NSE

The reaction networks in Microphysics have the ability to use NSE instead of integrating the entire network when the conditions are appropriate. There are 2 different implementations of NSE in Microphysics, that have slightly different use cases.

• Tabulated NSE and aprox19 : this uses a table of NSE abundances given \((\rho, T, Y_e)\) generate from a large network (125 isotopes). The table also returns \(dY_e/dt\) resulting from electron-captures, to allow for the NSE state to evolve. This is meant to be used in the cores of massive stars and works only with the aprox19 reaction network.

  Furthermore, since the table can achieve \(Y_e\) and \(\bar{A}\) that are not representable by the 19 isotopes in aprox19, this table requires that we use the auxiliary composition and advect \(Y_e\), \(\bar{A}\), and \(\langle B/A\rangle\). All of the EOS calls will work with these quantities.

  This is enabled via USE_NSE_TABLE

• Self-consistent NSE : this adds an NSE solver to the network that can be called to find the equilibrium abundances of each of the species defined in the network. It works with any of the pynucastro-generated networks. Unlike the tabulated NSE, there is no need to advect the auxiliary composition, since this only deals with the isotopes defined in the main reaction network.

  This is enabled via USE_NSE_NET

Both solvers define a number of preprocessor variables, and both will provide a function in_nse() that can be used to determine if a state is currently in NSE.

make option	preprocessor variables set
USE_NSE_TABLE	NSE, NSE_TABLE, AUX_THERMO
USE_NSE_NET	NSE, NSE_NET

The directive NSE should be used whether the specific implementation of NSE does not matter.

These two NSE solvers are described below.

Tabulated NSE and aprox19

The aprox19 network can be run in a manner where we blends the standard aprox19 network with a table for nuclear statistic equilibrium resulting from a much larger network at high density and temperatures. This option is enabled by building with:

NETWORK_DIR=aprox19 USE_NSE_TABLE=TRUE

Composition and EOS

The NSE table was generated using a 125 nuclei reaction network (described in [MaWoosleyMalone+13]), and includes electron-capture rates, so the compositional quantities it carries, \(\bar{A}\) and \(Y_e\) and not representable from the 19 isotopes we carry in the network. In particular, it can attain a lower \(Y_e\) than aprox19 can represent.

For this reason, when we are using the NSE network, we always take the composition quantities in the EOS directly from eos_state.aux[] instead of from eos_state.xn[]. The AUX_THERMO preprocessor variable is enabled in this case, and the equations of state interpret this to use the auxiliary data for the composition. This is described in Auxiliary Composition.

NSE Table Outputs

The NSE table provides values for the auxiliary composition, \(Y_e\), \(\bar{A}\), and \(\langle B/A \rangle\) resulting from the full 125 nuclei network. It also provides a set of 19 \(X_k\) that map into the isotopes carried by aprox19.

These three quantities are stored as aux data in the network and are indexed as iye, iabar, and ibea. Additionally, when coupling to hydrodynamics, we need to advect these auxiliary quantities.

For Strang split coupling of hydro and reactions, \(DX_k/Dt = 0\), and our evolution equations are:

\[\begin{split}\begin{align*} \frac{DY_e}{Dt} &= \sum_k \frac{Z_k}{A_k} \frac{DX_k}{Dt} = 0 \\ \frac{D}{Dt} \frac{1}{\bar{A}} &= - \frac{1}{\bar{A}^2} \frac{D\bar{A}}{Dt} = \sum_k \frac{1}{A_k} \frac{DX_k}{Dt} = 0 \rightarrow \frac{D\bar{A}}{Dt} = 0 \\ \frac{D}{Dt} \left (\frac{B}{A} \right ) &= \sum_k \frac{B_k}{A_k} \frac{DX_k}{Dt} = 0 \end{align*}\end{split}\]

Therefore each of these auxiliary equations obeys an advection equation in the hydro part of the advancement.

NSE Flow

The basic flow of a simulation using aprox19 + the NSE table is as follows:

• initialize the problem, including \(X_k\)

• fill the initial aux data with \(Y_e\), \(\bar{A}\), and \((B/A)\)

• in hydro, we will update these quantities simply via advection (for Strang-split evolution)

• for the reactive update:

  • check if NSE applies (see below)

  • if we are in an NSE region:

    • use \(\rho\), \(T\), and \(Y_e\) to call the table. This returns: \(dY_e/dt\), \((B/A)_{\rm out}\), and \(\bar{A}_{\rm out}\).

    • update \(Y_e\) [1] :

      \[(Y_e)_{\rm out} = (Y_e)_{\rm in} + \Delta t \frac{dY_e}{dt}\]

    • \(\bar{A}_{\rm out}\) is simply the value returned from the table

    • the energy generation rate, \(e_{\rm nuc}\) is:

      \[e_{\rm nuc} = \eta \left [ \left ( \frac{B}{A} \right )_{\rm out} - \left ( \frac{B}{A} \right )_{\rm in} \right ] * \frac{1.602 \times 10^{-6} {\rm erg}}{{\rm MeV}} N_A \frac{1}{\Delta t}\]

      where \(\eta\) is an inertia term < 1 to prevent the energy changing too much in one set.

    • the new binding energy for the zone is then:

      \[\left ( \frac{B}{A} \right )_{\rm out} = \left ( \frac{B}{A} \right )_{\rm in} + \eta \left [ \left ( \frac{B}{A} \right )_{\rm out} - \left ( \frac{B}{A} \right )_{\rm in} \right ]\]

    • update the mass fractions, \(X_k\), using the values from the table

  • if we are not in NSE:

    • integrate the aprox19 network as usual

    • update the aux quantities at the end of the burn

NSE check

For a zone to be consider in NSE, we require \(\rho\) > rho_nse and either

• \(T\) > T_nse together with the composition check

• \(T\) > T_always_nse

where we assume that T_always_nse > T_nse.

The composition check considers the following nuclei groups:

• He_group: atomic numbers 1 to 2 (H to He)

• C_group: atomic numbers 6 to 7 (C to N)

• O_group: atomic number 8 (O)

• Si_group: atomic number 14 (Si)

• Fe_group: atomic numbers 24 to 30 (Cr to Zn)

and we then say that a composition supports NSE if:

• \(X(C_group)\) < C_nse

• \(X(O_group)\) < O_nse

• \(X(Si_group)\) < Si_nse

• \(X(Fe_group) + X(He_group)\) > He_Fe_nse

NSE table ranges

The NSE table was created for:

• \(9 < \log_{10}(T) < 10.4\)

• \(7 < \log_{10}(\rho) < 10\)

• \(0.4 < Y_e < 0.5\)

Self-consistent NSE

The self-consistent NSE approach uses only the nuclei in the main reaction network. It solves for the chemical potentials of the proton and neutron and from there gets the abundances of each of the nuclei under the assumption of NSE, following the procedure outlined in [CTS+07].

The solve is done using a port of the hybrid Powell method from MINPACK (we ported the solver to templated C++).

The advantage of this approach is that it can be used with any reaction network, once the integration has reached NSE.

This solver is enabled by compiling with

USE_NSE_NET=TRUE

The functions to find the NSE state are then found in nse_solver.H.

Dynamic NSE Check

We have implemented a dynamic NSE check for the self-consistent nse procedure that tells us whether the network has reached the NSE state. The overall procedure is outlined in [KushnirKatz20]. The overall usage comes down to a single function in_nse(state). By supplying the current state, this function returns a boolean that tells us whether we’re in NSE or not. The current status of this functionality only works for pynucastro-generated network since aprox networks have slightly different syntax.

The overall framework is constructed following [KushnirKatz20] with slight variations. The overview of the steps we take are the following:

• Minimum Temperature Check: require T > T_min_nse, where T_min_nse is a runtime parameter with a default value T_min_nse = 4.0e9.

• Mass Abundance Check: compare the current mass abundances of the nuclei to the NSE mass fractions. A detailed criteria are the following:

  We first determine whether the current molar fraction is close to NSE with a criteria of:

  \[\frac{r - r_{NSE}}{r_{NSE}} < 0.5\]

  where \(r = Y_\alpha/(Y_p^2 Y_n^2)\) and \(r_{NSE} = \left(Y_\alpha/(Y_p^2 Y_n^2)\right)_{NSE}\) if there is neutron in the network.

  \[\frac{r - r_{NSE}}{r_{NSE}} < 0.25\]

  where \(r = Y_\alpha/(Y_p^2)\) and \(r_{NSE} = \left(Y_\alpha/(Y_p^2)\right)_{NSE}\) if neutron is not in the network.

  If the molar check above failed, then we proceed with an overall molar fraction check:

  \[\epsilon_{abs} = Y^i - Y^i_{NSE} < \mbox{nse_abs_tol}\]
  \[\epsilon_{rel} = \frac{\epsilon_{abs}}{Y^i} < \mbox{nse_rel_tol}\]

  where nse_rel_tol = 0.2 and nse_abs_tol = 0.005 by default.

• Removed [KushnirKatz20] also requires a fast reaction cycle that exchanges 1 \(\alpha\) particle with 2 \(p\) and 2 \(n\) particles. We used to have this check, but currently removed as we think it is not necessary. However, the description is as following: This reaction cycle should have the following reactions or their reverse:

  • 1 \((\alpha, \gamma)\), 2 \((\gamma, p)\), 2 \((\gamma, n)\)

  • 1 \((\alpha, p)\), 1 \((\gamma, p)\), 2 \((\gamma, n)\)

  • 1 \((\alpha, n)\), 2 \((\gamma, p)\), 1 \((\gamma, n)\)

  To consider to be fast reaction cycle, every step in the cycle to have \(Y_i/\textbf{min}(b_f, b_r) < \epsilon t_s\) for \(i = n, p, \alpha\) participated in this step, where \(b_f\) and \(b_r\) are the forward and reverse rate of the reaction, \(\epsilon\) is a tolerance which has a default value of \(0.1\), and \(t_s\) is the sound crossing time of a simulation cell.

  An example of such reaction cycle would be:

  \[\isotm{S}{32} (\gamma, p)(\gamma, p)(\gamma, n)(\gamma, n) \isotm{Si}{28} (\alpha, \gamma) \isotm{S}{32}\]

• NSE Grouping Process: Initially, \(p\), \(n\), and \(\alpha\) are grouped into a single group called the light-isotope-group, or LIG. Other isotopes belong to their own group, which only contains themselves. We need to start the grouping process with the reaction rate that has the fastest (smallest) timescale. In the original [KushnirKatz20] paper, they use the group molar fraction for evaluating the reaction timescale. This complicates things because now reaction timescale changes after each successful grouping. We’ve determined that the result is roughly the same even if we just use the molar fraction of the isotope that is involved in the actual reaction. Therefore, instead of using \(t_{i,k} = \tilde{Y}_i/\textbf{min}(b_f(k), b_r(k))\), to evaluate the reaction timescale of the reaction, \(k\), where \(\tilde{Y}_i\) represents the sum of molar fractions of the group that isotope \(i\) belongs to, we simply use the \(Y_i\), which is the molar fraction of the isotope \(i\), which is the isotope involved in the reaction that is different from \(p\), \(n\), and \(\alpha\). After we settle on calculating the timescale, since \(Y_i\) doesn’t change, we can calculate all timescale at once and sort the reaction to determine the order at which we want to start merging.

  There are two requirements for us to check whether this reaction can be used to group the nuclei involved, which are:

  • at least 1 isotope, \(i\), that passes:

    \[t_{i,k} < \epsilon t_s\]
  • \[2|b_f(k) - b_r(k)|/(b_f(k) + b_r(k) < \epsilon\]

  Here we only consider two cases of reactions:

  • There are exactly two isotopes involved in reaction, \(k\), that are not in the light-isotope-group. In this case, if the reaction passes the two criteria mentioned above, we merge the groups containing those two isotopes if they’re not yet in the same group.

  • There is only one isotope involved in reaction, \(k\), that is not in the light-isotope-group, which is not necessarily isotope \(i\) that passes the first criteria. In this case, we merge the isotope that is not in LIG into LIG.

  Here we skip over reactions of the following due to obvious reasons:

  • Reactions that have no reverse rates.

  • Reactions that involve more than 2 reactants and products

  • Reactions that have more than 2 non-light-isotope-group.

  • The nuclei that participate in the reaction is either in LIG or in another group. This means that the non-LIG nuclei have already merged.

  At the end of the grouping process, we define that the current state have reached NSE when there is only a single group left, or there are two groups left where one of them is the light-isotope-group.

  When there is no neutron in the network, it can be difficult for isotopes to form a single group due to the missing neutron rates. Therefore, there is an alternative criteria of defining a “single group” when neutron is not present in the network: for isotopes, \(Z >= 14\), isotopes with odd and even \(N\) form two distinct groups.

Additional Options

Here we have some runtime options to allow a more cruel estimation to the self-consistent nse check:

• nse.nse_dx_independent = 1 in the input file allows the nse check to ignore the dependency on the cell size, dx, which calculates the sound crossing time, t_s. Naturally, we require the timescale of the rates to be smaller than t_s to ensure the states have time to achieve equilibrium. However, sometimes this check can be difficult to achieve, so we leave this as an option for the user to explore.

• nse.nse_molar_independent = 1 in the input file allows the user to use the nse mass fractions for nse check after the first check (the one that ensures we’re close enough to the nse mass fractions to get reasonable results) is passed. This allows the subsequent checks to only rely on the thermodynamic conditions instead of mass fractions.

• nse.nse_skip_molar = 1 in the input file allows the user to skip the molar fraction check after the integration has failed. This option is used to completely forgo the requirement on molar fractions and allow the check to only dependent on the thermodynamic conditions. By only applying this after option after the integration failure, we hope the integrator has evolved the system to the NSE state the best it can. By turning on this option, we hope to give relief to the integrator if the system is in NSE thermodynamically, which is likely the case.

• nse.T_nse_net in the input file allows the user to define a simple temperature threshold to determine the NSE state instead of using the complicated procedure that looks for a balance between the forward and the reverse rates. Once this quantity is set to a positive value, then in_nse returns true if the current temperature is higher than T_nse_net, and false if the current temperature is lower than T_nse_net. Note that we still perform a simple molar fraction check to ensure that the current state is close enough to the NSE state.

• nse.ase_tol is the tolerance that determines the equilibrium condition for forward and reverse rates. This is set to 0.1 by default.

• nse.nse_abs_tol is the absolute tolerance of checking the difference between current molar fraction and the NSE molar fraction. This is set to 0.005 by default.

• nse.nse_rel_tol is the relative tolerance of checking the difference between current molar fraction and the NSE molar fraction. This is set to 0.2 by default.

• nse.T_min_nse is the minimum temperature required to consider the subsequent NSE checks. This is mainly to avoid unnecessary computations of computing the NSE mass fractions when the current temperature is too low. This is set to 4.0e9 by default.

Footnotes

[1]

The table actually provides the weak rate, which is the sum of all electron capture and positron decay rates times the appropriate abundances minus a similar rate for the beta decay and positron capture, [wrate] = [rectot] + [rpdtot] - [redtot] - [rpctot]

So if electron capture dominates, then [wrate] is positive and this should be subtracted from \(Y_e\).