Comments (5)
not at the moment.
while the Python interface to Agama provides some basic unit conversion facilities, the standalone codes such as mkspherical, phaseflow and raga all work in the N-body units (i.e. G=1). in practice you choose your length and velocity units, and the mass unit follows from it (for instance, 1 kpc, 1 km/s => 232500 Msun is a common choice).
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So when we are evolving a system with binary black holes, should the total mass of the stellar particles be set to 1 or the total mass of the stellar particles + mass of the black holes be set to 1?
Thank you.
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this doesn't matter; you can choose any base unit for two out of three independent dimensional quantities (mass, length and velocity), and the third one should be determined from the condition G=1. So for instance you may use 1Msun as mass unit and 1 pc as length unit, then the velocity unit should be 0.0656 km/s, etc.
from agama.
Thank you! That really helped me!
from agama.
by the way, the convention G=1 is referred to as "N-body units" in the docs. In the most strict definition (e.g. Heggie&Mathieu 1986), this name also implies that the total mass of the system is 1 and the total energy is -1/4, which fully determines all three base dimensional units, but both these conditions are not required in Agama when using just G=1, which establishes only one relation between three base units (length, mass and time). In fact, the whole library is carefully designed to be as scale-invariant as possible, so that even measuring distances in km and time in seconds should not break the internal algorithms - if this doesn't hold promise, that is a bug.
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Related Issues (20)
- Allow setting value of G HOT 4
- Simulation with binary SMBH and two component mass function crashing after some timesteps
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- Discrepancy with Galpy when using agama.GalpyPotential HOT 2
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