Eccentric Binary System Research Articles

On the nature of the eccentric eclipsing binary star SY Phe with a pulsating γ Dor component

ABSTRACT Spectroscopic observations of the eccentric binary system SY Phe were made at the South African Astronomical Observatory in 2019, 2020, and 2021, and its mid-resolution spectra were obtained. The radial velocities of the component stars were measured using the cross-correlation method and Fourier disentangling of the spectra. The spectral type (hence the effective temperature) of the primary star was determined from a model-atmosphere analysis. The radial velocity and Transiting Exoplanet Survey Satellite (TESS) light curves of the system were analysed, and its absolute parameters were derived. A strong (5.2 mmag) periodic signal with a frequency typical of $\gamma$ Dor stars (1.169 cycles per day) dominates the Fourier spectrum of the light curve between the eclipses. Apsidal motion parameters of SY Phe were calculated by studying eclipse timing variations. The Geneva evolution models indicate an evolutionary age of 2 Gyr and solar metallicity for the primary component; however, although the position of the secondary component in the H–R diagram matches the isochrone of 2 Gyr, it appears to have a larger radius and higher effective temperature than expected for its determined mass. Here, the secondary component has too large a radius, which is in accordance with the radius discrepancy problem that has been encountered in other studies, especially in late-type dwarfs, and has not been solved for half a century.

Exploring extreme brightness variations in blue supergiant MACHO 80.7443.1718: Evidence for companion-driven enhanced mass loss

Context. The evolution of massive stars is dominated by interactions within binary and multiple systems. In order to accurately model this evolution, it is necessary to investigate all possible forms of an interaction in binary systems that may affect the evolution of the components. One of the “laboratories” plausible for this kind of investigation is the massive eccentric binary system MACHO 80.7443.1718 (ExtEV), which exhibits an exceptionally large amplitude of light variability close to the periastron passage of its 32.8-day orbit. Aims. We examine whether the light variability of ExtEV can be explained by a wind-wind collision (WWC) binary system model. We also critically review other models proposed to explain the light curve of ExtEV. Methods. We conducted an analysis of (i) the broadband multicolor photometry of ExtEV spanning a wide range of wavelengths from the ultraviolet to near-infrared, (ii) the time-series space photometry from the Transiting Exoplanet Survey Satellite (TESS), (iii) ground-based Johnson UBV photometry, and (iv) time-series high-resolution spectroscopy. To derive the parameters of the primary component of the system, we fit the spectral energy distribution (SED) and calculated evolutionary models of massive stars that included mass loss. Using radial-velocity data, we determined the spectroscopic parameters of the system. We also fit an analytical model of light variations to the TESS light curve of ExtEV. Results. The ExtEV system exhibits an infrared excess, indicating an increased mass-loss rate. The system does not match the characteristics of B[e] stars, however. We rule out the possibility of the presence of a Keplerian disk around the primary component. We also argue that the scenario with periodic Roche-lobe overflow at periastron may not be consistent with the observations of ExtEV. Analysis of the SED suggests that the primary component has a radius of about 30 R⊙ and a luminosity of ∼6.6 × 105 L⊙. With the analysis of the radial-velocity data, we refine the orbital parameters of ExtEV and find evidence for the presence of a tertiary component in the system. Using evolutionary models we demonstrate that the primary component’s mass is between 25 and 45 M⊙. We successfully reproduced the light curve of ExtEV with our analytical model, showing that the dominant processes shaping its light curve can be attributed to the atmospheric eclipse and light scattered in the WWC cone. We also estimate the primary’s mass loss rate due to stellar wind for 4.5 × 10−5 M⊙ yr−1. Conclusions. ExtEV is most likely not an extreme eccentric ellipsoidal variable, but rather an exceptional WWC binary system. The mass loss rate we derived exceeds theoretical predictions by up to two orders of magnitude. This implies that the wind in the system is likely enhanced by tidal interactions, rotation, and possibly also tidally excited oscillations. Therefore, ExtEV represents a rare evolutionary phase of a binary system that may help to understand the role of companion-driven enhanced mass loss in the evolution of massive binary systems.

Gravitational radiation from eccentric binary black hole system in dynamical Chern-Simons gravity

Dynamical Chern-Simons (DCS) gravity, a typical parity-violating gravitational theory, modifies both the generation and propagation of gravitational waves from general relativity (GR). In this work, we derive the gravitational waveform radiated from a binary slowly-rotating black hole system with eccentric orbits under the spin-aligned assumption in the DCS theory. Compared with GR, DCS modification enters the second-order post-Newtonian (2PN) approximation, affecting the spin-spin coupling and monopole-quadrupole coupling of binary motion. This modification produces an extra precession rate of periastron. This effect modulates the scalar and gravitational waveform through a quite low frequency. Additionally, the dissipation of conserved quantities results in the secular evolution of the semimajor axis and the eccentricity of binary orbits. Finally, the frequency-domain waveform is given in the post-circular scheme, requiring the initial eccentricity to be ≲ 0.3. This ready-to-use template will benefit the signal searches and improve the future constraint on DCS theory.

Relativistic Effects on Circumbinary Disk Evolution: Breaking the Polar Alignment around Eccentric Black Hole Binary Systems

We study the effects of general relativity (GR) on the evolution and alignment of circumbinary disks around binaries on all scales. We implement relativistic apsidal precession of the binary into the hydrodynamics code phantom. We find that the effects of GR can suppress the stable polar alignment of a circumbinary disk, depending on how the relativistic binary apsidal precession timescale compares to the disk nodal precession timescale. Studies of circumbinary disk evolution typically ignore the effects of GR, which is an appropriate simplification for low-mass or widely separated binary systems. In this case, polar alignment occurs, provided that the disks initial misalignment is sufficiently large. However, systems with a very short relativistic precession timescale cannot polar align and instead move toward coplanar alignment. In the intermediate regime where the timescales are similar, the outcome depends upon the properties of the disk. Polar alignment is more likely in the wavelike disk regime (where the disk viscosity parameter is less than the aspect ratio, α < H/r), since the disk is in good radial communication. In the viscous disk regime, disk breaking is more likely. Multiple rings can destructively interact with one another, resulting in short disk lifetimes and the disk moving toward coplanar alignment. Around main-sequence star or stellar mass black hole binaries, polar alignment may be suppressed far from the binary, but in general, the inner parts of the disk can align to polar. Polar alignment may be completely suppressed for disks around supermassive black holes for close binary separations.

Probing the physics in the core boundary layers of the double-lined B-type binary KIC 4930889 from its gravito-inertial modes

Context. Stellar evolution models of B-type stars are still uncertain in terms of internal mixing properties, notably in the area between the convective core and the radiative envelope. This impacts age determination of such stars in addition to the computation of chemical yields produced at the end of their life. Aims. We investigated the thermal and chemical structure and rotation rate in the near-core boundary layer of the double-lined B-type binary KIC 4930889 from its four-year Kepler light curve, ground-based spectroscopy, and Gaia astrometry. Methods. We computed grids of 1D stellar structure and evolution models for different mixing profiles and prescriptions of the temperature gradient in the near-core region. We examined the preferred prescription and the near-core rotation rate using 22 prograde dipole modes detected by Kepler photometry of KIC 4930889. We employed a Mahalanobis distance merit function and considered various nested stellar model grids, rewarding goodness of fit but penalising model complexity. Results. We were able to constrain the near-core rotation rate of the pulsator to Ωrot = 0.73−0.06+0.02 d−1. Furthermore, we found a preference for either an exponentially decaying mixing profile in the near-core region or absence of additional near-core mixing, but found no preference among the various options for the temperature gradient in this region. The frequency (co)variances of our theoretical predictions are much larger than the errors on the observed frequencies. This forms the main limitation on further constraining the individual parameters of our models. A combination of spectroscopic, astrometric, binary, and asteroseismic information was used to achieve these constraints. Additionally, non-adiabatic pulsation computations of our best models indicate a need for opacity enhancements to accurately reproduce the observed mode excitation. Conclusions. The eccentric close binary system KIC 4930889 proves to be a promising target to investigate additional physics in close binaries by developing new modelling methods with the capacity to include the effect of tidal interactions for full exploitation of all detected oscillation modes.

Observational investigation of the dynamical tides in massive eccentric binaries

ABSTRACT One of the basics of astrophysics is the construction of a theory to explain tidal interactions and evolution. The accuracy of the theoretical models produced by these theories is relied upon by various other astrophysical fields. As a result, putting observational models to the test is of utmost importance. The foundation of this premise rests on the comparison between predicted models and observed stellar quantities. In order to maximize the potential of the topic, a comprehensive analysis of five massive eccentric binary systems is presented in this study, which is further compared with other sample sets to evaluate the impact of tidal interaction in massive eccentric systems. The amalgamation of radial velocity measurements with photometry has yielded a comprehensive collection of both physical and orbital parameters. The spectra of each binary system were evaluated using a standard atmospheric analysis in order to ascertain the temperatures and metallicities of each component in the binaries. The utilization of high-precision analysis enables us to ascertain the evolutionary status of the components that have been deduced from models of stellar evolution, by examining the positions of these components in the T$_{\rm eff}/\log g\,{\rm Kiel}$ diagram. The examination of the influence of tidal interaction on stars possessing radiative envelopes constitutes an important part of our research. The interpretation of our analysis is situated within the framework of the temporal scales of tidal synchronization and circularization.

FX UMa: A New Heartbeat Binary System with Linear and Nonlinear Tidal Oscillations and δ Sct Pulsations

We present a detailed analysis of an eclipsing double-lined binary FX UMa based on TESS photometry and newly acquired spectroscopic observations. The radial velocities and atmospheric parameters for each component star are obtained from the SONG high-resolution spectra. Combined with the radial-velocity measurements, our light-curve modeling yields absolute masses and radii of the two components. The Fourier amplitude spectrum of the residual light curve reveals a total of 103 frequencies with signal-to-noise ratio (S/N) ≥ 4, including 12 independent frequencies, 17 multiples of the orbital frequency (Nf orb), and 74 combination frequencies. Ten Nf orb peaks with S/N > 10 have very high amplitudes and are likely due to tidally excited oscillations (TEOs). The remaining Nf orb peaks (4 ≤ S/N ≤ 10) may be originated from the imperfect removal, or they are actually real TEOs. Four anharmonic frequencies can pair up and sum to give exact harmonics of the orbital frequency, suggesting the existence of nonlinear tidal processes in the eccentric binary system FX UMa. Eight independent frequencies in the range of 20–32 day−1 are typical low-order pressure modes of δ Scuti pulsators.

DMPP-3: confirmation of short-period S-type planet(s) in a compact eccentric binary star system, and warnings about long-period RV planet detections

ABSTRACT We present additional HARPS radial velocity observations of the highly eccentric (e ∼ 0.6) binary system DMPP-3AB, which comprises a K0V primary and a low-mass companion at the hydrogen burning limit. The binary has a 507 d orbital period and a 1.2 au semimajor axis. The primary component harbours a known 2.2 M⊕ planet, DMPP-3A b, with a 6.67-d orbit. New HARPS measurements constrain periastron passage for the binary orbit and add further integrity to previously derived solutions for both companion and planet orbits. Gaia astrometry independently confirms the binary orbit and establishes the inclination of the binary is 63.89 ± 0.78°. We performed dynamical simulations that establish that the previously identified ∼800 d RV signal cannot be attributed to an orbiting body. The additional observations, a deviation from strict periodicity, and our new analyses of activity indicators suggest the ∼800 d signal is caused by stellar activity. We conclude that there may be long-period planet ‘detections’ in other systems, which are similar misinterpreted stellar activity artefacts. Without the unusual eccentric binary companion to the planet-hosting star, we could have accepted the ∼800 d signal as a probable planet. Further monitoring of DMPP-3 will reveal which signatures can be used to most efficiently identify these imposters. We also report a threshold detection (0.2 per cent FAP) of a ∼2.26 d periodicity in the RVs, potentially attributed to an Earth-mass S-type planet interior to DMPP-3A b.

Superorbital periods of Be/X-ray binaries driven by stellar spin precession

ABSTRACT Superorbital periods are observed in the optical light curves of many Be/X-ray binaries yet their origin has remained somewhat elusive. We suggest that precession of the spin axis of the Be star can drive superorbital periods, particularly for short orbital period binaries. We consider the short orbital period ($P_{\rm orb}=16.6\, \rm d$) and highly eccentric (eb = 0.72) Be/X-ray binary A0538−66 that has a superorbital period of $421\, \rm d$. First, we show that the spin axis precession time-scale is about twice the observed superorbital period. Then, with hydrodynamic simulations we show that the Be star decretion disc can remain locked to the equator of the precessing Be star. At each periastron passage of the neutron star, material is accreted into a disc around the neutron star. The neutron star disc nodally precesses on the same time-scale as the Be star disc and therefore both discs can contribute to the observed superorbital period. For wider and less eccentric binary systems, the Be star disc can have a larger radial extent and more complex behaviour is expected as a result of disc warping and breaking.

New insight into the massive eccentric binary HD 165052: self-consistent orbital solution, apsidal motion, and fundamental parameters

ABSTRACT HD 165052 is a short-period massive eccentric binary system that undergoes apsidal motion. As the rate of apsidal motion is directly related to the internal structure constants of the binary components, its study allows getting insight into the internal structure of the stars. We use medium- and high-resolution spectroscopic observations of HD 165052 to provide constraints on the fundamental properties of the binary system and the evolutionary state of its components. We apply a spectral disentangling code to reconstruct artefact-free spectra of the individual stars and derive the radial velocities (RVs) at the times of the observations. We perform the first analysis of the disentangled spectra with the non-local thermodynamic equilibrium model atmosphere code CMFGEN to determine the stellar properties. We derive the first self-consistent orbital solution of all existing RV data, including those reported in the literature, accounting for apsidal motion. We build, for the very first time, dedicated stellar evolution tracks with the Clés code requesting the theoretical effective temperatures and luminosities to match those obtained from our spectroscopic analysis. The binary system HD 165052, consisting of an O6.5 V((f)) primary ($T_\text{eff,P}=37\, 500 \pm 1000$ K) and an O7 V((f)) secondary ($T_\text{eff,S}=36\, 000 \pm 1000$ K), displays apsidal motion at a rate of $(11.30^{+0.64}_{-0.49})^\circ$ yr−1. Evolutionary masses are compared to minimum dynamical masses to constrain the orbital inclination. Evolutionary masses $M_\text{ev,P}=24.8\pm 1.0\, \mathrm{M}_\odot$ and $M_\text{ev,S}=20.9\pm 1.0\, \mathrm{M}_\odot$ and radii $R_\text{ev,P}=7.0^{+0.5}_{-0.4}\, \mathrm{R}_\odot$ and $R_\text{ev,S}=6.2^{+0.4}_{-0.3}\, \mathrm{R}_\odot$ are derived, and the inclination is constrained to 22.1° ≤ i ≤ 23.3°. Theoretical apsidal motion rates, derived assuming an age of 2.0 ± 0.5 Myr for the binary, are in agreement with the observational determination. The agreement with theoretical apsidal motion rates enforces the inferred values of the evolutionary stellar masses and radii.

Theoretical investigation of the occurrence of tidally excited oscillations in massive eccentric binary systems

Context. Massive and intermediate-mass stars reside in binary systems at a much higher rate than low-mass stars. At the same time, binaries containing massive main-sequence (MS) component(s) are often characterised by eccentric orbits, and can therefore be observed as eccentric ellipsoidal variables (EEVs). The orbital phase-dependent tidal potential acting on the components of EEVs can induce tidally excited oscillations (TEOs), which can affect the evolution of the binary system. Aims. We investigate how the history of resonances between the eigenmode spectra of the EEV components and the tidal forcing frequencies depends on the initial parameters of the system, limiting our study to the MS. Each resonance is a potential source of TEO. We are particularly interested in the total number of resonances, their average rate of occurrence, and their distribution in time. Methods. We synthesised 20 000 evolutionary models of the EEVs across the MS using Modules for Experiments in Stellar Astrophysics (MESA) software for stellar structure and evolution. We considered a range of masses of the primary component from 5 to 30 M⊙. Later, using the GYRE stellar non-adiabatic oscillations code, we calculated the eigenfrequencies for each model recorded by MESA. We focused only on the l = 2, m = 0, +2 modes, which are suspected of being dominant TEOs. Knowing the temporal changes in the orbital parameters of simulated EEVs and the changes in the eigenfrequency spectra for both components, we were able to determine so-called resonance curves, which describe the overall chance of a resonance occurring and therefore of a TEO occurring. We analysed the resonance curves by constructing basic statistics for them and analysing their morphology using machine learning methods, including the Uniform Manifold Approximation and Projection (UMAP) tool. Results. The EEV resonance curves from our sample are characterised by a striking diversity, including the occurrence of exceptionally long resonances or the absence of resonances for long evolutionary times. We find that the total number of resonances encountered by components in the MS phase ranges from ∼102 to ∼103, mostly depending on the initial eccentricity. We also noticed that the average rate of resonances is about an order of magnitude higher (∼102 Myr−1) for the most massive components in the assumed range than for EEVs with intermediate-mass stars (∼101 Myr−1). The distribution of resonances over time is strongly inhomogeneous, and its shape depends mainly on whether the system is able to circularise its orbit before the primary component reaches the terminal-age main sequence (TAMS). Both components may be subject to increased resonance rates as they approach the TAMS. Thanks to the low-dimensional UMAP embeddings performed for the resonance curves, we argue that their morphology changes smoothly across the resulting manifold for different initial EEV conditions. The structure of the embeddings allowed us to explore the whole space of resonance curves in terms of their morphology and to isolate some extreme cases. Conclusions. Resonances between tidal forcing frequencies and stellar eigenfrequencies cannot be considered rare events for EEVs with massive and intermediate-mass MS stars. On average, we should observe TEOs more frequently in EEVs that contain massive components than those that contain intermediate-mass ones. The TEOs will be particularly well pronounced for EEVs whose component(s) are close to the TAMS, which calls for observational verification. Given the total number of resonances and their rates, TEOs may play an important role in the transport of angular momentum within massive and intermediate-mass stars (mainly near the TAMS).

Repeating periodic eruptions of the supernova impostor SN 2000ch

ABSTRACT We analyse photometric observations of the supernova (SN) impostor SN 2000ch in NGC 3432 covering the time since its discovery. This source was previously observed to have four outbursts in 2000–2010. Observations now reveal at least three additional outbursts in 2004–2007, and 16 outbursts in 2010–2022. Outburst light curves are irregular and multipeaked, exhibiting a wide variety of peak magnitude, duration, and shape. The outbursts after 2008 repeat with a period of 200.7 ± 2 d, while the outburst in 2000 seems to match with a shorter period. The next outburst should occur around January/February 2023. We propose that these periodic eruptions arise from violent interaction around times of periastron in an eccentric binary system, similar to the periastron encounters of η Carinae leading up to its Great Eruption, and resembling the erratic pre-SN eruptions of SN 2009ip. We attribute the irregularity of the eruptions to the interplay between the orbit and the variability of the luminous blue variable (LBV) primary star, wherein each successive periastron pass may have a different intensity or duration due to the changing radius and mass-loss rate of the LBV-like primary. Such outbursts may occasionally be weak or undetectable if the LBV is relatively quiescent at periastron but can be much more extreme when the LBV is active. The observed change in orbital period may be a consequence of mass lost in outbursts. Given the similarity to the progenitor of SN 2009ip, SN 2000ch deserves continued attention in the event it is headed for a stellar merger or an SN-like explosion.

Precession and polar alignment of accretion discs in triple (or multiple) stellar systems

ABSTRACTWe investigate the mechanism of polar alignment for accretion discs in hierarchical systems (HSs) with more than two stars. In eccentric binary systems, low-mass discs that are sufficiently tilted to the binary orbit align in a polar configuration with respect to the binary plane by aligning their angular momentum to the binary eccentricity vector. In HSs, secular evolution of the orbital parameters makes the eccentricity vector of the system precess with time. This precession undermines the stability of the polar orbit for accretion discs hosted in HSs. We analytically show that the binary criteria for polar alignment derived in the literature are necessary but not sufficient conditions for polar alignment in HSs. Then, we derive an analytical criterion for polar alignment in HSs. In general, we find that discs orbiting the innermost level of an HS can go polar. Conversely, radially extended discs orbiting the outer levels of an HS cannot polarly align and evolve as orbiting around a circular binary. We confirm our findings through detailed numerical simulations. Also, our results are compatible with the observed distribution of disc–orbit mutual inclination. Finally, we compare the observed distribution of disc inclinations in the binary and in the HS populations. Binaries host mainly coplanar discs, while HSs show a wide range of disc inclinations. We suggest that the wider range of inclinations in HSs results from the secular oscillation of their orbital parameters (such as Kozai–Lidov oscillations), rather than from a different initial condition or evolution between HSs and binaries.

Apsidal motion and absolute parameters of GV Nor and V881 Sco based on mid-resolution spectroscopy and TESS photometry

ABSTRACT The veracity of stellar evolution models and theoretical internal structure constants may be subjected to stringent tests by using physical parameters obtained from spectroscopic and photometric observations of eclipsing binary systems that exhibit apsidal motion. Two eclipsing binary stars from the southern hemisphere with no prior published spectroscopic analyses were selected for this study: GV Nor and V881 Sco. Spectroscopic observations of these two eccentric binary systems were made at the South African Astronomical Observatory (SAAO) in 2018 and 2019, and their mid-resolution spectra were obtained. The radial velocities were measured using the cross-correlation method as well as by disentangling the spectra. The rotational broadening of the disentangled spectra of the components was also determined. The absolute parameters of these two systems were obtained by analyzing the SAAO-based spectroscopic data in conjunction with photometric data from Transiting Exoplanet Survey Satellite. Apsidal motion parameters for these two eccentric binary systems were calculated by analysing eclipse timing variations. Granada and Padova evolution models indicated ages of 340 ± 35 Myr for GV Nor and 760 ± 15 Myr for V881 Sco, in agreement with the observational results. The theoretical internal structure constants estimated from the Granada models were compatible with the observational values within the error limits. However, for both systems, it was found that their circularization and synchronization timescales were shorter than their respective evolutionary ages. Therefore, the fact that the components rotate faster than their synchronized values and still have rather large eccentric orbits (e ∼ 0.11) challenges present theories of circularization and spin–orbit synchronization.

Preferential accretion and circumbinary disc precession in eccentric binary systems

ABSTRACT We present a suite of high-resolution hydrodynamic simulations of binaries immersed in circumbinary accretion discs (CBDs). For the first time, we investigate the preferential accretion rate as a function of both eccentricity eb and mass ratio qb in a densely sampled parameter space, finding that when compared with circular binaries, (i) mass ratios grow more efficiently in binaries on moderately eccentric orbits (0.0 ≲ eb ≲ 0.4), and (ii) high eccentricities (eb ≳ 0.6) suppress mass ratio growth. We suggest that this non-monotonic preferential accretion behaviour may produce an observable shift in the mass ratio distributions of stellar binaries and massive black hole binaries. We further find that the response of a CBD can be divided into three regimes, depending on eccentricity and mass ratio: (i) CBDs around circular binaries always precess freely, whereas CBDs around eccentric binaries either (ii) undergo forced precession or (iii) remain locked at an angle with respect to the binary periapsis. Forced precession in eccentric binaries is associated with strong modulation of individual accretion rates on the precession time-scale, a potentially observable signature in accreting binaries with short orbital periods. We provide CBD locking angles and precession rates as a function of eb and qb for our simulation suite.

Study of eccentric binaries in Horndeski gravity

We study the orbital evolution of eccentric binary systems in Horndeski gravity. This particular theory provides a testbed to give insightful comparisons with data. We compute the rate of energy loss and the rate of change of angular momentum for the binaries by calculating the multipole moments of the radiation fields. We have used appropriate parameters for the eccentric binaries to compute the decay rates of its orbital eccentricity and semi-major axis. We then compare this decay rate with that of GR.

Apsidal motion in massive eccentric binaries: The case of CPD-41° 7742, and HD 152218 revisited

Context. This paper is part of a study of the apsidal motion in close eccentric massive binary systems, which aims to constrain the internal structure of the stars. We focus on the binary CPD-41° 7742 and briefly revisit the case of HD 152218. Aims. Independent studies of CPD-41° 7742 in the past showed large discrepancies in the longitude of periastron of the orbit, hinting at the presence of apsidal motion. We here perform a consistent analysis of all observational data, explicitly accounting for the rate of change of the longitude of periastron. Methods. We make use of the extensive set of spectroscopic and photometric observations of CPD-41° 7742 to infer values for the fundamental parameters of the stars and of the binary. Applying a disentangling method to the spectra allows us to simultaneously derive the radial velocities (RVs) at the times of observation and reconstruct the individual spectra of the stars. The spectra are analysed by means of the CMFGEN model atmosphere code to determine the stellar properties. We determine the apsidal motion rate in two ways: First, we complement our RVs with those reported in the literature, and, second, we use the phase shifts between the primary and secondary eclipses. The light curves are further analysed by means of the Nightfall code to constrain the orbital inclination and, thereby, the stellar masses. Stellar structure and evolution models are then constructed with the Clés code for the two stars with the constraints provided by the observations. Different prescriptions for the mixing inside the stars are adopted in the models. Newly available photometric data of HD 152218 are analysed, and stellar structure and evolution models are built for the system as for CPD-41° 7742. Results. The binary system CPD-41° 7742, made of an O9.5 V primary (MP = 17.8 ± 0.5 M⊙, RP = 7.57 ± 0.09 R⊙, Teff, P = 31 800 ± 1000 K, Lbol,P = 5.28−0.68+0.67 × 104 L⊙) and a B1–2 V secondary (MS = 10.0 ± 0.3 M⊙, RS = 4.29−0.06+0.04 R⊙, Teff, S = 24 098 ± 1000 K, Lbol,S = 5.58−0.94+0.93 × 103 L⊙), displays apsidal motion at a rate of 15.°38−0.51+0.42 yr−1. Initial masses of 18.0 ± 0.5 M⊙ and 9.9 ± 0.3 M⊙ are deduced for the primary and secondary stars, respectively, and the binary’s age is estimated to be 6.8 ± 1.4 Myr. Regarding HD 152218, initial masses of 20.6 ± 1.5 and 15.5 ± 1.1 M⊙ are deduced for the primary and secondary stars, respectively, and the binary’s age of 5.2 ± 0.8 Myr is inferred. Conclusions. Our analysis of the observational data of CPD-41° 7742 that explicitly accounts for the apsidal motion allows us to explain the discrepancy in periastron longitudes pointed out in past studies of this binary system. The age estimates are in good agreement with estimates obtained for other massive binaries in NGC 6231. This study confirms the need for enhanced mixing in the stellar evolution models of the most massive stars to reproduce the observational stellar properties; this points towards larger convective cores than usually considered.

Onset of common envelope evolution during a core helium flash by rapid envelope expansion

ABSTRACT We suggest that the vigorous core convection during core helium flash on the tip of the red giant branch (RGB) of low-mass stars excites waves that carry energy to the envelope and inflate it for few years to increase the number of extreme horizontal branch (EHB; sdB and sdO) stars with masses of $\simeq 0.47 \, \mathrm{M}_\odot$ with respect to canonical binary evolution. Using the open-source mesa-binary, we follow the evolution of a number of eccentric binary systems with an initial primary stellar mass of $1.6 \, \mathrm{M}_\odot$. The energy that the waves carry to the envelope leads to envelope expansion at the tip of the RGB. The inflated RGB star engulfs many secondary stars to start a CEE that otherwise would not occur. If the secondary star manages to remove most of the RGB envelope the primary evolves to become an EHB star with a mass of $\simeq 0.47 \, \mathrm{M}_\odot$. However, we expect that in most cases the secondary star does not have time to spiral-in to close orbits. It rather ends at a large orbit and leaves a massive enough envelope for the primary star to later evolve along the asymptotic giant branch and to engulf the secondary star, therefore forming a non-spherical planetary nebula.

Detailed analysis of an eccentric TESS binary star with solar-type components: TIC 284613090

Eclipsing binary stars are significant objects for understanding stellar evolution, stellar population synthesis, and galaxy dynamics. Thanks to the increasing number of high-quality photometric data of space observations, the sensitivity of the fundamental astrophysical parameters (mass M, radius R) has risen and this situation has highlighted the importance of eclipsing binary stars once again. There are known relations between M and luminosity (L) for the eclipsing binary systems, especially for the detached binaries. However, in these M−L relations, some break-points appear and those points need to be re-examined with accurate M and L parameters of stars located at these areas. One of these break-points presents around the Solar mass. Therefore, this study focuses on a TESS field object TIC 284613090 which is an eccentric binary system containing two solar-type components. In the study, the fundamental parameters of the binary component stars were precisely obtained by a simultaneous analysis of the radial velocities and the TESS light curve. As a result, the masses and radii of the primary and secondary binary components were found to be M1=1.030 (8) M⊙, M2=1.019 (8) M⊙ and R1=1.611 (5)R⊙, R2=1.485 (5) R⊙, respectively. When the evolutionary status of the component stars was examined, it turned out that they are approaching the end of their main sequence evolution and the age of the system is 9.65 (20) Gyr. As a result of binary evolutionary models, it was also found that TIC 284613090 started its evolution with an orbital period of 11.83 days and an eccentric orbit with a value of e = 0.227. The position of the binary components in the M−L relation was examined as well and showed consistency with the relation.

Apsidal motion in massive eccentric binaries in NGC 6231

Context. The measurement of the apsidal motion in close eccentric massive binary systems provides essential information to probe the internal structure of the stars that compose the system. Aims. Following the determination of the fundamental stellar and binary parameters, we make use of the tidally induced apsidal motion to infer constraints on the internal structure of the stars composing the binary system HD 152219. Methods. The extensive set of spectroscopic, photometric, and radial velocity observations allowed us to constrain the fundamental parameters of the stars together with the rate of apsidal motion of the system. Stellar structure and evolution models were further built with the Clés code testing different prescriptions for the internal mixing occurring inside the stars. The effect of stellar rotation axis misalignment with respect to the normal to the orbital plane on our interpretation of the apsidal motion in terms of internal structure constants is investigated. Results. Made of an O9.5 III primary star (M1 = 18.64 ± 0.47 M⊙, R1 = 9.40−0.15+0.14 R⊙, Teff,1 = 30 900 ± 1000 K, Lbol,1 = (7.26 ± 0.97)×104 L⊙) and a B1-2 V-III secondary star (M2 = 7.70 ± 0.12 M⊙, R2 = 3.69 ± 0.06 R⊙, Teff,2 = 21 697 ± 1000 K, Lbol,2 = (2.73 ± 0.51)×103 L⊙), the binary system HD 152219 displays apsidal motion at a rate of (1.198 ± 0.300)° yr−1. The weighted-average mean of the internal structure constant of the binary system is inferred: k̄2 = 0.00173 ± 0.00052. For the Clés models to reproduce the k2-value of the primary star, a significantly enhanced mixing is required, notably through the turbulent mixing, but at the cost that other stellar parameters cannot be reproduced simultaneously. Conclusions. The difficulty to reproduce the k2-value simultaneously with the stellar parameters as well as the incompatibility between the age estimates of the primary and secondary stars are indications that some physics of the stellar interior are still not completely understood.