488 research outputs found
Polarimetric Calibration of Large-Aperture Telescopes II: The sub-aperture method
A new method for absolute polarimetric calibration of large telescopes is
presented. The proposed method is highly accurate and is based on the
calibration of a small sub-aperture, which is then extended to the full system
by means of actual observations of an astronomical source. The calibration
procedure is described in detail along with numerical simulations that explore
its robustness and accuracy. The advantages and disadvantages of this technique
with respect to other possible alternatives are discussed.Comment: Journal of the Optical Society of America-A, submitte
Statistical analysis of the very quiet Sun magnetism
The behavior of the observed polarization amplitudes with spatial resolution
is a strong constraint on the nature and organization of solar magnetic fields
below the resolution limit. We study the polarization of the very quiet Sun at
different spatial resolutions using ground- and space-based observations. It is
shown that 80% of the observed polarization signals do not change with spatial
resolution, suggesting that, observationally, the very quiet Sun magnetism
remains the same despite the high spatial resolution of space-based
observations. Our analysis also reveals a cascade of spatial scales for the
magnetic field within the resolution element. It is manifest that the Zeeman
effect is sensitive to the microturbulent field usually associated to Hanle
diagnostics. This demonstrates that Zeeman and Hanle studies show complementary
perspectives of the same magnetism.Comment: Accepted for publication in Ap
Multi-line Stokes inversion for prominence magnetic-field diagnostics
We present test results on the simultaneous inversion of the Stokes profiles
of the He I lines at 587.6 nm (D_3) and 1083.0 nm in prominences (90-deg
scattering). We created datasets of synthetic Stokes profiles for the case of
quiescent prominences (B<200 G), assuming a conservative value of 10^-3 of the
peak intensity for the polarimetric sensitivity of the simulated observations.
In this work, we focus on the error analysis for the inference of the magnetic
field vector, under the usual assumption that the prominence can be assimilated
to a slab of finite optical thickness with uniform magnetic and thermodynamic
properties. We find that the simultaneous inversion of the two lines
significantly reduces the errors on the inference of the magnetic field vector,
with respect to the case of single-line inversion. These results provide a
solid justification for current and future instrumental efforts with multi-line
capabilities for the observations of solar prominences and filaments.Comment: 14 pages, 5 figures, 1 tabl
Polarimetric Calibration of Large-Aperture Telescopes I: The Beam-Expansion Method
This paper describes a concept for the high-accuracy absolute calibration of
the instrumental polarization introduced by the primary mirror of a
large-aperture telescope. This procedure requires a small aperture with
polarization calibration optics (e.g., mounted on the dome) followed by a lens
that opens the beam to illuminate the entire surface of the mirror. The Jones
matrix corresponding to this calibration setup (with a diverging incident beam)
is related to that of the normal observing setup (with a collimated incident
beam) by an approximate correction term. Numerical models of parabolic on-axis
and off-axis mirrors with surface imperfections are used to explore its
accuracy.Comment: Journal of the Optical Society of America-A, in pres
Steady dynamos in finite domains: an integral equation approach
The paper deals with the integral equation approach to steady kinematic
dynamo models in finite domains based on Biot-Savart's law. The role of the
electric potential at the boundary is worked out explicitly. As an example, a
modified version of the simple spherical -effect dynamo model proposed
by Krause and Steenbeck is considered in which the -coefficient is no
longer constant but may vary with the radial coordinate. In particular, the
results for the original model are re-derived. Possible applications of this
integral equation approach for numerical simulations of dynamos in arbitrary
geometry and for an ''inverse dynamo theory'' are sketched.Comment: 18 pages, submitted to Astron. Nach
Spectral type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070
We show individual high resolution spectra of components A, B, and C of the
nearby late-M type multiple system LHS 1070. Component A is a mid-M star, B and
C are known to have masses at the threshold to brown dwarfs. From our spectra
we measure rotation velocities and the mean magnetic field for all three
components individually. We find magnetic flux on the order of several
kilo-Gauss in all components. The rotation velocities of the two late-M objects
B and C are similar (vsini = 16km/s), the earlier A component is spinning only
at about half that rate. This suggests weakening of net rotational braking at
late-M spectral type, and that the lack of slowly rotating late-M and L dwarfs
is real. Furthermore, we found that magnetic flux in the B component is about
twice as strong as in component C at similar rotation rate. This indicates that
rotational braking is not proportional to magnetic field strength in fully
convective objects, and that a different field topology is the reason for the
weak braking in low mass objects.Comment: accepted for publication as A&A Lette
Identification of strong photometric activity in the components of LHS 1070
Activity in low-mass stars is an important ingredient in the evolution of
such objects. Fundamental physical properties such as age, rotation, magnetic
field are correlated with activity. Aims: We show that two components of the
low-mass triple system LHS 1070 exhibit strong flaring activity. We identify
the flaring components and obtained an improved astrometric solution for the
LHS 1070 A/(B+C) system. Methods: Time-series CCD observations were used to
monitor LHS 1070 in the B and I_C bands. H-band data were used to obtain
accurate astrometry for the LHS 1070 A/(B+C) system. Results: We have found
that two components of the triple system LHS 1070 exhibit photometric activity.
We identified that components A and B are the flaring objects. We estimate the
total energy, ~2.0 x 10^{33} ergs, and the magnetic field strength, ~5.5 kG, of
the flare observed in LHS 1070 B. This event is the largest amplitude, \Delta B
> 8.2 mag, ever observed in a flare star.Comment: 5 pages, 5 figures, accepted for publication in A&
Fine structure of the chromospheric activity in Solar-type stars - The Halpha Line
A calibration of H-alpha as both a chromospheric diagnostic and an age
indicator is presented, complementing the works previously done on this subject
(Herbig 1985, Pasquini & Pallavicini 1991. The chromospheric diagnostic was
built with a statistically significant sample, covering nine years of
observations, and including 175 solar neighborhood stars. Regarding the age
indicator, the presence of stars for which very accurate ages are determined,
such as those belonging to clusters and kinematic groups, lends confidence to
our analysis. We also investigate the possibility that stars of the same age
might have gone through different tracks of chromospheric decay, identifying -
within the same age range - effects of metallicity and mass. These parameters,
however, as well as age, seem to be significant only for dwarf stars, losing
their meaning when we analyze stars in the subgiant branch. This result
suggests that, in these evolved stars, the emission mechanism cannot be
magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al.
2002c, and references therein). The Sun is found to be a typical star in its
H-alpha chromospheric flux, for its age, mass and metallicity. As a byproduct
of this work, we developed an automatic method to determine temperatures from
the wings of H-alpha, which means the suppression of the error inherent to the
visual procedure used in the literature.Comment: 10 pages, 10 figures, accepted for publication in Astronomy &
Astrophysics. Nature of replacement: match astro-ph and ADS title (greek
letter
X-Ray Evidence for Flare Density Variations and Continual Chromospheric Evaporation in Proxima Centauri
Using the XMM-Newton X-ray observatory to monitor the nearest star to the
Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically
active star ever observed. Correlated X-ray and optical variability provide
strong support for coronal energy and mass supply by a nearly continuous
sequence of rapid explosive energy releases. Variable emission line fluxes were
observed in the He-like triplets of OVII and NeIX during a giant flare. They
give direct X-ray evidence for density variations, implying densities between
2x10^{10} - 4x10^{11} cm^{-3} and providing estimates of the mass and the
volume of the line-emitting plasma. We discuss the data in the context of the
chromospheric evaporation scenario.Comment: 10 pages, 2 figures, accepted by The Astrophysical Journal, Letters;
improved calculations of radiative loss of cool plasma (toward end of paper
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