TY - JOUR
T1 - Disruption of circumstellar discs by large-scale stellar magnetic fields
AU - ud-Doula, Asif
AU - Owocki, Stanley P.
AU - Kee, Nathaniel Dylan
N1 - Funding Information:
A.uD acknowledges support by NASA through Chandra Award numbers GO5-16005X and TM7-18001X issued by the Chandra X-ray Observatory Center which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060.
Publisher Copyright:
© 2018 The Author(s).
PY - 2018/8/1
Y1 - 2018/8/1
N2 - Spectropolarimetric surveys reveal that 8-10 per cent ofOBAstars harbor large-scalemagnetic fields, but thus far no such fields have been detected in any classical Be stars. Motivated by this, we present here magnetohydrodynamical simulations for how a pre-existing Keplerian disc - like that inferred to form from decretion of material from rapidly rotating Be stars - can be disrupted by a rotation-aligned stellar dipole field. For characteristic stellar and disc parameters of a near critically rotating B2e star, we find that a polar surface field strength of just 10 G can significantly disrupt the disc, while a field of 100 G, near the observational upper limit inferred for most Be stars, completely destroys the disc over just a few days. Our parameter study shows that the efficacy of this magnetic disruption of a disc scales with the characteristic plasma beta (defined as the ratio between thermal and magnetic pressure) in the disc, but is surprisingly insensitive to other variations, e.g. in stellar rotation speed, or the mass-loss rate of the star's radiatively driven wind. The disc disruption seen here for even a modest field strength suggests that the presumed formation of such Be discs by decretion of material from the star would likely be strongly inhibited by such fields; this provides an attractive explanation for why no large-scale fields are detected from such Be stars.
AB - Spectropolarimetric surveys reveal that 8-10 per cent ofOBAstars harbor large-scalemagnetic fields, but thus far no such fields have been detected in any classical Be stars. Motivated by this, we present here magnetohydrodynamical simulations for how a pre-existing Keplerian disc - like that inferred to form from decretion of material from rapidly rotating Be stars - can be disrupted by a rotation-aligned stellar dipole field. For characteristic stellar and disc parameters of a near critically rotating B2e star, we find that a polar surface field strength of just 10 G can significantly disrupt the disc, while a field of 100 G, near the observational upper limit inferred for most Be stars, completely destroys the disc over just a few days. Our parameter study shows that the efficacy of this magnetic disruption of a disc scales with the characteristic plasma beta (defined as the ratio between thermal and magnetic pressure) in the disc, but is surprisingly insensitive to other variations, e.g. in stellar rotation speed, or the mass-loss rate of the star's radiatively driven wind. The disc disruption seen here for even a modest field strength suggests that the presumed formation of such Be discs by decretion of material from the star would likely be strongly inhibited by such fields; this provides an attractive explanation for why no large-scale fields are detected from such Be stars.
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U2 - 10.1093/MNRAS/STY1228
DO - 10.1093/MNRAS/STY1228
M3 - Article
AN - SCOPUS:85056821734
SN - 0035-8711
VL - 478
SP - 3049
EP - 3055
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 3
ER -