Session 2
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University of Manchester, Manchester, United Kingdom
We present the first radio map of the 'engraved hourglass nebula', MyCn18, made famous by the WFPC2 HST picture taken by Sahai & Trauger in 1995. The 3 cm observations were made using the six telescopes of the Australia Telescope Compact Array. The double-ring structure of the nebula is resolved.
High-resolution MERLIN maps of NGC 7027 and BD +30 3639 are also presented showing small-scale knotty structure and the presence of hotspots that have been shown to mainly coincide with those in optical images. In some cases the expected optical emission is missing presumably obscured by dust.
Noelle Daly
noelle@astro.gla.ac.uk
University of Glasgow, Great Britain
Interplanetary Scintillation (IPS) is caused by the diffracting nature of the turbulent inner heliosphere. Here, the density and velocity structure of the solar wind can be studied by examining the scintillation experienced by radio waves from distant radio sources, as they propagate through this random medium. Hence, daily maps can be drawn, revealing large-scale density fluctuations in the solar wind. This view of the 'big picture' is a unique one, as in-situ spacecraft can only give a localised measurement. This can also be used to predict the conditions of the near-earth space environment. This predictive power is important in many fields, including the protection of orbiting satellites from dangerously large fluxes of high energy particles which can damage their electrical components.
This phenemenon has been observed by the 3.6 hectare IPS dipole array at Cambridge. For the first time, all the primary data from the 1990-94 IPS survey is available as a single data set, and is currently being analysed at Glasgow. This body of data has now been restructured and cleaned up, taking into account interference, the skewing of the array from perfect alignment, etc. Important matters to be determined are the 'average' scintillation of a particular source, and the characteristic scintillation behaviour of a source in relation to its elongation from the sun. Therefore, areas of enhanced and decreased scintillation can be discerned, and used to produce 3-dimensional maps of structure in the solar wind.
Sona Ehlerová
sona@ig.cas.cz
Astronomical Institute of the Czech Academy of Science, Czech Republic
Energy release from massive stars in OB associations, either in the form of stellar winds or supernova explosions, leads to the creation of shocks expanding around the associations.Evolution of these shocks in a stratified gaseous medium under the influence of the galactic gravitational field is simulated using the infinitesimally thin shell approximation. Numerical simulations are compared with HI observations made by the Effelsberg radiotelescope.
Vincent de Heij
deheij@strw.leidenuniv.nl
Leiden Observatory, The Netherlands
The extend and filamentary structure of the high-lattitude (b > 10 deg.) galactic cirrus has been mapped by several surveys, e.g. the IRAS and DIRBE observations. Because these surveys only measure a part of the continuum radiation, they do not contain any kinematical information of the cirrus. But by using the 21 cm line emission of the diffuse galactic gas, it is possible to get insight into the kinematics of the dust. The gas and dust seems to be correlated quite well, making it possible to unravel the velocity dependence of the continuum emission. Results of the unraveling for several of these high lattitude cirrus complexes, show a great kinematic activity.
Karl Joulain1, Edith Falgarone1,
Guillaume Pineau des
Forets2, and David Flower3
joulain@ensapa.ens.fr
1Radioastronomie, CNRS, Ecole Normale Supérieure, Paris, France
2DAEC, CNRS, Observatoire de Paris, Meudon France
3Physics Department, The University, Durham UK
We study the time-dependent chemical evolution of a piece of atomic gas trapped in a long-lived vortex, characteristic of the dissipative structures of turbulence. Magnetic field is present and is close to be aligned with the vorticity. The chemical evolution is controlled by the sharp temperature rise following the passage through layers where viscous dissipation is intense, and by the ion-neutral drifts in layers where the orthoradial velocity of the neutrals is large. We compare the chemical content of a gas processed by these dissipative structures with observations. In particular, we naturally reproduce the observed and unexplained abundance ratio of OH and HCO+ in diffuse interstellar gas.
Csaba Kiss1, Attila Moor1, Viktor
L. Tóth1 and Silvana Nikolic1, 2
pkisscs@innin.elte.hu
1Dept. of Astronomy, Lóránd Eötvös University,
Budapest, Hungary
2Observatory of Belgrade, Belgrade, Yugoslavia
Optical observations of the dark cloud Khav15 (Khavtassi 1955) resulted in distance and extinction estimations. An extinction layer appear in the direction of the cloud at distance modulus 6.7 mag, i.e., it is located at a distance of 220±60 pc, with an average extinction of AV = 0.35 mag. CO observations revealed a cometary shaped small cloud with a dense 13CO core of 12 MSun with peak column density of N(13CO) = 4 x 1015 cm-2. The total gas mass in Khav15 from 13CO measurements is 27 MSun . The small, non-star-forming dark cloud seems to be connected to a large FIR shell. Its velocity, and density structure reflects a past shock passage, possibly due to the same effect which formed the shell.
Tomasz Kobak and Michal Ostrowski
kto@oa.uj.edu.pl
Jagiellonian University Observatory, Kraków, Poland
The role of MHD turbulence in the cosmic ray acceleration process in a volume with the reconnecting magnetic field is considered by the method of Monte Carlo simulations. We performed simulations of proton acceleration with the 3D analytic model of stationary reconnection of Craig et al. (1995) providing the unperturbed background conditions. Perturbations of particle trajectories due to a turbulent magnetic field component were modeled as the small-amplitude pitch-angle momentum scattering. Comparison of the acceleration process in the model involving particle trajectory perturbations to the unperturbed one reveals that the turbulence can substantially increase the acceleration efficiency, enabling particles to reach much higher final energies.
Vincent Minier
vincent@oso.chalmers.se
Onsala Space Observatory, 439 92 Onsala, Sweden
An extensive survey of northern hemisphere masers for 6.7 GHz methanol emission has been undertaken using the newly equipped Onsala 25m telescope. The 51 -> 60 A+ transition at 6.668 GHz is the strongest Class II methanol maser emission and appears to be a very good tracer of massive star formation regions. It is generally believed that this maser emission arises from circumstellar disc containing warm molecular gas. Observations in Australia have shown that the maser spots lie in lines or arcs and several exhibit velocity gradient indicative of Keplerian rotation in edge-on rotating discs. Recently, we have conducted VLBI observations at 6.7 GHz as well as 12 GHz to investigate these exciting structures of the maser spots through five bright sources never observed by VLBI before. In addition, we will observe at higher frequencies (between 80 and 112 GHz) using the Onsala 20m telescope to verify the theoretical model of infrared pumping mechanisms which explains the existence of 6.7 GHz and 12 GHz masers
Ramachandran Rajagopalan
ramach@nfra.nl
Netherlands Foundation for Research in Astronomy, Dwingeloo, The Netherlands
With the knowledge of many new measurements of scatter broadening of pulse profiles, the dependence of scatter broadening on dispersion measure will be discussed. Distances will be estimated to a few pulsars in certain specific directions in the Galaxy on the basis of the measured interstellar scattering parameters.
Kirill V. Semenkov
kirill@dpc.asc.rssi.ru
Astro Space Center of Lebedev's Physical Institute, Moscow, Russia
The problem of simulation of interstellar scattering effects on image of compact radiosources in framework of one-dimensional phase screen model solves.
Software simulating this process is written. As a result:
At present we work on appliance of the simulations to explain observations.
Ruslan Shalashov
vay@rf.unn.runnet.ru
Nizny Novgorod State University, Russia
The investigation of partial polarisation of the galactic synchrotron radio emission is an effective tool to get information on the interstellar medium and magnetic field in the Galaxy. The interpretation of polarisation measurement results leads to the problems of two types: direct and inverse. The direct problems in polarimetry of the galactic radio emission consist in the consideration of generation and propagation of radiation on the basis of some models on galactic spatial distribution of relativistic electrons with a given energetic spectrum, ionised interstellar gas and magnetic fields. The inverse problem consist in a direct determination of parameters of galactic synchrotron radio emission sources, the characteristics of interstellar ionised gas (concentration, spatial distribution) and the structure of the magnetic field in the Galaxy according to observations intensity and linear polarisation of cosmic radio emission. Here by virtue of an ambiguity of inverse problem solutions it is necessary to use a priori information on physical conditions in the interstellar space. Correct solutions of inverse problems give in principle more perfect interpretation of the experimental data. There exist different methods to solve the inverse problems in different frequency bands. When solving inverse problems the essential data on the galactic magnetic field and interstellar medium can be obtained using the correlation functions of the sky brightness temperature distribution and polarisation parameters of the galactic radio emission.