation temperature and the local radiation
field. In practice, the actual
distance beyond which the dust exists, say r_{fit}, is determined by
the model fitting of the observed SED, by
radiative transport calculations through the dust.
The r_{fit} is often much larger than r_{subl}.

Hence, whether one encounters a dusty Stromgren sphere or not,
is determined by the
type of the star / integrated spectrum of the cluster;
radial density distribution around the central star; and r_{fit}.
We call it a Case A, if the ionized region
extends into the region where the gas the and dust co-exist.
The other case of entire ionized region devoid of any dust grains
is termed Case B.
So for Case B, the extent of the H II region can be obtained by solving the
equation,

where, *N(r)* is the Lyman continuum photon flux,
_{2} is
the recombination coefficient for hydrogen (for recombinations to all states
except the ground state) and *n _{e}* is the number
density of electrons or

For m = 0, 1, 2 equation(A1) can be solved easily by using the boundary condition

where, *N _{Lyc}* is the total number of Lyman continuum
photons emitted per second by the embedded exciting star / star cluster
and

In case A however, the ionizing (Lyman continuum) photons experience further attenuation due to direct absorption by the dust, so the modified radiation transfer equation would be,

where _{Lyc} refers
to the optical depth of dust at
< 912 Å.
We solve the above equation, using the boundary conditions,

where *N*_{1} is determined by using equation (A1) and