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Search Results: 1 - 10 of 461697 matches for " A. Lazarian "
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A. Lazarian
Revista mexicana de astronomía y astrofísica , 2009,
Alignment of suprathermally rotating grains
A. Lazarian
Physics , 1995, DOI: 10.1093/mnras/277.4.1235
Abstract: It is shown, that mechanical alignment can be efficient for suprathermally rotating grains, provided that they drift with supersonic velocities. Such a drift should be widely spread due to both Alfv\'{e}nic waves and ambipolar diffusion. Moreover, if suprathermal rotation is caused by grain interaction with a radiative flux, it is shown, that mechanical alignment may be present even in the absence of supersonic drift. This means that the range of applicability of mechanical alignment is wider that it is generally accepted and it can rival the paramagnetic one. We also study the latter mechanism and reexamine the interplay between poisoning of active sites and desorption of molecules blocking the access to the active sites of H$_{2}$ formation in order to explain the observed poor alignment of small grains and good alignment of large grains. To have a more comprehensive picture of alignment we briefly discuss the alignment by radiation fluxes and caused by grain magnetic moments.
Turbulence in Atomic Hydrogen
A. Lazarian
Physics , 1998,
Abstract: Understanding the properties of interstellar turbulence is a great intellectual challenge and the urge to solve this problem is partially motivated by a necessity to explain the star formation mystery. This review deals with a recently suggested inversion technique as applied to atomic hydrogen. This technique allows to determine 3D turbulence statistics through the variations of 21 cm intensity. We claim that a radio interferometer is an ideal tool for such a study as its visibility function is directly related to the statistics of galactic HI. Next, we show how galactic rotation curve can be used to study the turbulence slice by slice and relate the statistics given in galactic coordinates and in the velocity space. The application of the technique to HI data reveals a shallow spectrum of the underlying HI density that is not compatible with a naive Kolmogorov picture. We show that the random density corresponding to the found spectrum tends to form low contrast filaments that are elongated towards the observer.
Davis-Greenstein alignment of non-spherical grains
A. Lazarian
Physics , 1995, DOI: 10.1086/176382
Abstract: Paramagnetic alignment of non-spherical dust grains rotating at thermal velocities is studied. The analytical solution is found for the alignment measure of oblate grains. Perturbative approach is used for solving the problem. It is shown that even the first approximation of the suggested iteration procedure provides the accuracy well within one percent of the expected measure of alignment. The results obtained are applicable both to paramagnetic and to superferromagnetic grains.
Mechanical alignment of suprathermally rotating grains
A. Lazarian
Physics , 1995, DOI: 10.1086/176252
Abstract: It is shown that mechanical alignment of grains can be efficient for grains rotating suprathermally, i.e. with kinetic energy substantially exceeding $k$ (the Boltzmann constant) over any temperature in the system. The paper studies suprathermal rotation caused by H$_{2}$ formation and the alignment that takes place due to crossover events. Gaseous bombardment in the course of a crossover as well as both gaseous friction and poisoning of active sites are shown to produce alignment. The first type of alignment happens due to the angular momentum deposited by a corpuscular flux with a grain, the second is caused by the change of the mean time back to crossover due to the interaction with a gaseous flux. We show that the two processes act as to decrease the grain cross section in respect to the flux and we find the Rayleigh reduction factor for the joint action of the two processes as well as the range of applicability of each of the processes. Our study indicates that mechanical alignment can be more widely spread than it is generally accepted.
Paramagnetic alignment of fractal grains
A. Lazarian
Physics , 1994,
Abstract: Paramagnetic alignment of fractal suprathermally rotating grains is discussed. It is shown that if the concentration of H$_{2}$formation sites is low and resurfacing is active, fractal structure of grains enhances their alignment. Studying the influence of grain surface physics and chemistry on the alignment we found that there exist two critical values of grain sizes, and the alignment of smaller grains is expected to decrease as compared to the predictions of the Purcell's theory (1979). One of the critical sizes is due to intensified poisoning of active sites, while the other is a result of a finite value of the imaginary part of magnetic susceptibility in the limit of high angular velocities. On the contrary, if active sites completely cover grain surface, suprathermal rotation, and therefore efficient alignment, is possible only for a limited range of grain sizes.
Paramagnetic alignment of thermally rotating dust
A. Lazarian
Physics , 1997, DOI: 10.1093/mnras/288.3.609
Abstract: Paramagnetic alignment of thermally rotating oblate dust grains is studied analytically for finite ratios of grain to gas temperatures. For such ratios, the alignment of angular momentum J in respect to the grain axis of maximal inertia is only partial. We treat the alignment of J using perturbative methods and disentangle the problem of J alignment in grain body axes from that of J alignment in respect to magnetic field. This enables us to find the alignment of grain axes to magnetic field and thus relate our theory to polarimetric observations.Our present results are applicable to the alignment of both paramagnetic and superparamagnetic grains.
Gold alignment & internal dissipation
A. Lazarian
Physics , 1997, DOI: 10.1086/304201
Abstract: The measures of mechanical alignment were obtained for both prolate and oblate grains when their temperature is comparable with grain kinetic energy devided by k, the Boltzmann constant. For such grains, the alignment of angular momentum, J, with the axis of maximal inertia, a, is only partial. This substantially alters the alignment as compared with the results in Lazarian (1995) and Roberge, Hanany & Messinger (1996) obtained on the assumption of perfect alignment. We also describe the Gold alignment when the Barnett dissipation is suppressed and derive an analytical expression which relates the measure of alignment with parameters of grain nonsphericity and the direction of the gas - grain drift. This solution provides the lower limit for the alignment measure, while the upper limit is given by the analytics derived in Lazarian (1994). Using results of a recent study of incomplete internal relaxation in Lazarian & Roberge (1996), we find measures of alignment for the whole range of ratios of grain rotational energy to k over T_s, where T_s is the grain temperature. To describe alignment for mildly supersonic drifts, we suggest an analytical approach which provides good correspondence with the results of direct numerical simulations in Roberge, Hanany & Messinger (1995). We also extend our approach to account for the simultaneous action of the Gold and Davis-Greenstein mechanisms.
Physics of Grain Alignment
Lazarian A
Physics , 2000,
Abstract: Aligned grains provide one of the easiest ways to study magnetic fields in diffuse gas and molecular clouds. How reliable our conclusions about the inferred magnetic field depends critically on our understanding of the physics of grain alignment. Although grain alignment is a problem of half a century standing recent progress achieved in the field makes us believe that we are approaching the solution of this mystery. I review basic physical processes involved in grain alignment and show why mechanisms that were favored for decades do not look so promising right now. I also discuss why the radiative torque mechanism ignored for more than 20 years looks right now the most powerful means of grain alignment.
Heat Transfer and Reconnection Diffusion in Turbulent Magnetized Plasmas
A. Lazarian
Physics , 2011,
Abstract: It is well known that magnetic fields constrain motions of charged particles, impeding the diffusion of charged particles perpendicular to magnetic field direction. This modification of transport processes is of vital importance for a wide variety of astrophysical processes including cosmic ray transport, transfer of heavy elements in the interstellar medium, star formation etc. Dealing with these processes one should keep in mind that in realistic astrophysical conditions magnetized fluids are turbulent. In this review we single out a single transport process, namely, heat transfer and consider how it occurs in the presence of the magnetized turbulence. We show that the ability of magnetic field lines to constantly change topology and connectivity is at the heart of the correct description of the 3D magnetic field stochasticity in turbulent fluids. This ability is ensured by fast magnetic reconnection in turbulent fluids and puts forward the concept of reconnection diffusion at the core of the physical picture of heat transfer in astrophysical plasmas. Appealing to reconnection diffusion we describe the ability of plasma to diffuse between different magnetized eddies explaining the advection of the heat by turbulence. Adopting the structure of magnetic field that follows from the modern understanding of MHD turbulence, we also discuss thermal conductivity that arises as electrons stream along stochastic magnetic field lines. We compare the effective heat transport that arise from the two processes and conclude that in many astrophysically-motivated cased eddy advection of heat dominates. Finally, we discuss the concepts of sub and superdiffusion and show that the subdiffusion requires rather restrictive settings. At the same time, accelerated diffusion or superdiffusion of heat is possible on the scales less than the injection scale of the turbulence.
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