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At least 1,423 records · Page 79
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DOE OSTI2012

THE UNIVERSAL INITIAL MASS FUNCTION IN THE EXTENDED ULTRAVIOLET DISK OF M83

We report deep Subaru H{alpha} observations of the extended ultraviolet (XUV) disk of M83. These new observations enable the first complete census of very young stellar clusters over the entire XUV disk. Combining Subaru and Galaxy Evolution Explorer data with a stellar population synthesis model, we find that (1) the standard, but stochastically sampled, initial mass function (IMF) is preferred over the truncated IMF because there are low-mass stellar clusters (10{sup 2-3} M{sub Sun }) that host massive O-type stars; (2) the standard Salpeter IMF and a simple aging effect explain the counts of far-UV (FUV)-bright and H{alpha}-bright clusters with masses >10{sup 3} M{sub Sun }; and (3) the H{alpha}-to-FUV flux ratio over the XUV disk supports the standard IMF. To reach conclusion (2), we assumed instantaneous cluster formation and a constant cluster formation rate over the XUV disk. The Subaru Prime Focus Camera covers a large area even outside the XUV disk-far beyond the detection limit of the H I gas. This enables us to statistically separate the stellar clusters in the disk from background contamination. The new data, model, and previous spectroscopic studies provide overall consistent results with respect to the internal dust extinction (A{sub V} {approx} 0.1 mag) and low metallicity ({approx}0.2 Z{sub Sun }) using the dust extinction curve of the Small Magellanic Cloud. The minimum cluster mass for avoiding the upper IMF incompleteness due to stochastic sampling and the spectral energy distributions of O, B, and A stars are discussed in the Appendices.

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DOE OSTI2012

COSMOS: STOCHASTIC BIAS FROM MEASUREMENTS OF WEAK LENSING AND GALAXY CLUSTERING

In the theory of structure formation, galaxies are biased tracers of the underlying matter density field. The statistical relation between galaxy and matter density field is commonly referred to as galaxy bias. In this paper, we test the linear bias model with weak-lensing and galaxy clustering measurements in the 2 deg{sup 2} COSMOS field. We estimate the bias of galaxies between redshifts z = 0.2 and z = 1 and over correlation scales between R = 0.2 h{sup -1} Mpc and R = 15 h{sup -1} Mpc. We focus on three galaxy samples, selected in flux (simultaneous cuts I{sub 814W} < 26.5 and K{sub s} < 24) and in stellar mass (10{sup 9} < M{sub *} < 10{sup 10} h{sup -2} M{sub Sun} and 10{sup 10} < M{sub *} < 10{sup 11} h{sup -2} M{sub Sun }). At scales R > 2 h{sup -1} Mpc, our measurements support a model of bias increasing with redshift. The Tinker et al. fitting function provides a good fit to the data. We find the best-fit mass of the galaxy halos to be log (M{sub 200}/h{sup -1} M{sub Sun }) = 11.7{sup +0.6}{sub -1.3} and log (M{sub 200}/h{sup -1} M{sub Sun }) = 12.4{sup +0.2}{sub -2.9}, respectively, for the low and high stellar-mass samples. In the halo model framework, bias is scale dependent with a change of slope at the transition scale between the one and the two halo terms. We detect a scale dependence of bias with a turndown at scale R = 2.3 {+-} 1.5 h{sup -1} Mpc, in agreement with previous galaxy clustering studies. We find no significant amount of stochasticity, suggesting that a linear bias model is sufficient to describe our data. We use N-body simulations to quantify both the amount of cosmic variance and systematic errors in the measurement.

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DOE OSTI2012

ANALYZING STAR CLUSTER POPULATIONS WITH STOCHASTIC MODELS: THE HUBBLE SPACE TELESCOPE/WIDE FIELD CAMERA 3 SAMPLE OF CLUSTERS IN M83

The majority of clusters in the universe have masses well below 10{sup 5} M{sub Sun }. Hence, their integrated fluxes and colors can be affected by the presence or absence of a few bright stars introduced by stochastic sampling of the stellar mass function. Specific methods are being developed to extend the analysis of cluster energy distributions into the low-mass regime. In this paper, we apply such a method to real observations of star clusters, in the nearby spiral galaxy M83. We reassess the ages and masses of a sample of 1242 clusters for which UBVIH{alpha} fluxes were obtained from observations with the Wide Field Camera 3 instrument on board the Hubble Space Telescope. Synthetic clusters with known properties are used to characterize the limitations of the method (valid range and resolution in age and mass, method artifacts). The ensemble of color predictions of the discrete cluster models are in good agreement with the distribution of observed colors. We emphasize the important role of the H{alpha} data in the assessment of the fraction of young objects, particularly in breaking the age-extinction degeneracy that hampers an analysis based on UBVI data only. We find the mass distribution of the cluster sample to follow a power law of index -2.1 {+-} 0.2, and the distribution of ages a power law of index -1.0 {+-} 0.2 for log (M/ M{sub Sun }) > 3.5, and ages between 10{sup 7} and 10{sup 9} yr. An extension of our main method, which makes full use of the probability distributions of age and mass obtained for the individual clusters of the sample, is explored. It produces similar power-law slopes and will deserve further investigation. Although the properties derived for individual clusters significantly differ from those obtained with traditional, non-stochastic models in about 30% of the objects, the first-order aspect of the age and mass distributions is similar to those obtained previously for this M83 sample in the range of overlap of the studies. We extend the power-law description to lower masses with better mass and age resolution and without most of the artifacts produced by the classical method.

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DOE OSTI2012

LOCAL ANISOTROPY, HIGHER ORDER STATISTICS, AND TURBULENCE SPECTRA

Correlation anisotropy emerges dynamically in magnetohydrodynamics (MHD), producing stronger gradients across the large-scale mean magnetic field than along it. This occurs both globally and locally, and has significant implications in space and astrophysical plasmas, including particle scattering and transport, and theories of turbulence. Properties of local correlation anisotropy are further documented here by showing through numerical experiments that the effect is intensified in more localized estimates of the mean field. The mathematical formulation of this property shows that local anisotropy mixes second-order with higher order correlations. Sensitivity of local statistical estimates to higher order correlations can be understood in connection with the stochastic coordinate system inherent in such formulations. We demonstrate this in specific cases, and illustrate the connection to higher order statistics by showing the sensitivity of local anisotropy to phase randomization, after which the global measure of anisotropy is recovered at all scales of averaging. This establishes that anisotropy of the local structure function is not a measure of anisotropy of the energy spectrum. Evidently, the local enhancement of correlation anisotropy is of substantial fundamental interest and must be understood in terms of higher order correlations, specifically fourth-order and above.

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DOE OSTI2012

A GENERALIZED DIFFUSION TENSOR FOR FULLY ANISOTROPIC DIFFUSION OF ENERGETIC PARTICLES IN THE HELIOSPHERIC MAGNETIC FIELD

The spatial diffusion of cosmic rays in turbulent magnetic fields can, in the most general case, be fully anisotropic, i.e., one has to distinguish three diffusion axes in a local, field-aligned frame. We reexamine the transformation for the diffusion tensor from this local to a global frame, in which the Parker transport equation for energetic particles is usually formulated and solved. Particularly, we generalize the transformation formulae to allow for an explicit choice of two principal local perpendicular diffusion axes. This generalization includes the 'traditional' diffusion tensor in the special case of isotropic perpendicular diffusion. For the local frame, we describe the motivation for the choice of the Frenet-Serret trihedron, which is related to the intrinsic magnetic field geometry. We directly compare the old and the new tensor elements for two heliospheric magnetic field configurations, namely the hybrid Fisk and Parker fields. Subsequently, we examine the significance of the different formulations for the diffusion tensor in a standard three-dimensional model for the modulation of galactic protons. For this, we utilize a numerical code to evaluate a system of stochastic differential equations equivalent to the Parker transport equation and present the resulting modulated spectra. The computed differential fluxes based on the new tensor formulation deviate from those obtained with the 'traditional' one (only valid for isotropic perpendicular diffusion) by up to 60% for energies below a few hundred MeV depending on heliocentric distance.

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MINIMUM REQUIREMENTS FOR DETECTING A STOCHASTIC GRAVITATIONAL WAVE BACKGROUND USING PULSARS

We assess the detectability of a nanohertz gravitational wave (GW) background in a pulsar timing array (PTA) program by considering the shape and amplitude of the cross-correlation function summed over pulsar pairs. The distribution of correlation amplitudes is found to be non-Gaussian and highly skewed, which significantly influences detection and false-alarm probabilities. When only white noise combines with GWs in timing data, our detection results are consistent with those found by others. Contamination by red noise from spin variations and from any uncorrected interstellar plasma effects significantly increases the false-alarm probability. The number of arrival times (and thus the observing cadence) is important only as long as the residuals are dominated by white noise. When red noise and GWs dominate, the statistical significance of the correlation estimate can be improved only by increasing the number of pulsars. We characterize plausible detection regimes by evaluating the number of millisecond pulsars (MSPs) that must be monitored in a high-cadence, five-year timing program to detect a GW background spectrum h{sub c} (f) = A(f/f{sub 0}){sup -2/3} with f{sub 0} = 1 yr{sup -1} and A = 10{sup -15}. Our results indicate that a sample of 20 super-stable MSPs-those with rms timing residuals {sigma}{sub r} {approx}< 20 ns(A/10{sup -15}) from red-noise contributions over a five-year span-will allow detection of the GW background and study of its spectrum. However, a timing program on {approx}> 50-100 MSPs is likely needed for a complete PTA program, particularly if red noise is generally present in MSPs.

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DOE OSTI2012

COLLISIONS BETWEEN GRAVITY-DOMINATED BODIES. II. THE DIVERSITY OF IMPACT OUTCOMES DURING THE END STAGE OF PLANET FORMATION

Numerical simulations of the stochastic end stage of planet formation typically begin with a population of embryos and planetesimals that grow into planets by merging. We analyzed the impact parameters of collisions leading to the growth of terrestrial planets from recent N-body simulations that assumed perfect merging and calculated more realistic outcomes using a new analytic collision physics model. We find that collision outcomes are diverse and span all possible regimes: hit-and-run, merging, partial accretion, partial erosion, and catastrophic disruption. The primary outcomes of giant impacts between planetary embryos are approximately evenly split between partial accretion, graze-and-merge, and hit-and-run events. To explore the cumulative effects of more realistic collision outcomes, we modeled the growth of individual planets with a Monte Carlo technique using the distribution of impact parameters from N-body simulations. We find that fewer planets reached masses >0.7 M{sub Earth} using the collision physics model compared to simulations that assumed every collision results in perfect merging. For final planets with masses >0.7 M{sub Earth}, 60% are enriched in their core-to-mantle mass fraction by >10% compared to the initial embryo composition. Fragmentation during planet formation produces significant debris ({approx}15% of the final mass) and occurs primarily by erosion of the smaller body in partial accretion and hit-and-run events. In partial accretion events, the target body grows by preferentially accreting the iron core of the projectile and the escaping fragments are derived primarily from the silicate mantles of both bodies. Thus, the bulk composition of a planet can evolve via stochastic giant impacts.

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DOE OSTI2012

AGE AND MASS FOR 920 LARGE MAGELLANIC CLOUD CLUSTERS DERIVED FROM 100 MILLION MONTE CARLO SIMULATIONS

We present new age and mass estimates for 920 stellar clusters in the Large Magellanic Cloud (LMC) based on previously published broadband photometry and the stellar cluster analysis package, MASSCLEANage. Expressed in the generic fitting formula, d{sup 2} N/dMdt{proportional_to}M{sup {alpha}} t{sup {beta}}, the distribution of observed clusters is described by {alpha} = -1.5 to -1.6 and {beta} = -2.1 to -2.2. For 288 of these clusters, ages have recently been determined based on stellar photometric color-magnitude diagrams, allowing us to gauge the confidence of our ages. The results look very promising, opening up the possibility that this sample of 920 clusters, with reliable and consistent age, mass, and photometric measures, might be used to constrain important characteristics about the stellar cluster population in the LMC. We also investigate a traditional age determination method that uses a {chi}{sup 2} minimization routine to fit observed cluster colors to standard infinite-mass limit simple stellar population models. This reveals serious defects in the derived cluster age distribution using this method. The traditional {chi}{sup 2} minimization method, due to the variation of U, B, V, R colors, will always produce an overdensity of younger and older clusters, with an underdensity of clusters in the log (age/yr) = [7.0, 7.5] range. Finally, we present a unique simulation aimed at illustrating and constraining the fading limit in observed cluster distributions that includes the complex effects of stochastic variations in the observed properties of stellar clusters.

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DOE OSTI2012

EFFECTS OF NON-ISOTROPIC SCATTERING, MAGNETIC HELICITY, AND ADIABATIC FOCUSING ON DIFFUSIVE TRANSPORT OF SOLAR ENERGETIC PARTICLES

Transport of solar energetic particles in interplanetary space is analyzed. A new systematic derivation of the diffusion approximation is given, which incorporates the effects of non-isotropic scattering, magnetic helicity, and adiabatic focusing in a non-uniform large-scale magnetic field. The derivation is based on a system of stochastic differential equations, equivalent to the Fokker-Planck equation, and the new method is a generalization of the Smoluchowski approximation in the theory of the Brownian motion. Simple, physically transparent expressions for the transport coefficients are derived. Different results of earlier treatments of the problem are related to the assumptions regarding the evolving particle distribution.

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DOE OSTI2012

THE RELATIVE ABUNDANCE OF ISOLATED CLUSTERS AS A PROBE OF DARK ENERGY

Those galaxy clusters that do not belong to superclusters are referred to as isolated clusters. Their relative abundance at a given epoch may be a powerful constraint of the dark energy equation of state since it depends strongly on how fast the structures grow on the largest scale in the universe. We note that the mass function of isolated clusters can be separately evaluated through modification of the recently developed Corasaniti-Achitouv (CA) theory according to which the stochastic collapse barrier is quantified by two coefficients: the drifting average coefficient ({beta}) and the diffusion coefficient (D{sub B} ). Regarding {beta} in the CA formalism as an adjustable parameter and assuming that the formation of isolated clusters corresponds to the case of D{sub B} = 0, we determine the mass function of isolated clusters by fitting the numerical results from the MICE simulations to the modified CA formula. It is found that the best-fit value of {beta} changes with redshift and that the CA mass function with D{sub B} = 0 agrees very well with the numerical results at various redshifts. Defining the relative abundance of isolated clusters, {xi}{sub I}, as the ratio of the cumulative mass function of isolated clusters to that of non-isolated clusters at a given epoch, we finally show how sensitively {xi}{sub I} changes with the dark energy equation of state. It is also discussed how {xi}{sub I} can help break the degeneracy between the dark energy equation of state and the other key cosmological parameters.

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DOE OSTI2012

DETECTING GRAVITATIONAL WAVE MEMORY WITH PULSAR TIMING

We compare the detectability of gravitational bursts passing through the solar system with those passing near each millisecond pulsar in an N-pulsar timing array. The sensitivity to Earth-passing bursts can exploit the correlation expected in pulse arrival times while pulsar-passing bursts, though uncorrelated between objects, provide an N-fold increase in overall time baseline that can compensate for the lower sensitivity. Bursts with memory from mergers of supermassive black holes produce step functions in apparent spin frequency that are the easiest to detect in pulsar timing. We show that the burst rate and amplitude distribution, while strongly dependent on inadequately known cosmological evolution, may favor detection in the pulsar terms rather than the Earth timing perturbations. Any contamination of timing data by red spin noise makes burst detection more difficult because both signals grow with the length of the time data span T. Furthermore, the different bursts that could appear in one or more data sets of length T Almost-Equal-To 10 yr also affect the detectability of the gravitational wave stochastic background that, like spin noise, has a red power spectrum. A burst with memory is a worthwhile target in the timing of multiple pulsars in a globular cluster because it should produce a correlated signal with a time delay of less than about 10 years in some cases.

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DOE OSTI2012

STAR FORMATION LAWS: THE EFFECTS OF GAS CLOUD SAMPLING

Recent observational results indicate that the functional shape of the spatially resolved star formation-molecular gas density relation depends on the spatial scale considered. These results may indicate a fundamental role of sampling effects on scales that are typically only a few times larger than those of the largest molecular clouds. To investigate the impact of this effect, we construct simple models for the distribution of molecular clouds in a typical star-forming spiral galaxy and, assuming a power-law relation between star formation rate (SFR) and cloud mass, explore a range of input parameters. We confirm that the slope and the scatter of the simulated SFR-molecular gas surface density relation depend on the size of the sub-galactic region considered, due to stochastic sampling of the molecular cloud mass function, and the effect is larger for steeper relations between SFR and molecular gas. There is a general trend for all slope values to tend to {approx}unity for region sizes larger than 1-2 kpc, irrespective of the input SFR-cloud relation. The region size of 1-2 kpc corresponds to the area where the cloud mass function becomes fully sampled. We quantify the effects of selection biases in data tracing the SFR, either as thresholds (i.e., clouds smaller than a given mass value do not form stars) or as backgrounds (e.g., diffuse emission unrelated to current star formation is counted toward the SFR). Apparently discordant observational results are brought into agreement via this simple model, and the comparison of our simulations with data for a few galaxies supports a steep (>1) power-law index between SFR and molecular gas.

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A SECOND-ORDER BIAS MODEL FOR THE LOGARITHMIC HALO MASS DENSITY

We present an analytic model for the local bias of dark matter halos in a {Lambda}CDM universe. The model uses the halo mass density instead of the halo number density and is searched for various halo mass cuts, smoothing lengths, and redshift epochs. We find that, when the logarithmic density is used, the second-order polynomial can fit the numerical relation between the halo mass distribution and the underlying matter distribution extremely well. In this model, the logarithm of the dark matter density is expanded in terms of log halo mass density to the second order. The model remains excellent for all halo mass cuts (from M{sub cut} = 3 Multiplication-Sign 10{sup 11} to 3 Multiplication-Sign 10{sup 12} h{sup -1} M{sub Sun }), smoothing scales (from R = 5 h{sup -1} Mpc to 50 h{sup -1} Mpc), and redshift ranges (from z = 0 to 1.0) considered in this study. The stochastic term in the relation is found to be not entirely random, but a part of the term can be determined by the magnitude of the shear tensor.

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DOE OSTI2012

NON-STANDARD ENERGY SPECTRA OF SHOCK-ACCELERATED SOLAR PARTICLES

We consider a numerical model for the shock acceleration of energetic ions in the magnetic environment of the solar corona. The model is motivated by observations of the deka-to-hecto-MeV proton energy spectra, ion and electron timing, and abundances in the beginning of major solar energetic particle (SEP) events, prior to the event's main phase associated with coronal mass ejection (CME) driven shock in the solar wind. Inasmuch as the obliquity of the CME-liftoff-associated shocks in solar corona and hence the seed-particle supply for the shock acceleration are essentially time dependent, a steady state energy spectrum of accelerated protons near the shock could not be attained. Energy spectrum of the SEP emission depends on the spatial and energy distribution of seed particles for the coronal shock acceleration, on the shock wave history, and on the location and scenario of the energetic particle escape into the interplanetary medium. We use a numerical model of the shock acceleration on a semicircular magnetic field line to learn a significance of different effects. If the shock geometry in a particular magnetic tube changes from nearly parallel to perpendicular, the resulting SEP spectrum in most distant sections of the tube, e.g., at the top of a transequatorial loop, resembles a wide beam, which is very different from the standard power-law spectrum that would be expected in a steady state. Possible escape of the shock-accelerated particles from more than one coronal location, stochastic re-acceleration, and the magnetic tube expansion can make the SEP spectra even more complicated.

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PRACTICAL METHODS FOR CONTINUOUS GRAVITATIONAL WAVE DETECTION USING PULSAR TIMING DATA

Gravitational waves (GWs) are tiny ripples in the fabric of space time predicted by Einstein's general relativity. Pulsar timing arrays (PTAs) are well poised to detect low-frequency (10{sup -9}-10{sup -7} Hz) GWs in the near future. There has been a significant amount of research into the detection of a stochastic background of GWs from supermassive black hole binaries (SMBHBs). Recent work has shown that single continuous sources standing out above the background may be detectable by PTAs operating at a sensitivity sufficient to detect the stochastic background. The most likely sources of continuous GWs in the pulsar timing frequency band are extremely massive and/or nearby SMBHBs. In this paper we present detection strategies including various forms of matched filtering and power spectral summing. We determine the efficacy and computational cost of such strategies. It is shown that using an optimal matched filter explicitly including the poorly constrained pulsar distances with a grid-based method is computationally infeasible. We show that an Earth-term-matched filter constructed using only the correlated signal terms is robust, computationally viable and highly sensitive to GW signals. We further show that a simple power spectral summing technique is nearly equivalent to the Earth-term-matched filter in terms of the minimum detectable amplitude. Both of these techniques are only a factor of two less sensitive than the computationally unrealizable optimal matched filter. We also show that a pairwise matched filter, taking the pulsar distances into account, is comparable to the optimal matched filter for the single template case and comparable to the Earth-term-matched filter for many search templates. Finally, using simulated data optimal quality, we place a theoretical minimum detectable strain amplitude of h > 2 Multiplication-Sign 10{sup -15} from continuous GWs at frequencies on the order {approx}1/T{sub obs}.

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COSMIC-RAY DIFFUSION IN A SECTORED MAGNETIC FIELD IN THE DISTANT HELIOSHEATH

Very high intensities of galactic cosmic rays measured by Voyager 1 in the heliosheath appear to be incompatible with the presence of a modulation 'wall' near the heliopause produced by a pile up of the heliospheric magnetic field. We propose that the modulation wall is a structure permeable to cosmic rays as a result of a sectored magnetic field topology compressed by plasma slowdown on approach to the heliopause and stretched to high latitudes by latitudinal flows in the heliosheath. The tightly folded warped current sheet permits efficient cosmic-ray transport in the radial direction via a drift-like mechanism. We show that when stochastic variations in the sector widths are taken into account, particle transport becomes predominantly diffusive both along and across the magnetic sectors. Using a test-particle model for cosmic rays in the heliosheath we investigate the dependence of the diffusion coefficients on the properties of the sector structure and on particle energy.

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DOE OSTI2012

A THEORY OF BIMODAL ACCELERATION OF PICKUP IONS BY COMPRESSIVE SOLAR WIND TURBULENCE UNDER PRESSURE BALANCE

Recently, it was demonstrated that stochastic acceleration of particles going through a series of compressive plasma waves can be efficient and fast. It could be too fast so that the pressure built up by the accelerated particles may in turn modify the amplitude of waves to prevent the particles from having an exploding pressure. We call this condition pressure balance. In this paper, we take into account the fact that active acceleration of particles only occupies a limited volume of space due to a possible intermittent nature of plasma waves or turbulence. We develop a bimodal acceleration theory that treats the populations of particles in the active and inactive acceleration regions separately and allows the two populations to exchange particles efficiently. Under the requirement of the pressure balance condition, we show that the system automatically produces a solution of v {sup -5} steady state distribution for the accelerated particles. It is found that the v {sup -5} distribution is more robust and easier to achieve with a small volume of intense particle acceleration. These properties explain why the v {sup -5} distribution is commonly observed in space. We apply our model to pickup ion propagation and acceleration throughout the entire heliosphere. Our results can reproduce various observations in some great detail. We also found that this mechanism could be responsible for producing anomalous cosmic rays deep in the heliosheath.

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THE EFFECTS OF WAVE ESCAPE ON FAST MAGNETOSONIC WAVE TURBULENCE IN SOLAR FLARES

One of the leading models for electron acceleration in solar flares is stochastic acceleration by weakly turbulent fast magnetosonic waves ({sup f}ast waves{sup )}. In this model, large-scale flows triggered by magnetic reconnection excite large-wavelength fast waves, and fast-wave energy then cascades from large wavelengths to small wavelengths. Electron acceleration by large-wavelength fast waves is weak, and so the model relies on the small-wavelength waves produced by the turbulent cascade. In order for the model to work, the energy cascade time for large-wavelength fast waves must be shorter than the time required for the waves to propagate out of the solar-flare acceleration region. To investigate the effects of wave escape, we solve the wave kinetic equation for fast waves in weak turbulence theory, supplemented with a homogeneous wave-loss term. We find that the amplitude of large-wavelength fast waves must exceed a minimum threshold in order for a significant fraction of the wave energy to cascade to small wavelengths before the waves leave the acceleration region. We evaluate this threshold as a function of the dominant wavelength of the fast waves that are initially excited by reconnection outflows.

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