Statistical Estimation and Inference Improvements for Exoplanet Discovery

The radial velocity method has been widely used by astronomers since the 1990\u27s for discovering extra-solar planets, often referred to as simply exoplanets . This method involves estimating the radial velocity of a distant star over time using the stellar light, followed by modeling such radial velocity estimates as a function of time using Keplerian-orbital equations with parameters that describe the exoplanet. While a number of approaches exist for estimating the radial velocity from the stellar light, we introduce a new approach for this that uses Hermite-Gaussian functions to reduce the estimation to linear least-squares regression. Furthermore, we demonstrate that this new approach, compared to the commonly used cross-correlation approach, provides an approximate 21% reduction of statistical risk in simulation studies as well as in applications to recently collected data. We then extend this linear model to include additional terms that represent the effect of stellar activity on the observed light, an effect known to both hide and imitate the signal of exoplanets. The F-statistic for the fitted coefficients of these additional terms is found to have higher statistical power than many traditional stellar activity indicators at detecting the presence of stellar activity. Finally, we also use the linear model in a Bayesian framework to merge both traditional steps of the radial velocity method into one that estimates the exoplanet\u27s orbital parameters directly from the time series of observed stellar light

On the magnetic structure and wind parameter profiles of Alfven wave driven winds in late-type supergiant stars

Cool stars at giant and supergiant evolutionary phases present low velocity and high density winds, responsible for the observed high mass-loss rates. Although presenting high luminosities, radiation pressure on dust particles is not sufficient to explain the wind acceleration process. Among the possible solutions to this still unsolved problem, Alfven waves are, probably, the most interesting for their high efficiency in transfering energy and momentum to the wind. Typically, models of Alfven wave driven winds result in high velocity winds if they are not highly damped. In this work we determine self-consistently the magnetic field geometry and solve the momentum, energy and mass conservation equations, to demonstrate that even a low damped Alfven wave flux is able to reproduce the low velocity wind. We show that the magnetic fluxtubes expand with a super-radial factor S>30 near the stellar surface, larger than that used in previous semi-empirical models. The rapid expansion results in a strong spatial dilution of the wave flux. We obtained the wind parameter profiles for a typical supergiant star of 16 M_sun. The wind is accelerated in a narrow region, coincident with the region of high divergence of the magnetic field lines, up to 100 km/s. For the temperature, we obtained a slight decrease near the surface for low damped waves, because the wave heating mechanism is less effective than the radiative losses. The peak temperature occurs at 1.5 r_0 reaching 6000 K. Propagating outwards, the wind cools down mainly due to adiabatic expansion.Comment: to appear in the MNRA

The Psychiatrization of Poverty: Rethinking the Mental Health-Poverty Nexus

The positive association between ‘mental illness’ and poverty is one of the most well established in psychiatric epidemiology. Yet, there is little conclusive evidence about the nature of this relationship. Generally, explanations revolve around the idea of a vicious cycle, where poverty may cause mental ill health, and mental ill health may lead to poverty. Problematically, much of the literature overlooks the historical, social, political, and cultural trajectories of constructions of both poverty and ‘mental illness’. Laudable attempts to explore the social determinants of mental health sometimes take recourse to using and reifying psychiatric diagnostic categories that individualize distress and work to psychiatrically reconfigure ‘symptoms’ of oppression, poverty, and inequality as ‘symptoms’ of ‘mental illness’. This raises the paradoxical issue that the very tools that are used to research the relationship between poverty and mental health may prevent recognition of the complexity of that relationship. Looking at the mental health–poverty nexus through a lens of psychiatrization (intersecting with medicalization, pathologization, and psychologization), this paper recognizes the need for radically different tools to trace the messiness of the multiple relationships between poverty and distress. It also implies radically different interventions into mental health and poverty that recognize the landscapes in which lived realities of poverty are embedded, the political economy of psychiatric diagnostic and prescribing practices, and ultimately to address the systemic causes of poverty and inequality

Propagation of non-linear circularly polarised Alfven waves in a homogeneous medium

We study the evolution of non-linear circularly polarised Alfven waves by solving numerically the time-dependent equations of magnetohydrodynamics (MHD) in one dimension. We examine the behaviour of the waves and find that different physical mechanisms are relevant in different ranges of beta. In a low beta plasma the wave may undergo a parametric decay. This is because the wave excites a density enhancement that travels slower than the wave itself and thus interacts with the wave. When beta is greater or equal to one the density enhancement does not interact with the wave and no decay takes place, instead the Alfven wave is reflected against the density enhancement. The reflection zone propagates with the speed 1/n of the Alfven speed. Because of that the magnetic flux is conserved which results in an amplification of the oscillating magnetic field by a factor 1/n. We find that n depends on beta, and that in particular it is less or equal to one for values of beta ~ 1 and is greater or equal to one for beta >> 1. We discuss the relevance of these mechanisms to the acceleration of the solar wind, and the triggering of MHD turbulence in the polar wind region. In particular these simulations can explain the presence of inward propagating Alfven waves in the solar corona

Alfven waves as a driving mechanism in stellar winds

Alfven waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the HR diagram. We discuss here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds.Comment: Accepted for publication in Advances in Space Research. Presented at the World Space Environment Forum 2007, Egypt. 9 pages, 2 figure

The Evolution of Amastin Surface Glycoproteins in Trypanosomatid Parasites

Amastin is a transmembrane glycoprotein found on the cell surfaces of trypanosomatid parasites. Encoded by a large, diverse gene family, amastin was initially described from the intracellular, amastigote stage of Trypanosoma cruzi and Leishmania donovani. Genome sequences have subsequently shown that the amastin repertoire is much larger in Leishmania relative to Trypanosoma. However, it is not known when this expansion occurred, whether it is associated with the origins of Leishmania and vertebrate parasitism itself, or prior to this. To examine the timing of amastin diversification, as well as the evolutionary mechanisms regulating gene repertoire and sequence diversity, this study sequenced the genomic regions containing amastin loci from two related insect parasites (Leptomonas seymouri and Crithidia sp.) and estimated a phylogeny for these and other amastin sequences. The phylogeny shows that amastin includes four subfamilies with distinct genomic positions, secondary structures, and evolution, which were already differentiated in the ancestral trypanosomatid. Diversification in Leishmania was initiated from a single ancestral locus on chromosome 34, with rapid derivation of novel loci through transposition and accelerated sequence divergence. This is absent from related organisms showing that diversification occurred after the origin of Leishmania. These results describe a substantial elaboration of amastin repertoire directly associated with the origin of Leishmania, suggesting that some amastin genes evolved novel functions crucial to cell function in leishmanial parasites after the acquisition of a vertebrate host

A comparison of the acceleration mechanisms in YSO and AGN jets

We examine the hypothesis that there exists a simple scaling between the observed velocities of jets found in Young Stellar Objects (YSOs) and jets found in Active Galactic Nuclei (AGN). We employ a very simplified physical model of the jet acceleration process. We use time-dependent, spherically symmetric wind models in Newtonian and relativistic gravitational fields to ask whether the energy input rates required to produce the jet velocities observed in YSOs (of about 2 $\times$ the escape velocity from the central object) can also produce AGN jet velocities (Lorentz factors of $\gamma \sim$ 10). Such a scaling would be expected if there is a common production mechanism for such jets. We demonstrate that such a scaling does exist, provided that the energy input process takes place sufficiently deep in the gravitational potential well, enabling physical use to be made of the speed of light as a limiting velocity, and provided that the energy released in the accretion process is imparted to a small fraction of the available accreting material.Comment: 19 pages, 11 figures, accepted to MNRA