466 research outputs found
Detection of polarization from the E^4\Pi-A^4\Pi system of FeH in sunspot spectra
Here we report the first detection of polarization signals induced by the
Zeeman effect in spectral lines of the E^4\Pi-A^4\Pi system of FeH located
around 1.6 m. Motivated by the tentative detection of this band in the
intensity spectrum of late-type dwarfs, we have investigated the full Stokes
sunspot spectrum finding circular and linear polarization signatures that we
associate with the FeH lines of the E^4\Pi-A^4\Pi band system. We investigate
the Zeeman effect in these molecular transitions pointing out that in Hund's
case (a) coupling the effective Land\'e factors are never negative. For this
reason, the fact that our spectropolarimetric observations indicate that the
Land\'e factors of pairs of FeH lines have opposite signs, prompt us to
conclude that the E^4\Pi-A^4\Pi system must be in intermediate angular momentum
coupling between Hund's cases (a) and (b). We emphasize that theoretical and/or
laboratory investigations of this molecular system are urgently needed for
exploiting its promising diagnostic capabilities.Comment: 11 pages, 4 figures, accepted for publication in Astrophysical
Journal Letter
Uncertainties in the solar photospheric oxygen abundance
The purpose of this work is to better understand the confidence limits of the
photospheric solar oxygen abundance derived from three-dimensional models using
the forbidden [OI] line at 6300 \AA , including correlations with other
parameters involved. We worked with a three-dimensional empirical model and two
solar intensity atlases. We employed Bayesian inference as a tool to determine
the most probable value for the solar oxygen abundance given the model chosen.
We considered a number of error sources, such as uncertainties in the continuum
derivation, in the wavelength calibration and in the abundance/strength of Ni.
Our results shows correlations between the effects of several parameters
employed in the derivation. The Bayesian analysis provides robust confidence
limits taking into account all of these factors in a rigorous manner. We obtain
that, given the empirical three-dimensional model and the atlas observations
employed here, the most probable value for the solar oxygen abundance is
. However, we note that this uncertainty does
not consider possible sources of systematic errors due to the model choice.Comment: Accepted for publication in Astronomy and Astrophysic
Penumbral thermal structure below the visible surface
. The thermal structure of the penumbra below its visible surface
(i.e., ) has important implications for our present understanding
of sunspots and their penumbrae: their brightness and energy transport, mode
conversion of magneto-acoustic waves, sunspot seismology, and so forth. .
We aim at determining the thermal stratification in the layers immediately
beneath the visible surface of the penumbra: ( km below the visible continuum-forming layer). . We analyzed
spectropolarimetric data (i.e., Stokes profiles) in three Fe \textsc{i} lines
located at 1565 nm observed with the GRIS instrument attached to the 1.5-meter
solar telescope GREGOR. The data are corrected for the smearing effects of
wide-angle scattered light and then subjected to an inversion code for the
radiative transfer equation in order to retrieve, among others, the temperature
as a function of optical depth . . We find that the
temperature gradient below the visible surface of the penumbra is smaller than
in the quiet Sun. This implies that in the region the penumbral
temperature diverges from that of the quiet Sun. The same result is obtained
when focusing only on the thermal structure below the surface of bright
penumbral filaments. We interpret these results as evidence of a thick
penumbra, whereby the magnetopause is not located near its visible surface. In
addition, we find that the temperature gradient in bright penumbral filaments
is lower than in granules. This can be explained in terms of the limited
expansion of a hot upflow inside a penumbral filament relative to a granular
upflow, as magnetic pressure and tension forces from the surrounding penumbral
magnetic field hinder an expansion like this.Comment: 5 pages; 2 figures; accepted for publication in Astronomy and
Astrophysics Letter
Estimating the magnetic field strength from magnetograms
A properly calibrated longitudinal magnetograph is an instrument that
measures circular polarization and gives an estimation of the magnetic flux
density in each observed resolution element. This usually constitutes a lower
bound of the field strength in the resolution element, given that it can be
made arbitrarily large as long as it occupies a proportionally smaller area of
the resolution element and/or becomes more transversal to the observer and
still produce the same magnetic signal. Yet, we know that arbitrarily stronger
fields are less likely --hG fields are more probable than kG fields, with
fields above several kG virtually absent-- and we may even have partial
information about its angular distribution. Based on a set of sensible
considerations, we derive simple formulae based on a Bayesian analysis to give
an improved estimation of the magnetic field strength for magnetographs.Comment: 8 pages, 7 figures, accepted for publication in A&
Analytical maximum likelihood estimation of stellar magnetic fields
The polarised spectrum of stellar radiation encodes valuable information on
the conditions of stellar atmospheres and the magnetic fields that permeate
them. In this paper, we give explicit expressions to estimate the magnetic
field vector and its associated error from the observed Stokes parameters. We
study the solar case where specific intensities are observed and then the
stellar case, where we receive the polarised flux. In this second case, we
concentrate on the explicit expression for the case of a slow rotator with a
dipolar magnetic field geometry. Moreover, we also give explicit formulae to
retrieve the magnetic field vector from the LSD profiles without assuming mean
values for the LSD artificial spectral line. The formulae have been obtained
assuming that the spectral lines can be described in the weak field regime and
using a maximum likelihood approach. The errors are recovered by means of the
hermitian matrix. The bias of the estimators are analysed in depth.Comment: accepted for publication in MNRA
MOLPOP-CEP: An Exact, Fast Code for Multi-Level Systems
We present MOLPOP-CEP, a universal line transfer code that allows the exact
calculation of multi-level line emission from a slab with variable physical
conditions for any arbitrary atom or molecule for which atomic data exist. The
code includes error control to achieve any desired level of accuracy, providing
full confidence in its results. Publicly available, MOLPOP-CEP employs our
recently developed Coupled Escape Probability (CEP) technique, whose
performance exceeds other exact methods by orders of magnitude. The program
also offers the option of an approximate solution with different variants of
the familiar escape probability method. As an illustration of the MOLPOP-CEP
capabilities we present an exact calculation of the Spectral Line Energy
Distribution (SLED) of the CO molecule and compare it with escape probability
results. We find that the popular large-velocity gradient (LVG) approximation
is unreliable at large CO column densities. Providing a solution of the
multi-level line transfer problem at any prescribed level of accuracy,
MOLPOP-CEP is removing any doubts about the validity of its final results.Comment: 10 pages, 3 figures, accepted for publication in A&
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