496 research outputs found
Más allá del conocimiento: los desafíos epistemológicos y sociales de la enseñanza mediante el cambio conceptual
The focus of this paper is on the generic assessment practices teachers need to adopt in order to support the shift in perspective about classroom management. This research show that actions, practices, products and discourse patterns of the classroom can provide the necessary assessment information in the epistemic, cognitive, and social domains teachers need to facilitate science learning
Spinning compact binary inspiral II: Conservative angular dynamics
We establish the evolution equations of the set of independent variables
characterizing the 2PN rigorous conservative dynamics of a spinning compact
binary, with the inclusion of the leading order spin-orbit, spin-spin and mass
quadrupole - mass monopole effects, for generic (noncircular, nonspherical)
orbits. More specifically, we give a closed system of first order ordinary
differential equations for the orbital elements of the osculating ellipse and
for the angles characterizing the spin orientations with respect to the
osculating orbit. We also prove that (i) the relative angle of the spins stays
constant for equal mass black holes, irrespective of their orientation, and
(ii) the special configuration of equal mass black holes with equal, but
antialigned spins, both laying in the plane of motion (leading to the largest
recoil found in numerical simulations) is preserved at 2PN level of accuracy,
with leading order spin-orbit, spin-spin and mass quadrupolar contributions
included.Comment: v2: 19 pages, extended, improved, published versio
Intrinsic Size OF Sgr A*: 72 Schwarzschild Radii
Recent proper motion studies of stars at the very center of the Galaxy
strongly suggest that Sagittarius (Sgr) A*, the compact nonthermal radio source
at the Galactic Center, is a 2.5 million solar mass black hole. By means of
near-simultaneous multi-wavelength Very Long Baseline Array measurements, we
determine for the first time the intrinsic size and shape of Sgr A* to be 72
Rsc by < 20 Rsc, with the major axis oriented essentially north-south, where
Rsc (= 7.5 x 10^{11} cm) is the Schwarzschild radius for a 2.5 million solar
mass black hole. Contrary to previous expectation that the intrinsic structure
of Sgr A* is observable only at wavelengths shorter than 1 mm, we can discern
the intrinsic source size at 7 mm because (1) the scattering size along the
minor axis is half that along the major axis, and (2) the near simultaneous
multi-wavelength mapping of Sgr A* with the same interferometer makes it
possible to extrapolate precisely the minor axis scattering angle at 7 mm. The
intrinsic size and shape place direct constraints on the various emission
models for Sgr A*. In particular, the advection dominated accretion flow model
may have to incorporate a radio jet in order to account for the structure of
Sgr A*.Comment: 15 pages including 2 ps figures and 1 table, to appear in ApJ Letter
The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion
We present a semi-analytic model for shaping the nebula around eta Carinae
that accounts for the simultaneous production of bipolar lobes and an
equatorial disk through a rotating surface explosion. Material is launched
normal to the surface of an oblate rotating star with an initial kick velocity
that scales approximately with the local escape speed. Thereafter, ejecta
follow ballistic orbital trajectories, feeling only a central force
corresponding to a radiatively reduced gravity. Our model is conceptually
similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we
modify it to an explosion instead of a steady line-driven wind, we include a
rotationally-distorted star, and we treat the dynamics somewhat differently.
Continuum-driving avoids the disk inhibition that normally operates in
line-driven winds. Our model provides a simple method by which rotating hot
stars can simultaneously produce intrinsically bipolar and equatorial mass
ejections, without an aspherical environment or magnetic fields. Although
motivated by eta Carinae, the model may have generic application to other LBVs,
B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less
influential, our model generalizes to produce bipolar morphologies without
disks, as seen in many PNe.Comment: ApJ accepted, 9 page
Ram pressure stripping and galaxy orbits: The case of the Virgo cluster
We investigate the role of ram pressure stripping in the Virgo cluster using
N-body simulations. Radial orbits within the Virgo cluster's gravitational
potential are modeled and analyzed with respect to ram pressure stripping. The
N-body model consists of 10000 gas cloud complexes which can have inelastic
collisions. Ram pressure is modeled as an additional acceleration on the clouds
located at the surface of the gas distribution in the direction of the galaxy's
motion within the cluster. We made several simulations changing the orbital
parameters in order to recover different stripping scenarios using realistic
temporal ram pressure profiles. We investigate systematically the influence of
the inclination angle between the disk and the orbital plane of the galaxy on
the gas dynamics. We show that ram pressure can lead to a temporary increase of
the central gas surface density. In some cases a considerable part of the total
atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk
after the stripping event. A quantitative relation between the orbit parameters
and the resulting HI deficiency is derived containing explicitly the
inclination angle between the disk and the orbital plane. The comparison
between existing HI observations and the results of our simulations shows that
the HI deficiency depends strongly on galaxy orbits. It is concluded that the
scenario where ram pressure stripping is responsible for the observed HI
deficiency is consistent with all HI 21cm observations in the Virgo cluster.Comment: 29 pages with 21 figures. Accepted for publication in Ap
Public perception and knowledge on nanotechnology: A study based on a citizen science approach
Even with the widespread use of nanomaterials (NMs) in everyday life, consumer knowledge about the functionality, benefits, and possible dangers of nanotechnology (NT) is still modest. As with any developing technology, its public perception has direct implications on future policies and has to be taken into account by academia and industry alike. As part of the “Nan-O-Style” interdisciplinary research project, an online survey was conducted using a Citizen Science-guided approach. The main goal was to evaluate the current levels of knowledge and the attitude towards NT among the general Austrian public and to determine how differing sociodemographic factors may affect these. Over the course of 17 months, a total of 1067 responses were collected and quantitatively analysed. We found that while Austrians display a generally optimistic view and a positive attitude towards NT, there are still remaining concerns about its safety and possible risks. Participants expressed great desire for more information about NT and its applications, as well as for clear labelling and transparency of products containing NMs. Notably, we found that age did not affect the general attitude towards NT nor the levels of NT awareness. While participants with a university degree were generally more knowledgeable on this specific topic, surprisingly, there were no significant differences in the attitude towards NT among people from different educational backgrounds. Similar to previous studies, we observed that male participants demonstrated a more positive attitude towards NT and scored slightly higher in our NT quiz compared to female participants. However, female participants voiced greater desire for more information and transparency regarding NMs. Interestingly, while participants with a negative attitude towards NT scored lowest on the NT quiz, they also expressed the least interest in receiving more NT-related information. This illustrates a difficulty in mitigating public aversion solely by providing more information
Resolved Mid-Infrared Imaging of AGN: An Isotropic Measure of Intrinsic Power
We present a strong correlation between 12μm mid-IR and intrinsic X-ray (2–10 keV) luminosities of local Seyferts. This work is based on new diffraction-limited mid-IR observations with the 8-m Very Large Telescope (VLT), resulting in the least-contaminated core fluxes of 42 Seyferts to date
HI deficiency in the galaxy cluster ACO 3627. ATCA observations in the Great Attractor region
ATCA 21 cm HI observations of the rich galaxy cluster ACO 3627 in the Great
Attractor region are presented. Three fields of 30' diameter located within one
Abell radius of ACO 3627 were observed with a resolution of 15'' and an rms
noise of \sim 1 mJy/beam. Only two galaxies were detected in these fields. We
compare their HI distribution to new optical R-band images and discuss their
velocity fields. The first galaxy is a gas-rich unperturbed spiral whereas the
second shows a peculiar HI distribution. The estimated 3-sigma HI mass limit of
our observations is \sim 7 x 10^8 M_{\odot} for a line width of 150 km s^{-1}.
The non-detection of a considerable number of luminous spiral galaxies
indicates that the spiral galaxies are HI deficient. The low detection rate is
comparable to the HI deficient Coma cluster (Bravo-Alfaro et al. 2000). ACO
3627 is a bright X-ray cluster. We therefore suspect that ram pressure
stripping is responsible for the HI deficiency of the bright cluster spirals.Comment: 9 pages with 7 figures; A&A, in pres
Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center
Sgr A* at the Galactic Center is a puzzling source. It has a mass
M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole
candidate. Observations of stellar winds and other gas flows in its vicinity
suggest a mass accretion rate approximately few x 10^{-6} solar masses per
year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s
if the radiative efficiency is the usual 10 percent, whereas observations
indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is
unusual, with emission extending over many decades of wavelength. We present a
model of Sgr A* which is based on a two-temperature optically-thin
advection-dominated accretion flow. The model is consistent with the estimated
mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray
bands as well as upper limits in the sub-mm and infrared bands; the fit is less
good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low
luminosity of Sgr A* is explained naturally in the model by means of advection.
Most of the viscously dissipated energy is advected into the central mass by
the accreting gas, and therefore the radiative efficiency is extremely low,
approximately 5 x 10^{-6}. A critical element of the model is the presence of
an event horizon at the center which swallows the advected energy. The success
of the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised
Version changes to text, tables and figures. ApJ, 492, in pres
Mechanisms of particles in sensitization, effector function and therapy of allergic disease
Humans have always been in contact with natural airborne particles from many sources including biologic particulate matter (PM) which can exhibit allergenic properties. With industrialization, anthropogenic and combustion-derived particles have become a major fraction. Currently, an ever-growing number of diverse and innovative materials containing engineered nanoparticles (NPs) are being developed with great expectations in technology and medicine. Nanomaterials have entered everyday products including cosmetics, textiles, electronics, sports equipment, as well as food, and food packaging. As part of natural evolution humans have adapted to the exposure to particulate matter, aiming to protect the individual's integrity and health. At the respiratory barrier, complications can arise, when allergic sensitization and pulmonary diseases occur in response to particle exposure. Particulate matter in the form of plant pollen, dust mites feces, animal dander, but also aerosols arising from industrial processes in occupational settings including diverse mixtures thereof can exert such effects. This review article gives an overview of the allergic immune response and addresses specifically the mechanisms of particulates in the context of allergic sensitization, effector function and therapy. In regard of the first theme (i), an overview on exposure to particulates and the functionalities of the relevant immune cells involved in allergic sensitization as well as their interactions in innate and adaptive responses are described. As relevant for human disease, we aim to outline (ii) the potential effector mechanisms that lead to the aggravation of an ongoing immune deviation (such as asthma, chronic obstructive pulmonary disease, etc.) by inhaled particulates, including NPs. Even though adverse effects can be exerted by (nano)particles, leading to allergic sensitization, and the exacerbation of allergic symptoms, promising potential has been shown for their use in (iii) therapeutic approaches of allergic disease, for example as adjuvants. Hence, allergen-specific immunotherapy (AIT) is introduced and the role of adjuvants such as alum as well as the current understanding of their mechanisms of action is reviewed. Finally, future prospects of nanomedicines in allergy treatment are described, which involve modern platform technologies combining immunomodulatory effects at several (immuno-)functional levels
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