Description of the SERT 2 spacecraft and mission

Performance and orbital flight plan of SERT 2 ion thruster spacecraf

Spectroscopic and photometric studies of white dwarfs in the Hyades

The Hyades cluster is known to harbour ten so-called classical white dwarf members. Numerous studies through the years have predicted that more than twice this amount of degenerate stars should be associated with the cluster. Using the PPMXL catalog of proper motions and positions, a recent study proposed 17 new white dwarf candidates. We review the membership of these candidates by using published spectroscopic and photometric observations, as well as by simulating the contamination from field white dwarfs. In addition to the ten classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades origin and three more objects that have an uncertain membership status due to their unknown or imprecise atmospheric parameters. Among those, two to three are expected as field stars contamination. Accurate radial velocity measurements will confirm or reject the candidates. One consequence is that the longstanding problem that no white dwarf older than ~340 Myr appears to be associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and Astrophysics journa

Unsupervised discovery of temporal sequences in high-dimensional datasets, with applications to neuroscience.

Identifying low-dimensional features that describe large-scale neural recordings is a major challenge in neuroscience. Repeated temporal patterns (sequences) are thought to be a salient feature of neural dynamics, but are not succinctly captured by traditional dimensionality reduction techniques. Here, we describe a software toolbox-called seqNMF-with new methods for extracting informative, non-redundant, sequences from high-dimensional neural data, testing the significance of these extracted patterns, and assessing the prevalence of sequential structure in data. We test these methods on simulated data under multiple noise conditions, and on several real neural and behavioral datas. In hippocampal data, seqNMF identifies neural sequences that match those calculated manually by reference to behavioral events. In songbird data, seqNMF discovers neural sequences in untutored birds that lack stereotyped songs. Thus, by identifying temporal structure directly from neural data, seqNMF enables dissection of complex neural circuits without relying on temporal references from stimuli or behavioral outputs

Positive regulation of meiotic DNA double-strand break formation by activation of the DNA damage checkpoint kinase Mec1(ATR)

During meiosis, formation and repair of programmed DNA double-strand breaks (DSBs) create genetic exchange between homologous chromosomes-a process that is critical for reductional meiotic chromosome segregation and the production of genetically diverse sexually reproducing populations. Meiotic DSB formation is a complex process, requiring numerous proteins, of which Spo11 is the evolutionarily conserved catalytic subunit. Precisely how Spo11 and its accessory proteins function or are regulated is unclear. Here, we use Saccharomyces cerevisiae to reveal that meiotic DSB formation is modulated by the Mec1(ATR) branch of the DNA damage signalling cascade, promoting DSB formation when Spo11-mediated catalysis is compromised. Activation of the positive feedback pathway correlates with the formation of single-stranded DNA (ssDNA) recombination intermediates and activation of the downstream kinase, Mek1. We show that the requirement for checkpoint activation can be rescued by prolonging meiotic prophase by deleting the NDT80 transcription factor, and that even transient prophase arrest caused by Ndt80 depletion is sufficient to restore meiotic spore viability in checkpoint mutants. Our observations are unexpected given recent reports that the complementary kinase pathway Tel1(ATM) acts to inhibit DSB formation. We propose that such antagonistic regulation of DSB formation by Mec1 and Tel1 creates a regulatory mechanism, where the absolute frequency of DSBs is maintained at a level optimal for genetic exchange and efficient chromosome segregation

Astrometry of OH/IR stars using 1612 MHz hydroxyl masers. I. Annual parallaxes of WX Psc and OH138.0+7.2

We report on the measurement of the trigonometric parallaxes of 1612 MHz hydroxyl masers around two asymptotic giant branch stars, WX Psc and OH138.0+7.2, using the NRAO Very Long Baseline Array with in-beam phase referencing calibration. We obtained a 3-sigma upper limit of <=5.3 mas on the parallax of WX Psc, corresponding to a lower limit distance estimate of >~190 pc. The obtained parallax of OH138.0+7.2 is 0.52+/-0.09 mas (+/-18%), corresponding to a distance of 1.9(+0.4,-0.3) kpc, making this the first hydroxyl maser parallax below one milliarcsecond. We also introduce a new method of error analysis for detecting systematic errors in the astrometry. Finally, we compare our trigonometric distances to published phase-lag distances toward these stars and find a good agreement between the two methods.Comment: Preprint, accepted for publication in The Astronomical Journal (January 17, 2017

Freezing of the quantum Hall liquid at ν=\nu = 1/7 and 1/9

We compare the free energy computed from the ground state energy and low-lying excitations of the 2-D Wigner solid and the fractional quantum Hall liquid, at magnetic filling factors $\nu = 1/7$ and 1/9. We find that the Wigner solid melts into the fractional quantum Hall liquid at roughly the same temperature as that of some recent luminescence experiments, while it remains a solid at the lower temperatures characteristic of the transport experiments. We propose this melting as a consistent interpretation of both sets of experiments.Comment: uses RevTeX 2.0 or 3.

Dissipation, topology, and quantum phase transition in a one-dimensional Joesphson junction array

We study the phase diagram and quantum critical properties of a resistively shunted Josephson junction array in one dimension from a strong coupling analysis. After mapping the dissipative quantum phase model to an effective sine-Gordon model we study the renormalization group flow and the phase diagram. We try to bridge the phase diagrams obtained from the weak and the strong coupling renormalization group calculations to extract a more comprehensive picture of the complete phase diagram. The relevance of our theory to experiments in nanowires is discussed.Comment: 13 pages, 3 figures, A few typos are correcte

Quantum Correlated Interstitials and the Hall Resistivity of the Magnetically Induced Wigner Crystal

We study a trial wavefunction for an interstitial in a Wigner crystal. We find that the electron correlations, ignored in a conventional Hartree-Fock treatment, dramatically lower the interstitial energy, especially at fillings close to an incompressible liquid state. The correlation between the interstitial electron and the lattice electrons at $\nu <1/m$ is introduced by constructing a trial wave- function which bears a Jastrow factor of a Laughlin state at $\nu=1/m$. For fillings close to but just below $\nu=1/m$, we find that a perfect Wigner crystal becomes unstable against formation of such interstitials. It is argued that conduction due to correlated interstitials in the presence of weak disorder leads to the {\it classical} Hall resistivity, as seen experimentally.Comment: 10 pages, RevTe

The quantized Hall conductance of a single atomic wire: A proposal based on synthetic dimensions

We propose a method by which the quantization of the Hall conductance can be directly measured in the transport of a one-dimensional atomic gas. Our approach builds on two main ingredients: (1) a constriction optical potential, which generates a mesoscopic channel connected to two reservoirs, and (2) a time-periodic modulation of the channel, specifically designed to generate motion along an additional synthetic dimension. This fictitious dimension is spanned by the harmonic-oscillator modes associated with the tightly-confined channel, and hence, the corresponding "lattice sites" are intimately related to the energy of the system. We analyze the quantum transport properties of this hybrid two-dimensional system, highlighting the appealing features offered by the synthetic dimension. In particular, we demonstrate how the energetic nature of the synthetic dimension, combined with the quasi-energy spectrum of the periodically-driven channel, allows for the direct and unambiguous observation of the quantized Hall effect in a two-reservoir geometry. Our work illustrates how topological properties of matter can be accessed in a minimal one-dimensional setup, with direct and practical experimental consequences.