The Solar-Type Contact Binary BX Pegasi Revisited

We present the results of new CCD photometry for the contact binary BX Peg, made during three successive months beginning on September 2008. As do historical light curves, our observations display an O'Connell effect and the November data by themselves indicate clear evidence for very short-time brightness disturbance. For these variations, model spots are applied separately to the two data set of Group I (Sep.--Oct.) and Group II (Nov.). The former is described by a single cool spot on the secondary photosphere and the latter by a two-spot model with a cool spot on the cool star and a hot one on either star. These are generalized manifestations of the magnetic activity of the binary system. Twenty light-curve timings calculated from Wilson-Devinney code were used for a period study, together with all other minimum epochs. The complex period changes of BX Peg can be sorted into a secular period decrease caused dominantly by angular momentum loss due to magnetic stellar wind braking, a light-travel-time (LTT) effect due to the orbit of a low-mass third companion, and a previously unknown short-term oscillation. This last period modulation could be produced either by a second LTT orbit with a period of about 16 yr due to the existence of a fourth body or by the effect of magnetic activity with a cycle length of about 12 yr.Comment: 23 pages, including 6 figures and 8 tables, accepted for publication in PAS

Physical Properties of the Transiting Planetary System TrES-3

We present four new transits of the planetary system TrES-3 observed between 2009 May and 2010 June. Among these, the third transit by itself indicates possible evidence for brightness disturbance, which might be the result of the planet blocking a cool starspot on the stellar surface. A total of 109 transit times, including our measurements, were used to determine the improved ephemeris with a transit epoch of 2454185.910944$\pm$0.000072 HJED and an orbital period of 1.30618700$\pm$0.00000015 d. We analyzed the transit light curves using the JKTEBOP code and adopting the quadratic limb-darkening law. In order to derive the physical properties of the TrES-3 system, the transit parameters are combined with the empirical relations from eclipsing binary stars and stellar evolutionary models. The stellar mass and radius obtained from a calibration using $T_A$, log $\rho_{\rm A}$ and [Fe/H] are consistent with those from the isochrone analysis. We found that the exoplanet TrES-3b has a mass of 1.93$\pm$0.07 M$_{\rm Jup}$, a radius of 1.30$\pm$0.04 R$_{\rm Jup}$, a surface gravity of log $g_{\rm b}$=3.45$\pm$0.02, a density of 0.82$\pm$0.06 $\rho_{\rm Jup}$, and an equilibrium temperature of 1641$\pm$23 K. The results are in good agreement with theoretical models for gas giant planets.Comment: 15 pages, including 4 figures and 4 tables, accepted for publication in PAS

Numerical Simulation of Vortex-Dominated Flows Using the Penalized VIC Method

Vorticity plays a key role in determining fluid flow dynamics, especially in vortex-dominated flows. Vortex methods, which are based on the vorticity-based formulation of the Navier-Stokes equations, have provided deeper insight into physical reality in a variety of flows using vorticity as a primary variable. The penalized vortex-in-cell (VIC) method is a state-of-the-art variant of vortex methods. In the penalized VIC method, Lagrangian fluid particles are traced by continuously updating their position and strength from solutions at an Eulerian grid. This hybrid method retains beneficial features of pure Lagrangian and Eulerian methods. It offers an efficient and effective way to simulate unsteady viscous flows, thereby enabling application to a wider range of problems in flows. This article presents the fundamentals of the penalized VIC method and its implementations

IMPACT OF LAND SURFACE VEGETATION CHANGE OVER THE LA PLATA BASIN ON THE REGIONAL CLIMATIC ENVIRONMENT: A STUDY USING CONVENTIONAL LAND-COVER/LAND-USE AND NEWLY DEVELOPED ECOSYSTEM FUNCTIONAL TYPES

Naturally occurring or human induced changes in land surface vegetation have been recognized as one of the important factors influencing climate change. The La Plata Basin in South America has experienced significant changes in structural land-cover/land-use types, and those changes can involve changes in the surface physical properties such as albedo and roughness length, evapotranspiration, infiltration, and water storage eventually affecting the development of precipita-tion and the hydroclimate of the basin. In this study, the Weather Research and Forecast (WRF) modeling system was employed to investigate the role of changing land surface conditions in the La Plata Basin. For this purpose, ensembles of seasonal simulations were prepared for a control case and two extreme land cover scenarios: the first one assumes an expansion of the agricultural activities and the second one assumes a "natural" vegetation cover where no croplands are present. An extreme anthropogenic land-cover change -simulating an extensive agricultural practice- implies that the northern part of the basin, where croplands replace forests and savannah, would experience an overall increase in albedo and reduced surface friction. The two changes lead to a reduction of sensible heat and surface temperature, and a somewhat higher evapotranspiration due to decreased stomatal resistance and stronger near-surface winds. The effect on sensible heat seems to dominate and leads to a reduction in convective instability. The stronger low level winds due to reduced friction also imply a larger amount of moisture advected out of the basin, and thus resulting in reduced moisture flux convergence (MFC) within the basin. The two effects, increased stability and reduced MFC, result in a reduction of precipitation. On the other hand, the southern part of the basin exhibits the opposite behavior, as crops would replace grasslands, resulting in reduced albedo, a slight increase of surface temperature and increased precipitation. Notably, the results are not strictly local, as advective processes tend to modify the circulation and precipitation patterns downstream over the South Atlantic Ocean. A newly developed land surface classification, so-called Ecosystem Functional Types (EFTs, systems that share homogeneous energy and mass exchanges with the atmosphere), is implemented in the WRF model to explore its usefulness in regional climate simulations of surface and atmospheric variables. Results show that use of the EFT data improves the climate simulation of 2-m temperature and precipitation, making EFTs a good alternative to land cover types in numerical climate models. An additional advantage of EFTs is that they can be calculated on a yearly basis, thus representing the interannual variability of the surface states. During dry years the 2-m temperature and 10-m wind are more sensitive to changes in EFTs, while during wet years the sensitivity is larger for the 2-m water vapor mixing ratio, convective available potential energy, vertically-integrated moisture fluxes and surface precipitation. This indicates that the impact of land-cover and land-use changes on the climate of the LPB is dependent not only on the wetness of the year, but also on the meteorological or climate variables. Comparisons with observations show that the simulated precipitation difference induced by EFT changes resembles the overall pattern of observed precipitation changes for those same years over the LPB. In the case of the 2-m temperature, the simulated changes due to EFT changes are similar to the observed changes in the eastern part and the southern part of the basin (especially in Uruguay), where t he strongest EFT changes occurred