Enzo v2.0 documentation

Star Particle Class

Purpose

To give star particles more functionality and interaction with the grids, it was useful to create a new class for a generic particle type that can represent, e.g., stars, black holes, sink particles.

Main features

  • merging
  • accretion
  • conversion to a radiation source
  • adding feedback spheres to the grid, e.g. mass removal from accretion, supernovae.
  • different behaviors for different star types
  • multiple types of star particles
  • “active” and “inactive” stars

Approach

Star particle class flowchart

A flowchart of the logic of the star particle class. View PDF.

We keep the original implementation of the particles that are stored in the pointers, ParticlePosition, ParticleVelocity, ParticleMass, ParticleNumber, ParticleType, and ParticleAttribute. Star particles are still created in the FORTRAN routines, e.g. star_maker2.F. In the current version, the star class is a layer on top of these particles. Thus we must keep the particle pointers and objects synchronized when their quantities change.

Particles created in the FORTRAN routines that will be converted into a star object initially have a negative particle type. This indicates that the star is not “born” yet, which is also used to flag various feedback spheres, such as mass removal from the grid. The stars are activated, i.e. positive particle type, in Star::ActivateNewStar() after it has been checked for mergers, accretion, and feedback.

We store the star objects as a linked list in grid class. Because a star object can affect multiple grids (over multiple processors) when adding feedback sphere, processors other than the one hosting the star particle needs to know about this star object. Currently for convenience, we create a global list of star objects on all processors. For not many stars (< 100k), this does not consume that much memory. However in the future, we might have to reconsider how star particles are communicated across processors.

Feedback spheres

Any event can be set in Star::SetFeedbackFlag to add a feedback sphere. This sphere can be of any size, and its properties are set in Star::CalculateFeedbackParameters() and grid::AddFeedbackSphere(). Because they can cover grids on multiple levels, we have to ensure that they are all at the same time. In Star::FindFeedbackSphere(), we check if sphere is completely contained within grids on the current level. If true, we can safely add the sphere. If it’s not imperative that the grids are completely synchronized, one can add the feedback sphere immediate after the star object is flagged for feedback.

Accretion / Mass Loss

Star objects can store up to 100 (#define MAX_ACCR) accretion rates as a function of time. Alternatively, currently in the black hole particles, they can have an instantaneous accretion rate. This is done in Star::CalculateMassAccretion. The actual accretion to the star object is done in Star::Accrete().

How to add a new particle type

  1. Set the particle type to the negative of the particle type in the star maker routine. Be sure not to overwrite the type like what’s done in the regular star_maker.F routines.

  2. Add the particle type to the if-statement in grid::FindNewStarParticles.

  3. Then the particles merge if any exist within StarClusterCombineRadius. This is not restricted to only star cluster (radiating) particles. Even if there is any merging, the particle shouldn’t disappear.

  4. At the end of StarParticleInitialize(), the routine checks if any stars should be activated in Star_SetFeedbackFlag. This is where you should check first for errors or omissions. You’ll have to add a new case to the switch statement. Something as simple as

    case NEW_PARTICLE_TYPE:
    if (this->type < 0)
       this->FeedbackFlag = FORMATION;
    else
       this->FeedbackFlag = NO_FEEDBACK;
    

    will work.

    After this, the particle is still negative but will be flipped after the feedback to the grid is applied in Star_ActivateNewStar() that’s called from StarParticleFinalize. Here for Pop II and III stars, we use a mass criterion. For Pop III stars, we set the mass to zero in the pop3_maker() f77 routine, then only set the mass after we’ve applied the feedback sphere.

  5. The grid feedback is added in StarParticleAddFeedback that is called in StarParticleFinalize(). In Star_CalculateFeedbackParameters(), you’ll want to add an extra case to the switch statement that specifies the radius of the feedback sphere and its color (metal) density.

  6. If the feedback sphere is covered by grids on the level calling StarParticleAddFeedback() (i.e. all of the cells will be at the same time), then Grid_AddFeedbackSphere() will be called. Here you’ll have to add another if-block to add your color field to the grid.