PyREC scaffolding (from YREC development, 7be4133)/ ytime.fGlossary

ytime.f

181 lines · 1 definitions · on GitHub (7be4133)

Python version: ytime.py, 154 lines, on the right (switch to Python above the code).

Variables on this page (6)

atime, clsun, cmsun, json, lexcom, tcut

Purpose

YTIME gives the time step allowed by helium burning. Its comment: stars with a helium luminosity have a time step limited by the time to burn ATIME(4) of $Y$ at the temperature maximum, or the fraction ATIME(5) of $Y$ there; a high central density is taken as the sign of a giant in the helium flash rather than a horizontal-branch star.

How it fits

  • Inputs: the model (arguments: $\log\rho$ HD, HCOMP, $L$, $m$, $\log T$, JCORE, M, JXBEG), work arrays for the rates, and from modules the limits ATIME, the temperature cuts TCUT and the extended-composition switch LEXCOM (NMLPAR_MOD), and $L_\odot$ CLSUN and $M_\odot$ CMSUN (CONSTANTS_MOD).
  • Who calls it: only HTIMER.
  • What it calls: ENGEB, for the energy rates at the temperature maximum (and, below ATIME(1), at each helium-burning point), which also returns EALPCA and ENU (and a $^7$Be mass fraction this routine does not use).
  • Outputs: DELTSY, the step (in seconds, by its use in HTIMER), and EG(1..6), the energy rates at the temperature maximum.

The method

  • Helium flash ($\log\rho_c \ge 5$): find the shell of maximum temperature, compute its energy rates with ENGEB, and take $$\Delta t = \frac{10^{15}}{\epsilon_5},$$ with $\epsilon_5$ = EG(5), ENGEB's fifth argument E3AL, the triple-α rate (settled from the code, Nov 9, 2026: line 104↗; line 115↗; engeb.f, line 4↗), at least $10^{-22}$.

  • Otherwise: with $Y_\mathrm{core}$ at JCORE and $q = 5.85\times10^{17}$ erg/g (the commented-out Q3A), if $Y_\mathrm{core} \ge$ ATIME(1), $$\Delta Y = \min(\texttt{ATIME(4)},\ \texttt{ATIME(5)}\,Y_\mathrm{core}),\qquad \Delta t = \frac{q}{L_\odot}\,\frac{m}{L/L_\odot}\,\Delta Y$$ at JCORE. ATIME(1) is one threshold for both elements by design: XTIME compares the central $X$ with it, this routine $Y_\mathrm{core}$; REMAP's comment calls it the "minimum central abundance to use core burning dt criterion," and the documentation of TIME_CORE_MIN, the "minimum X,Y for using core vs shell burning criteria" (settled from the code and the documentation, Nov 9, 2026: line 123↗; remap.f, lines 260–261↗).

Otherwise (the shell branch) the step is the smallest of three limits, starting from $10^{20}$ s: - burning ATIME(14) $M_\odot$ of helium at the luminosity just below the hydrogen shell, $\Delta t = (q/L_\odot)\,\texttt{ATIME(14)}\,M_\odot/(L/L_\odot)$ at JXBEG-1 (when JXBEG > 1 and that $L > 0$); - burning the fraction ATIME(12) of $Y$ at any point with $Y \ge$ ATIME(1) and $\log T \ge$ TCUT(4), $\Delta t = \texttt{ATIME(12)}\,Y/\dot Y$, with $\dot Y = \epsilon_{3\alpha}/q + \epsilon_{\alpha\mathrm{cap}}/(1.7276\times10^{18}\ \mathrm{erg/g})$ from ENGEB (EGG55 and EALPCA, negative values taken as zero); - while $Y_\mathrm{core}$ > YFLOOR ($10^{-5}$, below every END_YCEN in use) and $L > 0$ at JCORE, the core criterion without its cap: $\Delta t = (q/L_\odot)\,\texttt{ATIME(5)}\,Y_\mathrm{core}\,m/(L/L_\odot)$ at JCORE, which resolves central helium exhaustion down to the END_YCEN stop.

Definitions

subroutineYTIME

lines 4–181 · GitHub

The helium-burning time step: during the helium flash $10^{15}/\epsilon_{3\alpha}$ at the temperature maximum; otherwise the time to burn a fraction of the core's helium, or, below ATIME(1), the smallest of the shell and core limits.

Uses modules CONSTANTS_MOD, DIMENSIONS_MOD, NMLPAR_MOD

Called by HTIMER

Flowchart of its calls, in order (2)
if (HD(1).GE.5.0D0), lines 69–119
ENGEB104
else, lines 120–178
else, lines 129–176
for J = 1,M (DO 220), lines 147–170
ENGEB162

Change log

The changes since rp_1

Changed since rp_1. EALPCA and ENU come back from ENGEB as arguments (/NEWEPS/ retired), with the new BE7MFLOC for ENGEB's $^7$Be mass fraction (formerly /BE7/); COMMON blocks to modules, JSON from DIMENSIONS_MOD, IMPLICIT NONE.

Changed since a8a5950. The shell branch (below ATIME(1)) has real limits: It used to rescale DELTSY, which held HTIMER's helium step in seconds, as if it were an amount of $Y$, so the step grew through core helium exhaustion; it now takes the smallest of the ATIME(14), ATIME(12), and core limits above, so ATIME(12) and ATIME(14), read but never used before, now count (this settles former question 2).