Two-stage fragmentation in a rotating protostellar cloud

We discuss the problem of fragmentation in a rotating cloud, showing that different physical models can result in widely different results. We also show that results of our turbulent-rotation model agree qualitatively with those of numerical calculations by Larson (1978a) and suggest that fragmentation in a rotating nebula takes place at two different stages.     

Fragmentation of a nonisothermal protostellar cloud

The collapse of a very low thermal energy, rotating cloud results in fragmentation to a binary protostellar system even in the nonisothermal regime. The solar system therefore probably did not form from a fragmentation hierarchy involving ejection of the presolar nebula from a multiple system.     

The magnetic field of a rotating cloud and magneto-rotational explosions

The contraction of a massive rotating plasma cloud with the magnetic field perpendicular to the axis of rotation and extending to infinity is considered. At some stage the contracting cloud reaches a state of dynamic quasi-stationary equilibrium. The change of the magnetic field in the cloud atmosphere before its arrival at the quasi-stationary state (stage I) and also in the process of quasi-equilibrium (stage II) are studied.At stage I an essential change of the external magnetic field geometry occurs, namely the formation of zero (neutral) lines and the transformation of the field into a quasi-radial one. Given certain conditions, the reconnection of the field lines in neutral X-type points may occur with the formation of closed loops. In this case the flux of field lines, which connect the contracting cloud with infinity, decreases asymptotically as (R/R _i)^2/3, whereR/R _i is the ratio of the present radius to initial one.After the cloud arrives at the state of dynamic equilibrium (stage II) a considerable increasing of the magnetic field occurs due to twisting of the field lines by rotation. The field strength increases up to some threshold after which instability suddenly occurs. As a result of cumulation occurring in the zero-line direction, and the subsequent dynamic dissipation, the ejection of relativistic particles and plasma in both directions along the rotational axis takes place. The magnetic field restores itself rapidly due to the continual twisting and this leads to the appearance of repeated explosions.The tension of the magnetic field lines as well as plasma outflow carry away the angular momentum. Its diminution determines the rate of secular gravitational contraction. During the contraction the rotational energy increases, so that recurrent bursts, being of magneto-rotational nature are based, finally, on the gravitational energy reservoir.According to our calculations of the time-interval for repetition of explosions, the energy output and certain other parameters, we are able to explain repeated bursts in the nuclei of galaxies and quasars observed, in particular, in the appearance of radio-variability.Extended version of the paper read at All-Moscow Astrophysical Seminar in Sternberg Astronomical Institute 3rd April 1969.     

The dynamics of dust in a collapsing protostellar cloud and its possible role in planet formation

First results are presented of a calculation describing the collapse of a rotating dusty protostellar cloud. The dust and gas components are calculated separately, although their interaction (e.g., radiation transport, friction, etc.) is taken into account. In the early stages of the collapse the dust is dynamically unimportant. The evolution of the dust cloud is strongly influenced by dust-dust collisions: rapid sedimentation into an equatorial dust disc is found to take place as a result of accumulative dust-dust collisions and the corresponding grain growth. Treating the dust separately from the gas allows us to compare our results with solar-system cosmochemical measurements, with celestial mechanics information and to draw conclusions about the time and place of planet formation in the collapsing cloud.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

The flow field in a force-free magnetic field

We have obtained a complete set of the zeroth-order equations for a force-free field at large magnetic Reynolds numbers. One of the equations has often been overlooked in previous works. We discuss the question of uniqueness of solution of the Cauchy problem and outline a general method of solution in the plane and axisymmetric cases.     

SMA observations of the magnetic fields around a low-mass protostellar system

Observations of the submillimeter polarized dust emission is an important tool to study the role of the magnetic fields in the evolutions of molecular clouds and in the star formation processes. The Submillimeter Array (SMA) is the first imaging submillimeter interferometer. The installation of quarter wave plates in front of the 345 GHz receivers has allowed to carry out polarimetric observations. We present high angular resolution 345 GHz SMA observations of polarized dust emission towards the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this system the observed magnetic field morphology is in agreement with the standard theoretical models of formation of low-mass stars in magnetized molecular clouds at scales of a few hundred AU; gravity has overcome magnetic support and the magnetic field traces a clear hourglass shape. The magnetic field is substantially more important than turbulence in the evolution of the system and the initial misalignment of the magnetic and spin axes may have been important in the formation of the binary system.     

Matter infall in collapsing molecular cloud cores with an axial magnetic field

The magnetic fields affect collapse of molecular cloud cores. Here, we consider a collapsing core with an axial magnetic field and investigate its effect on infall of matter and formation of accretion disk. For this purpose, the equations of motion of ions and neutral infalling particles are numerically solved to obtain the streamlines of trajectories. The results show that in a non-steady state of ionization and ion–neutral coupling, which is not unexpected in the case of infall, the radius of accretion disk will be larger as a consequence of axial magnetic field.     

CO abundances in a protostellar cloud: freeze-out and desorption in the envelope and outflow of L483

CO isotopes are able to probe the different components in protostellar clouds. These components, core, envelope and outflow have distinct physical conditions, and sometimes more than one component contributes to the observed line profile. In this study, we determine how CO isotope abundances are altered by the physical conditions in the different components. We use a 3D molecular line transport code to simulate the emission of four CO isotopomers, ¹²CO   J = 2 → 1, ¹³CO J = 2 → 1  , C¹⁸O   J = 2 → 1  and C¹⁷O   J = 2 → 1  from the Class 0/1 object L483, which contains a cold quiescent core, an infalling envelope and a clear outflow. Our models replicate James Clerk Maxwell Telescope (JCMT) line observations with the inclusion of freeze-out, a density profile and infall. Our model profiles of ¹²CO and ¹³CO have a large linewidth due to a high-velocity jet. These profiles replicate the process of more abundant material being susceptible to a jet. C¹⁸O and C¹⁷O do not display such a large linewidth as they trace denser quiescent material deep in the cloud.     

The density and magnetic field of the dense cores of the molecular cloud L 1630

The density and magnetic field strength of the dense cores in the Orion B molecular cloud are derived from the observed radius and FWHM line width based on the model of a uniformly magnetic sphere. We obtain the average magnetic field strength of 110μG and the average density of 8 × 10⁴/cm³ for the 39 cores, which agree closely with the observations. The method for deriving the density and magnetic field strength is applicable to the cores with R>0.2pc.     

The interstellar chemistry of protostellar disks

A study has been undertaken of the gas-grain chemistry of protostellar disks which are sufficiently cool that in the outer regions, where the gas density is less than 10¹³ cm^–3 and the ionization rate highest, a bimolecular chemistry resembling that of dark clouds can occur. Since the gas-grain collision rate is so high, outgassing mantle molecules effectively determine the gas phase composition at any position in the disk. In contrast to previous work, a detailed gas phase chemistry is considered along with the accretion and desorption of mantle species which is controlled locally by the dust temperature.     

Line formation in a magnetic field

The effect of noncoherent scattering is examined for an absorption line formed in a uniform magnetic field. It is shown that the Stokes parameters of the line radiation may be computed by using the line source function in the absence of a magnetic field as a first approximation for that in the presence of a magnetic field.     

The mean energy of a decaying magnetic field in MHD-turbulence

A magnetic field which is contained in a turbulent, electrically conducting fluid is supposed to decay. The decay time of the mean energy of the fluctuating part of the magnetic field – in contrast with the energy of the mean field – is shown to be determined by the molecular magnetic diffusivity and the correlation length of the turbulence. The investigation is carried out for weak magnetic fields in homogeneous isotropic mirror-symmetric turbulence and in the framework of the second-order correlations approximation.     

The ion H 3 + in a strong magnetic field

A first detailed study of the low-lying electronic states of the H ₃ ⁺ molecular ion in linear configuration, parallel to a magnetic field, is carried out for B=0–4.414×10¹³ G in the Born-Oppenheimer approximation. The variational method is employed with a single, physically adequate trial function which includes, in particular, explicitly the electronic correlation term in the form exp?(γ r ₁₂), where γ is a variational parameter. The state of the lowest total energy (ground state) depends on the magnetic field strength. It evolves from spin-singlet ¹Σ _g for small magnetic fields B?5×10⁸ G to weakly-bound spin-triplet ³Σ _u for intermediate fields and eventually to spin-triplet ³Π _u state for B?5×10¹⁰ G.     

Pair production in a strong time-depending magnetic field: The effect of a strong gravitational field

We present the calculation of the probability production of an electron–positron pair in the presence of a strong magnetic field with time-varying strength. The calculation takes into account the presence of a strong, constant and uniform gravitational field in the same direction of the magnetic field. The results show that the presence of the gravitational field in general enhances very much the production of pairs. In particular, high-energy pairs are more likely produced in the presence of the gravitational field than in Minkowski spacetime.     

The periodicity of a charged particle motion in a magnetic dipole field

The author's purpose is to present in this article the evidence of his intuition on the periodic behavior of a charged particle, moving in a magnetic dipole field, that is achieved by adopting the Alfvén's analysis of the motion into the composite of two motions and studying the contribution of these motions into the general behavior of the charged particle, separately.     

The large-scale solar magnetic field

The large-scale photospheric magnetic field, measured by the Mt. Wilson magnetograph, has been analyzed in terms of surface harmonics (P _n ^m )()cosm and P _n ^m ()sinm) for the years 1959 through 1972. Our results are as follows. The single harmonic which most often characterized the general solar magnetic field throughout the period of observation corresponds to a dipole lying in the plane of the equator (2 sectors, n = m = 1). This 2-sector harmonic was particularly dominant during the active years of solar cycles 19 and 20. The north-south dipole harmonic (n = 1, m = 0) was prominent only during quiet years and was relatively insignificant during the active years. (The derived north-south dipole includes magnetic fields from the entire solar surface and does not necessarily correlate with either the dipole-like appearance of the polar regions of the Sun or with the weak polar magnetic fields.) The 4-sector structure (n = m = 2) was prominent, and often dominant, at various times throughout the cycle. A 6-sector structure (n = m = 3) occasionally became dominant for very brief periods during the active years. Contributions to the general solar magnetic field from harmonics of principal index 4 n 9 were generally relatively small throughout this entire solar cycle with one outstanding exception. For a period of several months prior to the large August 1972 flares, the global photospheric field was dominated by an n = 5 harmonic; this harmonic returned to a low value shortly after the August 1972 flare events. Rapid changes in the global harmonics, in particular, relative and absolute changes in the contributions of harmonics of different principal index n to the global field, imply that the global solar field is not very deep or that very strong fluid flows connect the photosphere with deeper layers.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The charged particle distribution due to emission in a magnetic field

Charged particles emission in a magnetic field on the basis of Boltzman equation with source term is studied. The work develops the results of Narasimha (1962), where the neutral gas expansion into a vacuum is studied. The different specific cases of emission are given for moving sources. The results may be of use when we study phenomena which happen in plasma close to the Earth and in space, as well as for studies of phenomena which take place due to moving satellites in the ionosphere and in interplanetary or interstellar plasma.     

The magnetic field in the solar atmosphere

This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.