What are Super Velocity Stars?
A runaway star, a runaway star, is one that moves with an anomalous
The proper motion of such a star is oftenis indicated precisely in relation to the stellar association, of which she once had to become a member before being thrown out of it. Our Sun is just one of 400 billion stars in our galaxy, the Milky Way.
The galaxy rotates slowly, making oneturnover over 250 million years. Most stars in the Milky Way keep pace with its slow rotation: the Sun, for example, has a speed of 19.4 km/s relative to other stars. But there are also “runaway stars” in the galaxy: their speed relative to other stars is up to 200 km/s.
About 10-30% of stars of spectral class O and 5-10%all stars of spectral type B have velocities of a similar order. All of them are relatively young inhabitants of the galaxy - up to 50 million years old, and during this time they travel relatively short distances in space - from hundreds of parsecs to several kiloparsecs, so it sometimes seems possible to determine the cluster in which they were born.
Runaway stars and bow shock
Some runaway stars produce a bowa shock wave of compressed matter, which is very similar to the 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 runaway star moves at high speedthrough the interstellar medium (a very thin mixture of gas and dust) at supersonic speeds, the interstellar matter becomes visible as a bow shock wave.
The term "Supersonic Speed" means thatthe 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 speed is about 330 m/s, in almost empty interstellar space its value is approximately 10 km/s.
Thus, the detection of the bow shock wavearound an OB star means that it is traveling at supersonic speed, and thus it can be reliably identified as a runaway star, even if its speed has not been measured directly.
Hubble Space Telescope images of runaway stars between October 2005 and July 2006. Source: NASA
Characteristics of the stars
At a distance of 750 pc from the Sun, 56 are knownrunaway stars. These stars are almost no different from the rest of the stars in the disk component of the galaxy in all their parameters, except for their high spatial velocity. Four stars from this group have a mass greater than 25 solar (for them the mass is determined by the type of spectrum with not very high accuracy).
It is now assumed that such starsare formed either during the dynamic evolution of the clusters and associations in which they were born (the most likely reason is a close triple encounter), or as a result of the disintegration of a binary system during a supernova explosion, when a running star receives the initial impulse from the explosion of a companion star.
While both are theoretically possiblemechanism, astronomers in practice are usually inclined to the hypothesis of a supernova explosion. R. Hoogerwerth and his colleagues at the Leiden Observatory in the Netherlands used data from the Hipparcos satellite to track the movements of 56 runaway stars through time and found evidence to support both theories.
The authors traced the movement of these stars in the galaxyand for most of them (including all four massive ones), we found when and from which association these stars flew out, as well as which of the two possible ejection mechanisms operated for each particular star (most of the stars were ejected during the decay of binaries).
Most likely, all four massive runaway stars acquired their high spatial speed as a result of supernova explosions in binary systems.
The authors provide several arguments in favor of this conclusion:
- These stars are very massive.In order to be thrown out of the cluster (association), they had to fly near not much less massive stars. Otherwise, according to the law of conservation of momentum, it would be less massive stars that would have been thrown out of the system. And there are very few such massive stars - this is a direct consequence of Salpeter's law. The close flyby of several massive stars turns out to be an extremely rare event, compared to the rather rare close triple encounters of low-mass stars.
- Massive stars only live a few millionyears old. This fact imposes an additional limitation on the described rare event - the approach must have time to occur before massive stars exploded as supernovae.
- These stars fly at speeds several timeshigher than the dispersion of the velocities of the associations in which they were born. By itself, this fact does not contradict anything; after a successful close approach, the stars can acquire sufficiently 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 own spatial velocity after its disintegration due to a supernova explosion.
Determining the percentage of the first and second mechanisms in the formation of runaway stars imposes strong restrictions on theories of cluster formation and stellar evolution.
Numerical simulations done in 2000 showed that the number of stars escaping could help determine, for example, the number of binary pairs born in clusters.
Radial velocities measured for just oneone-third of the O-B stars of the Hipparcos catalog. According to the available data, we can say that both mechanisms are roughly 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 their age and their initial velocities.
- Runaway Star α Giraffe
The star is located in the constellation Giraffe and is distant fromEarth is four thousand light years away. 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 500 thousand times brighter than the Sun.
The runaway star α Giraffe creates a heada shock wave that travels at a speed of 60 km/s and compresses the interstellar medium along its path. The head wave is about ten light years away from the star itself.
The star also emits a powerful stellar wind.Astronomers have long believed that α Giraffe was ejected from a nearby cluster of young, hot stars due to gravitational interactions with other members of the cluster. According to another hypothesis, the star could have acquired speed (having flown out of the binary system) as a result of the explosion of a massive companion star as a supernova.
- Runaway Star ζ Ophiuchus
As ζ Ophiuchis moves, it forms an arcing wave of interstellar material ahead of it, clearly visible in this colorful infrared image taken by the WISE spacecraft.
Pictured in artificial colors ζ Ophiuchuslooks 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 that of the sun. A strong stellar wind flies in front of the star, compressing and heating interstellar matter and forming a bow shock.
- Runaway Star AE Charioteer
AE Aurigae is the bright star just below and to the left of center in this colorful portrait of the nebula IC 405, also known as the Flaming Star Nebula.
Hot Variable Surrounded by a Space Clouda star of spectral type O, with its energetic radiation, makes hydrogen, located along the gas filaments, glow. The star's blue light reflects off interstellar dust. The star AE Charioteer was born in a completely different cloud that it highlights.
Infrared image of a bow shock wave (yellow arc) created by the escape star ζ Ophiuchus in an interstellar cloud of dust and gas
Speed record
One of the fastest escaping stars is US 708.constellation Ursa Major. It was discovered in 1982 and rediscovered in 2005. For a long time it was believed that it, like other objects of this type, was thrown out by a supermassive black hole from the center of the Galaxy.
S5-HSV1 set a new US 708 record in 2019in the constellation Crane. The object was discovered in the survey of the Anglo-Australian telescope. Its speed is 1.7 thousand kilometers per second. It is now the only highly proven supervelocity star to have come from the center of the Milky Way.
How Can Super Velocity Stars Help?
Now high-velocity stars are being discovered in the halo- beyond the visible part of the galaxy. In contrast to the typical outskirt population of red, fading old stars, these are young, hot, blue giants.
Such are born in the center of the Milky Way, where it goesactive star formation. However, the strong gravitational field of the supermassive black hole tears them away from the pair and accelerates them to the speed of escape from the Galaxy. They are very bright and therefore easier to detect.
The existence of a halo of scientists brought an anomalythe speed of rotation of the outer regions of the Galaxy. It cannot be explained if there were only stars. A lot of extra mass is needed. It is called dark matter because we do not directly detect it.
What is the shape of the halo, spherical or flattened,unknown, but the trajectories of supervelocity stars will help answer this question. Analyzing this data, Professor Avi Loeb of Harvard and colleagues estimated the mass of the Milky Way, along with dark matter, at 1.2-1.9 trillion times the mass of the Sun.
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