arXiv subjects

Wafa Gull

Publications and source records attributed to Wafa Gull.

7 recordsLinked to original sources

arXiv2026

Fourier Morphology Diagnostics of Delta Cephei MESA-RSP Light Curves

Fourier morphology provides a compact test of whether a nonlinear Cepheid model reproduces the observed harmonic content and asymmetric light-curve shape, not only the pulsation period. We apply Fourier and time-domain morphology diagnostics to observed and synthetic Delta Cephei light curves. The observed Johnson V-band light curve is represented by a Fourier template and compared with MIST bolometric-correction V-band light curves generated from the calibrated MESA-RSP model sequence. This study is framed as a target-specific morphology assessment rather than as a new Fourier-decomposition method. The observed template has Delta V = 0.8390 mag, R21 = 0.3401, R31 = 0.1943, rise fraction = 0.2775, and asymmetry index = 0.4450. The final accepted model, with RSP_alfam = 0.400 and RSP_alfat = 0.095, gives Delta V_syn = 0.0367 mag, R21 = 0.0185, R31 = 0.0028, rise fraction = 0.4860, and asymmetry index = 0.0280. The accepted calibration sequence therefore improves the synthetic amplitude only modestly and leaves the synthetic curve much too sinusoidal. The result shows that the accepted sequence is period-calibrated and morphology-tested, but it is not yet an observed-amplitude solution for Delta Cephei.

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arXiv2026

Evolutionary Period-Change Modeling of Delta Cephei with MESA Tracks

Cepheid period changes provide a direct evolutionary diagnostic because the pulsation period responds to changes in stellar radius as a star crosses the instability strip. We present a controlled MESA evolutionary-track analysis for Delta Cephei using nonrotating, no-wind models. The period is estimated from the period-mean-density relation, P = Q [(R/Rsun)^3/(M/Msun)]^(1/2), with Q = 0.033 d. The adopted comparison values are P = 5.366531 d and dP/dt = -0.1006 s yr^-1. A solar-metallicity mass and overshoot grid identifies clean blueward solutions with negative dP/dt near the observed period, but the best such case has dP/dt = -0.6813 s yr^-1, a factor of 6.77 too large in magnitude. A metallicity pilot grid improves the result substantially, with a best clean-blueward model at M = 5.90 Msun, Z = 0.012, and fov,core = 0.010, giving P = 5.370677 d and dP/dt = -0.2460 s yr^-1. A local refinement around this solution does not improve beyond the same model. Thus, metallicity refinement reduces the period-change mismatch by a factor of about 2.77 relative to the solar-metallicity baseline, but the final nonrotating, no-wind, fixed-Q model still overpredicts the observed magnitude of dP/dt by a factor of about 2.45. The result supports a blueward evolutionary interpretation while identifying rotation, mass loss, binary-related effects, and structure-dependent pulsation periods as the natural scope of a follow-up study.

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arXiv2026

Nonlinear MESA-RSP Modeling of Delta Cephei: Period Matching, Amplitude Control, and Model-Acceptance Diagnostics

Delta Cephei is the prototype classical Cepheid and a useful target for testing whether a one-dimensional nonlinear radial-pulsation workflow can satisfy more than a period constraint. We present a target-specific, reproducible workflow using the Modules for Experiments in Stellar Astrophysics (MESA) Radial Stellar Pulsation (RSP) capability, hereafter MESA-RSP, to construct, tune, extend, and classify nonlinear radial-pulsation calculations for Delta Cephei. A tuned 200-period calculation with M = 5.0 solar masses, Teff = 6050 K, L = 2422.5 solar luminosities, X = 0.73, and Z = 0.007 gives P_model = 5.366622 d for the adopted target P_obs = 5.366531 d, a difference of 9.1 x 10^-5 d, or approximately 7.9 s. This agreement shows that the mean-density structure can be tuned successfully, but extended calculations demonstrate that period matching alone is not an adequate nonlinear model-acceptance criterion. When the period-matched model was continued with the default/weak-damping setup, the amplitude and cycle behavior became nonstationary. We therefore explored the MESA-RSP eddy-viscous damping parameter RSP_alfam as a first amplitude-control experiment. The RSP_alfam = 0.60 case is retained as a period-stable reference, while the RSP_alfam = 0.425, 500-period calculation is classified as the clean amplitude-enhanced candidate. A 700-period continuation is retained only as a diagnostic case because it produced an energy-error warning. The main contribution is a reproducible Delta Cephei workflow and a transparent classification scheme showing that acceptable nonlinear Cepheid models require period agreement, interpretable late-cycle amplitude behavior, controlled surface-velocity diagnostics, and the absence of serious numerical warnings.

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arXiv2026

Synthetic Observed-Band Light Curves of Delta Cephei from MESA-RSP Models

Observed-band light curves provide a stronger test of Cepheid pulsation models than period matching alone, because the measured photometric amplitude depends on the phase-dependent luminosity, temperature, radius, and passband transformation. We construct synthetic observed-band light curves for delta Cephei from nonlinear radial pulsation models computed with MESA-RSP and transform the model outputs L(phi), Teff(phi), and R(phi) into bolometric magnitudes and MIST bolometric-correction magnitudes in Bessell and Gaia passbands. The adopted AAVSO Johnson V-band template has peak-to-peak amplitude Delta V_obs = 0.8390 mag. The period-stable reference model with RSP alfam = 0.60 gives a MIST-BC V-band amplitude of only Delta V_syn approximately 0.0106 mag. Amplitude-enhanced models increase the synthetic amplitude, with RSP alfam = 0.425 giving 0.0302 mag, RSP alfam = 0.400 giving 0.0344 mag, and the accepted RSP alfam = 0.400, RSP alfat = 0.095 model giving 0.0367 mag. The final accepted model therefore reaches only 4.4 percent of the observed Johnson V-band amplitude. These results show that MIST-BC transformations are necessary for a physically meaningful observed-band comparison, but they do not remove the amplitude discrepancy in the present model sequence. The remaining mismatch indicates that the dominant limitation is the small nonlinear pulsation amplitude of the models rather than the bolometric-correction transformation itself.

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arXiv2026

Native-Opacity Sensitivity of a Fixed Delta Cephei MESA-RSP Pulsation Model

Radiative opacity is one of the central microphysical inputs controlling the thermal response of Cepheid envelopes and the driving or damping of radial pulsations. We present a controlled opacity-sensitivity experiment for a fixed delta Cephei nonlinear radial pulsation model computed with the MESA Radial Stellar Pulsation module. The stellar and pulsation parameters are held fixed at M = 5.0 solar masses , Teff = 6050 K, L = 2360 solar luminosities , X = 0.73, Z = 0.007, and RSP_alfam = 0.425, while the high-temperature opacity source is varied among native MESA opacity configurations: OPAL-A09, OP-A09, and OPLIB-AGSS09. The low-temperature opacity prefix, C/O-dependent opacity prefix, and all other RSP parameters are kept fixed so that the comparison isolates the effect of the adopted high-temperature opacity table. Verification integrations were performed at 20, 100, and 300 pulsation cycles, followed by photo-restarted continuations to 500 cycles. At 500 cycles, OPAL-A09 gives the closest period agreement, PRSP = 5.366986 d, only about 39 s longer than Pobs = 5.366531 d. OP-A09 gives the largest amplitude-growth diagnostics, with Delta Mag = 0.037307 and Delta R = 0.293677, corresponding to increases of 42.5% and 43.9% relative to OPAL-A09. OPLIB-AGSS09 gives a systematically longer period, P_RSP = 5.403926 d, with more modest amplitude-growth changes. The same ordering is reflected in the MESA history-column diagnostic rsp_GREKM, defined by the MESA defaults as the fractional growth of kinetic energy per pulsation period. These results show that native opacity choice measurably affects period matching, pulsation growth diagnostics, and nonlinear amplitude growth in this fixed {\delta} Cephei model. However, the tested opacity choices do not by themselves resolve the known observed-amplitude discrepancy.

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arXiv2026

A Reproducible AAVSO Johnson-V Fourier Template for the Prototype Cepheid Delta Cephei

We present an empirical Fourier reconstruction of the observed Johnson-V light curve of the prototype Classical Cepheid Delta Cephei. The goal is not to infer a full physical stellar model but to establish a reproducible observed-light-curve benchmark for later comparison with nonlinear pulsation, synthetic photometry, Baade-Wesselink/SPIPS, GYRE-supported, and independent hydrodynamic calculations. Using an adopted period of Pobs = 5.366531 d, 244 AAVSO Johnson-V measurements were filtered to a cleaned sample of 242 points after rejecting two extreme outliers. The cleaned data span 355.09259 d and were phase folded using an empirical bright-maximum epoch of JD = 2460851.395800. We fit a low-order Fourier model to the phased light curve and adopt a third-order template as the preferred empirical morphology representation. The adopted N = 3 fit gives A0 = 3.9031, A1 = 0.3434 mag, A2 = 0.1428 mag, and A3 = 0.0531 mag, corresponding to R21 = 0.4159 and R31 = 0.1547. The template has a full Johnson-V amplitude of Delta V = 0.8544 mag, a rise fraction of frise = 0.2885, and an asymmetry index of Aasym = 0.4230. Bootstrap uncertainties are reported in the manuscript. Fourier orders N = 4-6 reduce the RMS residual by only about 0.0012 mag relative to the N = 3 model, so the third-order representation is retained as the simplest adequate empirical template. Observer-level residual diagnostics show that the remaining scatter is not purely random. This paper provides an observational morphology target rather than a physical explanation of the pulsation.

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arXiv2026

Semi-Empirical Pulsation Reconstruction of Delta Cephei with Photometry, Radial Velocities, and Temperature Constraints

We present a semi-empirical reconstruction of the pulsation behavior of the classical Cepheid delta Cephei using observed Johnson-V photometry, HARPS-N radial velocities, and published SPIPS pulsation-model curves. A cleaned AAVSO Johnson-V data set was phase folded with P = 5.366531 d, corrected for observer zero-point offsets, clipped for residual outliers, and fitted with a three-harmonic Fourier template. The resulting empirical template has Delta V about 0.833 mag, R21 about 0.382, R31 about 0.168, and a rise fraction about 0.287. After phase and vertical alignment, the published SPIPS V-band curve reproduces the empirical Johnson-V morphology with an RMS residual about 0.023 mag. A Fourier representation of the HARPS-N radial velocities gives a peak-to-peak velocity span about 40.82 km s^-1, corresponding to a preliminary radius-displacement amplitude about 4.81 solar radii for p = 1.317. Combining this radius curve with the SPIPS Teff(phi) curve yields a hybrid luminosity curve close to the published SPIPS luminosity curve. The adopted R0 = 43.7 solar radii scale gives a mean luminosity ratio L_hybrid/L_SPIPS about 1.04, while R0 = 44.9 solar radii gives about 1.10. A direct radius-scale comparison also favors the lower adopted radius scale, with an RMS offset of 0.864 solar radii relative to the SPIPS-implied radius, compared with 2.060 solar radii for the larger-radius case. These results show that the observed visual morphology, radial displacement, and temperature-driven luminosity variation are mutually consistent when placed on a common phase convention, while the absolute radius scale remains the dominant systematic.

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