Sinks: Get Subregions

Selecting sinks by shape on a catalogue you have already loaded: spheres, cylinders, boxes, and the hollow versions of the first two.

A sink is a point mass. That single fact explains most of what follows: there is no cell to clip, so a sink is either inside a region or outside it, and the keywords that exist to decide what to do with a straddling cell do nothing here.

The run below carries six sinks placed at known offsets from the box centre, so every answer on this page can be checked by hand.

Setup

using Mera, Printf

MERA_EXAMPLES = get(ENV, "MERA_EXAMPLES", "/Volumes/FASTStorage/Simulations/Mera-Tests")
path = "$MERA_EXAMPLES/RAMSES/sinks3d_multi"

info  = getinfo(2, path, verbose=false)
sinks = getsinks(info, verbose=false)

ids(o) = sort(Int.(getvar(o, :id)))

id = Int.(getvar(sinks, :id))
rs = getvar(sinks, :r_sphere,   :pc, center=[:bc])
rc = getvar(sinks, :r_cylinder, :pc, center=[:bc])
dz = getvar(sinks, :z, :pc) .- info.boxlen * info.scale.pc / 2

@printf("%-4s %10s %10s %12s %14s\n", "id", "r_sphere", "r_cyl(z)", "z-offset", "M [Msol]")
for k in sortperm(id)
    @printf("%-4d %10.2f %10.2f %12.1f %14.2f\n",
            id[k], rs[k], rc[k], dz[k], getvar(sinks, :msink, :Msol)[k])
end

*__   __ _______ ______   _______
|  |_|  |       |    _ | |   _   |
|       |    ___|   | || |  |_|  |
|       |   |___|   |_||_|       |
|       |    ___|    __  |       |
| ||_|| |   |___|   |  | |   _   |
|_|   |_|_______|___|  |_|__| |__|
Mera v1.8.0 | Julia 1.12.7 | 4 threads
id     r_sphere   r_cyl(z)     z-offset       M [Msol]
1          0.08       0.07          0.0        2698.33
2         49.90      49.90          0.0        2366.31
3         59.91      59.91          0.0        2166.53
4         69.93       0.02         69.9        1921.63
5         84.81      84.81          0.0        1604.87
6         69.19      56.50         39.9        1140.60

Keep that table beside you. Every selection below is checkable against it: a sphere of radius 55 pc about the centre must contain exactly the sinks whose r_sphere is under 55.

A sphere

subregion takes the shape as its second argument. radius is measured from center, in range_unit.

inner = subregion(sinks, :sphere, radius=55, center=[:bc], range_unit=:pc, verbose=false)
println("sphere r = 55 pc  -> ", ids(inner))
println("expected from the table: the sinks with r_sphere < 55")

sphere r = 55 pc  -> [1, 2]
expected from the table: the sinks with r_sphere < 55

inverse=true gives the complement, which is how you take everything except a region.

outer = subregion(sinks, :sphere, radius=55, center=[:bc], range_unit=:pc,
                  inverse=true, verbose=false)
println("outside  -> ", ids(outer))
println("the two together account for every sink: ",
        length(inner.data) + length(outer.data) == length(sinks.data))

outside  -> [3, 4, 5, 6]
the two together account for every sink: true

A shell

shellregion is the hollow form: an inner and an outer radius. This is the selection that a single-sink catalogue cannot demonstrate at all, since it needs one object inside the shell and another outside it.

shell = shellregion(sinks, :sphere, radius=[55., 75.], center=[:bc], range_unit=:pc, verbose=false)
println("shell 55-75 pc -> ", ids(shell))
println("its inverse    -> ", ids(shellregion(sinks, :sphere, radius=[55., 75.], center=[:bc],
                                              range_unit=:pc, inverse=true, verbose=false)))

shell 55-75 pc -> [3, 4, 6]
its inverse    -> [1, 2, 5]

Note that a shell needs both radii to be nonzero. Asking for an inner radius of zero is an error rather than a silent fall-back to a solid sphere, because the two are different questions.

try
    shellregion(sinks, :sphere, radius=[0., 75.], center=[:bc], range_unit=:pc, verbose=false)
catch e
    println("refused: ", first(split(sprint(showerror, e), "\n")))
end

refused: [Mera]: shellregion(:sphere) needs nonzero inner and outer radii — got radius = [0.0, 75.0].

A cylinder, and why it differs

A cylinder selects on the cylindrical radius and a half-height, so it answers a different question from the sphere: distance from an axis rather than from a point.

cyl = subregion(sinks, :cylinder, radius=55, height=100, center=[:bc],
                range_unit=:pc, verbose=false)
println("cylinder r = 55 pc, h = 100 pc -> ", ids(cyl))
println("sphere   r = 55 pc             -> ", ids(inner))

cylinder r = 55 pc, h = 100 pc -> [1, 2, 4]
sphere   r = 55 pc             -> [1, 2]

The two disagree by exactly one sink, and the table says why. Sink 4 sits almost on the cylinder's axis but a long way up it: r_cylinder is 0.02 pc while r_sphere is 69.9 pc. The sphere of radius 55 pc excludes it on distance from the centre; the cylinder keeps it, because it is within 55 pc of the axis and its z-offset of 69.9 pc is inside the 100 pc half-height.

Sink 6 shows the opposite lever: r_cylinder 56.5 pc puts it just outside this cylinder, even though it is closer to the centre in three dimensions than sink 4 is.

That is the argument for having both shapes rather than one. A sphere asks how far from a point; a cylinder asks how far from an axis, and how far along it. One sink sits almost exactly on the cylinder's axis with a large z-offset: its r_sphere puts it outside the sphere, while its r_cylinder is nearly zero, so the cylinder keeps it as long as the half-height reaches. Another sits at a moderate radius in all three coordinates and falls outside the cylinder while being inside the larger sphere.

half = subregion(sinks, :cuboid, xrange=[-125., 0.], yrange=[-125., 125.], zrange=[-125., 125.],
                 center=[:bc], range_unit=:pc, verbose=false)
println("the x < centre half of the box -> ", ids(half))

the x < centre half of the box -> [5]

What does not apply to points

One keyword exists on the region functions for cell data and is meaningless here:

• cell= decides whether a cell straddling the border is taken whole or by its centre. A point does not straddle anything, so it is accepted and ignored.

And one limitation that does apply, and that no keyword fixes:

The result is a catalogue

Every region function hands back a SinkDataType, not a bare table, so the result is usable everywhere the original was: getvar, another region, savedata.

println("type       : ", typeof(shell))
println("units kept : ", shell.used_descriptors[:units][:msink])
println("mass in the shell: ", round(sum(getvar(shell, :msink, :Msol)), digits=2), " Msol")

# regions compose: the shell, then the half-box
both = subregion(shell, :cuboid, xrange=[-125., 0.], yrange=[-125., 125.], zrange=[-125., 125.],
                 center=[:bc], range_unit=:pc, verbose=false)
println("shell, then x < centre -> ", ids(both))

type       : Mera.SinkDataType
units kept : m
mass in the shell: 5228.77 Msol
shell, then x < centre -> Int64[]

Summary

All of them return a catalogue you can keep working with, and all of them ignore cell=, because a sink is a point.

Next steps

• Sinks: Load Selections, restricting at read time, and accretion
• Sinks: First Inspection, the catalogue and its units
• Clumps: Get Subregions, the same shapes on the other point-like type

The same steps on the other data types: Hydro · Gravity · Particles · Clumps