Solar and Stellar Photospheric Abundances

The determination of photospheric abundances in late-type stars from spectroscopic observations is a well-established field, built on solid theoretical foundations. Improving those foundations to refine the accuracy of the inferred abundances has proven challenging, but progress has been made. In parallel, developments on instrumentation, chiefly regarding multi-object spectroscopy, have been spectacular, and a number of projects are collecting large numbers of observations for stars across the Milky Way and nearby galaxies, promising important advances in our understanding of galaxy formation and evolution. After providing a brief description of the basic physics and input data involved in the analysis of stellar spectra, a review is made of the analysis steps, and the available tools to cope with large observational efforts. The paper closes with a quick overview of relevant ongoing and planned spectroscopic surveys, and highlights of recent research on photospheric abundances.Comment: Invited review to appear in Living Reviews in Solar Physics. 39 pages, 7 figure

Fronthaul evolution: From CPRI to Ethernet

It is proposed that using Ethernet in the fronthaul, between base station baseband unit (BBU) pools and remote radio heads (RRHs), can bring a number of advantages, from use of lower-cost equipment, shared use of infrastructure with fixed access networks, to obtaining statistical multiplexing and optimised performance through probe-based monitoring and software-defined networking. However, a number of challenges exist: ultra-high-bit-rate requirements from the transport of increased bandwidth radio streams for multiple antennas in future mobile networks, and low latency and jitter to meet delay requirements and the demands of joint processing. A new fronthaul functional division is proposed which can alleviate the most demanding bit-rate requirements by transport of baseband signals instead of sampled radio waveforms, and enable statistical multiplexing gains. Delay and synchronisation issues remain to be solved

FBMC system: an insight into doubly dispersive channel impact

It has been claimed that filter bank multicarrier (FBMC) systems suffer from negligible performance loss caused by moderate dispersive channels in the absence of guard time protection between symbols. However, a theoretical and systematic explanation/analysis for the statement is missing in the literature to date. In this paper, based on one-tap minimum mean square error (MMSE) and zero-forcing (ZF) channel equalizations, the impact of doubly dispersive channel on the performance of FBMC systems is analyzed in terms of mean square error of received symbols. Based on this analytical framework, we prove that the circular convolution property between symbols and the corresponding channel coefficients in the frequency domain holds loosely with a set of inaccuracies. To facilitate analysis, we first model the FBMC system in a vector/matrix form and derive the estimated symbols as a sum of desired signal, noise, intersymbol interference (ISI), intercarrier interference (ICI), interblock interference (IBI), and estimation bias in the MMSE equalizer. Those terms are derived one-by-one and expressed as a function of channel parameters. The numerical results reveal that under harsh channel conditions, e.g., with large Doppler spread or channel delay spread, the FBMC system performance may be severely deteriorated and error floor will occur

Infrared Studies of Molecular Shocks in the Supernova Remnant HB 21: II. Thermal Admixture of Shocked H2_2 Gas in the South

We present near- and mid-infrared observations on the shock-cloud interaction region in the southern part of the supernova remnant HB 21, performed with the InfraRed Camera (IRC) aboard AKARI satellite and the Wide InfraRed Camera (WIRC) at the Palomar 5 m telescope. The IRC 4 um (N4), 7 um (S7), and 11 um (S11) band images and the WIRC H2 v=1->0 S(1) 2.12 um image show similar diffuse features, around a shocked CO cloud. We analyzed the emission through comparison with the H2 line emission of several shock models. The IRC colors are well explained by the thermal admixture model of H2 gas--whose infinitesimal H2 column density has a power-law relation with the temperature $T$, $dN\sim T^{-b}dT$--with n(H2) $\sim3.9\times10^4$ cm^{-2}, $b\sim4.2$, and N(H2;T>100K) $\sim2.8\times10^{21}$ cm^{-2}. We interpreted these parameters with several different pictures of the shock-cloud interactions--multiple planar C-shocks, bow shocks, and shocked clumps--and discuss their weaknesses and strengths. The observed H2 v=1->0 S(1) intensity is four times greater than the prediction from the power-law admixture model, the same tendency as found in the northern part of HB 21 (Paper I). We also explored the limitation of the thermal admixture model with respect to the derived model parameters.Comment: 35 pages, 10 figures, Accepted in "Advances in Space Research", higher resolution @ http://astro.snu.ac.kr/~jhshinn/asr-20090921-submitted_arxiv.pdf ; rev.2 - deletion of section 6.4 and the related content