Running star

Some escaping stars produce a head shock wave of compressed matter, which is very similar to a head wave around a boat floating on water. This wave has the same physical nature as the shock wave created by a fighter jet in the air . When a fugitive star moves at high speed through an interstellar medium (a very thin mixture of gas and dust) at a supersonic speed , then the interstellar matter becomes noticeable in the form of a head shock wave. The term " Supersonic speed " means that the speed of a moving object is higher than the speed of sound in the environment. While in the lower layer of the Earth’s atmosphere this velocity is about 330 m / s, then in an almost empty interstellar space its value is about 10 km / s <- no! the lower the density of the substance, the slower the sound propagates in the medium, therefore the speed of sound propagation in space is much lower than in the surface layers of the atmosphere ->. Thus, the detection of a head shock wave around an OB star means that it moves with supersonic speed, and thus it can be reliably identified as a runaway star, even if its speed was not measured directly ^[4] .

At a distance of 750 pc from the Sun, 56 runaway stars are known. These stars almost do not differ from other stars of the disk component of the Galaxy in all their parameters, except for high spatial velocity. Four stars from this group have a mass above 25 solar (it should be noted that for these stars the mass is determined by the shape of the spectrum with not very high accuracy) ^[5] :

Now it is assumed that such stars are formed either during the dynamic evolution of the clusters and associations in which they were born (the most probable reason is close triple convergence), or as a result of the decay of the binary system in a supernova explosion , when a traveling star receives an initial impulse in the explosion of a companion star ^{[ 5]} . While both mechanisms are theoretically possible, astronomers usually tend to the supernova explosion hypothesis in practice. R. Hoogerwerf and his colleagues from the Leiden Observatory in the Netherlands used the data obtained by the Hipparcos satellite to track the motion of 56 runaway stars in time and found evidence to support both theories ^[6] . The authors traced the movement of these stars in the Galaxy and for most of them (including all 4 massive ones) found when and from which association these stars departed, as well as which of the two possible ejection mechanisms acted for each particular star (most of the stars were ejected during double decay). Most likely, all four massive runaway stars acquired their high spatial velocity as a result of supernova explosions in binary systems . The authors give several arguments in favor of this conclusion ^[6] :

• These stars are very massive. In order to be ejected from the cluster (association) they had to fly near not much less massive stars. Otherwise, according to the law of conservation of momentum , the less massive stars would be thrown out of the system. And there are very few such massive stars - this is a direct consequence of Salpeter's law . The close flight of several massive stars turns out to be an extremely rare event, in comparison with the rather rare close triple proximity of stars of small masses.
• Massive stars live only a few million years. This fact imposes an additional restriction on the described rare event - the rapprochement must have time to happen before the massive stars exploded like supernovae .
• These stars fly at speeds several times higher than the variance of the velocities of those associations in which they were born. By itself, this fact does not contradict anything, after a successful close rapprochement, stars can acquire fairly high speeds. However, this happens only in rare cases, the average value of the speed acquired in such processes is significantly lower. Thus, with a very high probability each of these four stars was part of a fairly close massive binary system and acquired its spatial velocity after its decay due to a supernova explosion ^[5] .

Determining the percentage of the first and second mechanism in the formation of runaway stars imposes strong restrictions on the theory of cluster formation and star evolution. A numerical simulation performed in 2000 showed ^[6] that the number of runaway stars can help determine, for example, the number of binary pairs produced in clusters. Radial velocities are measured for just one third of the stars in the Hipparcos catalog . According to available data, it can be said that both mechanisms are approximately equivalent. With an increase in the number of runaway stars, for which the speed and position in space will be determined, it will be possible to find their parent clusters, as well as age and their initial speeds ^[5] .

The Escaping Star α Giraffe

The star is in the constellation Giraffe and is four thousand light-years distant from Earth . Its mass exceeds the mass of the Sun by 25-30 times, it is five times hotter than the Sun (its temperature is 30 thousand degrees) and five hundred thousand times brighter than the Sun. The escaping star α Giraffe creates a head shock wave , which propagates at a speed of 60 km / s and compresses the interstellar medium in its path. The head wave is about ten light years from the star itself. A star also emits a powerful stellar wind . Astronomers have long believed that the α Giraffe was ejected from the nearest cluster of young hot stars due to gravitational interaction with other members of the cluster. According to another hypothesis, a star could gain speed (flying out of a binary system) as a result of the explosion of a massive companion star as a supernova ^[7] .

The Escaping Star ζ Ophiuchus

When ζ moves, Ophiuchus forms in front of itself an arcuate wave of interstellar matter, which is clearly visible in this colorful infrared image taken by the WISE spacecraft. In the photograph in artificial colors, the ζ Ophiuchus looks bluish. It is located near the center of the picture and moves upward at a speed of 24 km / s. The mass of the star is 20 times greater than the solar mass . A strong stellar wind flies ahead of the star, compressing and heating interstellar matter and forming a head shock wave . Around are clouds of relatively undisturbed matter. Probably, ζ Ophiuchus was once a member of the binary star system, its companion was much more massive and earlier ended its life path. When a companion star exploded like a supernova , catastrophically losing mass, ζ Ophiuchus was swept away from the system. ζ Ophiuchus is located 460 light-years from us. Its luminosity is 65,000 times brighter than the sun . She would be one of the brightest stars in the sky if she had not been surrounded by a dense light-absorbing medium. This photograph, created by the WISE telescope, extends to 1.5 degrees , which covers about 12 light years ^[8] .

The Escaping Star AE of the Charioteer

AE Voznichy is a bright star just below and to the left of the center of this colorful portrait of the IC 405 nebula, also known as the Flaming Star Nebula ^[10] . Surrounded by a cosmic cloud, a hot variable star of spectral type O, with its energetic radiation, makes hydrogen glow along the gas fibers. The blue light of a star is reflected from interstellar dust . The star AE of the Ascendant was not born at all in the cloud that she highlights. Restoring the motion of a star in space, astronomers concluded that it was most likely born in the Orion nebula about 2.7 million years ago ^[11] . Gravitational interactions with the nearest stars more than two million years ago knocked her out of her native places along with another O-star - μ Dove . Runaway stars drifted in different directions, moving away from each other by 200 km / s. Currently, the angular distance between them is 70º ^[3] .

Three running stars are known in the OB1 association of Orion — apart from AE of the Ascendant and μ Dove , also 53 Aries ^[12] . The first two are almost identical in color, mass and age, and move at speeds of up to 100 km / s each, leaving the OB1 Orion association 2.5 million years ago. Astronomers Blaau and Morgan in 1954 suggested ^[13] that both stars gained such high speed due to any one event. Jies and Bolton in 1986 came to the conclusion ^[14] that AE of Voznichy , μ Dove , as well as a pair of massive stars with large eccentricities of the orbits called ι Orion (O and B giants) are the result of a two-on-two interaction, which caused the appearance of running stars. Neither AE Voznichy ’s nor Pigeon’s μ show signs of mass exchange in the past (this is judged by the amount of helium ), and therefore, most likely, the reason that these two stars were ejected from the cluster is precisely the dynamic scenario. Researchers, extrapolating to the past, found that the stars were ejected from the Trapezium of Orion about 2.7 million years ago ^[10] .

In 1952, Blaau discovered ^[15] that the Zeta Ophiuchus once belonged to the OB2 Scorpio association. She could fly either from the group of the Upper Scorpion 1 million years ago or from the group of the Upper Centaurus Wolf 3 million years ago. Ophiuchus ζ properties (such as helium quantity and rotational speed ) indicate that it was once part of a close binary system . Astronomers have tested radio pulsars within a radius of 1 kpc - supernova remnants for which their relative motions can be reliably measured. As a result, a pulsar PSR J193211059 , up to 3 million years old, was discovered, which left the Upper Scorpio group 1 million years ago with a radial velocity of about 200 km / s. All this gave strong confirmation that they were once a couple, and the exploding star threw the Ophiuchus ζ in one direction, and itself flew in the other.

About half of the known OB stars are members of binary systems. Modern evolutionary scenarios for such systems were developed by Edward van den Heuvel ^[16] . He realized that during the evolution of the close binary system , there is a phase of intense mass transfer, as a result of which the substance flows from a heavy star to its lighter satellite. This has important implications for the further development of the system. Mass transfer occurs several million years or less, if a heavy, and consequently the fastest growing star increases in size and becomes a supergiant many times larger than our Sun. The transfer rate of a substance can become so large that it is initially a heavy star, in the end, it becomes lighter than its companion. The stage of mass transfer will not change the ultimate fate of the supergiant , and it will still be the first of two to explode as a supernova . An important result of the mass transfer process, however, is that the central remainder after a supernova explosion , that is, a neutron star or a black hole, will remain gravitationally bound and will remain in orbit around the OB star, even after it receives a high escape velocity.

Thus, from what is known about the evolution of heavy stars in binary systems, an OB star that was ejected from the OB association by a supernova explosion should be accompanied by a compact stellar remnant. However, in the past, many astronomers have carefully examined runaway OB stars for the presence of a neutron star or black hole , but nothing of the kind has been found. This negative observation, obviously, does not confirm the scenario of a supernova explosion . But based on new observations, a group of astronomers , led by Lex Kaper of ESO , found ^[17] that the well-known binary system Vela X-1 , consisting of an OB star and a neutron star, has all the characteristics of a runaway star. Vela X-1 is the brightest X-ray source in the constellation Sails . It consists of the so-called X - ray pulsar ^[18] , which, of course, is a neutron star formed as a result of a supernova explosion and a companion - an OB star.

The image taken at the ESO in the vicinity of the relatively bright OB star HD 77581 and its satellite Vela X-1 (optically invisible) was taken with the 1.54-m telescope at La Silla Observatory , through the narrow-band H-alpha filter, it clearly shows the presence of a typical shock the head wave, thus immediately confirming the status of this system as “runaway” ^[19] . In fact, this is one of the most “perfect” shock waves of a parabolic shape, which has never been so clearly seen around a runaway OB star ^[20] . In addition, the orientation of the shock wave indicates that the system is moving north, and its origin, respectively, should lie south of its current position. It is there that the well - known OB association Vel OB1 is located.

Based on the measurement of the distance to Vel OB1 (about 6,000 light-years ) and the observed proper motion and radial speed of HD 77581, it can be calculated that the Vela X-1 system moves at a speed of 90 km / s. At this speed, the HD 77581 and its compact satellite would take about 2.5 million years to fly out of the Vel OB1 association and take their current position. This corresponds exactly to the expected time that has passed since the explosion of the supernova progenitor ^[4] .