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Statement concerning the work of the laureates:
Studies of the structure and evolution of close binary stars belong to the most important fields of modern astrophysics. These studies lead to understanding the formation of a large variety of objects, such as X-ray and gamma-ray sources, sources of gravitational radiation, binary neutron stars, binary black holes and type Ia Supernovae. Also, the chemical evolution of the Universe is strongly influenced by binary star evolution.
Massive close binaries, which produce relativistic objects - neutron stars (NS) and black holes (BH) - gained special attention in late 1960s-early 1970s after discovery of galactic X-ray sources, radio pulsars, and especially, the first binary radio pulsar. In the early 1970s the binary nature of many X-ray sources was revealed, and it was discovered that they harbor NSs and BHs, which produce X-rays due to the release of gravitational energy by matter accreted from a companion. At about the same time, extensive computations of evolution of components of close binaries became possible. These allowed to trace the evolution quantitatively up to the birth of a NS or a BH.
Motivated by these discoveries and the new possibilities, van den Heuvel with co-authors on the one hand and Tutukov & Yungelson on the other, independently conducted ambitious research programs to explain the origins, population sizes of the binaries with relativistic objects and the rates of the phenomena associated with them.
They clarified the role of most of the physical processes governing the evolution of these binaries and laid the foundation for a large and active field in astrophysics. They also made predictions of the expected rates of gravitational wave events produced by the mergers of double compact objects.
The detection of the first gravitational wave event GW150914 in 2015, and the subsequent LIGO-VIRGO events confirmed the fundamental predictions of general relativity and gave credence to the modern theory of binary stellar evolution.
By means of evolutionary computations and analysis of observations, in 1972 -1975 van den Heuvel, as well as Tutukov & Yungelson and their co-authors traced the sequence of evolutionary transformations of massive close binaries, which envisioned loss of the hydrogen-rich envelope by the initially more massive component of the system and its transformation into the less massive component (a helium star/Wolf-Rayet star) accompanied by an O-B star.
Subsequently, the explosion of the helium star as a SN led to the formation of a relativistic object and, if the pair remained bound after the SN explosion, it evolved into a high-mass X-ray binary (HMXB), due to accretion of companion matter by the relativistic object. Next, the loss of the hydrogen-rich envelope by the optical O,B star in the HMXB, accompanied by a large loss of orbital angular momentum, turns it into Wolf-Rayet (WR) star, moving in a narrow orbit with the compact star, and the system may become an X-ray source again due to accretion of WR-star wind matter. The second SN explosion happens later, leaving behind two single relativistic objects or a bound eccentric-orbit pair of them.
The figure gives a graphic representation of this evolutionary sequence of a massive close binary.
Figure 1. Model of the evolution of a massive close binary from the main-sequence to the formation of a pair of relativistic objects and a gravitational wave event. (Time runs from the top to the bottom of the figure). The right-hand branch at the bottom of the plot represents the still hypothetical merger of a NS with its (super)giant companion (so called Thorne-Zytkow object), leading to formation of a single NS. T denotes the duration of the evolution phase, N the expected number of objects in this phase in the Galaxy (this type of graphic representation of the evolution of a relativistic binary was introduced by van den Heuvel in 1972).
The viability of this "evolutionary scenario" was confirmed by the discovery of binary radio pulsars. The concept of such evolutionary scenarios has become the basis of the numerical binary population synthesis (BPS) method, which allows, combining statistical data on the initial distributions of binaries over masses of components, their ratios, orbital separations, eccentricities, with scenarios, to model populations of binaries of different kinds and related objects and find the rates of the phenomena associated with them in the Galaxy and Universe. In later years, the expected rates of detection of mergers of NS+NS, NS+BH, BH+BH binaries by LIGO were estimated in this way. It was predicted that the mergers of BH pairs will, most probably, be the first gravitational wave events to be detected, which was confirmed by the detection since September 2015 of the GW bursts produced by merging pairs of BH.
Even long before the detection of GW bursts, short gamma-ray bursts, the most powerful electro-magnetic events in the Universe, were suggested to be due to the mergers of NSs in binaries (Blinnikov et al. 1984); such a merger of a double NS, accompanied by both a GW burst and gamma-ray burst, was detected by LIGO-VIRGO on August 17, 2017.
After their works of the early and mid-1970s, van den Heuvel, Tutukov and Yungelson continued to study stars in the particular stages of suggested by their evolutionary scenario for massive binaries, like HMXB, WR-stars, pulsars, formation and evolution of relativistic binaries, and their work also branched out into other directions.
Van den Heuvel with co-authors put forward in 1992 the model of super-soft X-ray Sources, as binaries harboring white dwarfs with surface nuclear fusion. Tutukov and Yungelson applied Binary Population Synthesis to model the population of these objects, and of other types of binary white dwarfs, especially as possible precursors of Type Ia Supernovae.
To summarize, van den Heuvel, Tutukov and Yungelson made fundamental contributions to the understanding of the evolution of massive close binary stars, especially the formation of relativistic objects in them, of X-ray binaries and sources of outbursts of gravitational waves and gamma-ray radiation. The concepts elaborated by them form the basis of the current huge activity in this field of stellar astrophysics.
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