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Link a unit system to a pNbody object

There is different way to link a system of units to a pNbody object. In all cases, a system of unit is defined by providing three numbers, corresponding to the value of respectively centimeter (UnitLength_in_cm), gramm (UnitMass_in_g) and centimeter per second (UnitVelocity_in_cm_per_s) in the desired system of unit.

This may be done using the following possibilites.

Defining units explicitely

The first possibility to define units is to simply gives the explicit values for the variables UnitLength_in_cm, UnitMass_in_g, UnitVelocity_in_cm_per_s. If nb is your pNbody object, this is achieved with the following command:

nb.set_local_system_of_units(UnitLength_in_cm=1,UnitVelocity_in_cm_per_s=1,UnitMass_in_g=1)

Another equivalent possibility is to use a dictionary:

params = {}
params['UnitLength_in_cm']         = 1.
params['UnitVelocity_in_cm_per_s'] = 1.
params['UnitMass_in_g']            = 1.
nb.set_local_system_of_units(params)

Defining units using a units parameter file

Here we use a units parameter file (here called ./unitsparameters), containing the following lines:

UnitLength_in_cm
UnitLength in cm
Float
3e+21

UnitMass_in_g
UnitMass in g
Float
3e+43

UnitVelocity_in_cm_per_s
UnitVelocity in cm per s
Float
20000000.

In this case, if nb is your pNbody object, the following command link a system of unit:

nb.set_local_system_of_units(unitparameterfile='./unitsparameters')

Defining units using a gadget parameter file

Gadget parameter files contains the definition for a system of units. In such files, we usually find the following lines:

UnitLength_in_cm                              3.085678e+21
UnitMass_in_g                                 1.989e+43
UnitVelocity_in_cm_per_s                      100000.0

In this case, if nb is your pNbody object, the following command link a system of unit defined in the Gadget file params:

nb.set_local_system_of_units(gadgetparameterfile='./params')

Defining units when opening a file

pNbody offers also the possibility to define a unit system when opening a file. In this case we directely pass either a units parameter:

nb = Nbody("../snap.dat",ftype='gadget',unitsfile='unitsparameters')

or a gadget parameter file:

nb = Nbody("../snap.dat",ftype='gadget',unitsfile='params')

to the class constructor.