Geoscience Reference
In-Depth Information
out that a geometric picture may be too simple to be true because diffrac-
tion effect may increase the event rate drastically.
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The number of TNO
occultation events is dominated by the faint-end objects where, near the
diffraction limit, Fresnel diffraction applies. The criterion of an occultation
event is really an artificial one. An arbitrary cut-off may introduce an error
which could be as big as the number count itself. Moreover, the so-called
“occultation event rate” depends on the system and the algorithm used.
We'd say that the geometric picture is good for an estimate, but, some-
thing else is needed for an interpretation. A simulator is just what we need
to understand the underlying astronomy and astrophysics.
The following steps are taken to develop the simulator. First, an ensem-
ble of light curves are derived from a specific astronomical or astrophysi-
cal model. Next, all these light curves will be run through a specific data
pipeline with a photometric algorithm applied. An eciency test is also
required here. By adding a fake signal, the eciency of an algorithm can
be obtained. The final result can now be compared with an observation to
justify the model assumed. To derive an ensemble of light curves from the
first principles, the physical process of TNO occultation can be simulated
with the following four levels of consideration.
(1)
Star level
: The spectral type, luminosity class, apparent magnitude (or
angular size) and, if available, a limb darkening model of a target star
determine the physical properties of the diffracted light source.
(2)
TNO level
: The size, distance and shape of a TNO together with the
astronomical or astrophysical assumption in the total population, size
distribution and orbital distribution of TNOs specify the obstacle in
a diffraction calculation. A diffractive shadow can be derived with the
above two levels of parameters.
(3)
Shadow level
: Impact parameter and shadow velocity should be involved
here. An occultation event does not have to be a center-crossing event.
Nevertheless, the impact parameter can only be a random number that
leads to a certain distribution. The shadow ground velocity will be
coupled to the TNO distance by assuming a circular orbit. This might
be a good approximation for the classical TNOs. Scattered objects have
a larger dispersion in their shadow velocities.
(4)
Light curve level
: Filter is introduced in this level. It is proper to asso-
ciate a specific term “configuration” with everything mentioned above.
A configuration corresponds to the old geometric picture which relates
to an ensemble of light curves with a certain probability. In addition,
