Stellar magnetic field


Stellar magnetic field

A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a main sequence (hydrogen-burning) star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. This creates starspots on the surface, and the related phenomenon of coronal loops. [cite web
last=Brainerd | first=Jerome James | date=July 6, 2005
url=http://www.astrophysicsspectator.com/topics/observation/XRayCorona.html
title=X-rays from Stellar Coronas
publisher=The Astrophysics Spectator
accessdate= 2007-06-21
]

Measurement

The magnetic field of a star can be measured by means of the Zeeman effect. Normally the atoms in a star's atmosphere will absorb certain frequencies of energy in the electromagnetic spectrum, producing characteristic dark absorption lines in the spectrum. When the atoms are within a magnetic field, however, these lines become split into multiple, closely-space lines. The energy also becomes polarized with an orientation that depends on orientation of the magnetic field. Thus the strength and direction of the star's magnetic field can be determined by examination of the Zeeman effect lines. [cite conference
last=Wade | first=Gregg A.
title=Stellar Magnetic Fields: The view from the ground and from space
booktitle=The A-star Puzzle: Proceedings IAU Symposium No. 224
publisher=Cambridge University Press
pages=235-243 | date=July 8-13, 2004
location=Cambridge, England
url=http://journals.cambridge.org/production/action/cjoGetFulltext?fulltextid=280661
accessdate=2007-06-21
] [cite journal
last = Basri | first = Gibor
title=Big Fields on Small Stars
journal=Science | year=2006 | volume=311
issue=5761 | pages=618–619
url=http://www.sciencemag.org/cgi/content/full/sci;311/5761/618
accessdate=2007-02-04
doi=10.1126/science.1122815
pmid=16456068
]

A stellar spectropolarimeter is used to measure the magnetic field of a star. This instrument consists of a spectrograph combined with a polarimeter. The first instrument to be dedicated to the study of stellar magnetic fields was NARVAL, which was mounted on the Bernard Lyot Telescope at the Pic du Midi de Bigorre in the French Pyrenees mountains. [cite news
author=Staff
title=NARVAL: First Observatory Dedicated To Stellar Magnetism
publisher=Science Daily | date=February 22, 2007
url=http://www.sciencedaily.com/releases/2007/02/070208131656.htm
accessdate=2007-06-21
]

Various measurements—including magnetometer measurements over the last 150 years; [cite journal
author=Lockwood, M.; Stamper, R.; Wild, M. N.
title=A Doubling of the Sun's Coronal Magnetic Field during the Last 100 Years
journal=Nature | year=1999 | doi=10.1038/20867
volume=399 | issue=6735 | pages=437–439
url=http://adsabs.harvard.edu/abs/1999Natur.399..437L
accessdate=2008-08-21
] 14C in tree rings; and 10Be in ice cores [cite journal
last=Beer | first=Jürg
title=Long-term indirect indices of solar variability
journal=Space Science Reviews | year=2000
volume=94 | issue=1/2 | pages=53–66
url=http://adsabs.harvard.edu/abs/2000SSRv...94...53B
accessdate=2008-08-21
] —have established substantial magnetic variability of the Sun on decadel, centennial and millennial time scales.cite journal
first=Jasper | last=Kirkby
authorlink=Jasper Kirkby
title=Cosmic Rays and Climate
journal=Surveys in Geophysics | year=2007
volume=28 | pages=333–375
doi=10.1007/s10712-008-9030-6
url=http://arxiv.org/abs/0804.1938v1
accessdate=2008-08-21
]

Field generation

Stellar magnetic fields are believed to be caused within the convective zone of the star. The convective circulation of the conducting plasma functions like a dynamo. This activity destroys the star's primordial magnetic field, then generates a dipolar magnetic field. As the star undergoes differential rotation—rotating at different rates for various latitudes—the magnetism is wound into a toroidal field of "flux ropes" that become wrapped around the star. The fields can become highly concentrated, producing activity when they emerge on the surface. [cite journal
last=Piddington | first=J. H.
title=On the origin and structure of stellar magnetic fields
journal=Astrophysics and Space Science
year=1983 | volume=90 | issue=1 | pages=217–230
url=http://adsabs.harvard.edu/abs/1983Ap&SS..90..217P
accessdate=2007-06-21
doi=10.1007/BF00651562
]

urface activity

Starspots are regions of intense magnetic activity on the surface of a star. (On the Sun they are termed sunspots.) These form a visible component of magnetic flux tubes that are formed within a star's convection zone. Due to the differential rotation of the star, the tube becomes curled up and stretched, inhibiting convection and producing zones of lower than normal temperature. [cite news
first=Jonathan | last=Sherwood
title=Dark Edge of Sunspots Reveal Magnetic Melee
publisher=University of Rochester
date=December 3, 2002
url=http://www.rochester.edu/news/show.php?id=290
accessdate=2007-06-21
] Coronal loops often form above starspots, forming from magnetic field lines that stretch out into the corona. These in turn serve to heat the corona to temperatures over a million kelvins. [cite journal
author=Hudson, H. S.; Kosugi, T.
title=How the Sun's Corona Gets Hot
journal=Science | year=1999 | volume=285
issue=5429 | pages=849
url=http://www.sciencemag.org/cgi/content/full/285/5429/849
accessdate=2007-06-21
doi=10.1126/science.285.5429.849
]

The magnetic fields linked to starspots and coronal loops are linked to flare activity, and the associated coronal mass ejection. The plasma is heated to tens of millions of kelvins, and the particles are accelerated away from the star's surface at extreme velocities. [cite web
last=Hathaway | first=David H. | date=January 18, 2007
url=http://solarscience.msfc.nasa.gov/flares.shtml
title=Solar Flares | publisher=NASA
accessdate=2007-06-21
]

Surface activity appears to be related to the age and rotation rate of main sequence stars. Young stars with a rapid rate of rotation exhibit strong activity. By contrast middle-aged, Sun-like stars with a slow rate of rotation show low levels of activity that varies in cycles. Some older stars display almost no activity, which may mean they have entered a lull that is comparable to the Sun's Maunder minimum. Measurements of the time variation in stellar activity can be useful for determining the differential rotation rates of a star. [cite web
last = Berdyugina | first = Svetlana V. | year=2005
url =http://solarphysics.livingreviews.org/Articles/lrsp-2005-8/
title =Starspots: A Key to the Stellar Dynamo
publisher =Living Reviews
accessdate = 2007-06-21
]

Magnetic stars

A T Tauri star is a type of pre-main sequence star that is being heated through gravitational contraction and has not yet begun to burn hydrogen at its core. They are variable stars that are magnetically active. The magnetic field of these stars is thought to interact with its strong stellar wind, transferring angular momentum to the surrounding protoplanetary disk. This allows the star to brake its rotation rate as it collapses. [cite journal
author=Küker, M.; Henning, T.; Rüdiger, G.
title=Magnetic Star-Disk Coupling in Classical T Tauri Systems
journal=The Astrophysical Journal | year=2003
volume=589 | pages=397–409
doi=10.1086/374408
]

Small, M-class stars (with 0.1–0.6 solar masses) that exhibit rapid, irregular variability are known as flare stars. These fluctuations are believed to be caused by flares, although the activity is much stronger relative to the size of the star. The flares on this class of stars can extend up to 20% of the circumference, and radiate much of their energy in the blue and ultraviolet portion of the spectrum. [cite web
last=Templeton | first=Matthew | date=Autumn 2003
url=http://www.aavso.org/vstar/vsots/fall03.shtml
title=Variable Star Of The Season: UV Ceti
publisher=AAVSO | accessdate=2007-06-21
]

Planetary nebulae are created when a red giant star ejects its outer envelope, forming an expanding shell of gas. However it remains a mystery why these shells are not always spherically symmetrical. 80% of planetary nebulae do not have a spherical shape; instead forming bipolar or elliptical nebulae. One hypothesis for the formation of a non-spherical shape is the effect of the star's magnetic field. Instead of expanding evenly in all directions, the ejected plasma tends to leave by way of the magnetic poles. Observations of the central stars in at least four planetary nebulae have confirmed that they do indeed possess powerful magnetic fields. [cite news
author=Jordan, S.; Werner, K.; O'Toole, S.
title=First Detection Of Magnetic Fields In Central Stars Of Four Planetary Nebulae
publisher=Space Daily | date=January 06, 2005
url=http://www.spacedaily.com/news/stellar-chemistry-05a.html
accessdate=2007-06-23
]

After some massive stars have ceased thermonuclear fusion, a portion of their mass collapses into a compact body of neutrons called a neutron star. These bodies retain a significant magnetic field from the original star, but the collapse in size causes the strength of this field to increase dramatically. The rapid rotation of these collapsed neutron stars results in a pulsar, which emits a narrow beam of energy that can periodically point toward an observer.

An extreme form of a magnetized neutron star is the magnetar. These are formed as the result of a core-collapse supernova. [cite web
last=Duncan | first=Robert C. | year=2003
url=http://solomon.as.utexas.edu/~duncan/magnetar.html
title='Magnetars', Soft Gamma Repeaters, and Very Strong Magnetic Fields
publisher=University of Texas at Austin
accessdate=2007-06-21
] The existence of such stars was confirmed in 1998 with the measurement of the star SGR 1806-20. The magnetic field of this star has increased the surface temperature to 18 million K and it releases enormous amounts of energy in gamma ray bursts. [cite news
author=Isbell, D.; Tyson, T.
title=Strongest Stellar Magnetic Field yet Observed Confirms Existence of Magnetars
publisher=NASA/Goddard Space Flight Center
date=May 20, 1998
url=http://heasarc.gsfc.nasa.gov/docs/asca/science/magnetar.html
accessdate=2006-05-24
]

ee also

* Alpha2 Canum Venaticorum variable
* Dynamo theory
* Intermediate polar
* Peculiar star
* Polar (cataclysmic variable)
* SX Arietis variable

References

External links

*cite web
last=Donati | first=Jean-François | date=June 16, 2003
url=http://www.ast.obs-mip.fr/users/donati/field.html
title=Surface magnetic fields of non degenerate stars
publisher=Laboratoire d’Astrophysique de Toulouse
accessdate = 2007-06-23

*cite web
last=Donati | first=Jean-François | date=November 5, 2003
url=http://www.ast.obs-mip.fr/users/donati/diffrot.html
title=Differential rotation of stars other than the Sun
publisher=Laboratoire d’Astrophysique de Toulouse
accessdate = 2007-06-24


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