A proposal for regularly updated review/survey articles: "Perpetual Reviews"

We advocate the publication of review/survey articles that will be updated regularly, both in traditional journals and novel venues. We call these "perpetual reviews." This idea naturally builds on the dissemination and archival capabilities present in the modern internet, and indeed perpetual reviews exist already in some forms. Perpetual review articles allow authors to maintain over time the relevance of non-research scholarship that requires a significant investment of effort. Further, such reviews published in a purely electronic format without space constraints can also permit more pedagogical scholarship and clearer treatment of technical issues that remain obscure in a brief treatment.Comment: This is a draft white paper and we seek comments from the communit

The Chemical Composition of an Extrasolar Minor Planet

We report the relative abundances of 17 elements in the atmosphere of the white dwarf star GD 362, material that, very probably, was contained previously in a large asteroid or asteroids with composition similar to the Earth/Moon system. The asteroid may have once been part of a larger parent body not unlike one of the terrestrial planets of our solar system.Comment: ApJ, in pres

Hardness of approximation for quantum problems

The polynomial hierarchy plays a central role in classical complexity theory. Here, we define a quantum generalization of the polynomial hierarchy, and initiate its study. We show that not only are there natural complete problems for the second level of this quantum hierarchy, but that these problems are in fact hard to approximate. Using these techniques, we also obtain hardness of approximation for the class QCMA. Our approach is based on the use of dispersers, and is inspired by the classical results of Umans regarding hardness of approximation for the second level of the classical polynomial hierarchy [Umans, FOCS 1999]. The problems for which we prove hardness of approximation for include, among others, a quantum version of the Succinct Set Cover problem, and a variant of the local Hamiltonian problem with hybrid classical-quantum ground states.Comment: 21 pages, 1 figure, extended abstract appeared in Proceedings of the 39th International Colloquium on Automata, Languages and Programming (ICALP), pages 387-398, Springer, 201

The International Affiliation Network of YouTube Trends

Online video, a ubiquitous, visual, and highly shareable medium, is well-suited to crossing geographic, cultural, and linguistic barriers. Trending videos in particular, by virtue of reaching a large number of viewers in a short span of time, are powerful as both influencers and indicators of international communication flows. In this work, we study a large set of videos trending across 57 nations, collected from YouTube over a 7-month period. We consider the set as a network of content flowing between nations, then develop conditional co-affiliation, a nation-nation co-affiliation index that enables a meaningful interpretation of network path length and the application of betweenness centrality. We observe a highly-interlinked network with remarkably similar co-affiliation levels between very different nations. However, Arabic-speaking nations appear more isolated, with the U.A.E. emerging as a key bridge. By analyzing video trend lifespans, we show that nations having many globally-popular video trends are reliably not the nation where those trends are strongest: we see no evidence to support the widely discussed idea of cultural exporter or trendsetter nations. We model correlations between co-affiliation and a selection of contextual factors. We note a surprisingly complex interaction between migration and shared video trends. Consistent with existing work on video popularity, we find that long trending times within one nation do not necessarily translate to reaching a wide global audience. This work expands on previous studies of the geographic popularity of videos by incorporating trending data and extending our analysis from video-nation affiliations to nation-nation co-affiliations. Characterizing these relationships is key to understanding the international cultural impact and potential of online video

The epigenetic regulator ATF7ip inhibits Il2 expression, regulating Th17 responses.

T helper 17 cells (Th17) are critical for fighting infections at mucosal surfaces; however, they have also been found to contribute to the pathogenesis of multiple autoimmune diseases and have been targeted therapeutically. Due to the role of Th17 cells in autoimmune pathogenesis, it is important to understand the factors that control Th17 development. Here we identify the activating transcription factor 7 interacting protein (ATF7ip) as a critical regulator of Th17 differentiation. Mice with T cell-specific deletion of Atf7ip have impaired Th17 differentiation secondary to the aberrant overproduction of IL-2 with T cell receptor (TCR) stimulation and are resistant to colitis in vivo. ChIP-seq studies identified ATF7ip as an inhibitor of Il2 gene expression through the deposition of the repressive histone mark H3K9me3 in the Il2-Il21 intergenic region. These results demonstrate a new epigenetic pathway by which IL-2 production is constrained, and this may open up new avenues for modulating its production

Giant Planet Companion to 2MASSW J1207334-393254

We report new VLT/NACO imaging observations of the young, nearby brown dwarf 2MASSW J1207334-393254 and its suggested planetary mass companion (2M1207 b). Three epochs of VLT/NACO measurements obtained over nearly one year show that the planetary mass companion candidate shares the same proper motion and, with a high confidence level, is not a stationary background object. This result confirms the status of 2M1207 b as of planetary mass (5 times the mass of Jupiter) and the first image of a planetary mass companion in a different system than our own. This discovery offers new perspectives for our understanding of chemical and physical properties of planetary mass objects as well as their mechanisms of formation.Comment: 4 pages, 2 figures. accepted in Astronomy & Astrophysic

Lithium Depletion Boundary in a Pre-Main Sequence Binary System

A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a \~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature is detected from the primary (~M4). The physical companionship of the two stars is confirmed from common proper motions. Current theoretical pre-main sequence evolutionary models cannot simultaneously match the observed colors, brightnesses, and Li depletion patterns of this binary system. At the age upper limit of 20 Myr, contemporary theoretical evolutionary models predict too slow Li depletion. If true Li depletion is a faster process than predicted by theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391) estimated from the Li depletion boundary method are all overestimated. Because of the importance of the open cluster age scale, development of self-consistent theoretical models to match the HIP 112312 data is desirable.Comment: Accepted in ApJL. 5 pages total (3 tables, 3 figures

Echoes of a decaying planetary system: the gaseous and dusty disks surrounding three white dwarfs

We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission towards Ton 345 and allows us to refine models for the circumstellar dust around all three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a "Z II" region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.Comment: 43 pages, 9 tables, 9 figures. Accepted to Ap

Mapping the Shores of the Brown Dwarf Desert III: Young Moving Groups

We present the results of an aperture masking interferometry survey for substellar companions around 67 members of the young (~8-200Myr) nearby (~5-86pc) AB Doradus, Beta Pictoris, Hercules-Lyra, TW Hya, and Tucana-Horologium stellar associations. Observations were made at near infrared wavelengths between 1.2-3.8 microns using the adaptive optics facilities of the Keck II, VLT UT4, and Palomar Hale Telescopes. Typical contrast ratios of ~100-200 were achieved at angular separations between ~40-320mas, with our survey being 100% complete for companions with masses below 0.25\msolar across this range. We report the discovery of a $0.52 \pm 0.09$\msolar companion to HIP14807, as well as the detections and orbits of previously known stellar companions to HD16760, HD113449, and HD160934. We show that the companion to HD16760 is in a face-on orbit, resulting in an upward revision of its mass from $M_2 \sin i \sim 14$\mjupiter to $M_2 = 0.28 \pm 0.04$\msolar. No substellar companions were detected around any of our sample members, despite our ability to detect companions with masses below 80\mjupiter for 50 of our targets: of these, our sensitivity extended down to 40\mjupiter around 30 targets, with a subset of 22 subject to the still more stringent limit of 20\mjupiter. A statistical analysis of our non-detection of substellar companions allows us to place constraints on their frequency around ~0.2-1.5\msolar stars. In particular, considering companion mass distributions that have been proposed in the literature, we obtain an upper limit estimate of ~9-11% for the frequency of 20-80\mjupiter companions between 3-30AU at 95% confidence, assuming that their semimajor axes are distributed according to $d\mathcal{N}/da \propto a^{-1}$ in this range.Comment: Accepted by Ap