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Article: Directivity of the radio emission from the K1 dwarf star AB Doradus

TitleDirectivity of the radio emission from the K1 dwarf star AB Doradus
Authors
KeywordsPolarization
Radiation Mechanisms: Nonthermal
Radio Continuum: Stars
Stars: Activity
Stars: Individual (Ab Doradus)
Stars: Late-Type
Issue Date1994
PublisherInstitute of Physics Publishing Ltd. The Journal's web site is located at http://iopscience.iop.org/2041-8205
Citation
Astrophysical Journal Letters, 1994, v. 430 n. 1, p. 332-341 How to Cite?
AbstractWe present measurements of the spectrum and polarization of the flaring radio emission from the K1 dwarf star AB Doradus, together with previously reported single frequency measurements (with no polarization information) on 3 other days. On all 4 days spanning a 6 month period, the emission was strong and, when folded with the stellar rotation period, showed similar time variations with two prominent peaks at phase 0.35 and 0.75. These peaks coincide in longitude with two large starspots identified from the stellar optical light curve and have half-power widths as small as 0.1 rotations and no larger than 0.2 rotations. The modulated emission shows no measurable circular polarization, and its two peaks have different turnover frequencies. We discuss four contrasting models that can reproduce the observed properties of the modulated emission. Each model places a stringent upper limit on the source size and therefore a lower limit on its brightness temperature. In the first model the modulation is produced purely by the geometrical effects of occultation and requires sources with brightness temperature up to 10 13 K. In the second model we include the effects of limb darkening but find that neither the required source dimension nor its brightness temperature is significantly changed. In the third model the emission has a high directivity imposed extrinsically by coronal structures that absorb the radiation along all but the radial direction (to the stellar surface) and requires sources with brightness temperatures up to 10 11 K. In the fourth model the directivity is intrinsic to the emission process itself combined with a magnetic structure of a particular shape and requires sources with brightness temperatures up to 6 × 10 10 K. The high brightness temperatures, the broad-band and unpolarized nature of the radiation, and the coincidence of the radio peaks with large starspots suggest that the modulated emission of AB Dor is produced by gyroemission from ultrarelativistic electrons, that is synchrotron emission. This is in contrast to the Sun, which even in flares produces relatively few such energetic electrons. If as Readhead (1994) suggests the maximum (intrinsic) brightness temperature attainable by synchrotron emission is ∼10 11 K, then only models incorporating emission with high directivity - imposed extrinsically or, more likely, intrinsic to the source - can explain all the observed properties of the modulated emission. This then is the first indirect evidence that the incoherent radio emission of active stars can be highly directive.
Persistent Identifierhttp://hdl.handle.net/10722/174915
ISSN
2015 Impact Factor: 5.487
2015 SCImago Journal Rankings: 3.369

 

DC FieldValueLanguage
dc.contributor.authorLim, Jen_US
dc.contributor.authorWhite, SMen_US
dc.contributor.authorNelson, GJen_US
dc.contributor.authorBenz, AOen_US
dc.date.accessioned2012-11-26T08:48:06Z-
dc.date.available2012-11-26T08:48:06Z-
dc.date.issued1994en_US
dc.identifier.citationAstrophysical Journal Letters, 1994, v. 430 n. 1, p. 332-341en_US
dc.identifier.issn2041-8205en_US
dc.identifier.urihttp://hdl.handle.net/10722/174915-
dc.description.abstractWe present measurements of the spectrum and polarization of the flaring radio emission from the K1 dwarf star AB Doradus, together with previously reported single frequency measurements (with no polarization information) on 3 other days. On all 4 days spanning a 6 month period, the emission was strong and, when folded with the stellar rotation period, showed similar time variations with two prominent peaks at phase 0.35 and 0.75. These peaks coincide in longitude with two large starspots identified from the stellar optical light curve and have half-power widths as small as 0.1 rotations and no larger than 0.2 rotations. The modulated emission shows no measurable circular polarization, and its two peaks have different turnover frequencies. We discuss four contrasting models that can reproduce the observed properties of the modulated emission. Each model places a stringent upper limit on the source size and therefore a lower limit on its brightness temperature. In the first model the modulation is produced purely by the geometrical effects of occultation and requires sources with brightness temperature up to 10 13 K. In the second model we include the effects of limb darkening but find that neither the required source dimension nor its brightness temperature is significantly changed. In the third model the emission has a high directivity imposed extrinsically by coronal structures that absorb the radiation along all but the radial direction (to the stellar surface) and requires sources with brightness temperatures up to 10 11 K. In the fourth model the directivity is intrinsic to the emission process itself combined with a magnetic structure of a particular shape and requires sources with brightness temperatures up to 6 × 10 10 K. The high brightness temperatures, the broad-band and unpolarized nature of the radiation, and the coincidence of the radio peaks with large starspots suggest that the modulated emission of AB Dor is produced by gyroemission from ultrarelativistic electrons, that is synchrotron emission. This is in contrast to the Sun, which even in flares produces relatively few such energetic electrons. If as Readhead (1994) suggests the maximum (intrinsic) brightness temperature attainable by synchrotron emission is ∼10 11 K, then only models incorporating emission with high directivity - imposed extrinsically or, more likely, intrinsic to the source - can explain all the observed properties of the modulated emission. This then is the first indirect evidence that the incoherent radio emission of active stars can be highly directive.en_US
dc.languageengen_US
dc.publisherInstitute of Physics Publishing Ltd. The Journal's web site is located at http://iopscience.iop.org/2041-8205en_US
dc.relation.ispartofAstrophysical Journal Lettersen_US
dc.subjectPolarizationen_US
dc.subjectRadiation Mechanisms: Nonthermalen_US
dc.subjectRadio Continuum: Starsen_US
dc.subjectStars: Activityen_US
dc.subjectStars: Individual (Ab Doradus)en_US
dc.subjectStars: Late-Typeen_US
dc.titleDirectivity of the radio emission from the K1 dwarf star AB Doradusen_US
dc.typeArticleen_US
dc.identifier.emailLim, J: jjlim@hku.hken_US
dc.identifier.authorityLim, J=rp00745en_US
dc.description.naturelink_to_subscribed_fulltexten_US
dc.identifier.scopuseid_2-s2.0-12044256908en_US
dc.identifier.volume430en_US
dc.identifier.issue1en_US
dc.identifier.spage332en_US
dc.identifier.epage341en_US
dc.publisher.placeUnited Kingdomen_US
dc.identifier.scopusauthoridLim, J=7403453870en_US
dc.identifier.scopusauthoridWhite, SM=7404079906en_US
dc.identifier.scopusauthoridNelson, GJ=16459833800en_US
dc.identifier.scopusauthoridBenz, AO=7005864760en_US

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