A. Lazarian | University of Wisconsin-Madison (original) (raw)
Papers by A. Lazarian
The Astrophysical Journal, 1996
We present linear spectropolarimetry in the wavelength range 4.5-4.8 m of the embedded infrared s... more We present linear spectropolarimetry in the wavelength range 4.5-4.8 m of the embedded infrared source W33A. Our observations show for the first time the presence of polarization associated with the CO and XCN ice features, demonstrating that the absorbers reside in or on aligned grains. Both narrow and broad components of the solid CO feature near 4.67 m are polarized. The detection of polarization associated with the narrow CO component is particularly significant, as the ices responsible are thought to exist only in dense, cold regions of molecular clouds, where gas and grain temperatures are expected to be closely coupled and traditional grain alignment mechanisms should become ineffective. We discuss the significance of this result with regard to current grain alignment theories. Mechanisms in which grain rotational energy is enhanced by interaction with cosmic rays merit further investigation.
AIP Conference Proceedings, 2002
We present a statistical analysis of the interstellar medium (ISM) polarization from the largest ... more We present a statistical analysis of the interstellar medium (ISM) polarization from the largest compilation available of starlight data, which comprises ∼ 5500 stars. The measured correlation between the mean polarization degree and extinction indicates that ISM dust grains are not fully aligned with the uniform component of the large-scale Galactic magnetic field. Moreover, we estimate the ratio of the uniform to the random plane-of-thesky components of the magnetic field to be B u /B r ≈ 0.8. From the analysis of starlight polarization degree and position angle we find that the magnetic field broadly follows Galactic structures on large-scales. On the other hand, the angular power spectrum C ℓ of the polarization degree for Galactic plane data is found to be consistent with a power-law, C ℓ ∝ ℓ −1.5 (where ℓ ≈ 180 • /θ is the multipole order), for angular scales θ > 10 ′. We argue that this data set can be used to estimate diffuse polarized emission at microwave frequencies.
AIP Conference Proceedings, 2005
In this presentation we review the link between the statistics of intensity fluctuations in spect... more In this presentation we review the link between the statistics of intensity fluctuations in spectral line data cubes with underlying statistical properties of turbulence in the interstellar medium. Both the formalism of Velocity Channel Analysis for optically thin lines and its extension to the lines with self-absorption is described. We demonstrate that by observing optically thin lines from cold gas in sufficiently narrow (thin) velocity channels one may recover the scaling of the stochastic velocities from turbulent cascade, in particular, Kolmogorov velocities give K −2.7 contribution to the intensity power spectrum. Synthetically increasing the channel thickness separates out the underlying density inhomogeneities of the gas. Effects of self absorption, on the other hand, retain the velocity signature even for integrated lines. As a result, intensity fluctuations tend to show universal but featureless scaling of the power ∝ K −3 over the range of scales.
Proceedings of the International Astronomical Union, 2009
We discuss a new technique of studying magnetic fields in diffuse astrophysical media, e.g. inter... more We discuss a new technique of studying magnetic fields in diffuse astrophysical media, e.g. interstellar and intergalactic gas/plasma. This technique is based on the angular momentum alignment of atoms and ions in their ground or metastable states. The alignment reveals itself in terms of the polarization of the absorbed and emitted light. The corresponding studies of magnetic fields can be performed with multiband spectropolarimetry, from UV, optical, to IR/radio. A unique feature of these studies is that they can reveal the 3D orientation of magnetic field. We mention several cases of interplanetary, circumstellar and interstellar magnetic fields for which the studies of magnetic fields using ground state atomic alignment effect are promising.
![Research paper thumbnail of Erratum: "Enhancement and Suppression of Heat Transfer by MHD Turbulence" ([URL ADDRESS="/cgi-bin/resolve?2006ApJ...645L..25L" STATUS="OKAY"]ApJ, 645, L25 [2006][/URL])](https://attachments.academia-assets.com/109831468/thumbnails/1.jpg)
](The Astrophysical Journal, 2007
In the above-mentioned Letter, equation (8) has the wrong power of the Alfvén Mach number M A , w... more In the above-mentioned Letter, equation (8) has the wrong power of the Alfvén Mach number M A , which is defined as the ratio of the injection velocity to the Alfvén velocity. The correct expression is
The Astrophysical Journal, 2007
We study scaling relations of compressible strongly magnetized turbulence. We find a good corresp... more We study scaling relations of compressible strongly magnetized turbulence. We find a good correspondence of our results with the Fleck (1996) model of compressible hydrodynamic turbulence. In particular, we find that the density-weighted velocity, i.e. u ≡ ρ 1/3 v, proposed in Kritsuk et al. (2007) obeys the Kolmogorov scaling, i.e. E u (k) ∼ k −5/3 for the high Mach number turbulence. Similarly, we find that the exponents of the third order structure functions for u stay equal to unity for the all the Mach numbers studied. The scaling of higher order correlations obeys the She-Lévêque (1994) scalings corresponding to the two-dimensional dissipative structures, and this result does not change with the Mach number either. In contrast to v which exhibits different scaling parallel and perpendicular to the local magnetic field, the scaling of u is similar in both directions. In addition, we find that the peaks of density create a hierarchy in which both physical and column densities decrease with the scale in accordance to the Fleck (1996) predictions. This hierarchy can be related ubiquitous small ionized and neutral structures (SINS) in the interstellar gas. We believe that studies of statistics of the column density peaks can provide both consistency check for the turbulence velocity studies and insight into supersonic turbulence, when the velocity information is not available.
The Astrophysical Journal, 2008
In this paper, genus statistics have been applied to an HI column density map of the Small Magell... more In this paper, genus statistics have been applied to an HI column density map of the Small Magellanic Cloud in order to study its topology. To learn how topology changes with the scale of the system, we provide the study of topology for column density maps at varying resolution. To evaluate the statistical error of the genus we randomly reassign the phases of the Fourier modes while keeping the amplitudes. We find, that at the smallest scales studied (40 pc ≤ λ ≤ 80 pc) the genus shift is in all regions negative, implying a clump topology. At the larger scales (110 pc ≤ λ ≤ 250 pc) the topology shift is detected to be negative in 4 cases and positive ("swiss cheese" topology) in 2 cases. In 4 regions there is no statistically significant topology shift at large scales.
Monthly Notices of the Royal Astronomical Society, 2007
By applying recent results for the slab correlation timescale on to cosmic ray scattering theory,... more By applying recent results for the slab correlation timescale on to cosmic ray scattering theory, we compute cosmic ray parallel mean free paths within the quasi-linear limit. By employing these results on to charged particle transport in the Solar system, we demonstrate that much larger parallel mean free paths can be obtained in comparison to previous results. A comparison with solar wind observations is also presented to show that the new theoretical results are much closer to the observations than the previous results.
Astrophysics and Space Science, 1994
A new approach to the galactic seed magnetic field problem is briefly discussed. It is shown that... more A new approach to the galactic seed magnetic field problem is briefly discussed. It is shown that, in early stages of galactic evolution, the hierarchial agglomeration and fragmentation processes can account for the generation of a dynamically important magnetic field. The amplification of this field follows an inverse cascade since a non-zero average value of the field amplified on a smaller scale serves as a seed field on the next (earlier) hierarchial scale. In such a scenario, a problem of how to get things started never occurs as any infinitesimally small battery generated seed field (Lazarian 1992a) can be efficiently amplified passing by through a sufficient number of amplification cascades.
Astrophysics and Space Science, 1994
The properties of the ISM indicate that it is turbulent. However, the ISM turbulence is radically... more The properties of the ISM indicate that it is turbulent. However, the ISM turbulence is radically different from that in incompressible fluids. That is why it is so important to study it through observations. The relevant study still poses a challenging problem. In the present paper recent results based on a statistical approach to the problem are surveyed. Although it was pointed out long ago (see Kaplanet al., 1970) that random 3D motions of the ISM gas result in fluctuations of the observed electromagnetic emission, it is only recently that the problem of recovering statistical properties of the ISM turbulence from the line integrated data was given an adequate mathematical treatment. Here by the example of studying turbulence in HI, it is shown that the inverse problem can be solved uniquely using a realistic model of the ISM. The application of theoretical conclusions to existing data explains some facts which used to be considered inconsistent with turbulent behaviour and reveals unexpected features of the ISM turbulence.
Reconnection is a basic process that is necessary to understand in order to deal with MHD. I disc... more Reconnection is a basic process that is necessary to understand in order to deal with MHD. I discuss a reconnection model which was developed in collaboration with E. Vishniac. In this model reconnection is dramatically enhanced due to the presence of weak turbulence. I consider reconnection both in idealized conducting fluid and in a realistic interstellar medium. I show that in the latter case the damping of turbulence via ion-neutral collisions may limit the reconnection rate.
Turbulent mixing layers have important implications for gas temperatures along ISM phase interfac... more Turbulent mixing layers have important implications for gas temperatures along ISM phase interfaces. Currently, however, mixing layer theory is based on many questionable assumptions and approximations, which may limit its accuracy. Here, we developed comprehensive MHD code to simulate the dynamics and cooling of mixing layers with high precision. Our code is fully three dimensional, includes non-equilibrium cooling, is capable
The Astrophysical Journal, 1996
We present linear spectropolarimetry in the wavelength range 4.5-4.8 m of the embedded infrared s... more We present linear spectropolarimetry in the wavelength range 4.5-4.8 m of the embedded infrared source W33A. Our observations show for the first time the presence of polarization associated with the CO and XCN ice features, demonstrating that the absorbers reside in or on aligned grains. Both narrow and broad components of the solid CO feature near 4.67 m are polarized. The detection of polarization associated with the narrow CO component is particularly significant, as the ices responsible are thought to exist only in dense, cold regions of molecular clouds, where gas and grain temperatures are expected to be closely coupled and traditional grain alignment mechanisms should become ineffective. We discuss the significance of this result with regard to current grain alignment theories. Mechanisms in which grain rotational energy is enhanced by interaction with cosmic rays merit further investigation.
AIP Conference Proceedings, 2002
We present a statistical analysis of the interstellar medium (ISM) polarization from the largest ... more We present a statistical analysis of the interstellar medium (ISM) polarization from the largest compilation available of starlight data, which comprises ∼ 5500 stars. The measured correlation between the mean polarization degree and extinction indicates that ISM dust grains are not fully aligned with the uniform component of the large-scale Galactic magnetic field. Moreover, we estimate the ratio of the uniform to the random plane-of-thesky components of the magnetic field to be B u /B r ≈ 0.8. From the analysis of starlight polarization degree and position angle we find that the magnetic field broadly follows Galactic structures on large-scales. On the other hand, the angular power spectrum C ℓ of the polarization degree for Galactic plane data is found to be consistent with a power-law, C ℓ ∝ ℓ −1.5 (where ℓ ≈ 180 • /θ is the multipole order), for angular scales θ > 10 ′. We argue that this data set can be used to estimate diffuse polarized emission at microwave frequencies.
AIP Conference Proceedings, 2005
In this presentation we review the link between the statistics of intensity fluctuations in spect... more In this presentation we review the link between the statistics of intensity fluctuations in spectral line data cubes with underlying statistical properties of turbulence in the interstellar medium. Both the formalism of Velocity Channel Analysis for optically thin lines and its extension to the lines with self-absorption is described. We demonstrate that by observing optically thin lines from cold gas in sufficiently narrow (thin) velocity channels one may recover the scaling of the stochastic velocities from turbulent cascade, in particular, Kolmogorov velocities give K −2.7 contribution to the intensity power spectrum. Synthetically increasing the channel thickness separates out the underlying density inhomogeneities of the gas. Effects of self absorption, on the other hand, retain the velocity signature even for integrated lines. As a result, intensity fluctuations tend to show universal but featureless scaling of the power ∝ K −3 over the range of scales.
Proceedings of the International Astronomical Union, 2009
We discuss a new technique of studying magnetic fields in diffuse astrophysical media, e.g. inter... more We discuss a new technique of studying magnetic fields in diffuse astrophysical media, e.g. interstellar and intergalactic gas/plasma. This technique is based on the angular momentum alignment of atoms and ions in their ground or metastable states. The alignment reveals itself in terms of the polarization of the absorbed and emitted light. The corresponding studies of magnetic fields can be performed with multiband spectropolarimetry, from UV, optical, to IR/radio. A unique feature of these studies is that they can reveal the 3D orientation of magnetic field. We mention several cases of interplanetary, circumstellar and interstellar magnetic fields for which the studies of magnetic fields using ground state atomic alignment effect are promising.
The Astrophysical Journal, 2007
In the above-mentioned Letter, equation (8) has the wrong power of the Alfvén Mach number M A , w... more In the above-mentioned Letter, equation (8) has the wrong power of the Alfvén Mach number M A , which is defined as the ratio of the injection velocity to the Alfvén velocity. The correct expression is
The Astrophysical Journal, 2007
We study scaling relations of compressible strongly magnetized turbulence. We find a good corresp... more We study scaling relations of compressible strongly magnetized turbulence. We find a good correspondence of our results with the Fleck (1996) model of compressible hydrodynamic turbulence. In particular, we find that the density-weighted velocity, i.e. u ≡ ρ 1/3 v, proposed in Kritsuk et al. (2007) obeys the Kolmogorov scaling, i.e. E u (k) ∼ k −5/3 for the high Mach number turbulence. Similarly, we find that the exponents of the third order structure functions for u stay equal to unity for the all the Mach numbers studied. The scaling of higher order correlations obeys the She-Lévêque (1994) scalings corresponding to the two-dimensional dissipative structures, and this result does not change with the Mach number either. In contrast to v which exhibits different scaling parallel and perpendicular to the local magnetic field, the scaling of u is similar in both directions. In addition, we find that the peaks of density create a hierarchy in which both physical and column densities decrease with the scale in accordance to the Fleck (1996) predictions. This hierarchy can be related ubiquitous small ionized and neutral structures (SINS) in the interstellar gas. We believe that studies of statistics of the column density peaks can provide both consistency check for the turbulence velocity studies and insight into supersonic turbulence, when the velocity information is not available.
The Astrophysical Journal, 2008
In this paper, genus statistics have been applied to an HI column density map of the Small Magell... more In this paper, genus statistics have been applied to an HI column density map of the Small Magellanic Cloud in order to study its topology. To learn how topology changes with the scale of the system, we provide the study of topology for column density maps at varying resolution. To evaluate the statistical error of the genus we randomly reassign the phases of the Fourier modes while keeping the amplitudes. We find, that at the smallest scales studied (40 pc ≤ λ ≤ 80 pc) the genus shift is in all regions negative, implying a clump topology. At the larger scales (110 pc ≤ λ ≤ 250 pc) the topology shift is detected to be negative in 4 cases and positive ("swiss cheese" topology) in 2 cases. In 4 regions there is no statistically significant topology shift at large scales.
Monthly Notices of the Royal Astronomical Society, 2007
By applying recent results for the slab correlation timescale on to cosmic ray scattering theory,... more By applying recent results for the slab correlation timescale on to cosmic ray scattering theory, we compute cosmic ray parallel mean free paths within the quasi-linear limit. By employing these results on to charged particle transport in the Solar system, we demonstrate that much larger parallel mean free paths can be obtained in comparison to previous results. A comparison with solar wind observations is also presented to show that the new theoretical results are much closer to the observations than the previous results.
Astrophysics and Space Science, 1994
A new approach to the galactic seed magnetic field problem is briefly discussed. It is shown that... more A new approach to the galactic seed magnetic field problem is briefly discussed. It is shown that, in early stages of galactic evolution, the hierarchial agglomeration and fragmentation processes can account for the generation of a dynamically important magnetic field. The amplification of this field follows an inverse cascade since a non-zero average value of the field amplified on a smaller scale serves as a seed field on the next (earlier) hierarchial scale. In such a scenario, a problem of how to get things started never occurs as any infinitesimally small battery generated seed field (Lazarian 1992a) can be efficiently amplified passing by through a sufficient number of amplification cascades.
Astrophysics and Space Science, 1994
The properties of the ISM indicate that it is turbulent. However, the ISM turbulence is radically... more The properties of the ISM indicate that it is turbulent. However, the ISM turbulence is radically different from that in incompressible fluids. That is why it is so important to study it through observations. The relevant study still poses a challenging problem. In the present paper recent results based on a statistical approach to the problem are surveyed. Although it was pointed out long ago (see Kaplanet al., 1970) that random 3D motions of the ISM gas result in fluctuations of the observed electromagnetic emission, it is only recently that the problem of recovering statistical properties of the ISM turbulence from the line integrated data was given an adequate mathematical treatment. Here by the example of studying turbulence in HI, it is shown that the inverse problem can be solved uniquely using a realistic model of the ISM. The application of theoretical conclusions to existing data explains some facts which used to be considered inconsistent with turbulent behaviour and reveals unexpected features of the ISM turbulence.
Reconnection is a basic process that is necessary to understand in order to deal with MHD. I disc... more Reconnection is a basic process that is necessary to understand in order to deal with MHD. I discuss a reconnection model which was developed in collaboration with E. Vishniac. In this model reconnection is dramatically enhanced due to the presence of weak turbulence. I consider reconnection both in idealized conducting fluid and in a realistic interstellar medium. I show that in the latter case the damping of turbulence via ion-neutral collisions may limit the reconnection rate.
Turbulent mixing layers have important implications for gas temperatures along ISM phase interfac... more Turbulent mixing layers have important implications for gas temperatures along ISM phase interfaces. Currently, however, mixing layer theory is based on many questionable assumptions and approximations, which may limit its accuracy. Here, we developed comprehensive MHD code to simulate the dynamics and cooling of mixing layers with high precision. Our code is fully three dimensional, includes non-equilibrium cooling, is capable