8,360 research outputs found
Gigantios o Sigantios? Origine e sviluppi di un falso problema. A proposito di Greg. Naz. Epigr. 1-2 (PG 38, 81-83)
L\u2019esame completo della tradizione manoscritta e a stampa permette di risolvere il (falso) problema del destinatario, di rettificare in alcuni punti il testo fornito dai Padri maurini e di sciogliere un nodo interpretativo di un certo riliev
Stat mulier, sicut steterat prius\u2026 La metamorfosi della moglie di Loth nell\u2019Hamartigenia di Prudenzio
Rilettura del secondo paradigma addotto da Prudenzio nell\u2019Hamartigenia (vv. 723-776) per illustrare la dottrina del libero arbitrio, paradigma avente per oggetto l\u2019episodio biblico della fuga da Sodoma ed il diverso comportamento tenuto da Loth e dalla moglie davanti all\u2019ordine di non voltarsi indietro. Ridefinizione del ruolo dei personaggi e ridistribuzione degli spazi narrativi rispetto al modello scritturistico. Discussione dell\u2019interpretazione di alcuni passi data da Chr. Gnilka
Massive black hole factories: Supermassive and quasi-star formation in primordial halos
Supermassive stars and quasi-stars (massive stars with a central black hole)
are both considered as potential progenitors for the formation of supermassive
black holes. They are expected to form from rapidly accreting protostars in
massive primordial halos. We explore how long rapidly accreting protostars
remain on the Hayashi track, implying large protostellar radii and weak
accretion luminosity feedback. We assess the potential role of energy
production in the nuclear core, and determine what regulates the evolution of
such protostars into quasi-stars or supermassive stars. We follow the
contraction of characteristic mass scales in rapidly accreting protostars, and
infer the timescales for them to reach nuclear densities. We compare the
characteristic timescales for nuclear burning with those for which the extended
protostellar envelope can be maintained. We find that the extended envelope can
be maintained up to protostellar masses of 3.6x10^8 \dot{m}^3 solar, where
\dot{m} denotes the accretion rate in solar masses per year. We expect the
nuclear core to exhaust its hydrogen content in 7x10^6 yrs. If accretion rates
\dot{m}>>0.14 can still be maintained at this point, a black hole may form
within the accreting envelope, leading to a quasi-star. Alternatively, the
accreting object will gravitationally contract to become a main-sequence
supermassive star. Due to the limited gas reservoir in dark matter halos with
10^7 solar masses, the accretion rate onto the central object may drop at late
times, implying the formation of supermassive stars as the typical outcome of
direct collapse. However, if high accretion rates are maintained, a quasi-star
with an interior black hole may form.Comment: 9 pages, 4 figures, submitted to A&A. Comments are welcom
Formation of the First Stars by Accretion
The process of star formation from metal-free gas is investigated by
following the evolution of accreting protostars with emphasis on the properties
of massive objects. The main aim is to establish the physical processes that
determine the upper mass limit of the first stars. Although the consensus is
that massive stars were commonly formed in the first cosmic structures, our
calculations show that their actual formation depends sensitively on the mass
accretion rate and its time variation. Even in the rather idealized case in
which star formation is mainly determined by dot{M}acc, the characteristic mass
scale of the first stars is rather uncertain. We find that there is a critical
mass accretion rate dot{M}crit = 4 10^{-3} Msun/yr that separates solutions
with dot{M}acc> 100 Msun can form,
provided there is sufficient matter in the parent clouds, from others
(dot{M}acc > dot{M}crit) where the maximum mass limit decreases as dot{M}acc
increases. In the latter case, the protostellar luminosity reaches the
Eddington limit before the onset of hydrogen burning at the center via the
CN-cycle. This phase is followed by a rapid and dramatic expansion of the
radius, possibly leading to reversal of the accretion flow when the stellar
mass is about 100Msun. (abridged)Comment: 34 pages, 12 figures. ApJ, in pres
Plant Essential Oils as Biocides in Sustainable Strategies for the Conservation of Cultural Heritage
Biodeterioration is a complex network of interactions between macro/micro-biological systems and organic/inorganic substrates involving physical and chemical alterations, strictly related to their metabolic activities. Concerning microbial deterioration, finding a correct approach to counteract this process is often difficult, requiring an understanding of the kind of alterations and the use of methods that respect artwork and human and environmental health. Specific conservative
and remedial methods are used for this aim. They comprise physical, mechanical, and chemical methods, as well as, frequently, synthetic chemical biocides, which have obvious limitations because of their toxicity to operators or because they contain polluting substances that persist in the natural environment. New and alternative research has strongly focused on strategies to replace the use of toxic methods with natural products that do not have undesired effects, as well as implementing safe, novel compounds. Several plants contain natural chemical compounds such as oils, phenols,
flavonoids, alkaloids, coumarins, tannins, etc., commonly used as drugs, bioactive molecules, and nutrients. Essential oils extracted from plants can be the correct way to prevent the biodeterioration of cultural heritage in a safe manner. This review aims to summarize the latest research on the use of natural essential oils in restoration procedures for cultural heritage considering them sustainable means with respect to the environment and human health
Elastic properties of hydrogenated graphene
There exist three conformers of hydrogenated graphene, referred to as chair-,
boat-, or washboard-graphane. These systems have a perfect two-dimensional
periodicity mapped onto the graphene scaffold, but they are characterized by a
orbital hybridization, have different crystal symmetry, and otherwise
behave upon loading. By first principles calculations we determine their
structural and phonon properties, as well as we establish their relative
stability. Through continuum elasticity we define a simulation protocol
addressed to measure by a computer experiment their linear and nonlinear
elastic moduli and we actually compute them by first principles. We argue that
all graphane conformers respond to any arbitrarily-oriented extention with a
much smaller lateral contraction than the one calculated for graphene.
Furthermore, we provide evidence that boat-graphane has a small and negative
Poisson ratio along the armchair and zigzag principal directions of the carbon
honeycomb lattice (axially auxetic elastic behavior). Moreover, we show that
chair-graphane admits both softening and hardening hyperelasticity, depending
on the direction of applied load.Comment: submitted on Phys.Rev.
A Multi-Color Optical Survey of the Orion Nebula Cluster. Part I: the Catalog
We present U, B, V, I broad-band, 6200A TiO medium-band and Halpha photometry
of the Orion Nebula Cluster obtained with the WFI imager at the ESO/MPI 2.2
telescope. The nearly-simultaneous observations cover the entire ONC in a field
of about 34x34 arcmin. They enable us to determine stellar colors avoiding the
additional scatter in the photometry induced by stellar variability typical of
pre-main sequence stars. We identify 2,612 point-like sources in I band, 58%,
43% and 17% of them detected also in V, B and U, respectively. 1040 sources are
identified in Halpha band. In this paper we present the observations, the
calibration techniques, and the resulting catalog. We show the derived CMD of
the population and discuss the completeness of our photometry. We define a
spectro-photometric TiO index from the fluxes in V, I, and TiO-band. We find a
correlation between the index and the spectral type valid for M-type stars,
that is accurate to better than 1 spectral sub-class for M3-M6 types and better
than 2 spectral subclasses for M0-M2 types. This allows us to newly classify
217 stars. We subtract from our Halpha photometry the photospheric continuum at
its wavelength, deriving calibrated line excess for the full sample. This
represents the largest Halpha star catalog obtained to date on the ONC. This
data set enables a full re-analysis of the properties of the Pre-Main Sequence
population in the Orion Nebula Cluster to be presented, in an accompanying
paper.Comment: 15 pages, 15 figures. To be published in The Astrophysical Journal
Supplement Serie
Statistical Agent Based Modelization of the Phenomenon of Drug Abuse
We introduce a statistical agent based model to describe the phenomenon of
drug abuse and its dynamical evolution at the individual and global level. The
agents are heterogeneous with respect to their intrinsic inclination to drugs,
to their budget attitude and social environment. The various levels of drug use
were inspired by the professional description of the phenomenon and this
permits a direct comparison with all available data. We show that certain
elements have a great importance to start the use of drugs, for example the
rare events in the personal experiences which permit to overcame the barrier of
drug use occasionally. The analysis of how the system reacts to perturbations
is very important to understand its key elements and it provides strategies for
effective policy making. The present model represents the first step of a
realistic description of this phenomenon and can be easily generalized in
various directions.Comment: 12 pages, 5 figure
A multi-color optical survey of the orion nebula cluster. II. The H-R diagram
We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical
photometry and spectroscopy.We study the color–color diagrams in BVI, plus a narrowband filter centered at 6200 Å, finding evidence that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC in the M
spectral-type range, while a better agreement is found employing intrinsic colors derived from synthetic photometry, constraining the surface gravity value as predicted by a pre-main-sequence isochrone.We refine these model colors even further, empirically, by comparison with a selected sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and extend this sample with the addition of 65 newly classified stars from slit spectroscopy and 182 M-type from narrowband photometry; in this way, we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive the stellar reddening and the optical excess due to accretion from the location of each star in the BVI color–color diagram. This enables us to accurately determine the extinction of the ONC members, together with an estimate of their accretion luminosities. We adopt a lower distance for the Orion Nebula than previously assumed, based on recent parallax measurements. With a careful choice of also the spectral-type–temperature transformation, we produce the new Hertzsprung–Russell diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a slightly lower luminosity for early types. We determine the age distribution of the population, peaking from ~2 to ~3 Myr depending on the model. We study the distribution of the members in the mass–age plane and find that taking into account selection effects due to incompleteness,
removes an apparent correlation between mass and age.We derive the initial mass function for low- and intermediate mass members of the ONC, which turns out to be model dependent and shows a turnover at M ≲ 0.2 M_⊙
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