Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass

We present our findings based on a detailed analysis of the binaries of the Hyades, in which the masses of the components are well known. We fit the models of the components of a binary system to observations so as to give the observed total V and B − V of that system and the observed slope of the main sequence in the corresponding parts. According to our findings, there is a very definite relationship between the mixing-length parameter and the stellar mass. The fitting formula for this relationship can be given as  α= 9.19( M /M_⊙− 0.74)^0.053− 6.65  , which is valid for stellar masses greater than  0.77 M_⊙  . While no strict information is gathered for the chemical composition of the cluster, as a result of degeneracy in the colour–magnitude diagram, by adopting   Z = 0.033  and using models for the components of 70 Tau and θ² Tau we find the hydrogen abundance to be   X = 0.676  and the age to be 670 Myr. If we assume that   Z = 0.024  , then   X = 0.718  and the age is 720 Myr. Our findings concerning the mixing-length parameter are valid for both sets of the solution. For both components of the active binary system V818 Tau, the differences between radii of the models with   Z = 0.024  and the observed radii are only about 4 per cent. More generally, the effective temperatures of the models of low-mass stars in the binary systems studied are in good agreement with those determined by spectroscopic methods.     

The low-mass white dwarf companion to β Crateris

We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1 III star β  Crt, which shows that it has an unusually low mass,     , and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in the proper motion of β  Crt as measured by Hipparcos suggest that the period could be as high as ∼10 yr. However, a low-mass white dwarf in a system with a period ≳3 yr is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.     

The Algol-type eclipsing binaries RW CrB and VZ Leo: new RI photometric study and search for pulsations

In this study we present the new R and I light curve solutions for the eclipsing binary systems RW CrB and VZ Leo, which for VZ Leo is the first one in the literature. Our new observations have been analyzed using the Wilson-Devinney code from which new geometric and photometric elements are derived. The geometry of both stars is that of a semi-detached binary system where the secondary component fills its Roche lobe while the primary component is well inside. In the case of RW CrB, asymmetry in the light curve was explained by a spot model. The orbital period changes of both systems were studied and the results indicated a period decrease which can be explained by angular momentum loss. We also investigated the possibility of pulsations of the primary components since these systems are mentioned as candidates of δ Sct type pulsation. However, a time-series analysis of the residual curves in the filter I does not indicate any evidence of periodic light variation for both systems. Finally, we compared the results obtained for both binary stars to those of similar systems.     

The orbit of the visual binary ADS 8630 (γ Vir)

We present a new orbit for the visual binary ADS 8630 = γ Vir. Although it is one of the first visual double stars discovered, its orbital elements were still poorly known. Indeed the very high eccentricity of the orbit and the difficulty of observing the pair at periastron passage in 1836 has meant that it is only now that sufficient measures of the recent close approach in 2005 have allowed an orbital analysis which predicts the angular motion to an acceptable degree of accuracy. We present a series of 35 speckle measurements of ADS 8630 obtained with PISCO in Merate between 2004 and 2006. Those measures have been crucial for determining the new orbital elements since they cover an arc of 130 degrees in the apparent orbit and include the periastron passage of 2005. The masses of the individual F0V components of the binary are found to be 1.40 M_⊙ with an accuracy of about 3%. We also investigate in detail the possibility of the presence of a third body in the system, that was proposed by other authors. The high‐angular resolution infra‐red image of γ Vir that we obtained in June 2006 with the LuckyCam instrument on the ESO NTT shows the absence of any companion as faint as a M0V star at a distance larger than 0.4″. Combined with the analysis of the residuals of our orbit, the values found for the masses of the individual components and the radial velocity measurements, this observation rules out the presence in the system of a third companion with a mass larger than 0.3 M_⊙. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radial velocity and light curves analysis of the eclipsing binary NN Vir

The eclipsing binary NN Vir is a short period system showing an EW‐type light curve. Photometric observations of NN Vir were done by Gomez‐Ferrellad & Garcia‐Melendo (1997) at Esteve Duran Observatory. We used photometric data of NN Vir for light curve analysis. The available spectroscopic data of NN Vir is new and we also used the first radial velocity data of this system obtained by Rusinski & Lu (1999) for analysis. The radial velocity and light curves analysis was made with the latest version ofWilson program(1998) and the geometric and physical elements of the system are derived. By searching the simultaneous solutions of the system, we have determined the masses and radii of the components : 1.89(M_⊙) and 1.65(R_⊙) for the primary component; 0.93(M_⊙) and 1.23(R_⊙) for the secondary component. We estimated effective temperatures of 7030(K) for the primary and 6977(K) for the secondary component. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A catalogue of close binaries located in the δ Scuti region of the Cepheid instability strip

A catalogue of close eclipsing binary systems (detached and semidetached) with at least one of the components located in the δ Scuti region of the Cepheid instability strip is presented. The positions of the stars in the instability strip are determined by their accurate temperatures and luminosities. Observationally detected binaries (20 semidetached, four detached and one unclassified) with oscillating components were included in the catalogue as a separate table. The primaries of the oscillating Algols tend to be located near the blue edge of the instability strip. Using reliable luminosities and temperatures determined by recent photometric and spectroscopic studies, we have found that at least one or two components of 71 detached and 90 semidetached systems are located in the δ Scuti region of the Cepheid instability strip. In addition, 36 detached or semidetached systems discovered by the Hipparcos satellite were also given as a separate list. One of their components is seen in the δ Scuti region, according to their spectral type or   B − V   colours. They are potential candidate binaries with the δ Scuti-type pulsating components which need further photometric and spectroscopic studies in better precision. This catalogue covers information and literature references for 25 known and 197 candidate binaries with pulsating components.     

Composite spectra Paper 15: HD 216572, a triple system containing a short-period, double-lined secondary

We analyse 81 optical spectra of the composite-spectrum binary HD 216572, and show that the primary is a cool giant of type G8 III while the secondary is a double-lined binary consisting of two nearly identical B9 dwarfs in a 1.18-d orbit. The inner system undergoes partial eclipses, whose photometry we model to derive the physical parameters of both secondary stars. The outer system does not eclipse. We isolate the combined spectrum of the secondary by spectral subtraction, and from 48 separate radial-velocity measurements of both secondary components we obtain a triple-lined orbit solution from which we determine the individual masses of all three stars and the inclinations of both the inner and the outer orbits. The period of the outer system is 55 d, which is surprisingly short for a giant star, and our detection of small but non-negligible amounts of variable chromospheric emission in the Ca  ii K line is not unlike that detected in other systems with comparably short periods. The secondary components are in a circular orbit and are rotating at about  95 ± 10 km s⁻¹  ; although their surface-to-surface separation is only  4 R_⊙  the stars are not noticeably distorted geometrically by such close proximity. All three stars appear to be in synchronous rotation in their respective orbits. We derive fairly accurate Hertzsprung–Russell diagram positions for all three stars and compare them to evolutionary tracks calculated for the respective stellar masses, but cannot reconcile the age of the cool giant with that of the B stars.     

The first magnetic maps of a pre-main-sequence binary star system – HD 155555

We present the first maps of the surface magnetic fields of a pre-main-sequence binary system. Spectropolarimetric observations of the young, 18 Myr, HD 155555 (V824 Ara, G5IV+K0IV) system were obtained at the Anglo-Australian Telescope in 2004 and 2007. Both data sets are analysed using a new binary Zeeman–Doppler imaging (ZDI) code. This allows us to simultaneously model the contribution of each component to the observed circularly polarized spectra. Stellar brightness maps are also produced for HD 155555 and compared to previous Doppler images.
Our radial magnetic maps reveal a complex surface magnetic topology with mixed polarities at all latitudes. We find rings of azimuthal field on both stars, most of which are found to be non-axisymmetric with the stellar rotational axis. We also examine the field strength and the relative fraction of magnetic energy stored in the radial and azimuthal field components at both epochs. A marked weakening of the field strength of the secondary star is observed between the 2004 and 2007 epochs. This is accompanied by an apparent shift in the location of magnetic energy from the azimuthal to radial field. We suggest that this could be indicative of a magnetic activity cycle. We use the radial magnetic maps to extrapolate the coronal field (by assuming a potential field) for each star individually – at present ignoring any possible interaction. The secondary star is found to exhibit an extreme tilt (≈75°) of its large-scale magnetic field to that of its rotation axis for both epochs. The field complexity that is apparent in the surface maps persists out to a significant fraction of the binary separation. Any interaction between the fields of the two stars is therefore likely to be complex also. Modelling this would require a full binary field extrapolation.     

X-ray Emission from the Pre-Main Sequence Systems FU Orionis and T Tauri

We present new results from recent X-ray observations of the accreting pre-main sequence stars FU Orionis and T Tauri. XMM-Newton observations of the close binary system FU Ori reveal an unusual X-ray spectrum consisting of a cool moderately-absorbed component and a hot component viewed through much higher absorption. The two components thus originate in physically distinct regions. The double absorption spectrum is qualitatively different than observed in typical coronal sources and may signal either non-coronal emission or separate unresolved X-ray contributions from more than one star in the system. High-resolution Chandra imaging of the T Tau triple system shows that its X-ray emission is dominated by the optically-revealed northern component T Tau N. X-ray spectra of T Tau obtained with XMM can be acceptably fitted with a moderately absorbed two-temperature thermal plasma model. Its spectral properties are similar to those seen in coronal X-ray sources.     

Radio-to-TeV γ-ray emission from PSR B1259–63

We discuss the implications of the recent X-ray and TeV γ-ray observations of the PSR B1259–63 system (a young rotation powered pulsar orbiting a Be star) for the theoretical models of interaction of pulsar and stellar winds. We show that previously considered models have problems to account for the observed behaviour of the system. We develop a model in which the broad band emission from the binary system is produced in result of collisions of GeV–TeV energy protons accelerated by the pulsar wind and interacting with the stellar disk. In this model the high energy γ-rays are produced in the decays of secondary neutral pions, while radio and X-ray emission are synchrotron and inverse Compton emission produced by low-energy (≤100 MeV) electrons from the decays of secondary charged π ^± mesons. This model can explain not only the observed energy spectra, but also the correlations between TeV, X-ray and radio emission components.      

Binaries in the universe. possible dynamical mechanisms of formation,evolution and disruption of different kinds of constituents of the universe

We study the dynamics of extended shells of relatively low-mass particles around and inside the orbit of two heavy centres of gravity (a binary) by computer simulations. The binary components are surrounded byN = 16 000 small mass particles in uniform random distribution on few spherical envelopes with different radii expanding with respective velocities. Some shells are inside the orbit of binary.We apply this model to binary galaxy systems with baryonic dark matter, e.g., massive black holes. In principle, we can apply this model to different kinds of objects (from binary star systems until superclusters of galaxies).It is shown that the shell expands homologously with a decreasing velocity and then, falls back into the binary system forming zones of compressed matter. At some moment of time there could be a collapse of these particles on to the heavier component of the binary. Further in time, some part of particles which were outside the binary orbit escape from the system. Other particles which were initially inside of the orbit are captured by binary components.We consider a number of different models with different initial parameters. For models with smaller radii of shells, about one-half of the particles escape from systems; whereas for larger values the shell disrupts as a whole. Escaping particles form collimated flows in planes of orbits of binaries. Positions of flows and directions of motion depend on positions of heavier components of binaries at the moment of a closest approach of particles and on ratios of masses of binary components.We show that during evolution of our models different kinds of structures of systems often are very similar to the observed structures of galaxies: spiral and elliptical galaxies, interacting galaxies, different kinds of flows and jets. Totally systems are expanding - after 40 periods of rotation of the binary the system expands by 300 times.     

uvbyβ photometry of the short-period binary VV Ursae Majoris

We present well-sampled uvby light curves, supplemented by a few β filter measurements, of the Algol binary VV UMa. The light curves are analysed using two different codes to derive the orbital and absolute stellar parameters of this binary. We find reasonably good fits to the light curves and determine the stellar effective temperatures T _eff,1≃9000–9600 K , and T _eff,2≃5300–5600 K with a mass ratio q ≃0.35 . From the light-curve fits we discard the possibility of an anomalous gravity-darkening exponent for the secondary star of this system, as previously suggested.
We find evidence of short-term, small-amplitude variations in the brightness of the system. Two periodicities of about 1.10 and 0.51 h seem to be present in the data for at least two different nights, even within the secondary eclipse. This suggests that VV UMa may be a new Algol binary with a low-amplitude variable primary star, but new data collected during longer observing runs are necessary to confirm the pulsating nature of the brightness variations.     

Photometric study of the active binary star V1430 Aquilae

New BVR light curves and a photometric analysis of the eclipsing binary star V1430 Aql are presented. The light curves were obtained at the Çanakkale Onsekiz Mart University Observatory in 2004. The light curves are generally those of detached eclipsing binaries, but there are large asymmetries between maxima. New BVR light curves were analysed with an ILOT procedure. Light curve asymmetries of the system were explained in terms of large dark starspots on the primary component. The primary star shows a long‐lived and quasi‐poloidal spot distribution with active longitudes in opposite hemispheres. Absolute parameters of the system were derived.We also discuss the evolution of the system: the components are likely to be pre‐main sequence stars, but a post‐main sequence stage cannot be ruled out. More observations are needed to decide this point. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modelling the binary progenitor of Supernova 1993J

We have developed a detailed stellar evolution code capable of following the simultaneous evolution of both stars in a binary system, together with their orbital properties. To demonstrate the capabilities of the code, we investigate potential progenitors for the Type IIb Supernova 1993J, which is believed to have been an interacting binary system prior to its primary exploding. We use our detailed binary stellar evolution code to model this system to determine the possible range of primary and secondary masses that could have produced the observed characteristics of this system, with particular reference to the secondary. Using the luminosities and temperatures for both stars (as determined by Maund et al.) and the remaining mass of the hydrogen envelope of the primary at the time of explosion, we find that if mass transfer is 100 per cent efficient, the observations can be reproduced by a system consisting of a  15 M_⊙  primary and a  14 M_⊙  secondary in an orbit with an initial period of 2100 days. With a mass transfer efficiency of 50 per cent, a more massive system consisting of a  17 M_⊙  primary and a  16 M_⊙  secondary in an initial orbit of 2360 days is needed. We also investigate some of the uncertainties in the evolution, including the effects of tidal interaction, convective overshooting and thermohaline mixing.     

Atmospheric analysis of the M/L and M/T dwarf binary systems LHS 102 and Gliese 229

We present  0.9–2.5 μm  spectroscopy with   R ∼800  and  1.12–1.22 μm  spectroscopy with   R ∼5800  for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS 102B. We also report IZJHKL ' photometry for both components of the LHS 102 system, and L ' photometry for Gl 229A. The data are combined with previously published spectroscopy and photometry to produce flux distributions for each component of the kinematically old disc M/L dwarf binary system LHS 102 and the kinematically young disc M/T dwarf binary system Gliese 229. The data are analysed using synthetic spectra generated by the latest 'AMES-dusty' and 'AMES-cond' models by Allard & Hauschildt. Although the models are not able to reproduce the overall slope of the infrared flux distribution of the L dwarf, most likely because of the treatment of dust in the photosphere, the data for the M dwarfs and the T dwarf are well matched. We find that the Gl 229 system is metal-poor despite having kinematics of the young disc, and that the LHS 102 system has solar metallicity. The observed luminosities and derived temperatures and gravities are consistent with evolutionary model predictions if the Gl 229 system is very young  (age∼30 Myr)  with masses (A,B) of (0.38,≳0.007) M_⊙, and the LHS 102 system is older, aged  1–10 Gyr  with masses (A,B) of (0.19,0.07) M_⊙.     

Cyclic light and period variations of the UV Leonis system

Solutions of the new standard V‐light curves for the EA type binary UV Leo are obtained using the PHOEBE code (0.31a version). Absolute parameters of the stellar components were then determined, enabling them to be positioned on the absolute magnitude‐color (l.e. M_V vs. B – V) isochrones diagram, based on which the age of the system is estimated to be >4×10⁹ yr. Also times of minima data (“O – C curve”) have been analyzed. Apart from an almost sinusoidal variation with a period of 29.63 yr, which modulates the orbital period, and was attributed to a third body orbiting around the system, other cyclic variation in the orbital period and also brightness, with time scales of 24.25 and 22.77 yr were found, respectively. We associate this with a magnetic activity cycle newly reported here for UV Leo (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

CK Bootis: a W UMa system with a small mass ratio

We present an analysis of BV R light curves of an eclipsing binary CK Bootis, a system with a very small mass ratio. The light curves appear to exhibit a typical O'Connell effect. The light curves are analyzed by means of the latest version of the WD program. The asymmetry of the light curves is explained by a cool star spot model. The simultaneous BV R synthetic light curve analysis gives a tiny mass ratio of 0.12, an extremely large fill‐out factor of 0.65, and a very small difference between the component temperatures of 90 K. The absolute parameters of the system were also derived by combining the photometric solutions with the radial velocity data. The mass of the secondary is very low (0.15 M_⊙) and it continues losing mass. Thirty seven new times of minimum are reported. It is found that the orbital period of the system has a quasi periodic variation, superimposed on a period increase. The long‐term period increase rate is deduced to be dP/dt = 3.54x10^–7 d yr^–1, which can be interpreted as being due to mass transfer from the less massive star to the more massive component. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time series photometric spot modelling – V. Phase coherence of spots on UZ Librae

We present spot models for nine years of continuous VI_c photometry of UZ Lib from 1993–2001. The relatively stable double‐humped light curve shape suggests extreme phase coherence. From the spot‐modelling analysis, we found that the major spots or spot groups are always located on the hemisphere facing the secondary star and exactly in the opposite hemisphere anti‐facing the secondary. Several single‐humped light curves and our suggested binary scenario rule out a pure ellipsoidal variability as the cause of the double‐humped light curve shape. We try to explain this preferred spot pattern with a magnetic‐field structure that connects the two components, as suggested earlier for RS CVn stars in general. A possible 4.8 years spot cycle is found from the long‐term brightness variations but needs confirmation. We rediscuss the basic astrophysical data of UZ Lib. The Hipparcos parallax is likely wrong, a possible reason could be that UZ Lib is e.g., a triple system.     

Possible connection between period change and magnetic activity of the very short‐period binary VZ Piscium

New times of light minimum of the short‐period (P = 0^d.26) close binary system, VZ Psc, are presented. A period investigation of the binary star, by combining the three new eclipse times with the others collected from the literatures, shows that the variation of the period might be in an alternate way. Under the hypothesis that the variation of the orbital period is cyclic, a period of 25 years and an amplitude of 0.^d0030 for the cyclic change are determined. If this periodic variation is caused by the presence of a third body, the mass of the third body (m₃) should be no less than 0.081M_⊙. Since both components of VZ Psc are strong chromospherically active and the level of activity of the secondary component is higher than that of the primary one, the period may be more plausibly explained by cyclic magnetic activity of the less massive component. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Differential rotation on both components of the pre-main-sequence binary system HD 155555

We present the first measurements of surface differential rotation on a pre-main-sequence binary system. Using intensity (Stokes I) and circularly polarized (Stokes V) time-series spectra, taken over 11 nights at the Anglo-Australian Telescope (AAT), we incorporate a solar-like differential rotation law into the surface imaging process. We find that both components of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) binary system show significant differential rotation. The equator–pole lap times as determined from the intensity spectra are 80 d for the primary star and 163 d for the secondary. Similarly, for the magnetic spectra we obtain equator–pole lap times of 44 and 71 d, respectively, showing that the shearing time-scale of magnetic regions is approximately half of that found for stellar spots. Both components are therefore found to have rates of differential rotation similar to those of the same spectral-type main-sequence single stars. The results for HD 155555 are therefore in contrast to those found in other, more evolved, binary systems where negligible or weak differential rotation has been discovered. We discuss two possible explanations for this: first that at the age of HD 155555 binary tidal forces have not yet had time to suppress differential rotation and secondly that the weak differential rotation previously observed on evolved binaries is a consequence of their large convection zone depths. We suggest that the latter is the more likely solution and show that both temperature and convection zone depth (from evolutionary models) are good predictors of differential rotation strength. Finally, we also examine the possible consequences of the measured differential rotation on the interaction of binary star coronae.