Validation: src/zephyrus/collision.py
This page tracks the @pytest.mark.reference_pinned tests that anchor the behaviour of zephyrus.collision against the published giant-impact atmospheric erosion scaling law.
| Test id | Reference | Source page | Scope |
|---|---|---|---|
tests/test_collision.py::test_scaling_law_pins_the_kegerreis_closed_form |
Kegerreis et al. (2020), ApJL 901, L31, Eqn. 1 (erosion scaling law, closed form) | ADS 2020ApJ...901L..31K | Pins the loss fraction for two identical Earth-like bodies head-on at 1.0 and 1.5 mutual escape speeds, where the law collapses to X = 0.64 * (v_ratio^2 / sqrt(2))^0.65 with no geometry left. |
tests/test_collision.py::test_scaling_law_reproduces_kegerreis_table2_simulations |
Kegerreis et al. (2020), ApJL 901, L31, Tables 1 and 2 (SPH simulation suite) | ADS 2020ApJ...901L..31K | Pins the law against three simulated loss fractions from the paper's first suite (b = 0.7, v_c = 3 v_esc, impactor:target mass ratio 10^-0.5) at 20% tolerance, the paper's stated simulation-to-law scatter. |
Re-derivation note
collision.mass_loss returns the fractional atmospheric mass loss of the target,
X = min(0.64 * [ (v_c/v_esc)^2 * (M_i/M_tot)^(1/2) * (rho_i/rho_t)^(1/2) * f_M(b) ]^0.65, 1)
with the mutual escape speed v_esc = sqrt(2 G (M_t + M_i) / (R_t + R_i)) and the fractional interacting mass f_M of the paper's Eqn. B1: density-weighted spherical caps of common height d = (R_t + R_i)(1 - b), normalised by the density-weighted body volumes. At equal bulk densities f_M reduces exactly to the interacting volume f_V of Eqn. B2, which the test suite verifies as a property; a dedicated pin separates the two forms for an iron-rich impactor, where they differ by 0.0056 in X against a pin tolerance two orders of magnitude tighter.
The closed-form pin uses identical twin bodies so every bracket ratio except M_i/M_tot = 1/2 is unity, giving X = 0.64 * 0.5^0.325 = 0.510911 at contact speed equal to the mutual escape speed. The discrimination guards re-evaluate the law with the historical wrong mass-ratio denominator (M_i/M_t, giving 0.640000), a wrong outer exponent (0.5, giving 0.807261 at 1.5 v_esc), and a wrong velocity exponent (1, giving 0.664974); each sits far outside the rel = 1e-4 pin.
The Table 2 pins reproduce the paper's own SPH results to within 4% in the fast grazing regime the authors report fits tightest; the 20% tolerance covers their stated scatter (9% median, about 20% for slow head-on impacts). The three scenarios share one mass ratio across a factor of 5.6 in total mass, so the pinned cluster also exercises the paper's finding that the loss is independent of the system mass at fixed impactor:target ratio.
Anchor type
Published benchmark (SPH simulation suite and fitted scaling law from the same paper), plus the closed-form analytical limit of the fitted law.