Gamma ray astronomy and baryonic dark matter

Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse $\gamma$-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which $\gamma$-rays are produced through the interaction of high-energy cosmic-ray protons with cold $H_2$ clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo $\gamma$-ray flux turns out to be in remarkably good agreement with the discovery by Dixon et al. We also address future prospects to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999

Constraints on Cold H_2 Clouds from Gravitational Microlensing Searches

It has been proposed that the Galaxy might contain a population of cold clouds in numbers sufficient to account for a substantial fraction of the total mass of the Galaxy. These clouds would have masses of the order of 10^{-3} Solar mass and sizes of the order of 10 AU. We consider here the lensing effects of such clouds on the light from background stars. A semianalytical formalism for calculation of the magnification event rate produced by such gaseous lensing is developed, taking into account the spatial distribution of the dark matter in the Galaxy, the velocity distribution of the lensing clouds and source stars, and motion of the observer. Event rates are calculated for the case of gaseous lensing of stars in the Large Magellanic Cloud and results are directly compared with the results of the search for gravitational microlensing events undertaken by the MACHO collaboration. The MACHO experiment strongly constrains the properties of the proposed molecular clouds, but does not completely rule them out. Future monitoring programs will either detect or more strongly constrain this proposed population.Comment: 36 pages, 9 figures, 1 table, typos corrected, minor change

Thermal stability of cold clouds in galaxy halos

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria which are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10^-6 Msun necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z > 1 when the temperature of the microwave background was substantially larger than its current value; the formation and survival of such clouds to the present epoch therefore remain open questions.Comment: 5 pp incl 2 figs, LaTeX, emulateapj.sty; ApJ Letters in press. Significant revisions, results qualitatively unchange

Hyperglycemia selectively increases the expression of cycloxygenase-2 in human aortic endothelial cells

The conversion of arachidonic acid to vasoactive prostanoids including prostacyclin, prostaglandins and tromboxanes is mediated by cycloxygenase (COX). Two isoforms of enzyme have been shown: a constitutive (COX-1) and an inducible form (COX-2). Products of the arachidonic acid metabolism may be involved in the impairment of endothelium-dependent vasodilatation observed both in experimental models and in patients with diabetes mellitus. To determine the effect of hyperglycemia on COX-1 and COX-2 expression, human aortic endothelial cells (HAEC) were exposed to normal (5.5mM) and high (22.2mM) concentrations of glucose for 5 days. Cells were also treated with mannitol (22.2 mM) to rule out an effect due to osmolality changes. COX-1 and COX-2 mRNA and protein expressions were analyzed by Southern and Western blotting, respectively. Treatment with high glucose was associated with a two-fold increase of both COX-2 mRNA (P<0.05) and protein levels (P<0.05), whereas no changes were observed for COX-1. Moreover high concentration of mannitol did not exert any significant effect. The present study demonstrates that both isoforms of COX are normally expressed in HAEC, but only COX-2 was stimulated after exposure to high glucose. The results of the present study may provide molecular basis to understand hyperglycemia-induced endothelial dysfunctio

Gravitational wave scintillation by a stellar cluster

The diffraction effects on gravitational waves propagating through a stellar cluster are analyzed in the relevant approximation of Fresnel diffraction limit. We find that a gravitational wave scintillation effect - similar to the radio source scintillation effect - comes out naturally, implying that the gravitational wave intensity changes in a characteristic way as the observer moves.Comment: 9 pages, in press in IJMP

A Search for Stellar Obscuration Events due to Dark Clouds

The recent detections of a large population of faint submillimetre sources, an excess halo gamma-ray background, and the extreme scattering events observed for extragalactic radio sources have been explained as being due to baryonic dark matter in the form of small, dark, gas clouds. In this paper we present the results of a search for the transient stellar obscurations such clouds are expected to cause. We examine the Macho project light curves of 48 x 10^6 stars toward the Galactic bulge, LMC and SMC for the presence of dark cloud extinction events. We find no evidence for the existence of a population of dark gas clouds with Av > 0.2 and masses between ~ 10^-4 and 10^-2 M_solar in the Galactic disk or halo. However, it is possible that such dark cloud populations could exist if they are clustered in regions away from the observed lines of sight.Comment: 13 pages, 9 figures, submitted to Ap

Hyperglycemia selectively increases the expression of cycloxygenase-2 in human aortic endothelial cells

The conversion of arachidonic acid to vasoactive prostanoids including prostacyclin, prostaglandins and tromboxanes is mediated by cycloxygenase (COX). Two isoforms of enzyme have been shown: a constitutive (COX-1) and an inducible form (COX-2). Products of the arachidonic acid metabolism may be involved in the impairment of endothelium-dependent vasodilatation observed both in experimental models and in patients with diabetes mellitus. To determine the effect of hyperglycemia on COX-1 and COX-2 expression, human aortic endothelial cells (HAEC) were exposed to normal (5.5mM) and high (22.2mM) concentrations of glucose for 5 days. Cells were also treated with mannitol (22.2 mM) to rule out an effect due to osmolality changes. COX-1 and COX-2 mRNA and protein expressions were analyzed by Southern and Western blotting, respectively. Treatment with high glucose was associated with a two-fold increase of both COX-2 mRNA (P<0.05) and protein levels (P<0.05), whereas no changes were observed for COX-1. Moreover high concentration of mannitol did not exert any significant effect. The present study demonstrates that both isoforms of COX are normally expressed in HAEC, but only COX-2 was stimulated after exposure to high glucose. The results of the present study may provide molecular basis to understand hyperglycemia-induced endothelial dysfunctio

Introduzione

L'introduzione espone le ragioni scientifiche della pubblicazione del volume contenente gli Atti del IV seminario per giovani studiosi di letteratura latina con contributi su Livio, Cesare, Apuleio, Lucrezio, Varrone, Plauto, Ps. Acrone, Servio, Lucano, Manilio