J/ApJ/854/78 Magnetohydrodynamic (MHD) simulations. II. (Finley+, 2018)
The effect of combined magnetic geometries on thermally driven winds.
II. Dipolar, quadrupolar, and octupolar topologies.
Finley A.J., Matt S.P.
<Astrophys. J., 854, 78 (2018)>
=2018ApJ...854...78F 2018ApJ...854...78F
ADC_Keywords: Magnetic fields; Models
Keywords: magnetohydrodynamics (MHD) ; stars: low-mass ; stars: magnetic field ;
stars: rotation ; stars: winds, outflows
Abstract:
During the lifetime of Sun-like or low-mass stars a significant amount
of angular momentum is removed through magnetized stellar winds. This
process is often assumed to be governed by the dipolar component of
the magnetic field. However, observed magnetic fields can host strong
quadrupolar and/or octupolar components, which may influence the
resulting spin-down torque on the star. In Paper I (Finley & Matt
2017ApJ...845...46F 2017ApJ...845...46F), we used the magnetohydrodynamic (MHD) code PLUTO
Mignone+ 2007ApJS..170..228M 2007ApJS..170..228M ; Mignone 2009MSAIS..13...67M 2009MSAIS..13...67M) to compute
steady-state solutions for stellar winds containing a mixture of
dipole and quadrupole geometries. We showed the combined winds to be
more complex than a simple sum of winds with these individual
components. This work follows the same method as Paper I, including
the octupole geometry, which not only increases the field complexity
but also, more fundamentally, looks for the first time at combining
the same symmetry family of fields, with the field polarity of the
dipole and octupole geometries reversing over the equator (unlike the
symmetric quadrupole). We show, as in Paper I, that the lowest-order
component typically dominates the spin-down torque. Specifically, the
dipole component is the most significant in governing the spin-down
torque for mixed geometries and under most conditions for real stars.
We present a general torque formulation that includes the effects of
complex, mixed fields, which predicts the torque for all the
simulations to within 20% precision, and the majority to within ∼5%.
This can be used as an input for rotational evolution calculations in
cases where the individual magnetic components are known.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 45 128 Input parameters and results from simulations
with one & two magnetic components
table4.dat 45 32 Input parameters and results from simulations
with three magnetic components
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See also:
J/MNRAS/390/567 : Magnetic field and velocity of mid M dwarfs (Morin+, 2008)
J/ApJ/695/679 : Stellar rotation in M35 (Meibom+, 2009)
J/MNRAS/413/2218 : Stellar rotation in Hyades and Praesepe (Delorme+, 2011)
J/ApJ/733/115 : Rotation periods and membership in M34 (Meibom+, 2011)
J/ApJ/737/L35 : Pulsed Alfven waves in the solar wind (Gosling+, 2011)
J/ApJ/738/119 : Conversion from magnetoacoustic to Alfven waves
(Cally+, 2011)
J/MNRAS/432/1203 : Rotation periods of M-dwarf stars (McQuillan+, 2013)
J/ApJ/776/67 : Rotational tracks (van Saders+, 2013)
J/MNRAS/441/2361 : Stellar magnetism, age and rotation (Vidotto+, 2014)
J/AJ/152/115 : Pleiades members with K2 LCs. III. (Stauffer+, 2016)
Byte-by-byte Description of file: table[24].dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Case [1/160] Case
5- 7 F3.1 --- Rdip [0/1] Ratio, dipole to total field strength
9- 11 F3.1 --- Rquad [0/1] Ratio, quadrupole to total
field strength
13- 16 F4.1 --- Roct [-0.9/1] Ratio, octupole to total
field strength
18- 21 F4.1 --- va/vesc [0.5/20] Ratio, Alfven speed to
escape velocity
23- 26 F4.1 --- Ra/R* [2.7/28.1] Ratio, Alfven to stellar radius
28- 33 I6 --- Upsilon [4/549000] Wind magnetisation (Υ)
35- 40 I6 --- UpsilonOpen [194/156000] Open flux wind magnetisation
42- 45 F4.2 --- v(Ra)Avg/vesc [0.1/3.2] Average wind speed at the Alfven
surface to escape velocity
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History:
From electronic version of the journal
References:
Finley & Matt Paper I. 2017ApJ...845...46F 2017ApJ...845...46F
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 09-Nov-2018