TRACER_*

Implementation of tracer particles that allow one to follow the flow of baryonic mass between gas cells (as well as stars/BHs if relevant) in a quasi-Lagrangian sense. Otherwise, because gas cells exchange mass each timestep in the finite volume scheme of AREPO, it is fundamentally impossible to determine the Lagrangian history (i.e. thermal or dynamical history) of any gas parcel.

There are three different tracer particle modules implemented, and one passive scalar field:

1. TRACER_MC are “Monte Carlo tracers” than belong to parents (and so exist only as children to those cells/particles), without phase space coordinates (no positions or velocities). They are exchanged between parents in proportion to mass fluxes.

2. TRACER_PARTICLE are “Velocity field tracers” which have positions and velocities and are advected through space according to the local (linear gradient interpolated) gas velocity field.

3. TRACER_TRAJECTORY is a customized/extended version of velocity field tracers, intended primarily for recording tracer properties through time for e.g. nucleosynthesis postprocessing. It is not generalized.

TRACER_MC

Usage

Tracer “particles” which are probabilistically transfered between gas cells and star/BH particles based on the mass flux through each face (gas<->gas), the mass ratio of the spawned star (gas->star), the return of mass due to stellar mass loss (star->gas), and the accreted mass for sink type particles (gas->BH).

Note

For on-the-fly group finding, although MC tracers are included in GroupLenType and SubhaloLenType, they are not output in group order. However this is not generally a drawback, since following tracers across snapshots anyways requires a global snapshot search in most cases.

To run a simulation with MC tracers use the TRACER_MC=N configuration flag, where N is an empty particle type that the tracers will output as in snapshots.

Currently, the only way to introduce MC tracers is uniformly in all gas cells during the read of the initial condition file. To do this, set TracerMCPerCell in the parameter file to the starting number of tracers per cell, and set the GENERATE_TRACER_MC_IN_ICS config flag. The note for velocity field tracers about SPLIT_PARTICLE_TYPE applies to Monte Carlo tracers as well.

Additional Parameters

  • TracerMCPerCell the number of Monte Carlo tracers to generate for each parent gas cell after reading the initial conditions file. Only applies if GENERATE_TRACER_MC_IN_ICS is enabled. Recommended value: from 1 to 10 (integer).

Additional Config.sh Options

TRACER_MC=3

Master switch for MC tracers. Value specifies particle type to use for output.

GENERATE_TRACER_MC_IN_ICS

If enabled, one or more tracers are created within each gas cell during the read of the initial conditions (RestartFlag == 0 only). The number of tracers desired is set by the TracerMCPerCell parameter. Note that using this option when gas cells have different initial masses, as in the case of multi-mass zoom simulations, violates the typical implicit assumption that all Monte Carlo tracers represent fluid elements of equal mass. For the typical (old) scheme, this requires only additional thought in post-processing. For the directional (new) scheme, this violates a key assumption of the scheme and will modify the evolution of tracers, with more thought required.

TRACER_MC_NUM_FLUID_QUANTITIES

The number of fluid quantities to store for each tracer. Must match the number of fields added to the TRACER_MC_STORE_WHAT bitmask below.

TRACER_MC_STORE_WHAT

A bitmask specifying which fluid quantities to store for each tracer. For example, 1+8 stores the maximum temperature and maximum density encountered. These values are generally recorded from the parent gas cell for each tracer, although some (e.g. “wind counter”) are calculated based on more complex metrics. Maximum-type fields (e.g. “Tmax” or “Tmax Time”) are reset to zero immediately after writing a snapshot unless the TRACER_NO_RESET_EACH_SNAP option is set. See allvars.h for bitmask options. See public Illustris documentation for some further field descriptions.

TRACER_NO_RESET_EACH_SNAP

For maximum-type fluid quantity fields (e.g. “Tmax” or “Tmax Time”), disable zero’ing immediately after writing a snapshot. Such quantities are then global over the entire simulation time up until that snapshot has been written. This makes post-processing easier, at the cost of loss of information.

TRACER_MC_CHECKS

Enable frequent (and somewhat expensive) consistency checks on the internal tracer linked list structures, terminating if any problems are encountered. Also increases memory usage.

OUTPUT_MCTRNUM

Record the number of tracers as a field in parent particle/cells in snapshots.

Authors

Usage Policy and Citation

There is no requirement to contact the authors before using these modules, or for co-authorship on publications. We are happy, however, to provide advice or actively collaborate with interested users. The only exception to the above is questions broadly related to “cosmological gas accretion from the IGM into galaxies”. On science related to this, please contact the developers prior to use of the tracer particles.

Papers to cite:

TRACER_PARTICLE

Usage

Tracer particles which are passively advected through the domain by interpolating the velocity field of the underlying gas. These particles live in P and in timebins, but have no effect on the simulation.

To run a simulation with “velocity field” tracers use the TRACER_PARTICLE=N configuration flag, where N is an empty particle type (1-NTYPES) that the tracers will use and output as in snapshots.

Tracers can be introduced directly into the initial conditions by creating them with the target particle type. In this case, only position is important, velocity is set by the code, and mass should not be included. If the ICs have non-uniform gas masses and the goal is to sample the gas density field with the tracers, their distribution must be constructed manually in the ICs. Likewise to increase the ratio of tracers to gas cells above one.

Alternatively, the GENERATE_TRACER_PARTICLE_IN_ICS config flag can be used to place one tracer exactly at the position of each gas point during the read of the initial conditions. Useful to match the density field of any complicated gas geometry in the ICs when the masses are uniform.

Note

GENERATE_TRACER_PARTICLE_IN_ICS generates the tracer particles directly from gas particles, regardless of whether those have been read from the ICs or generated using GENERATE_GAS_IN_ICS.

Note

FoF can be run either on the fly or post-processed with velocity field tracers, which are included explicitly in FoF groups (nearest DM particle attachment), but not in subfind groups.

Additional Parameters

  • MinimumTracerHsml is a minimum distance (in code units) used when a tracer searches for its nearest cell. This should be a rough estimate of the minimum cell size.

Additional Config.sh Options

TRACER_PARTICLE=2

Master switch for Velocity Field tracer particles. Value specifies particle type to use.

GENERATE_TRACER_PARTICLE_IN_ICS

If enabled, a single tracer particle is placed at the position of each gas cell position during the read of the initial conditions (RestartFlag == 0 only).

Authors

Usage Policy and Citation

There is no requirement to contact the authors before using this module, or for co-authorship on publications. We are happy, however, to provide advice or actively collaborate with interested users. The only exception to the above is questions broadly related to “cosmological gas accretion from the IGM into galaxies”. On science related to this, please contact the developers prior to use of the tracer particles.

Papers to cite:

TRACER_TRAJECTORY

Usage

Enable the TRACER_TRAJECTORY option and use either the auto-generation or the init file to specify the initial tracer positions. Output destination file and frequencies must be specified by the two parameters, respectively. Fields to record and save can be modified by hand in tracer_trajectory.c.

Additional Parameters

  • NumberOfTracersToGenerate is required only if TRACER_TRAJECTORY_GENERATE is set. In this case, this number of new velocity tracers (per gas cell) are generated at startup and randomly placed within each cell.

  • TracerInitFile is required if TRACER_TRAJECTORY_GENERATE is not set. In this case, previously saved tracer positions are read from this file. The format is simple binary: the total number of tracers (4 byte int) as a header, followed by xyz coordinate triplets (8 byte doubles each).

  • TracerOutputFile specifies a file name to write the (simple binary) output into. Seems not entirely used (“tracer.dat” still hard-coded).

  • TracerOutputConfFile specifies an external text file which controls the timing of tracer outputs. The first line is an integer, the total number of tracer output steps. Subsequent lines, one per step, each contain two spaced-separated floats, the first being an output time, the second being the output time period. For each output time in the file, tracers are written until that time with the cadence given by the output time period.

Additional Config.sh Options

TRACER_TRAJECTORY

Master switch to enable saving of tracer “trajectories” (parent gas cell properties) in time. Requires that TRACER_PARTICLE be also enabled.

TRACER_TRAJECTORY_GENERATE

If specified, velocity tracers are generated at startup within each gas cell. The number desired is given by the NumberOfTracersToGenerate parameter. If not specified, tracers are assumed to have been already generated in the past and saved, in which case they are read from the TracerInitFile.

TRACER_TRAJECTORY_EXTENDED_OUTPUT

Normally six values (x,y,z,rho,temp,u) are saved for each tracer, plus its unique ID. If this option is enabled, four additional values (v_x,v_y,v_z,dedt) as well as EOS_NSPECIES abundances are also saved.

Authors

Usage Policy and Citation

TODO (Ruediger).

Papers to cite:

  • todo.