115 research outputs found
Solar off-limb emission of the OI 7772 \AA\ line
The aim of this paper is to understand the formation of the OI line at 7772
\AA\ in the solar chromosphere. We used SST/CRISP observations to observe OI
7772 \AA\ in several places around the solar limb. We compared the observations
with synthetic spectra calculated with the RH code in the one-dimension
spherical geometry mode. New accurate hydrogen collisional rates were included
for the RH calculations. The observations reveal a dark gap in the lower
chromosphere, which is caused by variations in the line opacity as shown by our
models. The lower level of the 7772 \AA\ transition is populated by a downward
cascade from the continuum. We study the effect of Lyman- pumping and
hydrogen collisions between the triplet and quintet system in OI. Both have a
small but non-negligible influence on the line intensity.Comment: 9 pages, 12 figures, Accepted for publication in A&
The Observed Trend of Boron and Oxygen in Field Stars of the Disk
Oxygen abundances are derived in a sample of 13 field F and G dwarfs and
subgiants with metallicities in the range of -0.75 < [Fe/H] < +0.15. This is
the same sample of stars for which boron abundances have been derived earlier
from archived spectra obtained with the Hubble Space Telescope. In a log-log
comparison of the B versus the O abundances, a slope of m(BO)=1.39 is found,
indicating that in the disk, the abundance of B relative to O is intermediate
between primary and secondary production (hybrid behavior). This relation of B
versus O for disk stars is compared to the same relation for halo stars.Comment: 14 pages, 3 figures, 1 table. In press to The Astronomical Journal
(July 2001
Formation of Li I lines in photospheric granulation
The possibility of significant systematic errors due to the use of 1D
homogeneous atmospheres in lithium-abundance determinations of cool stars
motivates a study of non-local-thermodynamic-equilibrium (NLTE) effects on Li I
line formation in a 3D solar-granulation simulation snapshot. The NLTE effect
on the equivalent width of the 671 nm resonance line is small in 1D models or
in integrated light from the granulation model. The line-strength variations
over the granulation pattern are however markedly different in NLTE compared to
LTE -- observations of this may provide diagnostics to NLTE effects. The
effects of horizontal photon exchange found in the granulation model are
moderate and due entirely to bound-bound processes, ultraviolet overionization
is unimportant.Comment: 9 pages Latex (AASTeX using aaspp4.sty) with 3 figures (PS). The
former EPS figures have been replaced with safer PS due to technical problems
encountered by some users. No change in content. Accepted for publication in
Astrophysical Journal Letter
The Oxygen Abundance of HE 1327-2326
From a newly obtained VLT/UVES spectrum we have determined the oxygen
abundance of HE 1327-2326, the most iron-poor star known to date. UV-OH lines
yield a 1D LTE abundance of [O/Fe]_OH = 3.7 (subgiant case) and [O/Fe]_OH = 3.4
(dwarf case). Using a correction of -1.0 dex to account for 3D effects on OH
line formation, the abundances are lowered to [O/Fe] = 2.8 and [O/Fe] = 2.5,
respectively, which we adopt. Without 3D corrections, the UV-OH based abundance
would be in disagreement with the upper limits derived from the OI triplet
lines: [O/Fe]_trip < 2.8 (subgiant) and [O/Fe]_trip < 3.0 (dwarf). We also
correct the previously determined carbon and nitrogen abundances for 3D
effects. Knowledge of the O abundance of HE 1327-2326 has implications for the
interpretation of its abundance pattern. A large O abundance is in accordance
with HE 1327-2326 being an early Population II star which formed from material
chemically enriched by a first generation supernova. Our derived abundances,
however, do not exclude other possibilities such as a Population III scenario.Comment: 13 pages, accepted for publication in ApJ
Stable Umbral Chromospheric Structures
Aims. To understand the morphology of the chromosphere in sunspot umbra. We
investigate if the horizontal structures observed in the spectral core of the
Ca II H line are ephemeral visuals caused by the shock dynamics of more stable
structures, and examine their relationship with observables in the H-alpha
line. Methods. Filtergrams in the core of the Ca II H and H-alpha lines as
observed with the Swedish 1-m Solar Telescope are employed. We utilise a
technique that creates composite images and tracks the flash propagation
horizontally. Results. We find 0"15 wide horizontal structures, in all of the
three target sunspots, for every flash where the seeing was moderate to good.
Discrete dark structures are identified that are stable for at least two umbral
flashes, as well as systems of structures that live for up to 24 minutes. We
find cases of extremely extended structures with similar stability, with one
such structure showing an extent of 5". Some of these structures have a
correspondence in H-alpha but we were unable to find a one to one
correspondence for every occurrence. If the dark streaks are formed at the same
heights as umbral flashes then there are systems of structures with strong
departures from the vertical for all three analysed sunspots. Conclusions.
Long-lived Ca II H filamentary horizontal structures are a common and likely
ever-present feature in the umbra of sunspots. If the magnetic field in the
chromosphere of the umbra is indeed aligned with the structures, then the
present theoretical understanding of the typical umbra needs to be revisited.Comment: Accepted to Astronomy and Astrophysics. Online material (Fig3.mov and
Fig4.mov) will be available at A&
Oxygen lines in solar granulation. I. Testing 3D models against new observations with high spatial and spectral resolution
Aims: we seek to provide additional tests of the line formation of
theoretical 3D solar photosphere models. In particular, we set out to test the
spatially-resolved line formation at several viewing angles, from the solar
disk-centre to the limb and focusing on atomic oxygen lines. The purpose of
these tests is to provide additional information on whether the 3D model is
suitable to derive the solar oxygen abundance. We also aim to empirically
constrain the NLTE recipes for neutral hydrogen collisions, using the
spatially-resolved observations of the OI 777 nm lines. Methods: using the
Swedish 1-m Solar Telescope we obtained high-spatial-resolution observations of
five atomic oxygen lines (along with lines for other species) for five
positions on the solar disk. These observations have a high spatial and
spectral resolution, and a continuum intensity contrast up to 9% at 615 nm. The
theoretical line profiles were computed using the 3D model, with a full 3D NLTE
treatment for oxygen and LTE for the other lines. Results: at disk-centre we
find an excellent agreement between predicted and observed line shifts,
strengths, FWHM and asymmetries. At other viewing angles the agreement is also
good, but the smaller continuum intensity contrast makes a quantitative
comparison harder. We use the disk-centre observations we constrain S_H, the
scaling factor for the efficiency of collisions with neutral hydrogen. We find
that S_H=1 provides the best match to the observations. Conclusions: overall
there is a very good agreement between predicted and observed line properties
over the solar granulation. This further reinforces the view that the 3D model
is realistic and a reliable tool to derive the solar oxygen abundance.Comment: 16 pages, 16 figures, accepted for publication in A&
Boron in Very Metal-Poor Stars
We have observed the B I 2497 A line to derive the boron abundances of two
very metal-poor stars selected to help in tracing the origin and evolution of
this element in the early Galaxy: BD +23 3130 and HD 84937. The observations
were conducted using the Goddard High Resolution Spectrograph on board the
Hubble Space Telescope. A very detailed abundance analysis via spectral
synthesis has been carried out for these two stars, as well as for two other
metal-poor objects with published spectra, using both Kurucz and OSMARCS model
photospheres, and taking into account consistently the NLTE effects on the line
formation. We have also re-assessed all published boron abundances of old disk
and halo unevolved stars. Our analysis shows that the combination of high
effective temperature (Teff > 6000 K, for which boron is mainly ionized) and
low metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates of
boron abundances from the B I 2497 A line. This is the case of HD 84937 and
three other published objects (including two stars with [Fe/H] ~ -3), for which
only upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 and
logN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which a
firm measurement of the boron abundance presently exists. The evolution of the
boron abundance with metallicity that emerges from the seven remaining stars
with Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derived
using the same stellar parameters, shows a linear increase with a slope ~ 1.
Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in the
range -3<[Fe/H]<-1. These results point to spallation reactions of ambient
protons and alpha particles with energetic particles enriched in CNO as the
origin of boron and beryllium in halo stars.Comment: 38 pages, 11 Encapsulated Postscript figures (included), uses
aaspp4.sty. Accepted for publication in The Astrophysical Journal. The
preprint is also available at: http://www.iac.es/publicaciones/preprints.htm
A tilted interference filter in a converging beam
Context. Narrow-band interference filters can be tuned toward shorter
wavelengths by tilting them from the perpendicular to the optical axis. This
can be used as a cheap alternative to real tunable filters, such as
Fabry-P\'erot interferometers and Lyot filters. At the Swedish 1-m Solar
Telescope, such a setup is used to scan through the blue wing of the Ca II H
line. Because the filter is mounted in a converging beam, the incident angle
varies over the pupil, which causes a variation of the transmission over the
pupil, different for each wavelength within the passband. This causes
broadening of the filter transmission profile and degradation of the image
quality. Aims. We want to characterize the properties of our filter, at normal
incidence as well as at different tilt angles. Knowing the broadened profile is
important for the interpretation of the solar images. Compensating the images
for the degrading effects will improve the resolution and remove one source of
image contrast degradation. In particular, we need to solve the latter problem
for images that are also compensated for blurring caused by atmospheric
turbulence. Methods. We simulate the process of image formation through a
tilted interference filter in order to understand the effects. We test the
hypothesis that they are separable from the effects of wavefront aberrations
for the purpose of image deconvolution. We measure the filter transmission
profile and the degrading PSF from calibration data. Results. We find that the
filter transmission profile differs significantly from the specifications.We
demonstrate how to compensate for the image-degrading effects. Because the
filter tilt effects indeed appear to be separable from wavefront aberrations in
a useful way, this can be done in a final deconvolution, after standard image
restoration with MFBD/Phase Diversity based methods. We illustrate the
technique with real data
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