Absolute dimensions of detached eclipsing binaries. III. The metallic-lined system YZ Cassiopeiae

The bright binary system YZ Cassiopeiae is a remarkable laboratory for studying the Am phenomenon. It consists of a metallic-lined A2 star and an F2 dwarf on a circular orbit, which undergo total and annular eclipses. We present an analysis of 15 published light curves and 42 new high-quality echelle spectra, resulting in measurements of the masses, radii, effective temperatures and photospheric chemical abundances of the two stars. The masses and radii are measured to 0.5% precision: M_A = 2.263 +/- 0.012 Msun, M_B = 1.325 +/- 0.007 Msun, R_A = 2.525 +/- 0.011 Rsun and R_B = 1.331 +/- 0.006 Rsun. We determine the abundance of 20 elements for the primary star, of which all except scandium are super-solar by up to 1 dex. The temperature of this star (9520 +/- 120 K) makes it one of the hottest Am stars. We also measure the abundances of 25 elements for its companion (Teff = 6880 +/- 240 K), finding all to be solar or slightly above solar. The photospheric abundances of the secondary star should be representative of the bulk composition of both stars. Theoretical stellar evolutionary models are unable to match these properties: the masses, radii and temperatures imply a half-solar chemical composition (Z = 0.009 +/- 0.003) and an age of 490-550 Myr. YZ Cas therefore presents a challenge to stellar evolutionary theory.Comment: Accepted for publication in MNRAS. 15 pages, 9 tables, 7 figure

HD 183648: a Kepler eclipsing binary with anomalous ellipsoidal variations and a pulsating component

KIC 8560861 (HD 183648) is a marginally eccentric (e = 0.05) eclipsing binary with an orbital period of P_(orb) = 31.973 d, exhibiting mmag amplitude pulsations on time-scales of a few days. We present the results of the complex analysis of high- and medium-resolution spectroscopic data and Kepler Q0 – Q16 long cadence photometry. The iterative combination of spectral disentangling, atmospheric analysis, radial velocity and eclipse timing variation studies, separation of pulsational features of the light curve, and binary light curve analysis led to the accurate determination of the fundamental stellar parameters. We found that the binary is composed of two main-sequence stars with an age of 0.9 ± 0.2 Gyr, having masses, radii and temperatures of M_1 = 1.93 ± 0.12 M_⊙, R_1 = 3.30 ± 0.07 R_⊙, T_(eff1) = 7650 ± 100 K for the primary, and M_2 = 1.06 ± 0.08 M_⊙, R_2 = 1.11 ± 0.03 R_⊙, T_(eff2) = 6450 ± 100 K for the secondary. After substracting the binary model, we found three independent frequencies, two of which are separated by twice the orbital frequency. We also found an enigmatic half orbital period sinusoidal variation that we attribute to an anomalous ellipsoidal effect. Both of these observations indicate that tidal effects are strongly influencing the luminosity variations of HD 183648. The analysis of the eclipse timing variations revealed both a parabolic trend, and apsidal motion with a period of P^(obs)_(apse) = 10400 ± 3000 y, which is ten times faster than what is theoretically expected. These findings might indicate the presence of a distant, unseen companion

Tracing CNO exposed layers in the Algol-type binary system u Her

The chemical composition of stellar photospheres in mass-transferring binary systems is a precious diagnostic of the nucleosynthesis processes that occur deep within stars, and preserves information on the components’ history. The binary system u Her belongs to a group of hot Algols with both components being B stars. We have isolated the individual spectra of the two components by the technique of spectral disentangling of a new series of 43 high-resolution échelle spectra. Augmenting these with an analysis of the Hipparcos photometry of the system yields revised stellar quantities for the components of u Her. For the primary component (the mass-gaining star), we find MA = 7.88 ± 0.26 M⊙, RA = 4.93 ± 0.15 R⊙ and Teff, A = 21 600 ± 220 K. For the secondary (the mass-losing star) we find MB = 2.79 ± 0.12 M⊙, RB = 4.26 ± 0.06 R⊙ and Teff, B = 12 600 ± 550 K. A non-local thermodynamic equilibrium analysis of the primary star's atmosphere reveals deviations in the abundances of nitrogen and carbon from the standard cosmic abundance pattern in accord with theoretical expectations for CNO nucleosynthesis processing. From a grid of calculated evolutionary models the best match to the observed properties of the stars in u Her enabled tracing the initial properties and history of this binary system. We confirm that it has evolved according to case A mass transfer. A detailed abundance analysis of the primary star gives C/N = 0.9, which supports the evolutionary calculations and indicates strong mixing in the early evolution of the secondary component, which was originally the more massive of the two. The composition of the secondary component would be a further important constraint on the initial properties of u Her system, but requires spectra of a higher signal-to-noise ratio

Constrained Disentangling: Complementary spectroscopic and interferometric solution

The complete set of orbital elements of binary and multiple systems can be only determined from complementary observables. In spectral disentangling the orbital elements are optimised along with simultaneous determination of the individual spectra of components. Therefore, a posteriori combination with some other observables, i.e. astrometric measurements or historic RVs is principally incorrect. With advances in the instrumentation more and more binary and multiple systems are spatially and spectroscopically resolved which makes the solution of orbital elements more firm. Following this advances, we have developed a wrapper with optimisation routine which enables spectral disentangling of time-series of spectra with the constraints from interferometric (astrometric) measurements

Tracing CNO Exposed Layers in the Hot Algol-Type Binary System 68 Her

We present a spectroscopc study of the hot Algol-type system u Her (68 Her, B2 IV + B8 III). A new set of high-resolution échelle spectra has been obtained at Calar Alto Observatory. Spectral disentangling allowed isolation of the individual spectra of the components. A detailed spectroscopic analysis of the primary star indicates an abundance pattern resulting from CNO processing

Stellar modelling of Spica, a high-mass spectroscopic binary with a β Cep variable primary component

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