Early-type Binaries Research Articles

A Study of Ten Early-type Contact Binary Stars in the Small Magellanic Cloud

To study early-type binary systems in different evolutionary stages or environments, the Small Magellanic Cloud (SMC) is an ideal laboratory due to its low metallicity compared to that of the Milky Way. We conduct a study on the period changes of the close binary systems with B-type spectral classifications in the SMC using OGLE collections. Ten B-type binaries that show no significant periodic variations based on the current observational data were analyzed. Through O − C analysis, four of the ten early-type binaries show a long-term period decrease, one shows a long-term period increase, and the other five systems are ambiguous due to the limitations of the observational data. Among the period-decreasing systems, two may be mainly caused by mass transfer, while the rest may be caused by angular momentum loss. The Wilson-Devinney code (W-D method) is used to analyze their I-band photometric light curves. According to the W-D results, five early-type binaries are in the deep contact state, three are in the shallow contact state, while two are in the medium contact state, and the temperature ratios of these early-type binaries are all close to unity. The five deep contact binaries are highly unstable systems and therefore serve as important objects for the study of binary mergers. Finally, a discussion on the evolution of early-type binary stars is conducted by combining the analyses of the light curves and the periodic changes of ten early-type binaries, that implies that the majority of early-type binaries in the SMC may form contact binaries from a phase of rapid mass transfer.

High-precision broadband linear polarimetry of early-type binaries

Aims. DH Cephei is a well-known massive O+O-type binary system on the northern sky, situated at the center of young open cluster NGC 7380. Our high-precision multi-band polarimetry clearly reveals that variations of linear polarization in this system are synchronous with the phase of the orbital period. We used the observed variations of Stokes parameters q and u to derive the orbital inclination i, orientation Ω, and the direction of rotation. Moreover, in order to obtain a rough estimation of the interstellar polarization in the vicinity of DH Cep, we observed polarization arising from the neighboring stars in the cluster. Methods. We used the Dipo1–2 polarimeter in combination with the remotely controlled 60 cm Tohoku T60 telescope to obtain linear polarization measurements of DH Cep in the B, V, and R passbands at the accuracy level of ~0.003%. To obtain an estimation of interstellar polarization of DH Cep, we observed more than a dozen field stars identified as members of NGC 7380 and in the close proximity to DH Cep. A Lomb-Scargle period search was applied to the acquired polarization data to reveal the dominating frequency in polarization variations. We used a standard analytical method based on a two-harmonics Fourier fit to derive the inclination, orientation, and the direction of rotation of the binary orbit. Results. The variations of Stokes parameters in all three B, V, and R passbands clearly suggest an unambiguous periodic signal at 1.055 d with an amplitude of variations of ~0.2%, which corresponds to half of the known orbital period of 2.11 d. This type of polarization variability is expected for a binary system with light-scattering material distributed symmetrically with respect to the orbital plane. In addition to the regular polarization variability, there is a nonperiodic component, which is strongest in the B passband. In the V passband, we obtained our most reliable values for the orbital inclination i = 46° + 11°/ − 46° and an orientation of the orbit on the sky of Ω = 105° ± 55°, with 1σ confidence intervals. Using our best estimate of i and the polametric amplitude in the V passband, we estimated that the mass loss from the system is ~3.4 × 10−7 M⊙ yr−1. The direction of the binary system rotation on the plane of the sky is clockwise. Our polarimetric observations of neighboring stars of DH Cep in NGC 7380 reveal that the polarization of the cluster stars is most likley due to aligned interstellar dust in the foreground.

Structural constants of selected early-type binaries and their astrophysical implications

Theoretical values of the structural coefficient k2, have been calculated in dependance on mass and age. The results are relatively more accurately tested from observations of apsidal motion in young and massive binariy systems. For this context, values of k2 were also derived for eleven well-studied and observed eccentric eclipsing–binaries: CW Cep, V 478 Cyg, AG Per, V 453 Cyg, V 578 Mon, PV Cas, V 451 Oph, V 380 Cyg, IQ Per, HD 152,218 and NO Pup. Our new k2 values are calculated by integrating the Clairaut–Radau equation numerically using the ‘EZ-Web’ version of the Eggleton stellar modeling program. Models are in the range between 2.6 and 17.5 Mʘ, constructed with the two metallicities Z = 0.015 and Z = 0.02. From the observations, we obtained a linear regression formula relating system-mean k2 values for given logarithmic mass and age. This correlation can be used to test the match between apsidal motion results and the separately derived ages of the components of a binary system. From these findings, we conclude that empirically determined k2 values have better determined from young and massive binaries than binaries in general. Apsidal motion constants, when calculated from reliable data, can be used to test the ages of the stars. These checks can be made with our correlation for the system NO Pup, which is the subject of current studies.

First photometric investigation of OGLE-SMC-ECL-2063: A low-metallicity massive contact binary in the SMC

Abstract Studying massive binaries in different evolution stages or environments may help us to solve the problem of the evolution of massive binaries. The metallicity in the Small Magellanic Cloud (SMC) is much lower than that in our Milky Way, and binaries in the SMC are rarely studied. OGLE-SMC-ECL-2063 is a short-period early-type binary with a period of ${0{_{.}^{\circ}}6317643}$ in the SMC. We use the Wilson–Devinney code to analyze its light curves. The result shows that OGLE-SMC-ECL-2063 is an overcontact binary with a high mass ratio of 0.900 and a fill-out factor of $35.9\%$. The O − C curves of the period of OGLE-SMC-ECL-2063 show a long-term increase with a cyclic oscillation of amplitude A = 0.00503 d and period P3 = 14.80 yr. All the evidence above indicates that OGLE-SMC-ECL-2063 is in the Case A mass transfer evolutionary state. The mass transfer rate $\dot{M}_2 = -5.67 \times 10^{-7} M_{\odot }\:$yr−1 is derived and used to explain the continuous period increase. Because both components of OGLE-SMC-ECL-2063 are early-type stars, the existence of a third body may be the reason for the cyclic change in period. The mass of the third body is derived to be no less than 0.70 M⊙ and the orbital separation to be no more than 13.22 au. Combining the result of light-curve analysis, the third body tends to be a low-mass late-type star. Such high-mass-ratio binaries play an important role in the evolution of early-type binaries. Thus, researching OGLE-SMC-ECL-2063 provides the basis for us to study the formation and evolution of early-type contact binaries.

High-precision broadband linear polarimetry of early-type binaries

Aims. The fact that the O-type close binary star system AO Cassiopeiae exhibits variable phase-locked linear polarization has been known since the mid-1970s. In this work, we re-observe the polarization arising from this system more than 50 yr later to better estimate the interstellar polarization and to independently derive the orbital parameters, such as inclination, i, orientation, Ω, and the direction of the rotation for the inner orbit from the phase-folded polarization curves of the Stokes q and u parameters. Methods. The Dipol-2 polarimeter was used to obtain linear polarization measurements of AO Cassiopeiae in the B, V, and R passbands with the T60 remotely controlled telescope at an unprecedented accuracy level of ~0.003%. We have obtained the first proper quantification of the interstellar polarization in the direction heading towards AO Cas by observing the polarization of three neighboring field stars. We employed a Lomb-Scargle algorithm and detected a clear periodic signal for the orbital period of AO Cas. The standard analytical method based on a two-harmonics Fourier fit was used to obtain the inclination and orientation of the binary orbit. Results. Our polarimetric data exhibited an unambiguous periodic signal at 1.76 days, thus confirming the orbital period of the binary system of 3.52 days. Most of the observed polarization is of interstellar origin. The de-biased values of the orbital inclination are i = 63° + 2°/−3° and orientation of Ω = 29º(209º) ± 8°. The direction of the binary system rotation on the plane of the sky is clockwise.

Melnick 33Na: a very massive colliding-wind binary system in 30 Doradus

ABSTRACT We present spectroscopic analysis of the luminous X-ray source Melnick 33Na (Mk 33Na, HSH95 16) in the Large Magellanic Cloud (LMC) 30 Doradus region (Tarantula Nebula), utilizing new time-series Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectroscopy. We confirm Mk 33Na as a double-lined O-type spectroscopic binary with a mass ratio q = 0.63 ± 0.02, e = 0.33 ± 0.01, and orbital period of 18.3 ± 0.1 d, supporting the favoured period from X-ray observations obtained via the Tarantula – Revealed by X-rays survey. Disentangled spectra of each component provide spectral types of OC2.5 If* and O4 V for the primary and secondary, respectively. Unusually for an O supergiant the primary exhibits strong C iv 4658 emission and weak N v 4603-20, justifying the OC classification. Spectroscopic analysis favours extreme physical properties for the primary (Teff = 50 kK, log L/L⊙ = 6.15) with system components of M1 = 83 ± 19 M⊙ and M2 = 48 ± 11 M⊙ obtained from evolutionary models, which can be reconciled with results from our orbital analysis (e.g. M1sin 3i = 20.0 ± 1.2 M⊙) if the system inclination is ∼38° and it has an age of 0.9–1.6 Myr. This establishes Mk 33Na as one of the highest mass binary systems in the LMC, alongside other X-ray luminous early-type binaries Mk34 (WN5h+WN5h), R144 (WN5/6h+WN6/7h), and especially R139 (O6.5 Iafc + O6 Iaf).

Orbital and superorbital periods in ULX pulsars, disc-fed HMXBs, Be/X-ray binaries, and double-periodic variables

ABSTRACT We present evidence for a simple linear relationship between the orbital period and superorbital period in ultra-luminous X-ray (ULX) pulsars, akin to what is seen in the population of disc-fed neutron star supergiant X-ray binary and Be/X-ray binary systems. We argue that the most likely cause of this relationship is the modulation of precessing hotspots or density waves in an accretion or circumstellar disc by the binary motion of the system, implying a physical link between ULX pulsars and high-mass X-ray binary (HMXB) pulsars. This hypothesis is supported by recent studies of Galactic and Magellanic Cloud HMXBs accreting at super-Eddington rates, and the position of ULX pulsars on the spin period–orbital period diagram of HMXBs. An interesting secondary relationship discovered in this work is the apparent connection between disc-fed HMXBs, ULXs, and a seemingly unrelated group of early-type binaries showing so-called double-periodic variability. We suggest that these systems are good candidates to be the direct progenitors of Be/X-ray binaries.

High-precision broad-band linear polarimetry of early-type binaries

Aim. To study the binary geometry of the classic Algol-type triple system λ Tau, we have searched for polarization variations over the orbital cycle of the inner semi-detached binary, arising from light scattering in the circumstellar material formed from ongoing mass transfer. Phase-locked polarization curves provide an independent estimate for the inclination i, orientation Ω, and the direction of the rotation for the inner orbit.Methods. Linear polarization measurements of λ Tau in the B, V , and R passbands with the high-precision Dipol-2 polarimeter have been carried out. The data have been obtained on the 60 cm KVA (Observatory Roque de los Muchachos, La Palma, Spain) and Tohoku 60 cm (Haleakala, Hawaii, USA) remotely controlled telescopes over 69 observing nights. Analytic and numerical modelling codes are used to interpret the data.Results. Optical polarimetry revealed small intrinsic polarization in λ Tau with ~0.05% peak-to-peak variation over the orbital period of 3.95 d. The variability pattern is typical for binary systems showing strong second harmonic of the orbital period. We apply a standard analytical method and our own light scattering models to derive parameters of the inner binary orbit from the fit to the observed variability of the normalized Stokes parameters. From the analytical method, the average for three passband values of orbit inclination i = 76° + 1°∕−2° and orientation Ω = 15°(195°) ± 2° are obtained. Scattering models give similar inclination values i = 72–76° and orbit orientation ranging from Ω = 16°(196°) to Ω = 19°(199°), depending on the geometry of the scattering cloud. The rotation of the inner system, as seen on the plane of the sky, is clockwise. We have found that with the scattering model the best fit is obtained for the scattering cloud located between the primary and the secondary, near the inner Lagrangian point or along the Roche lobe surface of the secondary facing the primary. The inclination i, inferred from polarimetry, agrees with the previously made conclusion on the semi-detached nature of the inner binary, whose secondary component is filling its Roche lobe. The non-periodic scatter, which is also present in the polarization data, can be interpreted as being due to sporadic changes in the mass transfer rate.

Mind Your Ps and Qs: The Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars

Abstract We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q = /M 1. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P ≲ 20 days (separations a ≲ 0.4 au), the binaries have small eccentricities e ≲ 0.4, favor modest mass ratios , and exhibit a small excess of twins q > 0.95. Second, the companion frequency peaks at intermediate periods log P (days) ≈ 3.5 (a ≈ 10 au), where the binaries have mass ratios weighted toward small values q ≈ 0.2–0.3 and follow a Maxwellian “thermal” eccentricity distribution. Finally, companions with long orbital periods log P (days) ≈ 5.5–7.5 (a ≈ 200–5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q ≈ 0.1–1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% ± 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q > 0.1 and log P (days) < 8.0 per primary increases from 0.50 ± 0.04 for solar-type MS primaries to 2.1 ± 0.3 for O-type MS primaries. We fit joint probability density functions to the corrected distributions, which can be incorporated into binary population synthesis studies.

Synchrotron radiation and absence of linear polarization in the colliding wind binary WR 146

Several massive early-type binaries exhibit non-thermal emission which has been attributed to synchrotron radiation from particles accelerated by diffusive shock acceleration (DSA) in the wind-collision region (WCR). If the magnetic field in the strong shocks is ordered, its component parallel to the shock front should be enhanced, and the resultant synchrotron radiation would be polarized. However, such polarization has never been measured. We performed spatially-unresolved radio continuum observations of WR 146, a well known a WC6+O8 system, at 5 cm and 20 cm with the Karl G. Jansky Very Large Array. No linearly polarized radio emission was detected. The data constrain the fractional linear polarization to less than 0.6% between 1 to 8 GHz. This is compatible with a high level of turbulence and a dominant random component in the magnetic field. In this case the relativistic particles could be produced by turbulent magnetic reconnection (MR). In order for this scenario to satisfy the required non-thermal energy budget, we find through numerical modeling that the strength of the magnetic field in the WCR must be as high as ~150 mG. However, if the magnetic field is ordered and DSA is ongoing, then a combination of internal and external Faraday rotation could equally account for the depolarization of the emission. It is not clear whether it is possible to develop the high level of turbulence and strong magnetic fields required for efficient MR in a long-period binary such as WR 146. This scenario might also have trouble explaining the low-frequency cutoff in the spectrum. We therefore favor a scenario where particles are accelerated through DSA and the depolarization is produced by mechanisms other than a large ratio between random to regular magnetic fields.

Distance scale calibration based on early-type binaries

Distance scale calibration based on early-type binaries

High-precision broad-band linear polarimetry of early-type binaries

We investigate the structure of the O-type binary system HD 48099 by measuring linear polarization that arises due to light scattering process. Linear polarization measurements of HD 48099 in the B, V and R passbands with the high-precision Dipol-2 polarimeter have been carried out. The data have been obtained with the 60 cm KVA (Observatory Roque de los Muchachos, La Palma, Spain) and T60 (Haleakala, Hawaii, USA) remotely controlled telescopes during 31 observing nights. Polarimetry in the optical wavelengths has been complemented by observations in the X-rays with the SWIFT space observatory. Optical polarimetry revealed small intrinsic polarization in HD 48099 with 0.1% peak to peak variation over the orbital period of 3.08 days. The variability pattern is typical for binary systems, showing strong second harmonic of the orbital period. We apply our model code for the electron scattering in the circumstellar matter to put constraints on the system geometry. A good model fit is obtained for scattering of light on a cloud produced by the colliding stellar winds. The geometry of the cloud, with a broad distribution of scattering particles away from the orbital plane, helps in constraining the (low) orbital inclination. We derive from the polarization data the inclination i = 17 deg +/- 2deg and the longitude of the ascending node Omega = 82 deg +/- 1 deg of the binary orbit. The available X-ray data provide additional evidence for the existence of the colliding stellar winds in the system. Another possible source of the polarized light could be scattering from the stellar photospheres. The models with circumstellar envelopes, or matter confined to the orbital plane, do not provide good constraints on the low inclination, better than i < 27 deg, as is already suggested by the absence of eclipses.

EARLY-TYPE ECLIPSING BINARIES WITH INTERMEDIATE ORBITAL PERIODS

We analyze 221 eclipsing binaries (EBs) in the Large Magellanic Cloud with B-type main-sequence (MS) primaries ($M_1$ $\approx$ 4 - 14 M$_{\odot}$) and orbital periods $P$ = 20 - 50 days that were photometrically monitored by the Optical Gravitational Lensing Experiment. We utilize our three-stage automated pipeline to (1) classify all 221 EBs, (2) fit physical models to the light curves of 130 detached well-defined EBs from which unique parameters can be determined, and (3) recover the intrinsic binary statistics by correcting for selection effects. We uncover two statistically significant trends with age. First, younger EBs tend to reside in dustier environments with larger photometric extinctions, an empirical relation that can be implemented when modeling stellar populations. Second, younger EBs generally have large eccentricities. This demonstrates that massive binaries at moderate orbital periods are born with a Maxwellian "thermal" orbital velocity distribution, which indicates they formed via dynamical interactions. In addition, the age-eccentricity anticorrelation provides a direct constraint for tidal evolution in highly eccentric binaries containing hot MS stars with radiative envelopes. The intrinsic fraction of B-type MS stars with stellar companions $q$ = $M_2$/$M_1$ $>$ 0.2 and orbital periods $P$ = 20 - 50 days is (7 $\pm$ 2)%. We find early-type binaries at $P$ = 20 - 50 days are weighted significantly toward small mass ratios $q$ $\approx$ 0.2 - 0.3, which is different than the results from previous observations of closer binaries with $P$ $<$ 20 days. This indicates that early-type binaries at slightly wider orbital separations have experienced substantially less coevolution and competitive accretion during their formation in the circumbinary disk.

TOWARD UNDERSTANDING THE NATURE OF THE YOUNG DETACHED BINARY SYSTEM HD 350731

The young binary system HD 350731 is a noteworthy laboratory for studying early-type binaries with similar components. We present here the analysis of differential multi-color photometric and spectroscopic observations for the double-lined detached system. Accurate absolute parameters were determined from the simultaneous solution of light and radial velocity curves for the first time. HD 350731 consists of two B8V-type components having masses and radii respectively of $M_{1}=2.91\pm0.13$ M$_{\odot}$, $M_{2}=2.80\pm0.14$ M$_{\odot}$, $R_{1}= 2.11 \pm0.05$ R$_{\odot}$ and $R_{2}=2.07\pm0.05$ R$_{\odot}$. The effective temperatures were determined based on analysis of disentangled spectra of the components and derived to be $12000\pm250$ K and $11830\pm300$ K for the primary and secondary components, respectively. The measured projected rotational velocities, 69.2$\pm$1.5 km s$^{-1}$ for primary and 70.1$\pm$1.7 km s$^{-1}$ for secondary, were found closer to the pseudo-synchronous velocities of the components. Comparison with evolutionary models suggests an age of 120$\pm$35 Myr. Kinematic analysis of the unevolved binary system HD 350731 revealed that it belongs to the young thin-disc population of the Galaxy.

A test of an early-type contact binary model by comparing the relations obtained from theory with those from observations

Contact binary systems, including massive, middle-mass and low-mass systems, have been observed. It is necessary to construct the evolutionary model for contact binary systems with all different masses. The model introduced by Huang, Song and Bi indicates some special and unique understandings of the physical processes occurring in the contact binaries. It is necessary to test whether this model can be applied to the early-type contact binaries. The best way to test a theoretical model is to know whether this model can explain the observational phenomena of such binaries. For this aim, a comparison is performed for the relations of mass-luminosity, mass-radius, and the distribution in the HR diagram obtained from the model introduced by Huang et al. and those from the astronomical observations. A good result is obtained by the comparison. This means the model can be used in calculating the evolution of early-type contact binaries, and can explain the observational phenomena of such binaries.

The influence of magnetic field on short-wavelength instability of Riemann ellipsoids

We address the question of stability of the so-called S-type Riemann ellipsoids, i.e. a family of Euler flows in gravitational equilibrium with the vorticity and background rotation aligned along the principal axis perpendicular to the flow. The Riemann ellipsoids are the simplest models of self-gravitating, tidally deformed stars in binary systems, with the ellipticity of the flow modelling the tidal deformation. By the use of the WKB theory we show that mathematically the problem of stability of Riemann ellipsoids with respect to short-wavelength perturbations can be reduced to the problem of magneto-elliptic instability in rotating systems, studied previously by Mizerski and Bajer [K.A. Mizerski, K. Bajer, The magneto-elliptic instability of rotating systems, J. Fluid Mech. 632 (2009) 401–430]. In other words the equations describing the evolution of short-wavelength perturbations of the Riemann ellipsoids considered in Lagrangian variables are the same as those for the evolution of the magneto-elliptic-rotational ( MER) waves in unbounded domain. This allowed us to use the most unstable MER eigenmodes found in Mizerski et al. [K.A. Mizerski, K. Bajer, H.K. Moffatt, The α -effect associated with elliptical instability, J. Fluid Mech., 2010 (in preparation)] to provide an estimate of the characteristic tidal synchronization time in binary star systems. We use the idea of Tassoul [J.-L. Tassoul, On synchronization in early-type binaries, Astrophys. J. 322 (1987) 856–861] and that the interactions between perturbations significantly increase the effective viscosity and hence the energy dissipation in an Ekman-type boundary layer at the surface of the star. The results obtained suggest that if the magnetic field generated by (say) the secondary component of a binary system is strong enough to affect the flow dynamics in the primary, non-magnetized component, the characteristic tidal synchronization time can be significantly reduced.

Synchronization and circularization in early-type binaries on main sequence

We showed in a preceding paper based on an analysis of the observed rates of apsidal motion that synchronization in early-type eclipsing binaries continues on the main sequence, and the observed synchronization times, tsyn, agree with the Zahn's theory and are inconsistent with the shorter time-scale proposed by Tassoul. It follows from this that circularization in early-type binaries must also proceed in accordance with the Zahn's theory because the circularization times, tcirc, in both theories are rather tightly related to tsyn via relation tcirc≈ αtsyn, where α is the orbital-to-axial momentum ratio. To further investigate this problem, we compile a catalogue of 101 eclipsing binaries with early-type main-sequence components (M1,2 > 1.6 M⊙). We determine the ages, t, and circularization time-scales, tcirc, for all these systems in terms of the two competing theories by comparing observational data with modern models of stellar evolution of Claret and atmospheric models of Kurucz. We compute tcirc with the allowance for the evolutionary variations of the physical parameters of the components and, for the first time in such studies, also take into account the variations of the orbital parameters (P, a, e) in the process of circularization subject to the conservation of the total angular momentum. The results of these computations show that the mechanism of orbital circularization in early-type close binary systems (CBSs) suggested by Tassoul is, like in the case of synchronization, inconsistent with observational data. At the same time, the Zahn's mechanism, which is based on the dissipation of the energy of dynamic tides in the upper layers of the envelopes of CBSs components due to non-adiabaticity of these layers, agrees satisfactorily with observations.

Analysis of photometric light curves solution for massive contact OB binary stars. LY Aurigae, BH Centauri, SV Centauri

We searched for signs of the presence of circumstellar gaseous matter in photometric data for massive contact early-type binaries by analyzing residual curves (the dependence of the difference between the observed and theoretical brightness variations on the orbital-period phase) for three such stars. The residual curves make it possible to estimate the influence of gas in the common envelope on the observed light curves for different phase intervals and to qualitatively describe the character of the distortion of the light from the system’s components. Changes of the residual curves from filter to filter indicate varying conditions in the circumstellar matter. Changes of the residual curves from one observation epoch to another indicate varying conditions in the circumstellar matter. We compared the residual curves obtained for different photometric bands and epochs via a correlation analysis. The distortion of light from the components of LY Aurigae in the ultraviolet differs from that in the visual. The distortion of light from the components of SV Centauri is appreciable, but not selective, and does not vary in time, while the distortion of light from BH Centauri possesses a strong selective component. A comparison of the radii computed for the components of BH Centauri and SV Centauri shows that the gas distribution near these binaries varies in time.

An analysis ofvsin (i) correlations in early-type binaries

We use information on the v sin (i) values of early-type binaries in order to search for correlations which may constrain the relative orientation of the stellar spin axes in binary systems. We find correlations in the case of close binaries which suggest that tidal synchronization is effective for binaries whose separation exceeds the stellar radius by more than an order of magnitude, in line with the theoretical predictions of Goldreich & Nicholson and the previous observational analysis of Giuricin et al. In the case of wide binaries, the v sin (i) values are not well correlated, which requires that the magnitude of the spin speeds is not tightly correlated. Under this assumption, we then find that the data provide no significant constraints on the degree of alignment of spin axes. The data are therefore compatible with scenarios (such as disc fragmentation or capture) which differ widely in the expected degree of spin alignment.

Discovery of the strongly eccentric, short-period binary nature of the B-type system HD 313926 by the MOST satellite

Abstract The MOST photometric space mission discovered an eclipsing binary among its guide stars in 2006 June which combines a relatively large eccentricity e= 0.20 with an orbital period of only 2.27 d. HD 313926 appears to consist of two early-type stars of spectral type B3–B7. It has the largest eccentricity among known early-type binaries with periods less than 3.5 d. Despite the large components indicated by its spectral type and light curve model, and its short period, the orbit of HD 313926 has not yet circularized so it is probably very young, even compared with other young B stars.