Stellar kinematics

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Stellar kinematics is the study of the movement of stars without needing to understand how they acquired their motion. This differs from stellar dynamics, which takes into account gravitational effects. The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.

Space velocity

The components of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed. This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the galactic center at the mean velocity of those nearby stars with low velocity dispersion. The Sun's motion with respect to the LSR is called the peculiar solar motion.

The components of space velocity in the Milky Way's Galactic coordinate system are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic center, V positive in the direction of galactic rotation, and W positive in the direction of the North Galactic Pole. The peculiar motion of the Sun with respect to the LSR is U = 10.00 ± 0.36 km/s, V = 5.23 ± 0.62 km/s and W = 7.17 ± 0.38 km/s.

The stars in the Milky Way can be subdivided into two general populations, based on their metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that population I, higher metallicity stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy. Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds.

Within the Milky Way galaxy, there are three primary components of stellar kinematics: the disk, halo and bulge or bar. These groups are closely related to the stellar populations in the galaxy, forming a strong correlation between the motion and chemical composition and indicating different formation mechanisms. The halo may be further sub-divided into an inner and outer halo, with the inner halo having a net prograde rotation and the outer a net retrograde movement.

High velocity stars

Depending on the definition, a high velocity star is 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. The velocity is also sometimes defined as supersonic relative to the surrounding interstellar medium. The three types of high velocity stars are: runaway stars, halo stars and hypervelocity stars.

Runaway stars

Four runaway stars plowing through regions of dense interstellar gas and creating bright bow waves and trailing tails of glowing gas. The stars in these NASA Hubble Space Telescope images are among 14 young runaway stars spotted by the Advanced Camera for Surveys between October 2005 and July 2006

A runaway star is one which is moving through space with an abnormally high velocity compared to other stars around it. The velocity is supersonic relative to the surrounding interstellar medium. The proper motion of a runaway star often points exactly away from a stellar association, whose member it therefore once must have been before it was hurled out.

Two possible mechanisms may give rise to a runaway star:

• In the first scenario, a close encounter between two binary systems may result in the disruption of both systems, with some of the stars being ejected at high velocities.
• In the second scenario, a supernova explosion in a multiple star system can result in the remaining components moving away at high speed.

While both mechanisms are theoretically possible, astronomers generally favour the supernova hypothesis as more likely in practice.

One example of a related set of runaway stars is the case of AE Aurigae, 53 Arietis and Mu Columbae, all of which are moving away from each other at velocities of over 100 km/s (for comparison, the Sun moves through the galaxy at about 20 km/s faster than the local average). Tracing their motions back, their paths intersect near to the Orion Nebula about 2 million years ago. Barnard's Loop is believed to be the remnant of the supernova that launched the other stars.

Another example is the X-ray object Vela X-1, where photodigital techniques reveal the presence of a typical supersonic bow shock hyperbola.

Halo stars

See also: Galactic halo

High-velocity stars are very old stars that do not share the motion of the Sun or most other stars in the solar neighbourhood which are in similar circular orbits around the centre of the Galaxy. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Galaxy. Although their orbital velocities in the Galaxy may be no faster than the Sun’s, their different paths result in the high relative velocities.

Typical examples are the halo stars passing through the disk of the galaxy at steep angles. One of the nearest 45 stars, called Kapteyn's star, is an example of the high-velocity stars that lie near the Sun. Its observed radial velocity is -245 km/s, and the components of its space velocity are U = 19 km/s, V = -288 km/s, and W = -52 km/s.

Hypervelocity stars

Hypervelocity stars (HVSs) are stars with a velocity so great, that they are able to escape the gravitational pull of the galaxy. Hence also the name Exiled Stars. Ordinary stars in the galaxy have velocities on the order of 100 km/s, while hypervelocity stars (especially near the center of the galaxy, which is where most are "produced"), have velocities on the order of 1000 km/s.

HVSs were first theorized by J. Hills in 1988. Currently, ten HVSs are known. The first one found — discovered in 2005 by H. Edelmann et al. Warren Brown et al. from the Harvard-Smithsonian Center for Astrophysics— possibly originated from the Large Magellanic Cloud. In 2006 and 2007 seven more were discovered by Warren Brown et al. All of the stars are over 50,000 parsecs away and unbound to the galaxy.

It is believed that about 1000 HVSs exist in our Galaxy. Considering that there are around 100 billion stars in the Milky Way, this a minuscule fraction.

Production methods

The main production method for HVSs is summarized as thus: they are believed to originate by close encounters of binary stars with the supermassive black hole in the centre of the Milky Way. One of the two partners is captured by the black hole, while the other escapes with high velocity. Also, it is worth noting that "captured" does not necessarily mean "swallowed," for in all likelihood the companion to the HVS will never fall into the black hole.

Known HVSs are main sequence stars with masses a few times that of the Sun.

A team at Argentina's Cordoba Observatory believe that our HVS are a result of a merging with a collision between the Milky Way, and an orbiting dwarf galaxy. A dwarf galaxy that has been orbiting the Milky Way, passed through the centre of the Milky Way. When the dwarf galaxy made its closest approach to the centre of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space.

Some neutron stars are inferred to be traveling with similar speeds, however they are unrelated to both HVSs and the HVS ejection mechanism. Neutron stars are the remnants of supernova explosions, and their extreme speeds are very likely the result of an asymmetric supernova explosion. The neutron star RX J0822-4300 , which was measured to move at a record speed of over 1300 km/s (0.54% c) in 2007 by the Chandra X-ray Observatory, is thought to have been produced this way.

List of HVSs

• HVS 1 - (SDSS J090744.99+024506.8) (a.k.a. The Outcast Star)
• HVS 2 - (SDSS J093320.86+441705.4) or (US 708)
• HVS 3 - (HE 0437-5439) - possibly from the Large Magellanic Cloud
• HVS 4 - (SDSS J091301.00+305120.0)
• HVS 5 - (SDSS J091759.42+672238.7)
• HVS 6 - (SDSS J110557.45+093439.5)
• HVS 7 - (SDSS J113312.12+010824.9)
• HVS 8 - (SDSS J094214.04+200322.1)
• HVS 9 - (SDSS J102137.08-005234.8)
• HVS 10 - (SDSS J120337.85+180250.4)

Kinematic groups

A set of stars with similar space motion and ages is known as a kinematic group. Most stars are born within molecular clouds known as stellar nurseries. The stars formed within such a cloud compose gravitationally bound star clusters containing dozens to thousands of members with similar ages and compositions. These clusters dissociate with time. Groups of young stars that escape a cluster, or are no longer bound to each other, form stellar associations. As these stars age and disperse, their association is no longer readily apparent and they become moving groups of stars.

See also

• Astrometry
• Gaia probe
• Hipparcos
• n-body problem

Further reading

• Majewski, Steven R. (2006). "Stellar Motions". University of Virginia. Retrieved 2008-02-25.
• "The Space Velocity and its Components". University of Tennessee. Retrieved 2008-02-25.
• Blaauw A., Morgan W.W. (1954), The Space Motions of AE Aurigae and mu Columbae with Respect to the Orion Nebula, Astrophysical Journal, v.119, p.625
• Hoogerwerf R., de Bruijne J.H.J., de Zeeuw P.T. (2000), The Origin of Runaway Stars, Astrophysical Journal, v. 544, p.L133

External links

• ESO press release about runaway stars
• Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
• A Successful Targeted Search for Hypervelocity Stars
• Two Exiled Stars Are Leaving Our Galaxy Forever
• An Unbound Hypervelocity Main-Sequence B-Type Star
• Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
• Hypervelocity stars. I. The spectroscopic survey information on HV 6 & HV 7