Infrared properties of dust grains derived from IRAS observations

The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species

Cosmic Needles versus Cosmic Microwave Background Radiation

It has been suggested by a number of authors that the 2.7K cosmic microwave background (CMB) radiation might have arisen from the radiation from Population III objects thermalized by conducting cosmic graphite/iron needle-shaped dust. Due to lack of an accurate solution to the absorption properties of exceedingly elongated grains, in existing literature which studies the CMB thermalizing process they are generally modelled as (1) needle-like spheroids in terms of the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory. We show here that the Rayleigh approximation is not valid since the Rayleigh criterion is not satisfied for highly conducting needles. We also show that the available intergalactic iron dust, if modelled as infinite cylinders, is not sufficient to supply the required opacity at long wavelengths to obtain the observed isotropy and Planckian nature of the CMB. If appealing to the antenna theory, conducting iron needles with exceedingly large elongations (10^4) appear able to provide sufficient opacity to thermalize the CMB within the iron density limit. But the applicability of the antenna theory to exceedingly thin needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ

The Spectral Energy Distribution of Self-gravitating Interstellar Clouds I. Spheres

We derive the spectral energy distribution (SED) of dusty, isothermal, self gravitating, stable and spherical clouds externally heated by the ambient interstellar radiation field. For a given radiation field and dust properties, the radiative transfer problem is determined by the pressure of the surrounding medium and the cloud mass expressed as a fraction of the maximum stable cloud mass above which the clouds become gravitational unstable. To solve the radiative transfer problem a ray-tracing code is used to accurately derive the light distribution inside the cloud. This code considers both non isotropic scattering on dust grains and multiple scattering events. The dust properties inside the clouds are assumed to be the same as in the diffuse interstellar medium in our galaxy. We analyse the effect of the pressure, the critical mass fraction, and the ISRF on the SED and present brightness profiles in the visible, the IR/FIR and the submm/mm regime with the focus on the scattered emission and the thermal emission from PAH-molecules and dust grains.Comment: accepted for publication in ApJS, May 2008, v176n1 issu

Electron density and carriers of the diffuse interstellar bands

We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction to 13 stars. A linear relation was found, that allows the determination of the electron density from the Mg I and Mg II column densities in diffuse clouds. The comparison of normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with electron density varying in the range ne=0.01-2.5 cm^-3. Therefore the DIBs carriers (1) can be observed only in one ionisation stage, or (2) the DIBs are arising in cloud regions (eg. cores or cloud coronas) for which we can not determine the electron density

Interstellar Grains -- The 75th Anniversary

The year of 2005 marks the 75th anniversary since Trumpler (1930) provided the first definitive proof of interstellar grains by demonstrating the existence of general absorption and reddening of starlight in the galactic plane. This article reviews our progressive understanding of the nature of interstellar dust.Comment: invited review article for the "Light, Dust and Chemical Evolution" conference (Gerace, Italy, 26--30 September 2004), edited by F. Borghese and R. Saija, 2005, in pres

TGF-β1 Down-Regulation of NKG2D/DAP10 and 2B4/SAP Expression on Human NK Cells Contributes to HBV Persistence

The mechanism underlying persistent hepatitis B virus (HBV) infection remains unclear. We investigated the role of innate immune responses to persistent HBV infection in 154 HBV-infected patients and 95 healthy controls. The expression of NKG2D- and 2B4-activating receptors on NK cells was significantly decreased, and moreover, the expression of DAP10 and SAP, the intracellular adaptor proteins of NKG2D and 2B4 (respectively), were lower, which then impaired NK cell-mediated cytotoxic capacity and interferon-γ production. Higher concentrations of transforming growth factor-beta 1 (TGF-β1) were found in sera from persistently infected HBV patients. TGF-β1 down-regulated the expression of NKG2D and 2B4 on NK cells in our in vitro study, leading to an impairment of their effector functions. Anti-TGF-β1 antibodies could restore the expression of NKG2D and 2B4 on NK cells in vitro. Furthermore, TGF-β1 induced cell-cycle arrest in NK cells by up-regulating the expression of p15 and p21 in NK cells from immunotolerant (IT) patients. We conclude that TGF-β1 may reduce the expression of NKG2D/DAP10 and 2B4/SAP, and those IT patients who are deficient in these double-activating signals have impaired NK cell function, which is correlated with persistent HBV infection

Planck pre-launch status : The Planck mission

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