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Author name code: espagnet
ADS astronomy entries on 2022-09-14
author:"Espagnet, O."
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Title: Pollux: A weak dynamo-driven dipolar magnetic field and
implications for its probable planet
Authors: Aurière, M.; Petit, P.; Mathias, P.; Konstantinova-Antova,
R.; Charbonnel, C.; Donati, J. -F.; Espagnet, O.; Folsom, C. P.;
Roudier, T.; Wade, G. A.
2021A&A...646A.130A Altcode: 2021arXiv210102016A
Context. Pollux is considered as an archetype of a giant star hosting a
planet since its radial velocity (RV) presents very stable sinusoidal
variations with a period of about 590 d. We then discovered a weak
magnetic field at its surface using spectropolarimetry, questioning the
planetary hypothesis. <BR /> Aims: We followed up our investigations on
Pollux to characterize its magnetic field and to infer the effects of
magnetic activity on the RV variations. <BR /> Methods: We first used
ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes
V spectra of Pollux to study their variations for a duration of 4.25
years, that is, for more than two periods of the RV variations. We
used the least-squares deconvolution profiles to measure the
longitudinal magnetic field and to perform a Zeeman Doppler imaging
(ZDI) investigation. <BR /> Results: The longitudinal magnetic field
of Pollux is found to vary with a sinusoidal behavior and a period
similar to that of the RV variations. From the ZDI investigation a
rotation period of Pollux is determined to be equal to 660 ± 15 days
and possibly different than the period of variations of the RV. As
to the magnetic topology, the poloidal component is dominant and
almost purely dipolar with an inclination of 10.5° of the dipole
with respect to the rotation axis. The mean strength of the surface
magnetic field is 0.44 G. Pollux is found approximately as active as
the Sun observed as a star and this activity could induce moderate
RV variations. <BR /> Conclusions: As to the origin of the magnetic
field of Pollux, we favor the hypothesis that it is maintained through
contemporaneous dynamo action. Pollux appears as the representative
of a class of slowly rotating and weakly magnetic G-K red giants. To
explain the sinusoidal RV variations of Pollux, two scenarios are
proposed. If the RV period is different from the rotation period,
the observed periodic RV variations are due to the hosted planet and
the contribution of Pollux magnetic activity is not significantly
detected. In the peculiar case in which the two periods are equal,
we cannot discard the possibility that the activity of Pollux could
explain the total RV variations and that the planet hypothesis would
appear unnecessary. In any case magnetic activity could contribute
significantly to RV variations in some intermediate mass G-K red
giants hosting planets, particularly those with small amplitude RV
variations. <P />Based on observations obtained at the Télescope
Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and
Université de Toulouse, France, and the Canada-France-Hawaii Telescope
(CFHT), which is operated by the National Research Council of Canada,
CNRS/INSU and the University of Hawaii.
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Title: Pollux: a stable weak dipolar magnetic field but no planet?
Authors: Aurière, Michel; Konstantinova-Antova, Renada; Espagnet,
Olivier; Petit, Pascal; Roudier, Thierry; Charbonnel, Corinne; Donati,
Jean-François; Wade, Gregg A.
2014IAUS..302..359A Altcode: 2013arXiv1310.6907A
Pollux is considered as an archetype of a giant star hosting a planet:
its radial velocity (RV) presents sinusoidal variations with a period
of about 590 d, which have been stable for more than 25 years. Using
ESPaDOnS and Narval we have detected a weak (sub-gauss) magnetic
field at the surface of Pollux and followed up its variations with
Narval during 4.25 years, i.e. more than for two periods of the RV
variations. The longitudinal magnetic field is found to vary with a
sinusoidal behaviour with a period close to that of the RV variations
and with a small shift in phase. We then performed a Zeeman Doppler
imaging (ZDI) investigation from the Stokes V and Stokes I least-squares
deconvolution (LSD) profiles. A rotational period is determined, which
is consistent with the period of variations of the RV. The magnetic
topology is found to be mainly poloidal and this component almost
purely dipolar. The mean strength of the surface magnetic field is
about 0.7 G. As an alternative to the scenario in which Pollux hosts a
close-in exoplanet, we suggest that the magnetic dipole of Pollux can
be associated with two temperature and macroturbulent velocity spots
which could be sufficient to produce the RV variations. We finally
investigate the scenarii of the origin of the magnetic field which
could explain the observed properties of Pollux.
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Title: Spatial relation between the 5-minute oscillations and
granulation patterns.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, P.; Mein, N.;
Malherbe, J. M.
1996A&A...313..297E Altcode:
A white light, high resolution movie of the quiet photosphere has been
used to study the spatial relation between the solar granulation and
the 5-min oscillations of high degree mode (small spatial scale). The
granular and oscillatory brightness components have been separated with
the help of a specific filter, in the k-ω diagram. The surface of the
Sun appears to be covered with adjacent oscillating cells of apparent
size 2" (1500km) to 3" (2000km). Some cells are noticeable by their
amplitude which is much larger than average. These strong oscillations
usually appear in expanding intergranular spaces. Our observations
suggest that the 5-min oscillations are stochastically generated by
turbulent convection. However, the most energetic oscillations occur
in downflows only, well separated of each other in time and space.
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Title: Penetration of the solar granulation into the photosphere:
height dependence of intensity and velocity fluctuations.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P.
1995A&AS..109...79E Altcode:
A 16-min time series of two-dimensional "Multichanel Subtractive Double
Pass" spectrograms, recorded in the NaD_2_ 5690 line, has been used to
study the vertical structure of the photosphere and, in particular,
the penetration of granules. A statistical and a morphological
investigations have been made. The 5-min oscillations have been
carefully filtered in the k-ω space. Intensity fluctuations associated
to granules disappear in a very short distance, about 60 to 90km above
the continuum level. The intensity features in the upper photosphere
are not related to the granules. The velocity fluctuations associated
to the granulation, cross the whole thickness of the photosphere. The
photosphere can thus be separated into two distinct layers, but only
for the intensity features, not for the velocity features which are
the same from the bottom to the top of the stable atmosphere. In the
lower photosphere, below 90km, only granules are detected. In the upper
photosphere, our observations suggest that the weak pattern visible
there, is turbulent in origin. In addition to this weak pattern,
bright sinking plumes have been discovered; they are associated with
strong downflows, often adjacent to strong upflows. The size of most
plumes is 1" to 2"; however the most outstanding of them may be as
large as 4". Their origin, which may be photospheric or chromospheric,
is still unknown.
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Title: Peculiar interactions between granules and network bright
points in the solar photosphere
Authors: Roudier, T.; Espagnet, O.; Muller, R.; Vigneau, J.
1994A&A...287..982R Altcode:
The magnetic elements which form the photospheric network, outside
active regions, are visible as tiny bright features, smaller than 0.5",
called network bright points (NBPs). 15 to 20% of them elongate during
their life, when they are squeezed between two expanding granules,
or simply pushed by an expanding or a moving granule. Some statistical
properties are reported. The most interesting property is that their
size and brightness increases simultaneously indicating a strong
increase of radiative flux as they grow. They can become as large as
1000 km x 400 km and as bright as 1.25 the brightness of the average
photosphere. As theoretical models predict that magnetic flux tubes of
size larger than 300 km should be darker than the mean photosphere, our
observations suggest that during the elongation, some mechanical energy
should contribute in the heating of the tubes. This additional energy
may heat some features in the upper photosphere like chromospheric
spicules and transition zone jets or microflares, whose production
rates and lifetimes are about the same as those of elongation of
NBPS. Elongated NBPs are shaked by the turbulent granules, similarly
to the normal NBPs and thus should contribute to the heating of the
solar corona too.
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Title: La granulation solaire: origine, pénétration dans la
photosphère et interactions avec les oscillations de 5 minutes
Authors: Espagnet, O.
1994PhDT........47E Altcode:
No abstract at ADS
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Title: Turbulent power spectra of solar granulation.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.
1993A&A...271..589E Altcode:
High resolution "Multichannel Subtractive Double Pass" spectrograms
and white light photographs have been used to compute power spectra
of the solar granulation. The 5 min oscillations have been carefully
filtered, and the influence of the variation of the mean size of
granules at the mesogranulation scale has been taken into account. <P
/>It results that both the power spectra of velocity and intensity
fluctuations, displayed in a log P - log k scale, are characteristic
of a turbulent atmosphere: they present a discontinuity at 3" =
2000 km (which corresponds to the size of a large granules) and an
energy which decreases according to the Kolmogorov -5/3 power law
in the granulation range; another discontinuity, at 1".4 = 1000 km
(which corresponds approximately to a Peclet number Pe = 1, for which
thermal diffusion and advection are of equal importance) is present in
the intensity power spectrum, which decreases with a -17/3 power law,
in the range of sizes smaller than 1".4. <P />These results indicate
that granules are turbulent eddies. They have some convective characters
(like the correlation between velocities and brightness) because they
are formed in a strongly superadiabatic atmosphere.
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Title: Turbulence et granulation solaire.
Authors: Espagnet, O.; Muller, R.; Roudier, T.
1992JAF....43...35E Altcode:
No abstract at ADS
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Title: Dynamique de la photosphère solaire: granulation,
mésogranulation, oscillations.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P.
1992JAF....43...36E Altcode:
No abstract at ADS
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Title: Dynamics of the solar granulation. II - Statistical analysis:
Power spectra, coherence, phase
Authors: Roudier, T.; Vigneau, J.; Espagnet, O.; Muller, R.; Mein,
P.; Malherbe, J. M.
1991A&A...248..245R Altcode:
The solar granulation is analyzed statistically by the computation
of coherence, phase, and power spectra using data provided by the
Multichannel Subtractive Double Pass spectrograph of the Pic du Midi
Observatory. The main result of the analysis is that the velocity
power spectrum of the granulation changes in shape at 3 arcsec: the raw
spectrum features a slope discontinuity; the corrected spectra have a
power maximum at 3 arcsec. Between 3 arcsec and the resolution limit
(0.8 arcsec), the power decreases almost linearly, with a slope close
to -5/3, consistent with the Kolmogorov power law for a turbulent
energy cascade.
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Title: Results from high resolution solar images and spectra obtained
at the Pic du Midi Observatory (1986-1990)
Authors: Roudier, Th.; Muller, R.; Vigneau, J.; Auffret, H.; Espagnet,
O.; Simon, G. W.; Title, A. M.; Frank, Z.; Shine, R. A.; Tarbell,
T. D.; Mein, P.; Malherbe, J. M.
1991AdSpR..11e.205R Altcode: 1991AdSpR..11..205R
We present an overview of our recent results about solar granulation and
mesogranulation, obtained with Pic du Midi observations. These results
were obtained during 1986-1990 using image and spectrographic analysis
of high spatial resolution data. The study of the solar granulation,
with 2 Dim. “Multichannel Subtractive Double Pass” (M.S.D.P.) spectra,
shows a clear change of the dynamical regime at 3” (⋍ 2200 km)
of the photospheric velocity field when oscillatory components are
filtered out. <P />A three hour movie obtained on film at Pic du Midi
Observatory and analyzed at the Lockheed Research Laboratory and the
National Solar Observatory (Sacramento Peak) was used to calculate
the horizontal flow pattern. The mean lifetime of the diverging areas
related to mesogranulation is estimated at 3 hours; these diverging
areas are swept by the supergranulation flow towards the supergranule
boundary with a mean speed of 0.4 km/s.
