arXiv subjects

John Scalo

Publications and source records attributed to John Scalo.

33 records · Page 2Linked to original sources

arXiv2001

Pre-Existing Superbubbles as the Sites of Gamma-Ray Bursts

According to recent models, gamma-ray bursts apparently explode in a wide variety of ambient densities ranging from ~ 10^{-3} to 30 cm^{-3}. The lowest density environments seem, at first sight, to be incompatible with bursts in or near molecular clouds or with dense stellar winds and hence with the association of gamma-ray bursts with massive stars. We argue that low ambient density regions naturally exist in areas of active star formation as the interiors of superbubbles. The evolution of the interior bubble density as a function of time for different assumptions about the evaporative or hydrodynamical mass loading of the bubble interior is discussed. We present a number of reasons why there should exist a large range of inferred afterglow ambient densities whether gamma-ray bursts arise in massive stars or some version of compact star coalescence. We predict that many gamma-ray bursts will be identified with X-ray bright regions of galaxies, corresponding to superbubbles, rather than with blue localized regions of star formation. Massive star progenitors are expected to have their own circumstellar winds. The lack of evidence for individual stellar winds associated with the progenitor stars for the cases with afterglows in especially low density environments may imply low wind densities and hence low mass loss rates combined with high velocities. If gamma-ray bursts are associated with massive stars, this combination might be expected for compact progenitors with atmospheres dominated by carbon, oxygen or heavier elements, that is, progenitors resembling Type Ic supernovae.

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arXiv2001

Wind-Driven Gas Networks and Star Formation in Galaxies: Reaction-Advection Hydrodynamic Simulations

The effects of wind-driven star formation feedback on the spatio-temporal organization of stars and gas in galaxies is studied using two-dimensional intermediate-representational quasi-hydrodynamical simulations. The model retains only a reduced subset of the physics, including mass and momentum conservation, fully nonlinear fluid advection, inelastic macroscopic interactions, threshold star formation, and momentum forcing by winds from young star clusters on the surrounding gas. Expanding shells of swept-up gas evolve through the action of fluid advection to form a ``turbulent'' network of interacting shell fragments whose overall appearance is a web of filaments (in two dimensions). A new star cluster is formed whenever the column density through a filament exceeds a critical threshold based on the gravitational instability criterion for an expanding shell, which then generates a new expanding shell after some time delay. A filament- finding algorithm is developed to locate the potential sites of new star formation. The major result is the dominance of multiple interactions between advectively-distorted shells in controlling the gas and star morphology, gas velocity distribution and mass spectrum of high mass density peaks, and the global star formation history. The gas morphology observations of gas in the LMC and in local molecular clouds. The frequency distribution of present-to-past average global star formation rate, the distribution of gas velocities in filaments (found to be exponential), and the cloud mass spectra (estimated using a structure tree method), are discussed in detail.

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arXiv2001

Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium

Multifractal scaling (MFS) refers to structures that can be described as a collection of interwoven fractal subsets which exhibit power-law spatial scaling behavior with a range of scaling exponents (concentration, or singularity, strengths) and dimensions. The existence of MFS implies an underlying multiplicative (or hierarchical, or cascade) process. Panoramic column density images of several nearby star- forming cloud complexes, constructed from IRAS data and justified in an appendix, are shown to exhibit such multifractal scaling, which we interpret as indirect but quantitative evidence for nested hierarchical structure. The relation between the dimensions of the subsets and their concentration strengths (the "multifractal spectrum'') appears to satisfactorily order the observed regions in terms of the mixture of geometries present: strong point-like concentrations, line- like filaments or fronts, and space-filling diffuse structures. This multifractal spectrum is a global property of the regions studied, and does not rely on any operational definition of "clouds.'' The range of forms of the multifractal spectrum among the regions studied implies that the column density structures do not form a universality class, in contrast to indications for velocity and passive scalar fields in incompressible turbulence, providing another indication that the physics of highly compressible interstellar gas dynamics differs fundamentally from incompressible turbulence. (Abstract truncated)

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arXiv2000

Chemical enrichment and star formation in the Milky Way disk; I. Sample description and chromospheric age-metallicity relation

The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-type dwarfs. This sample was built from the intersection of photometric catalogues with chromospheric activity surveys of the Mount Wilson group. For these stars, metallicities were estimated from uvby data, and ages were calculated from their chromospheric emission levels using a new metallicity-dependent chromospheric activity-age relation developed by Rocha-Pinto & Maciel (1998). A careful estimate of the errors in the chromospheric age is made. The errors in the chromospheric indices are shown to include partially the effects of the stellar magnetic cycles, although a detailed treatment of this error is still beyond our knowledge. It is shown that the results are not affected by the presence of unresolved binaries in the sample. We derive an age-metallicity relation which confirms the mean trend found by previous workers. The mean metallicity shows a slow, steady increase with time, amounting at least 0.56 dex in 15 Gyr. The initial metallicity of the disk is around -0.70 dex, in agreement with the G dwarf metallicity distribution. According to our data, the intrinsic cosmic dispersion in metal abundances is around 0.13 dex, a factor of two smaller than that found by Edvardsson et al. (1993). We show that chromospheric ages are compatible with isochrone ages, within the expected errors, so that the difference in the scatter cannot be caused by the accuracy of our ages and metallicities. This reinforces some suggestions that the Edvarsson et al.'s sample is not suitable to the determination of the age-metallicity relation.

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arXiv2000

Chemical enrichment and star formation in the Milky Way disk; II. Star formation history

A chromospheric age distribution of 552 late-type dwarfs is transformed into a star formation history by the application of scale height corrections, stellar evolutionary corrections and volume corrections. We show that the disk of our Galaxy has experienced enhanced episodes of star formation at 0-1 Gyr, 2-5 Gyr and 7-9 Gyr ago, although the reality of the latter burst is still uncertain. The star sample birthsites are distributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate. These results are compared with the metal-enrichment rate, given by the age-metallicity relation, with the expected epochs of close encounters between our Galaxy and the Magellanic Clouds, and with previous determinations of the star formation history. Simulations are used to examine the age-dependent smearing of the star formation history due to age uncertainties, and the broadening of the recovered features, as well as to measure the probability level that the history derived to be produced by statistical fluctuations of a constant star formation history. We show with more than 98% level of confidence that the the star formation of our Galaxy was not constant.

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arXiv2000

The Galactic Shock Pump: A Source of Supersonic Internal Motions in the Cool Interstellar Medium

We propose that galactic shocks propagating through interstellar density fluctuations provide a mechanism for the intermittent replenishment, or "pumping," of the supersonic motions and internal density enhancements observed pervasively within cool atomic and molecular interstellar structures, without necessarily requiring the presence of self-gravity, magnetic fields, or young stars. The shocks are assumed to be due to a variety of galactic sources on a range of scales. An analytic result for the kinematic vorticity generated by a shock passing through a radially-stratified two-dimensional isobaric model cloud is derived, assuming that the Mach number is not so large that the cloud is disrupted, and neglecting the shock curvature and cloud distortion. Two-dimensional lattic gas hydrodynamic simulations at modest Mach numbers were used to verify the analytic result. The induced internal velocities are initially a significant fraction of the shock speed divided by the square root of the density contrast, accounting for both the observed linewidth amplitudes and the apparent cloud-to-cloud linewidth-density scaling. The linewidth-size relation could then be interpreted in terms of the well-known power spectrum of a system of shocks. The induced vortical energy should quickly be converted to compressible and MHD modes, and so would be difficult to observe directly, even though it would still be the power source for the other modes. The shockpump thus produces density structure without the necessity of any sort of instability. We argue that the shockpump should lead to nested shock-induced structures, providing a cascade mechanism for supersonic "turbulence" and a physical explanation for the fractal-like structure of the cool interstellar medium.

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arXiv2000

An Intermittent Star Formation History in a 'Normal' Disk Galaxy: The Milky Way

The star formation rate history of the Milky Way is derived using the chromospheric age distribution for 552 stars in the solar neighborhood. The stars sample birthsites distributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate history. The derivation incorporates the metallicity dependence of chromospheric emission at a given age, and corrections to account for incompleteness, scale height-age correlations, and stellar evolutionary effects. The results show that the Milky Way has undergone fluctuations in the global star formation rate with amplitudes greater than a factor of 2-3 on timescales less than 0.2-1 Gyr. The actual history is likely to be more bursty than found here because of the smearing effect of age uncertainties. There is some evidence for a slow secular increase in the star formation rate, perhaps a record of the accumulation history of our galaxy. A smooth nearly-constant star formation rate history is strongly ruled out, confirming the result first discovered by Barry (1988) using a smaller sample and a different age calibration. This result suggests that galaxies can fluctuate coherently on large scales.

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arXiv2000

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. Simulations of the instability alone show that the condenstion process which forms a dense phase (``clouds'') is highly dynamical, and that the boundaries of the clouds are accretion shocks, rather than static density discontinuities. The density histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak at low densities plus a slope change at high densities. Final static situations may be established, but the equilibrium is very fragile: small density fluctuations in the warm phase require large variations in the density of the cold phase, probably inducing shocks into the clouds. This result suggests that such configurations are highly unlikely. Simulations including turbulent forcing show that large- scale forcing is incapable of erasing the signature of the TI in the density PDFs, but small-scale, stellar-like forcing causes erasure of the signature of the instability. However, these simulations do not reach stationary regimes, TI driving an ever-increasing star formation rate. Simulations including magnetic fields, self-gravity and the Coriolis force show no significant difference between the PDFs of stable and unstable cases, and reach stationary regimes, suggesting that the combination of the stellar forcing and the extra effective pressure provided by the magnetic field and the Coriolis force overwhelm TI as a density-structure generator in the ISM. We emphasize that a multi-modal temperature PDF is not necessarily an indication of a multi-phase medium, which must contain clearly distinct thermal equilibrium phases.

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arXiv1999

Commentary on "The Theory of the Fluctuations in Brightness of the Milky Way. V" by S. Chandrasekhar and G. Munch (1952)

The series of papers by Chandrasekhar and Munch in the 1950s were concerned with the use of statistical models to infer the properties of interstellar clouds based on the observed spatial brightness fluctuations of the Milky Way. The present paper summarizes the subsequent influence of this work, concentrating on the departure from their earlier discrete cloud model to a continuous stochastic model in Paper V of the series. The contrast between the two models anticipated and parallels current tensions in the interpretation of interstellar structure, as well as intergalactic Lyman alpha clouds. The case of interstellar structure is discussed in some detail. Implications concerning the reification of models and the ability of scientific abstraction to model complex phenomena are also briefly discussed.

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arXiv1999

A Prescription for Star Formation Feedback: The Importance of Multiple Shell Interactions

The relation between the star formation rate and the kinetic energy increase in a region containing a large number of stellar sources is investigated as a possible prescription for star formation feedback in larger scale galaxy evolution simulations, and in connection with observed scaling relations for molecular clouds, extragalactic giant H II regions, and starburst galaxies. The kinetic energy increase is not simply proportional to the source input rate, but depends on the competition between stellar power input and dissipation due to interactions between structures formed and driven by the star formation. A simple one-zone model is used to show that, in a steady-state, the energy increase should be proportional to the two-thirds power of the stellar energy injection rate, with additional factors depending on the mean density of the region and mean column density of fragments. The scaling relation is tested using two-dimensional pressureless hydrodynamic simulations of wind-driven star formation, in which star formation occurs according to a threshold condition on the column density through a shell, and a large number of shells are present at any one time. The morphology of the simulations resembles an irregular network or web of dynamically-interacting filaments. A set of 16 simulations in which different parameters were varied agree remarkably with the simple analytical prescription for the scaling relation. Converting from wind power of massive stars to Lyman continuum luminosity shows that the cluster wind model for giant H II regions may still be viable.

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arXiv1998

The IMF in Young Galaxies: What Theory Might Tell Us

Observations have not yielded convincing results concerning the form of the stellar initial mass function (IMF) or its variations in space and time, so it is proposed that theoretical models may provide useful guidance. Several classes of theoretical models for the physics controlling the IMF are reviewed, emphasizing the dependence of the results on the probability distribution of velocity and density fluctuations, the spatial density distribution, the softness of the equation of state as controlled by the dominant heating and cooling processes, the ambient radiation field, and the star formation rate itself. One particularly relevant result is the ability of turbulent interactions to form high-density condensations, which should vary with decreasing redshift as the dominant cooling mechanism changes from H$_2$, to fine structure lines, and finally to CO and gas-grain collisions, due to corresponding changes in the effective polytropic exponent. A second theme is the ability of some of these models to produce a characteristic or turnover mass in the IMF which has nothing to do with the Jeans mass. Further development of such models should provide IMFs whose functional dependence can be used as input for simulations of galaxy formation, in which the energy input from massive stars plays a crucial role, and for calculations of the consequences of protogalactic star formation on the subequent evolution of the universe. The probable irrelevance of the concepts of pressure-confined clouds and the formation of condensations by thermal instability in a turbulent interstellar medium is also discussed.

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arXiv1998

Clouds as Turbulent Density Fluctuations. Implications for pressure confinement and spectral line data interpretation

We examine the idea that diffuse and giant molecular clouds and their substructure form as density fluctuations induced by large scale interstellar turbulence. We do this by investigating the topology of various fields in realistic simulations of the ISM. We find that a) the velocity field is continuous across threshold-defined cloud boundaries; b) such cloud boundaries are rather arbitrary, with no correspondence to any actual physical boundary, such as a density discontinuity; c) abrupt velocity jumps are coincident with the density maxima; d) the volume and surface kinetic terms in the Eulerian Virial Theorem for a cloud ensemble are comparable in general; e) the magnetic field exhibits bends and reversals highly correlated with similar density features. These results suggest that clouds are formed by colliding gas streams. Within this framework, 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. Turbulent pressure confinement appears self-defeating, because turbulence contains large-scale motions which necessarily distort cloud boundaries. Density-weighted velocity histograms show similar FWHMs and similar multi-component structure to those of observational line profiles, though the histogram features do not correspond to isolated "clumps", but rather to extended regions throughout a cloud. We argue that the results presented here may be also applicable to small scales with larger densities (molecular clouds and cores) and suggest that quasi- hydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic at late stages of collapse. We expect this to occur only at protostellar densities.

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arXiv1998

Clustering Properties of Stars in Simulations of Wind-Driven Star Formation

Several recent observational studies have shown that the clustering of young stars in local star-forming regions, and of Cepheids in the LMC, can be described by a power law two-point correlation function. We show by numerical simulations that the observed range in power law slopes can be accounted for by a model in which stellar winds drive expanding shells that are subjected to nonlinear fluid advection and interactions with other shells, and in which star formation occurs when a threshold shell column density is exceeded. The models predict how the power law slope should depend on the maximum age of the stellar sample and the average star formation rate, although a number of effects preclude a comparison with currently-available data. We also show how stellar migration flattens the power law slope below a scale that depends on the velocity dispersion and age of the sample, an effect which may explain the secondary breaks in the observed correlation functions of some regions at large separations. Problems with using the correlation function as a descriptor of clustering structure for statistically inhomogeneous data sets are discussed.

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arXiv1998

The IMF Revisited: A Case for Variations

A survey of results concerning the IMF derived from star counts is presented, including work up to, but not including, that presented in these proceedings. The situation regarding low-mass stars in the field and in clusters, high-mass stars and intermediate-mass stars in clusters and associations of the Milky Way and LMC, pre-main sequence objects in visible and embedded clusters, and the IMF in galaxies more distant than the Magellanic Clouds is discussed, with an emphasis on the sources of uncertainty. Most of these uncertainties, especially radial mass segregation and unresolved binaries, would steepen the true IMF relative to the apparent IMF. Several cases of apparently large variations in cluster IMFs are pointed out, and a graphical comparison of results for about 60 clusters shows a spread of at least unity in the logarithmic IMF index for all mass ranges above about 1 $M_{\sun}$. I conclude that either: 1. The uncertainties are so large that very little can be said about an average IMF or IMF variations; or 2. If the observations are taken at face value, there are strong indications of IMF variations, which do not seem to correlate with obvious environmental conditions like metallicity or stellar density. If there is an average IMF, I suggest that it is steepest at intermediate masses. If the variations are real, they offer a useful test of theoretical models.

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arXiv1997

Self-Organized Large-Scale Coherence in Simulations of Galactic Star Formation

It is often assumed that galaxies cannot generate large-scale coherent star- forming activity without some organizing agent, such as spiral density waves, bars, large-scale instabilities, or external perturbations due to encounters with other galaxies. We present simulations of a simple model of star formation in which local spatial couplings lead to large-scale coherent, and even synchronized, patterns of star formation without any explicit propagation or any separate organizing agent. At a given location, star formation is assumed to occur when the gas velocity dispersion falls below a critical value dependent on the density. Young stars inject energy into the gas in their neighborhood, increasing the velocity dispersion and inhibiting the instability. A dissipation function continually "cools" the gas. The stability of this local inhibitory feedback model is examined both analytically and numerically. A large number of two-dimensional simulations are used to examine the effect of spatial couplings due to energy injection into neighboring regions. We find that several distinct types of behavior can be demarcated in a phase diagram whose parameter axes are the density (assumed constant in most models) and spatial coupling strength. These "phases" include, with decreasing density, a spatially homogeneous steady state, oscillatory "islands," traveling waves of star formation or global synchronization, and scattered "patches" of star formation activity. The coherence effects are explained in terms of the ability of the energy injected near a star formation site to introduce phase correlations in the subsequent cooling curves of neighboring regions.

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