Resistive plate chambers for time-of-flight measurements

The applications of Resistive Plate Chambers (RPCs) have recently been extended by the development of counters with time resolution below 100 ps sigma for minimum ionising particles. Applications to HEP experiments have already taken place and many further applications are under study. In this work we address the operating principles of such counters along with some present challenges, with emphasis on counter aging.Comment: Presented at "PSD6 - 6th International Conference on Position Sensitive Detectors", 9-13 September 2002, Leicester, UK. Submitted to Nuclear Instruments and Methods

Rotation of Hot Horizontal Branch Stars in Galactic Globular Clusters

We present high resolution UVES+VLT spectroscopic observations of 61 stars in the extended blue horizontal branches of the Galactic globular clusters NGC 1904 (M79), NGC 2808, NGC 6093 (M80), and NGC 7078 M15). Our data reveal for the first time the presence in NGC 1904 of a sizable population of fast (v sin(i) >= 20 km/s) horizontal branch (HB) rotators, confined to the cool end of the EHB, similar to that found in M13. We also confirm the fast rotators already observed in NGC 7078. The cooler stars (T_eff < 11,500 K) in these three clusters show a range of rotation rates, with a group of stars rotating at ~ 15 km/s or less, and a fast rotating group at ~ 30 km/s. Apparently, the fast rotators are relatively more abundant in NGC 1904 and M13, than in NGC 7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the stars hotter than T_eff ~ 11,500 K have projected rotational velocities vsini< 12 km/s. The connection between photometric gaps in the HB and the change in the projected rotational velocities is not confirmed by the new data. However, our data are consistent with a relation between this discontinuity and the HB jump.Comment: 2 pages, 1 figure, A.S.P. Conf. Ser., in press in Vol. 296, 200

Revisited fluorine abundances in the globular cluster M22 (NGC 6656)

Fluorine is a fairly good tracer of formation histories of multiple stellar populations in globular clusters as already revealed by several studies. Large variations in fluorine abundance in red giant stars of the globular cluster M22 have been recently reported by two different groups. Futhermore, one of these studies claims that the abundance of fluorine is anti-correlated with sodium abundances in this cluster, leading to strong conclusions on the chemical history of M22. To validate this important finding, we re-examine the F abundance determinations of some of the previously studied stars. We have thus reanalysed some high-resolution VLT/CRIRES spectra of RGB stars found in M22 in order to re-estimate their fluorine abundance from the spectral synthesis of the HF line at 2.336microns. Unlike what has been previously estimated, we show that only upper limits or doubtful fluorine abundances with large uncertainties in M22 RGB stars can be derived. This is probably caused by an incorrect identification of continuum fluctuations as the HF signature combined with a wrong correction of the stellar radial velocity. Such continuum fluctuations could be the consequences of telluric residuals that are still present in the analysed spectra. Therefore, no definitive conclusions on the chemical pollution caused by the M22 first stellar generation can presently be drawn from the fluorine content of this cluster.Comment: A&A, in pres

The influence of leaf characteristics on epiphyllic cover : a test of hypotheses with artificial leaves

Studies of epiphyll ecology have been hindered by the biochemical and morphological variability of the leaf substrate. The use of artificial (plastic ribbon tape) leaves solved that problem in a study done at the Braulio Carrillo National Park, Costa Rica. It showed that after nine months of field exposure, relative epiphyll cover was similar in five leaf shapes and two sizes. Driptips do not affect epiphyll cover, which was four times higher under a clearing than in the shaded understory, for all leaf shapes and sizes

Fluorine abundances and the puzzle of globular cluster chemical history

The abundance of fluorine in a few Galactic globular clusters is known to strongly vary from star-to-star. These unexpected chemical properties are an additional confirmation of the chemical inhomogeneities already found in several GC, and probably caused by the first generations of stars formed in these systems. The aim of this article is to complement our understanding of the F-behaviour in GC stars and to look for new constraints on the formation histories of their multiple stellar populations. We have collected near-IR spectra of 15 RGB stars belonging to GC spanning a wide range of metallicity: 47 Tuc, M4, NGC6397 and M30. F, Na and Fe abundances have been estimated by spectral synthesis. No anticorrelation between F and Na abundances are found for the most metal-rich cluster of the sample (47 Tuc). In this GC, RGB stars indeed exhibit rather small differences in [F/Fe] unlike the larger ones found for the [Na/Fe] ratios. This reveals a rather inhomogeneous stellar system and a complex chemical evolution history for 47 Tuc . In M4, one star of our study confirms the previous Na-F distribution reported by another group in 2005. For the two very metal-poor GC (NGC6397 and M30), only upper limits of F abundances have been derived. We show that F abundances could be estimated in such metal-poor GC with current telescopes and spectrographs only if unexpected F-rich giants are found and/or exceptional observational conditions are met. The distribution of the F and Na abundances in GC reveal that their RGB members seem to belong to two well-separated regions. All the RGB stars analysed so far in the different GC are indeed found to be either F-rich Na-poor or F-poor Na-rich. Such well-separated bimodal regimes are consistent with the separate formation episodes suspected in most galactic GC.Comment: Astronomy & Astrophysics, in pres

Automated derivation of stellar atmospheric parameters and chemical abundances: the MATISSE algorithm

We present an automated procedure for the derivation of atmospheric parameters (Teff, log g, [M/H]) and individual chemical abundances from stellar spectra. The MATrix Inversion for Spectral SythEsis (MATISSE) algorithm determines a basis, B_\theta(\lambda), allowing to derive a particular stellar parameter \theta by projection of an observed spectrum. The B_\theta(\lambda) function is determined from an optimal linear combination of theoretical spectra and it relates, in a quantitative way, the variations in the spectrum flux with variations in \theta. An application of this method to the GAIA/RVS spectral range is described, together with its performances for different types of stars of various metallicities. Blind tests with synthetic spectra of randomly selected parameters and observed input spectra are also presented. The method gives rapid, accurate and stable results and it can be efficiently applied to the study of stellar populations through the analysis of large spectral data sets, including moderate to low signal to noise spectra

Counting Dyck paths by area and rank

The set of Dyck paths of length $2n$ inherits a lattice structure from a bijection with the set of noncrossing partitions with the usual partial order. In this paper, we study the joint distribution of two statistics for Dyck paths: \emph{area} (the area under the path) and \emph{rank} (the rank in the lattice). While area for Dyck paths has been studied, pairing it with this rank function seems new, and we get an interesting $(q,t)$-refinement of the Catalan numbers. We present two decompositions of the corresponding generating function: one refines an identity of Carlitz and Riordan; the other refines the notion of $\gamma$-nonnegativity, and is based on a decomposition of the lattice of noncrossing partitions due to Simion and Ullman. Further, Biane's correspondence and a result of Stump allow us to conclude that the joint distribution of area and rank for Dyck paths equals the joint distribution of length and reflection length for the permutations lying below the $n$-cycle $(12...n)$ in the absolute order on the symmetric group.Comment: 24 pages, 7 figures. Connections with work of C. Stump (arXiv:0808.2822v2) eliminated the need for 5 pages of proof in the first draf

On the number of pancake stacks requiring four flips to be sorted

Using existing classification results for the 7- and 8-cycles in the pancake graph, we determine the number of permutations that require 4 pancake flips (prefix reversals) to be sorted. A similar characterization of the 8-cycles in the burnt pancake graph, due to the authors, is used to derive a formula for the number of signed permutations requiring 4 (burnt) pancake flips to be sorted. We furthermore provide an analogous characterization of the 9-cycles in the burnt pancake graph. Finally we present numerical evidence that polynomial formulae exist giving the number of signed permutations that require $k$ flips to be sorted, with $5\leq k\leq9$.Comment: We have finalized for the paper for publication in DMTCS, updated a reference to its published version, moved the abstract to its proper location, and added a thank you to the referees. The paper has 27 pages, 6 figures, and 2 table