Silvia Casu

FORMATION PUMPING OF MOLECULAR HYDROGEN IN DARK CLOUDS

Many theoretical and laboratory studies predict H2 to be formed in highly excited rovibrational states. The consequent relaxation of excited levels via a cascade of infrared transitions might be observable in emission from suitable interstellar regions. In this work, we model H2 formation pumping in standard dense clouds, taking into account the H/H2 transition zone, through an accurate description of chemistry and radiative transfer. The model includes recent laboratory data on H2 formation, as well as the effects of the interstellar UV field, predicting the populations of gas-phase H2 molecules and their IR emission spectra. Calculations suggest that some vibrationally excited states of H2 might be detectable toward lines of sight where significant destruction of H2 occurs, such as X-ray sources, and provides a possible explanation as to why observational attempts resulted in no detections reported to date.

Sardinia Radio Telescope wide-band spectral-polarimetric observations of the galaxy cluster 3C 129

We present new observations of the galaxy cluster 3C 129 obtained with the Sardinia Radio Telescope in the frequency range 6000-7200 MHz, with the aim to image the large-angular-scale emission at high-frequency of the radio sources located in this cluster of galaxies. The data were acquired using the recently-commissioned ROACH2-based backend to produce full-Stokes image cubes of an area of 1 deg x 1 deg centered on the radio source 3C 129. We modeled and deconvolved the telescope beam pattern from the data. We also measured the instrumental polarization beam patterns to correct the polarization images for off-axis instrumental polarization. Total intensity images at an angular resolution of 2.9 arcmin were obtained for the tailed radio galaxy 3C 129 and for 13 more sources in the field, including 3C 129.1 at the galaxy cluster center. These data were used, in combination with literature data at lower frequencies, to derive the variation of the synchrotron spectrum of 3C 129 along the tail of the radio source. If the magnetic field is at the equipartition value, we showed that the lifetimes of radiating electrons result in a radiative age for 3C 129 of t_syn = 267 +/- 26 Myrs. Assuming a linear projected length of 488 kpc for the tail, we deduced that 3C 129 is moving supersonically with a Mach number of M=v_gal/c_s=1.47. Linearly polarized emission was clearly detected for both 3C 129 and 3C 129.1. The linear polarization measured for 3C 129 reaches levels as high as 70% in the faintest region of the source where the magnetic field is aligned with the direction of the tail.

OBSERVATIONAL EVIDENCE OF DUST EVOLUTION IN GALACTIC EXTINCTION CURVES

Although structural and optical properties of hydrogenated amorphous carbons are known to respond to varying physical conditions, most conventional extinction models are basically curve fits with modest predictive power. We compare an evolutionary model of the physical properties of carbonaceous grain mantles with their determination by homogeneously fitting observationally derived Galactic extinction curves with the same physically well-defined dust model. We find that a large sample of observed Galactic extinction curves are compatible with the evolutionary scenario underlying such a model, requiring physical conditions fully consistent with standard density, temperature, radiation field intensity, and average age of diffuse interstellar clouds. Hence, through the study of interstellar extinction we may, in principle, understand the evolutionary history of the diffuse interstellar clouds.

CHEMISTRY IN DIFFUSE CLOUDS WITH TRANSIENT MICROSTRUCTURE

Microstructure is observed on many lines of sight in the diffuse interstellar medium, mainly through variations in atomic line absorptions on timescales of a decade or less. This timescale implies that microstructure exists on a size scale comparable with that of the solar system; it is overpressured and transient. Both observations and theory confirm that a specific chemistry occurs in microstructure. We therefore explore a model of diffuse interstellar gas in which the chemistry in diffuse clouds is supplemented by chemistry in many transient and tiny perturbations. These perturbations are here assumed to be of unidentified origin, but it is assumed that ambipolar diffusion occurs within them. For plausible physical parameters, we find that this model can account for the range of molecular column densities observed in diffuse clouds, including species not usually accounted for by conventional models. Some molecular ions, predicted to be generated in the microstructure (including HS+, CH+ 2, CH+ 3, H2O+, and H3O+) but not yet observed in diffuse clouds, should be present at levels that may allow their detection.

EXCITATION OF C2 IN DIFFUSE INTERSTELLAR CLOUDS

We investigate the effects and the implications of incorporating new collision and radiative rates in modeling the excitation of diatomic carbon molecule. The present results suggest that diffuse and translucent interstellar clouds may present a structure in which regions with different densities and kinetic temperatures overlap along the line of sight, such as core-halo clouds, the nested structure of the molecular gas, and clumpiness. Such conclusion reflects the response of the C2 rotational ladder to the interplay of thermal and radiative conditions, with low and high rotational levels tracing different regions of the parameter space. To relieve constraints to the formation and excitation of C2 molecules, we propose a scenario in which the chemistry in diffuse clouds is supplemented by chemistry in many transient and tiny perturbations.

Detection of CS emission towards Cygnus OB2 No. 12

Aims.The CS (J = 2-1) line was searched at locations with brightest 13CO (J=1-0) emissions in the region in front of the star Cyg OB2 No. 12. Methods.The observations were made with the 20-m millimeter wave telescope at Onsala using a 1024-channel autocorrelator with a resolution of 50 kHz (0.14 km s-1). Results.The line was detected only in one position (core B) with main beam brightness temperature

Retrieving physical conditions from interstellar H2 emission lines: a non linear fitting technique

T_{\rm MB} = 0.15(2)

MODELING DUST IN THE MAGELLANIC CLOUDS

K at

The Sardinia Radio Telescope (SRT): A large modern radio telescope for observations from meter to mm wavelengths

v_{\rm LSR} = 11.4(1)

MODELING EXTRAGALACTIC EXTINCTION THROUGH GAMMA-RAY BURST AFTERGLOWS

km s-1. Conclusions.The line excitation analysis suggests that core B could be a Bok globule in consideration of its morphology, chemistry and physical conditions.