TURBULENCE STATISTICS FROM SPECTRAL LINE OBSERVATIONS

LAZARIAN A. The Korean Astronomical Society 2004 Journal of The Korean Astronomical Society Vol.37 No.5

Turbulence is a crucial component of dynamics of astrophysical fluids dynamics, including those of ISM, clusters of galaxies and circumstellar regions. Doppler shifted spectral lines provide a unique source of information on turbulent velocities. We discuss Velocity-Channel Analysis (VCA) and its offspring Velocity Coordinate Spectrum (VCS) that are based on the analytical description of the spectral line statistics. Those techniques are well suited for studies of supersonic turbulence. We stress that a great advantage of VCS is that it does not necessary require good spatial resolution. Addressing the studies of mildly supersonic and subsonic turbulence we discuss the criterion that allows to determine whether Velocity Centroids are dominated by density or velocity. We briefly discuss ways of going beyond power spectra by using of higher order correlations as well as genus analysis. We outline the relation between Spectral Correlation Functions and the statistics available through VCA and VCS.

Synchrotron Intensity Gradients as Tracers of Interstellar Magnetic Fields

Lazarian, A.,Yuen, Ka Ho,Lee, Hyeseung,Cho, J. American Astronomical Society 2017 The Astrophysical journal Vol.842 No.1

<P>On the basis of the modern understanding of MHD turbulence, we propose a new way of using synchrotron radiation: using synchrotron intensity gradients (SIGs) for tracing astrophysical magnetic fields. We successfully test the new technique using synthetic data obtained with 3D MHD simulations and provide the demonstration of the practical utility of the technique by comparing the directions of magnetic fields that are obtained with PLANCK synchrotron intensity data to the directions obtained with PLANCK synchrotron polarization data. We demonstrate that the SIGs can reliably trace magnetic fields in the presence of noise and can provide detailed maps of magnetic field directions. We also show that the SIGs are relatively robust for tracing magnetic fields while the low spatial frequencies of the synchrotron image are removed. This makes the SIGs applicable to the tracing of magnetic fields using interferometric data with single-dish measurement absent. We discuss the synergy of using the SIGs together with synchrotron polarization in order to find the actual direction of the magnetic fields and quantify the effects of Faraday rotation as well as with other ways of studying astrophysical magnetic fields. We test our method in the presence of noise and the resolution effects. We stress the complementary nature of the studies using the SIG technique and those employing the recently introduced velocity gradient techniques that trace magnetic fields using spectroscopic data.</P>

POLARIMETRIC STUDIES OF MAGNETIC TURBULENCE WITH AN INTERFEROMETER

Lee, Hyeseung,Lazarian, A.,Cho, Jungyeon American Astronomical Society 2016 The Astrophysical journal Vol.831 No.1

<P>We study statistical properties of synchrotron polarization emitted from media with magnetohydrodynamic (MHD) turbulence. We use both synthetic and MHD turbulence simulation data for our studies. We obtain the spatial spectrum and its derivative with respect to the wavelength of synchrotron polarization arising from both synchrotron radiation and Faraday rotation fluctuations. In particular, we investigate how the spectrum changes with frequency. We find that our simulations agree with the theoretical predication in Lazarian & Pogosyan. We conclude that the spectrum of synchrotron polarization and its derivative can be very informative tools to obtain. detailed information about the statistical properties of MHD turbulence from radio observations of diffuse synchrotron polarization. They are especially useful for. recovering. the statistics of a turbulent magnetic field as well as the turbulent density of electrons. We also simulate interferometric observations that incorporate the effects of noise and finite telescope beam size, and demonstrate how we recover statistics of underlying MHD turbulence.</P>

ALFVÉNIC TURBULENCE BEYOND THE AMBIPOLAR DIFFUSION SCALE

Burkhart, Blakesley,Lazarian, A.,Balsara, D.,Meyer, C.,Cho, J. IOP Publishing 2015 The Astrophysical journal Vol.805 No.2

<P>We investigate the nature of the Alfvenic turbulence cascade in two-fluid magnetohydrodynamic (MHD) simulations in order to determine if turbulence is damped once the ion and neutral species become decoupled at a critical scale called the ambipolar diffusion scale (L-AD). Using mode decomposition to separate the three classical MHD modes, we study the second-order structure functions of the Alfven mode velocity field of both neutrals and ions in the reference frame of the local magnetic field. On scales greater than L-AD we confirm that two-fluid turbulence strongly resembles single-fluid MHD turbulence. Our simulations show that the behavior of two-fluid turbulence becomes more complex on scales less than L-AD. We find that Alfvenic turbulence can exist past L-AD when the turbulence is globally super-Alfvenic, with the ions and neutrals forming separate cascades once decoupling has taken place. When turbulence is globally sub-Alfvenic and hence strongly anisotropic, with a large separation between the parallel and perpendicular decoupling scales, turbulence is damped at L-AD. We also find that the power spectrum of the kinetic energy in the damped regime is consistent with a k(-4) scaling (in agreement with the predictions of Lazarian et al.).</P>

Alignment of Irregular Grains by Mechanical Torques

Hoang, Thiem,Cho, Jungyeon,Lazarian, A. American Astronomical Society 2018 The Astrophysical journal Vol.852 No.2

<P>We study the alignment of irregular dust grains by mechanical torques due to the drift of grains through the ambient gas. We first calculate mechanical alignment torques (MATs) resulting from specular reflection of gas atoms for seven irregular shapes: one shape of mirror symmetry, three highly irregular shapes (HIS), and three weakly irregular shapes (WIS). We find that the grain with mirror symmetry experiences negligible MATs due to its mirror-symmetry geometry. Three HIS can produce strong MATs, which exhibit some generic properties as radiative torques (RATs), while three WIS produce less efficient MATs. We then study grain alignment by MATs for the different angles between the drift velocity and the ambient magnetic field, for paramagnetic and superparamagnetic grains assuming efficient internal relaxation. We find that for HIS grains, MATs can align subsonically drifting grains in the same way as RATs, with low-J and high-J attractors. For supersonic drift, MATs can align grains with low-J and high-J attractors, analogous to RAT alignment by anisotropic radiation. We also show that the joint action of MATs and magnetic torques in grains with iron inclusions can lead to perfect MAT alignment. Our results point out the potential importance of MAT alignment for HIS grains predicted by the analytical model of Lazarian & Hoang, although more theoretical and observational studies are required due to uncertainty in the shape of interstellar grains. We outline astrophysical environments where MAT alignment is potentially important.</P>

Statistical Tracing of Magnetic Fields: Comparing and Improving the Techniques

Yuen, Ka Ho,Chen, Junda,Hu, Yue,Ho, Ka Wai,Lazarian, A.,Lazarian, Victor,Yang, Bo,Burkhart, Blakesley,Correia, Caio,Cho, Jungyeon,Canto, Bruno,Medeiros, J. R. De American Astronomical Society 2018 The Astrophysical journal Vol.865 No.1

STRINGENT LIMITS ON THE POLARIZED SUBMILLIMETER EMISSION FROM PROTOPLANETARY DISKS

Hughes, A. Meredith,Wilner, David J.,Cho, Jungyeon,Marrone, Daniel P.,Lazarian, Alexandre,Andrews, Sean M.,Rao, Ramprasad IOP Publishing 2009 The Astrophysical journal Vol.704 No.2

<P>We present arcsecond-resolution Submillimeter Array (SMA) polarimetric observations of the 880 mu m continuum emission from the protoplanetary disks around two nearby stars, HD 163296 and TW Hydrae. Although previous observations and theoretical work have suggested that a 2%-3% polarization fraction should be common for the millimeter continuum emission from such disks, we detect no polarized continuum emission above a 3 sigma upper limit of 7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We compare the SMA upper limits with the predictions from the exploratory Cho & Lazarian model of polarized emission from T Tauri disks threaded by toroidal magnetic fields, and rule out their fiducial model at the similar to 10 sigma level. We explore some potential causes for this discrepancy, focusing on model parameters that describe the shape, magnetic field alignment, and size distribution of grains in the disk. We also investigate related effects like the magnetic field strength and geometry, scattering off of large grains, and the efficiency of grain alignment, including recent advances in grain alignment theory, which are not considered in the fiducial model. We discuss the impact each parameter would have on the data and determine that the suppression of polarized emission plausibly arises from rounding of large grains, reduced efficiency of grain alignment with the magnetic field, and/or some degree of magnetic field tangling ( perhaps due to turbulence). A poloidal magnetic field geometry could also reduce the polarization signal, particularly for a face-on viewing geometry like the TW Hya disk. The data provided here offer the most stringent limits to-date on the polarized millimeter-wavelength emission from disks around young stars.</P>

Statistics of velocity centroids: effects of density–velocity correlations and non-Gaussianity

Esquivel, A.,Lazarian, A.,Horibe, S.,Cho, J.,Ossenkopf, V.,Stutzki, J. Blackwell Publishing Ltd 2007 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.381 No.4

<P>ABSTRACT</P><P>We continue with our previous work on statistics of velocity centroids, to retrieve information about the scaling properties of an underlying turbulent velocity field from spectroscopic observations. We use synthetic data sets with extreme effects of velocity–density correlations that we create artificially, which also have a non-Gaussian distribution of fluctuations. We confirm that centroids can be used to obtain the scaling properties of the turbulent velocity when the ratio of the density dispersion to the mean density is less than unity, regardless of velocity–density correlations and non-Gaussianity. It was found that extreme velocity–density correlations can distort the statistics of velocity centroids, impeding the recovery of the turbulent velocity spectral index from centroids. We show that such correlations introduce high-order moments to the maps of centroids, which we disregarded in previous work, but that they are only important when the density dispersion is large in comparison with the mean density. It was also found that non-Gaussian velocity and/or density distort the statistics of centroids too, but to a lower degree than extreme cross-correlations.</P>

A UNIFIED MODEL OF GRAIN ALIGNMENT: RADIATIVE ALIGNMENT OF INTERSTELLAR GRAINS WITH MAGNETIC INCLUSIONS

Hoang, Thiem,Lazarian, A. American Astronomical Society 2016 The Astrophysical journal Vol.831 No.2

<P>The radiative torque (RAT) alignment of interstellar grains with ordinary paramagnetic susceptibilities has been supported by earlier studies. The alignment of such grains depends on the so-called RAT parameter q(max), which is determined by the grain shape. In this paper, we elaborate on our model of RAT alignment for grains with enhanced magnetic susceptibility due to iron inclusions, such that RAT alignment is magnetically enhanced, which we term the. MRAT mechanism. Such grains can be aligned with high angular momentum at the so-called high-J attractor points, achieving a high degree of alignment. Using our analytical model of RATs, we derive the critical value of the magnetic relaxation parameter delta(m) to produce high-J attractor points as functions of q(max) and the anisotropic radiation angle relative to the magnetic field psi. We find that if about 10% of the total iron abundance present in silicate grains is forming iron clusters, this. is sufficient to produce high-J attractor points for all reasonable values of q(max). To calculate the degree of grain alignment, we carry out numerical simulations of MRAT alignment by including stochastic excitations from gas collisions and magnetic fluctuations. We show that large grains can achieve perfect alignment when the high-J attractor point is present, regardless of the values of q(max). Our obtained results pave the way for. the. physical modeling of polarized thermal dust emission as well as magnetic dipole emission. We also find that millimeter-sized grains in accretion disks may be aligned with the magnetic field if they are incorporated with iron nanoparticles.</P>

Cross-sectional Alignment of Polycyclic Aromatic Hydrocarbons by Anisotropic Radiation

Hoang, Thiem,Lazarian, A. American Astronomical Society 2018 The Astrophysical journal Vol.860 No.2

<P>We study the effect of anisotropic radiation illumination on the alignment of polycyclic aromatic hydrocarbons (PAHs) and report that the cross-sectional mechanism of alignment earlier considered in terms of gas-grain interactions can also be efficient for the photon-grain interaction. We demonstrate this by first calculating the angle-dependence rotational damping and excitation coefficients by photon absorption followed by infrared emission. We then calculate the degree of PAH alignment for the different environments and physical parameters, including the illumination direction, ionization fraction, and magnetic field strength. For the reflection nebula (RN) conditions with unidirectional radiation field, we find that the degree of alignment tends to increase with increasing angle psi between the illumination direction and the magnetic field, as a result of the decrease of the cross section of photon absorption with psi. We calculate the polarization of spinning PAH emission using the obtained degree of alignment for the different physical parameters, assuming constant grain temperatures. We find that the polarization of spinning PAH emission from RNe can be large, between 5% and 20% at frequencies nu > 20 GHz, whereas the polarization is less than 3% for photodissociation regions. In realistic conditions, the polarization is expected to be lower owing to grain temperature fluctuations and magnetic field geometry. The polarization for the diffuse cold neutral medium is rather low, below 1% at nu > 20 GHz, consistent with observations by WMAP and Planck. Our results demonstrate that the RNe are the favored environment to observe the polarization of spinning dust emission and polarized mid-IR emission from PAHs.</P>