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Author name code: bard
ADS astronomy entries on 2022-09-14
author:"Bard, S.,"
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Title: Radiative Hydrodynamic Simulations of Acoustic Waves in
Sunspots
Authors: Bard, S.; Carlsson, M.
2010ApJ...722..888B Altcode:
We investigate the formation and evolution of the Ca II H line in a
sunspot. The aim of our study is to establish the mechanisms underlying
the formation of the frequently observed brightenings of small regions
of sunspot umbrae known as "umbral flashes." We perform fully consistent
NLTE radiation hydrodynamic simulations of the propagation of acoustic
waves in sunspot umbrae and conclude that umbral flashes result from
increased emission of the local solar material during the passage
of acoustic waves originating in the photosphere and steepening to
shock in the chromosphere. To quantify the significance of possible
physical mechanisms that contribute to the formation of umbral flashes,
we perform a set of simulations on a grid formed by different wave
power spectra, different inbound coronal radiation, and different
parameterized chromospheric heating. Our simulations show that the
waves with frequencies in the range 4.5-7.0 mHz are critical to the
formation of the observed blueshifts of umbral flashes while waves with
frequencies below 4.5 mHz do not play a role despite their dominance
in the photosphere. The observed emission in the Ca II H core between
flashes only occurs in the simulations that include significant inbound
coronal radiation and/or extra non-radiative chromospheric heating in
addition to shock dissipation.
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Title: Constructing Computationally Tractable Models of Si I for
the 1082.7 nm Transition
Authors: Bard, S.; Carlsson, M.
2008ApJ...682.1376B Altcode:
The Si I 1082.7 nm line is often observed together with the He
I 1083.0 nm. The silicon line is assumed to give a measure of the
photospheric driver of the chromospheric dynamics observed in the He
I 1083.0 nm line. To exploit the diagnostic potential of the silicon
line it is of interest to study its line formation. We here analyze
physical processes in the formation of the Si I 1082.7 nm line and
find that it is formed in non-LTE (NLTE). Its line core intensity is
lower than the corresponding LTE value because the source function is
below the Planck function due to photon losses in the line itself. In
order to make possible full NLTE dynamic calculations of this line
we develop a general method for reducing the size of atomic models
used in NLTE calculations by representing several physically similar
energy levels as one mean level. Our procedure preserves all the
important physical properties of the atom that have a bearing on the
observable spectrographic properties of the line under study. We
apply our procedure to the Si I atom for the 1082.7 nm transition
under solar conditions, and we are able to reduce the size of the
atomic model from 238 levels and 3152 transitions to 23 levels and
171 transitions with no significant change in the calculated emergent
intensity of the 1082.7 nm line. We calculate the response functions
for the Si I 1082.7 nm line both in the quiet Sun and above sunspot
umbrae. We find that the line-center Doppler shift has a mean response
height to velocity perturbations of 541 km in a quiet-Sun atmosphere
and 308 km in a sunspot atmosphere. These numbers depend on the model
atmosphere employed.
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Title: Radiative hydrodynamic simulations of acoustic waves in
sunspots
Authors: Bard, S.; Carlsson, M.
2004IAUS..223..445B Altcode: 2005IAUS..223..445B
We describe fully consistent NLTE radiation hydrodynamic simulations
of the propagation of acoustic waves in sunspot umbrae. In contrast
to the case of the quiet internetwork Sun, we find that the observed
behavior of the resonance lines of singly ionized calcium (H and K
lines) cannot be explained without adding non-acoustic heating. The
best agreement with observations is obtained with an extra heating
term as a function of height that is constant per volume.
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Title: Normal modes and discovery of high-order cross-frequencies
in the DBV white dwarf GD 358
Authors: Vuille, F.; O'Donoghue, D.; Buckley, D. A. H.; Massacand,
C. -M.; Solheim, J. E.; Bard, S.; Vauclair, G.; Giovannini, O.;
Kepler, S. O.; Kanaan, A.; Provencal, J. L.; Wood, M. A.; Clemens,
J. C.; Kleinman, S. J.; O'Brien, M. S.; Nather, R. E.; Winget, D. E.;
Nitta, A.; Klumpe, E. W.; Montgomery, M. H.; Watson, T. K.; Bradley,
P. A.; Sullivan, D. J.; Wu, K.; Marar, T. M. K.; Seetha, S.; Ashoka,
B. N.; Mahra, H. S.; Bhat, B. C.; Babu, V. C.; Leibowitz, E. M.;
Hemar, S.; Ibbetson, P.; Mashals, E.; Meištas, E.; Moskalik, P.;
Zoła, S.; Krzesiński, J.; Pajdosz, G.
2000MNRAS.314..689V Altcode:
We present a detailed mode identification performed on the 1994 Whole
Earth Telescope (WET) run on GD 358. The results are compared with that
obtained for the same star from the 1990 WET data. The two temporal
spectra show very few qualitative differences, although amplitude
changes are seen in most modes, including the disappearance of the
mode identified as k=14 in the 1990 data. The excellent coverage
and signal-to-noise ratio obtained during the 1994 run lead to
the secure identification of combination frequencies up to fourth
order, i.e. peaks that are sums or differences of up to four parent
frequencies, including a virtually complete set of second-order
frequencies, as expected from harmonic distortion. We show how the
third-order frequencies are expected to affect the triplet structure
of the normal modes by back-interacting with them. Finally, a search
for l=2 modes was unsuccessful, not verifying the suspicion that such
modes had been uncovered in the 1990 data set.
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Title: New Whole Earth Telescope observations of CD-24 7599: steps
towards delta Scuti star seismology
Authors: Handler, G.; Pikall, H.; O'Donoghue, D.; Buckley, D. A. H.;
Vauclair, G.; Chevreton, M.; Giovannini, O.; Kepler, S. O.; Goode,
P. R.; Provencal, J. L.; Wood, M. A.; Clemens, J. C.; O'Brien, M. S.;
Nather, R. E.; Winget, D. E.; Kleinman, S. J.; Kanaan, A.; Watson,
T. K.; Nitta, A.; Montgomery, M. H.; Klumpe, E. W.; Bradley, P. A.;
Sullivan, D. J.; Wu, K.; Marar, T. M. K.; Seetha, S.; Ashoka, B. N.;
Mahra, H. S.; Bhat, B. C.; Babu, V. C.; Leibowitz, E. M.; Hemar,
S.; Ibbetson, P. A.; Mashal, E.; Meistas, E. G.; Dziembowski, W. A.;
Pamyatnykh, A. A.; Moskalik, P.; Zola, S.; Pajdosz, G.; Krzesinski,
J.; Solheim, J. E.; Bard, S.; Massacand, C. M.; Breger, M.; Gelbmann,
M. J.; Paunzen, E.; North, P.
1997MNRAS.286..303H Altcode:
92h of new Whole Earth Telescope observations have been acquired for the
delta Scuti star CD-24 7599. All the seven pulsation modes reported by
Handler et al. are confirmed. However, significant amplitude variations
which are not caused by beating of closely spaced frequencies occurred
within two years. Analysing the combined data of both WET runs,
we detect six further pulsation modes, bringing the total number
up to 13. We also examine our data for high-frequency pulsations
similar to those exhibited by rapidly oscillating Ap stars, but we
do not find convincing evidence for variability in this frequency
domain. From new colour photometry and spectroscopy we infer that
CD-24 7599 is a hot main-sequence delta Scuti star with approximately
solar metallicity and vsini=52& plusmn2kms^-1. We cannot yet
propose a definite pulsation mode identification, but we report the
detection of a characteristic frequency spacing between the different
modes. We ascribe it to the simultaneous presence of l=1 and l=2
modes of consecutive radial order. A comparison of this frequency
spacing with frequencies of solar-metallicity models, as well as
stability analysis, allows us to constrain tightly the evolutionary
state of CD-24 7599. It is in the first half of its main-sequence
evolution, and has a mass of 1.85 +/-0.05 M_solar and a mean density of
rho^-=0.246+/-0.020rho^-_solar. This yields a seismological distance
of 650 +/- 70 pc, which is as accurate as distance determinations
for delta Scuti stars observed in clusters. Most of the pulsation
modes are pure p modes of radial order k=4-6, but the g_1 mode of
l=2 is likely to be excited and observed as well. Since a significant
contribution to this mode's kinetic energy comes from the outer part
of the convective core, CD-24 7599 becomes particularly interesting
for testing convective overshooting theories.
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Title: Shock Signature in Sunspots
Authors: Bard, S.; Carlsson, M.
1997ESASP.404..189B Altcode: 1997cswn.conf..189B
No abstract at ADS
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Title: Model spectra for the helium cataclysmic variable: AM CVn
Authors: Bard, S.
1996ASSL..208..133B Altcode: 1996IAUCo.158..133B; 1996cvro.coll..133B
No abstract at ADS
