Line-dependent veiling in very active T Tauri Stars

The T Tauri stars with active accretion disks show veiled photospheric spectra. This is supposedly due to non-photospheric continuum radiated by hot spots beneath the accretion shocks at stellar surface and/or chromospheric emission lines radiated by the post-shocked gas. The amount of veiling is often considered as a measure of the mass-accretion rate. We analysed high-resolution photospheric spectra of accreting T Tauri stars LkHa 321, V1331 Cyg, and AS 353 A with the aim of clarifying the nature of the line-dependent veiling. Each of these objects shows a highly veiled, strong emission line spectrum and powerful wind features indicating high rates of accretion and mass loss. Equivalent widths of hundreds of weak photospheric lines were measured in the observed spectra and compared with those in synthetic spectra with the same spectral type. We found that the veiling is strongly line-dependent: larger in stronger photospheric lines and weak or absent in the weakest ones. No dependence of veiling on excitation potential within 0 to 5 eV was found. Different physical processes responsible for these unusual veiling effects are discussed in the framework of the magnetospheric accretion model. The observed veiling has two origins: 1) an abnormal structure of stellar atmosphere heated up by the accreting matter, and 2) a non-photospheric continuum radiated by a hot spot with temperature lower than 10000 K. The true level of the veiling continuum can be derived by measuring the weakest photospheric lines with equivalent widths down to $\approx$10 m\AA. A limited spectral resolution and/or low signal-to-noise ratio results in overestimation of the veiling continuum. In the three very active stars, the veiling continuum is a minor contributor to the observed veiling, while the major contribution comes from the line-dependent veiling.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and Astrophysic

Vectorial channel estimation for uplink MC-CDMA in beyond 3G wireless systems

In beyond 3G wireless systems the bandwidth efficiency can be increased with the use of adaptive antenna arrays. This paper focus on a key issue for adaptive antenna arrays, that is, channel parameters estimation including Direction-Of-Arrival (DOA). In order to estimate DOA. the channel frequency responses for the links between the mobile users and each of base station array elements are estimated by pilot-aided minimum mean square error (MMSE) algorithm. This estimator is not sensitive to the channel statistics. Based on frequency response estimatives DOA's of impinging multipath components to the base station are estimated by a low complex Wlauimuni Likelihood (ML) approach. Furthermore an uplink burst structure with specifically designed midamble field for multiuser channel estimation in MC-CDMA is proposed. The performance is assessed in terms of channel estimation errors for a MC-CDMA TDD system over fast and slow fading mobile channels

High resolution DOA estimation technique for uplink TDD mode

This paper investigates a maximum likelihood (ML) approach for direction of arrival (DOA) estimation based on the algorithm suggested by the 3GPP for scalar channels in UTRA-TDD mode. The performance of the algorithm is assessed by resorting to simulations in typical UMTS scenarios. The results indicate that the proposed scheme overcomes the limitation of the angular resolution inherent to classical techniques like subspace or beam-forming methods. In particular, it is shown that waves exhibiting an arbitrarily small difference in azimuth and temporal spacing higher than one chip time interval can be easily separated. A threshold operation is included to select only the most significant paths, in terms of energy, of the estimated channel. The results show that a good performance can be achieved from low to high values of Eb/N0

Facing the wind of the pre-FUor V1331 Cyg

The mass outflows in T Tauri stars (TTS) are thought to be an effective mechanism to remove angular momentum during the pre-main-sequence contraction of a low-mass star. The most powerful winds are observed at the FUor stage of stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007 and 20-d series of spectra taken in 2012. For the first time the photospheric spectrum of the star is detected and stellar parameters are derived: spectral type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric spectrum is highly veiled, but the amount of veiling is not the same in different spectral lines, being lower in weak transitions and much higher in strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of metals are accompanied by a `shell' absorption at radial velocity of about -240 km/s. We show that these absorptions form in the post-shock gas in the jet, i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta indicate the terminal wind velocity of about 500 km/s, which vary on time-scales from several days to years. A model of the stellar wind is developed to interpret the observations. The model is based on calculation of hydrogen spectral lines using the radiative transfer code TORUS. The observed H-alpha and H-beta line profiles and their variability can be well reproduced with a stellar wind model, where the mass-loss rate and collimation (opening angle) of the wind are variable. The changes of the opening angle may be induced by small variability in magetization of the inner disc wind. The mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the accretion rate of 2.0x10^{-6} Msun/yr.Comment: 11 pages, 12 figures; accepted for publication in MNRAS. Typographical errors have been corrected after the proof stag

Sensing opportunities in UMTS spectrum

The UMTS radio frequency spectrum is a highly expensive commodity. While the UMTS standards make very efficient use of the allocated bands there is however opportunity for further advancements. This paper focuses on opportunistic use of the UMTS spectrum as a means of ensuring that the maximum possible use of this valuable resource is made. In particular we focus on the local detection of UMTS TDD signals through the use of a cyclostationary feature detector. Simulation results for the use of this detector in the presence of multipath propagation and shadowing effects are presented

Analysis of communication opportunities in UMTS cellular networks

A scenario based on an UMTS TDD opportunistic cellular system that operates over UMTS FDD licensed cellular networks is considered. Therefore a simulation tool was developed for the analysis and assessment of UMTS TDD opportunistic radio system in a coexistence environment with UMTS FDD primary cellular networks. The communication presents the scenario considered, the main features of the simulation tool, discusses and proposes metrics to evaluate the communication opportunities in UMTS FDD primary cellular networks, and presents numerical simulation results. These show that a moderate number of UMTS FDD available frequencies and a reasonable UMTS TDD opportunistic radio transmission power will allow the deployment of UMTS TDD opportunistic radio networks in a coexistence environment with existing licensed systems

Sensing UMTS bands using cyclostationary features and cooperation between opportunistic terminals

The Opportunistic Radio (OR) concept relies on the cognitive features of the OR terminals, namely the ability to adapt its transmitter parameters, based upon interaction with the RF environment in which it operates. An OR system operates in licensed frequency bands, exploiting opportunities and operating with a lower priority regarding the licensed system, implementing a spectrum pool mechanism. The most important constraint is that the OR network should always avoid harmful interference with the licensed system, therefore it should reliably detect licensed signals in the used band in order to avoid interfering with the licensed owner of that band. Given the importance of UMTS systems in current wireless communications, this paper is focused on 3G bands and addresses the problem of sensing weak UMTS signals. The proposed sensing algorithm exploits the cyclostationary features of UMTS signals and the cooperation between multiple OR terminals clustered in the OR network

Application of game theory in ad- hoc opportunistic radios

The application of mathematical analysis to the study of wireless ad hoc networks has met with limited success due to the complexity of mobility, traffic models and the dynamic topology. A scenario based UMTS TDD opportunistic cellular system with an ad hoc behaviour that operates over UMTS FDD licensed cellular network is considered. In this paper, we describe how ad hoc opportunistic radio can be modeled as a game and how we apply game theory based Power Control in ad-hoc opportunistic radio

Utilização de um aparelho de cortina de ar no isolamento térmico de duas divisões

Este trabalho versa o problema da vedação aerodinâmica de um espaço habitacional ou comercial climatizado, no qual se pretendem manter condições de conforto térmico, relativamente a um ambiente exterior, que poderá estar mais quente ou mais frio. Para a sua concretização recorreu-se a um conjunto de medições numa instalação experimental à escala real em que um aparelho de cortina de ar, de jacto plano descendente, não-recirculado, se encontra instalado por cima de uma porta que separa duas salas contíguas e iguais, que se pretendem manter com diferentes valores de temperatura. Após uma breve explicação sobre as técnicas e metodologias experimentais que foram aplicadas, apresentam-se alguns exemplos elucidativos da influência de alguns parâmetros geométricos e dinâmicos sobre a eficácia da vedação conseguida, a partir dos quais se estabelecem algumas recomendações sobre a selecção, instalação e operação deste tipo de aparelho. São ainda analisadas as poupanças energéticas conseguidas com a utilização deste tipo de aparelho como meio de vedação de espaços climatizados abertos ao público