Short-period Binary Stars Research Articles

COMMISSION 42: CLOSE BINARY STARS

Two meetings of interest to close binaries took place during the reporting period: A full day session on short-period binary stars – mostly CV's – (Milone et al. 2008) during the 2006 AAS Spring meeting in Calgary and the very broadly designed IAU Symposium No. 240 on Binary Stars as Critical Tools and Tests in Contemporary Astrophysics in Prague, 2006, with many papers on close binaries [Hartkopf et al. 2007]. In addition, the book by Eggleton (2006), which is a comprehensive summary of evolutionary processes in binary and multiple stars, was published.

Polarimetric Variations of Binary Stars. III. Periodic Polarimetric Variations of the Herbig A[CLC]e[/CLC]/B[CLC]e[/CLC] Star MWC 1080

We present polarimetric observations of a massive pre-main sequence short-period binary star of the Herbig Ae/Be type, MWC 1080. The mean polarization at 7660 A is 1.60% at 81.6 deg, or 0.6% at 139 deg if an estimate of the interstellar polarization is subtracted. The intrinsic polarization points to an asymmetric geometry of the circumstellar or circumbinary environment while the 139 deg intrinsic position angle traces the axis of symmetry of the system and is perpendicular to the position angle of the outflow cavity. The polarization and its position angle are clearly variable, at all wavelengths, and on time scales of hours, days, months, and years. Stochastic variability is accompanied by periodic variations caused by the orbital motion of the stars in their dusty environment. These periodic polarimetric variations are the first phased-locked ones detected for a pre-main sequence binary. The variations are not simply double-periodic (seen twice per orbit) but include single-periodic (seen once per orbit) and higher-order variations. The presence of single-periodic variations could be due to non equal mass stars, the presence of dust grains, an asymmetric configuration of the circumstellar or circumbinary material, or the eccentricity of the orbit. MWC 1080 is an eclipsing binary with primary and secondary eclipses occurring at phases 0.0 and 0.55. The signatures of the eclipses are seen in the polarimetric observations.

Gravitational wave emission by cataclysmic variables: numerical models of semi-detached binaries

Gravitational wave emission is considered to be the driving force for the evolution of short-period cataclysmic binary stars, making them a potential test for the validity of General Relativity. In spite of continuous refinements of the physical description, a 10% mismatch exists between the theoretical minimum period ($P_{\rm turn} \simeq 70$ min) and the short-period cut-off ($P_{\rm min} \simeq 80$ min) observed in the period distribution for cataclysmic variable binaries. A possible explanation for this mismatch was associated with the use of the Roche model. We here present a systematic comparison between self-consistent, numerically constructed sequences of hydrostatic models of binary stars and Roche models of semi-detached binaries. On the basis of our approach, we also derive a value for the minimum period of cataclysmic variable binaries. The results obtained through the comparison indicate that the Roche model is indeed very good, with deviations from the numerical solution which are of a few percent at most. Our results therefore suggest that additional sources of angular momentum loss or alternative explanations need to be considered in order to justify the mismatch.

Discovery of a planet orbiting a binary star system from gravitational microlensing

The properties of the recently discovered1,2 extrasolar planets were not anticipated by theoretical work on the formation of planetary systems, most models for which were developed to explain our Solar System. Indeed, the observational technique used to detect these planets (measurement of radial-velocity shifts in stellar spectral lines) do not yet have the sensitivity to detect planetary systems like our own3. Here we report observations and modelling of the gravitational microlensing event MACHO-97-BLG-41. We infer that the lens system consists of a planet of about 3 Jupiter masses orbiting a binary stellar system consisting of a late-K dwarf star and an M dwarf. The stars are separated by ∼1.8 astronomical units (1 AU is the Earth–Sun distance), and the planet is orbiting them at a distance of about 7 AU. We had expected to find first the microlensing signature of jovian planets around single stars, so this result suggests that such planets orbiting short-period binary stars may be common.

A new deep imaging survey of Planetary Nebulae with known binary central stars

Binarity in central stars is often thought to be the most likely explanation for the bipolar and axisymmetric shapes seen in planetary nebulae. Theoretical computations show that low mass stellar companions — possibly even planets, merged nuclei and long period systems will also produce bipolar-shaped nebulae. As such all known short period binary central stars are expected to have an axisymmetric nebula surrounding them.

A binary lensing event toward the LMC: Observations and dark matter implications

The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of $\tau_{meas} = 2.9 +1.4/-0.9 \times 10^{-7}$. This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: $\tau_{back} = 5.4\times 10^{-8}$, but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass $\sim 0.5 \msun$. One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved which allows us to estimate the distance to the lenses. If the source star is not a short period binary star, then we show that the lens system is very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.

Low Mass White Dwarfs in Binaries

The lowest mass white dwarfs that can have been produced by single star evolution in the lifetime of the Galaxy have masses of about 0.53 M⊙. There are however several white dwarfs known with significantly lower masses. Evolution in a binary provides a straightforward explanation as the star can lose its envelope before ever burning helium. The products are expected to be short-period binary stars. To test this we have looked at 15 such stars and have found that 8 of them are close binaries. Thus binary evolution does have a major role in the formation of low-mass white dwarfs. We discuss whether the non-detections could also be binary stars.

The discovery of a short-period double-degenerate binary star

We have found that the white dwarf PG 1101+364 is a double-lined white dwarf/white dwarf binary with an orbital period of 3.47 hours. PG 1101+364 is the shortest period detached double-degenerate yet found and gravitational radiation will cause it to merge in 2.5 X 10**9 years. PG 1101+364 has a mass ratio of 0.87 +/- 0.03. From the relative strengths of the narrow core of Halpha, the lighter and therefore larger star appears to contribute more of the light by an amount that indicates almost equal temperatures for the two stars. We chose to observe PG 1101+364 on the basis of its low spectroscopic mass (0.31 Msun), and the discovery of its binary nature confirms the belief that binary evolution is required to produce such low mass white dwarfs. The radial velocity semi-amplitudes (69.7 +/- 1.7 km/s and 80.3 +/- 1.6 km/s) and the spectroscopic mass show that we see the orbit of PG 1101+364 at a low inclination of ~ 25 deg.

QUANTITATIVE SPECTRAL TYPES FOR 19 ALGOL SECONDARIES

ABSTRACT Time-resolved spectra of 19 short-period Algol-type binary star systems obtained during total eclipse used to derive the temperature spectral class of the mass-losing secondary component. The spectral classifications employed a quantitative comparison of the strengths of absorption features in stars of known spectral class with those of the program stars. The luminosity spectral class cannot be determined from these data, so both main sequence and giant stars were used for the comparison. Our spectral types are compared with published types and found to be generally in good agreement, unless the published types are derived from the light curves. The photometrically-detemrined types are systematically later than our directly-determined types. This effect is shown to also exist in catalogs of Algol parameters.

Photoelectric photometry of the short-period binary AX draconis

In March-June 1986 we carried out photoelectric photometry of the short-period binary star AX Dra in Strömgren's uvby bands and in Hβ. Results of analysis show the binary to be a near-contact detached system with a photometric mass ratio q= 0.775±0.007. The spectral type of the main component is F1, while that of the secondary is probably K3. The orbital period is rather stable. The main component is probably at the evolutionary stage of having expanded to nearly filling its Roche lobe.

The evolution of binary stars into contact states

Good-quality empirical results on 62 short-period binary stars recently summarised by Hilditch & Bell (1987) and Hilditch, King & McFarlane (1988) are discussed in terms of evolutionary paths from detached to semi-detached and contact states. These data suggest two evolutionary paths to the contact binaries — from detached systems directly into contact to form initially shallow-contact systems, and via case A mass transfer to semi-detached states, thence to contact systems. These empirical results support previous arguments based on evolutionary models and less detailed observational data. Concern is expressed about the paucity of high-quality spectroscopic data, particularly for low-mass systems displaying EB-type light curves and the resultant limitations on analyses of those light curves. Such systems provide tests of evolution into contact for the first time, or of broken-contact phases for WUMa-type binaries. The crucial importance of long-term monitoring (decades) of times of minima as indicators of mass transfer rates amongst these interacting binaries is also noted.

The Evolution of Binary Stars into Contact States

AbstractGood-quality empirical results on 62 short-period binary stars recently summarised by Hilditch & Bell (1987) and Hilditch, King & McFarlane (1988) are discussed in terms of evolutionary paths from detached to semi-detached and contact states. These data suggest two evolutionary paths to the contact binaries - from detached systems directly into contact to form initially shallow-contact systems, and via case A mass transfer to semi-detached states, thence to contact systems. These empirical results support previous arguments based on evolutionary models and less detailed observational data.Concern is expressed about the paucity of high-quality spectroscopic data, particularly for low-mass systems displaying EB-type light curves and the resultant limitations on analyses of those light curves. Such systems provide tests of evolution into contact for the first time, or of broken-contact phases for WUMa-type binaries. The crucial importance of long-term monitoring (decades) of times of minima as indicators of mass transfer rates amongst these interacting binaries is also noted.

A 6 centimeter radio survey of short-period active binary stars

Thirteen binaries with periods in the range of 0.2-2.0 days have been observed at 6 cm wavelength with the VLA. Eight out of these thirteen systems were detected, of which seven are RS Canum Venaticorum systems and one is an Algol system, with observed fluxes in the range of 0.3-5.0 mJy. The individual characteristics of the detected sources are briefly discussed. As a group, relative to active binaries of longer orbital periods, the short-period active binaries have a slightly lower mean radio luminosity. There is also a clear correlation of high radio luminosity with high X-ray luminosity evident in these short-period systems, although a functional dependence cannot be determined from noncontemporary data.

A visible and infrared study of the eclipsing dwarf nova OY Carinae - I. The visible eclipses of the central object

This paper presents four visible light curves of the highly inclined, short-period cataclysmic binary star OY Carinae in quiescence. These light curves show that the red dwarf eclipses both its white dwarf companion and the accretion disc and hotspot, which originate from material transferred from the red dwarf to the white dwarf. An analysis of the eclipse of the white dwarf, under the assumption that its red dwarf companion fills its Roche lobe, gives the following properties of the system when combined with the observed radial velocity amplitude of the white dwarf: the inclination lies between 73.5° and 81°, the mass of the white dwarf lies between 0.4 M⊙ and 1.4 M⊙ and the red dwarf lies either on or slightly below the lower main-sequence mass–radius relation. This range of properties is somewhat larger than found in previous analyses, where the uncertainties in the poorly known semi-amplitude of the radial velocity of the white dwarf were not taken fully into account. The consequences of our findings are discussed in the light of current ideas about the evolution of cataclysmic variable stars.

Hard X-ray observations of white dwarf binary systems

THE ability of degenerate dwarfs, as opposed to neutron stars, to radiate at X-ray wavelengths has created much theoretical interest. Ariel 5 hard X-ray observations indicate that current theories seem inadequate to explain all aspects of the observations. Using the Imperial College hard X-ray scintillation telescope (ST) on Ariel 5, several short period (up to 12 h) binary star systems have been studied and are described here. This study was carried out after a very hard X-ray flux was detected from AM Herculis leading to a search for similar signals from optically similar star systems. The detector used consists of 8 cm2 of Csl crystal actively collimated to 8° FWHM and covers the energy range 26–1,200 keV. It is offset from the satellite spin axis, thereby enabling background subtraction using a modulation technique. The instrument and its operation are described fully elsewhere1.

HD 23631 : a short-period binary and early AM star.

HD 23631 : a short-period binary and early AM star.