65 research outputs found
The magnetic field configuration of a solar prominence inferred from spectropolarimetric observations in the He I 10830 A triplet
Context: The determination of the magnetic field vector in quiescent solar
prominences is possible by interpreting the Hanle and Zeeman effects in
spectral lines. However, observational measurements are scarce and lack high
spatial resolution. Aims: To determine the magnetic field vector configuration
along a quiescent solar prominence by interpreting spectropolarimetric
measurements in the He I 1083.0 nm triplet obtained with the Tenerife Infrared
Polarimeter installed at the German Vacuum Tower Telescope of the Observatorio
del Teide. Methods. The He I 1083.0 nm triplet Stokes profiles are analyzed
with an inversion code that takes into account the physics responsible of the
polarization signals in this triplet. The results are put into a solar context
with the help of extreme ultraviolet observations taken with the Solar Dynamic
Observatory and the Solar Terrestrial Relations Observatory satellites.
Results: For the most probable magnetic field vector configuration, the
analysis depicts a mean field strength of 7 gauss. We do not find local
variations in the field strength except that the field is, in average, lower in
the prominence body than in the prominence feet, where the field strength
reaches 25 gauss. The averaged magnetic field inclination with respect to the
local vertical is 77 degrees. The acute angle of the magnetic field vector with
the prominence main axis is 24 degrees for the sinistral chirality case and 58
degrees for the dextral chirality. These inferences are in rough agreement with
previous results obtained from the analysis of data acquired with lower spatial
resolutions.Comment: Accepted in A&
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
Rotating Disks and Non-Kinematic Double Peaks
Double-peaked line profiles are commonly considered a hallmark of rotating
disks, with the distance between the peaks a measure of the rotation velocity.
However, double-peaks can arise also from radiative transfer effects in
optically thick non-rotating sources. Utilizing exact solutions of the line
transfer problem we present a detailed study of line emission from
geometrically thin Keplerian disks. We derive the conditions for emergence of
kinematic double peaks in optically thin and thick disks, and find that it is
generally impossible to disentangle the effects of kinematics and line opacity
in observed double-peaked profiles. Unless supplemented by additional
information, a double-peaked profile alone is not a reliable indicator of a
rotating disk. In certain circumstances, triple and quadruple profiles might be
better indicators of rotation in optically thick disks.Comment: MNRAS, to be publishe
Observations of Ellerman bomb emission features in He I D3 and He I 10830 {\AA}
Context. Ellerman bombs (EBs) are short-lived emission features,
characterized by extended wing emission in hydrogen Balmer lines. Until now, no
distinct signature of EBs has been found in the He I 10830 {\AA} line, and
conclusive observations of EBs in He I D 3 have never been reported. Aims. We
aim to study the signature of EBs in neutral helium triplet lines. Methods. The
observations consist of 10 consecutive SST/TRIPPEL raster scans close to the
limb, featuring the H, He I D3 and He I 10830 {\AA} spectral regions. We
also obtained raster scans with IRIS and make use of the SDO/AIA 1700 {\AA}
channel. We use Hazel to invert the neutral helium triplet lines. Results.
Three EBs in our data show distinct emission signatures in neutral helium
triplet lines, most prominently visible in the He I D3 line. The helium lines
have two components: a broad and blue-shifted emission component associated
with the EB, and a narrower absorption component formed in the overlying
chromosphere. One of the EBs in our data shows evidence of strong velocity
gradients in its emission component. The emission component of the other two
EBs could be fitted using a constant slab. Our analysis hints towards thermal
Doppler motions having a large contribution to the broadening for helium and
IRIS lines. We conclude that the EBs must have high temperatures to exhibit
emission signals in neutral helium triplet lines. An order of magnitude
estimate places our observed EBs in the range of K.Comment: 15 pages, 14 figure
A PCA approach to stellar effective temperatures
The derivation of the effective temperature of a star is a critical first
step in a detailed spectroscopic analysis. Spectroscopic methods suffer from
systematic errors related to model simplifications. Photometric methods may be
more robust, but are exposed to the distortions caused by interstellar
reddening. Direct methods are difficult to apply, since fundamental data of
high accuracy are hard to obtain. We explore a new approach in which the
spectrum is used to characterize a star's effective temperature based on a
calibration established by a small set of standard stars. We perform principal
component analysis on homogeneous libraries of stellar spectra, then calibrate
a relationship between the principal components and the effective temperature
using a set of stars with reliable effective temperatures. We find that our
procedure gives excellent consistency when spectra from a homogeneous set of
observations are used. Systematic offsets may appear when combining
observations from different sources. Using as reference the spectra of stars
with high-quality spectroscopic temperatures in the Elodie library, we define a
temperature scale for FG-type disk dwarfs with an internal consistency of about
50 K, in excellent agreement with temperatures from direct determinations, but
distinct from widely used scales based on the infrared flux method.Comment: Pages 1-9 Article and pages 9-22 Table. 7 Figure
G-band Spectral Synthesis in Solar Magnetic Concentrations
Narrow band imaging in the G-band is commonly used to trace the small magnetic field concentrations of the Sun, although the mechanism that makes them bright has remained unclear. We carry out LTE syntheses of the G-band in an assorted set of semi-empirical model magnetic concentrations. The syntheses include all CH lines as well as the main atomic lines within the band-pass. The model atmospheres produce bright G-band spectra having many properties in common with the observed G-band bright points. In particular, the contrast referred to the quiet Sun is about twice the contrast in continuum wavelengths. The agreement with observations does not depend on the specificities of the model atmosphere, rather it holds from single fluxtubes to MIcro-Structured Magnetic Atmospheres. However, the agreement requires that the real G-band bright points are not spatially resolved, even in the best observations. Since the predicted G-band intensities exceed by far the observed values, we foresee a notable increase of contrast of the G-band images upon improvement of the angular resolution. According to the LTE modeling, the G-band spectrum emerges from the deep photosphere that produces the continuum. Our syntheses also predict solar magnetic concentrations showing up in continuum images but not in the G-band . Finally, we have examined the importance of the CH photo-dissociation in setting the amount of G-band absorption. It turns out to play a minor role.Peer reviewe
On the magnetic field of off-limb spicules
Determining the magnetic field related to solar spicules is vital for
developing adequate models of these plasma jets, which are thought to play a
key role in the thermal, dynamic and magnetic structure of the Chromosphere.
Here we report on the magnetic properties of off-limb spicules in a very quiet
region of the solar atmosphere, as inferred from new spectropolarimetric
observations in the He I 10830 A triplet obtained with the Tenerife Infrared
Polarimeter. We have used a novel inversion code for Stokes profiles caused by
the joint action of atomic level polarization and the Hanle and Zeeman effects
(HAZEL) to interpret the observations. Magnetic fields as strong as ~40G were
detected in a very localized area of the slit, which could represent a possible
lower value of the field strength of organized network spicules.Comment: Accepted for publication in ApJ, 24 pages, 5 Figure
Warm Water Vapor around Sagittarius B2
Several condensations heated externally by nearby hot stars are present in the Sgr B2 region for which H2O far-IR lines are expected to probe only an external low-density and high temperature section. Millimeter-wave lines can penetrate deeper into them (higher densities and lower Tk). We have conducted a study combining H2O lines in both spectral regions using the ISO (far-IR lines) and the IRAM 30m telescope (183 GHz line). The far-IRH2O lines, seen in absorption, are optically thick. They form in the outermost gas in front of the far-IR continuum sources, probing a maximum visual extinction of ~5–10 mag. IR photons from the dust play a dominant role in their excitation. We
conclude, based on observations of the COJ = 7-6 line at 806.65 GHz, and the lack of emission from the far-IR CO lines, that the gas density has to be below ~10^4 cm^-3. Using the gas kinetic temperature and density derived from OH, CO, and other molecular species, we derive a water column density of (9 ± 3) x 10^16 cm^-2 in the absorbing gas, implying an abundance of ~(1-2) x 10^-5 in this region. The resulting relatively low H2O/OH abundance ratio,
~2–4, is a signature of UV photon-dominated surface layers traced by far-IR observations. As a consequence, the
temperature of the absorbing gas is high, Tk ~ 300-500 K, which allows very efficient neutral-neutral reactions producing H2O and OH. Finally, the 183.31 GHz data allow one to trace the inner, denser (n(H2) > 10^5-10^6 cm^-3), and colder (Tk ~ 40K) gas. The emission is very strong toward the cores with an estimated water vapor abundance of
a few x 10^-7. There is also moderate extended emission around Sgr B2 main condensations, in agreement with the
water vapor abundance derived from far-IR H2O lines.We thank Spanish DGES and PNIE for funding support under grants PANAYA2000-1784, ESP2001-4516, AYA2002-10113-E, ESP2002-01627,AYA2003-02785-E and AYA2004-05792. CSO
observations were supported by NSF grant AST-9980846. J.R.G. was also supported by the French Direction de la Recherche in the
latest stages of the work.Peer reviewe
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