Cesare Cecchi-Pestellini

The far-ultraviolet signature of the 'missing' baryons in the Local Group of galaxies

The number of baryons detected in the low-redshift (z < 1) Universe is far smaller than the number detected in corresponding volumes at higher redshifts. Simulations of the formation of structure in the Universe show that up to two-thirds of the ‘missing’ baryons may have escaped detection because of their high temperature and low density. One of the few ways to detect this matter directly is to look for its signature in the form of ultraviolet absorption lines in the spectra of background sources such as quasars. Here we show that the amplitude of the average velocity vector of ‘high velocity’ O vi (O5+) absorption clouds detected in a survey of ultraviolet emission from active galactic nuclei decreases significantly when the vector is transformed to the frames of the Galactic Standard of Rest and the Local Group of galaxies. At least 82 per cent of these absorbers are not associated with any ‘high velocity’ atomic hydrogen complex in our Galaxy, and are therefore likely to result from a primordial warm–hot intergalactic medium pervading an extended corona around the Milky Way or the Local Group. The total mass of baryons in this medium is estimated to be up to ∼1012 solar masses, which is of the order of the mass required to dynamically stabilize the Local Group.

The role of the charge state of PAHs in ultraviolet extinction

Aims. We explore the relation between the charge state of polycyclic aromatic hydrocarbons (PAHs) and the extinction curve morphology. Methods. We fit extinction curves with a dust model including core-mantle spherical particles of mixed chemical composition (silicate core, sp 2 and sp 3 carbonaceous layers), and an additional molecular component. We use exact methods to calculate the extinction due to classical particles and accurate computed absorption spectra of PAHs in different charge states, for the contribution due to the molecular component, along five different lines of sight. Results. A combination of classical dust particles and mixtures of real PAHs satisfactorily matches the observed interstellar extinction curves. Variations of the spectral properties of PAHs in different charge states produce changes consistent with the varying relative strengths of the bump and non-linear far-UV rise.

Rotational and vibrational excitation of CO molecules by collisions with 4He atoms

Full close-coupled calculations are carried out of the cross sections for energy transfer between rotational levels of carbon monoxide in collision with 4He atoms with energies between 5 and 600 cm-1. At low energies, the cross sections are dominated by contributions from shape resonances. The calculated cross sections are in satisfactory agreement with the experimental data measured at an energy of 570 cm-1. Calculations using the infinite order sudden approximation are carried out of cross sections for energy transfer between vibrational levels of CO. Vibrational energy transfer is dominated by transitions in which the vibrational quantum number changes by unity.

The relative role of EUV radiation and X-rays in the heating of hydrogen-rich exoplanet atmospheres

Aims. We study the relative role of EUV and X-ray radiation in the heating of hydrogen-rich planet atmospheres with different composition and electron content. Methods. An accurate photo-ionization model has been used to follow the primary photo-electron energy deposition throughout the atmosphere. Results. Heating rates and efficiencies have been computed, together with column density cut-offs at which photons of given energies stop their heating production inside the atmosphere. Assuming 100 eV as the energy borderline between the extreme ultraviolet spectral range and X-rays we find that when the absorbing hydrogen column density is higher than 10 20 cm −2 only X-rays can heat the gas. Extreme ultraviolet photons heat the upper atmospheric layers. Conclusions. Using emission spectra from a sample of solar-type stars of different ages representative of the Sun’s main sequence lifetime, we have derived the corresponding heating rates. We find that the existence of an energetic cross-over in atmospheric heating is present for all stars in the sample.

OPTICAL PROPERTIES OF COMPOSITE INTERSTELLAR GRAINS: A MORPHOLOGICAL ANALYSIS

In the framework of the transition matrix approach, we calculate the relevant optical properties of cosmic dust grains of amorphous carbon and astronomical silicates, modeled as aggregates of spherical monomers. Two mechanisms of aggregation were considered, producing clusters with different structure and degree of fluffiness: ballistic particle-cluster aggregation (BPCA) and ballistic cluster-cluster aggregation (BCCA). Our results are very different from those obtained through computational approaches based on effective medium theories and might have major implications both on the modeling procedure and on the dust-mass balance in the interstellar medium.

Dehydrogenated polycyclic aromatic hydrocarbons and UV bump (Research Note)

Context. Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of polycyclic aromatic hydrocarbons (PAHs) in several ionisation states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C24Hn, n ≤ 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. Aims. To assess the observational consequences of the latter hypothesis we have undertaken a systematic theoretical study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. Methods. We performed calculations using state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time-dependent DFT to evaluate the electronic excited-state properties. Results. We computed the absorption cross-section of the species C24Hn (n = 12, 10, 8, 6, 4, 2, 0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fully dehydrogenated counterparts. Conclusions. π-electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the π → π ∗ collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.

Beyond Mie Theory: The Transition Matrix Approach in Interstellar Dust Modeling

We model cosmic dust grains as aggregates (clusters) of spheres of appropriate geometry, whose optical properties we calculate in the framework of the transition matrix method. The calculation is performed without resorting to any approximation and with a computational effort that is noticeably lighter than the one required by other methods. Whatever the geometry chosen to model the cosmic grains, the orientational averages that are necessary to describe the propagation of the electromagnetic radiation through a dispersion of clusters are easily handled by exploiting the transformation properties of the transition matrix elements under rotation of the coordinate frame. In this paper we focus on the potentialities of the cluster model by comparing the extinction spectrum of a sphere of astronomical silicates with those of aggregates containing the same mass of silicates and composed of up to 12 spheres. Our main result is that, when a given mass of silicates is subdivided into clustering spheres, the extinction increases in regions of the spectrum determined by the degree of subdivision. We also show to what extent the substitution of the material of some of the clustering spheres with the same volume of carbon changes the extinction signature of the clusters. Finally we show to what extent modification of the geometry of the clusters produces detectable changes in their optical signatures. Detailed analysis of our results leads us to the conclusion that modeling the dust grains as clusters of a single morphology is not sufficient to describe the extinction in the whole wavelength range of astrophysical interest. The cluster model may help emphasize the decisive role of morphology in the identification of sustainable structures for dust grains in the typical physical and chemical conditions of the diffuse interstellar medium.

SOFT X-RAY IRRADIATION OF METHANOL ICE: FORMATION OF PRODUCTS AS A FUNCTION OF PHOTON ENERGY

Pure methanol ices have been irradiated with monochromatic soft X-rays of 300 and 550 eV close to the 1s resonance edges of C and O, respectively, and with a broadband spectrum (250-1200 eV). The infrared (IR) spectra of the irradiated ices show several new products of astrophysical interest such as CH2OH, H2CO, CH4, HCOOH, HCOCH2OH, CH3COOH, CH3OCH3, HCOOCH3, and (CH2OH)2, as well as HCO, CO, and CO2. The effect of X-rays is the result of the combined interactions of photons and electrons with the ice. A significant contribution to the formation and growth of new species in the CH3OH ice irradiated with X-rays is given by secondary electrons, whose energy distribution depends on the energy of X-ray photons. Within a single experiment, the abundances of the new products increase with the absorbed energy. Monochromatic experiments show that product abundances also increase with the photon energy. However, the abundances per unit energy of newly formed species show a marked decrease in the broadband experiment as compared to irradiations with monochromatic photons, suggesting a possible regulatory role of the energy deposition rate. The number of new molecules produced per absorbed eV in the X-ray experiments has been compared to those obtained with electron and ultraviolet (UV) irradiation experiments.

Stellar X-ray heating of planet atmospheres

Aims. To investigate the effects of the stellar X-ray irradiation on planet atmospheres, we study the X-ray transfer and energy deposition in a hydrogen rich gas. Methods. We construct an accurate X-ray transfer model taking both photoionization and Compton scattering into account; the electron energy deposition is followed by tracking the discrete exchange processes between electrons and the gas mixture. Results. Exospheric heating rates are derived as functions of the pressure in model atmospheres using a wide range of X-ray luminosity, spectral hardness representative of different stellar ages, and distances from the parent star. The computed heating rates suggest that X-ray irradiation might be an important heating source in planetary exospheres even at large distances from the parent star.

Role of clays in protecting adsorbed DNA against X-ray radiation

We studied the effects of soft X-rays radiation on free and clay (montmorillonite, kaolinite) adsorbed DNA. The DNA samples were exposed to X-rays of 1.49, 4.51 and 8.04 keV for exposure times ranging from 2 min up to 16 h. The biological transformation technique was used to estimate the damage of the DNA molecules. Free and clay adsorbed DNA are differently affected by X-rays. The former is damaged by X-rays and the level of damage depends on the energy dose rather than the hardness of the radiation. The clay adsorbed DNA is not damaged by X-rays for energy doses up to 5.8×10 4 erg. Clays materials could have protected the building blocks of life on the primordial Earth when the solar X-ray emission was much stronger than today.