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Article: Magnetic field topology in low-mass stars: Spectropolarimetric observations of M dwarfs

TitleMagnetic field topology in low-mass stars: Spectropolarimetric observations of M dwarfs
Authors
KeywordsBrown dwarfs Stars: Magnetic fields Techniques: Polarimetric Techniques: Spectroscopic
Stars: Individual (G 164-31, Gl 890, LHS 473, KP Tau, Gl 896B, 2E 4498) Stars: Low-mass
Issue Date2009
PublisherInstitute of Physics Publishing Ltd. The Journal's web site is located at http://iopscience.iop.org/2041-8205
Citation
Astrophysical Journal Letters, 2009, v. 704 n. 2, p. 1721-1729 How to Cite?
AbstractThe magnetic field topology plays an important role in the understanding of stellar magnetic activity. While it is widely accepted that the dynamo action present in low-mass partially convective stars (e.g., the Sun) results in predominantly toroidal magnetic flux, the field topology in fully convective stars (masses below ∼ 0.35 M ⊙) is still under debate. We report here our mapping of the magnetic field topology of the M4 dwarf G 164-31 (or Gl 490B), which is expected to be fully convective, based on time series data collected from 20 hr of observations spread over three successive nights with the ESPaDOnS spectropolarimeter. Our tomographic imaging technique applied to time series of rotationally modulated circularly polarized profiles reveals an axisymmetric large-scale poloidal magnetic field on the M4 dwarf. We then apply a synthetic spectrum fitting technique for measuring the average magnetic flux on the star. The flux measured in G 164-31 is |Bf| = 3.2± 0.4 kG, which is significantly greater than the average value of 0.68 kG determined from the imaging technique. The difference indicates that a significant fraction of the stellar magnetic energy is stored in small-scale structures at the surface of G 164-31. Our Hα emission light curve shows evidence for rotational modulation suggesting the presence of localized structure in the chromosphere of this M dwarf. The radius of the M4 dwarf derived from the rotational period and the projected equatorial velocity is at least 30% larger than that predicted from theoretical models. We argue that this discrepancy is likely primarily due to the young nature of G 164-31 rather than primarily due to magnetic field effects, indicating that age is an important factor which should be considered in the interpretation of this observational result. We also report here our polarimetric observations of five other M dwarfs with spectral types from M0 to M4.5, three of them showing strong Zeeman signatures. © 2009. The American Astronomical Society. All rights reserved.
Persistent Identifierhttp://hdl.handle.net/10722/80745
ISSN
2015 Impact Factor: 5.487
2015 SCImago Journal Rankings: 3.369
ISI Accession Number ID
Funding AgencyGrant Number
Spanish Ministry of ScienceAYA2007-67458
Ministry of Education and the National Science Council of Taiwan
Funding Information:

N.P.-B. has been aided in this work by a Henri Chretien International Research Grant administered by the American Astronomical Society. E. M. acknowledges support from the Spanish Ministry of Science through the project AYA2007-67458. The authors extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Access to the CFHT was made possible by the Ministry of Education and the National Science Council of Taiwan as part of the Cosmology and Particle Astrophysics (CosPA) initiative.

References

 

DC FieldValueLanguage
dc.contributor.authorPhanBao, Nen_HK
dc.contributor.authorLim, Jen_HK
dc.contributor.authorDonati, JFen_HK
dc.contributor.authorJohnsKrull, CMen_HK
dc.contributor.authorMartín, ELen_HK
dc.date.accessioned2010-09-06T08:09:48Z-
dc.date.available2010-09-06T08:09:48Z-
dc.date.issued2009en_HK
dc.identifier.citationAstrophysical Journal Letters, 2009, v. 704 n. 2, p. 1721-1729en_HK
dc.identifier.issn2041-8205en_HK
dc.identifier.urihttp://hdl.handle.net/10722/80745-
dc.description.abstractThe magnetic field topology plays an important role in the understanding of stellar magnetic activity. While it is widely accepted that the dynamo action present in low-mass partially convective stars (e.g., the Sun) results in predominantly toroidal magnetic flux, the field topology in fully convective stars (masses below ∼ 0.35 M ⊙) is still under debate. We report here our mapping of the magnetic field topology of the M4 dwarf G 164-31 (or Gl 490B), which is expected to be fully convective, based on time series data collected from 20 hr of observations spread over three successive nights with the ESPaDOnS spectropolarimeter. Our tomographic imaging technique applied to time series of rotationally modulated circularly polarized profiles reveals an axisymmetric large-scale poloidal magnetic field on the M4 dwarf. We then apply a synthetic spectrum fitting technique for measuring the average magnetic flux on the star. The flux measured in G 164-31 is |Bf| = 3.2± 0.4 kG, which is significantly greater than the average value of 0.68 kG determined from the imaging technique. The difference indicates that a significant fraction of the stellar magnetic energy is stored in small-scale structures at the surface of G 164-31. Our Hα emission light curve shows evidence for rotational modulation suggesting the presence of localized structure in the chromosphere of this M dwarf. The radius of the M4 dwarf derived from the rotational period and the projected equatorial velocity is at least 30% larger than that predicted from theoretical models. We argue that this discrepancy is likely primarily due to the young nature of G 164-31 rather than primarily due to magnetic field effects, indicating that age is an important factor which should be considered in the interpretation of this observational result. We also report here our polarimetric observations of five other M dwarfs with spectral types from M0 to M4.5, three of them showing strong Zeeman signatures. © 2009. The American Astronomical Society. All rights reserved.en_HK
dc.languageengen_HK
dc.publisherInstitute of Physics Publishing Ltd. The Journal's web site is located at http://iopscience.iop.org/2041-8205en_HK
dc.relation.ispartofAstrophysical Journal Lettersen_HK
dc.rightsThe Astrophysical Journal. Copyright © Institute of Physics Publishing, Inc.en_HK
dc.subjectBrown dwarfs Stars: Magnetic fields Techniques: Polarimetric Techniques: Spectroscopicen_HK
dc.subjectStars: Individual (G 164-31, Gl 890, LHS 473, KP Tau, Gl 896B, 2E 4498) Stars: Low-massen_HK
dc.titleMagnetic field topology in low-mass stars: Spectropolarimetric observations of M dwarfsen_HK
dc.typeArticleen_HK
dc.identifier.openurlhttp://library.hku.hk:4550/resserv?sid=HKU:IR&issn=0004-637X&volume=704&issue=2&spage=1721&epage=1729&date=2009&atitle=Magnetic+field+topology+in+low-mass+stars:+Spectropolarimetric+observations+of+M+dwarfsen_HK
dc.identifier.emailLim, J: jjlim@hku.hken_HK
dc.identifier.authorityLim, J=rp00745en_HK
dc.description.naturelink_to_subscribed_fulltext-
dc.identifier.doi10.1088/0004-637X/704/2/1721en_HK
dc.identifier.scopuseid_2-s2.0-70649098382en_HK
dc.identifier.hkuros169475en_HK
dc.relation.referenceshttp://www.scopus.com/mlt/select.url?eid=2-s2.0-70649098382&selection=ref&src=s&origin=recordpageen_HK
dc.identifier.volume704en_HK
dc.identifier.issue2en_HK
dc.identifier.spage1721en_HK
dc.identifier.epage1729en_HK
dc.identifier.isiWOS:000270486200054-
dc.publisher.placeUnited Kingdomen_HK
dc.identifier.scopusauthoridPhanBao, N=8719607900en_HK
dc.identifier.scopusauthoridLim, J=7403453870en_HK
dc.identifier.scopusauthoridDonati, JF=7006720613en_HK
dc.identifier.scopusauthoridJohnsKrull, CM=7003590780en_HK
dc.identifier.scopusauthoridMartín, EL=7404092339en_HK

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