31,420 research outputs found
Optical observations of very low ionization HII regions in the large Magellanic cloud
Several very low ionization isolated HII regions were detected on a prism-objective plate of the Large Magellanic Cloud. Most of the objects show a very weak (OIII) lambda 5007 emission line and, on the other hand the (OII) lambda 6584 doublets are very intense. This kind of objects seem to be ideal in order to determine accurate N and O abundance, avoiding the use of large ionization correction factors in the N abundance determination. Spectrophotometric observations of these regions were carried out with the 4 m telescope and the 2-D Frutti spectrograph at Cerro Tololo, and with the 1.52 m and the Image Dissector Scanner (IDS) at La Silla, ESO. The wavelength range lambda lambda 3700 to 7000 A was covered. Calibrated fluxes of the emission lines detected were measured, and from these data preliminary results of physical conditions of the gas as well as some ionic abundances were derived. Comparisons of the observations with ionization structure models show that the effective temperatures of the ionizing stars are less than 35,000 K. Possible abundances gradients across the large megallanic cloud are discussed
Faint emission lines in the Galactic H II regions M16, M20 and NGC 3603
We present deep echelle spectrophotometry of the Galactic {\hii} regions M16,
M20 and NGC 3603. The data have been taken with the Very Large Telescope
Ultraviolet-Visual Echelle Spectrograph in the 3100 to 10400 \AA range. We have
detected more than 200 emission lines in each region. Physical conditions have
been derived using different continuum and line intensity ratios. We have
derived He, C and O abundances from pure recombination
lines as well as abundances from collisionally excited lines for a large number
of ions of different elements. We have obtained consistent estimations of the
temperature fluctuation parameter, {\ts}, using different methods. We also
report the detection of deuterium Balmer lines up to D (M16) and to
D (M20) in the blue wings of the hydrogen lines, which excitation
mechanism seems to be continuum fluorescence. The temperature fluctuations
paradigm agree with the results obtained from optical CELs and the more
uncertain ones from far IR fine structure CELs in NGC 3603, although, more
observations covering the same volume of the nebula are necessary to obtain
solid conclusions.Comment: 22 pages, 13 Tables, 7 Figures. Accepted for publication by MNRA
Vanishing chiral couplings in the large-N_C resonance theory
The construction of a resonance theory involving hadrons requires
implementing the information from higher scales into the couplings of the
effective Lagrangian. We consider the large-Nc chiral resonance theory
incorporating scalars and pseudoscalars, and we find that, by imposing LO
short-distance constraints on form factors of QCD currents constructed within
this theory, the chiral low-energy constants satisfy resonance saturation at
NLO in the 1/Nc expansion.Comment: 5 pages, 2 figures. Version published in Physical Review D. Some
equations to facilitate the discussion have been adde
Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates. V. Follow-up of 30 OGLE Transits. New Candidates
We used VLT/VIMOS images in the V band to obtain light curves of extrasolar
planetary transits OGLE-TR-111 and OGLE-TR-113, and candidate planetary
transits: OGLE-TR-82, OGLE-TR-86, OGLE-TR-91, OGLE-TR-106, OGLE-TR-109,
OGLE-TR-110, OGLE-TR-159, OGLE-TR-167, OGLE-TR-170, OGLE-TR-171. Using
difference imaging photometry, we were able to achieve millimagnitude errors in
the individual data points. We present the analysis of the data and the light
curves, by measuring transit amplitudes and ephemerides, and by calculating
geometrical parameters for some of the systems. We observed 9 OGLE objects at
the predicted transit moments. Two other transits were shifted in time by a few
hours. For another seven objects we expected to observe transits during the
VIMOS run, but they were not detected. The stars OGLE-TR-111 and OGLE-TR-113
are probably the only OGLE objects in the observed sample to host planets, with
the other objects being very likely eclipsing binaries or multiple systems. In
this paper we also report on four new transiting candidates which we have found
in the data.Comment: 11 pages, 17 figures, accepted for publication in A&
Detection of emission in the Si i 1082.7 nm line core in sunspot umbrae
We analyze spectropolarimetric sunspot umbra observations taken in the
near-infrared Si i 1082.7 nm line taking NLTE effects into account. The data
were obtained with the GRIS instrument installed at the German GREGOR
telescope. A point spread function (PSF) was constructed using prior Mercury
observations with GRIS and the information provided by the adaptive optics
system of the GREGOR telescope. The data were then deconvolved from the PSF
using a principal component analysis deconvolution method and were analyzed via
the NICOLE inversion code. The Si i 1082.7 nm line seems to be in emission in
the umbra of the observed sunspot after the effects of scattered light are
removed. We show how the spectral line shape of umbral profiles changes
dramatically with the amount of scattered light. Indeed, the continuum levels
range, on average, from 44% of the quiet Sun continuum intensity to about 20%.
The inferred levels are in line with current model predictions and empirical
umbral models. Current umbral empirical models are not able to reproduce the
emission in the deconvolved umbral Stokes profiles. The results of the NLTE
inversions suggests that to obtain the emission in the Si i 1082.7 nm line, the
temperature stratification should first have a hump located at about log tau -2
and start rising at lower heights when moving into the transition region. This
is, to our knowledge, the first time the Si i 1082.7 nm line is seen in
emission in sunspot umbrae. The results show that the temperature
stratification of current umbral models may be more complex than expected with
the transition region located at lower heights above sunspot umbrae. Our
finding might provide insights into understanding why the sunspot umbra
emission in the millimeter spectral range is less than that predicted by
current empirical umbral models
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