arXiv subjects

V. Hocdé

Publications and source records attributed to V. Hocdé.

At least 19 recordsLinked to original sources

arXiv2026

GIARPS simultaneous optical and infrared high-spectral-resolution observations of Cepheids I. Limb darkening from cross-correlated radial velocities

The Baade-Wesselink method, while theoretically applicable for calibrating the distance scale via Cepheids, is not currently employed in the cosmic distance ladder. There are various versions of this method. In all these approaches, the projection factor used to convert the radial into the pulsation velocity remains the limiting quantity. By comparing for the first time simultaneous cross-correlated optical and H-band radial velocities of five Cepheids, we aim to better understand the geometric part of the projection factor that is directly linked to the limb darkening of the stars. The five Cepheids were observed with the HARPS-N and GIANO-B instruments mounted at the Telescopio Nazionale Galileo in the GIARPS configuration. A two-parameter linear fit was applied to the simultaneous radial velocity measurements of each star in order to extract the amplitude ratio of both curves (fVH) as well as the difference in mean velocities. We find a mean value of fVH of 1.056 with a root mean square over the five stars of 0.016, which means that the V-band radial velocity amplitudes are around 5% larger than the ones obtained in the H band. In addition, we find that the H-band radial velocity curves have an average (i.e., a gamma-velocity) of about 1 km/s blueshifted compared to the visible ones, except for one star, X Cyg, which has a blueshift of 0.4 km/s. At first order, the fVH values are directly linked to the ratio of limb darkening coefficients in both bands (or equivalently the ratio of geometrical projection factors). The fVH values are found to be consistent with expectations from static stellar atmosphere models. This work shows that the differential limb darkening of Cepheids in visible and H bands, and consequently the geometrical part of the projection factor, follows what is expected from stellar atmosphere models.

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arXiv2026

Multi-chromatic observations of classical Cepheids using the CHARA Array interferometer: Surface brightness-colour relation, projection factor, and limb-darkening

The Baade-Wesselink (BW) method compares the linear and angular variations of Cepheids to derive their distance. This method is limited, however, by the projection factor, which relates the observed radial velocity to the true pulsation velocity of the star. Using simultaneous observations from the CHARA Array interferometer in the K, H, and R bands with the MYSTIC, MIRC-X, and SPICA combiners, respectively, we aim to understand the physics of Cepheid atmospheres better. Applying a specific method to multi-chromatic simultaneous interferometric observations of Cepheids, we derived robust limb-darkened angular diameters that were then used to calibrate the SBCR and study the projection factor. We also developed a strategy to measure the limb-darkening of Cepheids in R, H, and K bands. These measurements were then used to constrain the geometrical component of the projection factor. From the limb-darkened angular diameter curves of three Cepheids, we decreased the scatter of the SBCR in $V-K$ colour to 0.0011 magnitude and to 0.0040 for the SBCR in $G_{BP}-G_{RP}$. These SBCRs are particularly robust because they are based on multi-chromatic diameters and are homogeneous, which previous calibrations for Cepheids were not. For the very first time, we derived the limb-darkening of Cepheids in R, H, and K bands. For $δ$ Cep, we derived an R-based projection factor of $1.275 \pm 0.051$, and the geometrical part obtained from the measured limb-darkening coefficient in R band was estimated to $1.420 \pm 0.016$ (on average), as expected from stellar static and hydrodynamical atmosphere models. The limb-darkening coefficients obtained in H and K band are consistent with models. These results demonstrate that multi-chromatic interferometry can improve the accuracy of the BW method. It is therefore essential to continue the CHARA survey of Cepheids in the coming years.

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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

Cepheids with giant companions III. Evolutionary modeling of nine binary double Cepheids from the Milky Way and Magellanic Clouds

Binary double (BIND) Cepheids are systems comprising two Cepheid components. This feature provides important constraints that allow us to reveal the origin of Cepheids, trace their evolution, and test pulsation theory. Ten BIND Cepheids are now known, with only one having its parameters determined. We aim to estimate the physical parameters of the components of nine BIND Cepheids in the Magellanic Clouds and the Milky Way, investigate their evolutionary configurations, and formation scenarios. We also expand the parameter space of characterized individual Cepheids in mass, radius, period, and metallicity. We extended the recently introduced $q$-PED method to BIND Cepheids, combining observational constraints with theoretical pulsation and evolutionary models. We considered all consistent configurations (first-crossing, blue-loop, and mixed) as viable solutions. Probabilistic and observational constraints, including spectroscopic mass ratios for two systems, were then used to discriminate between them. We obtained new $q$-PED estimates of mass, radius, temperature, luminosity, and age for 18 Cepheids with previously unknown physical parameters. For one Galactic system, the spectroscopic mass ratio $q_s=0.84\pm0.04$ indicates a first-crossing plus a blue-loop Cepheid solution. This mass ratio, along with the predicted mass ratios lower than unity for two other systems, suggests past binary interactions and a likely merger origin for one component. We derive a new period--mass--radius relation and mass--luminosity relation covering the mass range $2.3-4.6$ M$_\odot$. This work provides the first mass estimates for Cepheids in the SMC, extending the lower Cepheid mass limit down to 2.3 M$_\odot$. Binary interactions in the past evolution of Cepheids may be common, affecting up to 40\% of our systems with two clear cases and two more if blue loop Cepheids are preferred.

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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

High-contrast imaging of Galactic Cepheids with VLT/SPHERE

Cepheids are key distance indicators and benchmarks for stellar evolution, yet most of them are members of binary or multiple systems. While spectroscopic surveys and Gaia proper-motion anomalies reveal a high binary fraction, the population of resolved companions remains poorly characterised. We aim to search for and characterise visual companions to bright Galactic Cepheids using high-contrast imaging and to derive quantitative limits on undetected companions to constrain the architecture of Cepheid multiple systems. We observed 47 Cepheids with SPHERE using the ZIMPOL instrument in classical imaging mode and the V, R, and I filters. The data were obtained in pupil-stabilised mode and analysed using PCA-based imaging technique. For detected companions, we injected negative fake companions in a Monte Carlo approach to measure the relative astrometry. For non-detections, synthetic companions were injected to compute 5sigma contrast curves as a function of separation. We detected companions with a signal-to-noise ratio of > 5 for 8 Cepheids, corresponding to about 17% of the sample. Our SPHERE imaging confirms previously known visual companions with improved astrometry and reveals new wide components for AP Pup, T Vel, and TX Del) at projected separations of 0.16-0.9". For the remaining Cepheids, we derived typical maximum contrasts of 10, 11, and 12mag at 0.25", 0.5", and > 1, respectively. For a sub-set of targets, these limits ruled out main sequence companions more massive than late-K dwarfs beyond 0.5". Our SPHERE survey provides the first homogeneous set of high-contrast optical constraints on wide companions of Galactic Cepheids. The low detection rate of visual companions compared to the high overall binary fraction implies that most companions inferred from radial velocities and Gaia astrometry are either closer than 20mas or significantly fainter than the limits reached here.

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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

Reconstruction of Cepheid Radial Velocity Curves from the shape of the V-band Light Curves

This paper aims to develop the first method to reconstruct the shape of the RV curves of short-period fundamental-mode Cepheids, based exclusively on their pulsation period and the morphology of their $V$-band light curves (LCs). We compiled a dataset of high-quality spectroscopic and photometric measurements from the literature for 81 short-period fundamental-mode Galactic Cepheids up to a pulsation period of 8\,days, enabling precise determination of the Fourier parameters and their uncertainties. We investigated correlations between LC and RV Fourier parameters and used these relations to reconstruct the RV curves. We further assessed the accuracy of these reconstructions by examining potential metallicity effects with an additional dataset of 23 metal-poor Cepheids. For pulsation periods between 3.5 and 7.0\,days, we found tight correlations between different combinations of LC and RV Fourier parameters up to order 7, in particular $R_{21}(RV)/R_{21}(LC)$ and $R_{31}(RV)/R_{31}(LC)$ are correlated with the pulsation period. These relationships enable the reconstruction of RV curves of Cepheids with their LC. The reconstructed curve has an uncertainty of about 0.60${\rm km\,s}^{-1}$ relative to the Fourier fit of true spectroscopic RV measurements. For individual Cepheids, the reconstructed RV curves integrated along the pulsation cycle (i.e. the linear radius variations) are accurate to less than 1\% and precise to within 4.16\% in comparison to the integrated true spectroscopic RV curves. This approach provides a valuable tool for the reconstruction of RV curves for extragalactic Cepheids through photometric data alone. It opens the road to a purely photometric parallax-of-pulsation method in the context of photometric surveys, such as the Vera Rubin Telescope.

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arXiv2026

Surface brightness-colour relations of Milky Way and Magellanic Clouds classical Cepheids based on Gaia magnitudes

Aims: We derive SBCRs for classical Cepheids in the Milky Way and in the Magellanic Clouds using the photometric data available in the Gaia database, and we quantify the metallicity effect. Methods: We first selected the data on the basis of a number of quality criteria and chose the best photometric data and the best parallaxes available in Gaia for Milky Way classical Cepheids. Secondly, we compiled an extensive list of period-radius (PR) relations available in the literature, and we also provide a new PR relation based on interferometric data in our previous work. Thirdly, combining the radius of classical Cepheids with distance estimates (based on Gaia parallaxes for the Milky Way and on eclipsing binaries for the Magellanic Clouds), we derived the surface brightness and colour of about 1700 classical Cepheids. Results: We first derived a new PR relation based on interferometric data and distances from the literature of seven classical Cepheids: $\mathrm{\log(R/R_{\odot}) = 1.133_{\pm 0.019} + 0.688_{\pm 0.016} log(P)}$. The metallicity does not affect the PR relations. Secondly, we calculated three different SBCRs for the Milky Way and Large and Small Magellanic Cloud classical Cepheids based on this new PR relation that clearly show the dependence of the metallicity on the SBCR based on Gaia magnitudes alone. Finally, we derived relations between the slopes, the zero points (ZP), and the metallicity ([Fe/H]) of these three SBCRs: $\mathrm{Slope_{SBCR}=-0.0663_{\pm 0.0121} [Fe/H] - 0.3010_{\pm 0.0030}}$ and $\mathrm{ZP_{SBCR}=-0.1016_{\pm 0.0091} [Fe/H] + 3.9988_{\pm 0.0029}}$. Conclusions: These new SBCRs, dedicated to classical Cepheids in the Milky Way and Magellanic Clouds, are of particular importance to apply the inverse Baade-Wesselink method to classical Cepheids observed by Gaia in a forthcoming study.

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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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arXiv2026

Unlocking the dynamics of Young Stellar Objects: Time-Domain Interferometry with six 4-m class telescopes

The dynamics of the inner regions of young stellar objects (YSOs) is driven by a variety of physical phenomena, from magnetospheres and accretion to the dust sublimation rim and inner disk flows. These inner environments evolve on timescales of hours to days, exactly when bursts, dips, and rapid structural changes carry the most valuable information about star and planet formations, but remain hardly reachable with current facilities. A better reactive infrastructure with six or more telescopes, combined with alerts from large time-domain surveys (e.g., at the era of LSST/Rubin type facilities), and equipped with instruments spanning from the V-band to the thermal infrared (N), would provide the instantaneous uv-coverage and spectral diagnostics needed to unambiguously interpret and image these events as they happen. Such a world's first time-domain interferometric observatory would enable qualitatively new science: directly linking optical and infrared variability to spatially resolved changes in magnetospheric accretion, inner-disk geometry, and dust and gas dynamics in the innermost astronomical unit. Crucially, connecting these processes to outer-scale unresolved information from JWST, ALMA, and the ELT would yield a complete tomography of the planet-forming region.

astro-ph.IM↗

arXiv2025

Surface brightness-colour relations of Cepheids calibrated by optical interferometry

Surface brightness-colour relations (SBCRs) are widely used to determine the angular diameters of stars. They are in particular used in the Baade-Wesselink (BW) method of distance determination of Cepheids. However, the impact of the SBCR on the BW distance of Cepheids is about 8%, depending on the choice of SBCR considered in the literature. We aim to calibrate a precise SBCR dedicated to Cepheids using the best quality interferometric measurements available as well as different photometric bands, including the Gaia bands. We selected interferometric and photometric data in the literature for seven Cepheids covering different pulsation periods. From the phased photometry in the different bands (VJHKG$\mathrm{G_{BP}G_{RP}}$) corrected from extinction and the interferometric limb-darkened angular diameters, we calculated the SBCR associated with each combination of colours. We first find that the seven Cepheids have consistent SBCRs as long as the two magnitudes considered are not too close in wavelengths. For the SBCR ($\mathrm{F_{V},V-K}$): $\mathrm{F_{V} = -0.1336_{\pm 0.0009}(V-K)_{0}+3.9572_{\pm 0.0015}}$, we obtain a root mean square (RMS) of 0.0040 mag, which is three times lower than the latest estimate from 2004. Also, for the first time, we present an SBCR dedicated to Cepheids based on Gaia bands only: $\mathrm{F_{G_{BP}} = -0.3001_{\pm 0.0030}(G_{BP}-G_{RP})_{0}+3.9977_{\pm 0.0029}}$, with an excellent RMS of 0.0061 mag. However, using theoretical models, we show that this SBCR is highly sensitive to metallicity. From this empirical multi-wavelength approach, we also show that the impact of the CircumStellar Environment (CSE) of Cepheids emission is not negligible and should be taken into account in the future.

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arXiv2025

Discovery of ionized circumstellar gas emission around the long-period Cepheid $\ell$ Carinae with ALMA

Cepheid circumstellar emissions have previously been detected via both infrared excess and infrared interferometric observations at a few stellar radii. Those studies have shown that these circumstellar emission can be produced by ionized gas, however there is no direct observational evidence to confirm this hypothesis. In this letter we explore the continuum emission and a spectrum of the bright and long-period Cepheid, $\ell$~Car ($P=35.56\,$day) at millimeter-wavelengths in order to detect possible effects of ionized gas emission. We presented ALMA observations of $\ell$ Car in two spectral setups in Band~6 (near 212 and 253\,GHz, respectively) and we compared the measured flux density to one expected for the stellar continuum. We also derived the spectral index and probed the presence of Radio Recombination Lines (RRL). We report statistically significant emission of about 3.5$\,$mJy in the two spectral ranges, which is about 2.5 times the stellar continuum emission. For the first time, we are also able to derive the spectral index of the flux density ($S_ν\propto ν^α$), $α=+1.26\pm$0.44 ($\sim$3$σ$ error), which is characteristic of partially optically thick ionized gas emission. Additionally, we discovered an emission line from a RRL of hydrogen H29$α$ centered on the stellar rest velocity, smaller in spatial extent than about 0\farcs2 ($\lesssim 100\,$AU), with a symmetric profile with a width at half power of 55.3$\pm$7.5\,\kms (1$σ$ error). It confirms the presence of ionized gas emission near $\ell$~Car. The millimeter emission detected from $\ell$ Car can be attributed to ionized gas emission from the Cepheid's chromosphere. Further radio interferometric observations are necessary to confirm the occurrence of these ionized gas envelopes around Cepheids of different pulsation periods.

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arXiv2025

Non-evolutionary effects on period change in Magellanic Cepheids

Classical Cepheids are a cornerstone class of pulsators, fundamental to testing stellar evolution and pulsation theories. Their secular period changes, characterized through $O-C$ (Observed minus Calculated) diagrams, offer valuable insights into their evolution. While evolutionary period changes are well understood from both observational and theoretical perspectives, shorter timescale period changes (on the order of ($\sim$ 10$^{2}$-10$^{4}$ days) - known as non-evolutionary period changes are yet to be systematically explored. In this work, we present a detailed and comprehensive search for non-evolutionary period changes using $O-C$ analysis of Magellanic Cloud (MC) Cepheids, based on 20+ years of OGLE photometry data. Our sample includes both the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) Cepheids, focusing on single radial mode Cepheids (both fundamental (FU) and first overtone (FO) modes). The results are grouped into two phenomena: (a) Cepheids in binary systems (b) Non-linear period changes.

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arXiv2024

Circumstellar emission of Cepheids across the instability strip: Mid-infrared observations with VLTI/MATISSE

The circumstellar envelopes (CSE) of Cepheids are still not well characterized despite their potential impact on distance determination via both the period-luminosity relation and the parallax-of-pulsation method. This paper aims to investigate Galactic Cepheids across the instability strip in the mid-infrared with MATISSE/VLTI in order to constrain the geometry and physical nature (gas and/or dust) of their CSEs. We secured observations of eight Galactic Cepheids from short up to long period of pulsation, with MATISSE/VLTI in $L$, $M$ and $N$-bands. The SED analysis in the mid-IR confirms the absence of dust spectral signature for all the star sample. For each star in $L$, $M$ and $N$-band we observe closure phases which are consistent with centro-symmetric geometry for the different targets. Finally, the visibilities in $L$, $M$ and $N$ bands are in agreement with the expected star angular diameter, although the observations are compatible with the presence of compact CSEs within the uncertainties. We provide 2$\,σ$ upper limits on the CSE flux contribution based on model residuals for several CSE radius, which yield to exclude models simultaneously large and bright ($R_\mathrm{CSE}\approx10\,R_\star$ and $f_\mathrm{CSE}\approx10\%$) for all the stars of the sample. Last, the visibilities in the $N$-band rule out CSE models with significant amount of different type of dust. The MATISSE observations of eight Cepheids with different pulsation period (from 7 up to 38$\,$day) and evolution stage, provide for the first time a comprehensive picture of Cepheids from mid-IR interferometry. We present additional evidences that circumstellar dust emission is negligible or absent around Cepheids for a wide range of stellar parameters in the instability strip. Further interferometric observations in the visible and the near-infrared will be necessary to disentangle the star and the CSE.

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arXiv2024

The orbital parameters of the del Cep inner binary system determined using 2019 HARPS-N spectroscopic data

An inner companion has recently been discovered orbiting the prototype of classical Cepheids, delta Cep, whose orbital parameters are still not fully constrained. We collected new precise radial velocity measurements of delta Cep in 2019 using the HARPS-N spectrograph mounted at the Telescopio Nazionale Galileo. Using these radial velocity measurements, we aimed to improve the orbital parameters of the system. We considered a template available in the literature as a reference for the radial velocity curve of the pulsation of the star. We then calculated the residuals between our global dataset (composed of the new 2019 observations plus data from the literature) and the template as a function of the pulsation phase and the barycentric Julian date. This provides the orbital velocity of the Cepheid component. Using a Bayesian tool, we derived the orbital parameters of the system. Considering priors based on already published Gaia constraints, we find for the orbital period a maximum a posteriori probability of Porb=9.32+/-0.03 years (uncertainties correspond to the 95% highest density probability interval), and we obtain an eccentricity e=0.71+/-0.02, a semimajor axis a=0.029 +/-0.003 arcsecond, and a center-of-mass velocity V0=-17.28+/-0.08 km/s, among other parameters. In this short analysis we derive the orbital parameters of the delta Cep inner binary system and provide a cleaned radial velocity curve of the pulsation of the star, which will be used to study its Baade-Wesselink projection factor in a future publication.

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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.

astro-ph.SR↗

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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