Binary Star Systems Research Articles

Moving Boundaries in Common Envelope Evolution

The common envelope phase in binary star systems is simulated using the 3-D moving-mesh hydrodynamic code MANGA. The companion object is modeled as a moving boundary condition in our simulations. We outline the methodology of implementing moving boundary conditions into MANGA, which are reactive to hydrodynamic and gravitational forces. The generation of initial conditions for our simulations is discussed, which must be modified near the location of the companion. We show that the size of the companion has a significant effect on the dynamics of the spiral-in, particularly with regard to the orbital separation and eccentricity. We note that our results are sensitive to the spatial resolution of our simulations. We conclude that the finite size of the companion object must be taken into account in future simulations of common envelope evolution.

Fate of twin stars on the unstable branch: Implications for the formation of twin stars

Hybrid hadron-quark equations of state that give rise a third family of stable compact stars have been shown to be compatible with the LIGO-Virgo event GW170817. Stable configurations in the third family are called hybrid hadron-quark stars. The equilibrium stable hybrid hadron-quark star branch is separated by the stable neutron star branch with a branch of unstable hybrid hadron-quark stars. The end-state of these unstable configurations has not been studied, yet, and it could have implications for the formation and existence of twin stars -- hybrid stars with the same mass as neutron stars but different radii. We modify existing hybrid hadron-quark equations of state with a first-order phase transition in order to guarantee a well-posed initial value problem of the equations of general relativistic hydrodynamics, and study the dynamics of non-rotating or rotating unstable twin stars via 3-dimensional simulations in full general relativity. We find that unstable twin stars naturally migrate toward the hadronic branch. Before settling into the hadronic regime, these stars undergo (quasi)radial oscillations on a dynamical timescale while the core bounces between the two phases. Our study suggests that it may be difficult to form stable twin stars if the phase transition is sustained over a large jump in energy density, and hence it may be more likely that astrophysical hybrid hadron-quark stars have masses above the twin star regime. We also study the minimum-mass instability for hybrid stars, and find that these configurations do not explode, unlike the minimum-mass instability for neutron stars. Additionally, our results suggest that oscillations between the two Quantum Chromodynamic phases could provide gravitational wave signals associated with such phase transitions in core-collapse supernovae and white dwarf-neutron star mergers.

Survey for Distant Solar Twins (SDST) – I. epic method for stellar parameter measurement

ABSTRACT Solar twins are stars of key importance to the field of astronomy and offer a multitude of science applications. Only a small number (≲200) of solar twins are known today, all of which are relatively close to our Sun (${\lesssim}{800}\, {\rm pc}$). The goal of our Survey for Distant Solar Twins (SDST) is to identify many more solar twin and solar analogue stars out to much larger distances (${\sim}{4}\, {\rm kpc}$). In this paper, we present a new method to identify solar twins using relatively low S/N, medium resolving power ($R\sim 28\, 000$) spectra that will be typical of such distant targets observed with HERMES on the ${3.9}\, {\rm m}$ Anglo-Australian Telescope (AAT). We developed a novel approach, namely epic, to measure stellar parameters (SPs) which we use to identify stars similar to our Sun. epic determines the stellar atmospheric parameters (effective temperature Teff, surface gravity log g, and metallicity [Fe/H]) using differential equivalent width (EW) measurements of selected spectroscopic absorption features and a simple model, trained on previously analysed spectra, that connects these EWs to the SPs. The reference for the EW measurements is a high S/N solar spectrum which is used to minimize several systematic effects. epic is fast, optimized for Sun-like stars and yields SP measurements with small enough uncertainties to enable spectroscopic identification of solar twin and analogue stars up to ${\sim}{4}\, {\rm kpc}$ away using AAT/HERMES, i.e. $\sigma \left(T_{\mathrm{eff}}, \log g, \textrm {[Fe/H]}\right) = \left({50}\, {\rm K}, {0.08}\, {\rm dex}, {0.03}\, {\rm dex}\right)$ on average at S/N = 25.

Orbital architectures of planet-hosting binaries – II. Low mutual inclinations between planetary and stellar orbits

ABSTRACT Planet formation is often considered in the context of one circumstellar disc around one star. Yet, stellar binary systems are ubiquitous, and thus a substantial fraction of all potential planets must form and evolve in more complex, dynamical environments. We present the results of a 5 yr astrometric monitoring campaign studying 45 binary star systems that host Kepler planet candidates. The planet-forming environments in these systems would have literally been shaped by the binary orbits that persist to the present day. Crucially, the mutual inclinations of star–planet orbits can only be addressed by a statistical sample. We describe in detail our sample selection and Keck/NIRC2 laser guide star adaptive optics observations collected from 2012 to 2017. We measure orbital arcs, with a typical accuracy of ∼0.1 mas yr−1, that test whether the binary orbits tend to be aligned with the edge-on transiting planet orbits. We rule out randomly distributed binary orbits at 4.7σ, and we show that low mutual inclinations are required to explain the observed orbital arcs. If the stellar orbits have a field binary-like eccentricity distribution, then the best match to our observed orbital arcs is a distribution of mutual inclinations ranging from 0° to 30°. We discuss the implications of such widespread planet–binary alignment in the theoretical context of planet formation and circumstellar disc evolution.

Electrically charged supermassive twin stars

By assuming that ultra dense hybrid neutron stars are endowed with a distribution of electric charge, we study the corresponding twin star solutions and their properties resulting from a sharp first order transition from confined hadronic to a deconfined quark phase. Two distinct quark matter equations of state with increasing stiffness are considered and the values for the maximum gravitational masses of the hadronic and hybrid twin configurations are obtained for different values of the total electric charge. Interestingly, our calculations indicate that sharp transitions make charged twin hybrid stars more massive than their neutral counterparts, and that the {2},{hbox {M}_{odot }} constraint from PSR J0740+6620 is surpassed for standard values of electric charge and can be considered stable only satisfying partial M/partial epsilon _0 > 0. In particular, our charged stellar models reach masses even higher than the unknown compact object measured in the GW190814 event.

Neutron Star–Neutron Star and Neutron Star–Black Hole Mergers: Multiband Observations and Early Warnings

Abstract The detections of gravitational waves (GWs) from binary neutron star systems and neutron star–black hole systems provide new insights into dense matter properties in extreme conditions and associated high-energy astrophysical processes. However, currently, information about the neutron star equation of state (EoS) is extracted with very limited precision. Meanwhile, the fruitful results from the serendipitous discovery of the γ-ray burst alongside GW170817 show the necessity of early warning alerts. Accurate measurements of the matter effects and sky location could be achieved by joint GW detection from space and ground. In our work, based on two example cases, GW170817 and GW200105, we use the Fisher information matrix analysis to investigate the multiband synergy between the space-borne decihertz GW detectors and the ground-based Einstein Telescope (ET). We especially focus on the parameters pertaining to the spin-induced quadrupole moment, tidal deformability, and sky localization. We demonstrate that (i) only with the help of multiband observations we can constrain the quadrupole parameter; and (ii) with the inclusion of decihertz GW detectors, the errors of tidal deformability would be a few times smaller, indicating that many more EoSs could be excluded; (iii) with the inclusion of ET, the sky localization improves by about 1 order of magnitude. Furthermore, we have systematically compared the different limits from four planned decihertz detectors and adopting two widely used waveform models.

Ionized Carbon around IRC+10216

Abstract Asymptotic giant branch stars create a rich inventory of molecules in their envelopes as they lose mass during later stages of their evolution. These molecules cannot survive the conditions in interstellar space, where they are exposed to ultraviolet photons of the interstellar radiation field. As a result, daughter molecules are the ones injected into space, and a halo of those molecules is predicted to exist around cool evolved stars. The most abundant molecule in the envelopes other than H2 is CO, which dissociates into C that is rapidly ionized into C+ in a halo around the star that is optically thin to the interstellar radiation field. We develop the specific predictions of the ionized carbon halo size and column density for the well-studied, nearby star IRC+10216. We compare those models to observations of the [C ii] 157.7 μm far-infrared fine structure line using the Stratospheric Observatory for Infrared Astronomy and Herschel. The combination of bright emission toward the star and upper limits to extended [C ii] is inconsistent with any standard model. The presence of [C ii] toward the star requires some dissociation and ionization in the inner part of the outflow, possibly due to a hot companion star. The lack of extended [C ii] emission requires that daughter products from CO photodissociation in the outer envelope remain cold. The [C ii] profile toward the star is asymmetric, with the blueshifted absorption due to the cold outer envelope.

UOCS – VII. Blue straggler populations of open cluster NGC 7789 with UVIT/AstroSat

ABSTRACT NGC 7789 is a ∼1.6 Gyr old, populous open cluster located at ∼2000 pc. We characterize the blue straggler stars (BSS) of this cluster using the Ultraviolet (UV) data from the UVIT/AstroSat. We present spectral energy distributions (SED) of 15 BSS candidates constructed using multiwavelength data ranging from UV to IR wavelengths. In 8 BSS candidates, a single temperature SED is found to be satisfactory. We discover hot companions in 5 BSS candidates. The hot companions with Teff ∼ 11750–15500 K, R ∼ 0.069–0.242 R⊙, and L ∼ 0.25–1.55 L⊙, are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than ∼0.18 M⊙, and thereby confirmed post mass transfer systems. We discuss the implication of this finding in the context of BSS formation mechanisms. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however we are unable to characterize them. We suggest that at least 5 of the 15 BSS candidates (33 per cent) studied in this cluster have formed via the mass-transfer mechanism.

Disk Material Inflates Gaia RUWE Values in Single Stars

Abstract An understanding of the dynamical evolution of binary star systems, and their effects on stellar and planetary evolution, requires well-characterized binary populations across stellar ages. However, the observational resources required to find and characterize binaries are expensive. With the release of high-precision Gaia astrometry, the re-normalized unit weight error (RUWE) statistic has been shown to reveal the presence of binary systems, with RUWE values greater than 1.2 indicating the presence of a stellar companion within ∼1″. Our goal is to assess whether this new diagnostic, which was developed for field-age systems (>1 Gyr), applies to young systems; specifically, those that host circumstellar disks. With a control sample of single-star systems, compiled from high-contrast imagining surveys of the Taurus and Upper Scorpius star-forming regions, we compare the RUWE values for systems with and without circumstellar disks. We show that the presence of a protoplanetary disk alone can result in inflated RUWE values. Based on the distribution of the RUWE for disk-bearing single stars, we suggest a more conservative single-star—binary threshold is warranted in the presence of disk material. We place this cutoff at the distribution’s 95th percentile, with RUWE = 2.5.

Gravitational waves from binary neutron stars

I review the current global status of research on gravitational waves emitted from mergers of binary neutron star systems, focusing on general-relativistic simulations and their use to interpret data from the gravitational-wave detectors, especially in relation to the equation of state of compact stars.

First TESS light curve solution and orbital period analysis of V567 And

The photometric light curve of V567 And observed by the Transiting Exoplanet Survey Satellite (TESS) was solved for the first time in this study. According to the final model parameters, it was concluded that the system is a semi-detached eclipsing binary star system. The mass and radius values of the primary and secondary components were estimated as M1=1.83±0.18 M⊙ and R1=3.16±0.21 R⊙, and M2=0.22±0.07 M⊙ and R2=2.63±0.43 R⊙, respectively. Orbital period variation of the system was also investigated for the first time in this study using one minima time given in the literature and 31 minima times calculated in this study. The downward parabolic variation of the O-C change of the system suggests a secular period decrease with a rate of 0.276±0.031 s/year. The O-C variation was interpreted in terms of the non-conservative mass transfer mechanism. The distance of V567 And was calculated to be 1229±61 pc from the distance modulus formula by taking into account interstellar extinction, which is in a good agreement with the distance value derived from the Gaia EDR3 parallax within error limits.

Binary companions triggering fragmentation in self-gravitating discs

ABSTRACT Observations of systems hosting close-in (<1 au) giant planets and brown dwarfs (M ≳ 7 MJup) find an excess of binary-star companions, indicating that stellar multiplicity may play an important role in their formation. There is now increasing evidence that some of these objects may have formed via fragmentation in gravitationally unstable discs. We present a suite of 3D smoothed particle hydrodynamics simulations of binary-star systems with circumprimary self-gravitating discs, which include a realistic approximation to radiation transport, and extensively explore the companion’s orbital parameter space for configurations that may trigger fragmentation. We identify a ‘sweet spot’ where intermediate separation binary companions (100 au ≲ a ≲ 400 au) can cause a marginally stable disc to fragment. The exact range of ideal binary separations is a function of the companion’s eccentricity, inclination, and mass. Heating is balanced by efficient cooling, and fragmentation occurs inside a spiral mode driven by the companion. Short separation, disc-penetrating binary encounters (a ≲ 100 au) are prohibitive to fragmentation, as mass stripping and disc heating quench any instability. This is also true of binary companions with high orbital eccentricities (e ≳ 0.75). Wide separation companions (a ≳ 500 au) have little effect on the disc properties for the set-up parameters considered here. The sweet spot found is consistent with the range of binary separations that display an excess of close-in giant planets and brown dwarfs. Hence, we suggest that fragmentation triggered by a binary companion may contribute to the formation of these substellar objects.

Binary Star System Decay by Graviton Interaction

The action of gravitons in a binary star system is modelled as the locus of points on an ellipse synchronous to the elliptic orbit of the binary star. In their interaction between the masses in the system the rotational energy of the gravitons is reduced by gravitational redshift, which accounts for the decay of the binary star orbital period. This model is able to fit a broad range of eccentricities of binary pulsar orbits and orbital period decay comparable to the General Relativistic gravitational wave model.

The Uncertain Future of Massive Binaries Obscures the Origin of LIGO/Virgo Sources

Abstract The LIGO/Virgo gravitational-wave observatories have detected at least 50 double black hole (BH) coalescences. This sample is large enough to have allowed several recent studies to draw conclusions about the implied branching ratios between isolated binaries versus dense stellar clusters as the origin of double BHs. It has also led to the exciting suggestion that the population is highly likely to contain primordial BHs. Here we demonstrate that such conclusions cannot yet be robust because of the large current uncertainties in several key aspects of binary stellar evolution. These include the development and survival of a common envelope, the mass and angular-momentum loss during binary interactions, mixing in stellar interiors, pair-instability mass loss, and supernova outbursts. Using standard tools such as the rapid population synthesis codes StarTrack and COMPAS and the detailed stellar evolution code MESA, we examine as a case study the possible future evolution of Melnick 34, the most massive known binary star system (with initial component masses of 144 M ⊙ and 131 M ⊙). We show that, despite its fairly well-known orbital architecture, various assumptions regarding stellar and binary physics predict a wide variety of outcomes: from a close BH–BH binary (which would lead to a potentially detectable coalescence), through a wide BH–BH binary (which might be seen in microlensing observations), or a Thorne–Żytkow object, to a complete disruption of both objects by a pair-instability supernova. Thus, because the future of massive binaries is inherently uncertain, sound predictions about the properties of BH–BH systems formed in the isolated binary evolution scenario are highly challenging at this time. Consequently, it is premature to draw conclusions about the formation channel branching ratios that involve isolated binary evolution for the LIGO/Virgo BH–BH merger population.

Gravitational waves at the first post-Newtonian order with the Weyssenhoff fluid in Einstein–Cartan theory

The generation of gravitational waves from a post-Newtonian source endowed with a quantum spin, modeled by the Weyssenhoff fluid, is investigated in the context of Einstein–Cartan theory at the first post-Newtonian level by resorting to the Blanchet–Damour formalism. After having worked out the basic principles of the hydrodynamics in Einstein–Cartan framework, we study the Weyssenhoff fluid within the post-Newtonian approximation scheme. The complexity of the underlying dynamical equations suggests to employ a discrete description via the point-particle limit, a procedure which permits the analysis of inspiralling spinning compact binaries. We then provide a first application of our results by considering binary neutron star systems.

Eccentric Dust-ring Formation in Kozai–Lidov Gas Disks

Abstract A highly misaligned gas disk around one component of a binary star system can undergo global Kozai–Lidov (KL) oscillations for which the disk inclination and eccentricity are exchanged. With hydrodynamical simulations of a gas and dust disk we explore the effects of these oscillations on the dust density distribution. For dust that is marginally coupled to the gas (St ≈ 1), we find that the dust undergoes dynamical behavior similar to that of the gas disk but the radial distribution of dust may be very different from that of the gas. The inward radial drift of the dust is faster in an eccentric disk leading to a smaller outer dust disk radius. The dust breaks into multiple narrow eccentric rings during the highly eccentric disk phase. Eccentric dust-ring formation may have significant implications for the formation of planets in misaligned disks. We suggest that multiple dust rings may generally occur within gas disks that have sufficiently strong eccentricity peaks at intermediate radii.

Studies of Connected Networks via Fractional Metric Dimension

Metric dimension is an effective tool to study different distance‐based problems in the field of telecommunication, robotics, computer networking, integer programming, chemistry, and electrical networking. In this paper, we study the latest form of metric dimension called fractional metric dimension of some connected networks such as circular diagonal ladder, double sun flower, and double path networks.

Astrometric excess noise in Gaia EDR3 and the search for X-ray binaries

ABSTRACT Astrometric noise (ϵ) in excess of parallax and proper motion is a potential signature of orbital wobble (ω) of individual components in binary star systems. The combination of X-ray selection with astrometric noise could then be a powerful tool for robustly isolating accreting binaries in large surveys. Here, we mine the Gaia EDR3 catalogue for Galactic sources with significant values of astrometric noise over the parameter space expected for known and candidate X-ray binaries (XRBs). Cross-matching our sample with the Chandra Source Catalogue returns a primary sample of ≈6500 X-ray sources with significant ϵ. X-ray detection efficiency for objects with significant ϵ is a factor of ≈4.5 times higher than in a matched control sample exhibiting low ϵ. The primary sample branches off the main sequence much more than control objects in colour–mag space, and includes a higher fraction of known binaries, variables, and young stellar object class types. However, values of ϵ reported in the Gaia pipeline releases so far can exceed expectations for individual XRBs with known semimajor axis size and other system parameters. It is likely that other factors (possibly attitude and modelling uncertainties, as well as source variability) currently dominate the observed excess noise in such systems. Confirmation of their nature must therefore await future Gaia releases. The full X-ray matched catalogue is released here to enable legacy follow-up.

Supernovae in colliding-wind binaries: observational signatures in the first year

ABSTRACT When a core-collapse supernova (SN) explodes in a binary star system, the ejecta might encounter an overdense shell, where the stellar winds of the two stars previously collided. In this work, we investigate effects of such interactions on SN light curves on time-scales from the early flash ionization signatures to approximately one year after the explosion. We construct a model of the colliding-wind shell in an orbiting binary star system and we provide an analytical expression for the shell thickness and density, which we calibrate with three-dimensional adaptive mesh refinement hydrodynamical simulations probing different ratios of wind momenta and different regimes of radiative cooling efficiency. We model the angle-dependent interaction of SN ejecta with the circumstellar medium and estimate the shock radiative efficiency with a realistic cooling function. We find that the radiated shock power exceeds typical Type IIP SN luminosity only for double red supergiant binaries with mass ratios q ≳ 0.9, wind mass-loss rates $\dot{M}\gtrsim 10^{-4}\, \rm M_\odot \, \text{yr}^{-1}$, and separations between about 50 and 1500 au. The required $\dot{M}$ increases for binaries with smaller q or primaries with faster wind. We estimate that ≪1 per cent of all collapsing massive stars satisfy the conditions on binary mass ratio and separation. Recombination luminosities due to colliding wind shells are at most a factor of 10 higher than for an otherwise unperturbed constant-velocity wind, but higher densities associated with wind acceleration close to the star provide much stronger signal.

Relativistic hybrid stars in light of the NICER PSR J0740+6620 radius measurement

We explore the implications of the recent radius determination of PSR J0740+6620 by the NICER experiment combined with the neutron skin measurement by the PREX-II experiment and the associated inference of the slope of symmetry energy, for the structure of hybrid stars with a strong first-order phase transition from nucleonic to quark matter. We combine a covariant density-functional nucleonic equation of state (EOS) with a constant-speed-of-sound EOS for quark matter. We show that the radius and tidal deformability ranges obtained from GW170817 can be reconciled with the implication of the PREX-II experiment if there is a phase transition to quark matter in the low-mass compact star. In the high-mass segment, the EoS needs to be stiff to comply with the large-radius inference for PSR J0740+6620 and J0030+0451 with masses $M\simeq 2M_{\odot}$ and $M\simeq 1.4M_{\odot}$. We show that twin stars are not excluded, but the mass and radius ranges (with $M \geq M_\odot$) are restricted to narrow domains $\Delta M_{\rm twin} \lesssim 0.05 M_\odot$ and $\Delta R_{\rm twin} \sim 1.0$~km. We also show that the existence of twin configurations is compatible with the light companion in the GW190814 event being a hybrid star in the case of values of the sound-speed square $s=0.6$ and $s=1/3$.