112 research outputs found
Random function tracer Patent
Design and development of random function tracer for obtaining coordinates of points on contour map
PHOENIX model chromospheres of mid- to late-type M dwarfs
We present semi-empirical model chromospheres computed with the atmosphere
code PHOENIX. The models are designed to fit the observed spectra of five mid-
to late-type M dwarfs. Next to hydrogen lines from the Balmer series we used
various metal lines, e. g. from Fe {\sc i}, for the comparison between data and
models. Our computations show that an NLTE treatment of C, N, O impacts on the
hydrogen line formation, while NLTE treatment of less abundant metals such as
nickel influences the lines of the considered species itself. For our coolest
models we investigated also the influence of dust on the chromospheres and
found that dust increases the emission line flux.
Moreover we present an (electronically published) emission line list for the
spectral range of 3100 to 3900 and 4700 to 6800 \AA for a set of 21 M dwarfs
and brown dwarfs. The line list includes the detection of the Na {\sc i} D
lines in emission for a L3 dwarf.Comment: 14 pages, 18 figure
Multiwavelength observations of a giant flare on CN Leonis I. The chromosphere as seen in the optical spectra
Flares on dM stars contain plasmas at very different temperatures and thus
affect a wide wavelength range in the electromagnetic spectrum. While the
coronal properties of flares are studied best in X-rays, the chromosphere of
the star is observed best in the optical and ultraviolet ranges. Therefore,
multiwavelength observations are essential to study flare properties throughout
the atmosphere of a star. We analysed simultaneous observations with UVES/VLT
and XMM-Newton of the active M5.5 dwarf CN Leo (Gl 406) exhibiting a major
flare. The optical data cover the wavelength range from 3000 to 10000 Angstrom.
From our optical data, we find an enormous wealth of chromospheric emission
lines occurring throughout the spectrum. We identify a total of 1143 emission
lines, out of which 154 are located in the red arm, increasing the number of
observed emission lines in this red wavelength range by about a factor of 10.
Here we present an emission line list and a spectral atlas. We also find line
asymmetries for H I, He I, and Ca II lines. For the last, this is the first
observation of asymmetries due to a stellar flare. During the flare onset,
there is additional flux found in the blue wing, while in the decay phase,
additional flux is found in the red wing. We interpret both features as caused
by mass motions. In addition to the lines, the flare manifests itself in the
enhancement of the continuum throughout the whole spectrum, inverting the
normal slope for the net flare spectrum.Comment: 15 pages, accepted by A&
Multi-wavelength observations of Proxima Centauri
We report simultaneous observations of the nearby flare star Proxima Centauri
with VLT/UVES and XMM-Newton over three nights in March 2009. Our optical and
X-ray observations cover the star's quiescent state, as well as its flaring
activity and allow us to probe the stellar atmospheric conditions from the
photosphere into the chromosphere, and then the corona during its different
activity stages. Using the X-ray data, we investigate variations in coronal
densities and abundances and infer loop properties for an intermediate-sized
flare. The optical data are used to investigate the magnetic field and its
possible variability, to construct an emission line list for the chromosphere,
and use certain emission lines to construct physical models of Proxima
Centauri's chromosphere.
We report the discovery of a weak optical forbidden Fe xiii line at 3388 AA
during the more active states of Proxima Centauri. For the intermediate flare,
we find two secondary flare events that may originate in neighbouring loops,
and discuss the line asymmetries observed during this flare in H i, He i, and
Ca ii lines. The high time-resolution in the H alpha line highlights strong
temporal variations in the observed line asymmetries, which re-appear during a
secondary flare event. We also present theoretical modelling with the stellar
atmosphere code PHOENIX to construct flaring chromospheric models.Comment: 19 pages, 22 figures, accepted by A&
The Ca II infrared triplet's performance as an activity indicator compared to Ca II H and K
Aims. A large number of Calcium Infrared Triplet (IRT) spectra are expected
from the GAIA- and CARMENES missions. Conversion of these spectra into known
activity indicators will allow analysis of their temporal evolution to a better
degree. We set out to find such a conversion formula and to determine its
robustness.
Methods. We have compared 2274 Ca II IRT spectra of active main-sequence F to
K stars taken by the TIGRE telescope with those of inactive stars of the same
spectral type. After normalizing and applying rotational broadening, we
subtracted the comparison spectra to find the chromospheric excess flux caused
by activity. We obtained the total excess flux, and compared it to established
activity indices derived from the Ca II H & K lines, the spectra of which were
obtained simultaneously to the infrared spectra.
Results. The excess flux in the Ca II IRT is found to correlate well with
and , as well as , if the
-dependency is taken into account. We find an empirical conversion formula
to calculate the corresponding value of one activity indicator from the
measurement of another, by comparing groups of datapoints of stars with similar
B-V.Comment: 16 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
A coronal explosion on the flare star CN Leonis
We present simultaneous high-temporal and high-spectral resolution
observations at optical and soft X-ray wavelengths of the nearby flare star CN
Leo. During our observing campaign a major flare occurred, raising the star's
instantaneous energy output by almost three orders of magnitude. The flare
shows the often observed impulsive behavior, with a rapid rise and slow decay
in the optical and a broad soft X-ray maximum about 200 seconds after the
optical flare peak. However, in addition to this usually encountered flare
phenomenology we find an extremely short (~2 sec) soft X-ray peak, which is
very likely of thermal, rather than non-thermal nature and temporally coincides
with the optical flare peak. While at hard X-ray energies non-thermal bursts
are routinely observed on the Sun at flare onset, thermal soft X-ray bursts on
time scales of seconds have never been observed in a solar nor stellar context.
Time-dependent, one-dimensional hydrodynamic modeling of this event requires an
extremely short energy deposition time scale of a few seconds to reconcile
theory with observations, thus suggesting that we are witnessing the results of
a coronal explosion on CN Leo. Thus the flare on CN Leo provides the
opportunity to observationally study the physics of the long-sought
"micro-flares" thought to be responsible for coronal heating.Comment: 7 pages, accepted by A&
Fe XIII coronal line emission in cool M dwarfs
We report on a search for the Fe xiii forbidden coronal line at 3388.1 \AA in
a sample of 15 M-type dwarf stars covering the whole spectral class as well as
different levels of activity. A clear detection was achieved for LHS 2076
during a major flare and for CN Leo, where the line had been discovered before.
For some other stars the situation is not quite clear. For CN Leo we
investigated the timing behaviour of the Fe xiii line and report a high level
of variability on a timescale of hours which we ascribe to microflare heating.Comment: 13 pages, 10 figure
Rapid magnetic flux variability on the flare star CN Leonis
We present UVES/VLT observations of the nearby flare star CN Leo covering the
Wing-Ford FeH band near 1mu with high spectral resolution. Some of the FeH
absorption lines in this band are magnetically sensitive and allow a
measurement of the mean magnetic flux on CN Leo. Our observations, covering
three nights separated by 48 hours each, allow a clear detection of a mean
magnetic field of Bf~2.2kG. The differential flux measurements show a
night-to-night variability with extremely high significance. Finally, our data
strongly suggest magnetic flux variability on time scales as low as 6 hours in
line with chromospheric variability.Comment: 4 pages, accepted for publication as A&A Lette
The high energy spectrum of Proxima Centauri simultaneously observed at X-ray and FUV wavelengths
The M dwarf Proxima~Centauri is known to be magnetically active and it hosts
a likely Earth-like planet in its habitable zone. Understanding the
characteristics of stellar radiation by understanding the properties of the
emitting plasma is of paramount importance for a proper assessment of the
conditions on Proxima~Centauri~b and exoplanets around M dwarfs in general. We
determine the temperature structure of the coronal and transition region plasma
of Proxima Centauri from simultaneous X-ray and far-ultraviolet (FUV)
observations. The differential emission measure distribution (DEM) was
constructed for flaring and quiescent periods by analysing optically thin X-ray
and FUV emission lines. Four X-ray observations of Proxima Centauri were
conducted by the LETGS instrument on board of the Chandra X-ray Observatory and
four FUV observations were carried out using the Hubble STIS spectrograph. From
the X-ray light curves, we determined a variation of the quiescent count rate
by a factor of two within 20\% of the stellar rotation period. To obtain the
DEM, 18 optically thin emission lines were analysed (12 X-ray and six FUV). The
flare fluxes differ from the quiescence fluxes by factors of 4-20 (FUV) and
1-30 (X-ray). The temperature structure of the stellar corona and transition
region was determined for both the quiescence and flaring state by fitting the
DEM(T) with Chebyshev polynomials for a temperature range T = 4.25 - 8.
Compared to quiescence, the emission measure increases during flares for
temperatures below 0.3\,MK (FUV dominated region) and beyond 3.6\,MK (X-ray
dominated region). The reconstructed DEM shape provides acceptable line flux
predictions compared to the measured values. We provide synthetic spectra at
1-1700 \AA, which may be considered as representative for the high-energy
irradiation of Proxima~Cen~b during quiescent and flare periods.Comment: 12 pages, 11 figures, accepted to A&
Detection and high-resolution spectroscopy of a huge flare on the old M9 dwarf DENIS 104814.7-395606.1
We report a flare on the M9 dwarf DENIS 104814.7-395606.1, whose mass places
it directly at the hydrogen burning limit. The event was observed in a spectral
sequence during 1.3 hours. Line shifts to bluer wavelengths were detected in H
alpha, H beta, and in the Na D lines, indicating mass motions. In addition we
detect a flux enhancement on the blue side of the two Balmer lines in the last
spectrum of our series. We interpret this as rising gas cloud with a projected
velocity of about 100 km/s which may lead to mass ejection. The higher Balmer
lines H gamma to H 8 are not seen due to our instrumental setup, but in the
last spectrum there is strong evidence for H 9 being in emission.Comment: 7 pages, 8 figures, accepted A&
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