19,712 research outputs found
Shadows and strong gravitational lensing: a brief review
For ultra compact objects (UCOs), Light Rings (LRs) and Fundamental Photon
Orbits (FPOs) play a pivotal role in the theoretical analysis of strong
gravitational lensing effects, and of BH shadows in particular. In this short
review, specific models are considered to illustrate how FPOs can be useful in
order to understand some non-trivial gravitational lensing effects. This paper
aims at briefly overviewing the theoretical foundations of these effects,
touching also some of the related phenomenology, both in General Relativity
(GR) and alternative theories of gravity, hopefully providing some intuition
and new insights for the underlying physics, which might be critical when
testing the Kerr black hole hypothesis.Comment: 32 pages, 9 figures; Review paper in the General Relativity and
Gravitation (GRG) Topical Collection "Testing the Kerr spacetime with
gravitational-wave and electromagnetic observations" (Guest Editor: Emanuele
Berti); v2: Typo corrected and two references adde
The Origin of Fluorine: Abundances in AGB Carbon Stars Revisited
Revised spectroscopic parameters for the HF molecule and a new CN line list
in the 2.3 mu region have been recently available, allowing a revision of the F
content in AGB stars. AGB carbon stars are the only observationally confirmed
sources of fluorine. Nowadays there is not a consensus on the relevance of AGB
stars in its Galactic chemical evolution. The aim of this article is to better
constrain the contribution of these stars with a more accurate estimate of
their fluorine abundances. Using new spectroscopic tools and LTE spectral
synthesis, we redetermine fluorine abundances from several HF lines in the
K-band in a sample of Galactic and extragalactic AGB carbon stars of spectral
types N, J and SC spanning a wide range of metallicities. On average, the new
derived fluorine abundances are systematically lower by 0.33 dex with respect
to previous determinations. This may derive from a combination of the lower
excitation energies of the HF lines and the larger macroturbulence parameters
used here as well as from the new adopted CN line list. Yet, theoretical
nucleosynthesis models in AGB stars agree with the new fluorine determinations
at solar metallicities. At low metallicities, an agreement between theory and
observations can be found by handling in a different way the
radiative/convective interface at the base of the convective envelope. New
fluorine spectroscopic measurements agree with theoretical models at low and at
solar metallicity. Despite this, complementary sources are needed to explain
its observed abundance in the solar neighbourhood.Comment: 9 pages, 4 figures, accepted in A&
De limites, fronteiras e construções mútuas
A ideia de fronteira, entendida na sua forte polissemia, tanto se nos tem oferecido como objecto de estudo, como tem constituĂdo um importante instrumento de análise, reporte-se ela a paĂses ou territĂłrios, contextos ou grupos, categorias ou ideias. Na medida em que enfatiza os factores de distinção e de contacto, dá conta das dinâmicas de interacção social, tanto numa perspectiva diacrĂłnica como sincrĂłnica, alĂ©m de ter, tambĂ©m, implicações polĂticas. Que potencialidades conserva ainda, nos dias de hoje, a noção de fronteira nas suas diferentes acepções? Quais os avatares contemporâneos de fronteiras espaciais, naturais, polĂticas ou simbĂłlicas? Como se conjuga no presente esta noção com as categorias de margem, centro e periferia? Como dĂŁo estas categorias a ver as intersecções? Qual a valia heurĂstica de todas elas face a outras ainda como complementaridade e paridade, que sugerem outros modelos de análise de uma interacção.Fundação para a CiĂŞncia e a Tecnologia (FCT).Fundação Calouste Gulbenkian
Inspection and diagnosis tests for structural safety evaluation: A case study
Diagnosis and assessment of existing structures is a developing area due to the appearance of a high number of building defects, structural and non-structural deterioration and precocious loss of quality, and, consequently, lower expected durability. With the aim of verifying the viability of rehabilitation or the need to demolish an existing fifteen year old parking building, several inspections and diagnostic non-destructive and destructive testing, visual inspection, were carried out to evaluate the structural safety conditions
A thorough analysis of the short- and mid-term activity-related variations in the solar acoustic frequencies
The frequencies of the solar acoustic oscillations vary over the activity
cycle. The variations in other activity proxies are found to be well correlated
with the variations in the acoustic frequencies. However, each proxy has a
slightly different time behaviour. Our goal is to characterize the differences
between the time behaviour of the frequency shifts and of two other activity
proxies, namely, the area covered by sunspots and the 10.7cm flux. We define a
new observable that is particularly sensitive to the short-term frequency
variations. We then compare the observable when computed from model frequency
shifts and from observed frequency shifts obtained with the Global Oscillation
Network Group (GONG) for cycle 23. Our analysis shows that on the shortest
time-scales the variations in the frequency shifts seen in the GONG
observations are strongly correlated with the variations in the area covered by
sunspots. However, a significant loss of correlation is still found. We verify
that the times when the frequency shifts and the sunspot area do not vary in a
similar way tend to coincide with the times of the maxima of the quasi-biennial
variations seen in the solar seismic data. A similar analysis of the relation
between the 10.7cm flux and the frequency shifts reveals that the short-time
variations in the frequency shifts follow even more closely those of the 10.7cm
flux than those of the sunspot area. However, a loss of correlation between
frequency shifts and 10.7cm flux variations is still found around the same
times.Comment: 7 pages, 6 figures, accepted for publication in MNRA
On the relation between activity-related frequency shifts and the sunspot distribution over the solar cycle 23
The activity-related variations in the solar acoustic frequencies have been
known for 30 years. However, the importance of the different contributions is
still not well established. With this in mind, we developed an empirical model
to estimate the spot-induced frequency shifts, which takes into account the
sunspot properties, such as area and latitude. The comparison between the model
frequency shifts obtained from the daily sunspot records and those observed
suggests that the contribution from a stochastic component to the total
frequency shifts is about 30%. The remaining 70% is related to a global,
long-term variation. We also propose a new observable to investigate the short-
and mid-term variations of the frequency shifts, which is insensitive to the
long-term variations contained in the data. On the shortest time scales the
variations in the frequency shifts are strongly correlated with the variations
in the total area covered by sunspots. However, a significant loss of
correlation is still found, which cannot be fully explained by ignoring the
invisible side of the Sun when accounting for the total sunspot area. We also
verify that the times when the frequency shifts and the sunspot areas do not
vary in a similar way tend to coincide with the times of the maximum amplitude
of the quasi-biennial variations found in the seismic data.Comment: 4 pages, 2 figures, proceedings of the Joint TASC2 - KASC9 Workshop -
SPACEINN - HELAS8 Conference "Seismology of the Sun and the Distant Stars
2016: Using Today's Successes to Prepare the Future". To be published by the
EPJ Web of Conference
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