Radiation astronomy/Hypervelocity stars
The image on the right shows four high-velocity, runaway stars plowing through their local interstellar medium.
"Resembling comets streaking across the sky, these four speedy stars are plowing through regions of dense interstellar gas and creating brilliant arrowhead structures and trailing tails of glowing gas."
"These bright arrowheads, or bow shocks, can be seen in these four images taken with NASA's Hubble Space Telescope. The bow shocks form when the stars' powerful stellar winds, streams of matter flowing from the stars, slam into surrounding dense gas. The phenomenon is similar to that seen when a speeding boat pushes through water on a lake."
"The stars in these images are among 14 runaway stars spotted by Hubble's Advanced Camera for Surveys. The stars appear to be young, just millions of years old. Their ages are based on their colors and the presence of strong stellar winds, a signature of youthful stars."
"Depending on their distance from Earth, the bullet-nosed bow shocks could be 100 billion to a trillion miles wide (the equivalent of 17 to 170 solar system diameters, measured out to Neptune's orbit). The bow shocks indicate that the stars are moving fast, more than 112,000 miles an hour (more than 180,000 kilometers an hour) with respect to the dense gas they are plowing through. They are traveling roughly five times faster than typical young stars, relative to their surroundings."
"The high-speed stars have traveled far from their birth places. Assuming their youthful phase lasts only a million years and they are moving at roughly 112,000 miles an hour, the stars have journeyed 160 light-years."
"The Hubble observations were taken between October 2005 and July 2006."
"Total velocities in the Galactic rest frame are computed correcting radial velocities and proper motions for the solar and the local standard of rest (LSR) motion (Schönrich 2012). In doing so, we assume that the distance between the Sun and the [Galactic Center] GC is d⊙ = 8.2 kpc, and that the Sun has an height above the stellar disk of z⊙ = 25 pc (Bland-Hawthorn & Gerhard 2016). We assume a rotation velocity at the Sun position vLSR = 238 km s−1 and a Sun’s peculiar velocity vector v⊙ = [U⊙,V⊙,W⊙] = [14.0,12.24,7.25] km s−1 (Schönrich et al. 2010; Schönrich 2012; Bland-Hawthorn & Gerhard 2016)."
Theoretical high velocity stars
Def. a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighbourhood is called a high-velocity star.
Def. a high-velocity star moving through space with an abnormally high velocity relative to the surrounding interstellar medium is called a runaway star.
Def. a star whose elliptical orbit takes it well outside the plane of [its galaxy] at steep angles is called a halo star.
Mechanisms that may give rise to a runaway star
- Gravitational interactions between stars in a stellar system can result in large accelerations of one or more of the involved stars. In some cases, stars may even be ejected. This can occur in seemingly stable star systems of only three stars, as described in studies of the three-body problem in gravitational theory.
- A collision or close encounter between stellar systems, including galaxies, may result in the disruption of both systems, with some of the stars being accelerated to high velocities, or even ejected. A large-scale example is the gravitational interaction between the Milky Way Galaxy and the Large Magellanic Cloud.
- A supernova explosion in a multiple star system can accelerate both the supernova remnant and/or remaining stars to high velocities.
High velocity stars
"Stars with extremely high velocities have been long studied to probe our Galaxy. The interest in the high velocity tail of the total velocity distribution of stars in our Milky Way is twofold. First, it flags the presence of extreme dynamical and astrophysical processes, especially when the velocity of a star is so high that it approaches (or even exceeds) the escape speed from the Galaxy at its position. Secondly, high velocity stars, spanning a large range of distances, can be used as dynamical tracers of integral properties of the Galaxy. The stellar high velocity distribution has for example been used to trace the local Galactic escape speed and the mass of the Milky Way (e.g. Smith et al. 2007; Gnedin et al. 2010; Piffl et al. 2014). To put the concept of high velocity in context, the value of the escape speed is found to be ∼ 530 km s−1 at the Sun position, it increases up to ∼ 600 km s−1 in the central regions of the Galaxy, and then falls down to ≲ 400 km s−1 at Galactocentric distances ∼ 50 kpc (Williams et al. 2017)."
On the right is an image of WR 124 a Wolf–Rayet star in the constellation of Sagitta surrounded by a ring nebula of expelled material known as M1-67.
Its a runaway star with a radial velocity around 200 km/s, discovered in 1938, and identified as a high velocity Wolf–Rayet star. It is listed in the General Catalogue of Variable Stars as QR Sagittae with a range of 0.08 magnitudes.
The Gaia Data Release 2 parallax is 0.1153±0.0365 mas, leading to a statistical distance estimate of pc.
The expansion rate of the M1-67 nebula expelled from the star has been directly measured using the Hubble WFPC2 camera images taken 11 years apart, and compared that rate to the expansion velocity measured by the Doppler shift of the nebular emission lines. The distance calculated from the nebular expansion rate is 3.35 kpc. Previous distances of 5kpc to 8.4kpc, have corresponding luminosities of 338,000-1,000,000 L☉.
WR stars of lower metallicity can form from lower mass progenitors and have lower luminosity, but this would be unusual for a Population I star within the Milky Way. A young highly massive and luminous WN8h star would still be burning hydrogen in its core, but a less luminous and older star would be burning helium in its core. The result of modelling the star purely from its observed characteristics is a luminosity of 1,000,000 L☉ and a mass of 33 M☉, corresponding to a relatively young hydrogen-burning star at around 8 kpc The mass loss rate is 10−5 - 10−4 M☉ per year, depending on the distance and properties determined for the star. WR 124 can be seen as a glowing body in the center of a gigantic fireball.
Fast halo stars
"A first class of objects that can be found in the high tail of the total velocity distribution is fast halo stars. Their measured dispersion velocity is around 150 km s−1 (Smith et al. 2009; Evans et al. 2016), therefore 3-σ outliers can exceed 450 km s−1, while remaining bound. Halo stars could also reach unbound velocities, when they are part of the debris of tidally disrupted satellite galaxies, like the Sagittarius Dwarf galaxy, that has not yet virialized (e.g. Abadi et al. 2009). Velocities outliers in the bulge and disk velocity distribution may also exist and become apparent in a large data set."
"Looking towards the constellation of Triangulum (The Triangle), in the northern sky, lies the galaxy pair MRK 1034. The two very similar galaxies, named PGC 9074 and PGC 9071, are close enough to one another to be bound together by gravity, although no gravitational disturbance can yet be seen in the image. These objects are probably only just beginning to interact gravitationally."
"Both are spiral galaxies, and are presented to our eyes face-on, so we are able to appreciate their distinctive shapes. On the left of the image, spiral galaxy PGC 9074 shows a bright bulge and two spiral arms tightly wound around the nucleus, features which have led scientists to classify it as a type Sa galaxy. Close by, PGC 9071 — a type Sb galaxy — although very similar and almost the same size as its neighbour, has a fainter bulge and a slightly different structure to its arms: their coils are further apart."
"The spiral arms of both objects clearly show dark patches of dust obscuring the light of the stars lying behind, mixed with bright blue clusters of hot, recently-formed stars. Older, cooler stars can be found in the glowing, compact yellowish bulge towards the centre of the galaxy. The whole structure of each galaxy is surrounded by a much fainter round halo of old stars, some residing in globular clusters."
"Gradually, these two neighbours will attract each other, the process of star formation will be increased and tidal forces will throw out long tails of stars and gas. Eventually, after maybe hundreds of millions of years, the structures of the interacting galaxies will merge together into a new, larger galaxy."
Variable type: BY Draconis variable (BY Dra), Temperature = 3550 ± 50 K, Rotational velocity = 9.15 km/s, Visibility: The star is at an apparent magnitude of 9 and is visible through binoculars or a telescope in the constellation of Pictor, in the southern sky.
Radial velocity (cz) = 245.29 ± 0.10 km/s, Spectral type: M1VIp, V* VZ Pic, HD 33793, IRAS 05100-4502, 2MASS J05114046-4501051 and Gaia DR2 4810594479417465600.
""Runaway stars" (RSs) form an another class of high velocity stars. They were originally introduced as O and B type stars ejected from the Galactic disk with velocities higher than 40 km s−1 (Blaauw 1961). Theoretically, there are two main formation channels: i) dynamical encounters between stars in dense stellar systems such as young star clusters (e.g. Poveda et al. 1967; Leonard & Duncan 1990; Gvaramadze et al. 2009), and ii) supernova explosions in stellar binary systems (e.g. Blaauw 1961; Portegies Zwart 2000). Both mechanisms have been shown to occur in our Galaxy (Hoogerwerf et al. 2001). Typical velocities attained by the two formation channels are of the order of a few tens of km s−1, and even if several hundreds of km s−1 can be attained for the most extreme systems (Portegies Zwart 2000; Przybilla et al. 2008; Gvaramadze et al. 2009; Gvaramadze & Gualandris 2011; Silva & Napiwotzki 2011), simulations indicate that the majority of runaway stars from dynamical encounters have ejection velocities ≲ 200 km s−1 (Perets & Šubr 2012). Recent results show that it is possible to achieve ejection velocities up to ∼ 1300 km s−1 for low-mass G/K type stars in very compact binaries (Tauris 2015). Nevertheless, the rate of production of unbound RSs, referred to as hyper runaway stars (HRSs), is estimated to be as low as 8 · 10−7 yr−1 (Perets & Šubr 2012; Brown 2015)."
"A heavy runaway star rushing away from a nearby stellar nursery at more than 400 000 kilometres per hour, a speed that would get you to the Moon and back in two hours [is shown in the image on the right]. The runaway is the most extreme case of a very massive star that has been kicked out of its home by a group of even heftier siblings. Tantalising clues from three observatories, including the NASA/ESA Hubble Space Telescope’s newly installed Cosmic Origins Spectrograph (COS), and some old-fashioned detective work, suggest that the star may have travelled about 375 light-years from its suspected home, a giant star cluster called R136."
"The homeless star is on the outskirts of the 30 Doradus Nebula, a raucous stellar breeding ground in the nearby Large Magellanic Cloud. The finding bolsters evidence that the most massive stars in the local Universe reside in 30 Doradus, making it a unique laboratory for studying heavyweight stars. 30 Doradus, also called the Tarantula Nebula, is roughly 170 000 light-years from Earth. Nestled in the core of 30 Doradus, R136 contains several stars topping 100 solar masses each."
"The observations offer insights into how massive star clusters behave."
"These results are of great interest because such dynamical processes in very dense, massive clusters have been predicted theoretically for some time, but this is the first direct observation of the process in such a region. Less massive runaway stars from the much smaller Orion Nebula Cluster were first found over half a century ago, but this is the first potential confirmation of more recent predictions applying to the most massive young clusters."
"Runaway stars can be made in a couple of ways. A star may encounter one or two heavier siblings in a massive, dense cluster and get booted out through a stellar game of pinball. Or, a star may get a “kick” from a supernova explosion in a binary system, with the more massive star exploding first."
"It is generally accepted, however, that R136 is young enough that the cluster’s most massive stars have not yet exploded as supernovae. This implies that the star must have been ejected through dynamical interaction."
"Hubble astronomers unexpectedly picked up another clue when they used the star as a target to calibrate the COS instrument, installed in May 2009 during Servicing Mission 4. Those ultraviolet spectroscopic observations, made in July 2009, showed that the wayward star is unleashing a fury of charged particles in one of the most powerful stellar winds known, a clear sign that it is extremely massive, perhaps as much as 90 times heavier than the Sun. The star, therefore, also must be very young, about one million to two million years old, because extremely massive stars only live for a few million years."
"Sifting through Hubble’s archive of images, astronomers found another important piece of evidence. An optical image of the star taken by the Wide Field Planetary Camera 2 in 1995 revealed that it is at one end of an egg-shaped cavity. The cavity’s glowing edges stretch behind the star and point in the direction of its home in 30 Doradus."
"Another spectroscopic study from the European Southern Observatory’s Very Large Telescope (VLT) at the Paranal Observatory in Chile revealed that the star’s velocity is constant and not a result of orbital motion in a binary system. Its velocity corresponds to an unusual motion relative to the star’s surroundings, evidence that it is a runaway star."
"The study also confirmed that the light from the runaway is from a single massive star rather than the combined light of two lower mass stars. In addition, the observation established that the star is about ten times hotter than the Sun, a temperature that is consistent with a high-mass object."
AE Aur is a runaway star that might have been ejected during a collision of two binary star groups, which also is credited with ejecting Mu Columbae and possibly 53 Arietis, and has been traced to the Trapezium cluster in the Orion Nebula two million years ago, where the binary Iota Orionis may have been the other half of this collision.
AE Aur is seen to light up the Flaming Star nebula, but it was not formed within it and is passing through the nebula at high speed producing a violent bow shock and high energy electromagnetic radiation.
Spectral class: O9 III + B0.8 III/IV, Variable type: (B) Orion, Radial velocity (cz) = 21.5 km/s , Temperature (ι Ori Aa) = 32,500 K, Rotational velocity = 122 km/s , Temperature (ι Ori Ab) = 27,000 K, Temperature (ι Ori B) = 18,000 K
Iota Orionis (ι Orionis, abbreviated ι Ori) is a multiple star system in the equatorial constellation of Orion the hunter, the eighth-brightest member of Orion with an apparent visual magnitude of 2.77, the brightest member of the asterism known as Orion's Sword, a member of the NGC 1980 open cluster, and from parallax measurements, is located at a distance of roughly 2,300 light-years (710 parsecs) from the Sun.
The system has three components designated Iota Orionis A, B and C, where Iota Orionis A is itself a massive spectroscopic binary, with components Iota Orionis Aa (officially named Hatysa and Ab, plus B and C.
Iota Orionis is dominated by Iota Orionis A whose two components are a stellar class O9 III star (blue giant) and a class B0.8 III/IV star about 2 magnitudes fainter. The collision of the stellar winds from this pair makes the system a strong X-ray source, but the two objects of this system appear to have different ages, with the secondary being about double the age of the primary, where in combination with the high eccentricity (e=0.764) of their 29-day orbit, the binary system may have been created through a capture, rather than by being formed together and undergoing a mass transfer, for example, through an encounter between two binary systems.
NGC 1980 contains few bright stars other than Iota Orionis, where only eighteen other stars are considered members in a survey down to 14th magnitude, most around 9th magnitude but including the 5th magnitude stars HR 1886 and 1887.
"As a class, the fastest stars in our Galaxy are expected to be hypervelocity stars (HVSs). These were first theoretically predicted by Hills (1988) as the result of a three-body interaction between a binary star and the massive black hole in the Galactic Centre (GC), Sagittarius A*. Following this close encounter, a star can be ejected with a velocity ∼ 1000 km s−1, sufficiently high to escape from the gravitational field of the Milky Way (Kenyon et al. 2008; Brown 2015). The first HVS candidate was discovered by Brown et al. (2005): a B-type star with a velocity more than twice the Galactic escape speed at its position. Currently about ∼ 20 unbound HVSs with velocities ∼ 300 - 700 km s−1 have been discovered by targeting young stars in the outer halo of the Milky Way (Brown et al. 2014)."
"HVSs are predicted to be ejected from the GC with an uncertain rate around 10−4 yr−1 (Yu & Tremaine 2003; Zhang et al. 2013), two orders of magnitude larger than the rate of ejection of runaway stars with comparable velocities from the stellar disk (Brown 2015). Because of their extremely high velocities, HVS trajectories span a large range of distances, from the GC to the outer halo."
Stars considered hypervelocity stars are those with radial velocity greater than |299| km/s, or 299 km/s < v < -299 km/s.
Bound hypervelocity stars
Stars "sharing the same formation scenario as HVSs, but with an ejection velocity which is not sufficiently high to escape from the whole Milky Way (e.g. Bromley et al. 2006). Most of the deceleration occurs in the inner few kpc due to the bulge potential (Kenyon et al. 2008), and the minimum velocity necessary at ejection to be unbound is of the order of ∼ 800 km s−1 (a precise value depends on the choice of the Galactic potential, Brown 2015; Rossi et al. 2017). If we consider the Hills mechanism , this population of bound stars is expected to be dominant over the sample of HVSs (Rossi et al. 2014; Marchetti et al. 2018)."
"At the moment, the fastest star discovered in our Galaxy is US 708, traveling away from the Milky Way with a total velocity ∼ 1200 km s−1 (Hirsch et al. 2005). Its orbit is not consistent with coming from the GC (Brown et al. 2015), and the most likely mechanism responsible for its acceleration is the explosion of a thermonuclear supernova in an ultra-compact binary in the Galactic disk (Geier et al. 2015)."
Radial velocity (cz) = -55.99 ± ~ km/s, ROSAT X-ray source 1RXS J011758.8+651730, Ariel X-ray source 3A 0114+650, SWIFT J0117.8+6516 X-ray source, and INTEGRAL1 1 gamma-ray source.
4U 1700-37 is one of the stronger binary X-ray sources in the sky, and is classified as a high-mass X-ray binary that was discovered by the Uhuru satellite.
Radial velocity = −75.00 ± 7.4 km/s.
Distance = 2120 ± 343 pc.
Variable type is Ellipsoidal]] + High-mass X-ray binaries (HMXB)
Optical component is HD 153919, Radial velocity (cz) = 33.38 ± 6.93 km/s, INTEGRAL1 39 (gamma-ray source), Einstein catalog 2E 1700.5-3746 (X-ray source), TD1 19850 (ultraviolet source).
The image on the left shows a high velocity binary star and the comet-like trail behind it is light years long. The binary is plowing through the interstellar medium. It's radiating, been radiated, and packs a punch!
"Ultra-violet studies of Mira by NASA's Galaxy Evolution Explorer (Galex) space telescope have revealed that it sheds a trail of material from the outer envelope, leaving a tail 13 light-years in length, formed over tens of thousands of years. It is thought that a hot bow-wave of compressed plasma/gas is the cause of the tail; the bow-wave is a result of the interaction of the stellar wind from Mira A with gas in interstellar space, through which Mira is moving at an extremely high speed of 130 kilometres/second (291,000 miles per hour). The tail consists of material stripped from the head of the bow-wave, which is also visible in ultra-violet observations. Mira's bow-shock will eventually evolve into a planetary nebula, the form of which will be considerably affected by the motion through the interstellar medium (ISM).
At second right is the only available X-ray image, by the Chandra X-ray Observatory, of Mira A on the right and Mira B (left). "Mira A is losing gas rapidly from its upper atmosphere [apparently] via a stellar wind. [Mira B is asserted to be a white dwarf. In theory] Mira B exerts a gravitational tug that creates a gaseous bridge between the two stars. Gas from the wind and bridge accumulates in an accretion disk around Mira B and collisions between rapidly moving particles in the disk produce X-rays."
At right "is a NASA Hubble Space Telescope image of the cool red giant star Mira A (right), officially called Omicron Ceti in the constellation Cetus, and its nearby hot companion (left) taken on December 11, 1995 in visible light using the European Space Agency's Faint Object Camera (FOC). The stars in this false-color picture are separated by an angular size of only 0.6 arcseconds (equal to 70 times the distance between Earth and the Sun), but clearly resolved by the FOC. Image reconstruction techniques have been used to further enhance the details in the Mira images."
Mira B, also known as VZ Ceti, is the companion star to the variable star Mira. Its orbit around Mira is poorly known; the most recent estimate listed in the Sixth Orbit Catalog of Visual Binary Stars gives an orbital period of roughly 500 years, with a periastron around the year 2285. Assuming the distance in the Hipparcos catalog and orbit are correct, Mira A and B are separated by an average of 100 AU.
Even more recently (2010) analysis of rapid optical brightness variations has indicated that Mira B is in fact a white dwarf.
Radial velocity (cz) = 338.45 ± 2.62 km/s, Spectral type: F2V, aka Gaia DR1 5489531875795980032, Gaia DR2 5489531880096156416, and 2MASS J07532122-5239133.
Radial velocity (cz) = 333.17 ± 0.35 km/s, Spectral type: G5V, 2MASS J07393021-5548171 is aka Gaia DR1 5487974692453649792, Gaia DR2 5487974692453649792 and OM 96.
Gaia DR2 1396963577886583296
"The extragalactic star with a highest probability of being unbound from our Galaxy is Gaia DR2 1396963577886583296, with a total velocity ∼ 700 km s−1, resulting in a probability Pub = 0.98. [...] This star is at ∼ 30 kpc from the GC, with an elevation of ∼ 25 kpc above the Galactic plane."
Gaia DR2 3736372993468775424
Radial velocity (cz) = 378.65 ± 1.11 km/s.
Gaia DR2 4554190291969378048
Radial velocity (cz) = -318.40 ± 0.59 km/s
Gaia DR2 4692184601887720064
Radial velocity (cz) = 319.79 ± 0.67 km/s, 2MASS J01095931-6808494 is aka OM 1, RAVE J010959.3-680849, UCAC2 2064411, UCAC3 44-2237, and UCAC4 110-001006.
Radial velocity (cz) = 300.36 ± 5.02 km/s, Spectral type: A5III, ASAS J060746-6658.6 is aka CSV 725, GCRV 26604, GSC 08905-00975, HIP 29055, 2MASS J06074571-6658388, Gaia DR1 5283957629860435072, Gaia DR2 5283957629860435072, OM 89, RAVE J060745.7-665839, SSTISAGEMC J060745.70-665838.9, SV* BV 458, SV* HV 7641, SV* HV 12250, TYC 8905-975-1, UCAC4 110-001006, and uvby98 620198089.
HD 271791 is a B-type hyperrunaway star ejected from the Galactic disk.
"HD 271791 (Heber et al. 2008) [is] a 11 ± 1 M⊙ B-giant stars established to be a run-away star with velocity similar to those of hypervelocity stars. HD 271791 is 21.8 ± 3.7 kpc away from the GC and −10.4 ± 2.0 kpc below the disk plane (Heber et al. 2008)".
Radial velocity (cz) = 366.22 ± ~ km/s, Spectral type: B2(III), 2MASS J06022786-6647286 is aka Gaia DR2 5284151216932205312, GCRV 26603, HIP 28618, OM 88, SSTISAGEMC J060227.86-664728.7, TYC 8905-1908-1, and uvby98 620198088.
"A hundred million years ago, a triple-star system was traveling through the bustling center of our Milky Way galaxy when it made a life-changing misstep. The trio wandered too close to the galaxy's giant black hole, which captured one of the stars and hurled the other two out of the Milky Way. Adding to the stellar game of musical chairs, the two outbound stars merged to form a super-hot, blue star."
"This story may seem like science fiction, but astronomers using NASA's Hubble Space Telescope say it is the most likely scenario for a so-called hypervelocity star, known as HE 0437-5439, one of the fastest ever detected. It is blazing across space at a speed of 1.6 million miles (2.5 million kilometers) an hour, three times faster than our Sun's orbital velocity in the Milky Way. Hubble observations confirm that the stellar speedster hails from the Milky Way's core, settling some confusion over where it originally called home."
"Most of the roughly 16 known hypervelocity stars, all discovered since 2005, are thought to be exiles from the heart of our galaxy. But this Hubble result is the first direct observation linking a high-flying star to a galactic center origin."
"Using Hubble, we can for the first time trace back to where the star comes from by measuring the star's direction of motion on the sky. Its motion points directly from the Milky Way center. These exiled stars are rare in the Milky Way's population of 100 billion stars. For every 100 million stars in the galaxy lurks one hypervelocity star."
"Studying these stars could provide more clues about the nature of some of the universe's unseen mass, and it could help astronomers better understand how galaxies form. Dark matter's gravitational pull is measured by the shape of the hyperfast stars' trajectories out of the Milky Way."
"The stellar outcast is already cruising in the Milky Way's distant outskirts, high above the galaxy's disk, about 200,000 light-years from the center. By comparison, the diameter of the Milky Way's disk is approximately 100,000 light-years. Using Hubble to measure the runaway star's direction of motion and determine the Milky Way's core as its starting point, [the] team calculated how fast the star had to have been ejected to reach its current location."
"The star is traveling at an absurd velocity, twice as much as the star needs to escape the galaxy's gravitational field. There is no star that travels that quickly under normal circumstances – something exotic has to happen."
"There's another twist to this story. Based on the speed and position of HE 0437-5439, the star would have to be 100 million years old to have journeyed from the Milky Way's core. Yet its mass – nine times that of our Sun – and blue color mean that it should have burned out after only 20 million years – far shorter than the transit time it took to get to its current location."
"The most likely explanation for the star's blue color and extreme speed is that it was part of a triple-star system that was involved in a gravitational billiard-ball game with the galaxy's monster black hole. This concept for imparting an escape velocity on stars was first proposed in 1988. The theory predicted that the Milky Way's black hole should eject a star about once every 100,000 years."
"The triple-star system contained a pair of closely orbiting stars and a third outer member also gravitationally tied to the group. The black hole pulled the outer star away from the tight binary system. The doomed star's momentum was transferred to the stellar twosome, boosting the duo to escape velocity from the galaxy. As the pair rocketed away, they went on with normal stellar evolution. The more massive companion evolved more quickly, puffing up to become a red giant. It enveloped its partner, and the two stars spiraled together, merging into one superstar – a blue straggler."
"While the blue straggler story may seem odd, you do see them in the Milky Way, and most stars are in multiple systems."
"This vagabond star has puzzled astronomers since its discovery in 2005 by the Hamburg/European Southern Observatory sky survey. Astronomers had proposed two possibilities to solve the age problem. The star either dipped into the Fountain of Youth by becoming a blue straggler, or it was flung out of the Large Magellanic Cloud, a neighboring galaxy."
"In 2008 a team of astronomers thought they had solved the mystery. They found a match between the exiled star's chemical makeup and the characteristics of stars in the Large Magellanic Cloud. The rogue star's position also is close to the neighboring galaxy, only 65,000 light-years away. The new Hubble result settles the debate over the star's birthplace."
"Astronomers used the sharp vision of Hubble's Advanced Camera for Surveys to make two separate observations of the wayward star 3 1/2 years apart. Team member Jay Anderson of the Space Telescope Science Institute in Baltimore, Md., developed a technique to measure the star's position relative to each of 11 distant background galaxies, which form a reference frame."
"Anderson then compared the star's position in images taken in 2006 with those taken in 2009 to calculate how far the star moved against the background galaxies. The star appeared to move, but only by 0.04 of a pixel (picture element) against the sky background."
"Hubble excels with this type of measurement. This observation would be challenging to do from the ground."
"The team is trying to determine the homes of four other unbound stars, all located on the fringes of the Milky Way."
"We are targeting massive 'B' stars, like HE 0437-5439. These stars shouldn't live long enough to reach the distant outskirts of the Milky Way, so we shouldn't expect to find them there. The density of stars in the outer region is much less than in the core, so we have a better chance to find these unusual objects."
"HE 0437-5439 is a B-type star, and is likely to be originated from the centre of the Large Magellanic Cloud (LMC) (Erkal et al. 2018)."
"HE 0437-5439 (a.k.a HVS 3; e.g., Edelmann et al. 2005; Bonanos et al. 2008; Przybilla et al. 2008), which is also a ∼9 M⊙ B-type star".
Radial velocity (cz) = 723.87 ± ~ km/s, Spectral type: sdB+F, [BGK2006] HV 3 is aka Gaia DR2 4777328613382967040.
Radial velocity (cz) = 361.60 ± 12.52 km/s, Spectral type: B, aka 2MASS J03211707+1907363 and Gaia DR2 56282900715073664.
Heliocentric distance = 22.32 ± 2.50 kpc, Radial velocity vlos = 361.38 ± 12.52 km/s, vrf = 408.33 ± 12.57 km/s, Spectral type B7V, Luminosity (L⊙) = 309, Teff = 14000 K, r = 29.59 ± 2.51 kpc, Z = −11.56 ± 1.30 kpc.
LAMOST-HVS3 is the lowest spectrum in the figures on the right.
In the figure on the left, "Values of radial velocity in the Galactic rest-frame vrf of 24 HVSs (blue circles; Brown et al. 2014; Brown 2015) discovered in other surveys and of LAMOST-HVS1,2,3 (red stars), plotted against the Galactocentric radius r. The blue circles represent the 21 B-type HVSs discovered from systematic searches (e.g. Brown et al. 2014) while the blue box, triangle and inverted triangle represent the three serendipitously discovered HVSs: US 708 (Hirsch et al. 2005), HD 271791 and HE 0437-5439 respectively. We note that the velocity plotted of star HD 271791 is the total velocity rather than the Galactic rest-frame radial velocity. Black long and short dashed lines represent the Galactic escape velocity curve predicted by the Milky Way mass model constructed by Huang et al. 2016 and 2017, respectively."
"According to Lu et al. 2010 and Zhang et al. (2010, 2013), for HVSs originating from the GC, their spatial distribution can be used to trace the parent population of their progenitors directly. Lu et al. (2010) suggest that HVSs identified hitherto may arise from the two young star-forming regions near the GC [...]: a clockwise young stellar disk (CWS) and a northern arm (Narm). Similar to Lu et al. (2010), [...] the spatial distribution in the Galactic coordinate [can be plotted] as viewed from the GC) [including] the three HVSs discovered by LAMOST. The planes (projected to infinity) of several young stellar structures near the GC [include] the CWS, the Narm, the counter-clockwise young stellar disk (CCWS), the outer warped CWS and the bar. As pointed out by Z14 [Zheng et al. (2014)], LAMOST-HVS1 is spatially very close to the outer CWS but also not far from the Narm. LAMOST-HVS2 is spatially very close to the CCWS but also not far from the outer CWS. LAMOST-HVS3 is very close to the bar. The spatial association may suggest that all the three LAMOST HVSs may originate from the GC and their progenitors are spatially associated with young stellar structures near the GC, including the outer CWS, the CCWS, and the bar. In addition, the discovery of the three LAMOST HVSs reduces the apparent anisotropy of spatial distribution of known HVSs on the sky."
Assuming "that the three LAMOST HVSs arise from the GC, one can calculate their flight times. It should be noted that the flight times derived here are actually upper limits since it is assumed that the observed radial velocities represent the full space motion of the HVSs. The [...] flight times of LAMOST-HVS1 and 2 are larger than their life times, implying that they do not have enough time to travel to their current positions from the GC. If their GC origin holds, the possible explanation is that these two stars are blue stragglers that have experienced a similar process to HVS HE 0437-5439. The flight time (∼ 58 Myr) of LAMOST-HVS3 is substantially smaller than its life time (∼ 80 Myr), suggesting that it has enough time to travel to its current position even there is a few million delay in its ejection from the GC (Brown et al. 2012)."
Radial velocity (cz) = 427.30 ± ~ km/s, Spectral type: A0, aka 2MASS J08060876+0633499.
Radial velocity (cz) = 428.3 ± ~ km/s, Spectral type: A5, aka 2MASS J08581989+1503526.
LAMOST J091206.52+091621.8 is LAMOST-HVS1.
The "first hypervelocity star (HVS) discovered from the LAMOST spectroscopic survey [...] is a B-type star with a heliocentric radial velocity about 620 km s−1, which projects to a Galactocentric radial velocity component of ∼477 km s−1. With a heliocentric distance of ∼13 kpc and an apparent magnitude of ∼13 mag, it is the nearest bright HVS currently known. With a mass of ∼9M⊙, it is one of the three most massive HVSs discovered so far."
Teff = (2.07 ± 0.12) × 104 K.
Metallicity [Fe/H] = −0.13 ± 0.07.
Radial velocity (cz) = 612.28 ± 4.63 km/s, 2MASS J09120652+0916216 and Gaia DR2 590511484409775360.
Radial velocity (cz) = 453.34 ± ~ km/s, Spectral type: A5, 2MASS J09184992-0053313.
Radial velocity (cz) = 306.16 ± - km/s, Spectral type: F5, 2MASS J09270711+2427530.
Radial velocity (cz) = 430.31 ± - km/s, Spectral type: F0, aka 2MASS J09412236-0008221.
"Only one late-type star, LAMOST J115209.12+120258.0 (Li et al. 2015), is most likely unbound, but the Hills mechanisms is ruled out as a possible explanation of its extremely high velocity."
"LAMOST J115209.12+120258.0 (Li et al. 2015) [...] moves on an unbound orbit not originating in the [Galactic Centre] GC."
"According to its atmospheric parameters, it is either a B-type main-sequence (MS) star or a blue horizontal branch (BHB) star. Its Galactocentric distance and velocity are 30.3 ± 1.6 kpc and 586 ± 7 kms−1 if it is an MS star, and they are 13.2 ± 3.7 kpc and 590 ± 7 kms−1 if a BHB star."
LAMOST J115209.12+120258.0 is an F-type runaway star "ejected from the Galactic disk".
"The star (J225837.56+400005.2; LAMOST-HVS4) was observed on November 21, 2017, and it has magnitudes (g/r/i/z/y) of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS; Kaiser et al. 2002, 2010; Chambers et al. 2016) larger than 16.7 mag. The heliocentric radial velocity is vr⊙ = 359 ± 7 km s−1, and we check it using the cross-correlation package RVSAO of IRAF (Kurtz & Mink 1998). The value is consistent with the radial velocity provided by LAMOST."
The "heliocentric radial velocity (vr⊙) [is translated] to a Galactocentric radial component of vrf = 585 ± 7 km s−1 according to
where "VLSR = 235 km s−1 for the motion of the local standard of rest (LSR) (Hogg et al. 2005; Bovy et al. 2012; Reid et al. 2014) and (U0, V0, W0) = (9.58, 10.52, 7.01) km s−1 for the peculiar motion of the Sun with respect to the LSR (Tian et al. 2015; c.f., Schönrich et al. 2010; Huang et al. 2015)."
Radial velocity (cz) = 359.22 ± 7.00 km/s, LAMOST-HVS4 is aka Gaia DR2 1928660566125735680.
LP 40-365 is a low-mass white dwarf in the constellation Ursa Minor that travels at high speed through the Milky Way, has a very unusual elemental composition, lacking hydrogen, helium or carbon and may have been produced in a subluminous supernova type Iax that failed to destroy its host star totally. The "LP" name is derived from the Luyten-Palomar proper motion catalogue in which it appeared in the 1960s. Another catalog name for this star is "GD 492". The star was cataloged as a Giclas object with the designation "GD 492" being assigned by Henry Giclas in 1970.
Radial velocity (cz) = 498.01 ± 1.10 km/s.
Radial velocity (cz) = 341.29 ± 7.79 km/s, Spectral type: sdB, LAMOST-HVS2 is aka 2MASS J16202076+3747399, Gaia DR2 1330715287893559936, KUV 16186+3755, and SDSS J162020.76+374740.0.
Spectral type: B2V, Radial velocity (cz) vlos = 341.10 ± 7.79 km/s, vrf = 502.33 ± 8.37 km/s, Luminosity (L⊙) = 2399, Teff = 20600 K, r 20.86 ± 4.57 kpc, Z 15.80 ± 3.25 kpc, Heliocentric distance 22.24 ± 4.57 kpc.
"The [image on the right] shows two observations of [the] neutron star [RX J0822-4300] obtained with the Chandra X-ray Observatory over the span of five years, between December 1999 [on the left] and April 2005 [on the right]. By combining how far it has moved across the sky with its distance from Earth [at about 7,000 light years], astronomers determined the cosmic cannonball is moving at over 3 million miles per hour, one of the fastest moving stars ever observed. At this rate, RX J0822-4300 [at (J2000) RA 08h 23m 08.16s Dec -42° 41' 41.40" in Puppis] is destined to escape from the Milky Way after millions of years, even though it has only traveled about 20 light years so far."
A recoil velocity of 672±115 km/s is much less problematic theoretically.
SDSS J013655.91+242546.0 is an A-type hyperrunaway star "ejected from the Galactic disk".
Radial velocity (cz) = 357.01 ± 2.80 km/s
Radial velocity (cz) = 350.8 ± 6.0 km/s, Gaia DR2 887676144156721664.
Radial velocity (cz) = 361.42 km/s, Spectral type: B8, [BGK2006] J074950.24+243841.2 is aka Gaia DR2 867705370863613056.
Sloan Digital Sky Survey (SDSS) J090744.99+024506.8 (SDSS 090745.0+024507), a short period variable star which has a Galactic rest-frame radial velocity of 709 km/s, may have been originally part of a binary system that was tidally disrupted by the supermassive black hole at the centre of the Milky Way, causing it to be ejected at high velocity, having an effective temperature of 10,500 K (spectral type B9) and age estimated at 350 million years, a heliocentric distance of 71 kpc, ejected from the centre of the galaxy less than 100 million years ago, which implies the existence of a population of young stars at the galactic centre less than 100 million years ago.
The Outcast Star is the first discovered member of a class of objects named hypervelocity stars, discovered in 2005 at the MMT Observatory of the Harvard-Smithsonian Center for Astrophysics (CfA).
Radial velocity (cz) = 832.36 ± 8.90 km/s, Spectral type: B, aka [BGK2006] HV 1 and Gaia DR2 577294697514301440.
Radial velocity (cz) = 605.6 ± 6.0 km/s, Spectral type: B, [BGK2006] J091301.01+305119.8, Gaia DR2 699811079173836928, and USNO-B1.0 1208-00168480.
Radial velocity (cz) = 553.51 ± 9.00 km/s, Spectral type: B, [BGK2006] HV 5 is aka [BGK2006] J091759.48+672238.3, Gaia DR2 1069326945513133952, and USNO-B1.0 1573-00143533.
Radial velocity (cz) = 512.64 ± 7.30 km/s, Spectral type: B, [BGK2006] HV 8 is aka USNO-B1.0 1100-00174712 and Gaia DR2 633599760258827776
Radial velocity (cz) = 467.7 ± 9.0 km/s, Spectral type: A, aka [BGK2006] HV 11 and Gaia DR2 3833516903071703040.
Radial velocity (cz) = 627.46 ± 9.20 km/s, Spectral type: B, aka USNO-B1.0 0891-00196913 and Gaia DR2 3830584196322129920.
Radial velocity (cz) = 504.42 ± 8.70 km/s, Spectral type: B, aka SDSS J103357.26-011507.4 and Gaia DR2 3782644733437017984.
Radial velocity (cz) = 355.11 ± 10.70 km/s, Spectral type: B, aka [BGK2006] HV 21 and Gaia DR2 834069905715968640.
Radial velocity (cz) = 448.34 ± 13.30 km/s, Spectral type: B, aka [BGK2006] HV 21 and Gaia DR2 3805700495839529088.
[BGK2006] HV 14 is a high-velocity star with radial velocity (cz) = 538.08 ± 9.30 km/s, Spectral type: B, aka Gaia DR2 3859275333773935488.
[BGK2006] HV 12 is a high-velocity star with radial velocity (cz) = 503.7 ± 24.0 km/s, Spectral type: B, aka Gaia DR2 3809777626689513216 and USNO-B1.0 0932-00228004.
Radial velocity (cz) = 579.26 ± 8.30 km/s, Spectral type: B, [BGK2006] HV 13 is aka Gaia DR2 3804790100211231104.
Radial velocity (cz) = 451.74 ± 9.60 km/s, Spectral type: A1, [BGK2006] J110224.37+025002.8 is aka Gaia DR2 3814622895259904256.
"Here we report the [...] recently discovered HVSs: SDSS J110557.45+093439.5 [...] traveling with Galactic rest-frame velocities at least +508+/-12 [...]."
Radial velocity (cz) = 625.25 ± 8.40 km/s, Spectral type: B, is aka [BGK2006] HV 6, [BGK2006] J110557.45+093439.5, Gaia DR2 3867267443277880320, and SDSS J110557.45+093439.4.
Radial velocity (cz) = 496.61 ± 9.80 km/s, Spectral type: B, is aka [BGK2006] HV 24 and Gaia DR2 3810351984075984768.
Gaia DR2 3590660794817798016 has a radial velocity (cz) = 482.39 ± 8.10 km/s, Spectral type: B.
"Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star’s place of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS 7."
"Here we report the [...] most recently discovered HVSs: [...] SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least [...] +418+/-10 km s-1 [...]."
In 2013 a team under N. Przybilla wrote that the star had a chemically peculiar photosphere, which masked its origins.
The star was first cataloged during the Sloan Digital Sky Survey and was identified as a hyper-velocity star in 2006.
Radial velocity (cz) = 518.6 ± 3.0 km/s, Spectral type: sdB, [BGK2006] HV 7 are aka [BGK2006] J113312.12+010824.9, EPIC 201540171, Gaia DR2 3799146650623432704, GALEX 2413439155581226272, and USNO-A2.0 0900-06954189.
Radial velocity (cz) = 464.7 ± 12.0 km/s, Spectral type: B, 2QZ J113341.0-012115, [BGK2006] HV 15 are aka [BGK2006] J113341.09-012114.3 and Gaia DR2 3794074603484360704.
Radial velocity (cz) = 597.60 ± ~ km/s, Spectral type: B, aka [BGK2006] HV 19 and Gaia DR2 3911105521632982400.
Radial velocity (cz) = 504.42 ± - km/s, Spectral type: B, aka [BGK2006] HV 20 and Gaia DR2 3800802102817768832.
Radial velocity (cz) = 424.40 ± 8.60 km/s, Spectral type: A1, aka 2QZ J115245.9-021117, [BGK2006] J115245.91-021116.2 and Gaia DR2 3602104614919092736.
[BGK2006] HV 10 aka Gaia DR2 3926757653770374272 or USNO-B1.0 1080-00234374 has a Radial velocity (cz) = 455.7 ± 9.0 km/s, Spectral type: B.
Radial velocity (cz) = 434.41 ± 9.90 km/s, Spectral type: B, [BGK2006] HV 16 is aka Gaia DR2 3708104343359742848.
Radial velocity (cz) = 363.22 ± 10.00 km/s, Spectral type: A1, 2MASS J14495559+3103510 is aka [BGK2006] J144955.58+310351.4, Gaia DR2 1283080527168129536 and USNO-B1.0 1210-00225097.
Radial velocity (cz) = -359.8 ± 9.0 km/s, Gaia DR2 4582309129318356352.
Radial velocity (cz) = -375.78 ± 2.95 km/s, 2MASS J01332653-2904447, Gaia DR2 5035174402313161472, and [SRA98] 8-1103.
"Hypervelocity stars (HVSs) travel with velocities so high that they exceed the escape velocity of the Galaxy."
"Scientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have discovered a star that breaks the galactic speed record, traveling with a velocity of about 2.7 million mph (1,200 km/s). This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion."
"US 708 is a helium-rich sub-dwarf O-type star with a mass of ∼ 0.3 M⊙".
Radial velocity (cz) = 708.84 ± - km/s, Spectral type: sdOHe, [BGK2006] HV 2 is aka Gaia DR2 815106177700219392, SDSS J093320.86+441705.5, USNO-B1.0 1342-00209722.
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