Search results for "010303 astronomy & astrophysics"
showing 10 items of 1094 documents
Ortho-H2 and the age of prestellar cores
2013
Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based…
Detecting filamentary pattern in the cosmic web : a catalogue of filaments for the SDSS
2014
The main feature of the spatial large-scale galaxy distribution is its intricate network of galaxy filaments. This network is spanned by the galaxy locations that can be interpreted as a three-dimensional point distribution. The global properties of the point process can be measured by different statistical methods, which, however, do not describe directly the structure elements. The morphology of the large scale structure, on the other hand, is an important property of the galaxy distribution. Here we apply an object point process with interactions (the Bisous model) to trace and extract the filamentary network in the presently largest galaxy redshift survey, the Sloan Digital Sky Survey (…
Analyzing observations of molecules in the ISM: Theoretical and experimental studies of energy transfer
2012
Our laboratories have recently calculated a set of collision coefficients characterizing the efficiency of energy transfer between helium and/or hydrogen and a large variety of interstellar molecules. We have considered with molecules ranging from light hydrides, observed by the Herschel Space Observatory, to medium size molecules, observed by mm antennas, to heavy complex organic molecules, observed also in the cm range. We present a review of recent theoretical results obtained in our laboratories, for various kinds of commonly observed molecules.
Sodium vapor cell laser guide star experiments for continuous wave model validation
2016
Recent numerical simulations and experiments on sodium Laser Guide Star (LGS) have shown that a continuous wave (CW) laser with circular polarization and re-pumping should maximize the fluorescent photon return flux to the wavefront sensor for adaptive optics applications. The orientation and strength of the geomagnetic field in the sodium layer also play an important role affecting the LGS return ux. Field measurements of the LGS return flux show agreement with the CW LGS model, however, fluctuations in the sodium column abundance and geomagnetic field intensity, as well as atmospheric turbulence, induce experimental uncertainties. We describe a laboratory experiment to measure the photon …
Preliminary assessment of the ATHENA/WFI non-X-ray background
2017
We present a preliminary assessment of the non-X-ray background for the WFI on board ATHENA conducted at IAAT in the context of the collaborative background and radiation damage working group activities. Our main result is that in the baseline configuration originally assumed for the camera the requirement on the level of non-X-ray background could not be met. In light of the results of Geant4 simulations we propose and discuss a possible optimization of the camera design and pinpoint some open issues to be addressed in the next phase of investigation. One of these concerns the possible contribution to the non-X-ray background from soft protons and ions funneled to the focal plane through t…
The filter wheel and filters development for the X-IFU instruments onboard Athena
2016
Athena is the large mission selected by ESA in 2013 to investigate the science theme “Hot and Energetic Universe” and presently scheduled for launch in 2028. One of the two instruments located at the focus of the 12 m-long Athena telescope is the X-ray Integral Field Unit (X-IFU). This is an array of TES microcalorimeters that will be operated at temperatures of 50 mK in order to perform high resolution spectroscopy with an energy resolution down to 2.5 eV at energies < 7 keV. In order to cope with the large dynamical range of X-ray fluxes spanned by the celestial objects Athena will be observing, the X-IFU will be equipped with a filter wheel. This will allow the user to fine tune the i…
The virial theorem and the dark matter problem in hybrid metric-Palatini gravity
2012
Hybrid metric-Palatini gravity is a recently proposed theory, consisting of the superposition of the metric Einstein-Hilbert Lagrangian with an f(R) term constructed a la Palatini. The theory predicts the existence of a long-range scalar field, which passes the Solar System observational constraints, even if the scalar field is very light, and modifies the cosmological and galactic dynamics. Thus, the theory opens new possibilities to approach, in the same theoretical framework, the problems of both dark energy and dark matter. In this work, we consider the generalized virial theorem in the scalar-tensor representation of the hybrid metric-Palatini gravity. More specifically, taking into ac…
Possibility of a dark matter interpretation for the excess in isotropic radio emission reported by ARCADE.
2011
The ARCADE 2 Collaboration has recently measured an isotropic radio emission which is significantly brighter than the expected contributions from known extra-galactic sources. The simplest explanation of such excess involves a ``new'' population of unresolved sources which become the most numerous at very low (observationally unreached) brightness. We investigate this scenario in terms of synchrotron radiation induced by weakly interacting massive particle (WIMP) annihilations or decays in extra-galactic halos. Intriguingly, for light-mass WIMPs with a thermal annihilation cross section, the level of expected radio emission matches the ARCADE observations.
Large-scale inhomogeneities may improve the cosmic concordance of supernovae
2010
We reanalyze the supernovae data from the Union Compilation including the weak lensing effects caused by inhomogeneities. We compute the lensing probability distribution function for each background solution described by the parameters Omega_M, Omega_L and w in the presence of inhomogeneities, approximately modeled with a single-mass population of halos. We then perform a likelihood analysis in the space of FLRW-parameters and compare our results with the standard approach. We find that the inclusion of lensing can move the best-fit model significantly towards the cosmic concordance of the flat LCDM model, improving the agreement with the constraints coming from the cosmic microwave backgro…
Accreting Pulsars: Mixing-up Accretion Phases in Transitional Systems
2018
In the last 20 years our understanding of the millisecond pulsar population changed dramatically. Thanks to the large effective area and good time resolution of the NASA X-ray observatory Rossi X-ray Timing Explorer, we discovered that neutron stars in Low Mass X-ray Binaries (LMXBs) spins at frequencies between 200 and 750 Hz, and indirectly confirmed the recycling scenario, according to which neutron stars are spun up to millisecond periods during the LMXB-phase. In the meantime, the continuous discovery of rotation-powered millisecond pulsars in binary systems in the radio and gamma-ray band (mainly with the Fermi Large Area Telescope) allowed us to classify these sources into two "spide…