The VLT-FLAMES tarantula survey:XII. Rotational velocities of the single O-type stars

Context. The 30 Doradus (30 Dor) region of the Large Magellanic Cloud, also known as the Tarantula nebula, is the nearest starburst region. It contains the richest population of massive stars in the Local Group, and it is thus the best possible laboratory to investigate open questions on the formation and evolution of massive stars. Aims. Using ground-based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Dor. The sample is large enough to obtain statistically significant information and to search for variations among subpopulations - in terms of spectral type, luminosity class, and spatial location - in the field of view. Methods. We measured projected rotational velocities, 3e sin i, by means of a Fourier transform method and a profile fitting method applied to a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, P(3e), of the equatorial rotational velocity, 3e. Results. The distribution of 3e sin i shows a two-component structure: a peak around 80 km s1 and a high-velocity tail extending up to 600 km s^-1 This structure is also present in the inferred distribution P(3e) with around 80% of the sample having 0 <3e ≤ 300 km s^-1 and the other 20% distributed in the high-velocity region. The presence of the low-velocity peak is consistent with what has been found in other studies for late O- and early B-type stars. Conclusions. Most of the stars in our sample rotate with a rate less than 20% of their break-up velocity. For the bulk of the sample, mass loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. If massive-star formation results in stars rotating at birth with a large portion of their break-up velocities, an alternative braking mechanism, possibly magnetic fields, is thus required to explain the present-day rotational properties of the O-type stars in 30 Dor. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Even though we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. That the highvelocity tail may be populated primarily (and perhaps exclusively) by post-binary interaction products has important implications for the evolutionary origin of systems that produce gamma-ray bursts. © 2013 Author(s).

Secrecy Capacity Analysis Over κ–μ Fading Channels: Theory and Applications

In this paper, we consider the transmission of confidential information over a κ-μ fading channel in the presence of an eavesdropper who also experiences κ-μ fading. In particular, we obtain novel analytical solutions for the probability of strictly positive secrecy capacity (SPSC) and a lower bound of secure outage probability (SOPL) for independent and non-identically distributed channel coefficients without parameter constraints. We also provide a closed-form expression for the probability of SPSC when the μ parameter is assumed to take positive integer values. Monte-Carlo simulations are performed to verify the derived results. The versatility of the κ-μ fading model means that the results presented in this paper can be used to determine the probability of SPSC and SOPL for a large number of other fading scenarios, such as Rayleigh, Rice (Nakagamin), Nakagami-m, One-Sided Gaussian, and mixtures of these common fading models. In addition, due to the duality of the analysis of secrecy capacity and co-channel interference (CCI), the results presented here will have immediate applicability in the analysis of outage probability in wireless systems affected by CCI and background noise (BN). To demonstrate the efficacy of the novel formulations proposed here, we use the derived equations to provide a useful insight into the probability of SPSC and SOPL for a range of emerging wireless applications, such as cellular device-to-device, peer-to-peer, vehicle-to-vehicle, and body centric communications using data obtained from real channel measurements.

Level-resolved quantum statistical theory of electron capture into many-electron compound resonances in highly charged ions

The strong mixing of many-electron basis states in excited atoms and ions with open f shells results in very large numbers of complex, chaotic eigenstates that cannot be computed to any degree of accuracy. Describing the processes which involve such states requires the use of a statistical theory. Electron capture into these “compound resonances” leads to electron-ion recombination rates that are orders of magnitude greater than those of direct, radiative recombination and cannot be described by standard theories of dielectronic recombination. Previous statistical theories considered this as a two-electron capture process which populates a pair of single-particle orbitals, followed by “spreading” of the two-electron states into chaotically mixed eigenstates. This method is similar to a configuration-average approach because it neglects potentially important effects of spectator electrons and conservation of total angular momentum. In this work we develop a statistical theory which considers electron capture into “doorway” states with definite angular momentum obtained by the configuration interaction method. We apply this approach to electron recombination with W²⁰⁺, considering 2×10⁶ doorway states. Despite strong effects from the spectator electrons, we find that the results of the earlier theories largely hold. Finally, we extract the fluorescence yield (the probability of photoemission and hence recombination) by comparison with experiment.