Close Binary Stars Research Articles

Close Binary Stars. III: Roche Model

Close Binary Stars. III: Roche Model

Close Binary Stars. VIII: Close Binary Star Systems in the Late Stages of Evolution

Close Binary Stars. VIII: Close Binary Star Systems in the Late Stages of Evolution

Close Binary Stars. VII: On the Evolution of Close Binary Systems

Close Binary Stars. VII: On the Evolution of Close Binary Systems

Close Binary Stars. II: Parametric Models in Inverse Problems of Astrophysics

Close Binary Stars. II: Parametric Models in Inverse Problems of Astrophysics

Close Binary Stars. VI: New Research Methods

Close Binary Stars. VI: New Research Methods

Close Binary Stars. V: Eclipsing Star Systems with Extended Atmospheres

Close Binary Stars. V: Eclipsing Star Systems with Extended Atmospheres

Surface magnetism of rapidly rotating red giants: Single versus close binary stars

According to dynamo theory, stars with convective envelopes efficiently generate surface magnetic fields, which manifest as magnetic activity in the form of starspots, faculae, and/or flares, when their rotation period is shorter than their convective turnover time. Most red giants, having undergone significant spin down while expanding, have slow rotation and no spots. However, based on a sample of about 4500 red giants observed by the NASA Kepler mission, a previous study showed that about 8% of them display spots, about 15% of which belong to close binary systems. Here, we shed light on a puzzling fact: for rotation periods less than 80 days, a red giant that belongs to a close binary system displays a photometric modulation about an order of magnitude larger than that of a single red giant with a similar rotational period and similar physical properties. We investigate whether binarity leads to larger magnetic fields when tides lock systems, or if a different spot distribution on single versus close binary stars can explain this fact. For this, we measured the chromospheric emission in the Ca II H & K lines of 3130 of the 4465 stars studied in a previous work thanks to the LAMOST survey. We show that red giants in a close-binary configuration with spin-orbit resonance display significantly larger chromospheric emission than single stars, suggesting that tidal locking leads to larger magnetic fields at a fixed rotational period. Beyond bringing interesting new observables to study the evolution of binary systems, this result could be used to distinguish single versus binary red giants in automatic pipelines based on machine learning.

New Close Binary Central Stars of Planetary Nebulae from Gaia DR3 Epoch Photometry

Close binary interactions perform a key role in the formation and shaping of planetary nebulae (PNe). However only a small fraction of Galactic PNe are known to host close binary systems. Many such systems are detectable through photometric variability. We searched recently published epoch photometry data from Gaia DR3 for planetary nebula central stars with periodic photometric variability indicative of binarity, uncovering four previously unknown close binaries.

Tidal Wave Breaking in the Eccentric Lead-in to Mass Transfer and Common Envelope Phases

The evolution of many close binary and multiple star systems is defined by phases of mass exchange and interaction. As these systems evolve into contact, tidal dissipation is not always sufficient to bring them into circular, synchronous orbits. In these cases, encounters of increasing strength occur while the orbit remains eccentric. This paper focuses on the outcomes of close tidal passages in eccentric orbits. Close eccentric passages excite dynamical oscillations about the stars’ equilibrium configurations. These tidal oscillations arise from the transfer of orbital energy into oscillation mode energy. When these oscillations reach sufficient amplitude, they break near the stellar surface. The surface wave-breaking layer forms a shock-heated atmosphere that surrounds the object. The continuing oscillations in the star’s interior launch shocks that dissipate into the atmosphere, damping the tidal oscillations. We show that the rapid, nonlinear dissipation associated with the wave breaking of fundamental oscillation modes therefore comes with coupled mass loss to the wave-breaking atmosphere. The mass ratio is an important characteristic that defines the relative importance of mass loss and energy dissipation and therefore determines the fate of systems evolving under the influence of nonlinear dissipation. The outcome can be rapid tidal circularization (q ≪ 1) or runaway mass exchange (q ≫ 1).

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.

Magnetic effect on equilibrium tides and its influence on the orbital evolution of binary systems

In the standard theory of equilibrium tides, hydrodynamic turbulence is considered. In this paper we study the effect of magnetic fields on equilibrium tides. We find that the turbulent Ohmic dissipation associated with a tidal flow is much stronger than the turbulent viscous dissipation such that a magnetic field can greatly speed up the tidal evolution of a binary system. We then apply the theory to three binary systems: the orbital migration of 51 Pegasi b, the orbital decay of WASP-12b, and the circularization of close binary stars. Theoretical predictions are in good agreement with observations, which cannot be clearly interpreted with hydrodynamic equilibrium tides.

The Equilibrium Tide: An Updated Prescription for Population Synthesis Codes

We present an updated prescription for the equilibrium tides suitable for population synthesis codes. A grid of 1D evolutionary models was created and the viscous timescale was calculated for each detailed model. A metallicity-dependent power-law relation was fitted to both the convective cores and convective envelopes of the models. The prescription was implemented into the population synthesis code Binary Star Evolution and predicts a 16.5% reduction in the overall number of merges, with those involving main-sequence stars most affected. The new prescription also reduces the overall supernova rate by 3.6% with individual channels being differently affected. The single degenerate Ia supernova occurrence is reduced by 12.8%. The merging of two carbon oxygen white dwarfs to cause a Ia supernova occurs 16% less frequently. The number of subsynchronously rotating stars in close binaries is substantially increased with our prescription, as is the number of noncircularized systems at the start of common-envelope evolution.

The Twisted Magnetic Field of the Protobinary L483

We present H-band (1.65 μm) and SOFIA HAWC+ 154 μm polarization observations of the low-mass core L483. Our H-band observations reveal a magnetic field that is overwhelmingly in the E–W direction, which is approximately parallel to the bipolar outflow that is observed in scattered IR light and in single-dish 12CO observations. From our 154 μm data, we infer a ∼45° twist in the magnetic field within the inner 5″ (1000 au) of L483. We compare these new observations with published single-dish 350 μm polarimetry and find that the 10,000 au scale H-band data match the smaller-scale 350 μm data, indicating that the collapse of L483 is magnetically regulated on these larger scales. We also present high-resolution 1.3 mm Atacama Large Millimeter/submillimeter Array data of L483 that reveals it is a close binary star with a separation of 34 au. The plane of the binary of L483 is observed to be approximately parallel to the twisted field in the inner 1000 au. Comparing this result to the ∼1000 au protostellar envelope, we find that the envelope is roughly perpendicular to the 1000 au HAWC+ field. Using the data presented, we speculate that L483 initially formed as a wide binary and the companion star migrated to its current position, causing an extreme shift in angular momentum thereby producing the twisted magnetic field morphology observed. More observations are needed to further test this scenario.

Tidal dynamo in solar-like close binary stars

Abstract Thermal convection is commonly believed to be the energy source of stellar or planetary dynamo. In this short paper we provide another possibility, namely large-scale tidal flow. In close binary stars, say, solar-like stars with orbital period at 2 or 3 days, large-scale tidal flow is comparable to or even stronger than convective flow, and it can induce magnetic dynamo action. Based on dynamo equation and tidal theory we estimate the magnetic energy induced by large-scale tidal flow, which is proportional to the cube of orbital frequency. Our estimation can be tested by the future spectropolarimetric observations and numerical simulations for close binary stars.

Close detached white dwarf + brown dwarf binaries: further evidence for low values of the common envelope efficiency

ABSTRACT Common envelope evolution is a fundamental ingredient in our understanding of the formation of close binary stars containing compact objects that include the progenitors of type Ia supernovae, short gamma-ray bursts, and most stellar gravitational wave sources. To predict the outcome of common envelope evolution, we still rely to a large degree on a simplified energy conservation equation. Unfortunately, this equation contains a theoretically rather poorly constrained efficiency parameter (αCE) and, even worse, it is unclear if energy sources in addition to orbital energy (such as recombination energy) contribute to the envelope ejection process. In previous works, we reconstructed the evolution of observed populations of post-common envelope binaries (PCEBs) consisting of white dwarfs with main-sequence star companions and found indications that the efficiency is rather small (αCE ≃ 0.2–0.3) and that extra energy sources are only required in very few cases. Here, we used the same reconstruction tool to investigate the evolutionary history of a sample of observed PCEBs with brown dwarf companions. In contrast to previous works, we found that the evolution of observationally well-characterized PCEBs with brown dwarf companions can be understood assuming a low common envelope efficiency (αCE = 0.24–0.41), similar to that required to understand PCEBs with main-sequence star companions, and that contributions from recombination energy are not required. We conclude that the vast majority of PCEBs form from common envelope evolution that can be parametrized with a small efficiency and without taking into account additional energy sources.

Asteroseismology Across the Hertzsprung–Russell Diagram

Asteroseismology has grown from its beginnings three decades ago to a mature field teeming with discoveries and applications. This phenomenal growth has been enabled by space photometry with precision 10–100 times better than ground-based observations, with nearly continuous light curves for durations of weeks to years, and by large-scale ground-based surveys spanning years designed to detect all time-variable phenomena. The new high-precision data are full of surprises, deepening our understanding of the physics of stars. ▪ This review explores asteroseismic developments from the past decade primarily as a result of light curves from the Kepler and Transiting Exoplanet Survey Satellite space missions for massive upper main sequence OBAF stars, pre-main-sequence stars, peculiar stars, classical pulsators, white dwarfs and subdwarfs, and tidally interacting close binaries. ▪ The space missions have increased the numbers of pulsators in many classes by an order of magnitude. ▪ Asteroseismology measures fundamental stellar parameters and stellar interior physics—mass, radius, age, metallicity, luminosity, distance, magnetic fields, interior rotation, angular momentum transfer, convective overshoot, core-burning stage—supporting disparate fields such as galactic archeology, exoplanet host stars, supernovae progenitors, gamma-ray and gravitational wave precursors, close binary star origins and evolution, and standard candles. ▪ Stars are the luminous tracers of the Universe. Asteroseismology significantly improves models of stellar structure and evolution on which all inference from stars depends.

V456 Cyg: An eclipsing binary with tidally perturbed g-mode pulsations

Context. Many well-known bright stars have been observed by the ongoing transiting exoplanet survey satellite (TESS) space mission. For several of them, these new data reveal previously unobserved variability, such as tidally perturbed pulsations in close binary stars. Aims. Using newly detected gravity-mode (g-mode) pulsations in V456 Cyg, we aim to determine the global stellar properties of this short-period eclipsing binary and evaluate the interaction between these pulsations and the tides. Methods. We model the binary orbit and determine the physical properties of the component stars using the TESS photometry and published spectroscopy. We then measure the pulsation frequencies from the residuals of the light curve fit using iterative prewhitening, and analyse them to determine the global asteroseismic stellar parameters. We evaluate the pulsation parameters as a function of the orbital phase. Results. We find that the pulsations belong to the secondary component of V456 Cyg and that this star likely has a uniform radial rotation profile, synchronous (νrot = 1.113 (14) d−1) with the binary orbit (νorb = 1.122091 (8) d−1). The observed g modes are amplified by almost a factor three in the stellar hemisphere facing the primary. We present evidence that this is caused by tidal perturbation of the pulsations, with the mode coupling being strongly affected. Conclusions. V456 Cyg is only the second object for which tidally perturbed high-order g-mode pulsations are identified, after π5 Ori. This opens up new opportunities for tidal g-mode asteroseismology, as it demonstrates another avenue in which g modes and tides can interact with each other.

Tidal Dissipation Due to Inertial Waves Can Explain the Circularization Periods of Solar-type Binaries

Abstract Tidal dissipation is responsible for circularizing the orbits and synchronizing the spins of solar-type close binary stars, but the mechanisms responsible are not fully understood. Previous work has indicated that significant enhancements to the theoretically predicted tidal dissipation rates are required to explain the observed circularization periods (P circ) in various stellar populations and their evolution with age. This was based partly on the common belief that the dominant mechanism of tidal dissipation in solar-type stars is turbulent viscosity acting on equilibrium tides in convective envelopes. In this paper, we study tidal dissipation in both convection and radiation zones of rotating solar-type stars following their evolution. We study equilibrium tide dissipation, incorporating a frequency-dependent effective viscosity motivated by the latest hydrodynamical simulations, and inertial wave (dynamical tide) dissipation, adopting a frequency-averaged formalism that accounts for the realistic structure of the star. We demonstrate that the observed binary circularization periods can be explained by inertial wave (dynamical tide) dissipation in convective envelopes. This mechanism is particularly efficient during pre-main-sequence phases, but it also operates on the main sequence if the spin is close to synchronism. The predicted P circ due to this mechanism increases with the main-sequence age in accordance with observations. We also demonstrate that both equilibrium tide and internal gravity-wave dissipation are unlikely to explain the observed P circ, even during the pre-main sequence, based on our best current understanding of these mechanisms. Finally, we advocate more realistic dynamical studies of stellar populations that employ tidal dissipation due to inertial waves.

The spins of compact objects born from helium stars in binary systems

ABSTRACT The angular momentum (AM) content of massive stellar cores helps us to determine the natal spin rates of neutron stars and black holes. Asteroseismic measurements of low-mass stars have proven that stellar cores rotate slower than predicted by most prior work, so revised models are necessary. In this work, we apply an updated AM transport model based on the Tayler instability to massive helium stars in close binaries, in which tidal spin-up can greatly increase the star’s AM. Consistent with prior work, these stars can produce highly spinning black holes upon core-collapse if the orbital period is less than $P_{\rm orb} \lesssim \! 1 \, {\rm d}$. For neutron stars, we predict a strong correlation between the pre-explosion mass and the neutron star rotation rate, with millisecond periods ($P_{\rm NS} \lesssim 5 \, {\rm ms}$) only achievable for massive ($M \gtrsim 10 \, M_\odot$) helium stars in tight ($P_{\rm orb} \lesssim 1 \, {\rm d}$) binaries. Finally, we discuss our models in relation to type Ib/c supernovae, superluminous supernove, gamma-ray bursts, and LIGO/Virgo measurements of black hole spins. Our models are roughly consistent with the rates and energetics of these phenomena, with the exception of broad-lined Ic supernovae, whose high rates and ejecta energies are difficult to explain.

Merger Rate Density of Binary Black Holes through Isolated Population I, II, III and Extremely Metal-poor Binary Star Evolution

Abstract We investigate the formation of merging binary black holes (BHs) through isolated binary evolution, performing binary population synthesis calculations covering an unprecedentedly wide metallicity range of Population (Pop) I, II, III, and extremely metal-poor (EMP) binary stars. We find that the predicted merger rate density and primary BH mass (m 1) distribution are consistent with the gravitational wave (GW) observations. Notably, Population III and EMP (<10−2 Z ⊙) binary stars yield most of the pair instability (PI) mass gap events with m 1 = 65–130 M ⊙. Population III binary stars contribute more to the PI mass gap events with increasing redshift, and all the PI mass gap events have the Population III origin at redshifts ≳8. Our result can be assessed by future GW observations in the following two points. First, there are no binary BHs with m 1 = 100–130 M ⊙ in our result, and thus the m 1 distribution should suddenly drop in the range of m 1 = 100–130 M ⊙. Second, the PI mass gap event rate should increase toward higher redshift up to ∼11, since those events mainly originate from the Population III binary stars. We find that the following three assumptions are needed to reproduce the current GW observations: a top-heavy stellar initial mass function and the presence of close binary stars for Population III and EMP binary stars, and inefficient convective overshoot in the main-sequence phase of stellar evolution. Without any of the above, the number of PI mass gap events becomes too low to reproduce current GW observations.