Electric Discharges in the Protoplanetary Nebula as a Source of Impulse Magnetic Fields to Promote Dust Aggregation

Lightning discharge generated in the protoplanetary nebula is viewed as a temporally isolated surge in the flow of electrically charged particles, similar to that of terrestrial lightning. If the current is intense enough, a powerful circular impulse magnetic field is generated around the instantaneous virtual electric conductor. Such magnetic field is capable of magnetizing dust grains containing ferromagnetic components present in its vicinity to their saturation levels. As a result, dust grains attract one another, forming the aggregates. This magnetically driven attraction suggests an important process possibly operational at an early stage of the planetary accretion. Based on both a classical model for electric conductor, and the theory of Lienard–Wiechert electromagnetic potentials, our calculations show that the magnetic impulse due to a discharge channel of a few cm in diameter transferring a charge of about 10⁴ electrons reaches as high as 10 T. At these magnetic fields, the ferromagnetic dust grains, and possibly the already-formed larger aggregates as well, are easily magnetized to the saturation levels, producing compact clusters exhibiting permanent magnetic moments.     

Wave Propagation in Pulsating Stars

Lissajous diagrams are used to detect phase lags between radial velocity curves associated with different ions. Present in many kind of variable stars, this effect provides an important tool to study the atmosphere dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of the Rosette Nebula using the Decameter Wave Radio Telescope at Gauribidanur

A map of Rosette Nebula in continuum absorption is made at 34.5 MHz using the Decameter Wave Radio Telescoe at Gauribidanur, India, with a resolution of 26×40, is presented. These observations are combined with the 2700 MHz measurements of Grahamet al. (1982) to derive the electron temperature distribution across the nebula. It is found that the temperatures in the southeastern parts of the nebula are around 5000 K and increase up to 8000 K towards the northwestern regions. It is suggested that the lower electron temperatures in the southeastern regions are due to the presence of more dust there compared to other regions in the nebula.     

Radio Wave Propagation in the Corona and the Interplanetary Medium

Radio wave propagation through an inhomogeneous, random plasma produces a variety of observable phenomena – group delay, Faraday rotation, refraction, angular broadening, spectral broadening, and scintillations in phase, amplitude, and frequency. These may be exploited to constrain the mean and fluctuating properties of the medium through a variety of remote sensing techniques. In the case of the solar corona and the solar wind, the mean density, magnetic field, solar wind speed, and the spatial spectrum of the density fluctuation scan all be constrained in regions that are inaccessible to in situmeasurements. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wave zone structure of NP 0532 and infrared radiation excess of Crab Nebula

At the distancer?10¹⁵ cm from NP 0532 the plasma concentration decreases so that the intense low-frequency wave (ν=30 Hz) can propagate. The interaction of this wave with the electrons ejected from the pulsar should result in the IR radiation withF _ν～10² fu at λ～10 μ. This flux is the order of the excess IR radiation from the Crab Nebula.     

原行星盘的研究进展

原行星盘是环绕在年轻星天体(如T Tauri型星,HAe/Be星)周围的气体尘埃盘,是具有初始角动量的分子云核在塌缩形成恒星过程中的自然结果,是行星系统的起源地。原行星盘研究不仅是恒星形成理论的重要组成部分,而且是行星形成理论的基础。首先介绍了盘的形成与演化规律;然后介绍了年轻星天体的能谱分布,盘的模型和参数(质量吸积率、质量、尺度、温度、寿命);随后讨论了尘埃颗粒在盘中生长的观测证据以及行星在盘中形成的大致过程;最后对原行星盘研究的现状和未来做了总结与展望。     

Modelling the Planetary Nebula

Deep narrow-band imaging and spectroscopy of the Planetary Nebula (PN)surrounding V4334 Sagittarii have been obtained since the onset of thethermal pulse in the star. Although of low surface brightness,narrow-band imaging suggests a ring structure maybepresent. Abundances, expansion rates and physical parameters for thenebula, derived from spectroscopy, show the plasma to be essentiallytypical of old evolved PN (within the errors of observation). The mostrecent data show little evidence of recombination indicating a verylow plasma electron density.Assuming no recombination has occurred, models of the PN spectrum wereconstructed using both simplistic ideas as well as sophisticatedphoto-ionisation codes. These show good agreement, and suggest that thepre-outburst UV radiation field would need to be powered by aprecursor with a surface temperature of T > 10⁵K. The luminosity ofthis star is less well constrained by the models, and cannot be usedsolely to accurately determine its distance. However, by excludingunrealistic models we expect 250 > L/L_odot > 1000 or so. When plotted ona HR diagram for evolved stars this shows good agreement with thatexpected for a pre-last thermal pulse object.     

Linear MHD Wave Propagation in Time-Dependent Flux Tube

The propagation of magnetohydrodynamic (MHD) waves is an area that has been thoroughly studied for idealised static and steady state magnetised plasma systems applied to numerous solar structures. By applying the generalisation of a temporally varying background density to an open magnetic flux tube, mimicking the observed slow evolution of such waveguides in the solar atmosphere, further investigations into the propagation of both fast and slow MHD waves can take place. The assumption of a zero-beta plasma (no gas pressure) was applied in Williamson and Erdélyi (Solar Phys. 2013, doi: 10.1007/s11207-013-0366-9 , Paper I) is now relaxed for further analysis here. Firstly, the introduction of a finite thermal pressure to the magnetic flux tube equilibrium modifies the existence of fast MHD waves which are directly comparable to their counterparts found in Paper I. Further, as a direct consequence of the non-zero kinetic plasma pressure, a slow MHD wave now exists, and is investigated. Analysis of the slow wave shows that, similar to the fast MHD wave, wave amplitude amplification takes place in time and height. The evolution of the wave amplitude is determined here analytically. We conclude that for a temporally slowly decreasing background density both propagating magnetosonic wave modes are amplified for over-dense magnetic flux tubes. This information can be very practical and useful for future solar magneto-seismology applications in the study of the amplitude and frequency properties of MHD waveguides, e.g. for diagnostic purposes, present in the solar atmosphere.     

Linear MHD Wave Propagation in Time-Dependent Flux Tube

MHD waves and oscillations in sharply structured magnetic plasmas have been studied for static and steady systems in the thin tube approximation over many years. This work will generalize these studies by introducing a slowly varying background density in time, in order to determine the changes to the wave parameters introduced by this temporally varying equilibrium, i.e. to investigate the amplitude, frequency, and wavenumber for the kink and higher order propagating fast magnetohydrodynamic wave in the leading order approximation to the WKB approach in a zero-β plasma representing the upper solar atmosphere. To progress, the thin tube and over-dense loop approximations are used, restricting the results found here to the duration of a number of multiples of the characteristic density change timescale. Using such approximations it is shown that the amplitude of the kink wave is enhanced in a manner proportional to the square of the Alfvén speed, $V_{\mathrm{A}}^{2}$ . The frequency of the wave solution tends to the driving frequency of the system as time progresses; however, the wavenumber approaches zero after a large multiple of the characteristic density change timescale, indicating an ever increasing wavelength. For the higher order fluting modes the changes in amplitude are dependent upon the wave mode; for the m=2 mode the wave is amplified to a constant level; however, for all m≥3 the fast MHD wave is damped within a relatively small multiple of the characteristic density change timescale. Understanding MHD wave behavior in time-dependent plasmas is an important step towards a more complete model of the solar atmosphere and has a key role to play in solar magneto-seismological applications.     

Towards the Frequency and Evolutionary Timescale of Protoplanetary Disks

In this poster we will present preliminary results from our ongoing near-infrared survey of very young clusters, the majority of them still embedded in their parental molecular cloud. We will draw general conclusions on issues of the frequency and timescales for the initial stages of planet formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Primordial substructure in the Orion Nebula Cluster

We use numerical N -body simulations of the Orion Nebula Cluster (ONC) to investigate the possibility of substructure in its formation. There is no substructure apparent in the ONC today. However, unless there was a remarkable degree of homogeneity in the molecular cloud from which it formed, it seems unlikely that this would have been true of the cluster in its earliest phase. More plausibly, the early structure of the cluster would have consisted of groups or clumps of subclusters, following the structure of the cloud itself. We have explored the extent to which such subclusters could subsequently have merged, and find that the age of the cluster is a critical factor. The most inhomogeneous initial conditions, comprising a small number of subclusters with many members, are ruled out by an age of 2 Myr or less. There is a considerable amount of freedom in the other direction, however, which suggests that fragmentation in the original cloud is more likely to have been on the scale of small clumps, each producing fewer than 100 stars. These initial subclusters could have been very dense – perhaps two or three orders of magnitude more dense than the core of the ONC today.     

Dynamics of the Orion Nebula

The collision of supersonic gas streams within anHII region is studied. A system of stationary shock waves is set up. Theoretical line profiles through these shocked regions are compared with those obtained in the bright central regions of the Orion Nebula. There is good agreement between the predicted and observed magnitude of line splitting in the forbidden lines of O⁺ and O⁺⁺.     

Observational Diagnostics of Gas in Protoplanetary Disks

Protoplanetary disks are composed primarily of gas (99% of the mass). Nevertheless, relatively few observational constraints exist for the gas in disks. In this review, I discuss several observational diagnostics in the UV, optical, near-IR, mid-IR, and (sub)-mm wavelengths that have been employed to study the gas in the disks of young stellar objects. I concentrate in diagnostics that probe the inner 20 AU of the disk, the region where planets are expected to form. I discuss the potential and limitations of each gas tracer and present prospects for future research.     

Kink Wave Propagation in Thin Isothermal Magnetic Flux Tubes

We investigated the propagation of kink waves in thin and isothermal expanding flux tubes in cylindrical geometry. By using the method of radial expansion for fluctuating variables we obtained a new kink wave equation. We show that including the radial component of the tube magnetic field leads to cutoff-free propagation of kink waves along thin flux tubes.     

Stellar X-ray Sources in the Rosette Nebula

We present optical photometric and spectroscopic studies of ROSAT X-ray stellar sources in the Rosette Nebula star-forming region. The brightest X-ray sources are either massive stars or active T Tauri stars associated with the open cluster NGC 2244, or are foreground stars. Some of the spectra of the young stars newly identified in the region are presented.     

On the Possible Explosion in the Carina Nebula

The consideration of the additional data on the distribution of OB stars, of the dust matter (the IRAS data at 100 m, and of the results of observations 120–300 m), of the X-ray emission, of the continuum radio emission, and of the data on the proper motions of stars in the Carina nebula, proved the made suggestion (Tovmassian et al., 1993, 1994), that the observed specific distribution of the mentioned objects in the Carina nebula could be due to some explosion which took place in its central region.