Astronomy Processing Research Articles

Imaging the warped dusty disk wind environment of SU Aurigae with MIRC-X

Context. T Tauri stars are low-mass young stars whose disks provide the setting for planet formation, which is one of the most fundamental processes in astronomy. Yet the mechanisms of this are still poorly understood. SU Aurigae is a widely studied T Tauri star and here we present original state-of-the-art interferometric observations with better uv and baseline coverage than previous studies. Aims. We aim to investigate the characteristics of the circumstellar material around SU Aur, and constrain the disk geometry, composition and inner dust rim structure. Methods. The MIRC-X instrument at CHARA is a six-telescope optical beam combiner offering baselines up to 331 m. We undertook image reconstruction for model-independent analysis, and fitted geometric models such as Gaussian and ring distributions. Additionally, the fitting of radiative transfer models constrained the physical parameters of the disk. Results. Image reconstruction reveals a highly inclined disk with a slight asymmetry consistent with inclination effects obscuring the inner disk rim through absorption of incident star light on the near side and thermal re-emission/scattering of the far side. Geometric models find that the underlying brightness distribution is best modelled as a Gaussian with a Full-Width Half-Maximum of 1.53 ± 0.01 mas at an inclination of 56.9 ± 0.4° and a minor axis position angle of 55.9 ± 0.5°. Radiative transfer modelling shows a flared disk with an inner radius at 0.16 au which implies a grain size of 0.14 μm assuming astronomical silicates and a scale height of 9.0 au at 100 au. In agreement with the literature, only the dusty disk wind successfully accounts for the near infrared excess by introducing dust above the mid-plane. Conclusions. Our results confirm and provide better constraints than previous inner disk studies of SU Aurigae. We confirm the presence of a dusty disk wind in the cicumstellar environment, the strength of which is enhanced by a late infall event which also causes very strong misalignments between the inner and outer disks.

Using mutual information to measure time lags from nonlinear processes in astronomy

Measuring time lags between time series or light curves at different wavelengths from a variable or transient source in astronomy is an essential probe of physical mechanisms causing multiwavelength variability. Time lags are typically quantified using discrete correlation functions (DCFs), which are appropriate for linear relationships. However, in variable sources such as x-ray binaries, active galactic nuclei (AGNs), and other accreting systems, the radiative processes and the resulting multiwavelength light curves often have nonlinear relationships. For such systems it is more appropriate to use nonlinear information-theoretic measures of causation such as mutual information, routinely used in other disciplines. We demonstrate with toy models the limitations of using the standard DCF and show improvements when using a discrete mutual information function (DMIF). For nonlinear correlations, the latter accurately and sharply identifies the lag components as opposed to the DCF, which can be erroneous. Following that, we apply the DMIF to the multiwavelength light curves of AGN NGC 4593. We find that x-ray fluxes are leading UVW2 fluxes by $\ensuremath{\sim}0.2$ days, closer to model predictions from reprocessing by the accretion disk than the DCF estimate. The uncertainties with the current light curves are too large, though, to rule out negative lags. Additionally, we find another delay component at approximately $\ensuremath{-}1$ day, i.e., UVW2 leading x rays, consistent with inward propagating fluctuations in the accretion disk scenario. This is not detected by the DCF. Keeping in mind the nonlinear relation between x ray and UVW2, this is worthy of further theoretical investigation. From both the toy models and real observations, it is clear that the mutual-information-based estimator is highly sensitive to complex nonlinear relations. With sufficiently high temporal resolution and signal-to-noise ratio, we will precisely detect each of the lag features corresponding to these relations.

SOB A CÚPULA DAS CONSTELAÇÕES: A TEORIA ATOR-REDE E O ENSINO DE ASTRONOMIA NO PLANETÁRIO

A abrangência e a magnitude dos conteúdos de astronomia despertam a curiosidade de crianças e de adultos, contribuem para a produção de conhecimentos e podem estimular o desenvolvimento de práticas docentes interdisciplinares. Nesse contexto, os Planetários oferecem possibilidades diferenciadas para os processos de ensino e de aprendizagem da astronomia. O objetivo desse artigo é analisar como os professores dos anos iniciais do ensino fundamental percebem o Planetário nos processos educativos. Para isto utilizamos o quadro teórico e metodológico da Teoria Ator-Rede (TAR) proposto por Bruno Latour (2012), especialmente a partir do conceito de actante, que pode ser um agente humano ou não humano. Os actantes podem assumir dois papéis distintos: de mediador, quando suas ações/associações na rede promovem mudanças; ou o de intermediário, quando o agente apenas transmite/reproduz as ações/associações existentes sem modificar as redes e os outros actantes. Assim, nos interessa responder à seguinte perguntanorteadora: em quais situações os professores concebem o Planetário como mediador dos processos de ensino e aprendizagem de astronomia ou como intermediário destes processos? Inicialmente, com o intuito de aproximação à temática, realizamos uma análise dos Parâmetros Curriculares Nacionais (PCN) sobre o ensino de astronomia. A seguir, examinamos 97 questionários dos professores dos diferentes níveis de ensino que frequentaram o Planetário com suas turmas durante o ano de 2014. Os resultados indicam que os professores dos anos iniciais, ao agirem de forma planejada e intencional no ensino de conteúdos de astronomia, percebem o Planetário como mediador, segundo a Teoria Ator-Rede, uma vez que, nestas situações, o Planetário - um dispositivo não humano – influencia, modifica e organiza as ações de ensino e aprendizagem de astronomia.

Spatial point processes in astronomy

We review several topics arising in astronomical research where the statistical analysis of spatial point processes plays a central role in scientific investigations. These include: the distribution of stars in the Milky Way; the distribution of galaxies in two and three dimensions; the location of clusters of photons from faint astronomical sources in detectors; the classification of objects in large multivariate databases; and the sky distribution of gamma-ray bursters. A variety of challenging methodological issues arise including anisotropic filamentary clustering, modeling of Poisson spatial processes, and weighted measures of clustering.

Two-dimensional stochastic processes in astronomy

In various astronomical applications, the impracticability of describing the phenomena under study with a relatively simple model leads to the failure of any data fitting adjustment. Our inability to relate the observable quantities to the unknown physical parameters means that the residuals appear larger than expected and that they are strongly correlated. A powerful tool to attack this problem is the method of least-squares collocation, borrowed from geodesy, combined with a technique for the estimation of the covariance function of the residuals

Interactive Computer Programs for Teaching Astronomy

Astronomy is the most popular and oldest of all sciences and it has had profound influence on human thought. Unfortunately, astronomy does not find an appropriate place in our school-college syllabi. This may be due to the fact that teaching of astronomy encounters a large number of problems with regard to visualization and practical experiments. Popularization of astronomy depends heavily on a large variety of astronomical events, such as the arrival of comets, eclipses, supernovae, etc. Visualization of dynamics in several directions, wide variance of time scales, concepts of space, etc., create problems in teaching/learning processes in astronomy. Our world of human experience is limited to within a narrow frame, whereas in astronomy we speak of size, time, and temperature in gigantic scales. To bring all these parameters onto the human level, one has to think of effective teaching aids and the right type of techniques. We have been using a large number of tools in teaching astronomy, including star charts, globes, models, photographs, slides, etc. Microcomputers act as an effective medium in teaching astronomy. They can even replace most of the above mentioned teaching aids (Hunt, 1986; Marx and Szucs, 1985; Sparkes, 1986). A microcomputer can also act as a textbook, a blackboard, or even a planetarium. The computer acts also as a mediator between the student and the model of some real-life situation. The process of building and using models, called simulation, helps us to investigate systems that would otherwise be inaccessible. We have developed a set of software to teach basic concepts in astronomy, such as the solar system, constellations, and the physics of stars. We have explored the possibilities of making the software interactive, using the observed data so that a real life situation can be experienced by the users, just as for practical experiments.

Atomic Processes in Astronomy

Atomic Processes in Astronomy