Intergalactic medium metal enrichment through dust sputtering

We study the motion of dust grains into the intergalactic medium (IGM) around redshift   z = 3  , to test the hypothesis that grains can efficiently pollute the gas with metals through sputtering. We use the results available in the literature for radiation-driven dust ejection from galaxies as initial conditions and follow the motion onwards. Via this mechanism, grains are ejected into the IGM with velocities  >100 km s⁻¹  ; as they move supersonically, grains can be efficiently eroded by non-thermal sputtering. However, Coulomb and collisional drag forces effectively reduce the charged grain velocity. Up-to-date sputtering yields for graphite and silicate (olivine) grains have been derived using the code transport of ions in matter ( trim ), for which we provide analytic fits. After training our method on a homogeneous density case, we analyse the grain motion and sputtering in the IGM density field as derived from a Λ cold dark matter (CDM) cosmological simulation at   z = 3.27  . We found that only large  ( a ≳ 0.1μm)  grains can travel up to considerable distances (few  ×100 kpc  physical) before being stopped. Resulting metallicities show a well-defined trend with overdensity δ. The maximum metallicities are reached for  10 < δ < 100  [corresponding to systems, in quasi-stellar object (QSO) absorption spectra, with  14.5 < log N (H  i ) < 16  ]. However the distribution of sputtered metals is very inhomogeneous, with only a small fraction of the IGM volume polluted by dust sputtering (filling factors of 18 per cent for Si and 6 per cent for C). For the adopted size distribution, grains are never completely destroyed; nevertheless, the extinction and gas photoelectric heating effects resulting from this population of intergalactic grains are well below current detection limits.     

Glimpsing through the high-redshift neutral hydrogen fog

We analyse the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ z _em≤ 6.42 to obtain tighter constraints on the volume-averaged neutral hydrogen fraction, x _{H  i} , at z ≈ 6. We study separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with a semi-analytical model of the Lyα forest, we conclude that x _{H  i} evolves smoothly from 10^−4.4 at   z = 5.3  to 10^−4.2 at   z = 5.6  , with a robust upper limit x _{H  i} < 0.36 at   z = 6.3  . The frequency and physical sizes of the peaks imply an origin in cosmic underdense regions and/or in H  ii regions around faint quasars or galaxies. In one case (the intervening H  ii region of the faint quasar RD J1148+5253 at   z = 5.70  along the line of sight of SDSS J1148+5251 at   z = 6.42  ) the increase of the peak spectral density is explained by the first-ever detected transverse proximity effect in the H  i Lyα forest; this indicates that at least some peaks result from a locally enhanced radiation field. We then obtain a strong lower limit on the foreground QSO lifetime of t _Q > 11 Myr. The observed widths of the peaks are found to be systematically larger than the simulated ones. Reasons for such discrepancy might reside either in the photoionization equilibrium assumption or in radiative transfer effects.     

Boron isotopic variations in fumarolic condensates and thermal waters from Vulcano Island, Italy: Implications for evolution of volcanic fluids

Temporal variation in the isotopic composition of boron has been monitored in fumarolic condensates collected over an extended time period (1970-1996) from La Fossa crater, Volcano Island. We also report comparative boron isotopic data for representative Vulcano lavas and for shallow hydrologic samples (seawater, wells, thermal springs) from the north flank of La Fossa. Combined with concurrent chemical and isotopic (δ¹⁸O, δD) data for the fumaroles, these results indicate that the fumarolic fluids record mixing relations between three distinct fluid end members: (1) a dominantly magmatic fluid (EM1); (2) a mixture of modified seawater with magmatic fluid (EM2); and (3) an aqueous fluid produced from seawater by extensive wall-rock reaction, evaporation, and boiling (AF). Differences between the latter two end members are most clearly accentuated on the basis of the boron isotopic data. Long-term compositional variations for crater fumaroles were dominated by EM1-AF mixing between 1979-88, with progressive decrease in EM1 contribution with time, and by EM2-AF mixing between 1988-96. The exact spatial distribution of these fluid reservoirs remains unclear, but all must have been present throughout the monitoring period to account for the observed variations. Moreover, the combined B-O-H data seem to preclude important contributions from shallow meteoric reservoirs. Marked short-term variations in δ¹¹B closely coincide with episodes of local seismicity, which presumably triggered reorganization of hydrothermal circulation patterns; gradual variations over periods up to 3-4 years are associated with relatively low seismicity during which fluid circulation was likely influenced by effects of mineral precipitation on permeability of the hydrologic system.     

The 6 h climatology of thunderstorms and rainfalls in the Friuli Venezia Giulia Plain

Friuli Venezia Giulia is a region located in the North-Eastern part of Italy. It has the Adriatic Sea (Gulf of Trieste) on the South and the Julian and Carnic Alps surrounding it on the North. For these geographical properties thunderstorms and precipitations are common events in the plain of this region.The climatology of thunderstorms and rainfalls, considering 6 h interval periods, is studied in this work. It is shown how the thunderstorm frequency, based on the recording of at least three lightning strikes during the 6 h period, is 16%. The occurrence frequency of at least 1 mm of rain accumulated in 6 h is 24%, while that of at least 5 mm in 6 h is 14%.The daily and monthly distributions of these events are then stratified in three classes, based on their “intensity” (weak, medium and strong), and the different behaviors are analysed. Finally, an explanation for the main monthly rain frequency is sought by looking at only two sounding-derived indices and in particular at their annual cycles. The two indices (related to the potential instability and to the water vapour flux) attempt to summarize the “convective” and “flux” mechanisms for producing rain. It is found that in some particular periods of the year the rain-originating process seems well identifiable, while in many others the two processes seem to be concomitant.     

Mercury exposure and elimination rates in captive bottlenose dolphins

Mercury concentrations in fish, faeces and exhaled air were investigated in order to evaluate total mercury exposure through the gut in captive bottlenose dolphin and excretion via intestine and pulmonary routes. Results showed that faeces account for elimination of 34-48% of dietary mercury; while only 0.9-1.2% of alimentary mercury is eliminated through exhaled air. The remaining 51.2-65.3% of ingested mercury, ranging approximately between 266 and 339 microg per day, is retained within the organism. The complexation of mercury with selenium, forming insoluble tiemannite granules, is discussed as an important mechanism, complementary to excretion, by which odontocetes are able to cope with elevated alimentary exposure to mercury.     

The fragmentation of pre-enriched primordial objects

Recent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. The question then arises which effect was responsible for this transition in the star formation properties. In this paper, we study the effect of metallicity on the evolution of the gas in a collapsing dark matter mini-halo. We model such a system as an isolated 3 σ peak of mass     that collapses at     , using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either     or 10⁻³ Z_⊙. We assume that H₂ has been radiatively destroyed by the presence of a soft UV background. Metals therefore provide the only viable cooling at temperatures below 10⁴ K. We find that the evolution proceeds very differently for the two cases. The gas in the lower metallicity simulation fails to undergo continued collapse and fragmentation, whereas the gas in the higher metallicity case dissipatively settles into the centre of the dark matter halo. The central gas, characterized by densities     , and a temperature,     , that closely follows that of the cosmic microwave background, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity,     , and its possible dependence on redshift. Compared with the pure H/He case, the fragmentation of the     gas leads to a larger relative number of low-mass clumps.