Publication archive

Publication archive

Saturn's moon Enceladus harbours a global water ocean, which lies under an ice crust and above a rocky core. Through warm cracks in the crust a cryo-volcanic plume ejects ice grains and vapour into space that contain materials originating from the ocean. Hydrothermal activity is suspected to occur deep inside the porous core, powered by tidal dissipation. So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material. Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus' organic inventory in enhanced concentrations.
Published: 29 June 2018

ESA's Concurrent Design Facility (CDF) has completed an assessment study of Small Planetary Platforms (SPP): small mission concepts that include a mothercraft and a swarm of small satellites that can be deployed. ESA's Science Directorate requested and managed the study as one of three topics selected for investigation following the "New Science Ideas" call for proposals.

The concept studied was a proposal to perform multi-point (and possibly multi-target) measurements around small bodies (asteroids and comets), as well as Mars or Venus, to gather information from different locations simultaneously.

The main goal of the study was not to design a specific mission but to provide a "tool-box" of technical building blocks the community can use to develop these new planetary mission architectures, in reply to future science calls.

The full CDF study report consists of three main reports, each available in pdf format below:

Reference Document
CDF-178(A) SPP assessment for Near Earth Object (NEO) Inactive Bodies
CDF-178(B) SPP assessment for Main Asteroid Belt (MAB) Active Bodies
CDF-178(C) SPP Executive Summary, compiling the main aspects of the two other reports, the system-level and main sub-system level trade-offs and covering the top level synthesis

 

Published: 12 June 2018
Observations in kinetic scale field line resonances, or eigenmodes of the geomagnetic field, reveal highly field-aligned plateaued electron distributions. By combining observations from the Van Allen Probes and Cluster spacecraft with a hybrid kinetic gyrofluid simulation we show how these distributions arise from the nonlocal self-consistent interaction of electrons with the wavefield. This interaction is manifested as electron trapping in the standing wave potential. The process operates along most of the field line and qualitatively accounts for electron observations near the equatorial plane and at higher latitudes. In conjunction with the highly field-aligned plateaus, loss cone features are also evident, which result from the action of the upward-directed wave parallel electric field on the untrapped electron populations.
Published: 12 June 2018
Magnetic reconnection is a key process that explosively accelerates charged particles, generating phenomena such as nebular flares, solar flares and stunning aurorae. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisc, where thin-current-sheet conditions are conducive to reconnection. The dayside magnetodisc is usually considered thicker than the nightside due to the compression of solar wind, and is therefore not an ideal environment for reconnection. In contrast, a recent statistical study of magnetic flux circulation strongly suggests that magnetic reconnection must occur throughout Saturn's dayside magnetosphere. Additionally, the source of energetic plasma can be present in the noon sector of giant planetary magnetospheres. However, so far, dayside magnetic reconnection has only been identified at the magnetopause. Here, we report direct evidence of near-noon reconnection within Saturn's magnetodisc using measurements from the Cassini spacecraft. The measured energetic electrons and ions (ranging from tens to hundreds of keV) and the estimated energy flux of ~2.6 mWm-2 within the reconnection region are sufficient to power aurorae. We suggest that dayside magnetodisc reconnection can explain bursty phenomena in the dayside magnetospheres of giant planets, which can potentially advance our understanding of quasi-periodic injections of relativistic electrons and auroral pulsations.
Published: 05 June 2018

This media kit contains background information about the Gaia mission. It has been prepared to accompany Gaia Data Release 2.

Contents:
Gaia – the billion star surveyor
Fast facts
Mapping the Galaxy with Gaia
Gaia's second data release – the Galactic census takes shape
Caveats and future releases
Science with Gaia's new data
Science highlights from Gaia's first data release
Making sense of it all – the role of the Gaia Data Processing and Analysis Consortium
Where is Gaia and why do we need to know?
From ancient star maps to precision astrometry
Appendices: Resources, Information about the press event, Media contacts

File updated 24 April 2018, to correct typo.

To download the pdf file click on the image or on the link to publication below.

Published: 21 April 2018
Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to ~50. Here we report an image of an individual star at redshift z = 1.49 (dubbed MACS J1149 Lensed Star 1) magnified by more than ×2,000. A separate image, detected briefly 0.26" from Lensed Star 1, is probably a counterimage of the first star demagnified for multiple years by an object of ≳3 solar masses in the cluster. For reasonable assumptions about the lensing system, microlensing fluctuations in the stars' light curves can yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhaloes or massive compact objects may help to account for the two images' long-term brightness ratio.
Published: 03 April 2018
Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052–DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.
Published: 30 March 2018
Circumstantial evidence suggests that magnetism and enhanced X-ray emission are likely correlated in early B-type stars: similar fractions of them (~10%) are strong and hard X-ray sources and possess strong magnetic fields. It is also known that some B-type stars have spots on their surface. Yet up to now no X-ray activity associated with spots on early-type stars was detected. In this Letter we report the detection of a magnetic field on the B2V star ρ Oph A. Previously, we assessed that the X-ray activity of this star is associated with a surface spot, herewith we establish its magnetic origin. We analyze spectra of ρ Oph A obtained with the FORS2 spectrograph at ESO Very Large Telescope (VLT) at two epochs, and detect a longitudinal component of the magnetic field of the order of ~500 G in one of the datasets. The detection of the magnetic field only at one epoch can be explained by stellar rotation which is also invoked to explain observed periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we derived the fundamental stellar parameters of ρ Oph A and further constrained its age. We conclude that ρ Oph A provides strong evidence for the presence of active X-ray emitting regions on young magnetized early type stars.
Published: 13 February 2018
In the months since the publication of the first results, the noise performance of LISA Pathfinder has improved because of reduced Brownian noise due to the continued decrease in pressure around the test masses, from a better correction of noninertial effects, and from a better calibration of the electrostatic force actuation. In addition, the availability of numerous long noise measurement runs, during which no perturbation is purposely applied to the test masses, has allowed the measurement of noise with good statistics down to 20  μHz. The Letter presents the measured differential acceleration noise figure, which is at (1.74±0.01)  fm s-2/√Hz above 2 mHz and (6±1)×10  fm s-2/√Hz at 20  μHz, and discusses the physical sources for the measured noise. This performance provides an experimental benchmark demonstrating the ability to realize the low-frequency science potential of the LISA mission, recently selected by the European Space Agency.

DOI: https://doi.org/10.1103/PhysRevLett.120.061101
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published: 05 February 2018
We examine X-rays from radiatively cooled shocks, focusing on how their thin-shell instability reduces X-ray emission. For 2D simulations of collision between equal expanding winds, we carry out a parameter study of such instability as a function of the ratio of radiative versus adiabatic-expansion cooling lengths. In the adiabatic regime, the extended cooling layer suppresses instability, leading to planar shock compression with X-ray luminosity that follows closely the expected ( LX∼M_dot2 ) quadratic scaling with mass-loss rate M_dot. In the strongly radiative limit, the X-ray emission now follows an expected linear scaling with mass-loss (LX∼M_dot), but the instability deforms the shock compression into extended shear layers with oblique shocks along fingers of cooled, dense material. The spatial dispersion of shock thermalization limits strong X-ray emission to the tips and troughs of the fingers, and so reduces the X-ray emission (here by about a factor 1/50) below what is expected from analytic radiative-shock models without unstable structure. Between these two limits, X-ray emission can switch between a high-state associated with extended shock compression, and a low-state characterized by extensive shear. Further study is needed to clarify the origin of this ‘shear mixing reduction factor’ in X-ray emission, and its dependence on parameters like the shock Mach number.
Published: 25 January 2014
We report on the first detection of a global change in the X-ray emitting properties of a wind–wind collision, thanks to XMM-Newton observations of the massive Small Magellenic Cloud (SMC) system HD 5980. While its light curve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the light-curve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind–wind collision, considering the varying wind properties of the eruptive component in HD 5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions.
Published: 02 February 2018
Under a Creative Commons license

We present a 3D orbit viewer application capable of displaying science data. 3DView, a web tool designed by the French Plasma Physics Data Center (CDPP) for the planetology and heliophysics community, has extended functionalities to render space physics data (observations and models alike) in their original 3D context. Time series, vectors, dynamic spectra, celestial body maps, magnetic field or flow lines, 2D cuts in simulation cubes, etc, are among the variety of data representation enabled by 3DView. The direct connection to several large databases, the use of VO standards and the possibility to upload user data makes 3DView a versatile tool able to cover a wide range of space physics contexts. The code is open source and the software is regularly used at Masters Degree level or summer school for pedagogical purposes. The present paper describes the general architecture and all major functionalities, and offers several science cases (simulation rendering, mission preparation, etc.) which can be easily replayed by the interested readers. Future developments are finally outlined.

Published: 31 January 2018
We present the Latmos Hybrid Simulation (LatHyS) database, which is dedicated to the investigations of planetary plasma environment. Simulation results of several planetary objects (Mars, Mercury, Ganymede) are available in an online catalogue. The full description of the simulations and their results is compliant with a data model developped in the framework of the FP7 IMPEx project. The catalogue is interfaced with VO-visualization tools such AMDA, 3DView, TOPCAT, CLweb or the IMPEx portal. Web services ensure the possibilities of accessing and extracting simulated quantities/data. We illustrate the interoperability between the simulation database and VO-tools using a detailed science case that focuses on a three-dimensional representation of the solar wind interaction with the Martian upper atmosphere, combining MAVEN and Mars Express observations and simulation results.
Published: 31 January 2018
The first estimation of the energy cascade rate |εC| of magnetosheath turbulence is obtained using the Cluster and THEMIS spacecraft data and an exact law of compressible isothermal magnetohydrodynamics turbulence. The mean value of |εC| is found to be close to 10-13  J m-3 s-1, at least 2 orders of magnitude larger than its value in the solar wind (~10-16  J m-3 s-1 in the fast wind). Two types of turbulence are evidenced and shown to be dominated either by incompressible Alfvénic or compressible magnetosoniclike fluctuations. Density fluctuations are shown to amplify the cascade rate and its spatial anisotropy in comparison with incompressible Alfvénic turbulence. Furthermore, for compressible magnetosonic fluctuations, large cascade rates are found to lie mostly near the linear kinetic instability of the mirror mode. New empirical power-laws relating |εC| to the turbulent Mach number and to the internal energy are evidenced. These new findings have potential applications in distant astrophysical plasmas that are not accessible to in situ measurements.
Published: 29 January 2018
Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). Aims: In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity (PL), period-Wesenheit (PW) relations for classical and Type II Cepheids and infrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity (MV-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. [Remainder of abstract truncated due to character limitations]
Published: 13 December 2017
We use the Gaia data release 1 (DR1) to study the proper motion (PM) fields of the Large and Small Magellanic Clouds (LMC, SMC). This uses the Tycho-Gaia Astrometric Solution (TGAS) PMs for 29 Hipparcos stars in the LMC and 8 in the SMC. The LMC PM in the West and North directions is inferred to be ({μ }W,{μ }N) =(-1.872+/- 0.045,0.224+/- 0.054) {mas} {{yr}}-1 , and the SMC PM ({μ }W,{μ }N)=(-0.874+/- 0.066,-1.229 +/- 0.047) {mas} {{yr}}-1 . These results have similar accuracy and agree to within the uncertainties with existing Hubble Space Telescope (HST) PM measurements. Since TGAS uses different methods with different systematics, this provides an external validation of both data sets and their underlying approaches. Residual DR1 systematics may affect the TGAS results, but the HST agreement implies this must be below the random errors. Also in agreement with prior HST studies, the TGAS LMC PM field clearly shows the clockwise rotation of the disk, even though it takes the LMC disk in excess of 108 years to complete one revolution. The implied rotation curve amplitude for young LMC stars is consistent with that inferred from line of sight (LOS) velocity measurements. Comparison of the PM and LOS rotation curves implies a kinematic LMC distance modulus m-M=18.54+/- 0.39, consistent but not yet competitive with photometric methods. These first results from Gaia on the topic of Local Group dynamics provide an indication of how its future data releases will revolutionize this field.
Published: 13 December 2017
Context. The first Gaia data release (DR1) delivered a catalogue of astrometry and photometry for over a billion astronomical sources. Within the panoply of methods used for data exploration, visualisation is often the starting point and even the guiding reference for scientific thought. However, this is a volume of data that cannot be efficiently explored using traditional tools, techniques, and habits. Aims. We aim to provide a global visual exploration service for the Gaia archive, something that is not possible out of the box for most people. The service has two main goals. The first is to provide a software platform for interactive visual exploration of the archive contents, using common personal computers and mobile devices available to most users. The second aim is to produce intelligible and appealing visual representations of the enormous information content of the archive. Methods. The interactive exploration service follows a client-server design. The server runs close to the data, at the archive, and is responsible for hiding as far as possible the complexity and volume of the Gaia data from the client. This is achieved by serving visual detail on demand. Levels of detail are pre-computed using data aggregation and subsampling techniques. For DR1, the client is a web application that provides an interactive multi-panel visualisation workspace as well as a graphical user interface. [Remainder of abstract truncated due to character limitations]
Published: 13 December 2017

Print out and build a paper model of CHEOPS, the CHaracterising ExOPlanet Satellite.

CHEOPS is a space science mission dedicated to the study of known exoplanets orbiting bright, nearby stars. It will use the technique of ultra-high precision photometry to measure accurate sizes of a large sample of Earth to Neptune-sized planets. By combining the accurate sizes determined by CHEOPS with existing mass measurements, it will be possible to establish the bulk density and composition of the planets; these, together with information on the host stars and the planets' orbits will be used to determine the planets' formation and evolutionary history.

CHEOPS is a small satellite with a total launch mass of approximately 300 kg and dimensions of 1.55m (height) × 1.49m (width, measured from solar array edge to edge) × 1.4m (depth).

The dark colours used in this paper model are representative of the true colours of the various spacecraft components. The paper model's scale is 1:15 when printed on DIN A4 paper.

CHEOPS is a partnership between the European Space Agency (ESA) and Switzerland.

Published: 19 March 2018
The three-dimensional motions of stars in small galaxies beyond our own are minute, yet they are crucial for understanding the nature of gravity and dark matter. Even for the dwarf galaxy Sculptor–one of the best-studied systems, which is inferred to be strongly dark matter dominated–there are conflicting reports on its mean motion around the Milky Way, and the three-dimensional internal motions of its stars have never been measured. Here, we present precise proper motions of Sculptor's stars based on data from the Gaia mission and Hubble Space Telescope. Our measurements show that Sculptor moves around the Milky Way on a high-inclination elongated orbit that takes it much further out than previously thought. For Sculptor's internal velocity dispersions, we find σR = 11.5 ± 4.3 km s−1 and σT = 8.5 ± 3.2 km s-1 along the projected radial and tangential directions. Thus, the stars in our sample move preferentially on radial orbits as quantified by the anisotropy parameter, which we find to be β~0.86+0.12-0.83 at a location beyond the core radius. Taken at face value, this high radial anisotropy requires abandoning conventional models for Sculptor's mass distribution. Our sample is dominated by metal-rich stars and for these we find βMR~0.95+0.04-0.27–a value consistent with multi-component spherical models where Sculptor is embedded in a cuspy dark halo, as might be expected for cold dark matter.
Published: 27 November 2017
22-Nov-2019 07:17 UT

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