ESA Science & Technology - Publication Archive

White dwarfs are stellar embers depleted of nuclear energy sources that cool over billions of years. These stars, which are supported by electron degeneracy pressure, reach densities of 10⁷ grams per cubic centimetre in their cores. It has been predicted that a first-order phase transition occurs during white-dwarf cooling, leading to the crystallization of the non-degenerate carbon and oxygen ions in the core, which releases a considerable amount of latent heat and delays the cooling process by about one billion years. However, no direct observational evidence of this effect has been reported so far. Here we report the presence of a pile-up in the cooling sequence of evolving white dwarfs within 100 parsecs of the Sun, determined using photometry and parallax data from the Gaia satellite. Using modelling, we infer that this pile-up arises from the release of latent heat as the cores of the white dwarfs crystallize. In addition to the release of latent heat, we find strong evidence that cooling is further slowed by the liberation of gravitational energy from element sedimentation in the crystallizing cores. Our results describe the energy released by crystallization in strongly coupled Coulomb plasmas, and the measured cooling delays could help to improve the accuracy of methods used to determine the age of stellar populations from white dwarfs.

Document reference: CDF-187(C)

A mission to the Ice Giants (Neptune and Uranus) will be among the ones examined by the next Planetary Sciences Decadal, which also fits with the potential launch opportunity, with a Jupiter swing-by, that would allow to reach both planets by launching in the early 2030s.

ESA is exploring potential contributions to a NASA-led mission to the ice giants aimed at understanding the interior structure and bulk composition of the planet(s), including isotopes and noble gases.

ESA and NASA agreed to study a palette of possible configurations of varying cost to ESA and complexity, keeping in mind the need for clear interfaces.

It is important to keep this background in mind and remember that this study is not analysing a specific science proposal but trying to understand potential contributions following a top-down approach.

Requested by SCI-FM and funded by GSP, the M* (Ice Giants) study was set to analyse the feasibility of "stand-alone" elements provided by ESA to be part of the NASA-led mission to Uranus, Neptune and their moons (M-class mission budget but not proposed following a Cosmic Vision Programme Call, hence M*).

The internal Phase 0 study of the M5 mission candidate THESEUS has been performed at ESA's Concurrent Design Facility (CDF) between June and November 2018. An internal final presentation has been prepared by the CDF Team, summarizing the outcome of the Phase 0 study. This presentation can be downloaded as a PDF (10 MB) by clicking the image to the right, or the 'link to publication' link below.

The internal final presentations of the internal Phase 0 studies of the other two M5 mission candidates, EnVision and SPICA, are also available.

The internal Phase 0 study of the M5 mission candidate SPICA has been performed at ESA's Concurrent Design Facility (CDF) between June and November 2018. An internal final presentation has been prepared by the CDF Team, summarizing the outcome of the Phase 0 study. This presentation can be downloaded as a PDF (6 MB) by clicking the image to the right, or the 'link to publication' link below.

The internal final presentations of the internal Phase 0 studies of the other two M5 mission candidates, EnVision and THESEUS, are also available.

The internal Phase 0 study of the M5 mission candidate EnVision has been performed at ESA's Concurrent Design Facility (CDF) between June and November 2018. An internal final presentation has been prepared by the CDF Team, summarizing the outcome of the Phase 0 study. This presentation can be downloaded as a PDF (10 MB) by clicking the image to the right, or the 'link to publication' link below.

The internal final presentations of the internal Phase 0 studies of the other two M5 mission candidates, SPICA and THESEUS, are also available.

The bulk of stars in galaxy clusters are confined within their constituent galaxies. Those stars do not trace the extended distribution of dark matter well as they are located in the central regions of the cluster's dark matter subhaloes. A small fraction of stars is expected, however, to follow the global dark matter shape of the cluster. These are the stars whose extended spatial distribution results from the merging activity of galaxies and form the intracluster light (ICL). In this work, we compare the bi-dimensional distribution of dark matter in massive galaxy clusters (as traced by gravitational lensing models) with the distribution of the ICL. To do that, we use the superb data from the Hubble Frontier Fields Initiative. Using the Modified Hausdorff distance (MHD) as a way of quantifying the similarities between the mass and ICL distributions, we find an excellent agreement (MHD ~25 kpc) between the two components. This result shows that the ICL exquisitely follows the global dark matter distribution, providing an accurate luminous tracer of dark matter. This finding opens up the possibility of exploring the distribution of dark matter in galaxy clusters in detail using only deep imaging observations.

Reference: ESA/SCI(2018)1

SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) was selected from a pool of 13 potential missions that were proposed to the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS) as a result of a joint call for mission concepts in June 2015.

Following its selection in November 2015, detailed studies were performed by ESA, CAS, three European industrial contractors and the Science Study Team to finalise the mission architecture, including the space and ground elements that are required to fulfil the science requirements. This report, also known as the Red Book, summarizes the SMILE definition study.

Based on a theoretical selection of pulsars as candidates for detection at X-ray energies, we present an analysis of archival X-ray observations performed with Chandra and XMM-Newton of PSR J1747–2958 (the pulsar in the "Mouse" nebula), PSR J2021+3651 (the pulsar in the "Dragonfly" nebula), and PSR J1826–1256. X-ray pulsations from PSR J1747–2958 and PSR J1826–1256 are detected for the first time, and a previously reported hint of an X-ray pulsation from PSR J2021+3651 is confirmed with a higher significance. We analyze these pulsars' spectra in regard to the theoretically predicted energy distribution, finding a remarkable agreement, and provide here a refined calculation of the model parameters taking into account the newly derived X-ray spectral data.

The bow shock is the first boundary the solar wind encounters as it approaches planets or comets. The Rosetta spacecraft was able to observe the formation of a bow shock by following comet 67P/Churyumov–Gerasimenko toward the Sun, through perihelion, and back outward again. The spacecraft crossed the newly formed bow shock several times during two periods a few months before and after perihelion; it observed an increase in magnetic field magnitude and oscillation amplitude, electron and proton heating at the shock, and the diminution of the solar wind further downstream. Rosetta observed a cometary bow shock in its infancy, a stage in its development not previously accessible to in situ measurements at comets and planets.

The assembly of our Galaxy can be reconstructed using the motions and chemistry of individual stars. Chemo-dynamical studies of the stellar halo near the Sun have indicated the presence of multiple components, such as streams and clumps, as well as correlations between the stars' chemical abundances and orbital parameters. Recently, analyses of two large stellar surveys revealed the presence of a well populated elemental abundance sequence, two distinct sequences in the colour–magnitude diagram and a prominent, slightly retrograde kinematic structure in the halo near the Sun, which may trace an important accretion event experienced by the Galaxy. However, the link between these observations and their implications for Galactic history is not well understood. Here we report an analysis of the kinematics, chemistry, age and spatial distribution of stars that are mainly linked to two major Galactic components: the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object that at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia–Enceladus. The stars that originate in Gaia–Enceladus cover nearly the full sky, and their motions reveal the presence of streams and slightly retrograde and elongated trajectories. With an estimated mass ratio of four to one, the merger of the Milky Way with Gaia–Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk, thus contributing to the formation of this component approximately ten billion years ago. These findings are in line with the results of galaxy formation simulations, which predict that the inner stellar halo should be dominated by debris from only a few massive progenitors.

Download this interactive media kit to learn more about the launch of BepiColombo on 20 October 2018, the spacecraft's seven year journey to Mercury, and the science goals of the mission.

Contents:
Introduction
Event Programme
Key messages
BepiColombo science themes
From Messenger to BepiColombo
Mercury Planetary Orbiter's science instruments
Mercury Magnetospheric Orbiter's science instruments
Launch and separation timeline
Mercury Transfer Module cameras
Journey to Mercury
Venus flyby science operations
Arrival at Mercury
Operating in extreme environments
Quick look Mercury facts
Principal investigators
Selected images
Selected videos
Media services

Update 10 October 2018: Updated figure on pages 8 and 12.
Update 18 October 2018: Inserted new page 11 (MTM cameras), updated figure on pages 8, 9, 13 and 16, changed "MMO-MAG" to "MMO-MGF" throughout, added link to cartoon video, changed email address for University of Leicester contact (page 22).
Update 08 April 2020: A new version was released containing an updated version of the journey graphic on p12.

To download the pdf file (14 MB) click on the image or on the link to publication below.

Exomoons are the natural satellites of planets orbiting stars outside our solar system, of which there are currently no confirmed examples. We present new observations of a candidate exomoon associated with Kepler-1625b using the Hubble Space Telescope to validate or refute the moon's presence. We find evidence in favor of the moon hypothesis, based on timing deviations and a flux decrement from the star consistent with a large transiting exomoon. Self-consistent photodynamical modeling suggests that the planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to Neptune. Since our inference is dominated by a single but highly precise Hubble epoch, we advocate for future monitoring of the system to check model predictions and confirm repetition of the moon-like signal.

We search for the fastest stars in the subset of stars with radial velocity measurements of the second data release (DR2) of the European Space Agency mission Gaia. Starting from the observed positions, parallaxes, proper motions, and radial velocities, we construct the distance and total velocity distribution of more than 7 million stars in our Milky Way, deriving the full 6D phase space information in Galactocentric coordinates. These information are shared in a catalogue, publicly available at http://home.strw.leidenuniv.nl/ marchetti/research.html. To search for unbound stars, we then focus on stars with a probability greater than 50% of being unbound from the Milky Way. This cut results in a clean sample of 125 sources with reliable astrometric parameters and radial velocities. Of these, 20 stars have probabilities greater than 80 % of being unbound from the Galaxy. On this latter sub-sample, we perform orbit integration to characterize the stars’ orbital parameter distributions. As expected given the relatively small sample size of bright stars, we find no hypervelocity star candidates, stars that are moving on orbits consistent with coming from the Galactic Centre. Instead, we find 7 hyper-runaway star candidates, coming from the Galactic disk. Surprisingly, the remaining 13 unbound stars cannot be traced back to the Galaxy, including two of the fastest stars (around 700 km s⁻¹). If conformed, these may constitute the tip of the iceberg of a large extragalactic population or the extreme velocity tail of stellar streams.

The first detected interstellar object 'Oumuamua that passed within 0.25au of the Sun on 2017 September 9 was presumably ejected from a stellar system. We use its newly determined non-Keplerian trajectory together with the reconstructed Galactic orbits of 7 million stars from Gaia DR2 to identify past close encounters. Such an "encounter" could reveal the home system from which 'Oumuamua was ejected. The closest encounter, at 0.60pc (0.53-0.67pc, 90% confidence interval), was with the M2.5 dwarf HIP 3757 at a relative velocity of 24.7km/s, 1Myr ago. A more distant encounter (1.6pc) but with a lower encounter (ejection) velocity of 10.7km/s was with the G5 dwarf HD 292249, 3.8Myr ago. Two more stars have encounter distances and velocities intermediate to these. The encounter parameters are similar across six different non-gravitational trajectories for 'Oumuamua. Ejection of 'Oumuamua by scattering from a giant planet in one of the systems is plausible, but requires a rather unlikely configuration to achieve the high velocities found. A binary star system is more likely to produce the observed velocities. None of the four home candidates have published exoplanets or are known to be binaries. Given that the 7 million stars in Gaia DR2 with 6D phase space information is just a small fraction of all stars for which we can eventually reconstruct orbits, it is a priori unlikely that our current search would find 'Oumuamua's home star system. As 'Oumuamua is expected to pass within 1pc of about 20 stars and brown dwarfs every Myr, the plausibility of a home system depends also on an appropriate (low) encounter velocity.

Saturn's moon Titan has a dense nitrogen-rich atmosphere, with methane as its primary volatile. Titan's atmosphere experiences an active chemistry that produces a haze of organic aerosols that settle to the surface and a dynamic climate in which hydrocarbons are cycled between clouds, rain and seas. Titan displays particularly energetic meteorology at equinox in equatorial regions, including sporadic and large methane storms. In 2009 and 2010, near Titan's northern spring equinox, the Cassini spacecraft observed three distinctive and short-lived spectral brightenings close to the equator. Here, we show from analyses of Cassini spectral data, radiative transfer modelling and atmospheric simulations that the brightenings originate in the atmosphere and are consistent with formation from dust storms composed of micrometre-sized solid organic particles mobilized from underlying dune fields. Although the Huygens lander found evidence that dust can be kicked up locally from Titan's surface, our findings suggest that dust can be suspended in Titan's atmosphere at much larger spatial scale. Mobilization of dust and injection into the atmosphere would require dry conditions and unusually strong near-surface winds (about five times more than estimated ambient winds). Such strong winds are expected to occur in downbursts during rare equinoctial methane storms—consistent with the timing of the observed brightenings. Our findings imply that Titan—like Earth and Mars—has an active dust cycle, which suggests that Titan's dune fields are actively evolving by aeolian processes.

The evolution of the Milky Way disk, which contains most of the stars in the Galaxy, is affected by several phenomena. For example, the bar and the spiral arms of the Milky Way induce radial migration of stars and can trap or scatter stars close to orbital resonances. External perturbations from satellite galaxies can also have a role, causing dynamical heating of the Galaxy, ring-like structures in the disk and correlations between different components of the stellar velocity. These perturbations can also cause 'phase wrapping' signatures in the disk, such as arched velocity structures in the motions of stars in the Galactic plane. Some manifestations of these dynamical processes have already been detected, including kinematic substructure in samples of nearby stars, density asymmetries and velocities across the Galactic disk that differ from the axisymmetric and equilibrium expectations, especially in the vertical direction, and signatures of incomplete phase mixing in the disk. Here we report an analysis of the motions of six million stars in the Milky Way disk. We show that the phase-space distribution contains different substructures with various morphologies, such as snail shells and ridges, when spatial and velocity coordinates are combined. We infer that the disk must have been perturbed between 300 million and 900 million years ago, consistent with estimates of the previous pericentric passage of the Sagittarius dwarf galaxy. Our findings show that the Galactic disk is dynamically young and that modelling it as time-independent and axisymmetric is incorrect.

The Second Small Astronomy Satellite (SAS-2) high-energy (in excess of 35 MeV) gamma-ray telescope has detected pulsed gamma-ray emission at the radio period from PSR 0833-45, the Vela pulsar, as well as an unpulsed flux from the Vela region. The pulsed emission consists of two peaks following the single radio peak by about 13 ms and 48 ms. The luminosity of the pulsed emission above 100 MeV from Vela is about 0.1 that of the pulsar NP 0532 in the Crab nebula, whereas the pulsed emission from Vela at optical wavelengths is less than 0.0002 that from the Crab. The relatively high intensity of the pulsed gamma-ray emission, and the double peak structure, compared with the single pulse in the radio emission, suggest that the high-energy gamma-ray pulsar emission may be produced under different conditions from those at lower energies.

Saturn's polar stratosphere exhibits the seasonal growth and dissipation of broad, warm vortices poleward of ~75° latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassini's reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly formed NPSV was bounded by a strengthening stratospheric thermal gradient near 78°N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn's long-lived polar hexagon—which was previously expected to be trapped in the troposphere—can influence the stratospheric temperatures some 300 km above Saturn's clouds.

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The young massive Jupiters discovered with high-contrast imaging provide a unique opportunity to study the formation and early evolution of gas giant planets. A key question is to what extent gravitational energy from accreted gas contributes to the internal energy of a newly formed planet. This has led to a range of formation scenarios from ‘cold’ to ‘hot’ start models. For a planet of a given mass, these initial conditions govern its subsequent evolution in luminosity and radius. Except for upper limits from radial velocity studies, disk modelling and dynamical instability arguments, no mass measurements of young planets are yet available to distinguish between these different models. Here, we report on the detection of the astrometric motion of Beta Pictoris, the ~21-Myr-old host star of an archetypical directly imaged gas giant planet, around the system’s centre of mass. Subtracting the highly accurate Hipparcos and Gaia proper motion from the internal 3 yr Hipparcos astrometric data reveals the reflex motion of the star, giving a model-independent planet mass of 11 ± 2 Jupiter masses. This is consistent with scenarios in which the planet is formed in a high-entropy state as assumed by hot start models. The ongoing data collection by Gaia will soon lead to mass measurements of other young gas giants and form a great asset to further constrain early-evolution scenarios.