J/A+A/629/A83 Complexity of magnetic fields on red dwarfs (Afram+, 2019)
Complexity of magnetic fields on red dwarfs.
Afram N., Berdyugina S.V.
<Astron. Astrophys. 629, A83 (2019)>
=2019A&A...629A..83A 2019A&A...629A..83A (SIMBAD/NED BibCode)
ADC_Keywords: Stars, M-type ; Polarization ; Line Profiles ; Magnetic fields
Keywords: stars: magnetic fields - polarization - molecular processes -
line: formation - radiative transfer
Abstract:
Generation, amplification, and evolution of magnetic fields in cool
stars can be investigated by measuring the Zeeman effect in atomic and
molecular lines observed in their spectra. In particular, Zeeman line
broadening and polarization have been used for detecting magnetic
fields in stellar atmospheres. Similar to the Sun, these fields are
complex and height-dependent (i.e., comprise 3D structures) and
require advanced diagnostics. Fortunately, many molecular lines
dominating M-dwarf spectra, such as FeH, CaH, MgH, and TiO, are
temperature- and Zeeman- sensitive and form at different atmospheric
heights, which makes them excellent probes of magnetic fields on M
dwarfs.
Our goal is to analyze the complexity of magnetic fields in M dwarfs.
We investigate how magnetic fields vary with the stellar temperature
(i.e., mass) and how "surface" inhomogeneities are distributed in
height - the dimension that is usually neglected in stellar magnetic
studies. This is achieved by including many atomic and molecular
species in our study.
We have determined effective temperatures of the photosphere and of
magnetic features, magnetic field strengths and filling factors for
nine M dwarfs (M1-M7). Our chi2 analysis is based on a comparison of
observed and synthetic intensity and circular polarization profiles
(Stokes I and V) of many magnetically sensitive atomic and molecular
lines in ten wavelength regions. Stokes profiles were calculated by
solving polarized radiative transfer equations under the local
thermodynamic equilibrium using model atmospheres.
We have found that properties of magnetic structures depend on the
analyzed atomic or molecular species and their formation heights
within the atmosphere. Two types of magnetic features similar to those
on the Sun have been found: one is cooler (starspots), while the other
one is hotter (network, small-scale magnetic features). The magnetic
field strength in both starspots and network is within 3kG to 6kG,
on average it is 5kG for the M1-M7 spectral class range. These fields
occupy a large fraction of M dwarf atmospheres at all heights, up to
100%. The plasma beta is less than one throughout the entire M dwarf
atmospheres, implying that they are highly magnetized stars.
A combination of many molecular and atomic species and a simultaneous
analysis of intensity and circular polarization spectra have allowed
us to better decipher the complexity of magnetic fields on M dwarfs,
including their dependence on the height within the atmosphere. This
work provides an opportunity to investigate a larger sample of M
dwarfs as well as L-type brown dwarfs.
Description:
Circular polarization and total intensity spectra obtained with
ESPaDOnS at the CFHT for the nine M dwarfs with the spectral class
range M1-M7. The spectra cover the wavelength range from 370nm to
1048nm.
All files were taken from the online data archive 'The Canadian
Astronomy Data Centre' www.cadc.hia.nrc.gc.ca/AdvancedSearch/ For the
polarimetric case, sets of 4 exposures produce one *p.fits file.
The p.fits files have:
COL1 = 'Wavelength' / Normalized
COL2 = 'Intensity' / Normalized
COL3 = 'Stokes ' / Normalized
COL4 = 'CheckN1 ' / Normalized
COL5 = 'CheckN2 ' / Normalized
COL6 = 'ErrorBar' / Normalized
COL7 = 'Wavelength' / UnNormalized
COL8 = 'Intensity' / UnNormalized
COL9 = 'Stokes ' / UnNormalized
COL10 = 'CheckN1 ' / UnNormalized
COL11 = 'CheckN2 ' / UnNormalized
COL12 = 'ErrorBar' / UnNormalized
COL13 = 'Wavelength' / Normalized, no autowave correction
COL14 = 'Intensity' / Normalized, no autowave correction
COL15 = 'Stokes ' / Normalized, no autowave correction
COL16 = 'CheckN1 ' / Normalized, no autowave correction
COL17 = 'CheckN2 ' / Normalized, no autowave correction
COL18 = 'ErrorBar' / Normalized, no autowave correction
COL19 = 'Wavelength' / UnNormalized, no autowave correction
COL20 = 'Intensity' / UnNormalized, no autowave correction
COL21 = 'Stokes ' / UnNormalized, no autowave correction
COL22 = 'CheckN1 ' / UnNormalized, no autowave correction
COL23 = 'CheckN2 ' / UnNormalized, no autowave correction
COL24 = 'ErrorBar' / UnNormalized, no autowave correction
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 118 9 List of fits files
fits/* . 9 Individual fits files
--------------------------------------------------------------------------------
See also:
https://www.cfht.hawaii.edu/Instruments/Spectroscopy/Espadons/CLASSICAL/ :
ESPaDOnS Processed Data
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 h RAh Right Ascension (J2000)
4- 5 I2 min RAm Right Ascension (J2000)
7- 11 F5.2 s RAs Right Ascension (J2000)
12 A1 --- DE- Declination sign (J2000)
13- 14 I2 deg DEd Declination (J2000)
16- 17 I2 arcmin DEm Declination (J2000)
19- 22 F4.1 arcsec DEs Declination (J2000)
24- 29 I6 --- Nx Number of pixels along X-axis
31- 32 I2 --- Ny Number of pixels along Y-axis
34- 38 I5 Kibyte size Size of FITS file
40- 52 A13 --- FileName Name of FITS file, in subdirectory fits
54-118 A65 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Acknowledgements:
Nadine Afram, nafram(at)hotmail.com
(End) Patricia Vannier [CDS] 01-Aug-2019