Algol Systems Research Articles

Indirect imaging of an accretion disk rim in the long-period interacting binary W Crucis

Light curves of the long-period Algols are known for their complex shape (asymmetry in the eclipse, light variations outside eclipse, changes from cycle-to-cycle), and their interpretation is not possible in the standard model of binary stars. Complex structures present in these active Algol systems could be studied with the eclipse-mapping method which was successfully applied to the new 7-color photometric observations in the Geneva system of W Cru, belonging to the isolated group of these active Algols. Several cycles of this long-period (198.5 days) eclipsing binary have been covered by observations. We have used a modified Rutten's approach to the eclipse-mapping. The optimization of the system's parameters and the recovery of the disk intensity distribution are performed using a genetic algorithm (GA). It is found that a hot component is hidden in the thick accretion disk which confirms previous findings. The mass of the component, M1 = 8.2 Ms indicates that it is a mid-B type star. The mass-losing component is filling its critical lobe which means it is a G-type supergiant with a mass M2 = 1.6 Ms. The disk is geometrically very extended and its outer radius is about 80% of the primary's critical lobe. A reconstructed image reveals a rather clumpy and nonuniform brightness distribution of an accretion disk rim in this almost edge-on seen system. This clumpyness accounts for light curve distortions and asymmetries, as well as for secular changes.

The Algol System XX Cephei

We present new five-color photometric and spectroscopic observations of the Algol system XX Cephei (V = 9.13, P = 2.3 days). This system is known to have a variable period. From analysis of both the period variations and the light curve we found no support for apsidal motion. However, there is a possible light-time effect. The period changes could also be episodic. Our spectroscopic analysis yielded a classification of A4. We found no evidence of features attributable to the cooler component or of active mass transfer. Our photometric solution, using the Wilson-Devinney modeling program, showed the system to be an Algol configuration with a mass ratio of 0.17. We determined the masses and radii, in solar units, to be M1 = 1.92, M2 = 0.33, R1 = 2.07, and R2 = 2.32, and the temperatures to be T1 = 8500 K and T2 = 4555 K. From the parallax given in the Hipparcos catalog, neglecting reddening, absolute magnitudes of MV = 1.73 and 4.43 were determined for the primary and secondary components, respectively.

Synthetic Spectra of Cool, Roche Lobe-filling Stars in Close Binary Systems

We present the details of LinBrod, a program that calculates synthetic spectra of cool Roche lobe-filling stars in close binary systems. The program has two modes of operation. In the primary mode it calculates the spectra by adding wavelength-dependent, velocity-shifted specific intensities from the distorted and gravity-darkened surface of the lobe-filling star. The wavelength-dependent specific intensities are calculated externally using the ATLAS9 stellar atmosphere program and a modified version of the MOOG spectrum synthesis program. In its secondary mode LinBrod calculates phase-dependent line-broadening functions that can be convolved with the spectra of nonrotating single stars to yield approximate synthetic spectra. We use the line-broadening functions to isolate and assess the effects of the physical processes that broaden the absorption lines in the spectra of lobe-filling stars. The synthetic spectra can be used to extract radial velocities, mass ratios, and chemical abundances from observed spectra of lobe-filling stars. Originally written to analyze observations of the secondary stars in X-ray binaries containing black holes, the program can also be used to analyze observations of the secondary stars in Algol systems, cataclysmic variables, and low-mass X-ray binaries containing neutron stars.

The Case for Third Bodies as the Cause of Period Changes in Selected Algol Systems

Many eclipsing binary star systems show long-term variations in their orbital periods, evident in their O - C (observed minus calculated period) diagrams. With data from the Robotic Optical Transient Search Experiment (ROTSE-I) compiled in the SkyDOT database, New Mexico State University 1 m data, and recent American Association of Variable Star Observers (AAVSO) data, we revisit Borkovits and Hegedus's best-case candidates for third-body effects in eclipsing binaries: AB And, TV Cas, XX Cep, and AK Her. We also examine the possibility of a third body orbiting Y Cam. Our new data support their suggestion that a third body is present in all systems except AK Her, as is revealed by the sinusoidal variations of the O - C residuals. Our new data suggest that a third body alone cannot explain the variations seen in the O - C residuals of AK Her. We also provide a table of 143 eclipsing binary systems that have historical AAVSO O - C data with new values computed from the SkyDOT database.

Optical Photometry and Spectroscopy of the Suspected "Cool Algol" AV Delphini: Determination of the Physical Properties

We present new spectroscopic and BVRI photometric observations of the double-lined eclipsing binary AV Del (period ¼ 3:85 days) conducted over six observing seasons. A detailed radial velocity and light-curve analysis of the optical data shows the system to be most likely semidetached, with the less massive and cooler star filling its Roche lobe. The system is probably a member of the rare class of ‘‘cool Algol’’ systems, which are distinguished from the ‘‘classical’’ Algol systems in that the mass-gaining component is also a late-type star rather than a B- or A-type star. By combining the spectroscopic and photometric analyses, we derive accurate absolute masses for the

Effects of chemical composition and thermohaline mixing on the accreting components for low-mass close binaries: application to blue stragglers

Blue stragglers (BSs) are important objects in cluster populations because of their peculiar properties. The colours and magnitudes of these objects are critical parameters in the population synthesis of the host cluster and may depend remarkably on the surface composition of BSs. Observations show that some BSs are short-orbital-period binaries, which may be accounted for by mass transfer in low-mass binaries. We have therefore studied the effects of surface composition and thermohaline mixing, caused by secular instability, on the accreting components for low-mass binaries, and we have applied the results to a short-orbital-period BS F190 in the old cluster M67. We examine thermohaline mixing in a low-mass accreting main-sequence star and find that, except for the redistribution of composition under the surface, the mixing affects the accretor very little during Roche lobe overflow unless thermohaline mixing is treated as an instantaneous process. A series of calculations is then carried out for low-mass binaries under different assumptions. The results indicate no distinction in surface composition between the models with and without thermohaline mixing during Roche lobe overflow, but we still see the divergences of evolutionary tracks on the Hertzsprung-Russell and colour-magnitude diagrams. The change of surface composition makes the gainer bluer and smaller than those with original surface composition, while thermohaline mixing lessens the effect slightly. If thermohaline mixing were to act instantaneously, the effect would be lessened more. Our calculation shows that case A and case B mass transfer may produce BSs in short- or relatively short-orbital-period binaries (including Algol systems), and that CNO abundance abnormalities could be observed in these products. This is consistent with the results of Monte Carlo simulations by previous studies. Our simulation of F190 shows that the primary's mass M-1i of the appropriate models is located in the range of 1.40-1.45 M-circle dot with initial mass ratio q(i) = 1.5 and initial orbital period P-i = 0.8 d, indicating that case A is a more likely evolutionary channel than case B to form this object. The simulation also shows that it is very likely that F190 is still in a slow stage of mass transfer. As a consequence, obvious CNO abundance abnormalities should be observed for the object.

Evidence for Superhumps in the Radio Light Curve of Algol and a New Model for Magnetic Activity in Algol Systems

Extensive radio data of two Algol systems and two RS CVn binaries were re-analyzed. We found evidence for a new periodicity that we interpret as a superhump in Algol, in which it may have been expected according to its semi-detached nature and low binary mass ratio. This is the first detection of the superhump phenomenon in the radio and the first observation of superhumps in Algol systems. According to our result, the accretion disk in Algol precesses in spite of its non-Keplerian nature and therefore this phenomenon is not restricted to the classical Keplerian disks in compact binaries.We propose that in Algol systems with short orbital periods, the disk is magnetically active as well as the secondary star. The magnetic field in the disk originates from amplification of the seed field in the magnetized material transferred from the secondary. The disk and stellar fields interact with each other, with reconnection of the field lines causing flares and particle acceleration. Relativistic particles are trapped in the field and directed toward the polar regions of the secondary star because of the dipole structure of its magnetic field. Our proposed model for the magnetic activity in Algol systems provides a simple explanation to the observed properties of Algol in the radio wavelengths, and to the presence of quiescent gyrosynchrotron emission near the polar region of the secondary star, where electrons are difficult to be confined if the field lines are open as in normal single magnetic stars. We propose that the superhump variation in the radio is generated by enhanced reconnection when the elongated side of the elliptic disk is the closest to the cool star. This leads to flares and enhancement in particle acceleration and is manifested as stronger gyrosynchrotron radiation.

Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei

We present an investigation of the Hα emission-line variations observed in the massive Algol binary RY Per. We give new radial velocity data for the secondary based on our optical spectra and for the primary based on high-dispersion UV spectra. We present revised orbital elements and an estimate of the primary's projected rotational velocity (which indicates that the primary is rotating 7 times faster than the synchronous rate). We use a Doppler tomography algorithm to reconstruct the individual primary and secondary spectra in the region of Hα, and we subtract the latter from each of our observations to obtain profiles of the primary and its disk alone. Our Hα observations of RY Per show that the mass-gaining primary is surrounded by a persistent but time-variable accretion disk. The profile that is observed outside eclipse has weak double-peaked emission flanking a deep central absorption, and we find that these properties can be reproduced by a disk model that includes the absorption of photospheric light by the band of the disk seen in projection against the face of the star. We developed a new method to reconstruct the disk surface density distribution from the ensemble of Hα profiles observed around the orbit, and this method accounts for the effects of disk occultation by the stellar components, the obscuration of the primary by the disk, and flux contributions from optically thick disk elements. The resulting surface density distribution is elongated along the axis joining the stars in the same way as seen in hydrodynamical simulations of gas flows that strike the mass gainer near the trailing edge of the star. This type of gas stream configuration is optimal for the transfer of angular momentum, and we show that rapid rotation is found in other Algol systems that have passed through a similar stage.

Doppler tomography of Algols

AbstractThe technique of Doppler tomography has been influential in the study of mass transfer in Algol‐type interacting binaries. The Algols contain a hot blue dwarf star with a magnetically‐active late‐type companion. In the close Algols, the gas stream flows directly into the photosphere of the blue mass‐gaining star because it does not have enough room to avoid impact with that star. Doppler tomograms of the Algols have been produced from over 2500 time‐resolved spectra at wavelengths corresponding to Hα, Hβ, He I (6678 Å), Si II (6371 Å) and Si IV (1394 ° A). These tomograms display images of accretion structures that include a gas stream, accretion annulus, accretion disk, stream‐star impact region, and occasionally a source of chromospheric emission associated with the cool, mass‐losing companion. Some Algol systems alternate between streamlike and disk‐like states, and provide direct evidence of active mass transfer within the Algols. This work produced the very first images of the gas stream for the entire class of interacting binaries, and demonstrated that the Algols are far more active than formerly believed, with variability on time scales of weeks to months. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Periodicities of Radio Flares in RS CVn and Algol Binaries

Periodicities of radio flaring activity from the Algol systems β Per and δ Lib and the RS CVn systems V711 Tau and UX Ari were determined from a continuous 5-year survey. The radio continuum fluxes at 2.3 GHz and 8.3 GHz were monitored with the NRAO—Green Bank Interferometer for 2096 days from 1995 January to 2000 October. The maximum detected flare strengths at 8.3 GHz were 1.17 Jy for β Per, 0.034 Jy for δ Lib, 1.44 Jy for V711 Tau, and 0.82 Jy for UX Ari. Power Spectrum Analysis and Phase Dispersion Minimization were used to determine the periodicity of flaring activity in each binary. The strongest periodicities found were 48.9 days for β Per, 120.7 days for V711 Tau, and 141.4 days for UX Ari. No significant periodicities were found for δ Lib. The continuous survey has demonstrated that there are active and quiescent flaring cycles in V711 Tau and β Per.

Cyclic Period Changes in the Algol WW Cygni

AbstractYear to decade-long cyclic period changes have been observed in many classes of close binaries. The Algol binary WW Cygni shows a cyclic change in its orbital period with an amplitude of slightly more than 0.02 days and a period of 56 years. A hypothetical third or fourth body does not satisfactorily explain the observed variation in the orbital period. The change in luminosity and color of the system at primary eclipse minimum are in agreement with the model proposed by Applegate for a magnetic cycle induced period change in WW Cygni. We have commenced monitoring 9 close binaries for evidence of the luminosity and color changes consistent with the magnetic cycle hypothesis. δ Librae is suggested as a case suitable for observation with an optical interferometer to test the third body proposed for this Algol system.

The eclipsing binary star RZ Cas

In a first report on the results of a multi-site campaign in 2001 of photometric and spectroscopic observations of the active semi-detached Algol-type system RZ Cas, we concentrate on the radial velocity (RV) variations. Using weak absorption lines we obtain an improved orbital solution for both components. In the orbital RV curve we observe a strong, asymmetric rotation effect. For the first time we detect rapid spectroscopic multi-mode pulsations in an Algol system. Whereas the photometrically observed oscillations were dominated by monoperiodic pulsations at frequency 64.19 c d - 1 until the year 2000, we find in 2001 a multiperiodic behaviour with two dominant frequencies of f 1 = 56.600 c d - 1 and f 2 = 64.189 c d - 1 . Both modes show amplitude variations over the orbital phase with a a minimum at orbital phases Φ = 0.6-0.8 and a maximum just after the primary minimum (f 2 ) and at Φ 0.25 (f 1 ). The different shape of amplitude modulation of the f 1 and f 2 modes points to different (l, m) mode structures. The modulation itself can be explained by assuming a variable extinction due to gas streams and an inhomogeneous accretion annulus that weakens the light from different regions of the primary depending on its orbital position. This assumption is well supported by the gas density distribution obtained in preliminary hydrodynamic simulations. We found strong variations and cycle-to-cycle variable shapes of the orbital RV-curves of Balmer lines that have maximum magnitude in the Ha line indicating a strong variability of mass-transfer rates and a non-stationary circumbinary envelope.

Physical parameters of the Algol system CF Puppis from simultaneous analysis of Geneva 7-colour light curves

The semi-detached eclipsing binary system CF Puppis ( P=7.65 days) has been analysed using the Wilson–Devinney program. Light curves have been obtained in the G eneva 7-colour photometric system, and radial velocity curves for both components have been measured with the cross-correlation technique. The physical and orbital parameters have been determined through a self-consistent simultaneous solution of light curves in seven colours and of the radial velocity curves of both components. The absolute elements of the components are, for the primary (mass gainer), M 1=1.84±0.04 M ⊙, R 1=3.06±0.01 R ⊙, M bol 1 =1.74±0.02, T eff 1 =6671±27 K, and for the secondary (mass loser), M 2=0.38±0.01 M ⊙, R 2=5.39 R ⊙, M bol 2 =2.51±0.02, T eff 2 =4202±17 K. The semi-major axis A of the relative orbit is 21.28±0.15 R ⊙. The estimated spectral types of the components are about F3 IV (primary) and K2/3 III. CF Pup has one of the coolest component A among the Algol systems. The equatorial rotational velocities are 24.8 (primary) and 37.0 km s −1. The secondary is clearly synchronized, and the primary is nearly synchronized ( v syn=20.2 km s −1). The distance to CF Pup is evaluated to 506 ± 16 pc, and the colour excess E[B2-V1] to 0.033 ± 0.017.

Optical Photometry and X-Ray Monitoring of the "Cool Algol" BD +05706: Determination of the Physical Properties

We present new photometric observations in the BVRI bands of the double-lined eclipsing binary BD +05°706 conducted over three observing seasons, as well as new X-ray observations obtained with ROSAT covering a full orbital cycle (P = 18.9 days). A detailed light-curve analysis of the optical data shows the system to be semidetached, confirming indications from an earlier analysis by Torres et al. (published in 1998), with the less massive and cooler star filling its Roche lobe. The system is a member of the rare class of cool Algol systems, which are different from the classical Algol systems in that the mass-gaining component is also a late-type star rather than a B- or A-type star. By combining the new photometry with a reanalysis of the spectroscopic observations reported by Torres et al., we derive accurate absolute masses for the components of M1 = 2.633 ± 0.028 M⊙ and M2 = 0.5412 ± 0.0093 M⊙, radii of R1 = 7.55 ± 0.20 R⊙ and R2 = 11.02 ± 0.21 R⊙, as well as effective temperatures of 5000 ± 100 and 4640 ± 150 K, for the primary and secondary, respectively. There are obvious signs of activity (spottedness) in the optical light curve of the binary. Our X-ray light curve clearly shows the primary eclipse but not the secondary eclipse, suggesting that the primary star is the dominant source of the activity in the system. The depth and duration of the eclipse allow us to infer some of the properties of the X-ray–emitting region around that star.

Double-periodic blue variables in the Magellanic Clouds

We report the discovery, based on an inspection of the OGLE-II database, of a group of blue variables in the Magellanic Clouds showing simultaneously two kinds of photometric variability: a short-term cyclic variability with typical amplitude mag and period P1 between 4 and 16 days and a sinusoidal, long-term cyclic oscillation with much larger amplitude mag with period P2 in the range of 150–1000 days. We find that both periods seems to be coupled through the relationship . In general, the short term variability is reminiscent of those shown by Algol-type binaries. We propose that the long-term oscillation could arise in the precession of a elliptical disc fed by a Roche-lobe filling companion in a low mass ratio Algol system.

Infrared light curves and absolute stellar parameters of the Algol system Librae: is Librae really an overmassive Algol binary?

We present the first infrared (IR) light curves in the J, H and K bands of δ Lib obtained at different observing runs from 1994 to 1998. These IR light curves, showing a secondary minimum much deeper than in visual bands, have been analysed in order to determine a new set of physical parameters. A new program using the Roche geometry, and stellar surface brightness from ATLAS model atmospheres, has been used in the analysis and allows us to determine the physical parameters and the distance of the binary. We compare these results with the Hipparcos parallax, with the spectroscopic masses of the components derived from the radial velocity curves, and with evolutionary stellar model predictions. The adoption of a mass M1≃ 2.85 M⊙ for the primary star would remove the discrepancies between our results and the Hipparcos distance, and the unusual location of the δ Lib components in the Teff− log g diagram. Our photometric analysis suggests that the stellar masses in δ Lib could be overestimated, casting doubt on the claim that the primary component of the system is overmassive and oversized compared with single main-sequence stars of similar masses.

Spectroscopic Binaries in a Sample of [ITAL]ROSAT[/ITAL] X-Ray Sources South of the Taurus Molecular Clouds

We report the results of our radial velocity monitoring of spectroscopic binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south of the Taurus-Auriga star-forming region. The original sample of ~120 sources by Neuhauser et al. was selected on the basis of their X-ray properties and the visual magnitude of the nearest optical counterpart, in such a way as to promote the inclusion of young objects. Roughly 20% of those sources have previously been confirmed to be very young. We focus here on the subset of the original sample that shows variable radial velocities (43 objects), a few of which have also been flagged previously as being young. New spectroscopic orbits are presented for 42 of those systems. Two of the binaries, RX J0528.9+1046 and RX J0529.3+1210, are indeed weak-lined T Tauri stars likely to be associated with the λ Ori region. Most of the other binaries are active objects of the RS Canum Venaticorum type, including several W Ursae Majoris and Algol systems. We detect a strong excess of short-period binaries compared with the field and an unusually large fraction of double-lined systems. This, along with the overall high frequency of binaries out of the original sample of ~120 sources, can be understood as a selection effect, since all these properties tend to favor the inclusion of the objects in a flux-limited X-ray survey such as this by making them brighter in X-rays. A short description of the physical properties of each binary is provided, and a comparison with evolutionary tracks is made using the stellar density as a distance-independent measure of evolution. We rely for this on our new determinations of the effective temperature and projected rotational velocities of all visible components of the binaries. A number of the systems merit follow-up observations, including at least four confirmed or probable eclipsing binaries. One of these, RX J0239.1-1028, consists of a pair of detached K dwarfs and may provide for a potentially important test of stellar evolution models once the absolute dimensions of the components are determined.

Optical/infrared spectroscopy and photometry of the short-period binary RX J1914+24

We present observations of the proposed double-degenerate polar RX J1914+24. Our optical and infrared spectra show no emission lines. This, coupled with the lack of significant levels of polarization, provides difficulties for a double-degenerate polar interpretation. Although we still regard the double-degenerate polar model as feasible, we have explored alternative scenarios for RX J1914+24. These include a double-degenerate Algol system, a neutron star-white dwarf pair and an electrically powered system. The latter model is particularly attractive since it accounts naturally for the lack of both emission lines and detectable polarization in RX J1914+24. The observed X-ray luminosity is consistent with the predicted power output. If true, then RX J1914+24 would be the first known stellar binary system radiating largely by electrical energy.

Variable Stars in the Field of the Globular Cluster NGC 3201

We report on the discovery and analysis of 14 short-period variable stars in the field of the southern globular cluster NGC 3201, located within roughly 2 mag on either side of the main-sequence turnoff. Eleven of these variable stars are eclipsing binaries, one is an RR Lyrae, and two are thus far unclassified systems. Among the eclipsing binary stars, nine are of the W Ursae Majoris (W UMa) type, one an Algol (EA) system, and one a detached system. Using spectroscopic follow-up observations, as well as analysis of the variables' locations in the color-magnitude diagram of the cluster, we find that only one variable star (a W UMa type blue straggler) is actually a member of NGC 3201. We present the phased photometry light curves for all the variable star systems, as well as their locations in the field of view and in the color-magnitude diagram.

The Interacting Binary Be Star HR 2142

AbstractCurrent information on the nature of the HR 2142 system is presented. The circumstellar material (CSM) including the mass flow toward and away from the mass-gaining B star has been studied from ∼ 100 IUESWP HIRES images obtained from 1979–95 and ground-based CCD data from KPNO acquired between 1985–2001. The strength and velocity behavior of the infall components to the UV Si ɪɪ lines seen during the primary shell phase (PSP) from Φ ∼ 0.70−0.98 resembles that observed in conventional Algol systems, but the inferred mass infall rate is too small to account for the massive Hα-emitting disk about the B primary. The cause for the mass outflow observed during the secondary shell phase (SSP) remains unknown, but a localized plasma that has been identified from phase-dependent variations in the emission component to Heɪ λ6678 may be part of the SSP phenomenon. Although the secondary remains undetected in the optical/IR spectral regions, UV data currently do not yield strong support for the presence of an O subdwarf as in the Φ Per system. HR 2142 may represent and earlier evolutionary stage of Φ Per and a possibly similar Be binary 59 Cyg.