The Power Spectrum of Supersonic Turbulence in Perseus

The Astrophysical Journal (Impact Factor: 5.99). 11/2006; 653(2). DOI: 10.1086/510620
Source: arXiv


We test a method of estimating the power spectrum of turbulence in molecular clouds based on the comparison of power spectra of integrated intensity maps and single-velocity-channel maps, suggested by Lazarian and Pogosyan. We use synthetic 13CO data from non-LTE radiative transfer calculations based on density and velocity fields of a simulation of supersonic hydrodynamic turbulence. We find that the method yields the correct power spectrum with good accuracy. We then apply the method to the Five College Radio Astronomy Observatory 13CO map of the Perseus region, from the COMPLETE website. We find a power law power spectrum with slope beta=1.81+-0.10. The values of beta as a function of velocity resolution are also confirmed using the lower resolution map of the same region obtained with the AT&T Bell Laboratories antenna. Because of its small uncertainty, this result provides a useful constraint for numerical codes used to simulate molecular cloud turbulence. Comment: 4 pages, 3 figures. ApJ Letters, in press

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    • "As new techniques for studying turbulence are being applied to observational data, the evidence of the turbulent nature of astrophysical media becomes really undeniable. For instance, the Velocity Channel Analysis (VCA) and Velocity Coordinate Spectrum (VCS) techniques (Lazarian and Pogosyan, 2000, 2004, 2006) provided unique insight into the velocity spectra of turbulence in molecular clouds (see Padoan et al, 2006, 2010), galactic and extragalactic atomic hydrogen (Stanimirovi´c and Lazarian (2001); Chepurnov et al (2010, 2015), see also the review by Lazarian (2009), where a compilation of velocity and density spectra obtained with contemporary HI and "
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    ABSTRACT: Realistic astrophysical environments are turbulent due to the extremely high Reynolds numbers. Therefore, the theories of reconnection intended for describing astrophysical reconnection should not ignore the effects of turbulence on magnetic reconnection. Turbulence is known to change the nature of many physical processes dramatically and in this review we claim that magnetic reconnection is not an exception. We stress that not only astrophysical turbulence is ubiquitous, but also magnetic reconnection itself induces turbulence. Thus turbulence must be accounted for in any realistic astrophysical reconnection setup. We argue that due to the similarities of MHD turbulence in relativistic and non-relativistic cases the theory of magnetic reconnection developed for the non-relativistic case can be extended to the relativistic case and we provide numerical simulations that support this conjecture. We also provide quantitative comparisons of the theoretical predictions and results of numerical experiments, including the situations when turbulent reconnection is self-driven, i.e. the turbulence in the system is generated by the reconnection process itself. We show how turbulent reconnection entails the violation of magnetic flux freezing, the conclusion that has really far reaching consequences for many realistically turbulent astrophysical environments. In addition, we consider observational testing of turbulent reconnection as well as numerous implications of the theory. The former includes the Sun and solar wind reconnection, while the latter include the process of reconnection diffusion induced by turbulent reconnection, the acceleration of energetic particles, bursts of turbulent reconnection related to black hole sources as well as gamma ray bursts. Finally, we explain why turbulent reconnection cannot be explained by turbulent resistivity or derived through the mean field approach.
    Full-text · Article · Dec 2015
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    • "In addition, it is now generally accepted that the " Big Power Law in the Sky " indicates the presence of turbulence on scales from tens of parsecs to thousands of kilometers (Armstrong et al., 1995; Chepurnov and Lazarian, 2010). Among other sources, evidence for this comes from studies of atomic hydrogen spectra in molecular clouds and galaxies (see Lazarian and Pogosyan, 1999; Stanimirovi´c and Lazarian, 2001; Padoan et al., 2006, 2009; Chepurnov et al., 2010, see also review by Lazarian, 2009 and references therein), as well as recent studies of emission lines and Faraday rotation (see Burkhart et al., 2010; Gaensler et al., 2011). LV99's model uses the properties of turbulence to predict broad outflows from extended current sheets. "
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    ABSTRACT: We study a model of fast magnetic reconnection in the presence of weak turbulence proposed by Lazarian and Vishniac (1999) using three-dimensional direct numerical simulations. The model has been already successfully tested in Kowal et al. (2009) confirming the dependencies of the reconnection speed $V_{rec}$ on the turbulence injection power $P_{inj}$ and the injection scale $l_{inj}$ expressed by a constraint $V_{rec} \sim P_{inj}^{1/2} l_{inj}^{3/4}$ and no observed dependency on Ohmic resistivity. In Kowal et al. (2009), in order to drive turbulence, we injected velocity fluctuations in Fourier space with frequencies concentrated around $k_{inj}=1/l_{inj}$, as described in Alvelius (1999). In this paper we extend our previous studies by comparing fast magnetic reconnection under different mechanisms of turbulence injection by introducing a new way of turbulence driving. The new method injects velocity or magnetic eddies with a specified amplitude and scale in random locations directly in real space. We provide exact relations between the eddy parameters and turbulent power and injection scale. We performed simulations with new forcing in order to study turbulent power and injection scale dependencies. The results show no discrepancy between models with two different methods of turbulence driving exposing the same scalings in both cases. This is in agreement with the Lazarian and Vishniac (1999) predictions. In addition, we performed a series of models with varying viscosity $\nu$. Although Lazarian and Vishniac (1999) do not provide any prediction for this dependence, we report a weak relation between the reconnection speed with viscosity, $V_{rec}\sim\nu^{-1/4}$.
    Full-text · Article · Mar 2012 · Nonlinear Processes in Geophysics
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    ABSTRACT: The Padoan and Nordlund model of the stellar initial mass function (IMF) is derived from low order statistics of supersonic turbulence, neglecting gravity (e.g. gravitational fragmentation, accretion and merging). In this work the predictions of that model are tested using the largest numerical experiments of supersonic hydrodynamic (HD) and magneto-hydrodynamic (MHD) turbulence to date (~1000^3 computational zones) and three different codes (Enzo, Zeus and the Stagger Code). The model predicts a power law distribution for large masses, related to the turbulence energy power spectrum slope, and the shock jump conditions. This power law mass distribution is confirmed by the numerical experiments. The model also predicts a sharp difference between the HD and MHD regimes, which is recovered in the experiments as well, implying that the magnetic field, even below energy equipartition on the large scale, is a crucial component of the process of turbulent fragmentation. These results suggest that the stellar IMF of primordial stars may differ from that in later epochs of star formation, due to differences in both gas temperature and magnetic field strength. In particular, we find that the IMF of primordial stars born in turbulent clouds may be narrowly peaked around a mass of order 10 solar masses, as long as the column density of such clouds is not much in excess of 10^22 cm^-2.
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