xtime.f

XTIME gives the time step allowed by hydrogen burning (its comment). While the central hydrogen abundance is at least ATIME(1), the step is the time to burn a given amount of the core's hydrogen; afterwards, in shell burning, the smaller of the time to burn a fraction of the star's hydrogen and the time to deplete a given amount of $X$ at the shell's midpoint.

• Inputs: the model (arguments: $\rho$, HCOMP, $L$, $m$, shell masses, $T$), the hydrogen-burning luminosity HYDLUM, the convective core's edge JCORE, the shell's midpoint JXMID; through modules the limits ATIME (NMLPAR_MOD), the constants (CONSTANTS_MOD).
• Who calls it: only HTIMER.
• What it calls: RATES and EQBURN, for $dX/dt$ at the shell's midpoint.
• Output: DELTSH, in seconds (by its use in HTIMER).
• Modules: NMLPAR_MOD (formerly COMMON/CTLIM/), CONSTANTS_MOD, DIMENSIONS_MOD.

With $q = 6.0\times10^{18}$ erg released per gram of hydrogen burned (the code's constant):

• Core burning ($X_c \ge$ ATIME(1)): $\Delta X = \min(\texttt{ATIME(2)},\ \texttt{ATIME(3)}\,X_\mathrm{core})$ and $$\Delta t = \frac{q}{L_\odot}\,\frac{m_\mathrm{core}}{L(m_\mathrm{core})/L_\odot}\,\Delta X,$$ with $m$ and $L$ at JCORE, so that a convective core counts as a whole (the comment). The header speaks of "the fraction ATIME(3) of the central X," and the code takes $X$ at JCORE: The two agree, since JCORE is the outermost point of the convective core, which is mixed, or 1 (the center) when there is no convective core or the star is fully convective (settled from the code, Nov 9, 2026: lines 53–55↗; line 66↗; findsh.f, line 65↗; findsh.f, lines 108–110↗; findsh.f, line 124↗).
• Shell burning: $\Delta t = q\,\texttt{ATIME(6)}\,X_s M_\odot/L_\odot\,/\,L_H$, with $X_s$ the surface $X$ and $L_H$ = HYDLUM; and, if $dX/dt < 0$ at the shell's midpoint and ATIME(7) > 0, $$\Delta t = \min\!\left(\Delta t,\ \left|\frac{\texttt{ATIME(7)}}{dX/dt}\right|\,\mathrm{CSECYR}\times10^9\right).$$ ATIME(6) is a mass in solar masses, whatever the star's mass: REMAP's comment calls it the "max permitted solar masses of X burned in timestep" (TIME_DX_TOTAL, "max absolute H burned per timestep (post-MS)" in the documentation); the code burns the hydrogen of ATIME(6) $M_\odot$ of matter at the surface $X$, and the header's "the mass fraction ATIME(6) of X in the entire star" is older wording (settled from the code and the documentation, Nov 9, 2026: lines 56–60↗; lines 73–74↗; remap.f, lines 266–267↗). The factor $\mathrm{CSECYR}\times10^9$ is there because EQBURN returns $dX/dt$ per Gyr: The rates of RATES are per $10^9$ yr per amu (settled from the code, Nov 9, 2026: lines 105–106↗; rates.f, lines 656–658↗).