3,834 research outputs found
Laboratory oscillator strengths of Sc I in the near-infrared region for astrophysical applications
Context. Atomic data is crucial for astrophysical investigations. To
understand the formation and evolution of stars, we need to analyse their
observed spectra. Analysing a spectrum of a star requires information about the
properties of atomic lines, such as wavelengths and oscillator strengths.
However, atomic data of some elements are scarce, particularly in the infrared
region, and this paper is part of an effort to improve the situation on near-IR
atomic data. Aims. This paper investigates the spectrum of neutral scandium, Sc
i, from laboratory measurements and improves the atomic data of Sc i lines in
the infrared region covering lines in R, I, J, and K bands. Especially, we
focus on measuring oscillator strengths for Sc i lines connecting the levels
with 4p and 4s configurations. Methods. We combined experimental branching
fractions with radiative lifetimes from the literature to derive oscillator
strengths (f - values). Intensity-calibrated spectra with high spectral
resolution were recorded with Fourier transform spectrometer from a hollow
cathode discharge lamp. The spectra were used to derive accurate oscillator
strengths and wavelengths for Sc i lines, with emphasis on the infrared region.
Results. This project provides the first set of experimental Sc i lines in the
near-infrared region for accurate spectral analysis of astronomical objects. We
derived 63 log(g f ) values for the lines between 5300{\AA} and 24300{\AA}. The
uncertainties in the f -values vary from 5% to 20%. The small uncertainties in
our values allow for an increased accuracy in astrophysical abundance
determinations.Comment: Accepted on 25 August 2015 by A&
Diversification Based Static Index Pruning - Application to Temporal Collections
Nowadays, web archives preserve the history of large portions of the web. As
medias are shifting from printed to digital editions, accessing these huge
information sources is drawing increasingly more attention from national and
international institutions, as well as from the research community. These
collections are intrinsically big, leading to index files that do not fit into
the memory and an increase query response time. Decreasing the index size is a
direct way to decrease this query response time.
Static index pruning methods reduce the size of indexes by removing a part of
the postings. In the context of web archives, it is necessary to remove
postings while preserving the temporal diversity of the archive. None of the
existing pruning approaches take (temporal) diversification into account.
In this paper, we propose a diversification-based static index pruning
method. It differs from the existing pruning approaches by integrating
diversification within the pruning context. We aim at pruning the index while
preserving retrieval effectiveness and diversity by pruning while maximizing a
given IR evaluation metric like DCG. We show how to apply this approach in the
context of web archives. Finally, we show on two collections that search
effectiveness in temporal collections after pruning can be improved using our
approach rather than diversity oblivious approaches
Exact Methods for Self Interacting Neutrinos
The effective many-body Hamiltonian which describes vacuum oscillations and
self interactions of neutrinos in a two flavor mixing scheme under the single
angle approximation has the same dynamical symmetries as the well known BCS
pairing Hamiltonian. These dynamical symmetries manifest themselves in terms of
a set of constants of motion and can be useful in formulating the collective
oscillation modes in an intuitive way. In particular, we show that a neutrino
spectral split can be simply viewed as an avoided level crossing between the
eigenstates of a mean field Hamiltonian which includes a Lagrange multiplier in
order to fix the value of an exact many-body constant of motion. We show that
the same dynamical symmetries also exist in the three neutrino mixing scheme by
explicitly writing down the corresponding constants of motion.Comment: To appear in the proceedings of CETUP* 201
An Exactly Solvable Model of Interacting Bosons
We introduce a class of exactly solvable boson models. We give explicit
analytic expressions for energy eigenvalues and eigenvectors for an sd-boson
Hamiltonian, which is related to the SO(6) chain of the Interacting Boson Model
Hamiltonian.Comment: 8 pages of LATE
- …