Tesis postgrado Astrofísica

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Examinando Tesis postgrado Astrofísica por Materia "ANALISIS DE FOURIER"

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  • Miniatura
    Ítem
    The effect of non-gaussian primordial perturbations on large-scale structure
    (Universidad de Valparaíso, 2021-12) Peña Pinto, Greco; Candlish, Graeme
    The late-time effect of primordial non-Gaussianity offers a window into the physics of inflation and the very early Universe. In this thesis we study the conse- quences of a particular class of primordial non-Gaussianity that is fully characterized by initial density fluctuations drawn from a non-Gaussian probability density func- tion, rather than by construction of a particular form for the primordial bispectrum. We numerically generate multiple realisations of cosmological structure and use the late-time matter power spectrum, bispectrum and trispectrum to determine the ef- fect of these modified initial conditions. We show that the initial non-Gaussianity has only a small imprint on the first three polyspectra, when compared to a stan- dard Gaussian cosmology. Furthermore, some of our models present an interesting scale-dependent deviation from the Gaussian case in the bispectrum and trispectrum, although the signal is at most at the percent level. The majority of our models are consistent with CMB constraints, while the others are only marginally excluded. Finally, we discuss further possible extensions of our study.
  • Miniatura
    Ítem
    Non-isotropic distributions of stellar rotational velocities
    (Universidad de Valparaíso, 2021-03) Solar Yáñez, Martín; Curé, Michel (Supervisor)
    Stellar rotation is defined from the angular motion of a star about its own rotational axis and study this phenomenon is useful to constrain models of stellar formation and evolution. The equatorial velocity v is defined as the surface angular speed at latitude 0o , however, the projected rotational velocity v sin i value is one of the most straightforward and cheapest (from an observational point of view) ways to obtain the rotational velocity of a star, where i is the inclination angle between the axis of rotation with respect to the observer. This research is separated in two parts; obtain this v sin i value via Fourier Transform in an automated procedure and apply a non- isotropic distribution model for the axes on the unit sphere of observation: 1)Obtaining v sin i: There are several methods to infer v sin i data, in the case of Fourier Transform, this method consists in obtain the first zero of the Fourier Trans- form domain from a rotational kernel for the observed absorption line profile. First, an automated procedure to obtain v sin i via Fourier Transform is constructed, for multi- ple absorption lines at different epochs. The method consists in fit a Gaussian profile to the respective line and select the signal from the curve. Monte Carlo simulations are performed where theoretical lines are constructed for specific rotational velocities. Then, the noise is added and multiple repetitions are computed to analyze the consis- tence of the method. Later, our automatized Fourier Transform method is used in the BeSOS database to obtain their v sin i values. Results are in global agreement with the literature. 2) How is distributed v sin i: An integral equation that governs the two distribu- tions v and v sin i (true and apparent rotational speed distributions respectively) is given by a Fredholm integral in which an α parameter is added into the kernel of the equation to model a non-isotropic distribution, where α < 0 yields inclination angles close to the polar axis, α > 0 a non-isotropic near to the equator and α = 0 is the typical isotropic axes distribution. The true distribution is calculated via Tikhonov regularization method from the observed distribution which is estimated by a kernel density estimator. As α parameter is unknown, we made a grid of α to compute the Fredholm integral. To obtain the best α value, we minimized the mean square error of the projected kernel density estimator distribution with respect to the new grid of solutions for the Fredholm integral equation. The Monte Carlo random sampling repetitions ensures that the method, in general terms, is reliable until α ∼ 1. Theprocedure is applied to several open stellar clusters and field stars to see the behav- ior of the non-isotropy finding effectively that the different databases of stars are not isotropic rotating with respect to Earth. In summary, we have developed two novel methods; measures the v sin i values from any stellar spectrum for multiple absorption lines at different epochs where the line is broadening mainly by rotation (high rotators); the second, obtains the non-isotropy from a sample of v sin i data without any convergence criteria. As fu- ture works we want to implement this last method to binary systems (star-star or exoplanet-star) discovered by transit and also an extra parameter β can be introduced into the Fredholm integral to compute more complex distribution of axes.