Two centuries of study of Algol systems

The periodicity of the light variation of Algol, discovered just over 200 years ago, may be regarded as the beginning of the study of eclipsing binary systems, especially those of the Algol type. Such studies, however, gained no real momentum until Vogel, 100 years ago, demonstrated by spectroscopy that the binary hypothesis of Algol's light changes is, in its essentials, correct. Three elements were needed to give us our modern notions of evolution by mass-transfer, namely: (i) results of combined analysis of light-curves and velocity-curves, (ii) evidence of circumstellar matter within binary systems and (iii) the notion that at least one component of an Algol system was near the limit of dynamical stability. All three entered the literature within about a decade, approximately halfway through the second century of eclipsing-binary studies; but it is the computational and instrumental developments of the last 25 years that have made real progress possible. We still lack commensurate theoretical developments, and the whole question of the contribution of Algol systems to the development of the Galaxy has barely been considered.     

Algols as limits on binary evolution scenarios

Evolutionary scenarios must account for Algol binaries surviving their first phase of mass transfer. The outcome of this phase is dependent upon the rapidity of the initial mass transfer, which can be estimated by calculating the radial reponse of potential progenitors to mass loss. Limits on the donor's evolutionary state, and its companion mass, can be placed on systems which would transfer mass on a thermal or dynamical timescale. Slower mass transfer rates are necessary for the successful transition to an Algol. Considering 1.5 and 5.0 M_⊙ models, the former succeed in case A and Br systems, while the latter can do so only in case A systems. To evolve into an Algol binary, all systems seem to require initial mass ratios near one.     

Algols as limits on binary evolution scenarios

Evolutionary scenarios must account for Algol binaries surviving their first phase of mass transfer. The outcome of this phase is dependent upon the rapidity of the initial mass transfer, which can be estimated by calculating the radial reponse of potential progenitors to mass loss. Limits on the donor's evolutionary state, and its companion mass, can be placed on systems which would transfer mass on a thermal or dynamical timescale. Slower mass transfer rates are necessary for the successful transition to an Algol. Considering 1.5 and 5.0 M models, the former succeed in case A and Br systems, while the latter can do so only in case A systems. To evolve into an Algol binary, all systems seem to require initial mass ratios near one.     

Algol, Beta Lyrae, and W Serpentis: Some new results for three well studied eclipsing binaries

The properties of the eclipsing binaries Algol, Beta Lyrae, and W Serpentis are discussed and new results are presented. The physical properties of the components of Algol are now well determined. High resolution spectroscopy of the H-alpha feature by Richards et al. and by Gillet et al. and spectroscopy of the ultraviolet resonance lines with the International Ultraviolet Explorer satellite reveal hot gas around the BBV primary. Gas flows also have been detected apparently originating from the low mass, cooler secondary component and flowing toward the hotter star through the Lagrangian L1 point. Analysis of 6 years of multi-bandpass photoelectric photometry of Beta Lyrae indicates that systematic changes in light curves occur with a characteristic period of -275 ± 25 days. These changes may arise from pulsations of the B8II star or from changes in the geometry of the disk component. Hitherto unpublished u, v, b, y, and H-alpha index light curves of W Ser are presented and discussed. W Ser is a very complex binary system that undergoes complicated, large changes in its light curves. The physical properties of W Ser are only poorly known, but it probably contains one component at its Roche surface, rapidly transfering matter to a component which is embedded in a thick, opaque disk. In several respects, W Ser resembles an upscale version of a cataclysmic variable binary system.     

The relation of Algols and W Serpentis stars

Evidence on the issues of whether the W Serpentis stars are a coherent class, and how they may interface with the Algol systems, is reviewed, with emphasis on the idea that they are semi-detached systems in the latter part of the rapid phase of mass transfer, with optically and geometrically thick disks of transferred gas around the (now) more massive star. We are interested in what will be seen when the gas clears away, and mainly examine the idea that it will be an Algol-type system. More particularly, consideration is given to centrifugally limited accretion as a mechanism to build up a substantial disk, and the presumed evolutionary sequence is from a W Ser to a rapidly rotating Algol to a normal Algol system. Systems such as V367 Cyg and RW Tau fit into this scheme only with difficulty. Because it is extremely difficult to measure the rotation of some W Ser (mass) primaries, it is natural to look at the rotation statistics of Algols to test this idea. The badly behaved light curves and spectroscopy of some Algols (eg. U Cep, RZ Sct) may be attributable to the double contact condition, and the ramifications of this possibility are discussed. If so, the rotation statistics of Algols should show two spikes, corresponding to the two special conditions into which a system should be driven by tidal braking and centrifugally limited spinup. Present rotation statistics do show these spikes. Algols should flip between these states fairly quickly, depending on the mass transfer rate. Thus, to the extent that the meager statistics can be accepted as meaningful, the new (fourth) morphological type of close binary (double contact) has attained demonstrable reality. The rotation statistics are presented in terms of a particular rotation parameter, R, which is zero for synchronism and unity for the centrifugal limit. Future work should develop rotation statistics to see if the rotational lobe-filling (R = 1) spike persists. It should also look into whether W Ser primaries are on the hydrogen burning main sequence, or in general what they are. We also need more light curves of W Ser type systems, high resolution line profiles for the (mass) primaries (with particular attention to the W Ser-Algol transition cases), and spectroscopy of low inclination W Serpentis systems, such as KX And.     

The relation of Algols and W Serpentis stars

Evidence on the issues of whether the W Serpentis stars are a coherent class, and how they may interface with the Algol systems, is reviewed, with emphasis on the idea that they are semi-detached systems in the latter part of the rapid phase of mass transfer, with optically and geometrically thick disks of transferred gas around the (now) more massive star. We are interested in what will be seen when the gas clears away, and mainly examine the idea that it will be an Algol-type system. More particularly, consideration is given to centrifugally limited accretion as a mechanism to build up a substantial disk, and the presumed evolutionary sequence is from a W Ser to a rapidly rotating Algol to a normal Algol system. Systems such as V367 Cyg and RW Tau fit into this scheme only with difficulty. Because it is extremely difficult to measure the rotation of some W Ser (mass) primaries, it is natural to look at the rotation statistics of Algols to test this idea. The badly behaved light curves and spectroscopy of some Algols (eg. U Cep, RZ Sct) may be attributable to the double contact condition, and the ramifications of this possibility are discussed. If so, the rotation statistics of Algols should show two spikes, corresponding to the two special conditions into which a system should be driven by tidal braking and centrifugally limited spinup. Present rotation statistics do show these spikes. Algols should flip between these states fairly quickly, depending on the mass transfer rate. Thus, to the extent that the meager statistics can be accepted as meaningful, the new (fourth) morphological type of close binary (double contact) has attained demonstrable reality. The rotation statistics are presented in terms of a particular rotation parameter, R, which is zero for synchronism and unity for the centrifugal limit. Future work should develop rotation statistics to see if the rotational lobe-filling (R = 1) spike persists. It should also look into whether W Ser primaries are on the hydrogen burning main sequence, or in general what they are. We also need more light curves of W Ser type systems, high resolution line profiles for the (mass) primaries (with particular attention to the W Ser-Algol transition cases), and spectroscopy of low inclination W Serpentis systems, such as KX And.     

Reverse Algols

An Algol is a binary system having a semidetached configuration where the less massive component is in contact with the critical equipotential surface. A reverse Algol is a binary system having a semidetached configuration where the more massive component is in contact with the critical equipotential surface. In 1985, Leung suggested 5 reverse Algol systems at the Beijing Colloquium. Two more such systems have been discovered recently. The spectral types of these systems range from early B to mid G. There is also a wide spread in mass ratio among these systems. There appear to be two types of reverse Algols, hot and cool systems. The hot systems have their more massive components as the hotter stars and the cool systems their more massive components as the cooler stars. The mass-radius relation of the reverse Algols is very similar to that of the contact and near-contact systems. It is believed that reverse Algols represent the pre-mass-reversal semidetached phase of close binary evolution. Since selection effects apply to both the regular Algols and reversed Algols in a similar manner, the evolutionary time scale between them would be simply the ratio of the number of confirmed systems of these two types of Algols.     

Algol,Beta Lyrae,and W Serpentis: Some new results for three well studied eclipsing binaries

The properties of the eclipsing binaries Algol, Beta Lyrae, and W Serpentis are discussed and new results are presented. The physical properties of the components of Algol are now well determined. High resolution spectroscopy of the H-alpha feature by Richards et al. and by Gillet et al. and spectroscopy of the ultraviolet resonance lines with the International Ultraviolet Explorer satellite reveal hot gas around the BBV primary. Gas flows also have been detected apparently originating from the low mass, cooler secondary component and flowing toward the hotter star through the Lagrangian L1 point. Analysis of 6 years of multi-bandpass photoelectric photometry of Beta Lyrae indicates that systematic changes in light curves occur with a characteristic period of -275 ± 25 days. These changes may arise from pulsations of the B8II star or from changes in the geometry of the disk component. Hitherto unpublished u, v, b, y, and H-alpha index light curves of W Ser are presented and discussed. W Ser is a very complex binary system that undergoes complicated, large changes in its light curves. The physical properties of W Ser are only poorly known, but it probably contains one component at its Roche surface, rapidly transfering matter to a component which is embedded in a thick, opaque disk. In several respects, W Ser resembles an upscale version of a cataclysmic variable binary system.     

Polarizing gas at small optical depths around Algols

The origins of visible-band linear polarimetry of Algols and objects related to them are reviewed. It is pointed out, not for the first time, that the polarization signals of these systems can vary sporadically by significant amounts. The difficulty of evaluating the interstellar component of the observed polarization is discussed and these components are evaluated anew for each object studied in this paper. With one possible exception, the polarization signals intrinsic to these binaries derive from electron scattering. A well-defined model is applied to the constant and variable (but phase-locked) polarization signals. Limits to the mass associated with a scattering disk in each binary are derived. Within broad limits, concentrations of scattering mass within and above or below the orbital plane are also developed as are the centroid longitudes of these concentrations within the system. It is pointed out that very few measures of visible-band, circular polarization have been made but that cm-wavelength measures of Algol itself have been very informative.     

Polarizing gas at small optical depths around Algols

The origins of visible-band linear polarimetry of Algols and objects related to them are reviewed. It is pointed out, not for the first time, that the polarization signals of these systems can vary sporadically by significant amounts. The difficulty of evaluating the interstellar component of the observed polarization is discussed and these components are evaluated anew for each object studied in this paper. With one possible exception, the polarization signals intrinsic to these binaries derive from electron scattering. A well-defined model is applied to the constant and variable (but phase-locked) polarization signals. Limits to the mass associated with a scattering disk in each binary are derived. Within broad limits, concentrations of scattering mass within and above or below the orbital plane are also developed as are the centroid longitudes of these concentrations within the system. It is pointed out that very few measures of visible-band, circular polarization have been made but that cm-wavelength measures of Algol itself have been very informative.     

High signal-to-noise ratio — The spectroscopic key to Algol systems

The usefulness of high signal-to-noise-ratio spectra for both radial-velocity and abundance studies of Algol systems is emphasised. It is shown that division by a hot star is a worthwhile step in pursuit of this objective. A preliminary analysis of high signalto-noise-ratio, red and near-infrared, Reticon observations of R CMa shows that its primary has solar CNO abundances within the 0.3 dex observational error. The low-mass (0.17 m_⊙) secondary of this Algol system must have lost a large fraction of its original mass. Some of this material would have been extensively processed during the secondary's main-sequence lifetime and would therefore have had a highly non-solar CNO-abundance distribution. The lack of serious contamination of the primary's abundances is consistent with most, but not all, plausible mass-transfer scenarios.     

Circumstellar matter in Algols and Serpentids

A systematic search is reported for phenomena associated with circumstellar matter in interacting binary systems of the Algol and W Serpentis type. Ultraviolet emission lines have been detected by the author in 10 Algols and 6 Serpentids. No conspicuous difference exists between systems with disk accretion and those with direct stream impact. Evidence is indicated that the lines are formed predominantly by scattering in an induced stellar wind. Relative intensity of Fe II and Fe III emissions appears to be correlated with the spectral type of the gainer. It is suggested that in the Serpentids we see a thick disk nearly edge-on, but that the emission lines indicate the nature of the gainer inside.     

Circumstellar matter in Algols and Serpentids

A systematic search is reported for phenomena associated with circumstellar matter in interacting binary systems of the Algol and W Serpentis type. Ultraviolet emission lines have been detected by the author in 10 Algols and 6 Serpentids. No conspicuous difference exists between systems with disk accretion and those with direct stream impact. Evidence is indicated that the lines are formed predominantly by scattering in an induced stellar wind. Relative intensity of Fe II and Fe III emissions appears to be correlated with the spectral type of the gainer. It is suggested that in the Serpentids we see a thick disk nearly edge-on, but that the emission lines indicate the nature of the gainer inside.     

A new light synthesis optimization program. Test on simulated Algol data

Simulation of light curve effects in Algol systems that are near contact or that may evolve into contact requires a different model than the tri-axial ellipsoid used in many current studies. A new light synthesis optimization program, based on the Roche model, satisfies the model requirements and determines system parameters with excellent accuracy, given a sufficiently large set of observations. A widely used criterion for stopping iterative solutions is inadequate. A working criterion is proposed as a replacement.     

A new light synthesis optimization program. Test on simulated Algol data

Simulation of light curve effects in Algol systems that are near contact or that may evolve into contact requires a different model than the tri-axial ellipsoid used in many current studies. A new light synthesis optimization program, based on the Roche model, satisfies the model requirements and determines system parameters with excellent accuracy, given a sufficiently large set of observations.A widely used criterion for stopping iterative solutions is inadequate. A working criterion is proposed as a replacement.     

Time-resolved spectroscopy of accretion disks in Algols

Time-resolved spectroscopy during the eclipse of short-period Algol systems, has shown their accretion disks to be small, turbulent structures with non-Keplerian velocity fields and asymmetries between the leading and trailing sides of the disk. These transient disks are produced by the impact of the gas stream on the mass-gaining star, and occur in systems where the star is just large enough to ensure the stream collision is complete. These emission line disks and the excess continuum emission do not always occur together. The permanent accretion disks in at least a few of the long-period Algol systems have features in common with the transient disks including non-Keplerian velocity fields.     

Time-resolved spectroscopy of accretion disks in Algols

Time-resolved spectroscopy during the eclipse of short-period Algol systems, has shown their accretion disks to be small, turbulent structures with non-Keplerian velocity fields and asymmetries between the leading and trailing sides of the disk. These transient disks are produced by the impact of the gas stream on the mass-gaining star, and occur in systems where the star is just large enough to ensure the stream collision is complete. These emission line disks and the excess continuum emission do not always occur together. The permanent accretion disks in at least a few of the long-period Algol systems have features in common with the transient disks including non-Keplerian velocity fields.     

Determining rotation rates from light curves: RW Mon and RW Tau

We present light curve solutions for the non-synchronously rotating Algols RW Mon and RW Tau, and we illustrate how rotation rates are determined from light curves. We find RW Mon's primary component to spin at about 5 times the synchronous rate, which confirms the indication of fast rotation from reported emission line activity. RW Tau turns out to be only a mildly rapidly rotating Algol system, and our light curve solutions do not yield any firm value for the rotation rate of the primary component. It is suggested that continued efforts should be made to do good quality line broadening studies in order to find rotation rates for systems with only modest degrees of rapid rotation, and in order to further test photometric rotation rates against those of line broadening studies.     

The evolution of moderately close and moderately wide binaries

We discuss evolutionary processes in binaries where the primary becomes a red giant with a deep convective envelope before it fills its Roche lobe. Such binaries (late Case B or late Case C, if they evolve conservatively) ought to suffer drastic mass transfer, on a hydrodynamic timescale. In some circumstances this may lead to a common envelope, spiral-in, and finally either a very short-period binary or coalescence. But there appear to be other circumstances in which the outcome is an ordinary Algol, or a wide binary with a white dwarf companion as in Barium stars and some symbiotics. We try to demonstrate that stellar-wind mass loss, enhanced one or two orders of magnitude by tidal interaction with a companion, can vitally affect the approach to RLOF, and indeed may prevent RLOF in binaries with periods over 1000 d. Such mass loss is probably accompanied by angular momentum loss, by magnetic braking combined with tidal friction. The result is that it will not be easy to predict definitively the outcome of evolution in a given zero-age binary.