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Title: SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS

Abstract

We have measured the surface magnetic flux on four accreting young brown dwarfs and one nonaccreting young very low mass (VLM) star utilizing high-resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, Two Micron All Sky Survey (2MASS) J1207334-393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilogauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3{sigma} upper limit for the magnetic flux in 2MASS J1207334-393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of 5 or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It ismore » interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently has an easy physical explanation.« less

Authors:
 [1];  [2]
  1. Institut fuer Astrophysik, Georg-August-Universitaet, Friedrich-Hund-Platz 1, 37077 Goettingen (Germany)
  2. Astronomy Department, University of California, Berkeley, CA 94720 (United States)
Publication Date:
OSTI Identifier:
21300582
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 697; Journal Issue: 1; Other Information: DOI: 10.1088/0004-637X/697/1/373; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; 99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; ABSORPTION; DWARF STARS; MAGNETIC FIELDS; MAGNETIC FLUX; MAGNETISM; T TAURI STARS; TEMPERATURE RANGE

Citation Formats

Reiners, A, Basri, G, and Christensen, U. R. SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS. United States: N. p., 2009. Web. doi:10.1088/0004-637X/697/1/373; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
Reiners, A, Basri, G, & Christensen, U. R. SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS. United States. https://doi.org/10.1088/0004-637X/697/1/373; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)
Reiners, A, Basri, G, and Christensen, U. R. 2009. "SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS". United States. https://doi.org/10.1088/0004-637X/697/1/373; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
@article{osti_21300582,
title = {SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS},
author = {Reiners, A and Basri, G and Christensen, U. R.},
abstractNote = {We have measured the surface magnetic flux on four accreting young brown dwarfs and one nonaccreting young very low mass (VLM) star utilizing high-resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, Two Micron All Sky Survey (2MASS) J1207334-393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilogauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3{sigma} upper limit for the magnetic flux in 2MASS J1207334-393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of 5 or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently has an easy physical explanation.},
doi = {10.1088/0004-637X/697/1/373; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)},
url = {https://www.osti.gov/biblio/21300582}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 697,
place = {United States},
year = {Wed May 20 00:00:00 EDT 2009},
month = {Wed May 20 00:00:00 EDT 2009}
}