Cosmological Evolution of Supergiant Star-Forming Clouds

We present the results of very high resolution CDM simulations of galaxy formation designed to follow the formation and evolution of self-gravitating, supergiant star-forming clouds. We find that the mass spectrum of these clouds is identical to that of globular clusters and GMCs; dN/dM ∝ M ^-1.7 ± 0.1. This revised version was published online in September 2006 with corrections to the Cover Date.     

Seasonal Sensitivity on COBEL-ISBA Local Forecast System for Fog and Low Clouds

Skillful low visibility forecasts are essential for air-traffic managers to effectively regulate traffic and to optimize air-traffic control at international airports. For this purpose, the COBEL-ISBA local numerical forecast system has been implemented at Paris CDG international airport. This local approach is robust owing to the assimilation of detailed local observations. However, even with dedicated observations and initialization, uncertainties remain in both initial conditions and mesoscale forcings. The goal of the research presented here is to address the sensitivity of COBEL-ISBA forecast to initial conditions and mesoscale forcings during the winter season 2002–2003. The main sources of uncertainty of COBEL-ISBA input parameters have been estimated and the evaluation of parameter uncertainty on the forecasts has been studied. A budget strategy is applied during the winter season to quantify COBEL-ISBA sensitivity. This study is the first step toward building a local ensemble prediction system based on COBEL-ISBA. The conclusions of this work point out the potential for COBEL-ISBA ensemble forecasting and quantify sources of uncertainty that lead to dispersion.     

Interactive Aspects of the Indian and the African Summer Monsoon Systems

This study addresses an understanding of the possible mutual interactions of sub-seasonal variability of the two neighboring regional monsoon systems through data analysis. The NCEP/NCAR re-analysis and OLR data for three years was used to reveal the large-scale organization of convective episodes on synoptic (~5 days) and low frequency (15–50 day) scales. It is found that synoptic scale organization over both the sectors is influenced by the eastward migration of large-scale convective episodes associated with the Madden Julian Oscillation (MJO) on the low frequency scale. The organization of convection associated with the African monsoon on the synoptic scale is influenced by the pulsatory character of lower mid-troposphere and upper troposphere wind regimes moving westward over the African sector. Over the Indian region formation of low pressure areas and depressions in the monsoon trough occur in an overlapping manner under an envelope of low frequency seasonal oscillation. We have also found some correspondence between the summer monsoon rainfall over tropical North Africa and India on a decadal basis, which would suggest a common mode of multi-decadal variability in the two monsoon systems. The study points out the need to organize simultaneous field campaigns over the Indian and the African monsoon regions so as to bring out observational features of possible interactions between the two neighboring systems, which could then be validated through modeling studies.     

漯河市降水量与降水云系的关系

采用1956-2006年漯河市观测站资料,运用一元回归分析漯河市降水的气候变化特征,讨论了3类降水云系与降水量的关系,为科学实施人工增雨(雪)作业和提高天气预报水平提供参考。     

A targeted survey for H i clouds in galaxy groups

Five galaxy groups with properties similar to those of the Local Group have been surveyed for H  i clouds with the Arecibo Telescope. In total 300 pointings have been observed on grids of approximately 2.5×1.5 Mpc² centred on the groups. The 4.5 σ detection limit on the minimal detectable H  i masses is approximately 7×10⁶ M_⊙ ( H ₀=65 km s⁻¹ Mpc⁻¹) . All detections could be attributed to optical galaxies; no significant detections of H  i clouds have been made. This null result leads to the conclusion that the total H  i mass of intragroup clouds must be less than 10 per cent of the total H  i mass of galaxy groups and less than 0.05 per cent of the dynamical mass. The recent hypothesis that Galactic high-velocity clouds are Local Group satellite galaxies is highly inconsistent with these observations.     

Carbon recombination lines from the Galactic plane at 34.5 & 328 MHz

We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission. A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular size of the line forming region. The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near 76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions. We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:T _e =20–40 K,n _e ∼ 0.1–0.3 cm⁻³ and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other hand, if the line forming regions are ∼ 2° in extent, then warmer gas (T _e ∼ 60–300 K) with lower electron densities (n _e ∼ 0.03–0.05 cm⁻³) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data. Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation regions.     

Advances in noctilucent cloud research in the space era

A summary of NLC research in the last two decades is presented. Results of NLC studies from the near-Earth space are discussed. It is shown that NLC can cover much larger territories than those estimated earlier and that there exists asymmetry in the coverage and also in the physical properties of NLC in the Northern and Southern Hemispheres. The mesopause often reveals a compelx multilayered structure. The pilot program of NLC research is discussed as a subprogram of the Middle Atmosphere Program and some vistas in NLC research are discussed.     

Calculations on the rates, mechanisms, and interstellar importance of the reactions between C and NH2 and between N and CH2

The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral–neutral reactions thought to be important in the formation of HNC and HCN, respectively – C+NH₂→HNC+H, and N+CH₂→HCN+H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.     

Helical fields and filamentary molecular clouds – I

We study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first apply the virial theorem to filamentary molecular clouds, including the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds.
We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We examine the effects of the equation of state by comparing 'isothermal' filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state.
Our theoretical models involve three parameters: two to describe the mass loading of the toroidal and poloidal fields, and a third that describes the radial concentration of the filament. We thoroughly explore our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r ^−1.8 to r ⁻², while logatropes have density profiles that range from r ⁻¹ to r ^−1.8. We find that purely poloidal fields produce filaments with steep radial density gradients that are not allowed by the observations.