Detection of Molecular Hydrogen Orbiting a "Naked" T Tauri Star

Astronomers have established that for a few million years newborn stars possess disks of orbiting gas and dust. Such disks, which are likely sites of planet formation, appear to disappear once these stars reach ages of 5-10 times 10^6 yr; yet, >= 10^7 yr is thought necessary for giant planet formation. If disks dissipate in less time than is needed for giant planet formation, such planets may be rare and those known around nearby stars would be anomalies. Herein, we report the discovery of H_2 gas orbiting a weak-lined T Tauri star heretofore presumed nearly devoid of circumstellar material. We estimate that a significant amount of H_2 persists in the gas phase, but only a tiny fraction of this mass emits in the near-infrared. We propose that this star possesses an evolved disk that has escaped detection thus far because much of the dust has coagulated into planetesimals. This discovery suggests that the theory that disks are largely absent around such stars should be reconsidered. The widespread presence of such disks would indicate that planetesimals can form quickly and giant planet formation can proceed to completion before the gas in circumstellar disks disperses.Comment: latex 12 pages, including 1 figur

Shifted convolution and the Titchmarsh divisor problem over F_q[t]

In this paper we solve a function field analogue of classical problems in analytic number theory, concerning the auto-correlations of divisor functions, in the limit of a large finite field.Comment: 22 pages, updated versio

Persistent currents of noninteracting electrons

We thoroughly study the persistent current of noninteracting electrons in one, two, and three dimensional thin rings. We find that the results for noninteracting electrons are more relevant for individual mesoscopic rings than hitherto appreciated. The current is averaged over all configurations of the disorder, whose amount is varied from zero up to the diffusive limit, keeping the product of the Fermi wave number and the ring's circumference constant. Results are given as functions of disorder and aspect ratios of the ring. The magnitude of the disorder-averaged current may be larger than the root-mean-square fluctuations of the current from sample to sample even when the mean free path is smaller, but not too small, than the circumference of the ring. Then a measurement of the persistent current of a typical sample will be dominated by the magnitude of the disorder averaged current.Comment: 10 pages, 4 figure

Pair-breaking effect on mesoscopic persistent currents

We consider the contribution of superconducting fluctuations to the mesoscopic persistent current (PC) of an ensemble of normal metallic rings, made of a superconducting material whose low bare transition temperature $T^{0}_{c}$ is much smaller than the Thouless energy $E_{c}$. The effect of pair breaking is introduced via the example of magnetic impurities. We find that over a rather broad range of pair-breaking strength $\hbar/\tau_{s}$, such that $T_c^0 \lesssim \hbar/\tau_s \lesssim E_c$, the superconducting transition temperature is normalized down to minute values or zero while the PC is hardly affected. This may provide an explanation for the magnitude of the average PC's in copper and gold, as well as a way to determine their $T^0_c$'s. The dependence of the current and the dominant superconducting fluctuations on $E_c\tau_s$ and on the ratio between $E_c$ and the temperature is analyzed. The measured PC's in copper (gold) correspond to $T^0_c$ of a few (a fraction of) mK

Time series aggregation, disaggregation and long memory

We study the aggregation/disaggregation problem of random parameter AR(1) processes and its relation to the long memory phenomenon. We give a characterization of a subclass of aggregated processes which can be obtained from simpler, "elementary", cases. In particular cases of the mixture densities, the structure (moving average representation) of the aggregated process is investigated

Le marigot houët à Bobo-Dioulasso : une question de santé publique ?

Le marigot Houët, unaffluent de la rivière Kou, traverse l’intérieur de la ville de Bobo-Dioulasso du Sud au Nord. Il abrite des poissons silures qui sont considérés comme sacrés par les populations riveraines et sert aussi de source d’activités socio-économiques pour ces populations. L’objectif de notre étude est de faire un état des lieux, partant, des fonctions du marigot, afin de contribuer à la protection de son écosystème et à l’amélioration de la santé et du cadre de vie des populations riveraines. Pour notre étude, nous avons fait des prélèvements d’échantillons d’eau en sept (7) points du marigot couvrant toutes les activités  socioéconomiques. Les analyses des paramètres physicochimiques de pollution (pH, conductivité, turbidité, matières en suspension, ortho-phosphates, nitrates et nitrites, sulfates et chlorophylle « a »), de la demande chimique en oxygène (DCO), de la demande biologique en oxygène (DBO5), des paramètres microbiologiques (E. coli, coliformes fécaux, streptocoques fécaux) et des paramètres organoleptiques montrent que l’eau du Houët est sujette à une pollution domestique et industrielle.Mots clés: écosystème, pollution, silures, ea

Permanence criteria for semi-free profinite groups

We introduce the condition of a profinite group being semi-free, which is more general than being free and more restrictive than being quasi-free. In particular, every projective semi-free profinite group is free. We prove that the usual permanence properties of free groups carry over to semi-free groups. Using this, we conclude that if k is a separably closed field, then many field extensions of k((x,y)) have free absolute Galois groups.Comment: 24 page

Production of Lambda and Sigma^0 hyperons in proton-proton collisions

This paper reports results on simultaneous measurements of the reaction channels pp -> pK+\Lambda and pp -> pK+\Sigma^0 at excess energies of 204, 239, and 284 MeV (\Lambda) and 127, 162, and 207 MeV (\Sigma^0). Total and differential cross sections are given for both reactions. It is concluded from the measured total cross sections that the high energy limit of the cross section ratio is almost reached at an excess energy of only about 200 MeV. From the differential distributions observed in the overall CMS as well as in the Jackson and helicity frames, a significant contribution of interfering nucleon resonances to the \Lambda production mechanism is concluded while resonant \Sigma^0-production seems to be of lesser importance and takes place only through specific partial waves of the entrance channel. The data also indicate that kaon exchange plays a minor role in the case of \Lambda- but an important role for \Sigma^0-production. Thus the peculiar energy dependence of the \Lambda-to-\Sigma^0 cross section ratio appears in a new light as its explanation requires more than mere differences between the p\Lambda and the p\Sigma^0 final state interaction. The data provide a benchmark for theoretical models already available or yet to come.Comment: 18 pages, 10 figures; accepted by The European Physical Journal A (EPJ A