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.     

Preliminary results on the fundamental parameters of the eclipsing binary V398 Lacertae

The Hipparcos Space Astrometry Mission photometric observations of V398 Lac, led to the discovery of its variability, allowing to classify it as an eclipsing binary with an orbital period of about 5.4 days. This prompted us to acquire highresolution échelle spectra with the aim of performing accurate radial velocity measurements and to determine the main physical parameters of the system's components. We present, for the first time, a double‐lined radial velocity curve and determine the orbital and physical parameters of the two components, that can be classified both as late B‐type stars. In particular, we obtained an orbital inclination i ∼ 85°. With this value of the inclination, we deduced masses M₁ = 3.83±0.35 M_⊙ andM₂ = 3.29±0.32 M_⊙, and radii R₁ = 4.89±0.18 R_⊙ and R₂ = 2.45±0.11 R_⊙ for the more massive and less massive components, respectively. Both components are well inside their own Roche lobes. The mass ratio is M₂/M₁ ∼ 0.86. We derived also the projected rotational velocities as v₁ sin i = 79±2 km s^–1 and v² sin i = 19±2 km s^–1. Our measurements indicate that the rotation of the primary star is essentially pseudo‐synchronized with the orbital velocity at the periastron, while the secondary appears to rotate very slowly and has not yet attained synchronization. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precession and nutation in the η Carinae binary system: evidence from the X-ray light curve

It is believed that η Carinae is actually a massive binary system, with the wind–wind interaction responsible for the strong X-ray emission. Although the overall shape of the X-ray light curve can be explained by the high eccentricity of the binary orbit, other features like the asymmetry near periastron passage and the short quasi-periodic oscillations seen at those epochs have not yet been accounted for. In this paper we explain these features assuming that the rotation axis of η Carinae is not perpendicular to the orbital plane of the binary system. As a consequence, the companion star will face η Carinae on the orbital plane at different latitudes for different orbital phases and, since both the mass-loss rate and the wind velocity are latitude dependent, they would produce the observed asymmetries in the X-ray flux. We were able to reproduce the main features of the X-ray light curve assuming that the rotation axis of η Carinae forms an angle of  29°± 4°  with the axis of the binary orbit. We also explained the short quasi-periodic oscillations by assuming nutation of the rotation axis, with an amplitude of about  5°  and a period of about 22 days. The nutation parameters, as well as the precession of the apsis, with a period of about 274 years, are consistent with what is expected from the torques induced by the companion star.     

Violent stellar merger model for transient events

We derive the constraints on the mass ratio for a binary system to merge in a violent process. We find that the secondary-to-primary stellar mass ratio should be  0.003 ≲ ( M ₂/ M ₁) ≲ 0.15  . A more massive secondary star will keep the primary stellar envelope in synchronized rotation with the orbital motion until merger occurs. This implies a very small relative velocity between the secondary star and the primary stellar envelope at the moment of merger, and therefore very weak shock waves, and low-flash luminosity. A too low-mass secondary will release small amount of energy, and will expel small amount of mass, which is unable to form an inflated envelope. It can, however, produce a quite luminous but short flash when colliding with a low-mass main-sequence star.
Violent and luminous mergers, which we term mergebursts, can be observed as V838 Monocerotis-type events, where a star undergoes a fast brightening lasting days to months, with a peak luminosity of up to  ∼10⁶ L_⊙  followed by a slow decline at very low effective temperatures.     

Spectral disentangling of the triple system DG Leo: orbits and chemical composition

DG Leo is a spectroscopic triple system composed of three stars of late-A spectral type, one of which was suggested to be a δ Scuti star. Seven nights of observations at high spectral and high time-resolution at the Observatoire de Haute-Provence with the ELODIE spectrograph were used to obtain the component spectra by applying a Fourier transform spectral disentangling technique. Comparing these with synthetic spectra, the stellar fundamental parameters (effective temperature, surface gravity, projected rotation velocity and chemical composition) are derived. The inner binary consists of two Am components, at least one of which is not yet rotating synchronously at the orbital period though the orbit is a circular one. The distant third component is confirmed to be a δ Scuti star with normal chemical composition.     

Spot patterns and differential rotation in the eclipsing pre-cataclysmic variable binary, V471 Tau

We present surface spot maps of the K2V primary star in the pre-cataclysmic variable binary system, V471 Tau. The spot maps show the presence of large high-latitude spots located at the sub-white dwarf longitude region. By tracking the relative movement of spot groups over the course of four nights (eight rotation cycles), we measure the surface differential rotation rate of the system. Our results reveal that the star is rotating rigidly with a surface shear rate,  dΩ= 1.6 ± 6 mrad d⁻¹  . The single active star AB Dor has a similar spectral type, rotation period and activity level as the K star in V471 Tau, but displays much stronger surface shear  (46 < dΩ < 58 mrad d⁻¹)  . Our results suggest that tidal locking may inhibit differential rotation; this reduced shear, however, does not affect the overall magnetic activity levels in active K dwarfs.     

Starspots and relativity: Applied Doppler imaging for the Gravity Probe B mission

We present Doppler images and surface differential rotation measurements for the primary of the RS CVn binary IM Pegasi, the guide star for the Gravity Probe B experiment. The data used is a subset of that taken during optical support of the mission and was obtained almost nightly over a near three year period from the Automatic Spectroscopic Telescope operated by Tennessee State University. Using the technique of least-squares deconvolution to increase the signal-to-noise ratio of the data, we have reconstructed 31 maximum entropy Doppler images of the star. The images show that the spot features are relatively stable for over a year (and possibly longer) with both a polar spot and lower latitude features. The most intense features are located on the side facing the secondary. In addition, we have incorporated a solar-like differential rotation law into the imaging process to determine the level of surface differential rotation for IM Peg for 22 epochs. A weighted least-squares average of the measurements gives a surface shear of 0.0142 ± 0.0007 rad/d, meaning that the equator takes ∼440 ± 20 days to lap the poles. Although the level of surface differential rotation was shown to vary over the period of the observations, this may indicate an underestimate in the errors of the method rather than any temporal evolution in the differential rotation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The ever challenging emission‐line binary β Lyrae

A brief history of investigations of Lyr, an emission‐line binary and one of the first ever discovered Be stars is presented. A rather fast progress in the understanding of this enigmatic object during the past fifteen years is then discussed in some detail. The current picture of β Lyr is that it is an eclipsing binary in a stage of mass transfer between the components. The mass‐losing star is a B6‐8II object, with a mass of about 3 M_⊙, which is filling the Roche lobe and sending material towards its more massive companion at a rate of about 2 × 10^—5 M_⊙ yr^—1. This leads to the observed rapid increase of the orbital period at a rate of 19 s per year. The mass‐gaining star is as early B star with a mass of about 13 M_⊙. It is completely hidden inside an opaque accretion disk, jet‐like structures, perpendicular to the orbital plane and a light‐scattering halo above the poles of the star. The observed radiation of the disk corresponds to an effective temperature which is much lower than what would correspond to an early B star. The disk shields the radiation of the central star in the directions along the orbital plane and redistributes it in the directions perpendicular to it. That is why the mass‐losing star appears brighter of the two in the optical region of the spectrum. At present, rather reliable estimates of all basic properties of the binary and its components are available. However, in spite of great progress in understanding the system in recent years, some disagreement between the existing models and observed phase variations still remains, both for continuum and line spectrum, which deserves further effort.     

Doin' the twist: secular changes in the surface differential rotation on AB Doradus

We present measurements of the rotation rates of individual starspots on the rapidly rotating young K0 dwarf AB Doradus, at six epochs between 1988 December and 1996 December. The equatorial rotation period of the star decreased from 0.5137 to 0.5129 d between 1988 December and 1992 January. It then increased steadily, attaining a value of 0.5133 d by 1996 December. The latitude dependence of the rotation rate mirrored the changes in the equatorial rotation rate. The beat period between the equatorial and polar rotation periods dropped from 140 to 70 d initially, then rose steadily. The most rigid rotation, in 1988 December, occurred when the starspot coverage was at a maximum. The time-dependent part of the differential rotation is found to have     , which should alter the oblateness of the star enough to explain the period changes observed in several close binaries via the Applegate mechanism.     

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.     

The EC 14026 stars – VIII. PG 1336−018: a pulsating sdB star in an HW Vir-type eclipsing binary

The sdB star PG 1336−018 is found to be a very short-period eclipsing binary system, remarkably similar to the previously unique system HW Vir. In addition, and unlike HW Vir, the sdB star in the PG 1336 system shows rapid oscillations of the type found in the recently discovered sdB pulsators, or EC 14026 stars. The orbital period, 0.101 0174 d, is one of the shortest known for a detached binary. Analysis of photoelectric and CCD photometry reveals pulsation periods near 184 and 141 s, with semi-amplitudes of ∼0.01 and ∼0.005 mag respectively. Both oscillations might have variable amplitude, and it is probable that other frequencies are present with amplitudes ∼0.003 mag or less. The 184- and 141-s pulsations are in the range of periods predicted by models for hot horizontal-branch stars. Analysis of medium-dispersion spectrograms yields T _eff=33 000±1000 K and log g =5.7±0.1 for the sdB primary star, a radial velocity semi-amplitude K ₁=78±3 km s⁻¹ and a system velocity γ=6±2 km s⁻¹. Spectrograms from the IUE Final Archive give T _eff=33 000±3000 K and E ( B − V )=0.05 for log g =6.0 models. The derived angular radius leads to a distance of 710±50 pc for the system, and an absolute magnitude for the sdB star of +4.1±0.2. A preliminary analysis of U , V and R light curves indicates the orbital inclination to be near 81° and the relative radii to be r ₁=0.19 and r ₂=0.205. Assuming the mass of the sdB primary to be 0.5 M⊙ leads to a mass ratio q =0.3 for the system, and indicates that the secondary is a late-type dwarf of type ∼M5. As with HW Vir, it is necessary to invoke small limb-darkening coefficients and high albedos for the secondary star to obtain reasonable fits to the observed light curves.     

HD 115781 and HD 116204—two RS CVn binaries

HD 115781 and HD 116204 (BL CVn and BM CVn) are shown to be RS CVn binaries with periods near 20 days. HD 115781 is double-lined; the primary type is about K1III, while the secondary is probably a late-type subgiant. The masses of the two components are equal within observational error. There is substantial photometric variability with a period half the orbital period; it is attributed to ellipsoidal variation. HD 116204 is also of type K1III. It shows exceptionally strong Ca II H and K emission, together with an emission-line spectrum typical of RS CVn stars in theIUE ultraviolet region, but Hα is an absorption line. The secondary star in the HD 116204 system has not been detected. The primary shows photometric variations, presumably due to starspots, with a period 5 per cent longer than the orbital period.     

Four‐colour photometry of EY Dra: A study of an ultra‐fast rotating active dM1‐2e star

We present more than 1000‐day long photometry of EY Draconis in BV (RI)_C passbands. The changes in the light curve are caused by the spottedness of the rotating surface. Modelling of the spotted surface shows that there are two large active regions present on the star on the opposite hemispheres. The evolution of the surface patterns suggests a flip‐flop phenomenon. Using Fourier analysis, we detect a rotation period of P_rot = 0.45875 d, and an activity cycle with P ≈ 350 d, similar to the 11‐year long cycle of the Sun. This cycle with its year‐long period is the shortest one ever detected on active stars. Two bright flares are also detected and analysed (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Composite spectra XIX: HD 208253, a long‐period binary whose hot secondary has enhanced strontium and barium

We separate and analyse the component spectra of the composite‐spectrum binary HD 208253. We find that the cool primary is an evolving star of spectral type G7 III, while its hot secondary is an early‐A dwarf. The giant is currently near the lowest point of the red‐giant branch and is slightly less luminous than its dwarf companion. We provide a set of precise radial‐velocity measurements for both stars. The double‐lined orbit which we derive from them shows that the component mass ratio is close to unity (q = 1.05 ± 0.01). We deduce the physical properties of both stars, determine their respective masses to be 2.75 ± 0.07 Me (giant) and 2.62 ± 0.07 Me (dwarf), and show that the orbit's inclination is within a degree or two of 68°. The spectrum of the A‐type component has quite component has quite narrow lines (we infer a rotational velocity of 18 km s^–1), though since the period of the orbit is well over 1 year that component cannot be in synchronous rotation. An intriguing property of the dwarf is its enhanced Sr and Ba, though it does not exhibit the other spectral peculiarities that would signal a classical Am star. While by no means unique amongst the multitude of oddities exhibited by A and early‐F stars, this dwarf which we have uncovered in a long‐period binary offers valuable constraints and challenges to stellar‐evolution theory. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Coordinated monitoring of the eccentric O-star binary Iota Orionis: optical spectroscopy and photometry

With the objective of investigating the windwind collision phenomenon and supporting contemporaneous X-ray observations, we have organized a large-scale, coordinated optical monitoring campaign of the massive, highly eccentric O9 III+B1 III binary Iota Orionis. Successfully separating the spectra of the components, we refine the orbital elements and confirm the rapid apsidal motion in the system. We also see strong interaction between the components during periastron passage and detect phase-locked variability in the spectrum of the secondary star. However, we find no unambiguous signs of the bow shock crashing on the surface of the secondary, despite the predictions of hydrodynamic simulations. Combining all available photometric data, we find rapid, phase-locked variations and model them numerically, thus restricting the orbital inclination to 50° i 70°.     

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.     

UBV photometry, UV spectroscopy and radio observations of the peculiar binary V Sagittae

We have performed high-speed UBV photometric observations on the peculiar binary V Sagittae. Using three new eclipse timings we update the orbital ephemeris and convert it to a dynamical time-scale (TDB). We also searched for quasi-periodic oscillations but did not detect them. Using the Wilson–Devinney algorithm we have modelled the light curve to find the stellar parameters of V Sge. We find that the system is a detached binary but that the primary star is very close to filling its Roche lobe, while the secondary star fills 90 per cent of its Roche lobe volume. We find temperatures of the primary and the secondary star to be T ₁=41 000 K and T ₂=22 000 K. We find i =72° and masses of 0.8 M_⊙ and 3.3 M_⊙ for the primary and secondary stars respectively. De-archived Hubble Space Telescope ( HST ) spectroscopy of V Sge shows evidence of mass loss via a wind or winds. In addition we report radio observations of V Sge during an optical high state at 2 cm, 3.6 cm and 6 cm wavelengths. The 3.6 cm emission is increased by a factor of more than six compared with an earlier detection in a previous optical high state.     

On the Physical Processes in Contact Binary Systems

Three importantphysical processes occurringin contact binarysystems are studied. The first one is the effect of spin, orbital rotation and tide on the structure of the components, which includes also the effect of meridian circulation on the mixing of the chemical elements in the components. The second one is the mass and energy exchange between the components. To describe the energy exchange, a new approach is introduced based on the understanding that the exchange is due to the release of the potential, kinetic and thermal energy of the exchanged mass. The third is the loss of mass and angular momentum through the outer Lagrangian point. The rate of mass loss and the angular momentum carried away by the lost mass are discussed. To show the effects of these processes, we follow the evolution of a binary system consisting of a 12M and a 5M star with mass exchange between the components and mass loss via the outer Lagrangian point, both with and without considering the effects of rotation and tide. The result shows that the effect of rotation and tide advances the start of the semi-detached and the contact phases, and delays the end of the hydrogen-burning phase of the primary. Furthermore, it can change not only the occurrence of mass and angular momentum loss via the outer Lagrangian point, but also the contact or semi-contact status of the system. Thus, this effect can result in the special phenomenon of short-term variations occurring over a slow increase of the orbital period. The occurrence of mass and angular momentum loss via the outer Lagrangian point can affect the orbital period of the system significantly, but this process can be influenced, even suppressed out by the effect of rotation and tide. The mass and energy exchange occurs in the common envelope. The net result of the mass exchange process is a mass transfer from the primary to the secondary during the whole contact phase.     

Supersonic wind accretion in massive close binary systems: a 3D SPH simulation of Cen X-3

In this paper we study the 3D SPH structure and dynamics of an accretion disc generated in a close binary system by supersonic wind accretion from a massive secondary on to a compact primary. The stellar masses and separation between the two components are characteristic of the Cen X-3 system: the secondary is a 19.1-M⊙ star not filling completely its Roche lobe, while the primary is a white dwarf or a neutron star of 1.4 M⊙.
An interesting result of our simulation is that, in a quasi-stationary state attained after ≃4 orbital periods, only about three-quarters of the particles released by the secondary penetrate the primary Roche lobe. The disc is remarkably elongated and thick, and consistent deviations from the 'standard model' of specific angular momentum and radial temperature distributions have been found. However, the most interesting result is that the azimuthal distribution of the radial Mach number shows oblique structures (spiral shocks), which persist from the outer edge to the inner regions, in contrast to the case of discs formed by the usual L1 accretion and even by wind accretion in much less massive systems.     

VZ CVn: a close binary system with γ Doradus-like variations

We present new photometric two-colour observations of the double-lined close binary star VZ CVn. Combining two data sets obtained in 1971–72 and in 2006, the physical parameters of both components were derived. After removing the light variations due to the eclipses and proximity effects, a periodic variation in the more massive component with a dominant period of 1.068 76 d could be detected in the first photometric data set. In accordance with its position on the Hertzsprung–Russell diagram and both its pulsation period and pulsation constant of 0.62 d, the primary component of VZ CVn should be an excellent γ Doradus candidate. The less massive secondary component seems to have smaller radius with respect to its mass. Both components appear to have lower luminosities with respect to their masses; hence, their radiative properties seem to be different. The evolutionary status of the components is also discussed.