arXiv subjects

F. K. Roepke

Publications and source records attributed to F. K. Roepke.

At least 19 recordsLinked to original sources

arXiv2026

Multi-dimensional NLTE radiative transfer for a double detonation Type Ia explosion model in the photospheric phase

Previous radiative transfer calculations have shown that both multi-dimensionality and NLTE (non local thermodynamic equilibrium) effects impact the synthetic observables predicted for Type Ia supernova explosion models. Here we carry out a 2D NLTE radiative transfer simulation in the photospheric phase for a double detonation model with a 1 M$_{\odot}$ carbon-oxygen core and 0.02 M$_{\odot}$ helium shell. The predicted observables demonstrate that departures from both spherical symmetry and local thermodynamic equilibrium are significant on scales relevant to comparisons with observations. The NLTE treatment leads to a bluer spectral energy distribution, more slowly declining optical-band light curves, changes to the near-infrared light curve shapes, and differences in the evolution of key spectral features. Although substantial viewing-angle variation is predicted for the model, the scale of the variation is not significantly impacted by NLTE effects up to peak. However, after peak, our 2D NLTE simulation retains a strong viewing-angle dependence in its spectra whereas our simulation of the same ejecta model using an approximate NLTE treatment predicts significantly reduced spectroscopic viewing-angle variation. The NLTE treatment leads to improved agreement with normal Type Ia supernovae, primarily due to the increased ionisation state of the simulation, although the absence of a secondary near-infrared maximum suggests that the ejecta remain over-ionised relative to observations after peak. Despite the improved agreement, many viewing angles still show excessive line blanketing, suggesting that the 0.02 M$_{\odot}$ helium-shell mass of the current model is too large to reproduce normal Type Ia supernovae over the majority of lines of sight.

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arXiv2026

Nebular Fingerprints of a Violent White Dwarf Merger: 3D NLTE Modelling of Type Ia Supernovae

Binary systems composed of two carbon-oxygen white dwarfs (WDs) are a leading progenitor candidate for Type Ia supernovae. One widely discussed scenario is the dynamically driven double-degenerate double-detonation (D$^6$) of a sub-Chandrasekhar-mass WD binary, where detonations are triggered by dynamical interaction. However, some systems are expected to undergo violent mergers, in which the primary ignites through direct carbon ignition as the secondary strikes its surface. We present the first 3D nebular-phase radiative-transfer calculations of a violent merger, using a $1.1 M_\odot$ and $0.7 M_\odot$ sub-Chandrasekhar binary. Our simulations employ a full NLTE (non local thermodynamic equilibrium) treatment of excitation and ionisation, including non-thermal electron contributions. By comparing 1D and 3D realisations, we show that multidimensional modelling improves the ionisation state and reveals features absent from 1D calculations, most notably [O I] from unburned material associated with the secondary. The model reproduces much of the panchromatic spectrum of the normal SN 2021aefx, but underpredicts [Ni II] while producing strong high-ionisation stable-Ni features, illustrating that stable-Ni signatures depend not only on abundance, but also on ionisation state. Although the model does not reproduce the strong [Ar II] and [Ne II] emission observed in the 03fg-like SN 2022pul, our calculations suggest that this event may require a similar merger configuration, involving full disruption of the secondary or a more massive companion with more extensive burning. Finally, viewing-angle variation is substantial, with signatures distinct from D$^6$-like scenarios, suggesting that JWST nebular samples, combined with multidimensional modelling, can discriminate between channels.

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arXiv2026

Multidimensional Nebular-Phase Calculations of Dynamically-Driven Double-Degenerate Double-Detonation Models for Type Ia Supernovae

The dynamically-driven double-degenerate double-detonation model has emerged as a promising progenitor candidate for Type Ia supernovae. In this scenario, the primary white dwarf ignites due to dynamical interaction with a companion white dwarf, which may also undergo a detonation. Consequently, two scenarios exist: one in which the secondary survives and another in which both white dwarfs detonate. In either case, substantial departures from spherical symmetry are imprinted on the ejecta. Here, we compute full non-local thermodynamic equilibrium nebular-phase spectra in 1D and 3D to probe the innermost asymmetries. Our simulations reveal that the multidimensional structures significantly alter the overall ionisation balance, width and velocity of features, especially when the secondary detonates. In this scenario, some element distributions may produce orientation-dependent line profiles that can be centrally peaked from some viewing-angles and somewhat flat-topped from others. Comparison to observations reveals that both scenarios produce most observed features from the optical to mid-infrared. However, the current model realisations do not consistently reproduce all line shapes or relative strengths, and yield prominent optical Ar III emission which is inconsistent with the data. When the secondary detonates, including 3D effects improves the average agreement with observations, however when compared to observations, particularly weak optical Co III emission and the presence of optical O I and near-infrared S I challenge its viability for normal Type Ia supernovae. Thus, overall, our comparisons with normal Type Ia's tentatively favour detonation of only the primary white dwarf, but we stress that more model realisations and mid-infrared observations are needed.

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arXiv2026

Multi-wavelength Signatures of Supernova Shock Breakout from Red Supergiants in Two Dimensions

We present new two-dimensional radiation hydrodynamic simulations of supernova shock breakout from red supergiants using the $\texttt{CASTRO}$ code. Our progenitors are 20 and 25 M$_{\odot}$ solar-metallicity stars evolved from the zero-age main sequence with $\texttt{MESA}$ and exploded in one dimension using $\texttt{FLASH}$. We consider a range of circumstellar media (CSM) produced by stellar winds to investigate how pre-explosion mass-loss affects shock breakout. The multigroup flux-limited diffusion scheme in $\texttt{CASTRO}$ captures the interaction between the explosion shock, its radiation precursor, and the surrounding CSM. We find that strong radiation precursors, generated by radiation leakage behind the shock, can drive fluid instabilities and move the effective photosphere outward before the shock reaches the stellar surface. The resulting breakout emissions reach peak luminosities of ${\sim}10^{44}$ erg s$^{-1}$ with full-width half-maximum durations of 1-3 hr, , fainter and longer than previous 1D models. The light-curve colors gradually evolve from blue to red after the peak. The 25 M$_{\odot}$ model with explosion energy $E \sim 1.69\times10^{51}$ erg produces ${\sim}$10-30\% higher maximum luminosity than the 20 M$_{\odot}$ model with $E \sim 1.09\times10^{51}$ erg. The dense CSM further extends the breakout rise time by increasing the photon diffusion. These results provide new constraints on red supergiant atmospheres and mass-loss histories prior to core collapse.

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arXiv2025

NLTE spectral modelling for a carbon-oxygen and helium white-dwarf merger as a Ca-rich transient candidate

We carry out NLTE (non local thermodynamic equilibrium) radiative transfer simulations to determine whether explosion during the merger of a carbon-oxygen (CO) white dwarf (WD) with a helium (He) WD can reproduce the characteristic Ca II/[Ca II] and He I lines observed in Ca-rich transients. Our study is based on a 1D representation of a hydrodynamic simulation of a 0.6 $M_{\odot}$ CO + 0.4 $M_{\odot}$ He WD merger. We calculate both photospheric and nebular-phase spectra including treatment for non-thermal electrons, as is required for accurate modelling of He I and [Ca II]. Consistent with Ca-rich transients, our simulation predicts a nebular spectrum dominated by emission from [Ca II] 7291, 7324 angstrom and the Ca II near-infrared (NIR) triplet. The photospheric-phase synthetic spectrum also exhibits a strong Ca II NIR triplet, prominent optical absorption due to He I 5876 angstrom and He I 10830 angstrom in the NIR, as is commonly observed for Ca-rich transients. Overall, our results therefore suggest that CO+He WD mergers are a promising channel for Ca-rich transients. However, the current simulation overpredicts some He I features, in particular both He I 6678 and 7065 angstrom and shows a significant contribution from Ti II, which results in a spectral energy distribution that is substantially redder than most Ca-rich transients at peak. Additionally the Ca II nebular emission features are too broad. Future work should investigate if these discrepancies can be resolved by considering full 3D models and exploring a range of CO+He WD binary configurations.

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arXiv2025

SEDONA-GesaRaT: an AI-Accelerated Radiative Transfer Program for 3-D Supernova Simulations

We present SEDONA-GesaRaT, a rapid code for supernova radiative transfer simulation developed based on the Monte-Carlo radiative transfer code SEDONA. We use a set of atomic physics neural networks (APNN), an artificial intelligence (AI) solver for the non-local thermodynamic equilibrium (NLTE) atomic physics level population calculation, which is trained and validated on 119 1-D type Ia supernova (SN Ia) radiative transfer simulation results showing great computation speed and accuracy. SEDONA-GesaRaT has been applied to the 3-D SN Ia explosion model N100 to perform a 3-D NLTE radiative transfer calculation. The spatially resolved linear polarization data cubes of the N100 model are successfully retrieved with a high signal-to-noise ratio using the integral-based technique (IBT). The overall computation cost of a 3-D NLTE spectropolarimetry simulation using SEDONA-GesaRaT is only $\sim$3000 core-hours, while the previous codes could only finish 1-D NLTE simulation, or 3-D local thermodynamic equilibrium (LTE) simulation, with similar computation resources. The excellent computing efficiency allows SEDONA-GesaRaT for future large-scale simulations that systematically study the internal structures of supernovae.

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arXiv2025

Explodability criteria for the neutrino-driven supernova mechanism

Massive stars undergoing iron core-collapse at the end of their evolution terminate their lives either in successful or failed supernovae (SNe). The physics of core-collapse supernovae (CCSNe) is complex, and their understanding requires computationally expensive simulations. Using these to predict CCSN outcomes over large, densely sampled parameter spaces of SN progenitors, as is needed e.g. for population synthesis studies, is thus not feasible. To remedy this situation, we present explodability criteria that allow us to predict the final fates of stars by evaluating stellar structure variables at the onset of core-collapse. The criteria are calibrated to predictions of a semi-analytical SN model, evaluated over a set of $\sim$~3,900 heterogeneous stellar progenitors (single, binary-stripped and accretor stars). Over these, the criteria achieve an accuracy of >99\% agreement with the semi-analytical model. The criteria are tested on 29 state-of-the-art 3D CCSN simulation outcomes from two different groups. Furthermore, we find that all explodability proxies needed for our pre-SN structure-based criteria have two distinct peaks and intervening valleys as a function of the carbon-oxygen (CO) core mass $M_\mathrm{CO}$, which coincide with failed and successful SNe, respectively. The CO core masses of explodability peaks shift systematically with metallicity, $Z$, and with timing of hydrogen-rich envelope removal by binary mass transfer. With these, we identify critical values in $M_\mathrm{CO}$ that define windows over which black holes form by direct collapse and formulate a CCSN recipe based on $M_\mathrm{CO}$ and $Z$, applicable for rapid binary population synthesis and other studies. Our explodability formalism is consistent with observations of Type~IIP, IIb/Ib and Ic supernova progenitors and partially addresses the missing Red Supergiant Problem by direct black hole formation.

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arXiv2025

Exploring the range of impacts of helium in the spectra of double detonation models for Type Ia supernovae

Models of sub-Chandrasekhar mass double detonations for Type Ia supernovae (SNe Ia) suggest a distinguishing property of this scenario is unburnt helium in the outer ejecta. However, modern explosion simulations suggest there may be significant variations in its mass and velocity distribution. We recently presented a NLTE (non local thermodynamic equilibrium) radiative transfer simulation for one realisation of the double detonation scenario with a modest He mass (0.018 M${\odot}$) present in the ejecta at relatively high velocities (${\sim}18000\,\mathrm{km}\,\mathrm{s}^{-1}$). That simulation predicted a He I 10830$\,Å$ feature blueward of Mg II 10927$\,Å$ consistent with near-infrared observations of "transitional" SNe Ia. To demonstrate the expected diversity in the helium signature, here we present a calculation for a double detonation model with a higher He mass (${\sim}$0.04 M${\odot}$) ejected at lower velocities (${\sim}13000\,\mathrm{km}\,\mathrm{s}^{-1}$). Despite our simulation predicting no clear optical or 2 micron helium features, a strong and persistent He I 10830$\,Å$ absorption is present. The feature appears at wavelengths consistent with the extended blue wing of the Mg II 10927$\,Å$ feature sometimes present in observations, suggesting this is a helium spectral signature (although for this particular model it is too strong and persistent to be consistent with normal SNe Ia). The significant differences in He I 10830$\,Å$ predicted by the two simulations suggests helium spectral signatures likely show significant variation throughout the SNe Ia population. This motivates further work to use this observable signature to test the parameter space for double detonation models.

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arXiv2024

Going from 3D common-envelope simulations to fast 1D simulations

One-dimensional (1D) methods for simulating the common-envelope (CE) phase offer advantages over three-dimensional (3D) simulations regarding their computational speed and feasibility. We present the 1D CE method from Bronner et al. (2024), including the results of the CE simulations of an asymptotic giant branch star donor. We further test this method in the massive star regime by computing the CE event of a red supergiant with a neutron-star mass and a black-hole mass companion. The 1D model can reproduce the orbital evolution and the envelope ejection from 3D simulations when choosing suitable values for the free parameters in the model. The best-fitting values differ from the expectations based on the low mass simulations, indicating that the free parameters depend on the structure of the giant star. The released recombination energy from hydrogen and helium helps to expand the envelope, similar to the low-mass CE simulations.

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arXiv2024

Heavy element abundances from a universal primordial distribution

We present a freeze-out approach to the formation of heavy elements in expanding nuclear matter. Applying concepts used in the description of heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and the Mott effect, which describes the dissolution of nuclei at high densities of nuclear matter. With this approach, we search for a universal primordial distribution in an equilibrium state from which the gross structure of the solar abundances of heavy elements freezes out via radioactive decay of the excited states. The universal primordial state is characterized by the Lagrangian parameters of temperature and chemical potentials of neutrons and protons. We show that such a state exists and determine a temperature of 5.266 MeV, a neutron chemical potential of 940.317 MeV and a proton chemical potential of 845.069 MeV, at a baryon number density of 0.013 fm$^{-3}$ and a proton fraction of 0.13. Heavy neutron-rich nuclei such as the hypothesized double-magic nucleus $^{358}$Sn appear in the primordial distribution and contribute to the observed abundances after fission. We discuss astrophysical scenarios for the realization of this universal primordial distribution for heavy element nucleosynthesis, including supernova explosions, neutron star mergers and the inhomogeneous Big Bang. The latter scenario may be of interest in the light of early massive objects observed with the James Webb Space Telescope and opens new perspectives to explain universality of the observed r-process patterns and the lack of observations of population III stars.

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arXiv2024

From spherical stars to disk-like structures: 3D common-envelope evolution of massive binaries beyond inspiral

Three-dimensional simulations usually fail to cover the entire dynamical common-envelope phase of gravitational wave progenitor systems due to the vast range of spatial and temporal scales involved. We investigated the common-envelope interactions of a $10\,M_\odot$ red supergiant primary star with a black hole and a neutron star companion, respectively, until full envelope ejection (${\gtrsim}\,97 \,\mathrm{\%}$ of the envelope mass). We find that the dynamical plunge-in of the systems determines largely the orbital separations of the core binary system, while the envelope ejection by recombination acts only at later stages of the evolution and fails to harden the core binaries down to orbital frequencies where they qualify as progenitors of gravitational-wave-emitting double-compact object mergers. As opposed to the conventional picture of a spherically symmetric envelope ejection, our simulations show a new mechanism: The rapid plunge-in of the companion transforms the spherical morphology of the giant primary star into a disk-like structure. During this process, magnetic fields are amplified, and the subsequent transport of material through the disk around the core binary system drives a fast jet-like outflow in the polar directions. While most of the envelope material is lost through a recombination-driven wind from the outer edge of the disk, about $7\,\mathrm{\%}$ of the envelope leaves the system via the magnetically driven outflows. We further explored the potential evolutionary pathways of the post-common-envelope systems given the expected remaining lifetime of the primary core ($2.97\,M_\odot$) until core collapse ($6{\times}10^{4}\,\mathrm{yr}$), most likely forming a neutron star. We find that the interaction of the core binary system with the circumbinary disk increases the likelihood of giving rise to a double-neutron star merger. (abridged)

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arXiv2024

Towards a self-consistent model of the convective core boundary in upper main sequence stars. Part I: 2.5D and 3D simulations

There is strong observational evidence that the convective cores of intermediate-mass and massive main sequence stars are substantially larger than those predicted by standard stellar-evolution models. However, it is unclear what physical processes cause this phenomenon or how to predict the extent and stratification of stellar convective boundary layers. Convective penetration is a thermal-timescale process that is likely to be particularly relevant during the slow evolution on the main sequence. We use our low-Mach-number Seven-League Hydro code to study this process in 2.5D and 3D geometries. Starting with a chemically homogeneous model of a $15\,\mathrm{M}_\odot$ zero-age main sequence star, we construct a series of simulations with the luminosity increased and opacity decreased by the same factor, ranging from $10^3$ to $10^6$. After reaching thermal equilibrium, all of our models show a clear penetration layer; its thickness becomes statistically constant in time and it is shown to converge upon grid refinement. The penetration layer becomes nearly adiabatic with a steep transition to a radiative stratification in simulations at the lower end of our luminosity range. This structure corresponds to the adiabatic `step overshoot' model often employed in stellar-evolution calculations. The simulations with the highest and lowest luminosity differ by less than a factor of two in the penetration distance. The high computational cost of 3D simulations makes our current 3D data set rather sparse. Depending on how we extrapolate the 3D data to the actual luminosity of the initial stellar model, we obtain penetration distances ranging from $0.09$ to $0.44$ pressure scale heights, which is broadly compatible with observations.

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arXiv2024

On the fate of the secondary white dwarf in double-degenerate double-detonation Type Ia supernovae -- II. 3D synthetic observables

A leading model for Type Ia supernovae involves the double-detonation of a sub-Chandrasekhar mass white dwarf. Double-detonations arise when a surface helium shell detonation generates shockwaves that trigger a core detonation; this mechanism may be triggered via accretion or during the merger of binaries. Most previous double-detonation simulations only included the primary white dwarf; however, the fate of the secondary has significant observational consequences. Recently, hydrodynamic simulations accounted for the companion in double-degenerate double-detonation mergers. In the merger of a 1.05$\text{M}_{\odot}$ primary white dwarf and 0.7$\text{M}_{\odot}$ secondary white dwarf, the primary consistently detonates while the fate of the secondary remains uncertain. We consider two versions of this scenario, one in which the secondary survives and another in which it detonates. We present the first 3D radiative transfer calculations for these models and show that the synthetic observables for both models are similar and match properties of the peculiar 02es-like subclass of Type Ia supernovae. Our calculations show angle dependencies sensitive to the companion's fate, and we can obtain a closer spectroscopic match to normal Type Ia supernovae when the secondary detonates and the effects of helium detonation ash are minimised. The asymmetry in the width-luminosity relationship is comparable to previous double-detonation models, but the overall spread is increased with a secondary detonation. The secondary detonation has a meaningful impact on all synthetic observables; however, multidimensional nebular phase calculations are needed to support or rule out either model as a likely explanation for Type Ia supernovae.

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arXiv2024

Type Ia supernova explosion models are inherently multidimensional

Theoretical and observational approaches to settling the important questions surrounding the progenitor systems and the explosion mechanism of normal Type Ia supernovae have thus far failed. With its unique capability to obtain continuous spectra through the near- and mid-infrared, JWST now offers completely new insights into Type Ia supernovae. In particular, observing them in the nebular phase allows us to directly see the central ejecta and thereby constrain the explosion mechanism. We aim to understand and quantify differences in the structure and composition of the central ejecta of various Type Ia supernova explosion models. We examined the currently most popular explosion scenarios using self-consistent multidimensional explosion simulations of delayed-detonation and pulsationally assisted, gravitationally confined delayed detonation Chandrasekhar-mass models and double-detonation sub-Chandrasekhar-mass and violent merger models. We find that the distribution of radioactive and stable nickel in the final ejecta, both observable in nebular spectra, are significantly different between different explosion scenarios. Therefore, comparing synthetic nebular spectra with JWST observations should allow us to distinguish between explosion models. We show that the explosion ejecta are inherently multidimensional for all models, and the Chandrasekhar-mass explosions simulated in spherical symmetry in particular lead to a fundamentally unphysical ejecta structure. Moreover, we show that radioactive and stable nickel cover a significant range of densities at a fixed velocity of the homologously expanding ejecta. Any radiation transfer postprocessing has to take these variations into account to obtain faithful synthetic observables; this will likely require multidimensional radiation transport simulations.

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arXiv2024

Including a Luminous Central Remnant in Radiative Transfer Simulations for Type Iax Supernovae

Type Iax supernovae (SNe Iax) are proposed to arise from deflagrations of Chandrasekhar mass white dwarfs (WDs). Previous deflagration simulations have achieved good agreement with the light curves and spectra of intermediate-luminosity and bright SNe Iax. However, the model light curves decline too quickly after peak, particularly in red optical and near-infrared (NIR) bands. Deflagration models with a variety of ignition configurations do not fully unbind the WD, leaving a remnant polluted with $^{56}\mathrm{Ni}$. Emission from such a remnant may contribute to the luminosity of SNe Iax. Here we investigate the impact of adding a central energy source, assuming instantaneous powering by $^{56}\mathrm{Ni}$ decay in the remnant, in radiative transfer calculations of deflagration models. Including the remnant contribution improves agreement with the light curves of SNe Iax, particularly due to the slower post-maximum decline of the models. Spectroscopic agreement is also improved, with intermediate-luminosity and faint models showing greatest improvement. We adopt the full remnant $^{56}\mathrm{Ni}$ mass predicted for bright models, but good agreement with intermediate-luminosity and faint SNe Iax is only possible for remnant $^{56}\mathrm{Ni}$ masses significantly lower than those predicted. This may indicate that some of the $^{56}\mathrm{Ni}$ decay energy in the remnant does not contribute to the radiative luminosity but instead drives mass ejection, or that escape of energy from the remnant is significantly delayed. Future work should investigate the structure of remnants predicted by deflagration models and the potential roles of winds and delayed energy escape, as well as extend radiative transfer simulations to late times.

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arXiv2023

Going from 3D to 1D: A one-dimensional approach to common-envelope evolution

The common-envelope (CE) phase is a crucial stage in binary star evolution because the orbital separation can shrink drastically while ejecting the envelope of a giant star. Three-dimensional (3D) hydrodynamic simulations of CE evolution are indispensable to learning about the mechanisms that play a role during the CE phase. While these simulations offer great insight, they are computationally expensive. We propose a one-dimensional (1D) model to simulate the CE phase within the stellar evolution code $\texttt{MESA}$ by using a parametric drag force prescription for dynamical drag and adding the released orbital energy as heat into the envelope. We compute CE events of a $0.97\,\mathrm{M}_\odot$ asymptotic giant-branch star and a point mass companion with mass ratios of 0.25, 0.50, and 0.75, and compare them to 3D simulations of the same setup. The 1D CE model contains two free parameters, which we demonstrate are both needed to fit the spiral-in behavior and the fraction of ejected envelope mass of the 1D method to the 3D simulations. For mass ratios of 0.25 and 0.50, we find good-fitting 1D simulations, while for a mass ratio of 0.75, we do not find a satisfactory fit to the 3D simulation as some of the assumptions in the 1D method are no longer valid. In all our simulations, we find that the released recombination energy is important to accelerate the envelope and drive the ejection.

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arXiv2023

Faint calcium-rich transient from the double-detonation of a $0.6\,M_\odot$ carbon-oxygen white dwarf star

We have computed a three-dimensional hydrodynamic simulation of the merger between a massive ($0.4\,M_\odot$) helium white dwarf (He WD) and a low-mass ($0.6\,M_\odot$) carbon-oxygen white dwarf (CO WD). Despite the low mass of the primary, the merger triggers a thermonuclear explosion as a result of a double detonation, producing a faint transient and leaving no remnant behind. This type of event could also take place during common-envelope mergers whenever the companion is a CO WD and the core of the giant star has a sufficiently large He mass. The spectra show strong Ca lines throughout the first few weeks after the explosion. The explosion only yields $<0.01\,M_\odot$ of $^{56}$Ni, resulting in a low-luminosity SN Ia-like lightcurve that resembles the Ca-rich transients within this broad class of objects, with a peak magnitude of $M_\mathrm{bol} \approx -15.7\,$mag and a rather slow decline rate of $Δm_{15}^\mathrm{bol}\approx 1.5\,$mag. Both, its lightcurve-shape and spectral appearance, resemble the appearance of Ca-rich transients, suggesting such mergers as a possible progenitor scenario for this class of events.

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arXiv2023

Scalable stellar evolution forecasting: Deep learning emulation vs. hierarchical nearest neighbor interpolation

Many astrophysical applications require efficient yet reliable forecasts of stellar evolution tracks. One example is population synthesis, which generates forward predictions of models for comparison with observations. The majority of state-of-the-art rapid population synthesis methods are based on analytic fitting formulae to stellar evolution tracks that are computationally cheap to sample statistically over a continuous parameter range. The computational costs of running detailed stellar evolution codes, such as MESA, over wide and densely sampled parameter grids are prohibitive, while stellar-age based interpolation in-between sparsely sampled grid points leads to intolerably large systematic prediction errors. In this work, we provide two solutions for automated interpolation methods that offer satisfactory trade-off points between cost-efficiency and accuracy. We construct a timescale-adapted evolutionary coordinate and use it in a two-step interpolation scheme that traces the evolution of stars from ZAMS all the way to the end of core helium burning while covering a mass range from ${0.65}$ to $300 \, \mathrm{M_\odot}$. The feedforward neural network regression model (first solution) that we train to predict stellar surface variables can make millions of predictions, sufficiently accurate over the entire parameter space, within tens of seconds on a 4-core CPU. The hierarchical nearest-neighbor interpolation algorithm (second solution) that we hard-code to the same end achieves even higher predictive accuracy, the same algorithm remains applicable to all stellar variables evolved over time, but it is two orders of magnitude slower. Our methodological framework is demonstrated to work on the MIST (Choi et al. 2016) data set. Finally, we discuss the prospective applications of these methods and provide guidelines for generalizing them to higher dimensional parameter spaces.

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