J/A+A/682/A170 AU Mic dynamic spectra of 42 radio observations (Bloot+, 2024)
Phenomenology and periodicity of radio emission from the stellar system
AU Microscopii.
Bloot S., Callingham J.R., Vedantham H.K., Kavanagh R.D., Pope B.J.S.,
Climent J.B., Guirado J.C., Pena-Monino L., Perez-Torres M.
<Astron. Astrophys. 682, A170 (2024)>
=2024A&A...682A.170B 2024A&A...682A.170B (SIMBAD/NED BibCode)
ADC_Keywords: Stars, variable ; Radio continuum ; Magnetic fields ; Spectroscopy
Keywords: stars: coronae - stars: individual (AU Mic) - stars: magnetic field -
radio continuum: stars
Abstract:
Stellar radio emission can measure a star's magnetic field strength
and structure, plasma density, and dynamics, and the stellar wind
pressure impinging on exoplanet atmospheres. However, properly
interpreting the radio data often requires temporal baselines that
cover the rotation of the stars, orbits of their planets, and any
longer-term stellar activity cycles.
Here we present our monitoring campaign on the young, active M dwarf
AU Microscopii with the Australia Telescope Compact Array between 1.1
and 3.1GHz. With over 250 hours of observations, these data represent
the longest radio monitoring campaign on a single main-sequence star
to date. We find that AU Mic produces a wide variety of radio
emission, for which we introduce a phenomenological classification
scheme predicated on the polarisation fraction and time-frequency
structure of the emission. Such a classification scheme is applicable
to radio emission from other radio-bright stars. The six types of
radio emission detected on AU Mic can be broadly categorised into
five distinct types of bursts, and broadband quiescent emission. We
find that the radio bursts are highly circularly polarised and
periodic with the rotation period of the star, implying that the
emission is beamed. It is therefore most likely produced by the
electron cyclotron maser instability. We present a model to show that
the observed pattern of emission can be explained by emission from
auroral rings on the magnetic poles. The total intensity of the
broadband emission is stochastic, but we show that its circular
polarisation fraction is also periodic with the rotation of the star.
Such a periodicity in the polarised fraction of emission has not been
observed on an M dwarf before.
We present a qualitative model to describe the periodicity in the
polarisation fraction of the broadband emission, using low-harmonic
gyromagnetic emission. Using a simple qualitative model, we infer a
magnetic obliquity of at least 20 degrees from the observed variation
in polarisation fraction. Finally, we show that the radio emission
might be evolving on long timescales, hinting at a potential stellar
magnetic activity cycle.
Description:
We present dynamic spectra of 42 radio observations of AU Mic, at 1.1
to 3.1GHz. For each observation, we include the baseline-averaged
Stokes V flux density, the Stokes I flux density, and the imaginary
component of both Stokes V and I. Timestamps in seconds of MJD are
included with each dynamic spectrum, as well as channel numbers, which
correspond to frequency. Channel 0 is the highest frequency at 3.1GHZ,
and channel 2048 is the lowest frequency at 1.1GHz.
Objects:
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RA (2000) DE Designation(s)
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20 45 09.53 -31 20 27.2 AU Mic = HD 197481
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 129 44 Summary of the observations used in this work
dynsp/* . 168 Individual dynamic spectrum files
burst.dat 35 23 The maximum flux density of each Stokes V
burst detected
list.dat 109 3 List of fits files
fits/* . 3 Individual fits files
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See also:
J/A+A/654/A159 : CHEOPS photometry of Au Mic b transits in 2020 (Szabo+, 2021)
J/A+A/659/L7 : AU Mic b and c transits with CHEOPS (Szabo+, 2022)
J/AJ/162/137 : Minerva-Australis radial velocity of AU Mic (Addison+, 2021)
J/AJ/164/110 : HST/COS far-UV spectra of AU Microscopii flares
(Feinstein+, 2022)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "date" Epoch Epoch (YYYY-MM-DD)
11 A1 --- n_Epoch [*y] Note on Epoch (1)
13- 17 A5 --- Array Array configuration
19- 22 F4.1 h Dur Duration
25- 47 A23 --- FileSI Name of Stokes I flux density datafile in
subdirectory dynsp (2)
49- 76 A28 --- FileSIi Name of Stokes I imaginary component datafile
in subdirectory dynsp (2)
78-100 A23 --- FileSV Name of Stokes V flux density datafile in
subdirectory dynsp (2)
102-129 A28 --- FileSVi Name of Stokes V imaginary component datafile
in subdirectory dynsp (2)
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Note (1): Note as follows:
* = epochs where the data was not salvageable.
y = All epochs used PKS 2058-297 as a phase calibrator, except the
epochs marked with y, which used PKS 2032-350.
Note (2): Data description of dynamic spectra:
---------------------------------------------------------------------
Row 1: Timestamps in seconds of MJD
other rows:
Column 1: Channel number
other columns: measured intensity (mJy) at corresponding timestamp
---------------------------------------------------------------------
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Byte-by-byte Description of file: burst.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "date" Epoch Epoch
13- 35 E23.18 mJy S Burst flux density
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Nx Number of pixels along X-axis
4- 6 I3 --- Ny Number of pixels along Y-axis
8- 9 I2 Kibyte size Size of FITS file
11- 42 A32 --- FileName Name of FITS file, in subdirectory fits
44-109 A66 --- Title Title of the FITS file
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Acknowledgements:
Sanne Bloot, bloot(at)astron.nl
(End) Sanne Bloot [the Netherlands], Patricia Vannier [CDS] 05-Feb-2024