arXiv subjects

R. Singh Rathour

Publications and source records attributed to R. Singh Rathour.

7 recordsLinked to original sources

arXiv2026

Pulsation periods reveal tension between theoretical and empirical radii for classical Cepheids in eclipsing binary systems

Context. With their precisely determined physical parameters, classical Cepheids in eclipsing binary systems are often used to constrain stellar evolution and pulsation theories. To this end, their position in the HR diagram, effective temperature and luminosity, and radius are commonly used when matching best-fitting evolutionary models. However, the pulsation period of Cepheids, the most precise observable, is rarely used in such studies. Aims. We explore how including the pulsation period as a constraint in matching evolutionary models affects the best-fitting solution. As the pulsation period follows the period-mean density relation, we examine whether it provides information consistent with that based on the stellar radius. Methods. We modeled four eclipsing binary systems with Cepheids from the LMC. We used $χ^2$ minimization to find the best-matching evolutionary model from a grid of models computed with MESA. The evolutionary models are supplemented with pulsation periods computed with RSP, with nonlinear period corrections taken into account. Results. Depending on whether the radius or the pulsation period is used to select the best-fitting model, discrepant solutions are obtained. In solutions selected based on the pulsation period, the stellar radius is systematically too low compared with observations. Conversely, for solutions based on the radius, the pulsation period is systematically too long. The tension amounts to a few sigma for stars with precisely determined radii and part of it is traced to a nonlinear increase in radius for large-amplitude pulsators, which has not been studied in detail in the literature. Conclusions. When using Cepheids in eclipsing binary systems to constrain stellar models, we recommend using the pulsation period instead of the radius. A systematic study of nonlinear effects on stellar radius in large-amplitude pulsators is needed.

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arXiv2026

Toward a Comprehensive Grid of Cepheid Models with MESA. IV. Modest Effects of Rotation on Blue Loops

Evolutionary tracks for $2-8M_\odot$ stars, with metallicities of $Z=0.014$, $0.006$, and $0.002$, including rotation, are computed with Modules for Experiments in Stellar Astrophysics (MESA). We study how rotation impacts the evolutionary properties of classical Cepheids. We examine whether rotation can offer a plausible explanation for the mass discrepancy problem when it is included in the evolutionary code using the fully diffusive approximation for rotationally induced mixing processes. We find that rotation barely influences the appearance and luminosity levels of the blue loops. While luminosity increases with increasing initial rotation rate, the increase does not exceed 0.04 dex, a fraction of the increase resulting from including the main sequence (MS) core overshooting of $0.2H_p$. As a consequence, rotation alone cannot resolve the mass discrepancy problem without simultaneously requiring significant MS core overshooting. Similar to the mass-luminosity relation, the period-radius and period-luminosity relations are barely affected by rotation, while the period-age relation predicts Cepheid ages to be only a few per cent longer compared with models without rotation. The predicted surface rotational velocities are too large compared with observations. These results are in contrast with those obtained with the Geneva code, which implements rotational mixing using the advective-diffusive scheme. In that approach, the luminosity levels of the loops are significantly higher, their luminosity extent increases, and the predicted rotation velocities are lower, compared with MESA models. The differences between the two approaches arise from significantly more efficient rotation-induced mixing during the MS evolution in models computed with the advective-diffusive scheme.

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arXiv2026

Toward a Comprehensive Grid of Cepheid Models with MESA. III. Evolutionary and Pulsation Relations for Models with Core and Envelope Overshooting

Evolutionary tracks for 2-8M$_\odot$ models, covering a [Fe/H]=$-$1.0 ($Z=0.0014$) to [Fe/H]=+0.2 ($Z=0.02$) metallicity range are computed with Modules for Experiments in Stellar Astrophysics, MESA, to investigate evolutionary and pulsation properties of classical, fundamental mode Cepheids. We examine in detail the effects of convective overshooting from the Main Sequence core, as well as from the convective envelope on the Red Giant Branch. Mass loss is also included in a few model sets. Linear pulsation properties are derived consistently with a module of MESA, Radial Stellar Pulsation, RSP. We provide edges of the classical Instability Strip, as well as ages, crossing times through the Instability Strip and period change rates. Period-Luminosity, Mass-Luminosity, Period-Radius and Period-Age relations are provided, both in analytical and tabular form. Their dependence on metallicity, crossing number and overshooting parameters are investigated. Qualitative comparisons with classical Cepheids in the Milky Way and Magellanic Clouds as well as other theoretical relations are presented. We find satisfactory agreement for most of the observables and good match with other theoretical work, however reproducing short-period Cepheids in the Small Magellanic Cloud as well as Cepheid mass discrepancy pose a challenge for the presented models. Considering metallicity effect of the Period-Luminosity relation, we find $γ\approx -0.20$ mag dex$^{-1}$, nearly independent on photometric pass band and in good agreement with recent observational studies. The magnitude of this effect depends on the underlying mass-luminosity relation, being stronger for relations that predict higher luminosities at a given mass.

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arXiv2026

Toward a Comprehensive Grid of Cepheid Models with MESA II. Impact of Physical and Numerical Assumptions on Elemental Abundances

Modern tools for modeling stellar evolution, such as MESA (Modules for Experiments in Stellar Astrophysics), offer state-of-the-art implementations of stellar theories. However, this parametric approach introduces many free parameters that are often not constrained by observations. This is particularly important for evolved stars, like classical Cepheids, because uncertainties increase with evolution time. In previous work, we studied the effect of varying microphysics, including solar abundance mixtures, nuclear networks, atmosphere models, mixing-length prescriptions, treatments of convective boundaries, and numerical setup on evolutionary tracks. Here, we extend this analysis to the surface abundances of the dominant elements H, He, C, N, O, Ne, and Mg. We establish a reference model and 22 variants for each mass and metallicity, evolving them from the Zero-Age Main Sequence to central helium exhaustion. Masses between 2 to 8 solar mass and metallicities Z=0.0014, 0.004, 0.014 are explored, spanning the range of classical Cepheids. Both canonical and overshooting models are computed and compared. We find that uncertainties in surface abundances are generally small, arising mainly from variations in the depth of the convective envelope during the first dredge-up. The size of the convective envelope is sensitive to many aspects, including mass and metallicity. The central C/O ratio, relevant for white dwarf evolution, can vary by about 0.15, driven largely by convective boundary treatments or by modifying the 12C(alpha,gamma)16O reaction rate during helium burning. Surface and central abundances for the considered models at several benchmark points during the evolution are provided online.

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arXiv2023

Pulsation modelling of the Cepheid Y Ophiuchi with RSP/MESA. Impact of the circumstellar envelope and a high projection factor on Baade-Wesselink method

Y~Ophiuchi (Y~Oph) is a classical Cepheid reported to be as dim as a Cepheid of about half its pulsation period, and exhibits a low radial velocity and light-curves amplitude. Our objective is to conduct hydrodynamical pulsation modeling of Y~Oph to derive its distance and provide physical insight to its low amplitude and luminosity, constrained by an extensive set of observations. We first perform a linear analysis on a grid of models using hydrodynamical pulsation code \texttt{MESA-RSP} in order to find the combinations of mass, metallicity, effective temperature and luminosity resulting in linear excitation of pulsations with period of about 17$\,$days. Then, for the best combinations of stellar parameters, we perform non-linear computations to obtain the full-amplitude pulsations of these models. Last, we compare the results to a complete set of observations along the pulsation cycle. We adjust simultaneously the distance, the color excess and circumstellar envelope (CSE) model to fit the light curves and the angular diameter. We find that all pulsation models at high effective temperature are in remarkable agreement with the observations along the pulsation cycle. This result suggests that the low amplitude of Y~Oph can be explained by its location close to the blue edge of the instability strip. We also find that a pulsational mass of about 7-8$\,\mathrm{M}_\odot$ is consistent with a non-canonical evolutionary model with moderate overshooting, PL relation and \textit{Gaia} parallax. However, a much lower mass below 5$\,$M$_\odot$ is required to match Baade-Wesselink (BW) distance measurements from the literature. We show that the combination of the impact of the CSE on the photometry together with a projection factor of about 1.5 explains the discrepant distance and luminosity values obtained from BW methods.

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arXiv2023

Precise Fourier parameters of Cepheid Radial Velocity Curves

The primary goal of this paper is to derive precise Fourier parameters of the radial velocity (RV) curves for fundamental and first-overtone Galactic Cepheids. For each star, we carefully selected RV measurements available in the literature which yield the highest precision of Fourier parameters. We performed a Fourier decomposition of the RV curves. We subtracted RV modulations caused by binary motion and have removed other residual trends. Finally, we have displayed and analyzed qualitatively the progressions found for Fourier parameters. We applied a standard identification of their pulsation mode based on their Fourier phase $ϕ_{21}$. Our final sample includes 178 fundamental-mode and 33 first overtone pulsators, as well as 7 additional Cepheids whose pulsation mode is uncertain or undetermined according to our criteria. For the fundamental-mode Cepheids, we improved the precision of Fourier parameters in comparison of previous results from the literature. We are able to firmly identify V495 Cyg as a new first-overtone Cepheid. We confirm first-overtone nature of several other stars. We also show that $α$ UMi should be definitely classified as a first-overtone pulsator. In 3 objects (VY Per, AQ Pup and QZ Nor) we found significant $γ$-velocity variations that we attribute to spectroscopic binarity. Finally, the analysis of the F-mode Fourier parameters up to 7th order reveals tight progression of Fourier phases. For $P<10\,$day we find a well defined upper limit for the Fourier amplitude $A_1$. The pulsation period coverage and the precision obtained, in particular for Fourier phase $ϕ_{21}$, will be useful for studying the dynamics of Cepheid pulsations with the help of hydrodynamical models. Further radial velocity measurements from modern high-resolution spectroscopic instruments will be important to improve these results.

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arXiv2023

Metallicity estimation of MW, SMC and LMC classical Cepheids from the shape of the $V$- and $I$-band light curves

Estimating metallicity of classical Cepheids is of prime importance for studying metallicity effect on stellar evolution, chemical evolution of galaxies, and ultimately its impact on period-luminosity relation used in the extragalactic distance scale. We aim at establishing new empirical relations for estimating the iron content of classical Cepheids for short and long-periods based on Fourier parameters from the $V$-band light curves. We calibrate new interrelations of Fourier parameters to convert $V$-band empirical relations into the $I$-band. Then we apply these relation in $V$ and $I$-bands to Cepheids from Milky Way (MW), Small and Large Magellanic Clouds (SMC and LMC) available in the literature. Last, we map the metallicity distribution in these galaxies for investigating potential application in galactic archeology. These empirical relations in $V$ and $I$ bands are able to derive the mean metallicity of a sample of MW, SMC and LMC Cepheids in agreement with literature values within 1$σ$. We also show that these relations are precise enough to reconstruct the radial metallicity gradients within the MW from OGLE data. The empirical relations in the $V$ and $I$ bands calibrated in this paper for short and long-period Cepheids provide a new useful tool to estimate the metallicity of Cepheids which are not accessible by spectroscopy. The calibration can be improved with further high-resolution spectroscopic observations of metal-poor Cepheids and homogeneous photometries in $V$ and $I$ bands.

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