Thermal Flipping and Thermal Trapping -- New Elements in Dust Grain Dynamics

Since the classical work by Purcell (1979) it has been generally accepted that most interstellar grains rotate suprathermally. Suprathermally rotating grains would be nearly perfectly aligned with the magnetic field by paramagnetic dissipation if not for ``crossovers'', intervals of low angular velocity resulting from reversals of the torques responsible for suprathermal rotation; during crossovers grains are susceptible to disalignment by random impulses. Lazarian and Draine (1997) identified thermal fluctuations within grain material as an important component of crossover dynamics. For grains of size less than 0.1 micron, these fluctuations ensure good correlation of angular momentum before and after crossover resulting in good alignment, in accord with observations of starlight polarization. In the present paper we discuss two new processes which are important for the dynamics of grains with a<0.1 micron. The first -- ``thermal flipping'' -- offers a way for small grains to bypass the period of greatly reduced angular momentum which would otherwise take place during a crossover, thereby enhancing the alignment of small grains. The second effect -- ``thermal trapping'' -- arises when thermal flipping becomes rapid enough to prevent the systematic torques from driving the grain to suprathermal rotation. This effect acts to reduce the alignment of small grains. The observed variation of grain alignment with grain size would then result from a combination of the thermal flipping process -- which suppresses suprathermal rotation of small grains -- and due to molecular hydrogen formation and starlight -- which drive large grains to suprathermal rotation rates.Comment: 16 pages, 2 figures, submitted ApJ

Studying Turbulence using Doppler-broadened lines: Velocity Coordinate Spectrum

We discuss a new technique for studying astrophysical turbulence that utilizes the statistics of Doppler-broadened spectral lines. The technique relates the power Velocity Coordinate Spectrum (VCS), i.e. the spectrum of fluctuations measured along the velocity axis in Position-Position-Velocity (PPV) data cubes available from observations, to the underlying power spectra of the velocity/density fluctuations. Unlike the standard spatial spectra, that are function of angular wavenumber, the VCS is a function of the velocity wave number k_v ~ 1/v. We show that absorption affects the VCS to a higher degree for small k_v and obtain the criteria for disregarding the absorption effects for turbulence studies at large k_v. We consider the retrieval of turbulence spectra from observations for high and low spatial resolution observations and find that the VCS allows one to study turbulence even when the emitting turbulent volume is not spatially resolved. This opens interesting prospects for using the technique for extragalactic research. We show that, while thermal broadening interferes with the turbulence studies using the VCS, it is possible to separate thermal and non-thermal contributions. This allows a new way of determining the temperature of the interstellar gas using emission and absorption spectral lines.Comment: 27 page, 3 figures, content extended and presentation reorganized to correspond to the version accepted to Ap

Turbulence Spectra from Doppler-shifted Spectral Lines

Turbulence is a key element of the dynamics of astrophysical fluids, including those of interstellar medium, clusters of galaxies and circumstellar regions. Turbulent motions induce Doppler shifts of observable emission and absorption lines. In the review we discuss new techniques that relate the spectra of underlying velocity turbulence and spectra of Doppler-shifted lines. In particular, the Velocity-Channel Analysis (VCA) makes use of the channel maps, while the Velocity Coordinate Spectrum (VCS) utilizes the fluctuations measured along the velocity axis of the Position-Position Velocity (PPV) data cubes. Both techniques have solid foundations based on analytical calculations as well as on numerical testings. Among the two the VCS, which has been developed quite recently, has two advantages. First of all, it is applicable to turbulent volumes that are not spatially resolved. Second, it can be used with absorption lines that do not provide good spatial sampling of different lags over the image of turbulent object. In fact, numerical testing shows that measurements of Doppler shifted absorption lines over a few directions is sufficient for a reliable recovering of the underlying spectrum of the turbulence. Our comparison of the VCA and the VCS with a more traditional technique of Velocity Centroids, shows that the former two techniques recover reliably the spectra of supersonic turbulence, while the Velocity Centroids may have advantages for studying subsonic turbulence. In parallel with theoretical and numerical work on the VCA and the VCS, the techniques have been applied to spectroscopic observations. We discuss results on astrophysical turbulence obtained with the VCA and the VCS.Comment: 15 pages, 4 figures, review talk at 18 International Conference on Spectral Line Shapes, to be published by AI

Radiative torques alignment in the presence of pinwheel torques

We study the alignment of grains subject to both radiative torques and pinwheel torques while accounting for thermal flipping of grains. By pinwheel torques we refer to all systematic torques that are fixed in grain body axes, including the radiative torques arising from scattering and absorption of isotropic radiation. We discuss new types of pinwheel torques, which are systematic torques arising from infrared emission and torques arising from the interaction of grains with ions and electrons in hot plasma. We show that both types of torques are long-lived, i.e. may exist longer than gaseous damping time. We compare these torques with the torques introduced by E. Purcell, namely, torques due to H$_2$ formation, the variation of accommodation coefficient for gaseous collisions and photoelectric emission. Furthermore, we revise the Lazarian & Draine model for grain thermal flipping. We calculate mean flipping timescale induced by Barnett and nuclear relaxation for both paramagnetic and superparamagnetic grains, in the presence of stochastic torques associated with pinwheel torques, e.g. the stochastic torques arising from H$_2$ formation, and gas bombardment. We show that the combined effect of internal relaxation and stochastic torques can result in fast flipping for sufficiently small grains and, because of this, they get thermally trapped, i.e. rotate thermally in spite of the presence of pinwheel torques. For sufficiently large grains, we show that the pinwheel torques can increase the degree of grain alignment achievable with the radiative torques by increasing the magnitude of the angular momentum of low attractor points and/or by driving grains to new high attractor points.Comment: 23 pages and 15 figures emulated ApJ style. Thermal flipping and trapping revised; paper accepted to Ap