12,636 research outputs found
A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 A whose intensity and polarization profiles are very
sensitive to the presence of hyperfine structure. A theoretical investigation
of the magnetic sensitivity of this line to the magnetic field uncovers several
interesting properties. The most important one is that the presence of strong
Paschen-Back perturbations due to the hyperfine structure produces an intensity
line profile whose shape changes according to the absolute value of the
magnetic field strength. A line ratio technique is developed from the intrinsic
variations of the line profile. This line ratio technique is applied to
spectropolarimetric observations of the quiet solar photosphere in order to
explore the probability distribution function of the magnetic field strength.
Particular attention is given to the quietest area of the observed field of
view, which was encircled by an enhanced network region. A detailed theoretical
investigation shows that the inferred distribution yields information on the
average magnetic field strength and the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field strength
and a tentative value of ~0.45" for the spatial scale at which the observed
magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical
Journal. Figures 1 and 9 are in JPG forma
Fluorescent Ly-alpha emission from the high-redshift intergalactic medium
We combine a high-resolution hydro-simulation of the LambdaCDM cosmology with
two radiative transfer schemes (for continuum and line radiation) to predict
the properties, spectra and spatial distribution of fluorescent Ly-alpha
emission at z~3. We focus on line radiation produced by recombinations in the
dense intergalactic medium ionized by UV photons. In particular, we consider
both a uniform background and the case where gas clouds are illuminated by a
nearby quasar. We find that the emission from optically thick regions is
substantially less than predicted from the widely used static, plane-parallel
model. The effects induced by a realistic velocity field and by the complex
geometric structure of the emitting regions are discussed in detail. We make
predictions for the expected brightness and size distributions of the
fluorescent sources.Our results account for recent null detections and can be
used to plan new observational campaigns both in the field (to measure the
intensity of the diffuse UV background) and in the proximity of bright quasars
(to understand the origin of high colum-density absorbers).Comment: 15 pages, 12 figures, accepted for publication in Ap
The GIRAFFE Inner Bulge Survey (GIBS). I. Survey Description and a kinematical map of the Milky Way bulge
The Galactic bulge is a massive, old component of the Milky Way. It is known
to host a bar, and it has recently been demonstrated to have a pronounced
boxy/peanut structure in its outer region. Several independent studies suggest
the presence of more than one stellar populations in the bulge, with different
origins and a relative fraction changing across the bulge area. This is the
first of a series of papers presenting the results of the Giraffe Inner Bulge
Survey, carried out at the ESO-VLT with the multifibre spectrograph FLAMES.
Spectra of ~5000 red clump giants in 24 bulge fields have been obtained at
resolution R=6500, in the infrared Calcium triplet wavelength region at 8500
{\AA}. They are used to derive radial velocities and metallicities, based on
new calibration specifically devised for this project. Radial velocities for
another ~1200 bulge red clump giants, obtained from similar archive data, have
been added to the sample. Higher resolution spectra have been obtained for 450
additional stars at latitude b=-3.5, with the aim of investigating chemical
abundance patterns variations with longitude, across the inner bulge. In total
we present here radial velocities for 6392 RC stars. We derive a radial
velocity, and velocity dispersion map of the Milky Way bulge, useful to be
compared with similar maps of external bulges, and to infer the expected
velocities and dispersion at any line of sight. The K-type giants kinematics is
consistent with the cylindrical rotation pattern of M-giants from the BRAVA
survey. Our sample enables to extend this result to latitude b=-2, closer to
the Galactic plane than probed by previous surveys. Finally, we find strong
evidence for a velocity dispersion peak at (0,-1) and (0,-2), possibly
indicative of a high density peak in the central 250 pc of the bulgeComment: A&A in pres
Non-myeloablative conditioning for allogeneic hematopoietic stem cell transplantation in patients with acute myeloid leukemia. The impact of the remission status
Upholding the unified model for Active Galactic Nuclei: VLT/FORS2 spectropolarimetry of Seyfert 2 galaxies
The origin of the unification model for Active Galactic Nuclei (AGN) was the detection of broad hydrogen recombination lines in the optical polarized spectrum of the Seyfert 2 galaxy (Sy2) NGC 1068. Since then, a search for the hidden broad-line region (HBLR) of nearby Sy2s started, but polarized broad lines have only been detected in ?30–40% of the nearby Sy2s observed to date. Here we present new VLT/FORS2 optical spectropolarimetry of a sample of 15 Sy2s, including Compton-thin and Compton-thick sources. The sample includes six galaxies without previously published spectropolarimetry, some of them normally treated as non-hidden BLR (NHBLR) objects in the literature, four classified as NHBLR, and five as HBLR based on previous data. We report ?4? detections of a HBLR in 11 of these galaxies (73% of the sample) and a tentative detection in NGC 5793, which is Compton-thick according to the analysis of X-ray data performed here. Our results confirm that at least some NHBLRs are misclassified, bringing previous publications reporting differences between HBLR and NHBLR objects into question. We detect broad H? and H? components in polarized light for 10 targets, and just broad H? for NGC 5793 and NGC 6300, with line widths ranging between 2100 and 9600 km s?1. High bolometric luminosities and low column densities are associated with higher polarization degrees, but not necessarily with the detection of the scattered broad components
Limits on excited tau leptons masses from leptonic tau decays
We study the effects induced by excited leptons on the leptonic tau decay at
one loop level. Using a general effective lagrangian approach to describe the
couplings of the excited leptons, we compute their contributions to the
leptonic decays and use the current experimental values of the branching ratios
to put limits on the mass of excited states and the substructure scale.Comment: 10 pages, 6 figures, to be published in Phys. Rev.
Informal collaborative learning (ICL) – student perspectives on the role of informal collaborative learning ICL in higher education
The Covid 19 pandemic requires critical re-evaluation of the way Higher Education can help students maximise their learning to adjust and adapt to a new reality. This requires a holistic approach which extends beyond consideration of student learning in formal settings to understanding the role informal learning can play. Whilst research exists on informal independent learning, less focus has been placed on how students collaborate informally as part of their learning process. In this study interviews with HE students in Spain and the UK offer insight into variable but beneficial informal collaborative learning (ICL) practices. The value attributed to ICL emphasises its potential contribution to the current educational landscape and the merit of recognising it as a distinct concept. Further, uneven access and the role of learner maturity signal the potential for universities to enhance learning and increase access by playing an active a role in developing ICL skills and personal networks
Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS II. The complete high resolution extinction map and implications for Bulge studies
We use the Vista Variables in the Via Lactea (VVV) ESO public survey data to
measure extinction values in the complete area of the Galactic bulge covered by
the survey at high resolution. We derive reddening values using the method
described in Paper I. This is based on measuring the mean (J-Ks) color of red
clump giants in small subfields of 2' to 6' in the following bulge area:
-10.3<b<+5.1 and -10<l<+10.4. To determine the reddening values E(J-Ks) for
each region, we measure the RC color and compare it to the (J-Ks) color of RC
stars measured in Baade's window, for which we adopt E(B-V)=0.55. This allows
us to construct a reddening map sensitive to small scale variations minimizing
the problems arising from differential extinction. The significant reddening
variations are clearly observed on spatial scales as small as 2'. We find a
good agreement between our extinction measurements and Schlegel maps in the
outer bulge, but, as already stated in the literature the Schlegel maps are not
reliable for regions within |b| < 6. In the inner regions we compare our
results with maps derived from DENIS and Spitzer surveys. While we find good
agreement with other studies in the corresponding overlapping regions, our
extinction map has better quality due to both higher resolution and a more
complete spatial coverage in the Bulge. We investigate the importance of
differential reddening and demonstrate the need for high resolution extinction
maps for detailed studies of Bulge stellar populations and structure. The
extinction variations on scales of up to 2'-6', must be taken into account when
analysing the stellar populations of the Bulge.Comment: Accepted for publication in A&
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