Shattering and coagulation of dust grains in interstellar turbulence

We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium within a few Myr for grains with radius   a ≳ 10⁻⁶  cm. We also show that shattering in warm neutral medium can limit the largest grain size in ISM  ( a ∼ 2 × 10⁻⁵ cm)  . On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with   a < 10⁻⁶ cm  coagulate in 10 Myr. In fact, the correlation among   R_V   , the carbon bump strength and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than ∼1/10 of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich environments.     

Modelling the turbulent magnetized ISM

Briefly stated, the interstellar matter (hereafter ISM) is a highlyturbulent flow, the turbulence mainly being driven by the violentstirring by supernova explosions, penetrated by dynamicallysignificant magnetic fields both on small and largescales. Sophisticated numerical models of such a turbulent flow cangive important information on some of the complicated processesoccurring in the ISM, e.g. on the regulation of the observedmultiphase structure, properties of interstellar turbulence and thegeneration and maintenance of magnetic fields by the flow itself; inthis paper such models and their most important results are reviewed.     

A statistical description of astrophysical turbulence

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.     

The generation and dissipation of solar and galactic magnetic fields

Turbulent diffusion of magnetic field plays an essential role in the generation of magnetic field in most astrophysical bodies. This paper reviews what can be proved, and what can be believed, about the turbulent diffusion of magnetic field. Observations indicate the dissipation of magnetic field at rates that can be understood only in terms of turbulent diffusion. Theory shows that a largescale weak magnetic field diffuses in a turbulent flow in the same way that smoke is mixed throughout the fluid by the turbulence. The small-scale fields (produced from the large-scale field by the turbulence) are limited in their growth by reconnection of field lines at neutral points, so that the turbulent mixing of field and fluid is not halted by them.Altogether, it appears that the mixing of field and fluid in the observed turbulent motions in the Sun and in the Galaxy is unavoidable. Turbulent diffusion causes decay of the general solar fields in a decade or so, and of the galactic field in 10⁸–10⁹ yr. We conclude that continual dynamo action is implied by the observed existence of the fields.This work was supported in part by the National Aeronautics and Space Administration under Grant NGL 14-001-001.     

What Drives Star Formation?

Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.     

The influence of fluctuation fields on the force-free field

In Paper I (Hu, 1982), we discussed the the influence of fluctuation fields on the force-free field for the case of conventional turbulence and demonstrated the general relationships. In the present paper, by using the approach of local expansion, the equation of average force-free field is obtained as (1+b)?×B ₀=(α#x002B;a)B ₀#x002B;a ⁽¹⁾×B ₀#x002B;K. The average coefficientsa,a ⁽¹⁾,b, andK show the influence of the fluctuation fields in small scale on the configurations of magnetic field in large scale. As the average magnetic field is no longer parallel to the average electric current, the average configurations of force-free fields are more general and complex than the usual ones. From the view point of physics, the energy and momentum of the turbulent structures should have influence on the equilibrium of the average fields. Several examples are discussed, and they show the basic features of the fluctuation fields and the influence of fluctuation fields on the average configurations of magnetic fields. The astrophysical environments are often in the turbulent state, the results of the present paper may be applied to the turbulent plasma where the magnetic field is strong.     

Mapping the roots of the galactic outflow in NGC1569

The exact nature of the interaction between hot, fast-flowing star-cluster winds and the surrounding clumpy interstellar medium (ISM) in starburst galaxies has very few observational constraints. Besides furthering our knowledge of ISM dynamics, detailed observations of ionised gas at the very roots of large-scale outflows are required to place limits on the current generation of high-resolution galactic wind models. To this end, we conduct a detailed investigation of the ionised gas environment surrounding the young star clusters in the starburst galaxy NGC1569. Using high spatial and spectral-resolution Gemini/GMOS integral-field unit observations, we accurately characterise the line-profile shapes of the optical nebular emission lines and find a ubiquitous broad (～300 km?s^?1) component underlying a bright narrower component. By mapping the properties of the individual line components, we find correlations that suggest that the broad component results from powerful cluster wind–gas clump interactions. We propose a model to explain the properties of the line components and the general turbulent state of the ISM.     

A Consistent one-Dimensional Model for the Turbulent Tachocline

The first consistent model for the turbulent tachocline is presented, with the turbulent diffusivity computed within the model instead of being specified arbitrarily. For the origin of the 3D turbulence a new mechanism is proposed. Owing to the strongly stable stratification, the mean radial shear is stable, while the horizontal shear is expected to drive predominantly horizontal, quasi-2D motions in thin slabs. Here I suggest that a major source of 3D overturning turbulent motions in the tachocline is the secondary shear instability due to the strong, random vertical shear arising between the uncorrelated horizontal flows in neighboring slabs. A formula for the vertical diffusivity due to this turbulence, Equation (9), is derived and applied in a simplified 1D model of the tachocline. It is found that Maxwell stresses due to an oscillatory poloidal magnetic field of a few hundred gauss are able to confine the tachocline to a thickness less than 5 Mm. The integral scale of the 3D overturning turbulence is the buoyancy scale, on the order of 10 km, and its velocity amplitude is a few m s^–1, yielding a vertical turbulent diffusivity on the order of 10⁸ cm² s^–1.     

Fragmentation and Structure Formation

I present an overview of the hierarchy of structures existing in the interstellar medium (ISM) and the possible mechanisms that cause the fragmentation of one level into the next, with the formation of stars as its last step. Within this framework, I then give an overview of the contributions to this session. Numerical work addresses, at the largest scales, the shaping and formation of structures in the ISM through turbulence driven by stellar energy injection, and the resulting star formation rate as a function of mean density. At the scales of molecular clouds, results comparing observational and numerical data on the density and velocity structure of turbulence-produced cores, as well as their mass spectra, are summarized, together with existing theories of core and star formation controlled by the turbulence. Observationally, an attempt to discriminate between the standard and turbulent models of star formation is presented, finding inconclusive results, but suggesting that both turbulence and the magnetic field are dynamically important in molecular clouds and their cores. Finally, various determinations of the magnetic field strength and geometry are also presented.     

First wavelet analysis of emission line variations in Wolf-Rayet stars

The quantification of stochastic substructures seen propagating away from the centers of emission lines of Wolf-Rayet (WR) stars is extended using the powerful, objective technique of wavelet analysis. Results for the substructures in one WR star so far show that the scaling laws between (a) flux and velocity dispersion and (b) lifetime and flux, combined with (c) their mass spectrum, strongly support the hypothesis that we are seeing the high mass tail-end distribution of full-scale supersonic compressible turbulence in the winds. This turbulence sets in beyond a critical radius from the star and shows remarkable similarity to the hierarchy of cloudlets seen in giant molecular clouds and other components of the ISM.The velocity dispersion is larger on average for substructures (interpreted as density enhanced turbulent eddies) propagating towards or away from the observer, suggesting that the turbulence is anisotropic. This is not surprising, since the most likely force which drives the windand the ensuing turbulence alike, radiation pressure, is directed outwards in all directions from the star. It is likely that a similar kind of turbulence prevails in the winds of all hot stars, of which those of WR stars are the most extreme.The consequences of clumping in winds are numerous. One of the most important is the necessary reduction in the estimate of the mass-loss rates compared to smooth outflow models.     

Chemodynamical Modelling of Galaxy Formation and Evolution

We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 10⁹ M_⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at no stage well mixed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Is the supersonic turbulence observed in HII regions sustained by the presence of magnetic fields?

We describe a set of Hα emission line profiles from populations of H II regions in nearby spiral galaxies. These are characterized by a strong Gaussian central peak, and lower intensity higher velocity wing features. From the peak we extract a non-thermal velocity component, due to the internal turbulence of the region. The plot of the widths of these non-thermal components against Hα luminosity shows a lower envelope in line width, which we assign to regions in, or close to virial equilibrium, although the region mass derived on this assumption is higher than the mass obtained by summing all known mass components. We speculate that this discrepancy, as well as the supersonicity of the line widths, can be explained by the presence of turbulent magnetic fields within the H II regions, and make a very rough estimate of the fields implied, of order a few tens of microgauss.     

Gas phase processes affecting galactic evolution

Gas processes affecting star formation are reviewed with an emphasis on gravitational and magnetic instabilities as a source of turbulence. Gravitational instabilities are pervasive in a multi-phase medium, even for sub-threshold column densities, suggesting that only an ISM with a pure-warm phase can stop star formation. The instabilities generate turbulence, and this turbulence influences the structure and timing of star formation through its effect on the gas distribution and density. The final trigger for star formation is usually direct compression by another star or cluster. The star formation rate is apparently independent of the detailed mechanisms for star formation, and determined primarily by the total mass of gas in a dense form. If the density distribution function is a log-normal, as suggested by turbulence simulations, then this dense gas mass can be calculated and the star formation rate determined from first principles. The results suggest that only 10^-4 of the ISM mass actively participates in the star formation process and that this fraction does so because its density is larger than 10⁵ cm^-3, at which point several key processes affecting dynamical equilibrium begin to break down. This revised version was published online in August 2006 with corrections to the Cover Date.     

The dynamo theory of plasma turbulence wave in rotating celestial bodies

The various modes of plasma turbulence waves (including MHD waves) are easily excited under cosmic circumstances. In this paper, if we consider that the celestial bodies rotate, there is a source term generated for the magnetic induced equation by the excited plasma turbulence waves. If we expand the turbulent field in the Fourier series and include rotation velocity, the dynamo equation for turbulent waves is obtained. We have also obtained the solutions of various wave forms corresponding to different rotation velocities and then we significantly discuss the magnetic fields in the Sun, planets, and other celestial bodies.     

Whistler wave cascades in solar wind plasma

Non-linear, three-dimensional, time-dependent fluid simulations of whistler wave turbulence are performed to investigate role of whistler waves in solar wind plasma turbulence in which characteristic turbulent fluctuations are characterized typically by the frequency and length-scales that are, respectively, bigger than ion gyrofrequency and smaller than ion gyroradius. The electron inertial length is an intrinsic length-scale in whistler wave turbulence that distinguishably divides the high-frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large-scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like   k ^−7/3  spectrum. By contrast, the small-scale turbulent fluctuations exhibit a Navier–Stokes-like evolution where characteristic turbulent eddies exhibit a typical   k ^−5/3  hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations.     

Properties of the negative effective magnetic pressure instability

As was demonstrated in earlier studies, turbulence can result in a negative contribution to the effective mean magnetic pressure, which, in turn, can cause a large‐scale instability. In this study, hydromagnetic mean‐field modelling is performed for an isothermally stratified layer in the presence of a horizontal magnetic field. The negative effective magnetic pressure instability (NEMPI) is comprehensively investigated. It is shown that, if the effect of turbulence on the mean magnetic tension force vanishes, which is consistent with results from direct numerical simulations of forced turbulence, the fastest growing eigenmodes of NEMPI are two‐dimensional. The growth rate is found to depend on a parameter β_* characterizing the turbulent contribution of the effective mean magnetic pressure for moderately strong mean magnetic fields. A fit formula is proposed that gives the growth rate as a function of turbulent kinematic viscosity, turbulent magnetic diffusivity, the density scale height, and the parameter β_*. The strength of the imposed magnetic field does not explicitly enter provided the location of the vertical boundaries are chosen such that the maximum of the eigenmode of NEMPI fits into the domain. The formation of sunspots and solar active regions is discussed as possible applications of NEMPI (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EURD observations of interstellar radiation

The hot interstellar medium (ISM) has far-reaching effect upon thestructure of galaxies. Although ISM heating processes are fairly wellunderstood, after decades of study, the processes that cool the hotinterstellar medium remain obscure. The EURD spectrograph was designed tomeasure the diffuse cosmic background from 350 to 1100 Å in order tostudy the hot ISM and the mechanisms by which it sheds its energy. Wepresent the first analysis of EURD observations of the cosmic background.These EURD observations have proven to be far more sensitive than previouswork; compared to previous results, we have improved the limits to theintensity of 450 to 900 Å line emission from the ISM by one to twoorders of magnitude. Our limit to OVI 1032 Å / 1038 Å doublet of 7900ph s^-1 cm^-2 str^-1 is the lowest yet reported. The EURDlimits to line emission are less intense than predicted by a varietytheoretical models of the local ISM.     

On the Polarization of Type I Radio Bursts

Circularly polarized radio radiation maintains its polarization even where the magnetic field reverses its sign relative to the ray (QT region) if the reversal is sufficiently abrupt (strong QT region). Bastian (1995) suggested that coronal turbulence scatters radiation, such as type I bursts, sufficiently to make the reversal abrupt where it would otherwise not be. However, the observed directivity of type I bursts sets an upper limit on the scattering. This limit implies that the turbulent scattering is not sufficient to maintain the circular polarization as in a strong QT region. The conclusion is strengthened by an analytical calculation of the polarization. Apparently, the fully polarized type I bursts, near disk center, encounter no horizontal magnetic fields, at least not until high enough in the corona that the QT region is strong anyway.     

Infrared spectroscopy of interstellar nanodiamonds from the Orgueil meteorite

Abstract— We present results from an ongoing study of the infrared (IR) and optical properties of nanodiamonds, an objective of which is to identify spectral features in the laboratory that could also be used telescopically to trace the presence of these particles in the interstellar medium (ISM). Fourier transform mid-and far-infrared spectra of nanodiamond residue extracted from the Orgueil (CI) chondrite were acquired. All of the mid-IR bands initially present were found to diminish, with the exception of a band at ～1100 cm^?1, following additional oxidation of the diamonds. The ～1100 cm^?1 band can be predominantly attributed to adsorbed species, especially an ether-type linkage, while the “oxidisable” features seem to be associated with less stable, surface-bonded species and residual carbonaceous material. We obtained three far-IR features but are uncertain about the origin of those at 475 and 188 cm^?1. We did not obtain a feature at ～120 cm^?1 reported by another group but do not discount the possibility that the band at 188 cm^?1 could be related to it. The weak absorption band at 475 cm^?1 (21 μm) is especially interesting because it may be strong in emission from hot nanodiamonds and, therefore, related to the unidentified infrared feature (UIF) observed at this wavelength in the spectra of some C-rich protoplanetary nebulae.     

Helical fields and filamentary molecular clouds – I

We study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first apply the virial theorem to filamentary molecular clouds, including the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds.
We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We examine the effects of the equation of state by comparing 'isothermal' filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state.
Our theoretical models involve three parameters: two to describe the mass loading of the toroidal and poloidal fields, and a third that describes the radial concentration of the filament. We thoroughly explore our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r ^−1.8 to r ⁻², while logatropes have density profiles that range from r ⁻¹ to r ^−1.8. We find that purely poloidal fields produce filaments with steep radial density gradients that are not allowed by the observations.