Inastrophysics,bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowingstellar wind.[1] Bow shock occurs when themagnetosphere of an astrophysical object interacts with the nearby flowing ambientplasma such as thesolar wind. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to themagnetopause. For stars, this boundary is typically the edge of theastrosphere, where the stellar wind meets theinterstellar medium.[1]
The defining criterion of ashock wave is that the bulk velocity of theplasma drops from "supersonic" to "subsonic", wherethe speed of sound cs is defined bywhere is theratio of specific heats, is thepressure, and is the density of the plasma.
A common complication in astrophysics is the presence of a magnetic field. For instance, the charged particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind (which is the component of the velocity parallel to the field lines about which the particles gyrate) drops below the speed at which the particles are gyrating.
The best-studied example of a bow shock is that occurring where the Sun's wind encountersEarth's magnetopause, although bow shocks occur around all planets, both unmagnetized, such asMars[2] andVenus[3] and magnetized, such asJupiter[4] orSaturn.[5] Earth's bow shock is about 17 kilometres (11 mi) thick[6] and located about 90,000 kilometres (56,000 mi) from the planet.[7]
Bow shocks form atcomets as a result of the interaction between the solar wind and the cometary ionosphere. Far away from the Sun, a comet is an icy boulder without an atmosphere. As it approaches the Sun, the heat of the sunlight causes gas to be released from thecometary nucleus, creating an atmosphere called acoma. The coma is partially ionized by the sunlight, and when the solar wind passes through this ion coma, the bow shock appears.
The first observations were made in the 1980s and 90s as several spacecraft flew by comets21P/Giacobini–Zinner,[8]1P/Halley,[9] and26P/Grigg–Skjellerup.[10] It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at Earth, for example. These observations were all made nearperihelion when the bow shocks already were fully developed.
TheRosetta spacecraft followed comet67P/Churyumov–Gerasimenko from far out in theSolar System, at a heliocentric distance of 3.6AU, in toward perihelion at 1.24 AU, and back out again. This allowedRosetta to observe the bow shock as it formed when the outgassing increased during the comet's journey toward the Sun. In this early state of development the shock was called the "infant bow shock".[11] The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.
For several decades, the solar wind has been thought to form a bow shock at the edge of theheliosphere, where it collides with the surrounding interstellar medium. Moving away from the Sun, the point where the solar wind flow becomes subsonic is thetermination shock, the point where the interstellar medium and solar wind pressures balance is theheliopause, and the point where the flow of the interstellar medium becomes subsonic would be the bow shock. This solar bow shock was thought to lie at a distance around 230 AU[12] from the Sun – more than twice the distance of the termination shock as encountered by the Voyager spacecraft.
However, data obtained in 2012 from NASA'sInterstellar Boundary Explorer (IBEX) indicates the lack of any solar bow shock.[13] Along with corroborating results from theVoyager spacecraft, these findings have motivated some theoretical refinements; current thinking is that formation of a bow shock is prevented, at least in the galactic region through which the Sun is passing, by a combination of the strength of the local interstellar magnetic-field and of the relative velocity of the heliosphere.[14]
In 2006, a far infrared bow shock was detected near theAGB starR Hydrae.[15]
Bow shocks are also a common feature inHerbig Haro objects, in which a much strongercollimated outflow of gas and dust from the star interacts with the interstellar medium, producing bright bow shocks that are visible at optical wavelengths.
TheHubble Space Telescope captured these images of bow shocks made of dense gasses and plasma in theOrion Nebula.
Bow shocks also appear around sixCataclysmic variable star (CVs) with luminousaccretion disks, which drive fast winds into the interstellar medium. These six CVs are:BZ Camelopardalis,[17][18]V341 Ara,[19][20]SY Cancri,[21] ASASSN-V J205457.73+515731.9,[22][23]LS Pegasi,[23] andFY Vulpeculae.[24]
Recent discoveries were partly made with the help ofamateur astronomers.[21][23][24] These nebulae usually appear indoubly ionized oxygen and are inside a largerH-alpha nebula.[18][20][24]
If a massive star is arunaway star, or if theinterstellar medium moves relative to the star, it can form aninfrared bow-shock that is detectable in 24 μm and sometimes in 8μm of theSpitzer Space Telescope or the W3/W4-channels ofWISE. In 2016 Kobulnicky et al. created the largest spitzer/WISE bow-shock catalog to date with 709 bow-shock candidates.[25][26] To get a larger bow-shock catalogThe Milky Way Project (aCitizen Science project) did map infrared bow-shocks in the galactic plane. The search identified 311 new bow shock candidates.[27] This larger catalog will help to understand the stellar wind of massive stars.[28]
The closest stars with infrared bow-shocks are (within 130 parsec):[26]
| Name | Distance (pc) | Spectral type | bow shock aligned/not aligned with star motion | Belongs to |
|---|---|---|---|---|
| Alpha Cephei | 15.04 | A8Vn | not aligned | |
| Beta Librae | 56.75 | B8Vn | not aligned | |
| Mimosa | 85.40 | B1IV | not aligned | Lower Centaurus–Crux subgroup |
| Alpha Muscae | 96.71 | B2IV | not aligned | Lower Centaurus–Crux subgroup |
| Acrux | 98.72 | B0.5IV+B1V | not aligned | Lower Centaurus–Crux subgroup |
| Beta Muscae | 104.71 | B2V | not aligned | Scorpius–Centaurus association |
| Pi Centauri | 109.65 | B5Vn | not aligned | Lower Centaurus–Crux subgroup |
| Zeta Ophiuchi | 112.23 | O9.2IVnn | aligned | Upper Scorpius subgroup |
| Maia | 117.51 | B8III | aligned | Pleiades |
| HD 110956 | 117.92 | B2/3V | not aligned | Lower Centaurus–Crux subgroup |
| HR 5906 | 128.87 | B5V | not aligned |
Most of them belong to theScorpius–Centaurus association.
A similar effect, known as the magnetic draping effect, occurs when a super-Alfvénic plasma flow impacts an unmagnetized object such as what happens when the solar wind reaches the ionosphere of Venus:[29] the flow deflects around the object draping themagnetic field along the wake flow.[30]
The condition for the flow to be super-Alfvénic means that the relative velocity between the flow and object,, is larger than the localAlfvén velocity which means a largeAlfvénic Mach number:. For unmagnetized andelectrically conductive objects, the ambient field createselectric currents inside the object, and into the surrounding plasma, such that the flow is deflected and slowed as the time scale of magneticdissipation is much longer than the time scale of magnetic fieldadvection. The induced currents in turn generate magnetic fields that deflect the flow creating a bow shock. For example, theionospheres of Mars and Venus provide the conductive environments for the interaction with the solar wind. Without an ionosphere, the flowing magnetized plasma is absorbed by the non-conductive body. The latter occurs, for example, when the solar wind interacts with theMoon which has no ionosphere. In magnetic draping, the field lines are wrapped and draped around the leading side of the object creating a narrow sheath which is similar to the bow shocks in the planetary magnetospheres. The concentrated magnetic field increases until theram pressure becomes comparable to themagnetic pressure in the sheath:
where is the density of the plasma, is the draped magnetic field near the object, and is the relative speed between the plasma and the object. Magnetic draping has been detected around planets, moons, solar coronal mass ejections, and galaxies.[31]
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