677 research outputs found
First polarimetric observations and modeling of the FeH F^4 Delta-X^4 Delta system
Lines of diatomic molecules are more temperature and pressure sensitive than
atomic lines, which makes them ideal tools for studying cool stellar
atmospheres an internal structure of sunspots and starspots. The FeH F^4
Delta-X^4 Delta system represents such an example that exhibits in addition a
large magnetic field sensitivity. The current theoretical descriptions of these
transitions including the molecular constants involved are only based on
intensity measurements because polarimetric observations have not been
available so far, which limits their diagnostic value. We present for the first
time spectropolarimetric observations of the FeH F^4 Delta-X^4 Delta system
measured in sunspots to investigate their diagnostic capabilities for probing
solar and stellar magnetic fields. We investigate whether the current
theoretical model of FeH can reproduce the observed Stokes profiles including
their magnetic properties. The polarimetric observations are compared with
synthetic Stokes profiles modeled with radiative transfer calculations. This
allows us to infer the temperature and the magnetic field strength of the
observed sunspots. We find that the current theory successfully reproduces the
magnetic properties of a large number of lines in the FeH F^4 Delta-X^4 Delta
system. In a few cases the observations indicate a larger Zeeman splitting than
predicted by the theory. There, our observations have provided additional
constraints, which allowed us to determine empirical molecular constants. The
FeH F^4 Delta-X^4 Delta system is found to be a very sensitive magnetic
diagnostic tool. Polarimetric data of these lines provide us with more direct
information to study the coolest parts of astrophysical objects.Comment: 4 pages, 3 figure
Flip-flop phenomenon: observations and theory
In many active stars the spots concentrate on two permanent active longitudes
which are 180 degrees apart. In some of these stars the dominant part of the
spot activity changes the longitude every few years. This so-called flip-flop
phenomenon has up to now been reported in 11 stars, both single and binary
alike, and including also the Sun. To explain this phenomenon, a
non-axisymmetric dynamo mode, giving rise to two permanent active longitudes at
opposite stellar hemispheres, is needed together with an oscillating
axisymmetric magnetic field. Here we discuss the observed characteristics of
the flip-flop phenomenon and present a dynamo solution to explain them.Comment: 4 pages, 5 figures, contribution to the conference "Dynamos of the
Sun, Stars and Planets", to be published in AN Volume 32
The first close-up of the "flip-flop" phenomenon in a single star
We present temperature maps of the active late-type giant FK Com which
exhibit the first imagining record of the ``flip-flop'' phenomenon in a single
star. The phenomenon, in which the main part of the spot activity shifts 180
degrees in longitude, discovered a decade ago in FK Com, was reported later
also in a number of RS CVn binaries and a single young dwarf. With the surface
images obtained right before and after the ``flip-flop'', we clearly show that
the ``flip-flop'' phenomenon in FK Com is caused by changing the relative
strengths of the spot groups at the two active longitudes, with no actual spot
movements across the stellar surface, i.e. exactly as it happens in other
active stars.Comment: 4 pages, accepted by A&A Letter
Paschen-Back effect in the CrH molecule and its application for magnetic field measurements on stars, brown dwarfs, and hot exoplanets
We investigated the Paschen-Back effect in the (0,0) band of the
A6{\Sigma}+-X6{\Sigma}+ system of the CrH molecule, and we examined its
potential for estimating magnetic fields on stars and substellar objects, such
as brown dwarfs and hot exoplanets. We carried out quantum mechanical
calculations to obtain the energy level structure of the
electronic-vibrational-rotational states considered both in the absence and in
the presence of a magnetic field. Level mixing due to magnetic field
perturbation (the Paschen-Back effect) was consistently taken into account.
Then, we calculated frequencies and strengths of transitions between magnetic
sublevels. Employing these results and solving numerically a set of the
radiative transfer equations for polarized radiation, we calculated Stokes
parameters for both the individual lines and the (0,0) band depending on the
strength and orientation of the magnetic field. We demonstrate that magnetic
splitting of the individual CrH lines shows a significant asymmetry due to the
Paschen-Back effect already at 1 G field. This leads to a considerable signal
in both circular and linear polarization, up to 30 percent at the magnetic
field strength of more than 3 kG in early L dwarfs. The polarization does not
cancel out completely even at very low spectral resolution and is seen as
broad-band polarization of a few percent. Since the line asymmetry depends only
on the magnetic field strength and not on the filling factor, CrH lines provide
a very sensitive tool for direct measurement of the stellar magnetic fields on
faint cool objects, such as brown dwarfs and hot Jupiters, observed with low
spectral resolution.Comment: 11 pages, 6 figures, to be published in A&
Molecules as magnetic probes of starspots
Stellar dynamo processes can be explored by measuring the magnetic field.
This is usually obtained using the atomic and molecular Zeeman effect in
spectral lines. While the atomic Zeeman effect can only access warmer regions,
the use of molecular lines is of advantage for studying cool objects. The
molecules MgH, TiO, CaH, and FeH are suited to probe stellar magnetic fields,
each one for a different range of spectral types, by considering the signal
that is obtained from modeling various spectral types. We have analyzed the
usefulness of different molecules (MgH, TiO, CaH, and FeH) as diagnostic tools
for studying stellar magnetism on active G-K-M dwarfs. We investigate the
temperature range in which the selected molecules can serve as indicators for
magnetic fields on highly active cool stars and present synthetic Stokes
profiles for the modeled spectral type. We modeled a star with a spot size of
10% of the stellar disk and a spot comprising either only longitudinal or only
transverse magnetic fields and estimated the strengths of the polarization
Stokes V and Q signals for the molecules MgH, TiO, CaH, and FeH. We combined
various photosphere and spot models according to realistic scenarios. In G
dwarfs, the molecules MgH and FeH show overall the strongest Stokes V and Q
signals from the starspot, whereas FeH has a stronger Stokes V signal in all G
dwarfs, with a spot temperature of 3800K. In K dwarfs, CaH signals are
generally stronger, and the TiO signature is most prominent in M dwarfs.
Modeling synthetic polarization signals from starspots for a range of G-K-M
dwarfs leads to differences in the prominence of various molecular signatures
in different wavelength regions, which helps to efficiently select targets and
exposure times for observations.Comment: 9 pages, 5 figures, 1 tabl
Spectropolarimetric observations of cool DQ white dwarfs
Following our recent discovery of a new magnetic DQ white dwarf (WD) with CH
molecular features, we report the results for the rest of the DQ WDs from our
survey. We use high signal-to-noise spectropolarimetric data to search for
magnetic fields in a sample of 11 objects. One object in our sample,
WD1235+422, shows the signs of continuum circular polarization that is similar
to some peculiar DQs with unidentified molecular absorption bands, but the low
S/N and spectral resolution of these data make more observations necessary to
reveal the true nature of this object.Comment: 4 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
Center-to-limb polarization in continuum spectra of F, G, K stars
Context. Scattering and absorption processes in stellar atmosphere affect the
center-to-limb variations of the intensity (CLVI) and the linear polarization
(CLVP) of stellar radiation. Aims. There are several theoretical and
observational studies of CLVI using different stellar models, however, most
studies of CLVP have concentrated on the solar atmosphere and have not
considered the CLVP in cooler non-gray stellar atmospheres at all. In this
paper, we present a theoretical study of the CLV of the intensity and the
linear polarization in continuum spectra of different spectral type stars.
Methods. We solve the radiative transfer equations for polarized light
iteratively assuming no magnetic field and considering a plane-parallel model
atmospheres and various opacities. Results. We calculate the CLVI and the CLVP
for Phoenix stellar model atmospheres for the range of effective temperatures
(4500K - 6900K), gravities (log g = 3.0 - 5.0), and wavelengths (4000 - 7000
{\AA}), which are tabulated and available at the Strasbourg astronomical Data
Center (CDS). In addition, we present several tests of our code and compare our
results with measurements and calculations of CLVI and the CLVP for the Sun.
The resulting CLVI are fitted with polynomials and their coefficients are
presented in this paper. Conclusions. For the stellar model atmospheres with
lower gravity and effective temperature the CLVP is larger.Comment: 10 pages, 8 figure
- …