975 research outputs found
Testing the Relation Between the Local and Cosmic Star Formation Histories
Recently, there has been great progress toward observationally determining
the mean star formation history of the universe. When accurately known, the
cosmic star formation rate could provide much information about Galactic
evolution, if the Milky Way's star formation rate is representative of the
average cosmic star formation history. A simple hypothesis is that our local
star formation rate is proportional to the cosmic mean. In addition, to specify
a star formation history, one must also adopt an initial mass function (IMF);
typically it is assumed that the IMF is a smooth function which is constant in
time. We show how to test directly the compatibility of all these assumptions,
by making use of the local (solar neighborhood) star formation record encoded
in the present-day stellar mass function. Present data suggests that at least
one of the following is false: (1) the local IMF is constant in time; (2) the
local IMF is a smooth (unimodal) function; and/or (3) star formation in the
Galactic disk was representative of the cosmic mean. We briefly discuss how to
determine which of these assumptions fail, and improvements in observations
which will sharpen this test.Comment: 14 pages in LaTeX (uses aaspp4.sty). 5 postscript figures. To appear
in the Astrophysical Journa
EEG correlates of social interaction at distance
This study investigated EEG correlates of social interaction at distance between twenty-five pairs of participants who were not connected by any traditional channels of communication.
Each session involved the application of 128 stimulations separated by intervals of random duration ranging from 4 to 6 seconds. One of the pair received a one-second stimulation from a light signal produced by an arrangement of red LEDs, and a simultaneous 500 Hz sinusoidal audio signal of the same length. The other member of the pair sat in an isolated sound-proof room, such that any sensory interaction between the pair was impossible.
An analysis of the Event-Related Potentials associated with sensory stimulation using traditional averaging methods showed a distinct peak at approximately 300 ms, but only in the EEG activity of subjects who were directly stimulated. However, when a new algorithm was applied to the EEG activity based on the correlation between signals from all active electrodes, a weak but robust response was also detected in the EEG activity of the passive member of the pair, particularly within 9 â 10 Hz in the Alpha range. Using the Bootstrap method and the Monte Carlo emulation, this signal was found to be statistically significant
Incidence and molecular characterization of flavescence dorée and stolbur phytoplasmas in grapevine cultivars from different viticultural areas of Serbia
The presence and distribution of grapevine phytoplasmas was investigated from 2003 to 2005 in some of the most important viticultural areas of Serbia, considering in particular the susceptibility and sensitiveness of both local and imported grapevine cultivars. Both flavescence dorĂ©e (FD) and bois noir (BN) phytoplasmas were detected using molecular techniques. The presence of FD phytoplasma at the moment seems limited, while BN phytoplasma appears to be present in the majority of grape growing regions in Serbia. Field surveys demonstrate that grapevine yellows (GY) epidemics in the vineyards inspected in Serbia spread very fast, indeed the incidence of symptomatic plants increased considerably year by year. In particular, the average rate of FD diffusion increased from 45.5 to 93.0 % in the SiÄevaÄko region, while the spread of BN resulted lower. The local cultivar 'Plovdina' appeared to be extremely sensitive to FD phytoplasma showing a percentage of infected plants ranging from 91 to 100 %. PCR-RFLP and phylogenetic analyses based on ribosomal protein (rp) and secY gene sequences performed on Serbian FD grapevine strains demonstrated their close relationship with the Italian FD-C strain present in north-east Italy. Based on both phylogenetic markers, Serbian FD strains represent a new distinct lineage and together with the FD-C strain form a major phylogenetic group within the elm yellows group.
Galactic chemical evolution of heavy elements: from Barium to Europium
We follow the chemical evolution of the Galaxy for elements from Ba to Eu,
using an evolutionary model suitable to reproduce a large set of Galactic
(local and non local) and extragalactic constraints. Input stellar yields for
neutron-rich nuclei have been separated into their s-process and r-process
components. The production of s-process elements in thermally pulsing
asymptotic giant branch stars of low mass proceeds from the combined operation
of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases
neutrons in radiative conditions during the interpulse phase, and the reaction
22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The
resulting s-process distribution is strongly dependent on the stellar
metallicity. For the standard model discussed in this paper, it shows a sharp
production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process
yields, we assume that the production of r-nuclei is a primary process
occurring in stars near the lowest mass limit for Type II supernova
progenitors. The r-contribution to each nucleus is computed as the difference
between its solar abundance and its s-contribution given by the Galactic
chemical evolution model at the epoch of the solar system formation. We compare
our results with spectroscopic abundances of elements from Ba to Eu at various
metallicities (mainly from F and G stars) showing that the observed trends can
be understood in the light of the present knowledge of neutron capture
nucleosynthesis. Finally, we discuss a number of emerging features that deserve
further scrutiny.Comment: 34 pages, 13 figures. accepted by Ap
Hypo- and hyper-virulence in apricot trees infected by European stone fruit yellows
An apricot orchard, located in an area of north eastern Italy under serious pressure from European stone fruit yellows (ESFY) infection, has been monitored since the year it was planted (1990). During this time, most of the trees displayed symptoms or were shown by PCR analyses to be infected. Two groups of apricot trees were particularly interesting: some trees were asymptomatically infected while others recovered from the symptoms but not from the pathogen. In order to isolate those strains of the phytoplasma characterised by varying virulence, each of the two groups was used as mother plants and propagated. The new trees were used to constitute experimental orchards, where they were observed for the presence of symptoms and in part were tested by PCR, starting in 2003. The results obtained confirmed the presence of strains of the pathogen characterised by varying virulence. The strains originally present in infected apricot trees which recovered from the symptoms of ESFY were seen to be hypovirulent; none of the propagated infected trees ever showed symptoms of the disease. Surprisingly, the strains present in asymptomatic apricot mother plants were hypervirulent and the propagated trees always displayed severe symptoms. In the propagated trees, the transmission of the pathogen was higher in the hypervirulent strains than in the hypovirulent ones. A graft transmission trial carried out in the greenhouse using some of the identified hypo- and hypervirulent strains, confirmed the results obtained in open field. Real time PCR analyses showed that in the trees infected by hypovirulent strains the colonisation of the pathogen was lower than in those infected by the hypervirulent strains. It is possible to affirm that the hypovirulent strains were present in those mother plants which had originally recovered. The research will continue with the aim of verifying the possibility of cross protection among the identified hypo- and hypervirulent strains.Keywords: âCandidatus Phytoplasma prunorumâ, real-time PCR, Prunu
Evolution of Li, Be and B in the Galaxy
In this paper we study the production of Li, Be and B nuclei by Galactic
cosmic ray spallation processes. We include three kinds of processes: (i)
spallation by light cosmic rays impinging on interstellar CNO nuclei (direct
processes); (ii) spallation by CNO cosmic ray nuclei impinging on interstellar
p and 4He (inverse processes); and (iii) alpha-alpha fusion reactions. The
latter dominate the production of 6Li and 7Li. We calculate production rates
for a closed-box Galactic model, verifying the quadratic dependence of the Be
and B abundances for low values of Z. These are quite general results and are
known to disagree with observations. We then show that the multi-zone
multi-population model we used previously for other aspects of Galactic
evolution produces quite good agreement with the linear trend observed at low
metallicities without fine tuning. We argue that reported discrepancies between
theory and observations do not represent a nucleosynthetic problem, but instead
are the consequences of inaccurate treatments of Galactic evolution.Comment: 26 pages, 5 figures, LaTeX. The Astrophysical Journal, in pres
Synthetic Molecular Clouds from Supersonic MHD and Non-LTE Radiative Transfer Calculations
The dynamics of molecular clouds is characterized by supersonic random
motions in the presence of a magnetic field. We study this situation using
numerical solutions of the three-dimensional compressible magneto-hydrodynamic
(MHD) equations in a regime of highly supersonic random motions. The non-LTE
radiative transfer calculations are performed through the complex density and
velocity fields obtained as solutions of the MHD equations, and more than
5x10^5 synthetic molecular spectra are obtained. We use a numerical flow
without gravity or external forcing. The flow is super-Alfvenic and corresponds
to model A of Padoan and Nordlund (1997). Synthetic data consist of sets of
90x90 synthetic spectra with 60 velocity channels, in five molecular
transitions: J=1-0 and J=2-1 for 12CO and 13CO, and J=1-0 for CS. Though we do
not consider the effects of stellar radiation, gravity, or mechanical energy
input from discrete sources, our models do contain the basic physics of
magneto-fluid dynamics and non-LTE radiation transfer and are therefore more
realistic than previous calculations. As a result, these synthetic maps and
spectra bear a remarkable resemblance to the corresponding observations of real
clouds.Comment: 33 pages, 12 figures included, 5 jpeg figures not included (fig1a,
fig1b, fig3, fig4 fig5), submitted to Ap
A study of gas contaminants and interaction with materials in RPC closed loop systems
Resistive Plate Counters (RPC) detectors at the Large Hadron Collider (LHC)
experiments use gas recirculation systems to cope with large gas mixture
volumes and costs. In this paper a long-term systematic study about gas
purifiers, gas contaminants and detector performance is discussed. The study
aims at measuring the lifetime of purifiers with unused and used cartridge
material along with contaminants release in the gas system. During the
data-taking the response of several RPC double-gap detectors was monitored in
order to characterize the correlation between dark currents, filter status and
gas contaminants
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Infrared, UV/VIS and Raman Spectroscopy of Comet Wild-2 Samples Returned by the Stardust Mission
Results from the preliminary examination of Stardust samples obtained using various spectroscopic methods will be presented
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