Página 11 dos resultados de 15310 itens digitais encontrados em 0.009 segundos
Resultados filtrados por Publicador: Universidade Cornell

Evolution of column density distributions within Orion~A

Stutz, A. M.; Kainulainen, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/04/2015 Português
Relevância na Pesquisa
153.59818%
We compare the structure of star-forming molecular clouds in different regions of Orion A to determine how the column density probability distribution function (N-PDF) varies with environmental conditions such as the fraction of young protostars. A correlation between the N-PDF slope and Class 0 protostar fraction has been previously observed in a low-mass star-formation region (Perseus) by Sadavoy; here we test if a similar correlation is observed in a high-mass star-forming region. We use Herschel data to derive a column density map of Orion A. We use the Herschel Orion Protostar Survey catalog for accurate identification and classification of the Orion A young stellar object (YSO) content, including the short-lived Class 0 protostars (with a $\sim$ 0.14 Myr lifetime). We divide Orion A into eight independent 13.5 pc$^2$ regions; in each region we fit the N-PDF distribution with a power-law, and we measure the fraction of Class 0 protostars. We use a maximum likelihood method to measure the N-PDF power-law index without binning. We find that the Class 0 fraction is higher in regions with flatter column density distributions. We test the effects of incompleteness, YSO misclassification, resolution, and pixel-scale. We show that these effects cannot account for the observed trend. Our observations demonstrate an association between the slope of the power-law N-PDF and the Class 0 fractions within Orion A. Various interpretations are discussed including timescales based on the Class 0 protostar fraction assuming a constant star-formation rate. The observed relation suggests that the N-PDF can be related to an "evolutionary state" of the gas. If universal...

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

Scalo, J.; Vazquez-Semadeni, E.; Chappell, D.; Passot, T.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/10/1997 Português
Relevância na Pesquisa
154.59691%
We investigate the form of the one-point probability distribution 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 and hydrodynamic turbulence, and of decaying Burgers turbulence, produce in all cases filamentary density structures and a power-law density pdf with logarithmic slope around -1.7. This suggests that the functional form 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 1D simulations of forced supersonic polytropic turbulence is used to resolve the discrepancy. They suggest that the pdf is lognormal only for effective polytropic indices $\gamma=1$ (or nearly lognormal for $\gamma\not=1$ if the Mach number is sufficiently small), while power laws develop at high densities if $\gamma<1$. 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 may occur at densities between 10$^3$ and at least 10$^4$ cm$^{-3}$. Several applications are examined. First...

Turbulence-Induced Relative Velocity of Dust Particles III: The Probability Distribution

Pan, Liubin; Padoan, Paolo; Scalo, John
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 03/07/2014 Português
Relevância na Pesquisa
153.59818%
Motivated by its important role in the collisional growth of dust particles in protoplanetary disks, we investigate the probability distribution function (PDF) of the relative velocity of inertial particles suspended in turbulent flows. Using the simulation from our previous work, we compute the relative velocity PDF as a function of the friction timescales, tau_p1 and tau_p2, of two particles of arbitrary sizes. The friction time of particles included in the simulation ranges from 0.1 tau_eta to 54T_L, with tau_eta and T_L the Kolmogorov time and the Lagrangian correlation time of the flow, respectively. The relative velocity PDF is generically non-Gaussian, exhibiting fat tails. For a fixed value of tau_p1, the PDF is the fattest for equal-size particles (tau_p2~tau_p1), and becomes thinner at both tau_p2tau_p1. Defining f as the friction time ratio of the smaller particle to the larger one, we find that, at a given f in 1/2>T_L). These features are successfully explained by the Pan & Padoan model. Using our simulation data and some simplifying assumptions...

An Unbiased Hessian Representation for Monte Carlo PDFs

Carrazza, Stefano; Forte, Stefano; Kassabov, Zahari; Latorre, Jose Ignacio; Rojo, Juan
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
153.59818%
We develop a methodology for the construction of a Hessian representation of Monte Carlo sets of parton distributions, based on the use of a subset of the Monte Carlo PDF replicas as an unbiased linear basis, and of a genetic algorithm for the determination of the optimal basis. We validate the methodology by first showing that it faithfully reproduces a native Monte Carlo PDF set (NNPDF3.0), and then, that if applied to Hessian PDF set (MMHT14) which was transformed into a Monte Carlo set, it gives back the starting PDFs with minimal information loss. We then show that, when applied to a large Monte Carlo PDF set obtained as combination of several underlying sets, the methodology leads to a Hessian representation in terms of a rather smaller set of parameters (CMC-H PDFs), thereby providing an alternative implementation of the recently suggested Meta-PDF idea and a Hessian version of the recently suggested PDF compression algorithm (CMC-PDFs). The mc2hessian conversion code is made publicly available together with (through LHAPDF6) a Hessian representations of the NNPDF3.0 set, and the CMC-H PDF set.; Comment: 27 pages, 17 figures; v2: replaced plots in Fig.1 which had a conflict with google chrome inline viewer; v3: final version...

Joint modeling of the probability distribution and power spectrum of the Lya forest: comparison with observations at z=3

Desjacques, Vincent; Nusser, Adi
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
153.59818%
We presents results of joint modeling of the probability distribution function (PDF) and the one-dimensional power spectrum (PS) of the Lya forest flux decrement. The sensitivity of these statistical measures to the shape and amplitude of the linear matter power spectrum is investigated using two variants of the LCDM cosmology. In the first model, the linear power spectrum has a scale-invariant spectral index, whereas in the second, it has a negative running index (RSI). We generate mock catalogs of QSO spectra, and compare their statistical properties to those of the observations at z=3. We perform a joint fit of the power spectrum and PDF. A scale-invariant model with \sigma_8=0.9 matches well the data if the mean IGM temperature is T\leq 15000K. For higher temperature, it tends to overestimate the flux power spectrum over scales k < 0.01s/km. The discrepancy is less severe when the PS alone is fitted. However, models matching the PS alone do not yield a good fit to the PDF. A joint analysis of the flux PS and PDF tightens the constraints on the model parameters and reduces systematic biases. The RSI model is consistent with the observed PS and PDF only if the temperature is T\geq 20000K. The best fit models reproduce the slope and normalisation of the column density distribution...

Proton structure at the LHC

Hartland, Nathan
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/11/2014 Português
Relevância na Pesquisa
153.59818%
A determination of Parton Distribution Functions (PDFs) from a global fit to a dataset including measurements from the LHC has been performed. The determinations have been carried out according to the NNPDF methodology, leading to a fit minimising parametrisation bias and with an accurate account of PDF uncertainty. In this thesis the importance of QCD measurements at the LHC to PDF extraction are discussed, and we summarise some of the technical difficulties in their inclusion into PDF fits. A number of methods are presented that permit the efficient inclusion of these observables into PDF determinations. Firstly a Bayesian reweighting procedure taking advantage of the Monte Carlo representation of PDF uncertainties in NNPDF sets is discussed, which is demonstrated by a study of the impact of early W production asymmetry measurements from ATLAS, CMS and LHCb upon an earlier PDF set. A package for the fast computation of observables in an automated NLO framework is presented, providing an interface between Monte Carlo event generators and NLO interpolation tools. Finally, a method of combining PDF evolution with interpolating codes for hadronic observable computation is described. This method largely overcomes the computational difficulties in performing fast perturbative QCD predictions for collider observables. The method has been applied to the determination of PDFs from a global dataset including vector boson production data from LHCb...

The Lyman alpha forest flux probability distribution at z>3

Calura, F.; Tescari, E.; D'Odorico, V.; Viel, M.; Cristiani, S.; Kim, T. -S.; Bolton, J. S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
154.59691%
We present a measurement of the Lyman alpha flux probability distribution function (PDF) measured from a set of eight high resolution quasar spectra with emission redshifts at 3.3 < z < 3.8. We carefully study the effect of metal absorption lines on the shape of the PDF. Metals have a larger impact on the PDF measurements at lower redshift, where there are fewer Lyman alpha absorption lines. This may be explained by an increase in the number of metal lines which are blended with Lyman alpha absorption lines toward higher redshift, but may also be due to the presence of fewer metals in the intergalactic medium with increasing lookback time. We also provide a new measurement of the redshift evolution of the effective optical depth, tau_eff, at 2.8 < z < 3.6, and find no evidence for a deviation from a power law evolution in the log(tau_eff)-log(1+z) plane. The flux PDF measurements are furthermore of interest for studies of the thermal state of the intergalactic medium (IGM) at z ~ 3 . By comparing the PDF to state-of-the-art cosmological hydrodynamical simulations, we place constraints on the temperature of the IGM and compare our results with previous measurements of the PDF at lower redshift. At redshift z=3, our new PDF measurements are consistent with an isothermal temperature-density relation...

Towards a separation of the elements in turbulence via the analyses within MPDFT

Arimitsu, Toshihico; Arimitsu, Naoko; Takechi, Kohei; Kaneda, Yukio; Ishihara, Takashi
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 18/11/2011 Português
Relevância na Pesquisa
154.59691%
The PDFs for energy dissipation rates created in a high resolution from $4096^3$ DNS for fully developed turbulence are analyzed in a high precision with the PDF derived within the formula of multifractal probability density function theory (MPDFT). MPDFT is a statistical mechanical ensemble theory constructed in order to analyze intermittent phenomena through the experimental PDFs with fat-tail. By making use of the obtained w-PDFs created from the whole of the DNS region, analyzed for the first time are the two partial PDFs, i.e., the max-PDF and the min-PDF which are, respectively, taken out from the partial DNS regions of the size $512^3$ with maximum and minimum enstropy. The main information for the partial PDFs are the following. One can find a w-PDF whose tail part can adjust the slope of the tail-part of a max-PDF with appropriate magnification factor. The value of the point at which the w-PDF multiplied by the magnification factor starts to overlap the tail part of the max-PDF coincides with the value of the connection point for the theoretical w-PDF. The center part of the min-PDFs can be adjusted quite accurately by the scaled w-PDFs with a common scale factor.; Comment: 15 pages, 16 figures, 1 table. The contents of the paper is based on the invited talk given at SigmaPhi 2011 at Larnaca in Cyprus

The Gamma-Ray-Flux Probability Distribution Function from Galactic Halo Substructure

Lee, Samuel K.; Ando, Shin'ichiro; Kamionkowski, Marc
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
153.59818%
One of the targets of the recently launched Fermi Gamma-ray Space Telescope is a diffuse gamma-ray background from dark-matter annihilation or decay in the Galactic halo. N-body simulations and theoretical arguments suggest that the dark matter in the Galactic halo may be clumped into substructure, rather than smoothly distributed. Here we propose the gamma-ray-flux probability distribution function (PDF) as a probe of substructure in the Galactic halo. We calculate this PDF for a phenomenological model of halo substructure and determine the regions of the substructure parameter space in which the PDF may be distinguished from the PDF for a smooth distribution of dark matter. In principle, the PDF allows a statistical detection of substructure, even if individual halos cannot be detected. It may also allow detection of substructure on the smallest microhalo mass scales, $\sim M_\oplus$, for weakly-interacting massive particles (WIMPs). Furthermore, it may also provide a method to measure the substructure mass function. However, an analysis that assumes a typical halo substructure model and a conservative estimate of the diffuse background suggests that the substructure PDF may not be detectable in the lifespan of Fermi in the specific case that the WIMP is a neutralino. Nevertheless...

A meta-analysis of parton distribution functions

Gao, Jun; Nadolsky, Pavel
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
154.59691%
A "meta-analysis" is a method for comparison and combination of nonperturbative parton distribution functions (PDFs) in a nucleon obtained with heterogeneous procedures and assumptions. Each input parton distribution set is converted into a "meta-parametrization" based on a common functional form. By analyzing parameters of the meta-parametrizations from all input PDF ensembles, a combined PDF ensemble can be produced that has a smaller total number of PDF member sets than the original ensembles. The meta-parametrizations simplify the computation of the PDF uncertainty in theoretical predictions and provide an alternative to the 2010 PDF4LHC convention for combination of PDF uncertainties. As a practical example, we construct a META ensemble for computation of QCD observables at the Large Hadron Collider using the next-to-next-to-leading order PDF sets from CTEQ, MSTW, and NNPDF groups as the input. The META ensemble includes a central set that reproduces the average of LHC predictions based on the three input PDF ensembles and Hessian eigenvector sets for computing the combined PDF+$\alpha_s$ uncertainty at a common QCD coupling strength of 0.118.; Comment: version to appear on JHEP