Web Vectorial Model: Description of Parameters
- Comet Name
- Any string of characters to identify the comet (used for plot lables and
header in tables). The only disallowed character is the single quote
('), which will be replaced by a backquote (`)
- Heliocentric Distance
- Distance of the comet from the Sun (in astronomical units) at the time
- Observer Distance
- Distance of the comet from the observer (in astronomical units) at the
time of observation.
Valid range: >0.
The only other consideraton for this parameter is that the spatial
resolution will be reduced if this parameter is large (this is due to the fact
that a fixed numbers of points are used to simulate the coma structure). The
user is advised to compare the spatial resolution at the center of coma (given
in the output after the calculation) to the instrument spatial resolution.
- Slit Sizes
- Two slit sizes are needed: the size of the slit (in arcseconds) in the
direction perpendicular to the sun-comet line, and the size along the sun-comet
line. In general, the geometry of the observation may not be aligned in this
way, but arbitrary cangles cannot be calculated so the closest orthogonal
orientation must be determined.
Although there are no explicit limits for the valid range, the
practical limit the projected slit width and length should not be smaller than
the spatial resolution.
- Production Rate Time Steps
- In most cases, one assumes a constant gas production rate together with a
time step long enough to allow the coma to establlish a permanent regimen.
(The code calculates this optimal time length.) In that case, enter a single Q
value and one time step, the time step being the start time in number of days before
the time of observation (T=0). However, it is possible to model a time-variable gas
production rate. In this case, up to 20 time steps can be pre-defined, and the
user must enter the estimated relative Q values (in molecules per second) and
the corresponding start time (in days before T=0) of each step. For example, entering:
Q = 1.0e28 6.3e28 2.5e29
TQ = 20.8 10.0 5.5
means that the modeled production rate will be:
Q=1.0e28 mol/sec for days T = -20.8 to -10.0 before the observation (duration = 10.8 days)
Q=6.3e28 mol/sec for days T = -10.0 to -5.5 before the observation (duration = 4.5 days)
Q=2.5e28 mol/sec for days T = -5.5 to 0 before the observation (duration = 5.5 days)
Valid range: There technically isn't a particular valid range of valid
values, but for the case of a single, established production rate, the rule of
thumb for the time step is at least 3 to 5 times the combined lifetimes of the
parent and daugther.
- Parent velocity
- This is the velocity of the parent molecule at 1 AU. The program
will internally adjust for the factor of sqrt(1/Rh).
- The total lifetime of the parent molecules
Valid range: < or = the dissociative lifetime of the parent molecules
- The dissociative lifetime of the parent molecules
Valid range: > or = the dissociative lifetime of the parent molecules
- The destruction level of the parent molecules
- The code needs to know how far the coma should be modelled. If the
destruction level is defined as 99%, the parent coma will have a diameter such
that 99% of the parent species are destroyed inside of it. That number of 99%
corresponds to approximately 3 parent scalelengths. This is enough in most
applications. This choice does not imply that the density that is
calculated is 99% of the total density. As a matter of fact, the parent
species density near the center of coma willbe very accurate. However, at the
edges of the coma, the parent density will be slightly underestimated. This
will have not adverse consequence on the final calculation, that of the
daugther species density.
- Name of daughter radical
- Any string of characters to identify the daughter species (used for plot
lables and header in tables).
- Excitation Rate g-factor of daughter radical (g)
- Velocity of daughter radical (Vd)
- It is assumed that the radicals are created from photolysis, and that
collisions are not a significant source of parent disassociation. Therefore,
Vd is independent of the heliocentric distance and no scaling for
distance is made by the program (unlike the case for the parent
- Total lifetime of the daughter radical (Tdt)
- The destruction level of the daughter radical
- This parameter defines the overall extent of the calculated coma. The
meaning of the parameter is exactly the same as that of the destruction level of the parent molecules. One must proceed by
trial and errors to determine wich value of (Pd) best fits the
user's needs. The spatial resolution depends on the choice of the two
destruction parameters (Pp) and Pd. The user should thus
have a look on this resolution and make sure it is not too large relative to
the slit size. If it is too large, this parameter (Pd) should be
decreased, and Pp should be kept constant. Then, a few runs with
various values of Pd will allow the user to compare the results and
find out what the best value is. For a molecule like OH, Pp=99% goes
combined to Pd=95% provides good results over most of the coma.
Michel Festou and Joel Parker