141 research outputs found
Hanle effect in the CN violet system with LTE modeling
Weak entangled magnetic fields with mixed polarity occupy the main part of
the quiet Sun. The Zeeman effect diagnostics fails to measure such fields
because of cancellation in circular polarization. However, the Hanle effect
diagnostics, accessible through the second solar spectrum, provides us with a
very sensitive tool for studying the distribution of weak magnetic fields on
the Sun. Molecular lines are very strong and even dominate in some regions of
the second solar spectrum. The CN system is
one of the richest and most promising systems for molecular diagnostics and
well suited for the application of the differential Hanle effect method. The
aim is to interpret observations of the CN
system using the Hanle effect and to obtain an estimation of the magnetic field
strength. We assume that the CN molecular layer is situated above the region
where the continuum radiation is formed and employ the single-scattering
approximation. Together with the Hanle effect theory this provides us with a
model that can diagnose turbulent magnetic fields. We have succeeded in fitting
modeled CN lines in several regions of the second solar spectrum to
observations and obtained a magnetic field strength in the range from 10--30 G
in the upper solar photosphere depending on the considered lines.Comment: Accepted for publication in Astronomy and Astrophysic
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
Modelling the molecular Zeeman effect in M-dwarfs: methods and first results
We present first quantitative results of the surface magnetic field
measurements in selected M-dwarfs based on detailed spectra synthesis conducted
simultaneously in atomic and molecular lines of the FeH Wing-Ford
transitions. A modified version of the Molecular
Zeeman Library (MZL) was used to compute Land\'e g-factors for FeH lines in
different Hund's cases. Magnetic spectra synthesis was performed with the
Synmast code. We show that the implementation of different Hund's case for FeH
states depending on their quantum numbers allows us to achieve a good fit to
the majority of lines in a sunspot spectrum in an automatic regime. Strong
magnetic fields are confirmed via the modelling of atomic and FeH lines for
three M-dwarfs YZ~CMi, EV~Lac, and AD~Leo, but their mean intensities are found
to be systematically lower than previously reported. A much weaker field
(~kG against ~kG) is required to fit FeH lines in the spectra of
GJ~1224. Our method allows us to measure average magnetic fields in very
low-mass stars from polarized radiative transfer. The obtained results indicate
that the fields reported in earlier works were probably overestimated by about
\%. Higher quality observations are needed for more definite results.Comment: Accepted by A&A, 13 pages, 7 figures, 1 tabl
First polarimetric measurements and modeling of the Paschen-Back effect in CaH transitions
We report the first spectropolarimetric observations and modeling of CaH
transitions in sunspots. We have detected strong polarization signals in many
CaH lines from the A-X system, and we provide the first successful fit to the
observed Stokes profiles using the previously developed theory of the
Paschen-Back effect in arbitrary electronic states of diatomic molecules and
polarized radiative transfer in molecular lines in stellar atmospheres. We
analyze the CaH Stokes profiles together with quasi-simultaneous observations
in TiO bands and conclude that CaH provides a valuable diagnostic of magnetic
fields in sunspots, starspots, cool stars, and brown dwarfs
NLTE modeling of Stokes vector center-to-limb variations in the CN violet system
The solar surface magnetic field is connected with and even controls most of
the solar activity phenomena. Zeeman effect diagnostics allow for measuring
only a small fraction of the fractal-like structured magnetic field. The
remaining hidden magnetic fields can only be accessed with the Hanle effect.
Molecular lines are very convenient for applying the Hanle effect diagnostics
thanks to the broad range of magnetic sensitivities in a narrow spectral
region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II
installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno
(IRSOL), we simultaneously observed intensity and linear polarization
center-to-limb variations in two spectral regions containing the (0,0) and
(1,1) bandheads of the CN B 2 {\Sigma} - X 2 {\Sigma} system. Here we present
an analysis of these observations. We have implemented coherent scattering in
molecular lines into a NLTE radiative transfer code. A two-step approach was
used. First, we separately solved the statistical equilibrium equations and
compute opacities and intensity while neglecting polariza- tion. Then we used
these quantities as input for calculating scattering polarization and the Hanle
effect. We have found that it is impossible to fit the intensity and
polarization simultaneously at different limb angles in the frame- work of
standard 1D modeling. The atmosphere models that provide correct intensity
center-to-limb variations fail to fit linear polar- ization center-to-limb
variations due to lacking radiation field anisotropy. We had to increase the
anisotropy by means of a specially introduced free parameter. This allows us to
successfully interpret our observations. We discuss possible reasons for
underestimating the anisotropy in the 1D modeling.Comment: 15 pages, 10 figures, accepted for publication in
Astronomy&Astrophysic
Aspects of Multi-Dimensional Modelling of Substellar Atmospheres
Theoretical arguments and observations suggest that the atmospheres of Brown
Dwarfs and planets are very dynamic on chemical and on physical time scales.
The modelling of such substellar atmospheres has, hence, been much more
demanding than initially anticipated. This Splinter
(http://star-www.st-and.ac.uk/~ch80/CS16/MultiDSplinter_CS16.html) has combined
new developments in atmosphere modelling, with novel observational techniques,
and new challenges arising from planetary and space weather observations.Comment: 12 pages, 5 figures, summery of Cool Stars 16 Splinter
'Multi-Dimensional Modelling of Substellar Atmospheres
Maximum Entropy Limit of Small-scale Magnetic Field Fluctuations in the Quiet Sun
The observed magnetic field on the solar surface is characterized by a very
complex spatial and temporal behavior. Although feature-tracking algorithms
have allowed us to deepen our understanding of this behavior, subjectivity
plays an important role in the identification and tracking of such features. In
this paper, we continue studies Gorobets, A. Y., Borrero, J. M., & Berdyugina,
S. 2016, ApJL, 825, L18 of the temporal stochasticity of the magnetic field on
the solar surface without relying either on the concept of magnetic features or
on subjective assumptions about their identification and interaction. We
propose a data analysis method to quantify fluctuations of the line-of-sight
magnetic field by means of reducing the temporal field's evolution to the
regular Markov process. We build a representative model of fluctuations
converging to the unique stationary (equilibrium) distribution in the long time
limit with maximum entropy. We obtained different rates of convergence to the
equilibrium at fixed noise cutoff for two sets of data. This indicates a strong
influence of the data spatial resolution and mixing-polarity fluctuations on
the relaxation process. The analysis is applied to observations of magnetic
fields of the relatively quiet areas around an active region carried out during
the second flight of the Sunrise/IMaX and quiet Sun areas at the disk center
from the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory satellite.Comment: 11 pages, 5 figures, The Astrophysical Journal Supplement Series
(accepted
Long-term chromospheric activity of non-eclipsing RS CVn-type stars
Context. The IUE database provides a large number of UV high and
low-resolution spectra of RS CVn-type stars from 1978 to 1996. In particular,
many of these stars were monitored continuously during several seasons by IUE.
Aims. Our main purpose is to study the short and long-term chromospheric
activity of the RS CVn systems most observed by IUE: HD 22468 (V711 Tau, HR
1099, K1IV+G5V), HD 21242 (UX Ari, K0IV+G5V) and HD 224085 (II Peg, K2IV).
Methods. We first obtain the Mount Wilson index S from the IUE high and
low-resolution spectra. Secondly, we analyse with the Lomb-Scargle periodogram
the mean annual index S and the amplitude of its rotational modulation.
Results. For HD 22468 (V711 Tau, HR 1099), we found a possible chromospheric
cycle with a period of 18 years and a shorter cycle with a period of 3 years,
which could be associated to a chromospheric "flip-flop" cycle. The data of HD
224085 (II Peg) also suggest a chromospheric cycle of 21 years and a flip-flop
cycle of 9 years. Finally, we obtained a possible chromospheric cycle of 7
years for HD 21242 (UX Ari).Comment: accepted for publication in Astronomy and Astrophysic
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