The Primeval Populations of the Ultra-Faint Dwarf Galaxies

We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter dominated, and least chemically-evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient globular cluster M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within ~1 Gyr of each other, as might be expected if their star formation was truncated by a global event, such as reionization.Comment: Accepted for publication in The Astrophysical Journal Letters. Latex, 5 pages, 2 color figures, 1 tabl

Late winter temperature response to large tropical volcanic eruptions in temperate western North America: Relationship to ENSO phases

February–March temperature reconstructions in western North America from 1500–1980 in the Common Era (CE) are used to evaluate, from a regional perspective, the hypothesis that radiative forcing by large tropical volcanic eruptions induces a tendency in the climate system towards an early post-event El Niño (EN) response followed by a delayed La Niña (LN) response. Post-event spatial composites using superposed epoch analysis (SEA) detect indications for an EN-like pattern in post-event Years 1–2; this result, however, is sensitive to the set of eruptions evaluated. Highly significant LN-like patterns are also observed for two eruptions during Year 1. In contrast, a clear and unique LN-like response is found in both evaluated eruption sets during Years 3–5; Year 3 in particular represents the time of strongest post-event response. No significant EN-like patterns occur during these years. The relative homogeneity of the SEA response for each post-event year is evaluated in terms of the ratio of the amplitude of the SEA composite to its standard deviation across the eruption events. In relation to the same metric determined from random-event-year SEAs, these signal-to-noise ratios are most highly significant in the portions of the domain with the strongest anomalies in Years 1–5, especially Year 3. The signal-to-noise ratios tend towards uniformly low and insignificant values beyond the first half-decade after the eruption, indicating generally reduced coherence across events. In relation to the larger-scale circulation, post-eruption 500mb February–March geopotential height composites from the 20th Century Reanalysis show ENSO-type features that are largely consistent with the SEA results from the primary eruption set during Year 1, but are inconsistent with the EN-like pattern exhibited by the second eruption set during Years 1–2. In Year 3, the pressure composite over North America and the adjacent Pacific and Atlantic is strongly LN-like, consistent with all SEA results; similarly, weakening coherence across events as time progresses beyond Year 3 is also consistent with more variable pressure composites noted after that time. The relatively robust character of the delayed LN-like response is evaluated in terms of the dynamic rebound of the climate system towards its initial energy balance as the radiative impact of immediate post-eruption aerosol cooling dissipates. The LN-like SEA temperature response in Years 3–5 exhibits a slight shift of its southern warm anomaly to the north and west relative to pure composite LN conditions, which is detected as a specifically post-eruption feature in the region

An Alternative Pathway for Okazaki Fragment Processing - Resolution of Fold-Back Flaps by Pif1 Helicase

Two pathways have been proposed for eukaryotic Okazaki fragment RNA primer removal. Results presented here provide evidence for an alternative pathway. Primer extension by DNA polymerase δ (pol δ) displaces the downstream fragment into an RNA-initiated flap. Most flaps are cleaved by flap endonuclease 1 (FEN1) while short, and the remaining nicks joined in the first pathway. A small fraction escapes immediate FEN1 cleavage and is further lengthened by Pif1 helicase. Long flaps are bound by replication protein A (RPA), which inhibits FEN1. In the second pathway, Dna2 nuclease cleaves an RPA-bound flap and displaces RPA, leaving a short flap for FEN1. Pif1 flap lengthening creates a requirement for Dna2. This relationship should not have evolved unless Pif1 had an important role in fragment processing. In this study, biochemical reconstitution experiments were used to gain insight into this role. Pif1 did not promote synthesis through GC-rich sequences, which impede strand displacement. Pif1 was also unable to open fold-back flaps that are immune to cleavage by either FEN1 or Dna2 and cannot be bound by RPA. However, Pif1 working with pol δ readily unwound a full-length Okazaki fragment initiated by a fold-back flap. Additionally, a fold-back in the template slowed pol δ synthesis, so that the fragment could be removed before ligation to the lagging strand. These results suggest an alternative pathway in which Pif1 removes Okazaki fragments initiated by fold-back flaps in vivo

Non-detection of previously reported transits of HD 97658b with MOST photometry

The radial velocity-discovered exoplanet HD 97658b was recently announced to transit, with a derived planetary radius of 2.93 \pm 0.28 R_{Earth}. As a transiting super-Earth orbiting a bright star, this planet would make an attractive candidate for additional observations, including studies of its atmospheric properties. We present and analyze follow-up photometric observations of the HD 97658 system acquired with the MOST space telescope. Our results show no transit with the depth and ephemeris reported in the announcement paper. For the same ephemeris, we rule out transits for a planet with radius larger than 2.09 R_{Earth}, corresponding to the reported 3\sigma lower limit. We also report new radial velocity measurements which continue to support the existence of an exoplanet with a period of 9.5 days, and obtain improved orbital parameters.Comment: 16 pages, 4 figures; 1 Table; accepted for publication in ApJL, includes changes made in response to the referee repor

Five Planets Orbiting 55 Cancri

We report 18 years of Doppler shift measurements of a nearby star, 55 Cancri, that exhibit strong evidence for five orbiting planets. The four previously reported planets are strongly confirmed here. A fifth planet is presented, with an apparent orbital period of 260 days, placing it 0.78 AU from the star in the large empty zone between two other planets. The velocity wobble amplitude of 4.9 \ms implies a minimum planet mass \msini = 45.7 \mearthe. The orbital eccentricity is consistent with a circular orbit, but modest eccentricity solutions give similar \chisq fits. All five planets reside in low eccentricity orbits, four having eccentricities under 0.1. The outermost planet orbits 5.8 AU from the star and has a minimum mass, \msini = 3.8 \mjupe, making it more massive than the inner four planets combined. Its orbital distance is the largest for an exoplanet with a well defined orbit. The innermost planet has a semi-major axis of only 0.038 AU and has a minimum mass, \msinie, of only 10.8 \mearthe, one of the lowest mass exoplanets known. The five known planets within 6 AU define a {\em minimum mass protoplanetary nebula} to compare with the classical minimum mass solar nebula. Numerical N-body simulations show this system of five planets to be dynamically stable and show that the planets with periods of 14.65 and 44.3 d are not in a mean-motion resonance. Millimagnitude photometry during 11 years reveals no brightness variations at any of the radial velocity periods, providing support for their interpretation as planetary.Comment: accepted to Ap

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS

Implementation of semi-discrete, non-staggered central schemes in a colocated, polyhedral, finite volume framework, for high-speed viscous flow