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Register a new userThe Araucaria Project: High-precision Cepheid astrophysics from the analysis of variables in double-lined eclipsing binaries
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Added by
Bogumi{\\l} Pilecki
Publication date
2018
fields
Physics
and research's language is
English
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Based on new observations and improved modeling techniques, we have reanalyzed seven Cepheids in the Large Magellanic Cloud. Improved physical parameters have been determined for the exotic system OGLE LMC-CEP-1718 composed of two first-overtone Cepheids and a completely new model was obtained for the OGLE LMC-CEP-1812 classical Cepheid. This is now the shortest period Cepheid for which the projection factor is measured. The typical accuracy of our dynamical masses and radii determinations is 1%. The radii of the six classical Cepheids follow period--radius relations in the literature. Our very accurate physical parameter measurements allow us to calculate a purely empirical, tight period--mass--radius relation that agrees well with theoretical relations derived from non-canonical models. This empirical relation is a powerful tool to calculate accurate masses for single Cepheids for which precise radii can be obtained from Baade--Wesselink-type analyses. The mass of the type-II Cepheid $kappa$ Pav, $0.56 pm 0.08 M_odot$, determined using this relation is in a very good agreement with theoretical predictions. We find large differences between the p-factor values derived for the Cepheids in our sample. Evidence is presented that a simple period--p-factor relation shows an intrinsic dispersion, hinting at the relevance of other parameters, such as the masses, radii, and radial velocity variation amplitudes. We also find evidence that the systematic blueshift exhibited by Cepheids, is primarily correlated with their gravity. The companion star of the Cepheid in the OGLE LMC-CEP-4506 system has a very similar temperature and luminosity, and is clearly located inside the Cepheid instability strip, yet it is not pulsating.
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We present here the first spectroscopic and photometric analysis of the double-lined eclipsing binary containing the classical, first-overtone Cepheid OGLE-LMC-CEP-2532 (MACHO 81.8997.87). The system has an orbital period of 800 days and the Cepheid is pulsating with a period of 2.035 days. Using spectroscopic data from three high-class telescopes and photometry from three surveys spanning 7500 days we are able to derive the dynamical masses for both stars with an accuracy better than 3%. This makes the Cepheid in this system one of a few classical Cepheids with an accurate dynamical mass determination (M_1=3.90 +/- 0.10 M_sun). The companion is probably slightly less massive (3.82 +/- 0.10 M_sun), but may have the same mass within errors (M_2/M_1= 0.981 +/- 0.015). The system has an age of about 185 million years and the Cepheid is in a more advanced evolutionary stage. For the first time precise parameters are derived for both stars in this system. Due to the lack of the secondary eclipse for many years not much was known about the Cepheids companion. In our analysis we used extra information from the pulsations and the orbital solution from the radial velocity curve. The best model predicts a grazing secondary eclipse shallower than 1 mmag, hence undetectable in the data, about 370 days after the primary eclipse. The dynamical mass obtained here is the most accurate known for a first-overtone Cepheid and will contribute to the solution of the Cepheid mass discrepancy problem.
Following the earlier discovery of classical Cepheid variables in the Sculptor Group spiral galaxy NGC 7793 from an optical wide-field imaging survey, we have performed deep near-infrared $J$- and $K$-band follow-up photometry of a subsample of these Cepheids to derive the distance to this galaxy with a higher accuracy than what was possible from optical photometry alone, by minimizing the effects of reddening and metallicity on the distance result. Combining our new near-infrared period-luminosity relations with the previous optical photometry we obtain a true distance modulus to NGC 7793 of $(27.66 pm 0.04)$ mag (statistical) $pm 0.07$ mag (systematic), i.e. a distance of $(3.40 pm 0.17)$ Mpc. We also determine the mean reddening affecting the Cepheids to be $E(B-V)=(0.08 pm 0.02)$ mag, demonstrating that there is significant dust extinction intrinsic to the galaxy in addition to the small foreground extinction. A comparison of the new, improved Cepheid distance to earlier distance determinations of NGC 7793 from the Tully-Fisher and TRGB methods yields agreement within the reported uncertainties of these previous measurements.
BVR light curves and radial velocities for the double-lined eclipsing binary V1135,Her were obtained. The brighter component of V1135,Her is a Cepheid variable with a pulsation period of 4.22433$pm$0.00026 days. The orbital period of the system is about 39.99782$pm$0.00233 days, which is the shortest value among the known Type,II Cepheid binaries. The observed B, V, and R magnitudes were cleaned for the intrinsic variations of the primary star. The remaining light curves, consisting of eclipses and proximity effects, are obtained. Our analyses of the multi-colour light curves and radial velocities led to the determination of fundamental stellar properties of both components of the interesting system V1135,Her. The system consists of two evolved stars, G1+K3 between giants and supergiants, with masses of M$_1$=1.461$pm$0.054 Msun ~and M$_2$=0.504$pm$0.040 {Msun} and radii of R$_1$=27.1$pm$0.4 {Rsun} and R$_2$=10.4$pm$0.2 {Rsun}. The pulsating star is almost filling its corresponding Roche lobe which indicates the possibility of mass loss or transfer having taken place. We find an average distance of d=7500$pm$450 pc using the BVR magnitudes and also the V-band extinction. Location in the Galaxy and the distance to the galactic plane with an amount of 1300 pc indicate that it probably belongs to the thick-disk population. Most of the observed and calculated parameters of the V1135,Her and its location on the color-magnitude and period-luminosity diagrams lead to a classification of an Anomalous Cepheid.
We present a method for solving the lightcurve of an eclipsing binary system which contains a Cepheid variable as one of its components as well as the solutions for three eclipsing Cepheids in the Large Magellanic Cloud (LMC). A geometric model is constructed in which the component stars are assumed to be spherical and on circular orbits. The emergent system flux is computed as a function of time, with the intrinsic variations in temperature and radius of the Cepheid treated self-consistently. Fitting the adopted model to photometric observations, incorporating data from multiple bandpasses, yields a single parameter set best describing the system. This method is applied to three eclipsing Cepheid systems from the MACHO Project LMC database: MACHO IDs 6.6454.5, 78.6338.24 and 81.8997.87. A best-fit value is obtained for each systems orbital period and inclination and for the relative radius, color and limb-darkening coefficients of each star. Pulsation periods and parameterizations of the intrinsic color variations of the Cepheids are also obtained and the amplitude of the radial pulsation of each Cepheid is measured directly. The system 6.6454.5 is found to contain a 4.97-day Cepheid, which cannot be definitely classified as Type I or Type II, with an unexpectedly brighter companion. The system 78.6338.24 consists of a 17.7-day, W Vir Class Type II Cepheid with a smaller, dimmer companion. The system 81.8997.87 contains an intermediate-mass, 2.03-day overtone Cepheid with a dimmer, red giant secondary.
Multi-color light curves and radial velocities for TYC,1031,1262,1 have been obtained and analyzed. TYC,1031,1262,1 includes a Cepheid with a period of 4.15270$pm$0.00061 days. The orbital period of the system is about 51.2857$pm$0.0174 days. The pulsation period indicates a secular period increase with an amount of 2.46$pm$0.54 min/yr. The observed B, V, and R magnitudes were cleaned for the intrinsic variations of the primary star. The remaining light curves, consisting of eclipses and proximity effects, are obtained and analyzed for orbital parameters. The system consists of two evolved stars, F8II+G6II, with masses of M$_1$=1.640$pm$0.151 {Msun} and M$_2$=0.934$pm$0.109 {Msun} and radii of R$_1$=26.9$pm$0.9 {Rsun} and R$_2$=15.0$pm$0.7 {Rsun}. The pulsating star is almost filling its corresponding Roche lobe which indicates the possibility of mass loss or transfer having taken place. We find an average distance of d=5070$pm$250,pc using the BVR and JHK magnitudes and also the V-band extinction. Kinematic properties and the distance to the galactic plane with an amount of 970 pc indicate that it belongs to the thick-disk population. Most of the observed and calculated parameters of the TYC,1031,1262,1 lead to a classification of an Anomalous Cepheid.
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