Modeling the HD32297 Debris Disk with Far-IR Herschel Data

HD32297 is a young A-star (~30 Myr) 112 pc away with a bright edge-on debris disk that has been resolved in scattered light. We observed the HD32297 debris disk in the far-infrared and sub-millimeter with the Herschel Space Observatory PACS and SPIRE instruments, populating the spectral energy distribution (SED) from 63 to 500{\mu}m. We aimed to determine the composition of dust grains in the HD32297 disk through SED modeling, using geometrical constraints from the resolved imaging to break degeneracies inherent in SED modeling. We found the best fitting SED model has 2 components: an outer ring centered around 110 AU, seen in the scattered light images, and an inner disk near the habitable zone of the star. The outer disk appears to be composed of grains > 2{\mu}m consisting of silicates, carbonaceous material, and water ice with an abundance ratio of 1:2:3 respectively and 90% porosity. These grains appear consistent with cometary grains, implying the underlying planetesimal population is dominated by comet-like bodies. We also discuss the 3.7{\sigma} detection of [C II] emission at 158{\mu}m with the Herschel PACS Spectrometer, making HD32297 one of only a handful of debris disks with circumstellar gas detected.Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical Journa

High-efficiency Dc to Dc Converter-regulators

High efficiency dc to dc converter regulators for spacecraft power suppl

Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk

We present Hubble Space Telescope STIS far-UV spectra of the edge-on disk around 49 Ceti, one of the very few debris disks showing sub-mm CO emission. Many atomic absorption lines are present in the spectra, most of which arise from circumstellar gas lying along the line-of-sight to the central star. We determined the line-of-sight CI column density, estimated the total carbon column density, and set limits on the OI column density. Surprisingly, no line-of-sight CO absorption was seen. We discuss possible explanations for this non-detection, and present preliminary estimates of the carbon abundances in the line-of-sight gas. The C/Fe ratio is much greater than the solar value, suggesting that 49 Cet harbors a volatile-rich gas disk similar to that of Beta Pictoris.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figure

Effet des facteurs température et nutrition azotée sur la croissance des plantes fourragères tropicales. I. Variation saisonnière de la croissance d'une graminée tropicale, Brachiaria mutica, au Sénégal

La croissance en climat sahélien subcanarien d'une graminée tropicale, Brachiaria mutica, a été étudiée pendant la saison sèche froide (novembre-m a rs), la saison sèche chaude (mars-mi-juillet) et la saison des pluies (mi-juillet-octobre). L'essentiel des résultats montre que cette graminée a une faible productivité en saison sèche froide, du fait de températures minimales très basses, contrairement à celles obtenues en saisons des pluies. Par ailleurs, il semble qu'il y ait une déplacement du facteur limitant (températures minimales basses) vers un effet azoté qui témoigne de la faible disponibilité en azote du sol pendant la saison froide et de l'importance d'une fertilisation azotée. Cette étude souligne l'intérêt d'introduire, lors de la saison sèche froide, des plantes fourragères des régions tempérées, moins exigeantes vis-à-vis de la température. (Résumé d'auteur

The Large Ultraviolet/Optical/Infrared Surveyor

Astronomy crossed a threshold three decades ago with the discovery of planets around other stars. Compared to scientists' previous expectations set by the Solar System, exoplanets are wonderfully abundant and varied. Indirect planet discovery techniques have shown that small rocky planets residing in stellar habitable zones, where such planets may have liquid water on their surfaces, are not rare. This revelation drives us to ask more ambitious and fundamental questions, that fascinate scientists and the public alike: are there other truly Earth-like planets out there and do any of them harbour life? Today, exoplanets are largely small black shadows' to us, with measurements of orbits, sizes and masses (all three in the best cases).The upcoming James Webb Space Telescope and future 30-m-class ground-based telescopes will characterize the atmospheres of habitable planet candidates orbit in glow-mass M dwarf stars. However, deeply probing atmospheres of the exoplanets most similar to the Earth, those around Sun-like stars, remains out of reach for currently planned observatories. Bringing them within our grasp is a primary motivation for the Large UV/Optical/Infrared Surveyor(LUVOIR) mission concept, currently the focus of a three-year NASA study

L'intérêt du Panicum maximum pour l'intensification fourragère au Sénégal. I. L'association maraîchage-élevage

L'intensification fourragère au Sénégal est envisagée, en association avec le maraîchage, où la graminée tropicale Panicum maximum permet une éradication des nématodes, un accroissement de la matière organique dans le sol et la constitution de réserves fourragères pour les éleveurs urbains de moutons. L'exploitation intensive de Panicum maximum est étudiée en fonction des variations climatiques saisonnières, pour définir une cinétique de croissance. L'incidence économique de la fertilisation est abordée sous l'aspect des exportations, du coefficient apparent d'utilisation et de la dilution de l'azote dans la matière sèche. (Résumé d'auteur

Chern-Simons number diffusion in (1+1)-dimensional Higgs theory

We study the Chern-Simons number diffusion rate in the (1+1)-dimensional latticeAbelian Higgs model at temperatures much higher than, as well as comparable to, the sphaleron energy. It is found that in the high-temperature limit the rate is likely to grow as power 2/3 of the temperature. In the intermediate-temperature regime, our numerical simulations show that very weak temperature dependence of the rate, found in previous work, persists at smaller lattice spacings. We discuss possibilities of relating the observed behavior of the rate to static finite-temperature properties of the model.Comment: 9 pages, LATeX + 4 figures included as postscript files, to be encapsulated using epsf. Text + figures uuencoded. Also available as a compressed postscript file by anonymous ftp from maggia.ethz.ch (login ftp, pw ftp; then: cd pub, binary, get ahm.ps.Z). IPS Research Report No. 94-0

High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk

High resolution FUV echelle spectra showing absorption features arising from CI and CO gas in the Beta Pictoris circumstellar (CS) disk were obtained on 1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS). An unsaturated spin-forbidden line of CI at 1613.376 A not previously seen in spectra of Beta Pictoris was detected, allowing for an improved determination of the column density of CI at zero velocity relative to the star (the stable component), N = (2-4) x 10^{16} cm^{-2}. Variable components with multiple velocities, which are the signatures of infalling bodies in the Beta Pictoris CS disk, are observed in the CI 1561 A and 1657 A multiplets. Also seen for the first time were two lines arising from the metastable singlet D level of carbon, at 1931 A and 1463 A The results of analysis of the CO A-X (0-0), (1-0), and (2-0) bands are presented, including the bands arising from {13}^CO, with much better precision than has previously been possible, due to the very high resolution provided by the STIS echelle gratings. Only stable CO gas is observed, with a column density N(CO) = (6.3 +/- 0.3) x 10^{14} cm{-2}. An unusual ratio of the column densities of {12}^CO to {13}^CO is found (R = 15 +/- 2). The large difference between the column densities of CI and CO indicates that photodissociation of CO is not the primary source of CI gas in the disk, contrary to previous suggestion.Comment: 13 pages, including 6 figures. LaTex2e (emulateapj5.sty). Accepted for publication in Ap

The Transiting Exocomets in the HD 172555 System

The Earth is thought to have formed dry, in a part of the Solar Nebula deficient in organic material, and to have acquired its organics and water through bombardment by minor bodies. Observations of this process in well-dated systems can provide insight into the probable origin and composition of the bombarding parent bodies. Transiting cometary activity has previously been reported in Ca II for the late-A member of the 241 Myr old Pictoris Moving Group member, HD 172555(Kiefer et al. 2014). We present HST STIS and COS spectra of HD 172555 demonstrating that the star has chromospheric emission and variable in falling gas features in transitions of silicon and carbon ions at times when no Fe II absorption is seen in the UV data, and no Ca II absorption is seen in contemporary optical spectra. The lack of CO absorption and stable gas absorption at the system velocity is consistent with the absence of a cold Kuiper belt analog (Riviere-Marichalar et al. 2012) in this system. The presence of infall in some species at one epoch and others at different epochs suggests that, like Pictoris, there may be more than one family of exocomets. If perturbed into star-grazing orbits by the same mechanism as for Pic, these data suggest that the wide planet frequency among A-early F stars in the PMG is at least 37.5, well above the frequency estimated for young moving groups independent of host star spectral type

Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti

We present ~0.4 resolution images of CO(3-2) and associated continuum emission from the gas-bearing debris disk around the nearby A star 49 Ceti, observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We analyze the ALMA visibilities in tandem with the broad-band spectral energy distribution to measure the radial surface density profiles of dust and gas emission from the system. The dust surface density decreases with radius between ~100 and 310 au, with a marginally significant enhancement of surface density at a radius of ~110 au. The SED requires an inner disk of small grains in addition to the outer disk of larger grains resolved by ALMA. The gas disk exhibits a surface density profile that increases with radius, contrary to most previous spatially resolved observations of circumstellar gas disks. While ~80% of the CO flux is well described by an axisymmetric power-law disk in Keplerian rotation about the central star, residuals at ~20% of the peak flux exhibit a departure from axisymmetry suggestive of spiral arms or a warp in the gas disk. The radial extent of the gas disk (~220 au) is smaller than that of the dust disk (~300 au), consistent with recent observations of other gas-bearing debris disks. While there are so far only three broad debris disks with well characterized radial dust profiles at millimeter wavelengths, 49 Ceti's disk shows a markedly different structure from two radially resolved gas-poor debris disks, implying that the physical processes generating and sculpting the gas and dust are fundamentally different.Comment: 20 pages, 8 figures, accepted for publication in ApJ March 31, 2017 (submitted Nov 2016