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The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, x-ray binaries, and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. More than 70% of all massive stars will exchange mass with a companion, leading to a binary merger in one-third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae.

An explanation for the formation of binary systems in which the components are extremely far apart is proposed: triple systems can break up and send one component far away by taking energy from the remaining binary, bringing the two stars so close together that from a distance they appear like one star. A third partner in wide binaries Very wide binary star systems—widely separated pairs of gravitationally bound stars—are relatively common in the Milky Way, but they challenge current theories of star formation. The problem is that their separation can exceed the typical size of the collapsing cloud of dust and gas from which the stars form. Recent observations have shown that very wide binaries are frequently members of triple systems and that close binaries often have a distant third companion. Bo Reipurth and Seppo Mikkola report N -body simulations of the dynamical evolution of newborn triple systems that demonstrate that although the triple systems are compact when they are born, they can develop extreme hierarchical architectures on timescales of millions of years as one component is dynamically scattered into a very distant orbit. The energy of ejection comes from shrinking the orbits of the other two stars, often making them look like a single star. Such loosely bound triple systems will therefore appear as very wide binaries. The formation of very wide binary systems 1 , 2 , 3 , such as the α Centauri system with Proxima (also known as α Centauri C) separated from α Centauri (which itself is a close binary A/B) by 15,000 astronomical units 4 (1  au is the distance from Earth to the Sun), challenges current theories of star formation, because their separation can exceed the typical size of a collapsing cloud core. Various hypotheses have been proposed to overcome this problem, including the suggestion that ultrawide binaries result from the dissolution of a star cluster—when a cluster star gravitationally captures another, distant, cluster star 5 , 6 , 7 . Recent observations have shown that very wide binaries are frequently members of triple systems 8 , 9 and that close binaries often have a distant third companion 10 , 11 , 12 . Here we report N -body simulations of the dynamical evolution of newborn triple systems still embedded in their nascent cloud cores that match observations of very wide systems 13 , 14 , 15 . We find that although the triple systems are born very compact—and therefore initially are more protected against disruption by passing stars 16 , 17 —they can develop extreme hierarchical architectures on timescales of millions of years as one component is dynamically scattered into a very distant orbit. The energy of ejection comes from shrinking the orbits of the other two stars, often making them look from a distance like a single star. Such loosely bound triple systems will therefore appear to be very wide binaries.

Because of their inherently high flux allowing the detection of clear signals, black hole x-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the International Gamma-Ray Astrophysics Laboratory Imager on Board the Integral Satellite (INTEGRAL/IBIS) telescope. Spectral modeling of the data reveals two emission mechanisms: The 250- to 400-keV (kilo-electron volt) data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400-keV to 2-MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band.

Millisecond pulsars are thought to be neutron stars that have been spun-up by accretion of matter from a binary companion. Although most are in binary systems, some 30% are solitary, and their origin is therefore mysterious. PSR J1719—1438, a 5.7-millisecond pulsar, was detected in a recent survey with the Parkes 64-meter radio telescope. We show that this pulsar is in a binary system with an orbital period of 2.2 hours. The mass of its companion is near that of Jupiter, but its minimum density of 23 grams per cubic centimeter suggests that it may be an ultralow-mass carbon white dwarf. This system may thus have once been an ultracompact low-mass x-ray binary, where the companion narrowly avoided complete destruction.

The Kepler spacecraft has been monitoring the light from 150,000 stars in its primary quest to detect transiting exoplanets. Here, we report on the detection of an eclipsing stellar hierarchical triple, identified in the Kepler photometry. KOI-126 [A, (B, C)], is composed of a low-mass binary [masses MB = 0.2413 ± 0.0030 solar mass (M[middle dot in circle]), MC = 0.2127 ± 0.0026 M[middle dot in circle]; radii RB = 0.2543 ± 0.0014 solar radius (R[middle dot in circle]), RC = 0.2318 ± 0.0013 R[middle dot in circle]; orbital period P₁ = 1.76713 ± 0.00019 days] on an eccentric orbit about a third star (mass MA = 1.347 ± 0.032 M[middle dot in circle]; radius RA = 2.0254 ± 0.0098 R[middle dot in circle]; period of orbit around the low-mass binary P₂ = 33.9214 ± 0.0013 days; eccentricity of that orbit e₂ = 0.3043 ± 0.0024). The low-mass pair probe the poorly sampled fully convective stellar domain offering a crucial benchmark for theoretical stellar models.

Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce x-ray emission, but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray emission (0.1 to 10 billion electron volts) from the recently detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce π° decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.

A distant eclipse glimpsed Every 27.1 years the bright binary star ε Aurigae dims as it undergoes an eclipse lasting about 18 months. Until now the body that transits the disk of the ε Aur system has been undetectable and the subject of much speculation. The preferred explanation is that the invisible companion is a tilted disk of opaque material surrounding a hidden star. Recent work implies that the system consists of a visible F-star, paired with a single B5V star enshrouded by a disk at a temperature of about 500K. Now interferometric observations made with the six-telescope CHARA Array in November and December 2009 have produced images of the eclipsing body in the infrared, revealing it to be an opaque disk, its elliptical shape suggesting that it is tilting as predicted. ε Aurigae is a bright, eclipsing binary star system but the cause of each 18-month-long eclipse has been unknown for nearly 190 years, because the companion was, until recently, undetectable. The preferred explanation has been a tilted disk of opaque material and here the authors report interferometric images that do indeed show an opaque disk of very low mass, tilted as expected, crossing the disk of the F star. Epsilon Aurigae (ε Aur) is a visually bright, eclipsing binary star system with a period of 27.1 years. The cause of each 18-month-long eclipse has been a subject of controversy for nearly 190 years 1 because the companion has hitherto been undetectable. The orbital elements imply that the opaque object has roughly the same mass as the visible component, which for much of the last century was thought to be an F-type supergiant star with a mass of ∼15 M ⊙ ( M ⊙ , mass of the Sun). The high mass-to-luminosity ratio of the hidden object was originally explained by supposing it to be a hyperextended infrared star 2 or, later, a black hole 3 with an accretion disk, although the preferred interpretation was as a disk of opaque material 4 , 5 at a temperature of ∼500 K, tilted to the line of sight 6 , 7 and with a central opening 8 . Recent work implies that the system consists of a low-mass (2.2 M ⊙ –3.3 M ⊙ ) visible F-type star, with a disk at 550 K that enshrouds a single B5V-type star 9 . Here we report interferometric images that show the eclipsing body moving in front of the F star. The body is an opaque disk and appears tilted as predicted 7 . Adopting a mass of 5.9 M ⊙ for the B star, we derive a mass of ∼(3.6 ± 0.7) M ⊙ for the F star. The disk mass is dynamically negligible; we estimate it to contain ∼0.07 M ⊕ ( M ⊕ , mass of the Earth) if it consists purely of dust.

The merger 1 of close binary systems containing two neutron stars should produce a burst of gravitational waves, as predicted by the theory of general relativity 2 . A reliable estimate of the double-neutron-star merger rate in the Galaxy is crucial in order to predict whether current gravity wave detectors will be successful in detecting such bursts. Present estimates of this rate are rather low 3 , 4 , 5 , 6 , 7 , because we know of only a few double-neutron-star binaries with merger times less than the age of the Universe. Here we report the discovery of a 22-ms pulsar, PSR J0737–3039, which is a member of a highly relativistic double-neutron-star binary with an orbital period of 2.4 hours. This system will merge in about 85 Myr, a time much shorter than for any other known neutron-star binary. Together with the relatively low radio luminosity of PSR J0737–3039, this timescale implies an order-of-magnitude increase in the predicted merger rate for double-neutron-star systems in our Galaxy (and in the rest of the Universe).

Herculean task achieved DI Herculis is well known to astrophysicists as a binary star system with an orbit that precesses (changes orientation) at a rate that seemingly cannot be accounted for by conventional physics and stellar models. Many theories have been offered to explain this anomaly, including a failure of general relativity, a 'circumbinary' planet and an unprecedentedly large tilt between the spin axes of the stars and the orbital axis. Now this long-standing mystery has been solved. Analysis of spectra obtained during a series of binary eclipses reveals that both stars in the binary are tipped over on their sides, rotating with their spin axes nearly perpendicular to the orbital axis. The slow precession arises from extra forces associated with the stars being on their 'sides'. For most binary stars, the theoretical and observed precession rates are in agreement, but the observed precession rate for the DI Herculis system is a factor of four slower than the theoretical rate, a disagreement that once was interpreted as evidence for a failure of general relativity. Here, both stars of DI Herculis are reported to rotate with their spin axes nearly perpendicular to the orbital axis, an observation that leads to the reconciliation of the theoretical and observed precession rates. The orbits of binary stars precess as a result of general relativistic effects, forces arising from the asphericity of the stars, and forces from any additional stars or planets in the system. For most binaries, the theoretical and observed precession rates are in agreement 1 . One system, however—DI Herculis—has resisted explanation for 30 years 2 , 3 , 4 . The observed precession rate is a factor of four slower than the theoretical rate, a disagreement that once was interpreted as evidence for a failure of general relativity 5 . Among the contemporary explanations are the existence of a circumbinary planet 6 and a large tilt of the stellar spin axes with respect to the orbit 7 , 8 . Here we report that both stars of DI Herculis rotate with their spin axes nearly perpendicular to the orbital axis (contrary to the usual assumption for close binary stars). The rotationally induced stellar oblateness causes precession in the direction opposite to that of relativistic precession, thereby reconciling the theoretical and observed rates.

I use recent observational and theoretical studies of type Ia supernovae (SNe Ia) to further constrain the viable SN Ia scenarios and to argue that there must be a substantial time delay between the end of the merger of the white dwarf (WD) with a companion or the end of mass accretion on to the WD and its terminal explosion. This merger/accretion to explosion delay (MED) is required to allow the binary system to lead to a more or less spherical explosion and to prevent a pre-explosion ionizing radiation. Considering these recent results and the required MED, I conclude that the core degenerate scenario is somewhat more favorable over the other scenarios, followed by the double degenerate scenario. Although the single degenerate scenario is viable as well, it is less likely to account for common (normal) SN Ia. As all scenarios require substantial MED, the MED has turned from a disadvantage of the core degenerate scenario to a challenge that theory should overcome. I hope that the requirement for a MED will stimulate the discussion of the different SN Ia scenarios and the comparison of the scenarios to each other.