HAL-INSU
Not a member yet
203706 research outputs found
Sort by
Mackinawite partial oxidation to green rust produces a large, abiotic uranium isotope fractionation
No full textInternational audienceLow-oxygen settings and transition zones between strictly anoxic and oxic conditions may have characterised large expanses of Precambrian continental margins, where oxygen-breathing, complex life emerged and diversified. Accurate reconstructions of oxygen levels in such conditions are therefore required, but current geochemical proxies fail in identifying transitional redox conditions. Uranium isotopes are an emerging palaeoredox proxy, as large isotope fractionations are recorded during the reduction of uranium into anoxic sediments. Their potential application to transitional conditions is, however, unclear, because the redox regulating mineralogy of such environments and the associated isotope fractionations are poorly constrained. Here, we explore the mineralogical transformations occurring during the partial oxidation of mackinawite. We show that green rust, a key mineral of Precambrian oceans, forms as a by-product of mackinawite oxidation, along with uraninite and polysulphide. We also demonstrate that this mechanism records a large abiotic uranium isotope fractionation during its reaction with aqueous iron sulphide
Mining the time axis with TRON. II. MeerKAT detects a stellar radio flare from a distant RS CVn candidate
No full textInternational audienceMedium-timescale (minutes to hours) radio transients are a relatively unexplored population. The wide field-of-view and high instantaneous sensitivity of instruments such as MeerKAT provides an opportunity to probe this class of sources, using image-plane detection techniques. The previous letter in this series describes our project and associated TRON pipeline designed to mine archival MeerKAT data for transient and variable sources. In this letter, we report on a new transient, a radio flare, associated with Gaia DR3 6865945581361480448, a G type star, whose parallax places it at a distance of 1334 pc. Its duration and high degree of circular polarization suggests electron cyclotron maser instability as the mechanism, consistent with an RS CVn variable
Multi-Scale Photogrammetric 3D Reconstruction of Apollo 17 Station 7
No full textInternational audienceThe Apollo 17 manned mission landed in the Taurus Littrow valley on the Moon in December 1972. During three days spent at the lunar surface, a total of 111 kg of well-documented rock and soil samples were collected from geologic waypoints and brought back to Earth [1, 2]. In a previous study, we used scanned photographs taken in situ with astronaut’s Hasselblad film cameras to reconstruct 3D models of boulders using Structurefrom-Motion photogrammetry [3, 4]. In addition to the 3D reconstruction of the boulders, we have shown in [5] that the lunar rock samples retrieved from these boulders could also be reconstructed in 3D using photogrammetry applied on archived photographs, in addition to the 360° panoramas. This allowed to setup a comprehensive 3D model of Staton 6, that can be explored in Virtual Reality (VR) [5]. The present work is focused on reconstructing the next waypoint, the “Station 7” geologic sampling site, also lying at the base of the North Massif
Protolith age, and timing of burial and exhumation of the UHP Chasteiran Unit (Dora-Maira Massif, Western Alps), constrained by zircon, garnet and rutile petrochronology
No full textInternational audienceThe Chasteiran Unit in the northern Dora-Maira Massif reached ultra-high-pressure conditions in the chloritoid-coesite stability field. The chemical and isotopic behaviours of zircon, garnet, and rutile from a metapelite were explored to reconstruct the metamorphic evolution of this Unit.Zircon crystals display detrital cores and thin (< 5 μm) metamorphic rims which cannot be dated. The dominant zircon population consists of Late Neoproterozoic (⁓600 Ma) magmatic grains whereas the youngest zircon cluster is Ordovician in age (⁓470 Ma).Garnet records three main growth stages: initial growth during a prograde pressure and temperature increase in the quartz stability field (2.5–2.7 GPa at 470–500 °C, inner core – stage 1), peak growth in the coesite stability field (2.7–2.8 GPa at 510–530 °C, outer core – stage 2), and final growth of the garnet rim between 2.3 GPa 520 °C and 1.5 GPa 510 °C (stage 3), contemporaneously with lawsonite consumption coupled with fluid production.LA-ICP-MS U-Pb dating of garnet indicates two distinct stages of growth for garnet cores and rims at ⁓61 Ma and ⁓43 Ma, respectively. The time interval separating the growth of garnet cores and rims is consistent with our thermodynamic modelling, which indicates the absence of garnet growth during the initial stage of exhumation, between 2.7. GPa and 2.3 GPa.Rutile is found both as inclusions in garnet and in the matrix. Rare inclusions of jadeite and Si-rich muscovite also indicate stability of rutile during burial at a minimum pressure of 2.0 GPa. Inclusions of rutile in garnet are commonly surrounded by fractures and some crystals display ilmenite exsolution lamellae, suggesting that despite their mode of occurrence, they might have behaved as an open system during retrograde metamorphism. Rutile consumption took place during exhumation, as suggested by the increase in Ti content in garnet and muscovite rims and thermodynamic modelling. Rutile in the matrix is partially replaced by ilmenite corona, which developed at P < 1.5 GPa, after garnet growth. SIMS U-Pb dating of rutile, irrespective of its petrographic mode of occurrence, yields a date of ⁓37 Ma.Our geochronological data suggest a different timing of burial and exhumation for the northern and southern Dora-Maira Massif and imply that the peak ultra-high-pressure episode in the northern Dora-Maira Massif is older than the peak eclogite-facies in the overlying oceanic units. Therefore, different extensional allochthons derived from the Briançonnais hyperextended palaeomargin may have been buried and/or detached from the downgoing slab at different times
High-Resolution Spectroscopy and Optical Photometry of MWC 349A and MWC 349B
No full textInternational audienceMWC 349A is a V ∼ 13 mag object with the B[e] phenomenon, a very strong optical emission-line spectrum, maser and laser line emission, and a radio-bright bipolar nebula, attenuated by ∼10 mag of interstellar extinction. MWC 349B is a visual companion of MWC 349A with no previously reported signs of emission. The physical connection of the pair has been the subject of debates toward revealing the evolutionary state of MWC 349A. Only low-resolution spectra reported for MWC 349B resulted in estimating its parameters with a large uncertainty. We report the results of high-resolution optical spectroscopy of both objects as well as their optical photometry
The Transition from Diffuse Atomic Gas to Molecular Cloud in Taurus
No full textInternational audienceWe study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions in the direction to stars behind the Taurus Molecular Cloud. New high-resolution spectra at visible and near infrared wavelengths of interstellar K I, CH, CH+, C2, CN, and CO toward HD 28975 and HD 29647 are combined with published results for HD 27778 and HD 30122. Gas densities and temperatures are inferred from analyses of C2, CN, and CO excitation. Our results for HD 29647 are noteworthy in that the CO column density is 1018 cm-2, our analysis of CO and C2 excitation reveal a temperature of 10 K and densities of about 1000 cm-3, and the CO excitation and radiation temperatures are the same, more like emission-line studies of dark molecular clouds. Similar results arise from our chemical analysis leading to CN through reactions involving the observed species CH and C2. The other directions are typical of molecule-rich diffuse clouds and can be considered CO-dark gas
The First Detection of a Strong Magnetic Field in a FS CMa Star
No full textInternational audienceWe report the first detection of a magnetic field in a star of the FS CMa type, a subgroup of objects showing the B[e] phenomenon. These stars are surrounded by an extended circumstellar medium, which is responsible for emission lines from both permitted and forbidden transitions, a large infrared excess due to dust, and a very complicated variability. The split of magnetically sensitive lines in the spectra of IRAS 17449+2320 unexpectedly revealed a strong magnetic field with strength of 6.2+/-0.2 kG. Our discovery indicates the importance of considering magnetic fields in studies of FS CMa stars, whose evolutionary status is still unknown.We found that IRAS 17449+2320 has an effective temperature of ~10,000 K. Several spectral lines affected by the circumstellar medium (e.g., O I 7772-7775 AA, Balmer lines) regularly exhibit redshifted absorption components with a period of 36.13 days. The same periodicity shows up in the equivalent width of the Halpha line.The analysis of our data suggests three possible scenarios for the nature and evolutionary status of the object: i) a magnetic A-type star primary with a hot dwarf secondary, ii) a binary merger, or iii) a magnetic Ap star near the terminal age main sequence. The latter one may represent an object which has been sought for decades and fills the gap in the observations and theory of magnetic Ap stars
Decadal variations of seeing above Maunakea
No full textInternational audienceI will present a preliminary analysis of seeing measurements done with the MaunaKea Atmospheric Monitor (MKAM) to infer if a variation can be seen due to global warming. MKAM and its instrument, a MASS-DIMM, was provided to the Maunakea Observatories by TMT after they completed their site studies. The instrument has been functionnal since 2009, taking several measurements of seeing every night it was in operation. In total, close to one million measurements were taken by the instrument which is still in operation. I will present the trends seen when looking at all seeing values. In addition, particular attention will be given to the trends shown by a "good seeing" sub-sample, limiting the sample to values that are considered acceptable when evaluating image quality of an astronomical observation. I will also report on seasonal trends
Impacts and Statistical Mitigation of Missing Data on the 21cm Power Spectrum: A Case Study with the Hydrogen Epoch of Reionization Array
No full textInternational audienceThe precise characterization and mitigation of systematic effects is one of the biggest roadblocks impeding the detection of the fluctuations of cosmological 21cm signals. Missing data in radio cosmological experiments, often due to radio frequency interference (RFI), poses a particular challenge to power spectrum analysis as it could lead to the ringing of bright foreground modes in Fourier space, heavily contaminating the cosmological signals. Here we show that the problem of missing data becomes even more arduous in the presence of systematic effects. Using a realistic numerical simulation, we demonstrate that partially flagged data combined with systematic effects can introduce significant foreground ringing. We show that such an effect can be mitigated through inpainting the missing data. We present a rigorous statistical framework that incorporates the process of inpainting missing data into a quadratic estimator of the 21cm power spectrum. Under this framework, the uncertainties associated with our inpainting method and its impact on power spectrum statistics can be understood. These results are applied to the latest Phase II observations taken by the Hydrogen Epoch of Reionization Array, forming a crucial component in power spectrum analyses as we move toward detecting 21cm signals in the ever more noisy RFI environment
Chemical Evolution of R-process Elements in Stars (CERES). IV. An observational run-up of the third r-process peak with Hf, Os, Ir, and Pt
No full textInternational audienceThe third r-process peak (Os, Ir, Pt) is poorly understood due to observational challenges, with spectral lines located in the blue or near-ultraviolet region of stellar spectra. These challenges need to be overcome for a better understanding of the r-process in a broader context. To understand how the abundances of the third r-process peak are synthesised and evolve in the Universe, it is necessary to carry out a homogeneous chemical analysis of metal-poor stars using high-quality data observed in the blue region of the electromagnetic spectrum (< 400 nm). We provide a homogeneous set of abundances for the third r-process peak (Os, Ir, Pt) and Hf, increasing their availability in the literature by up to one order of magnitude. We performed a classical 1D, local thermodynamic equilibrium (LTE) analysis of four elements (Hf, Os, Ir, Pt) using ATLAS model atmospheres to fit synthetic spectra on high signal-to-noise-ratio spectra of 52 red giants observed with UVES/VLT in high resolution (> 40,000). Due to the heavy line blending involved, we carefully determined upper limits and uncertainties. The observational results are compared with state-of-the-art nucleosynthesis models. Our sample displays larger abundances of Ir (Z=77) in comparison to Os (Z=76), both of which have been measured in a few stars in the past. The results also suggest decoupling between the abundances of third r-process peak elements with respect to Eu (a rare earth element) in Eu-poor stars. This seems to contradict a co-production scenario of Eu and the third r-process peak elements Os, Ir, and Pt in the progenitors of these objects. Our results are challenging to explain from a nucleosynthetic point of view: the observationally derived abundances indicate the need for an additional early, primary formation channel (or a non-robust r-process)