Algol-type Binaries Research Articles

Reverse Algols and hydrogen-rich Wolf-Rayet stars from very massive binaries

Massive star feedback affects the evolution of galaxies, where the most massive stars may have the largest impact. The majority of massive stars are born as members of close binary systems. In this work, we investigated detailed evolutionary models of very massive binaries (30−90 M⊙) with Large Magellanic Cloud (LMC) metallicity. We identify four effects defying the conventional knowledge of binary evolution, which are all related to the proximity of the models to the Eddington limit. We find that the majority of systems undergo mass transfer during core hydrogen burning. During the ensuing nuclear timescale evolution, many mass donors remain more massive than their companions (‘reverse Algols’), and nuclear timescale mass transfer may be interrupted or absent altogether. Furthermore, due to the elevated luminosity-to-mass ratio, many of the core-hydrogen-burning donors may develop Wolf-Rayet-type winds at luminosities where single stars would not. We identify observational counterparts of very massive reverse Algol binaries in the LMC and discuss their contribution to the observed hydrogen-rich Wolf-Rayet stars. We argue that understanding very massive Algol systems is key to predicting the advanced evolution of very massive binaries, including their ability to evolve into observable gravitational wave sources.

Semi-detached Algol type binaries with increasing orbital periods: Case studies of BD Vir, BO Gem, DG Lac, RW Cet, and RX Hya

This paper presents results from photometric investigations of five semi-detached binaries. Using the TESS and KEPLER (K2) databases, light curves of classical Algol systems BD Vir, BO Gem, DG Lac, RW Cet, and RX Hya were performed in detail. As a result of light curve analysis, the basic astrophysical parameters of the systems were estimated. The masses and radii of the primary components were estimated as 1.95(20) M⊙, 2.22(22) M⊙, 1.95(20) M⊙, 1.95(20) M⊙, 1.75(18) M⊙ and 2.29(3) R⊙, 2.21(2) R⊙, 2.09(1) R⊙, 1.86(2) R⊙, 1.79(3) R⊙ for BD Vir, BO Gem, DG Lac, RW Cet, and RX Hya, respectively. Likewise, the masses and radii of the secondary components were estimated as 0.45(4) M⊙, 0.47(4) M⊙, 0.26(2) M⊙, 0.50(5) M⊙, 0.47(2) M⊙ and 2.94(3) R⊙, 3.90(2) R⊙, 2.17(1) R⊙, 1.35(2) R⊙, 2.57(4) R⊙ for BD Vir, BO Gem, DG Lac, RW Cet, and RX Hya, respectively. It was determined that the orbital periods of the BD Vir, BO Gem, DG Lac, RW Cet, and RX Hya increased with rates of 8.4(3), 24.2(8), 8.1(3), 1.3(1) and 12.5(6) s century−1, respectively. In addition, sinusoidal changes were determined in the orbital period analysis and it was discussed that these sinusoidal changes could be caused by possible component(s) or magnetic activity around the binary. The positions of the components of BD Vir, BO Gem, DG Lac, RW Cet, and RX Hya in logTeff−logL and logM−logR diagrams are discussed.

Revisiting the high-mass transfer close binary star system AU Monocerotis

Context. AU Monocerotis is an eclipsing, double-lined spectroscopic binary with a period of 11 days that is in a state of extreme mass transfer, consisting of a main sequence B-type embedded in a thick accretion disk fed by a Roche lobe overflowing evolved G-type companion. It is also one of the double periodic variable Algol-type binaries. Aims. Our aim is to study the accretion environment and the origin of the long cycle in the system. We present revised properties of the gainer by including contributions from the accretion disk and its boundary layer, because the absorption lines used in previous works to estimate the parameters were contaminated by the disk absorption. Methods. We performed a multiwavelength spectroscopic study using archival high-resolution IUE ultraviolet (1200–3200 Å) spectra and optical spectra (from about 3700–9000 Å) from FEROS, HARPS, and SOPHIE. Results. Using the optical He I lines and the UV Si III, C II, Si IV lines, we derived new parameters for the temperature, gravity, and rotational velocity of the B star. The IUE spectra delineate a stratified environment around the gainer, with spectral lines such as O I, Mg II, Al II, and Si II formed in the outer accretion disk and a pseudo-photospheric boundary layer that alters the spectrum. Phase-limited discrete outflows, detected in the time-dependent absorption, trace the stream impact site and the disturbance it creates downstream in the disk. The long-term variability is due to changes in the accretion disk structure and circumstellar environment. Enhanced systemic mass outflow is observed at long cycle maximum, reaching at least 1000 km s−1. Conclusions. These results highlight the complex interplay between physical mechanisms that regulate the evolution of strongly interacting mass-exchanging binary stars.

Detailed models of interacting short-period massive binary stars

Context. The majority of massive stars are part of binary systems. In about a quarter of these, the companions are so close that mass transfer occurs while they undergo core hydrogen burning, first on the thermal and then on the nuclear timescale. The nuclear timescale mass transfer leads to observational counterparts: the semi-detached so-called massive Algol binaries. These systems may provide urgently needed tests of the physics of mass transfer. However, comprehensive model predictions for these systems are sparse. Aims. We use a large grid of detailed evolutionary models of short-period massive binaries and follow-up population synthesis calculations to derive probability distributions of the observable properties of massive Algols and their descendants. Methods. Our results are based on ∼10 000 binary model sequences calculated with the stellar evolution code MESA, using a metallicity suitable for the Large Magellanic Cloud (LMC), covering initial donor masses between 10 M⊙ and 40 M⊙ and initial orbital periods above 1.4 d. These models include internal differential rotation and magnetic angular momentum transport, non-conservative mass and angular momentum transfer between the binary components, and time-dependent tidal coupling. Results. Our models imply ∼30, or ∼3% of the ∼1000, core hydrogen burning O-star binaries in the LMC to be currently in the semi-detached phase. Our donor models are up to 25 times more luminous than single stars of an identical mass and effective temperature, which agrees with the observed Algols. A comparison of our models with the observed orbital periods and mass ratios implies rather conservative mass transfer in some systems, while a very inefficient one in others. This is generally well reproduced by our spin-dependent mass transfer algorithm, except for the lowest considered masses. The observations reflect the slow increase of the surface nitrogen enrichment of the donors during the semi-detached phase all the way to CNO equilibrium. We also investigate the properties of our models after core hydrogen depletion of the donor star, when these models correspond to Wolf-Rayet or helium+OB star binaries. Conclusions. A dedicated spectroscopic survey of massive Algol systems may allow to derive the dependence of the efficiency of thermal timescale mass transfer on the binary parameters, as well as the efficiency of semiconvective mixing in the stellar interior. This would be a crucial step towards reliable binary models up to the formation of supernovae and compact objects.

Stellar Chromospheric Variability

Cool stars with convective envelopes of spectral types F and later tend to exhibit magnetic activity throughout their atmospheres. The presence of strong and variable magnetic fields is evidenced by photospheric starspots, chromospheric plages and coronal flares, as well as by strong Ca ii H+K and Hα emission, combined with the presence of ultraviolet resonance lines. We review the drivers of stellar chromospheric activity and the resulting physical parameters implied by the observational diagnostics. At a basic level, we explore the importance of stellar dynamos and their activity cycles for a range of stellar types across the Hertzsprung–Russell diagram. We focus, in particular, on recent developments pertaining to stellar rotation properties, including the putative Vaughan–Preston gap. We also pay specific attention to magnetic variability associated with close binary systems, including RS Canum Venaticorum, BY Draconis, W Ursae Majoris and Algol binaries. At the present time, large-scale photometric and spectroscopic surveys are becoming generally available, thus leading to a resurgence of research into chromospheric activity. This opens up promising prospects to gain a much improved understanding of chromospheric physics and its wide-ranging impact.

OO Dra: An Algol-type Binary Formed through an Extremely Helium-poor Mass Accretion Revealed by Asteroseismology

Abstract Based on 2 minutes of Transiting Exoplanet Survey Satellite data, we analyzed intrinsic oscillations of the primary component and identified seven confident independent δ Scuti frequencies (f 1, f 2, f 3, f 4, f 7, f 11, and f 12). Both single-star evolutionary models and mass-accreting models are computed to reproduce the δ Scuti frequencies and the fitting results match well with each other. The stellar parameters of the primary star yielded by asteroseismology are M = 1.92 − 0.02 + 0.10 M ⊙, Z = 0.011 − 0.001 + 0.006 , R = 2.068 − 0.007 + 0.050 R ⊙, log g = 4.090 − 0.002 + 0.010 , T eff = 8346 − 320 + 244 K, and L = 18.65 − 2.82 + 3.31 L ⊙, which match well with the dynamic ones using the binary model. Furthermore, our asteroseismic results show that OO Dra is another Algol system that has just undergone the rapid mass-transfer stage. The fitting results of single-star evolutionary models indicate that the pulsator is a helium-poor star with an age of 8.22 − 1.33 + 0.12 Myr, and the further mass-accreting models show that the primary star looks like an almost unevolved star formed by an extremely helium-poor mass accretion in the Case A evolutionary scenario.

Say Hello to Algol's New Companion Candidates

Abstract Constant orbital period ephemerides of eclipsing binaries give the computed eclipse epochs (C). These ephemerides based on the old data cannot accurately predict the observed future eclipse epochs (O). Predictability can be improved by removing linear or quadratic trends from the O − C data. Additional companions in an eclipsing binary system cause light-time travel effects that are observed as strictly periodic O − C changes. Recently, Hajdu et al. estimated that the probability of detecting the periods of two new companions from the O − C data is only 0.00005. We apply the new discrete chi-square method to 236 yr of O − C data of the eclipsing binary Algol (β Persei). We detect the tentative signals of at least five companion candidates having periods between 1.863 and 219.0 yr. The weakest one of these five signals does not reveal a “new” companion candidate, because its 680.4 ± 0.4 day signal period differs only 1.4σ from the well-known 679.85 ± 0.04 day orbital period of Algol C. We detect these same signals also from the first 226.2 yr of data, and they give an excellent prediction for the last 9.2 yr of our data. The orbital planes of Algol C and the new companion candidates are probably coplanar because no changes have been observed in Algol’s eclipses. The 2.867 day orbital period has been constant since it was determined by Goodricke.

On the Modeling of Algol-Type Binaries

In earlier papers, we presented a binary evolutionary code for the purpose of reproducing the orbital parameters, masses, radii, and location in the Hertzsprung Russell diagram (abbreviated as HRD) of well-observed Algol systems. In subsequent versions, the effects of mass and angular momentum losses and tidal coupling were included in order to produce the observed distributions of orbital periods and mass ratios of Algol-type binaries. The mass loss includes stellar wind and possible liberal evolution, when the gainer star is not capable to absorb all of the matter during mass transfer from the donor star. We added magnetic braking to our code to better reproduce the observed equatorial velocities. Large equatorial velocities of mass-gaining stars are now lowered by tidal interaction and magnetic braking. Tides are mainly at work at short orbital periods, leaving magnetic braking alone at work during longer orbital periods. The observed values of the equatorial velocities of mass gainers in Algol-type binaries are mostly well reproduced by our code. According to our models, Algols have short periods with a strong magnetic field.

Magnetic braking at work in binaries

Context. In earlier papers, we aimed to reconstruct the progenitor systems of Algol-type semi-detached binaries. To this end, we developed a binary evolutionary code for the purpose of reproducing the orbital parameters, masses, and location in the HRD of well-observed Algol systems. In this code, the effects of mass and angular momentum losses and tidal coupling were included, but not magnetic braking at that point. In the present paper, we study the effects of magnetic braking on the rotation of the mass gainers in these systems. Aims. Equatorial velocities have been measured for a number of mass-gaining stars in interacting binaries. Tides tend to synchronize the rotation of the gainer, but many observed low equatorial velocities cannot be explained by tidal interactions alone. Methods. We added magnetic braking to our code to better reproduce the observed equatorial velocities. Results. Large equatorial velocities of mass-gaining stars are lowered by tidal interaction and magnetic braking. Tides are mainly at work at short orbital periods, leaving magnetic braking alone at work during longer orbital periods. Conclusions. Slow rotation of mass gainers in Algol-type binaries is mostly well reproduced by our code. However, (not observed) critical rotation of the gainer in some systems cannot be avoided by our calculations.

Testing stellar evolution models

AbstractIn this study, we have investigated the stellar evolution models using the open-source software instrument Modules for Experiments in Stellar Astrophysics. We examine the evolution of angular momentum and the stability of mass transfer in the evolution of Algol-type binaries through the inner Lagrangian point via the Roche lobe overflow. Also, we have determined the ongoing challenge of chemical mixing and exhibit improvements that make easier the simulation of Algol-type binaries evolution.

The first study of 54 new eccentric eclipsing binaries in our Galaxy

We present an analysis of the apsidal motion and light curve parameters of 54 never-before-studied galactic Algol-type binaries. This is the first analysis of such a large sample of eccentric eclipsing binaries in our Galaxy, and has enabled us to identify several systems that are worthy of further study. Bringing together data from various databases and surveys, supplemented with new observations, we have been able to trace the long-term evolution of the eccentric orbit over durations extending back up to several decades. Our present study explores a rather different sample of stars to those presented in the previously published catalogue of eccentric eclipsing binaries, sampling to fainter magnitudes, covering later spectral types, sensitive to different orbital periods with more than 50% of our systems having periods longer than six days. The typical apsidal motion in the sample is rather slow (mostly of order of centuries long), although in some cases this is less than 50 yr. All of the systems, except one, have eccentricities less than 0.5, with an average value of 0.23. Several of the stars also show evidence for additional period variability. In particular we can identify three systems in the sample, HD 44093, V611 Pup, and HD 313631, which likely represent relativistic apsidal rotators.

Algol as Horus in the Cairo Calendar: The Possible Means and the Motives of the Observations

Abstract An ancient Egyptian Calendar of Lucky and Unlucky Days, the Cairo Calendar (CC), assigns luck with the period of 2.850 days. Previous astronomical, astrophysical and statistical analyses of CC support the idea that this was the period of the eclipsing binary Algol three millennia ago. However, next to nothing is known about who recorded Algol’s period into CC and especially how. Here, we show that the ancient Egyptian scribes had the possible means and the motives for such astronomical observations. Their principles of describing celestial phenomena as activity of gods reveal why Algol received the title of Horus.

Low resolution spectroscopy of selected Algol systems

The analysis of spectroscopic data for 30 Algol-type binaries is presented. All these systems are short period Algols having primaries with spectral types B and A. Dominant spectral lines were identified for the spectra collected and their equivalent widths were calculated. All the spectra were examined to understand presence of mass transfer, a disk or circumstellar matter and chromospheric emission. We also present first spectroscopic and period study for few Algols and conclude that high resolution spectra within and outside the primary minimum are needed for better understanding of these Algol type close binaries.

A Monster CME Obscuring a Demon Star Flare

Abstract We explore the scenario of a coronal mass ejection (CME) being the cause of the observed continuous X-ray absorption of the 1997 August 30 superflare on the eclipsing binary Algol (the Demon Star). The temporal decay of the absorption is consistent with absorption by a CME undergoing self-similar evolution with uniform expansion velocity. We investigate the kinematic and energetic properties of the CME using the ice cream cone model for its three-dimensional structure in combination with the observed profile of the hydrogen column density decline with time. Different physically justified length scales were used that allowed us to estimate lower and upper limits of the possible CME characteristics. Further consideration of the maximum available magnetic energy in starspots leads us to quantify its mass as likely lying in the range g and kinetic energy in the range erg. The results are in reasonable agreement with extrapolated relations between flare X-ray fluence and CME mass and kinetic energy derived for solar CMEs.

Asteroseismology ofKeplerAlgol-type oscillating eclipsing binaries

Context. This research paper contains light curve modelling, spectroscopy and detailed asteroseismic studies for four out of five in total semidetached eclipsing binaries with a delta Scuti component, that have been detected to date through Kepler mission, namely KIC 06669809, KIC 10581918, KIC 10619109 and KIC 11175495. Aims. The goal is to study the pulsational characteristics of the oscillating stars of the systems as well as to estimate their absolute parameters and enrich the so far poor sample of this kind of systems. Methods. Ground based spectroscopic observations provide the means to estimate the spectral types of the primary components and to model the light curves with higher certainty. The photometric data are analysed using eclipsing binary modeling techniques, while Fourier analysis is applied on their residuals in order to reveal the pulsation frequency modes. Results. The results of analyses show that the primaries are pulsating stars of delta Scuti type and that all systems belong to the group of Algol type binaries with an oscillating star, namely oEA stars. The primaries of KIC 06669809, KIC 10581918, and KIC 10619109 pulsate in three, two, and five frequencies, respectively and in more than 200 other detected as combinations. The delta Scuti star of KIC 11175495 is the youngest and the fastest pulsator in binary systems that has ever been found and it oscillates in three main non radial frequencies, while other 153 are also found as depended ones. Moreover, a comparison of their properties with other systems of the same type as well as with theoretical models of pulsating stars are also presented and discussed.

BVRI Photometric Analysis and Spectra of the Dwarf Solar Type Algol Binary, V500 Pegasi

Observations and analyses of V500 Pegasi taken at the Dark Sky Observatory (DSO), the SARA North Observatory and the Dominion Astrophysical Observatory are presented. V500 Peg is an eclipsing binary with a period of 0.57983 d. The light curves have the appearance of a detached or semi-detached binary. Spectroscopy reveals that it is of F5V-type. A continuous 14-year period study may show an orbital period increase is occurring. This may be due to weak matter transfer with the primary component as the gainer. The light curve has a large difference in primary and secondary amplitudes that is typical of this class of binary. The final solution shows a total secondary eclipse. The solution is that of a classical Algol system (semi-detached with the secondary component filling its Roche lobe), but it is of solar type, with a magnetic spot. This may mean that magnetic braking is interfering in the nuclear evolution. The secondary component has a temperature of ∼4700 K (K3), which means it is somewhat overluminous for its mass.

A comparison between observed Algol-type double stars in the solar neighborhood and evolutionary computations of galactic case A binaries with a B-type primary at birth

We first discuss a large set of evolutionary calculations of close binaries with a B-type primary at birth and with a period such that the Roche lobe overflow starts during the core hydrogen burning phase of the primary (intermediate mass and massive case A binaries). The evolution of both components is followed simultaneously allowing us to check for the occurrence of contact binaries. We consider various models to treat a contact system and the influence of these models on the predicted Algol-type system population is investigated. We critically discuss the available observations of Algol-type binaries with a B-type primary at birth. Comparing these observations with the predictions allows us to put constraints on the contact star physics. We find that mass transfer in Algols is most probably not conservative, that contact during this phase does not necessarily lead to a merger, and that angular momentum loss must be moderate.

Hidden Population of Algols

Abstract We present the results of Monte Carlo simulation aiming to estimate the frequency of semi-detached Algol-type binaries among the stars observed as single. When an account is taken of various detection biases (mostly due to inclination of orbits), the fraction of Algols among the Galactic disk stars appears to be 0.1–0.2%. However, this number should be regarded as a lower limit only, since there are still unaccounted-for selection effects and other types of photometrically unresolved binaries. Hidden binarity appears to be an important phenomenon that should be taken into account when considering stellar statistics and constructing the fundamental relations between stellar parameters.

LUT REVEALS AN ALGOL-TYPE ECLIPSING BINARY WITH THREE ADDITIONAL STELLAR COMPANIONS IN A MULTIPLE SYSTEM

ABSTRACT A complete light curve of the neglected eclipsing binary Algol V548 Cygni in the UV band was obtained with the Lunar-based Ultraviolet Telescope in 2014 May. Photometric solutions are obtained using the Wilson–Devinney method. It is found that solutions with and without third light are quite different. The mass ratio without third light is determined to be q = 0.307, while that derived with third light is q = 0.606. It is shown that V548 Cygni is a semi-detached binary where the secondary component is filling the critical Roche lobe. An analysis of all available eclipse times suggests that there are three cyclic variations in the O–C diagram that are interpreted by the light travel-time effect via the presence of three additional stellar companions. This is in agreement with the presence of a large quantity of third light in the system. The masses of these companions are estimated as m sin i′ ∼ 1.09, 0.20, and 0.52 M ⊙. They are orbiting the central binary with orbital periods of about 5.5, 23.3, and 69.9 years, i.e., in 1:4:12 resonance orbit. Their orbital separations are about 4.5, 13.2, and 26.4 au, respectively. Our photometric solutions suggest that they contribute about 32.4% to the total light of the multiple system. No obvious long-term changes in the orbital period were found, indicating that the contributions of the mass transfer and the mass loss due to magnetic braking to the period variations are comparable. The detection of three possible additional stellar components orbiting a typical Algol in a multiple system make V548 Cygni a very interesting binary to study in the future.

Period studies and photometric models for two EB-type binaries EU Hya and AW Vul

New photometry for two Algol-type binaries, EU Hya (P = 0.7782 d) and AW Vul (P = 0.8065 d), was carried out using the 60-cm telescope at Xinglong station of National Astronomical Observatories, Chinese Academy of Sciences. With the updated Wilson-Devinney code, photometric elements were derived from the multi-color light curves. The modeled results indicate that the two systems are near-contact binaries, whose secondary components fill their Roche lobes. The fill-out factors of the primaries are fp = 81.2 (±0.2)% for EU Hya and fp = 82.4 (±0.3)% for AW Vul. Period analysis implies that there exists a downward parabola with a light-time orbit from the (O – C) curve. This kind of periodic oscillation may be attributed to the light-time orbit effect of a third companion. The long-term period decrease may be caused by mass and angular momentum loss. When the orbital period decreases, the fill-out factor of fp will increase. Our results indicate that the primaries will also eventually fill their Roche lobes. EU Hya and AW Vul may possibly evolve from semi-detached binaries into contact ones.