John Scalo

Interstellar Turbulence I: Observations and Processes

▪ Abstract Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...

Biosignatures from Earth-like planets around M dwarfs.

Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible/near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs-AD Leo and GJ 643-and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earths was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planets atmosphere has been suggested as strong evidence for life. Planets circling M stars may be good locations to search for such evidence.

Interstellar Turbulence II: Implications and Effects

▪ Abstract Interstellar turbulence has implications for the dispersal and mixing of the elements, cloud chemistry, cosmic ray scattering, and radio wave propagation through the ionized medium. This review discusses the observations and theory of these effects. Metallicity fluctuations are summarized, and the theory of turbulent transport of passive tracers is reviewed. Modeling methods, turbulent concentration of dust grains, and the turbulent washout of radial abundance gradients are discussed. Interstellar chemistry is affected by turbulent transport of various species between environments with different physical properties and by turbulent heating in shocks, vortical dissipation regions, and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered and accelerated in turbulent magnetic waves and shocks, and they generate turbulence on the scale of their gyroradii. Radio wave scintillation is an important diagnostic for small-scale turbulence in the ionized medium, giving information abou...

A Reappraisal of The Habitability of Planets around M Dwarf Stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

CLOUDS AS TURBULENT DENSITY FLUCTUATIONS: IMPLICATIONS FOR PRESSURE CONFINEMENT AND SPECTRAL LINE DATA INTERPRETATION

We examine the idea that diffuse H I and giant molecular clouds and their substructure form as density fluctuations induced by large-scale interstellar turbulence. We do this by closely investigating the topology of the velocity, density, and magnetic fields within and at the boundaries of the clouds emerging in high-resolution two-dimensional simulations of the interstellar medium (ISM) including self-gravity, magnetic fields, parameterized heating and cooling, and a simple model for star formation. We find that the velocity field is continuous across cloud boundaries for a hierarchy of clouds of progressively smaller sizes. Cloud boundaries defined by a density-threshold criterion are found to be quite arbitrary, with no correspondence to any actual physical boundary, such as a density discontinuity. Abrupt velocity jumps are coincident with the density maxima, which indicates that the clouds are formed by colliding gas streams. This conclusion is also supported by the fact that the volume and surface kinetic terms in the Eulerian virial theorem for a cloud ensemble are comparable in general and by the topology of the magnetic field, which exhibits bends and reversals where the gas streams collide. However, no unique trend of alignment between density and magnetic features is observed. Both sub- and super-Alfvenic motions are observed within the clouds. In light of these results, we argue that thermal pressure equilibrium is irrelevant for cloud confinement in a turbulent medium, since inertial motions can still distort or disrupt a cloud, unless it is strongly gravitationally bound. Turbulent pressure confinement appears self-defeating because turbulence contains large-scale motions that necessarily distort Lagrangian cloud boundaries or equivalently cause flux through Eulerian boundaries. We then discuss the compatibility of the present scenario with observational data. We find that density-weighted velocity histograms are consistent with observational line profiles of comparable spatial and velocity resolution, exhibiting similar FWHMs and similar multicomponent structure. An analysis of the regions contributing to each velocity interval indicates that the histogram features do not come from isolated clumps but rather from extended regions throughout a cloud, which often have very different total velocity vectors. Finally, we argue that the scenario presented here may also be applicable to small scales with larger densities (molecular clouds and their substructure, up to at least n~103-105 cm-3) and conjecture that quasi-hydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic. We demonstrate, using appropriate cooling rates, that this will not occur except for very small compressions (10-2 pc) or until protostellar densities are reached for collapse.

On the Probability Density Function of Galactic Gas. I. Numerical Simulations and the Significance of the Polytropic Index

We investigate the form of the one-point probability density function (pdf) for the density field of the interstellar medium using numerical simulations that successively reduce the number of physical processes included. Two-dimensional simulations of self-gravitating supersonic MHD turbulence, of supersonic self-gravitating hydrodynamic turbulence, and of decaying Burgers turbulence produce in all cases filamentary density structures and evidence for a power-law density pdf at large densities with logarithmic slope between -1.7 and -2.3. This suggests that a power-law shape of the pdf and the general filamentary morphology are the signature of the nonlinear advection operator. These results do not support previous claims that the pdf is lognormal. A series of one-dimensional simulations of forced supersonic polytropic turbulence is used to resolve the discrepancy. They suggest that the pdf is lognormal only for effective polytropic indices γ = 1 (or nearly lognormal for γ ≠ 1 if the Mach number is sufficiently small), while power laws develop for densities larger than the mean if γ < 1. We evaluate the polytropic index for conditions relevant to the cool interstellar medium using published cooling functions and different heating sources, finding that a lognormal pdf should probably occur at densities around 103 and is possible at larger densities, depending strongly on the role of gas-grain heating and cooling. Several applications are examined. First, we question a recent derivation of the initial mass function from the density pdf by Padoan, Nordlund, & Jones because (1) the pdf does not contain spatial information and (2) their derivation produces the most massive stars in the voids of the density distribution. Second, we illustrate how a distribution of ambient densities can alter the predicted form of the size distribution of expanding shells. Finally, a brief comparison is made with the density pdfs found in cosmological simulations.

A DECREASED PROBABILITY OF HABITABLE PLANET FORMATION AROUND LOW-MASS STARS

Smaller terrestrial planets (P0.3 M⊕) are less likely to retain the substantial atmospheres and ongoing tectonic activity probably required to support life. A key element in determining whether sufficiently massive sustainably habitable planets can form is the availability of solid planet-forming material. We use dynamical simulations of terrestrial planet formation from planetary embryos and simple scaling arguments to explore the implications of correlations between terrestrial planet mass, disk mass, and the mass of the parent star. We assume that the protoplanetary disk mass scales with stellar mass as M-disk ∝ fMh*(h), where f measures the relative disk mass and 1/2 0.3 M⊕ habitable planets decreases for low-mass stars for every realistic combination of parameters. This habitable fraction is small for stellar masses below a mass in the interval 0.5-0.8 M☉, depending on disk parameters, an interval that excludes most M stars. Radial mixing and therefore water delivery are inefficient in the lower mass disks commonly found around low-mass stars, such that terrestrial planets in the habitable zones of most low-mass stars are likely to be small and dry.

Perception of Interstellar Structure: Facing Complexity

This paper challenges some orthodox notions concerning the structure and evolution of star-forming regions, proposing that they arise largely by a dual process in which conceptual models are fashioned after categories which are in great part reflections of observational limitations, and the models are projected onto interpretations of data, an example of hypostatization of categories. Several examples are discussed. The need for internal support of molecular clouds is questioned. It is suggested that the inverse density-size relation often claimed for clouds and accounted for by several theoretical models is an artifact caused by limited dynamic range in column density detectability, selection bias, distance uncertainties, and internal density gradients, and is contradicted by several unbiased surveys. Limited spatial dynamic range (ratio of image size to resolution) in maps of column density structure results in a “Mr. Magoo effect” which tends to accommodate quasi-static evolutionary concepts. Column density structures mapped with a large spatial and column density dynamic range are dominated by irregular, connected, and nested forms on all scales. Contour shapes of both atomic and molecular clouds exhibit self-similar irregularity with a common fractal dimension over a large range in scale. These features and a technique for the quantification of complex structure are illustrated with a densely-sampled column density image of the Taurus region constructed from IRAS data. A comparison with 553 high-accuracy polarization vectors in the region is also given.

IS THERMAL INSTABILITY SIGNIFICANT IN TURBULENT GALACTIC GAS

We investigate numerically the role of thermal instability (TI) as a generator of density structures in the interstellar medium (ISM), both by itself and in the context of a globally turbulent medium. We consider three sets of numerical simulations: (1) —ows in the presence of the instability only; (2) —ows in the presence of the instability and various types of turbulent energy injection (forcing), and (3) models of the ISM including the magnetic —eld, the Coriolis force, self-gravity and stellar energy injection. Simula- tions in the —rst group show that the condensation process that forms a dense phase (ii clouds ˇˇ) is highly dynamical and that the boundaries of the clouds are accretion shocks, rather than static density discon- tinuities. The density histograms (probability density functions (PDFs)) of these runs exhibit either bimodal shapes or a single peak at low densities plus a slope change at high densities. Final static situ- ations may be established, but the equilibrium is very fragile: small density —uctuations in the warm phase require large variations in that of the cold phase, probably inducing shocks in the clouds. Com- bined with the likely disruption of the clouds by Kelvin-Helmholtz instability, this result suggests that such con—gurations are highly unlikely. Simulations in the second group show that large-scale turbulent forcing is incapable of erasing the signature of TI in the density PDFs, but small-scale, stellar-like forcing causes the PDFs to transit from bimodal to a single-slope power law, erasing the signature of the instability. However, these simulations do not reach stationary regimes, with TI driving an ever- increasing star formation rate. Simulations in the third group show no signi—cant diUerence between the PDFs of stable and unstable cases and reach stationary regimes, suggesting that the combination of the stellar forcing and the extra eUective pressure provided by the magnetic —eld and the Coriolis force over- whelm TI as a density-structure generator in the ISM, with TI becoming a second-order eUect. We emphasize that a multimodal temperature PDF is not necessarily an indication of a multiphase medium, which must contain clearly distinct thermal equilibrium phases, and that this ii multiphase ˇˇ terminology is often inappropriately used. Subject headings: instabilitiesISM: structureturbulence

THE TEMPERATURE DISTRIBUTION IN TURBULENT INTERSTELLAR GAS

We discuss the temperature distribution in a two-dimensional, thermally unstable numerical simulation of the warm and cold gas in the Galactic disk, including the magnetic field, self-gravity, the Coriolis force, stellar energy injection, and a realistic cooling function. We find that ~50% of the turbulent gas mass has temperatures in what would be the thermally unstable range if thermal instability were to be considered alone. This appears to be a consequence of there being many other forces at play than just thermal pressure, constituting a different process from that proposed in time-dependent models based on stochastic heating followed by cooling, although the latter mechanism may also be present. We also point out that a bimodal temperature probability distribution function is a simple consequence of the form of the interstellar cooling function and is not necessarily a signature of discontinuous phase transitions.