Algol Systems Research Articles

A comprehensive study of an oscillating eclipsing Algol: Y Camelopardalis

Abstract Y Camelopardalis (Y Cam) is classified as one of the oscillating eclipsing Algol (oEA) systems, which feature a $\delta$ Scuti-type pulsating component alongside mass transfer phenomena. oEA systems are invaluable for probing the evolutionary processes and internal structures of binary components, offering insights through their binary variations and oscillations. In this study, we conducted a comprehensive investigation of Y Cam utilizing high-quality photometric TESS data and high-resolution ELODIE spectra. Through our analysis, we examined the radial velocity variation, performed binary modeling, and calculated the effective temperature values of binary components. The fundamental stellar parameters, such as mass and radius, were determined with an accuracy of $\sim$2%–6%. Furthermore, we examined the orbital period variation to assess the amount of mass transfer using the available minima times of the system and three new minima times obtained from TESS light curves. Analyzing the pulsation structure of the system with the TESS data revealed the dominant pulsation period and amplitude of the pulsating component to be 0.066 d and 4.65 mmag, respectively. Notably, we observed frequency modulations with the orbital period’s frequency, along with variations in the amplitude of the highest amplitude frequency across different orbital phases. Remarkably, the amplitude reaches its peak at phases 0.5 and 1. These findings indicate a candidate for a tidally tilted pulsator. Consequently, we investigated the evolutionary status of the binary components using MESA binary evolution models, determining the age of the system to be 3.28 ± 0.09 Gyr.

Absolute properties of the oscillating eclipsing Algol XZ Ursae Majoris

Abstract It is known from archival TESS data that the semi-detached Algol system XZ Ursae Majoris (UMa) is one of the candidate binary stars exhibiting short-period oscillations. We secured new high-resolution spectroscopic observations for the program target to better understand its binary and pulsation properties. From the echelle spectra, the radial velocities (RVs) of the eclipsing pair were derived, and the atmosphere parameters of the primary component were measured to be vAsin i = 80 ± 7 km s−1, Teff, A = 7940 ± 120 K, and [M/H] = −0.15 ± 0.20. The combined solution of our double-lined RVs and the TESS data provides robust physical parameters for XZ UMa with mass and radius measurement precision of better than 2%. The outside-eclipse residuals from a mean light curve in the 0.002 phase bin were used for multifrequency analyses, and we extracted 32 significant frequencies (22 in <5.0 d−1 and 10 in 39–52 d−1). The low frequencies may be mostly aliasing sidelobes, while six of the high frequencies may be pulsation signals arising from the detached primary located inside the δ Sct domain. Their periods, pulsation constants, and pulsational–orbital-period ratios indicate that the mass-accreting primary star is a δ Sct pulsator and, hence, XZ UMa is an oscillating eclipsing Algol.

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.

Absolute Properties of the Oscillating Eclipsing Algol X Trianguli

We report results from the TESS photometric data and new high-resolution spectra of the Algol system X Tri showing short-period pulsations. From the echelle spectra, the radial velocities of the eclipsing pair were measured, and the rotational rate and effective temperature of the primary star were obtained to be v 1sini = 84 ± 6 km s−1 and T eff,1 = 7900 ± 110 K, respectively. The synthetic modeling of these observations implies that X Tri is in synchronous rotation and is physically linked to a visual companion TIC 28391715 at a separation of about 6.″5. The absolute parameters of our target star were accurately and directly determined to be M 1 = 2.137 ± 0.018 M ⊙, M 2 = 1.101 ± 0.010 M ⊙, R 1 = 1.664 ± 0.010 R ⊙, R 2 = 1.972 ± 0.010 R ⊙, L 1 = 9.67 ± 0.55 L ⊙, and L 2 = 2.16 ± 0.09 L ⊙. The phase-binned mean light curve was used to remove the binary effect from the observed TESS data. Multifrequency analysis of the residuals revealed 16 significant frequencies, of which the high-frequency signals between 37 day−1 and 48 day−1 can be considered probable pulsation modes. Their oscillation periods of 0.021−0.027 days and pulsation constants of 0.014−0.018 days are typical values of δ Sct variables. The overall results demonstrate that X Tri is an oEA star system consisting of a δ Sct primary and its lobe-filling companion in the semidetached configuration.

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.

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.

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.

OW Hydrae: A Newly Discovered SB2 Algol in a Triple Star, from DAO Spectrograms

Abstract Evidence is presented that the detached Algol system OW Hya is a double-lined spectroscopic binary (SB2). The components appear to be closely similar to each other, which is contradicted by the failure so far to detect the secondary eclipse. A sizeable asymmetry in the Mg ii λ4481 Å line is discussed. The interstellar line of Ca ii-K line is present, and the primary star appears to be a late-B giant. OW Hya is the short-period pair of the interferometric triple star, CHR 175, which appears to be hierarchical.

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.

KIC 10736223: An Algol-type Eclipsing Binary That Has Just Undergone the Rapid Mass-transfer Stage

Abstract This paper reports the discovery of an Algol system KIC 10736223 that just passed the rapid mass transfer stage. From the light-curve and radial-velocity modeling we find KIC 10736223 to be a detached Algol system with the less-massive secondary nearly filling its Roche lobe. Based on the short-cadence Kepler data, we analyzed intrinsic oscillations of the pulsator and identified six secured independent δ Scuti-type pulsation modes (f 1, f 3, f 9, f 19, f 42, and f 48). We compute two grids of theoretical models to reproduce the δ Scuti frequencies, and find that fitting results of mass-accreting models agree well with those of single-star evolutionary models. The fundamental parameters of the primary star yielded with asteroseismology are M ⊙, Z = 0.009 ± 0.001, R ⊙, , K, and L = L ⊙. The asteroseismic parameters match well with the dynamical parameters derived from the binary model. Moreover, our asteroseismic results show that the pulsator is an almost unevolved star with an age between 9.46 and 11.65 Myr for single-star evolutionary models and 2.67–3.14 Myr for mass-accreting models. Therefore, KIC 10736223 may be an Algol system that has just undergone the rapid mass-transfer process.

A strange phenomenon in XZ Andromedae: two Keplerian periods with a 1:3 ratio

Six mid-eclipse times of the eclipsing binary XZ And are obtained, which are analyzed together with others collected from the literature. Two sets of cyclic variations with periods of 33.43 and 100.4 yr are found if a double-Keplerian model is used to fit the data. The 1:3 ratio of the periods suggests that both cyclic variations arise from dynamic motions of two companions rather than magnetic activity of the eclipsing pair. According to the double-Keplerian model, the companions have masses of ∼ 1.32 M⊙ and ∼ 1.33 M⊙, respectively. Comparing with the total mass of the eclipsing pair of 3.12 M⊙, it is obvious that XZ And is a general N-body system. The strong gravitational perturbation between the two companions invalidates the double-Keplerian model. It is strange that two Keplerian periods with a 1:3 ratio are derived from the best fits with an “inappropriate” model. This illogical, but interesting phenomenon also appears in two other Algol systems, suggesting that our discoveries deserve attention from astronomers.

Period investigation of Algol systems TZ Eri and TU Her

TZ Eri and TU Her are both classic Algol-type systems (Algols). By observing and collecting times of minimum light, we constructed the O – C curves for the two systems. The long-time upward and downward parabolas shown in these diagrams are considered to be the result of the combination of mass transfer and angular momentum loss. The secular orbital period change rates are dP/dt = 4.74(± 0.12) × 10−7 d yr−1 and dP/dt = −2.33(± 0.01)× 10−6 d yr−1, respectively. There are also cyclic variations in their O – C curves which might be caused by the light-travel time effect (LTTE). A circumbinary star may exist in the TZ Eri system with a mass of at least 1.34 M⊙, while there are possibly two celestial bodies that almost follow a 2 : 1 resonance orbit around the TU Her binary pair. Their masses are at least 2.43 M⊙ and 1.27 M⊙.

Evidence for conservative mass transfer in the classical Algol system δ Librae from its surface carbon-to-nitrogen abundance ratio

Algol-type binary systems are the product of rapid mass transfer between the initially more massive component to its companion. It is still unknown whether the process is conservative, or whether substantial mass is lost from the system. The history of a system prior to mass exchange is imprinted in the photospheric chemical composition, in particular in the carbon-to-nitrogen (C/N) ratio. We use this to trace the efficiency of mass-transfer processes in the components of a classical Algol-type system, $\delta$ Librae. The present analysis is based on new spectroscopic data (ground-based high-resolution \'echelle spectra) and extracted archival photometric observations (space-based measurements from the STEREO satellites). In the orbital solution, non-Keplerian effects on the radial-velocity variations were taken into account. This reduces the primary's mass by 1.1 \Msun\ ($\sim$23\%) significantly in comparison to previous studies, and removes a long-standing discrepancy between the radius and effective temperature. A spectral disentangling technique is applied to the \'echelle observations and the spectra of the individual components are separated. Atmospheric and abundance analyses are performed for the mass-gaining component and we found C/N $= 1.55 \pm 0.40$ for this star. An extensive set of evolutionary models ($3.5\times10^6$) for both components are calculated from which the best-fitting model is derived. It is found that $\beta$, the parameter which quantifies the efficiency of mass-loss from a binary system, is close to zero. This means that the mass-transfer in $\delta$ Lib is mostly conservative with little mass loss from the system.

Seismic Study of the γ Doradus-type Pulsations in the Eclipsing Binary KIC 10486425

Abstract We present a photometric and spectroscopic analysis of the eclipsing binary KIC 10486425. Synthesis of the light curve and radial velocity measurements indicates that this binary is a circular, synchronized, detached system. The masses and radii of the two components are determined as , M 2 = 1.11 M ⊙, and , respectively. The cooler secondary star is more evolved, while the primary is still on the zero-age main sequence, suggesting that KIC 10486425 could be an Algol system. After removal of the binarity effects from the observed Kepler data, multiple frequency analysis is applied to the outside-eclipse light residuals, revealing that the primary component of KIC 10486425 is a γ Dor type pulsating star with a dominant period of 0.758 days. A total of 160 pulsation frequencies with significant confidence are extracted in the g-mode region, from which a period-spacing pattern is identified which clearly shows a downward slope. Synthesis of the period-spacing pattern leads to a mode identification of (l = 2, m = 2) for the pulsations with an asymptotic period spacing of ΔΠ l = 2 = 1803.5 s. The near-core rotation rate is derived as , about 15% slower than the orbital frequency of this synchronized binary, suggesting that the core and the envelope rotate differentially.

Spectroscopic time-series analysis of R Canis Majoris

R Canis Majoris is the prototype of a small group of Algol-type stars showing short orbital periods and low mass ratios. A previous detection of short-term oscillations in its light curve has not yet been confirmed. We investigate a new time series of high-resolution spectra with the aim to derive improved stellar and system parameters, to search for the possible impact of a third component in the observed spectra, to look for indications of activity in the Algol system, and to search for short-term variations in radial velocities. We disentangled the composite spectra into the spectra of the binary components. Then we analysed the resulting high signal-to-noise spectra of both stars. Using a newly developed program code based on an improved method of least-squares deconvolution, we were able to determine the radial velocities of both components also during primary eclipse. This allowed us to develop a better model of the system including the Rossiter–McLaughlin effect and to derive improved orbital parameters. Combining the results with those from spectrum analysis, we obtain accurate stellar and system parameters. We further deduce at least one oscillation frequency of 21.38 c d−1. It could be detected during primary eclipses only and confirms a previous photometric finding. Results point to an amplitude amplification of non-radial pulsation modes due to the eclipse mapping effect. The presence of a He I line in the spectra indicates mass transfer in the R CMa system. Calculations of its Roche geometry give evidence that the cool secondary component may fill its Roche lobe. No evidence of a third body in the system could be found in the observed spectra.

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.

Absolute Dimensions and Evolutionary Status of the Semi-detached Algol W Ursae Minoris

Abstract Double-lined eclipsing binaries allow accurate and direct determination of fundamental parameters such as mass and radius for each component, and they provide important constraints on the stellar structure and evolution models. In this study, we aim to determine a unique set of binary parameters for the Algol system W UMi and to examine its evolutionary status. New high-resolution time-series spectroscopic observations were carried out during 14 nights from 2008 April to 2011 March, and a total of 37 spectra were obtained using the Bohyunsan Optical Echelle Spectrograph. We measured the radial velocities (RVs) for both components, and the effective temperature of the primary star was found to be T eff,1 = 9310 ± 90 K by a comparison of the observed spectra and the Kurucz models. The physical parameters of W UMi were derived by an analysis of our RV data together with the multi-band light curves of Devinney et al. The individual masses, radii, and luminosities of both components are M 1 = 3.68 ± 0.10 M ⊙ and M 2 = 1.47 ± 0.04 M ⊙, R 1 = 3.88 ± 0.03 R ⊙ and R 2 = 3.13 ± 0.03 R ⊙, and L 1 = 102 ± 1 L ⊙ and L 2 = 7.3 ± 0.1 L ⊙, respectively. A comparison of these parameters with theoretical stellar models showed that the primary component lies in the main-sequence band, while the less massive secondary is noticeably evolved. The results indicate that the initially more massive star became the present secondary by losing most of its own mass via mass transfer to the companion (present primary).

Absolute Properties of the Pulsating Post-mass Transfer Eclipsing Binary OO Draconis

Abstract OO Dra is a short-period Algol system with a δ Sct-like pulsator. We obtained time-series spectra between 2016 February and May to derive the fundamental parameters of the binary star and to study its evolutionary scenario. The radial velocity (RV) curves for both components were presented, and the effective temperature of the hotter and more massive primary was determined to be K by comparing the disentangling spectrum and the Kurucz models. Our RV measurements were solved with the BV light curves of Zhang et al. using the Wilson-Devinney binary code. The absolute dimensions of each component are determined as follows: M 1 = 2.03 ± 0.06 , M 2 = 0.19 ± 0.01 , R 1 = 2.08 ± 0.03 , R 2 = 1.20 ± 0.02 , L 1 = 18 ± 2 , and L 2 = 2.0 ± 0.2 . Comparison with stellar evolution models indicated that the primary star resides inside the δ Sct instability strip on the main sequence, while the cool secondary component is noticeably overluminous and oversized. We demonstrated that OO Dra is an oscillating post-mass transfer R CMa-type binary; the originally more massive star became the low-mass secondary component through mass loss caused by stellar wind and mass transfer, and the gainer became the pulsating primary as the result of mass accretion. The R CMa stars, such as OO Dra, are thought to have formed by non-conservative binary evolution and ultimately to evolve into EL CVn stars.

Radial Velocities of 41 Kepler Eclipsing Binaries

Abstract Eclipsing binaries are vital for directly determining stellar parameters without reliance on models or scaling relations. Spectroscopically derived parameters of detached and semi-detached binaries allow us to determine component masses that can inform theories of stellar and binary evolution. Here we present moderate resolution ground-based spectra of stars in close binary systems with and without (detected) tertiary companions observed by NASA’s Kepler mission and analyzed for eclipse timing variations. We obtain radial velocities and spectroscopic orbits for five single-lined and 35 double-lined systems, and confirm one false positive eclipsing binary. For the double-lined spectroscopic binaries, we also determine individual component masses and examine the mass ratio distribution, which is dominated by binaries with like-mass pairs and semi-detached classical Algol systems that have undergone mass transfer. Finally, we constrain the mass of the tertiary component for five double-lined binaries with previously detected companions.