4,777 research outputs found
Organic Molecules in the Galactic Center. Hot Core Chemistry without Hot Cores
We study the origin of large abundances of complex organic molecules in the
Galactic center (GC). We carried out a systematic study of the complex organic
molecules CH3OH, C2H5OH, (CH3)2O, HCOOCH3, HCOOH, CH3COOH, H2CO, and CS toward
40 GC molecular clouds. Using the LTE approximation, we derived the physical
properties of GC molecular clouds and the abundances of the complex
molecules.The CH3OH abundance between clouds varies by nearly two orders of
magnitude from 2.4x10^{-8} to 1.1x10^{-6}. The abundance of the other complex
organic molecules relative to that of CH3OH is basically independent of the
CH3OH abundance, with variations of only a factor 4-8. The abundances of
complex organic molecules in the GC are compared with those measured in hot
cores and hot corinos, in which these complex molecules are also abundant. We
find that both the abundance and the abundance ratios of the complex molecules
relative to CH3OH in hot cores are similar to those found in the GC clouds.
However, hot corinos show different abundance ratios than observed in hot cores
and in GC clouds. The rather constant abundance of all the complex molecules
relative to CH3OH suggests that all complex molecules are ejected from grain
mantles by shocks. Frequent (similar 10^{5}years) shocks with velocities >6km/s
are required to explain the high abundances in gas phase of complex organic
molecules in the GC molecular clouds. The rather uniform abundance ratios in
the GC clouds and in Galactic hot cores indicate a similar average composition
of grain mantles in both kinds of regions. The Sickle and the Thermal Radio
Arches, affected by UV radiation, show different relative abundances in the
complex organic molecules due to the differentially photodissociation of these
molecules.Comment: 18 pages, 10 Postscript figures, uses aa.cls, aa.bst, 10pt.rtx,
natbib.sty, revsymb.sty revtex4.cls, aps.rtx and aalongtabl.sty. Accepted in
A&A 2006. version 2. relocated figures and tables. Language editor
suggestions. added reference
A decomposition procedure based on approximate newton directions
The efficient solution of large-scale linear and nonlinear optimization problems may require exploiting any special structure in them in an efficient manner. We describe and analyze some cases in which this special structure can be used with very little cost to obtain search directions from decomposed subproblems. We also study how to correct these directions using (decomposable) preconditioned conjugate gradient methods to ensure local convergence in all cases. The choice of appropriate preconditioners results in a natural manner from the structure in the problem. Finally, we conduct computational experiments to compare the resulting procedures with direct methods, as well as to study the impact of different preconditioner choices
Phase diagram of a polydisperse soft-spheres model for liquids and colloids
The phase diagram of soft spheres with size dispersion has been studied by
means of an optimized Monte Carlo algorithm which allows to equilibrate below
the kinetic glass transition for all sizes distribution. The system
ubiquitously undergoes a first order freezing transition. While for small size
dispersion the frozen phase has a crystalline structure, large density
inhomogeneities appear in the highly disperse systems. Studying the interplay
between the equilibrium phase diagram and the kinetic glass transition, we
argue that the experimentally found terminal polydispersity of colloids is a
purely kinetic phenomenon.Comment: Version to be published in Physical Review Letter
Separation and fractionation of order and disorder in highly polydisperse systems
Microcanonical Monte Carlo simulations of a polydisperse soft-spheres model
for liquids and colloids have been performed for very large polydispersity, in
the region where a phase-separation is known to occur when the system (or part
of it) solidifies. By studying samples of different sizes, from N=256 to N=864,
we focus on the nature of the two distinct coexisting phases. Measurements of
crystalline order in particles of different size reveal that the solid phase
segregates between a crystalline solid with cubic symmetry and a disordered
phase. This phenomenon is termed fractionation.Comment: 8 pages, 5 figure
The central parsecs of M87: jet emission and an elusive accretion disc
We present the first simultaneous spectral energy distribution (SED) of M87
core at a scale of 0.4 arcsec () across the electromagnetic
spectrum. Two separate, quiescent, and active states are sampled that are
characterized by a similar featureless SED of power-law form, and that are thus
remarkably different from that of a canonical active galactic nuclei (AGN) or a
radiatively inefficient accretion source. We show that the emission from a jet
gives an excellent representation of the core of M87 core covering ten orders
of magnitude in frequency for both the active and the quiescent phases. The
inferred total jet power is, however, one to two orders of magnitude lower than
the jet mechanical power reported in the literature. The maximum luminosity of
a thin accretion disc allowed by the data yields an accretion rate of , assuming 10% efficiency. This power
suffices to explain M87 radiative luminosity at the jet-frame, it is however
two to three order of magnitude below that required to account for the jet's
kinetic power. The simplest explanation is variability, which requires the core
power of M87 to have been two to three orders of magnitude higher in the last
200 yr. Alternatively, an extra source of power may derive from black hole
spin. Based on the strict upper limit on the accretion rate, such spin power
extraction requires an efficiency an order of magnitude higher than predicted
from magnetohydrodynamic simulations, currently in the few hundred per cent
range.Comment: 18 pages, 6 figures. Accepted for publication in MNRA
ISO observations of the Galactic center Interstellar Medium: neutral gas and dust
The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas
component with a kinetic temperature of 100-200 K. The bulk of this gas is not
associated to the well-known thermal radio continuum or far infrared sources
like Sgr A or Sgr B. How this gas is heated has been a longstanding problem.
With the aim of studying the thermal balance of the neutral gas and dust in the
GC, we have observed 18 molecular clouds located at projected distances far
from thermal continuum sources with the Infrared Space Observatory (ISO). In
this paper we present observations of several fine structure lines and the full
continuum spectra of the dust between 40 and 190 microns. A warm dust component
with a temperature between 27 and 42 K is needed to fit the spectra. We have
compared the gas and the dust emission with the predictions from J-type and
C-type shocks and photodissociation region (PDRs) models. We conclude that the
dust and the fine structure lines observations are best explained by a PDR with
a density of 10 cm^-3 and an incident far-ultraviolet field 10 times
higher than the local interstellar radiation field. PDRs can naturally explain
the discrepancy between the gas and the dust temperatures. However, these PDRs
can only account for 10-30% of the total H2 column density with a temperature
of ~ 150 K. We discuss other possible heating mechanisms (short version).Comment: Accepted for publication by A&
COVID-19 Lockdown and Disney+ strategy on social networks on its launch during the State of alarm in Spain
The launch of Disney+ in Spain in March 2020 coincided with the Covid-19 pandemic and a period of home confinement for many Spaniards. This study examines the advertising and engagement strategy used by Disney+ on Twitter and Instagram, analyzing 2,268 messages. The platform’s social media use is similar to that of its competitors, with Twitter used for user engagement and Instagram used for content promotion. However, the study finds that the platform missed the opportunity to emphasize its unique qualities and appeal to the brand’s values. Despite this, Disney+ generated significant engagement during its launch month, reflecting high levels of user anticipation. Popular original productions such as The Mandalorian and Aladdin, as well as acquired content like The Simpsons, indicate the brand’s focus on a family or broad-spectrum target audience, in contrast to the younger and niche audiences favored by its competitors.La llegada de Disney+ a España en marzo de 2020 estuvo marcada por la pandemia del Covid-19. El periodo de confinamiento domiciliario entre la población española coincidió con la irrupción del nuevo servicio streaming de una de las marcas más conocidas y queridas a nivel mundial. Este estudio analiza la estrategia publicitaria y de engagement empleada por Disney+ en Twitter e Instagram. El análisis de contenido incluye 2.268 mensajes. Se aprecia un uso de las redes sociales similar a marcas competidoras como Netflix con un uso de Twitter para informar y responder a los comentarios de los usuarios; mientras que de Instagram se promueven nuevos contenidos, en especial pelÃculas. Se observa que en el intento de destacar sus elementos diferenciales como servicio bajo demanda, se pierde la oportunidad de apelar a los valores propios de la marca del ratón. La plataforma consiguió un mejor engagement durante el mes de lanzamiento, lo que muestra la alta expectativa de los usuarios. Entre las publicaciones más virales se encuentran producciones originales como The Mandalorian o AladdÃn, pero también productos adquiridos como Los Simpson, y que apuntan al interés de la marca por un target familiar o de amplio espectro, frente a audiencias más jóvenes y de nicho valoradas por la competencia
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