Talks & Events
Ph.D. Thesis Defenses: 2006
Sedimentation and type I X-ray bursts
Neutron stars, with their strong surface gravity, have interestingly short timescales for the sedimentation of heavy elements. Recent observations of unstable thermonuclear burning (observed as X-ray bursts) on the surfaces of slowly accreting neutron stars (<0.01 of the Eddington rate) motivate us to examine how the sedimentation of CNO isotopes affects the ignition of these bursts.
In this thesis, we present the results of a study of the effect of sedimentation on the ignition of H and He in the envelope of an accreting neutron star. A diffusion code is developed for solving the diffusion and sedimentation of isotopes, which is coupled to an accreting neutron star envelope model for tracing the evolutions of isotopes before the thermonuclear instability occurs. We estimate the burst development using a simple one-zone model with a full reaction network. At accretion rates <0.003 Eddington, the ignition of H is sufficiently unstable that the rise in temperature ignites the triple-alpha reaction and produces a vigorous flash. At higher accretion rates (but still less than 0.01 Eddington), the H ignition, although unstable, does not heat the envelope enough to trigger the triple-alpha reaction, but instead manifests itself as a weak H flash. We propose that sources accreting at these rates will build up a massive He layer and produce a long burst, as seen from SLX 1737-282, SLX 1735-269, and GX 3+1. Intriguingly, even at accretion rates >0.1 Eddington sedimentation can still play a role. Although the H, He and CNO isotopes do not completely separate, the H abundance at the base of the accumulated layer is reduced. This changes the proton-to-seed ratio for the rapid-proton capture process. In the absence of convective mixing, the partial stratification would change the composition of the ashes and might enhance the abundance of 12 C---a necessary fuel for superbursts.
Model flames in a hydrostatic atmosphere
A model flame system based on the advection-diffusion-reaction method is defined and used to numerically study the problem of a flame propagating up an initially hydrostatic atmosphere, in 2-D. We identify and characterize the flame's steady states over a range of parameters, in the case where the gravitational scale height is much greater than the size of the flame, which itself is much greater than the flame's laminar width. We observe both laminar and turbulent steady flames and verify that, for strong enough gravity G, the turbulent flame speed is independent of the laminar flame speed and scales like the square root of GL, where L is the size of our domain. As this scaling law is commonly used to implement flame subgrid models, one of the aims of this thesis is to understand its robustness. We describe the flame geometry and discuss its relationship with the flame speed. The flow statistics inside turbulent flames are measured and found to be gaussian and isotropic, corresponding to strong mixing.
Curvature perturbations in the early universe: Theoretical models and observational tests
A very general prediction of inflation is that the power spectrum of density perturbations is characterized by a spectral index ns which is scale independent and approximately equal to unity. Drawing from the potential reconstruction method and adopting the slow-roll parameter expansion technique, we derive all possible single field inflationary potentials that would lead to a scale invariant density spectral index, consistent with current observations. In the process, a new method to determine the functional form of the inflationary potential in the slow roll approximation is devised, based on the reparametrization of the field dynamics with respect to the slow roll parameter epsilon which also allowed to show that under the assumptions made the investigation proved to be exhaustive and that no other solutions are available. Next, we focus on the fact that there exist a large class of inflationary models currently ruled out because the predicted production of curvature perturbations during the slow-roll stage results exponentially suppressed. We investigate whether an alternative mechanism for the generation of curvature perturbations can be devised for such a class of models. In the process, it is shown that it is sufficient for the inflationary potential to exhibit a broken symmetry to successfully convert isocurvature perturbations, which are excited during the slow-roll stage, into curvature perturbations thanks to an inhomogeneous decay stage. This conclusion is general, requiring as a sufficient condition only the fact that the inflation potential is characterized by a broken symmetry. Finally, we show that the perturbations thus produced are generally characterized by a non-negligible degree of non-gaussianity, which then provides a clear experimental signature for experimental detection or rejection.
Matter Power Spectrum 101
We modify the public PM code developed by Anatoly Klypin and Jon Holtzman to simulate cosmologies with arbitrary initial power spectrum and equation of state of dark energy. With this tool in hand, we perform the following studies on the matter power spectrum.
With an artificial sharp peak at k ~ 0.2 h Mpc -1 in the initial power spectrum, we find that the position of the peak is not shifted by nonlinear evolution. An upper limit of the shift at the level of 0.02% is achieved by fitting the power spectrum local to the peak using a power law plus a Gaussian. This implies that, for any practical purpose, the baryon acoustic oscillation peaks in the matter power spectrum are not shifted by nonlinear evolution which would otherwise bias the cosmological distance estimation. We also find that the existence of a peak in the linear power spectrum would boost the nonlinear power at all scales evenly. This is contrary to what HKLM scaling relation predicts, but roughly consistent with that of halo model.
We construct two dark energy models with the same linear power spectra today but different linear growth histories. We demonstrate that their nonlinear power spectra differ at the level of the maximum deviation of the corresponding linear power spectra in the past. Similarly, two constructed dark energy models with the same growth histories result in consistent nonlinear power spectra. This is hinting, not a proof, that linear power spectrum together with linear growth history uniquely determine the nonlinear power spectrum. Based on these results, we propose that linear growth history be included in the next generation fitting formulas of the nonlinear power spectrum.
For simple dark energy models parametrized by w 0 and w a , the existing nonlinear power spectrum fitting formulas, which are calibrated for ACDM model, work reasonably well. The corrections needed are at percent level for the power spectrum and 10% level for the derivative of the power spectrum. We find that, for Peacock & Dodds (1996) fitting formula, the corrections to the derivative of the power spectrum are independent of w a but changing with redshift. As a short term solution, a fitting form could be developed for w 0 , w a models based on this fact.
Cosmological Constraints from Photometrically-Selected Cluster Catalogs
We develop a new maximum likelihood method for estimating cosmological parameters using observed cluster abundances. Our model allows us to include and properly marginalize over systematic uncertainties associated with the cluster selection function. We apply our method to the maxBCG cluster catalog, and explicitly demonstrate that when the cluster selection function is accurately known, the maxBCG catalog can provide percent level constraints on particular combinations of cosmological and Halo Occupation Distribution (HOD) parameters. Even when the selection function is only known with poor accuracy, we show that our method can provide interesting constraints on s 8 if one assumes cosmological priors on O m h 2 and h as determined from the Cosmic Microwave Background (CMB) and supernovae data. Our best estimate for s 8 is s 8 = [Special characters omitted.] (1-s), though it is also subject to an additional 5% prior on the slope of the HOD.
Early stages of ultra high energy cosmic ray air showers as a diagnostic of exotic primaries
The nature of ultra high energy cosmic rays (UHECRs) remains an enigma. UHECR detection rate is increasing with new generation detectors which will speed up the process of understanding these energetic particles. After a review on the field of UHECRs, we focus on air shower characterisation of primaries. We study both common primaries as well as more exotic possibilities. One such case is the TeV black hole (BH) creation which can happen in models of large extra dimensions. High energy neutrinos interacting with air molecules may form these objects in the Earth's atmosphere, and a good way of discriminating them from other backgrounds is through air shower studies. Full scale Monte Carlo simulations of air shower cascades are the best way to predict air shower characteristics. However, these are very computer-time consuming. The first interactions of an air shower is instructive as it gives information on how the shower will develop without the full scale simulation. The first interaction study is a far less time consuming method that is advantageous for testing new models with many parameters. Hadronic models are used to interact particles in simulation softwares, where low energy experimental data are extrapolated up to high energies using various models, such as minijets and pomerons. Here, we study the first interactions of well studied cosmic ray primaries - photon, proton, iron nucleus - are performed with SIBYLL 2.1, a hadronisation Monte Carlo which uses the minijet model. "Templates" from common primaries can then be used to compare with new primaries. The TeV BHs are used as an example of a new primary. Air shower simulations have been carried out, which showed that these neutrino induced BH air shower resembles a hadronic air shower. Using the first interaction templates, we show that BHs indeed resemble protons closely.
The Galaxy Cross-Correlation Function as a Probe of the Spatial Distribution of Galactic Satellites
The spatial distribution of satellite galaxies around host galaxies can illuminate the relationship between satellites and dark matter subhalos and aid in developing and testing galaxy formation modes. The projected cross- correlation of bright and faint galaxies offers a promising avenue to putting constraints on the radial distribution of satellite galaxies. Previous efforts to constrain the distribution attempted to eliminate interlopers from the measured projected number density of satellites and found that the distribution is generally consistent with the expected dark matter halo profile of the parent hosts. The measured projected cross-correlation can be used to analyze contributions from satellites and interlopers together, using a halo occupation distribution (HOD) based analytic model for galaxy clustering. Tests on mock catalogs constructed from simulations show promise in this approach. Analysis of Sloan Digital Sky Survey (SDSS) data shows results generally consistent with interloper subtraction methods, although the radial distribution is poorly constrained with the current dataset and larger samples are required.
The ultra high energy cosmic ray flux from the southern Pierre Auger Observatory data
The Pierre Auger Observatory is currently the largest ground based cosmic ray observatory. Its goal is to characterize the properties of ultra high energy cosmic rays (of energies above ~ 10 18 eV) in order to understand their origin, composition and acceleration mechanisms.
In this thesis, the cosmic ray flux is calculated using data from the first two and a half years of the production phase of the Auger observatory. The Auger observatory is a hybrid observatory combining a ground array of particle detectors and a set of fluorescence telescopes that overlook this ground array. The constant intensity cut method is used to measure the attenuation curve from ground array data. The energy calibration curve is measured using events that the trigger both the ground array and the fluorescence detector. A simple proportionality between energy and signal at ground is found. The energy resolution is shown to be between 18% at low energies and 10% at higher energies, with an overall systematic uncertainty of 25%.
The resulting flux exhibits an ankle at energies around 3 - 1018 eV, and shows signs of a GZK cutoff at around 40 1018eV. Both these energy estimates are uncertain by 25%. The integrated flux of events above 10 19 eV is (25 � 5(stat) � 6(syst))/km 2 /sr/century.