Publication archive

Publication archive

We confirm the reality of the recently discovered Milky Way stellar cluster Gaia 1 using spectra acquired with the HERMES and AAOmega spectrographs of the Anglo-Australian Telescope. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the sky at visual wavelengths. Our observations identified 41 cluster members, and yielded an overall metallicity of [Fe/H]=−0.13±0.13 and barycentric radial velocity of vr = 58.30 ± 0.22 km s-1. These kinematics provide a dynamical mass estimate of 12.9+4.6-3.9×103 M. Isochrone fits to Gaia, 2MASS, and Pan-STARRS1 photometry indicate that Gaia 1 is an intermediate age (~3 Gyr) stellar cluster. Combining the spatial and kinematic data we calculate Gaia 1 has a circular orbit with a radius of about 12 kpc, but with a large out of plane motion: zmax=1.1+0.4-0.3 kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.
Published: 27 July 2017
Cassini discovered a plethora of neutral and ionized molecules in Titan's ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u q-1. In this Letter, we forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950–1300 km. We report on detections consistently centered between 25.8 and 26.0 u q-1 and between 49.0–50.1 u q-1 which are identified as belonging to the carbon chain anions, CN-/C3N- and/or C2H-/C4H-, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73–74 u q-1 could be attributed to the further carbon chain anions C5N-/C6H- but at lower altitudes and during further encounters extend over a higher mass/charge range. This, as well as further intermediary anions detected at >100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below <1100 km, the low-mass anions (<150 u q-1) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.
Published: 27 July 2017
We observed six He-clump stars of the intermediate-age stellar cluster Gaia1 with the MIKE/Magellan spectrograph. A possible extra-galactic origin of this cluster, recently discovered thanks to the first data release of the ESA Gaia mission, has been suggested, based on its orbital parameters. Abundances for Fe, α, proton- and neutron-capture elements have been obtained. We find no evidence of intrinsic abundance spreads. The iron abundance is solar ([FeI/H] = + 0.00 ± 0.01; σ = 0.03 dex). All the other abundance ratios are generally solar-scaled, similar to the Galactic thin disk and open cluster stars of similar metallicity. The chemical composition of Gaia1 does not support an extra-galactic origin for this stellar cluster, which can be considered as a standard Galactic open cluster.
Published: 20 July 2017
During the past five decades astronomers have been puzzled by the presence of strong absorption features including metal lines, observed in the optical and ultraviolet spectra of quasars, signaling inflowing and outflowing gas winds with relative velocities up to several thousands of km s-1. In particular, the location of these winds–close to the quasar, further out in its host galaxy, or in its direct environment–and the possible impact on their surroundings have been issues of intense discussion and uncertainty. Using our Herschel Space Observatory data, we report a tendency for this so-called associated metal absorption to occur along with prodigious star formation in the quasar host galaxy, indicating that the two phenomena are likely to be interrelated, that the gas winds likely occur on the kiloparsec scale and would then have a strong impact on the interstellar medium of the galaxy. This correlation moreover would imply that the unusually high cold dust luminosities in these quasars are connected with ongoing star formation. Given that we find no correlation with the AGN strength, the wind feedback that we establish in these radio-loud objects is most likely associated with their host star formation rather than with their black hole accretion.
Published: 01 July 2017
We present the results of a 140 ks XMM-Newton observation of the B2 star ρ Oph A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK (kT ~ 5 keV). From the analysis of its rise, we infer a magnetic field of ≥300 G and a size of the flaring region of ~ 1.4 − 1.9 × 1011 cm, which corresponds to ~25%–30% of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of ≥2.5 yr, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems. In both cases ρ Oph would constitute an extreme system that is worthy of further investigation.
Published: 21 June 2017
The origin of cometary matter and the potential contribution of comets to inner-planet atmospheres are long-standing problems. During a series of dedicated low-altitude orbits, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta spacecraft analyzed the isotopes of xenon in the coma of comet 67P/Churyumov-Gerasimenko. The xenon isotopic composition shows deficits in heavy xenon isotopes and matches that of a primordial atmospheric component. The present-day Earth atmosphere contains 22 ± 5% cometary xenon, in addition to chondritic (or solar) xenon.
Published: 10 June 2017
Context. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) was designed to measure the composition of the gas in the coma of comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency’s Rosetta mission. In addition to the volatiles, ROSINA measured refractories sputtered off the comet by the interaction of solar wind protons with the surface of the comet. Aims. The origin of different solar system materials is still heavily debated. Isotopic ratios can be used to distinguish between different reservoirs and investigate processes occurring during the formation of the solar system. Methods. ROSINA consisted of two mass spectrometers and a pressure sensor. In the ROSINA Double Focusing Mass Spectrometer (DFMS), the neutral gas of cometary origin was ionized and then deflected in an electric and a magnetic field that separated the ions based on their mass-to-charge ratio. The DFMS had a high mass resolution, dynamic range, and sensitivity that allowed detection of rare species and the known major volatiles. Results. We measured the relative abundance of all three stable silicon isotopes with the ROSINA instrument on board the Rosetta spacecraft. Furthermore, we measured 13C/12C in C2H4, C2H5, and CO. The DFMS in situ measurements indicate that the average silicon isotopic composition shows depletion in the heavy isotopes 29Si and 30Si with respect to 28Si and solar abundances, while 13C to 12C is analytically indistinguishable from bulk planetary and meteorite compositions. Although the origin of the deficiency of the heavy silicon isotopes cannot be explained unambiguously, we discuss mechanisms that could have contributed to the measured depletion of the isotopes 29Si and 30Si.
Published: 02 April 2017
The paucity of hypervelocity stars (HVSs) known to date has severely hampered their potential to investigate the stellar population of the Galactic Centre and the Galactic Potential. The first Gaia data release (DR1, 2016 September 14) gives an opportunity to increase the current sample. The challenge is the disparity between the expected number of hypervelocity stars and that of bound background stars. We have applied a novel data mining algorithm based on machine learning techniques, an artificial neural network, to the Tycho-Gaia astrometric solution (TGAS) catalogue. With no pre-selection of data, we could exclude immediately ∼99% of the stars in the catalogue and find 80 candidates with more than 90% predicted probability to be HVSs, based only on their position, proper motions, and parallax. We have cross-checked our findings with other spectroscopic surveys, determining radial velocities for 30 and spectroscopic distances for 5 candidates. In addition, follow-up observations have been carried out at the Isaac Newton Telescope for 22 stars, for which we obtained radial velocities and distance estimates. We discover 14 stars with a total velocity in the Galactic rest frame >400 km s-1, and 5 of these have a probability >50% of being unbound from the Milky Way. Tracing back their orbits in different Galactic potential models we find one possible unbound HVS with v ∼ 520 km s-1, 5 bound HVSs, and, notably, 5 runaway stars with median velocity between 400 and 780 km s-1. At the moment, uncertainties in the distance estimates and ages are too large to confirm the nature of our candidates by narrowing down their ejection location, and we wait for future Gaia releases to validate the quality of our sample. This test successfully demonstrates the feasibility of our new data mining routine.
Published: 26 May 2017
BepiColombo is Europe's first mission to Mercury, and consists of two scientific orbiters: ESA's Mercury Planetary Orbiter and JAXA's Mercury Magnetospheric Orbiter. It will study all aspects of Mercury, from the structure and dynamics of its magnetosphere and how it interacts with the solar wind, to the properties of its large iron core and the origin of its magnetic field. The data will improve our understanding of the planet and of the overall evolution of our Solar System.


  • Europe to Mercury
  • Mercury rising: coping with high temperatures
  • Building and testing BepiColombo
  • Introducing the fleet
  • Meeting Mercury
  • From Messenger to BepiColombo
  • An international endeavour

Published: 20 May 2017
We present the results of the very first search for faint Milky Way satellites in the Gaia data. Using stellar positions only, we are able to re-discover objects detected in much deeper data as recently as the last couple of years. While we do not identify new prominent ultrafaint dwarf galaxies, we report the discovery of two new star clusters, Gaia 1 and Gaia 2. Gaia 1 is particularly curious, as it is a massive (2.2 × 104 M⊙), large (~9 pc) and nearby (4.6 kpc) cluster, situated 10 arcmin away from the brightest star on the sky, Sirius! Even though this satellite is detected at significance in excess of 10, it was missed by previous sky surveys. We conclude that Gaia possesses powerful and unique capabilities for satellite detection, thanks to its unrivalled angular resolution and highly efficient object classification.
Published: 16 May 2017

The internal Phase 0 study of the Laser interferometer Space Antenna (LISA) mission has been performed at ESA's Concurrent Design Facility (CDF) and ran from 8 March to 5 May 2017. 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 (28 MB) by clicking the image to the right, or the 'link to publication' link below.

Contents of the presentation:

p.1 Introduction p.195 TT&C
p.6 CDF Study objectives p.207 Data handling
p.10 Science objectives p.220 Power
p.49 Systems p.238 Mechanisms
p.78 Payload p.255 Configuration
p.104 Mission analysis p.259 Structures
p.124 Ground segment and operations p.270 Thermal control
p.135 DFACS - AOCS p.282 Risk
p.156 Chemical propulsion p.306 Programmatics / AIV
p.172 Electric propulsion p.325 Conclusions

Published: 06 May 2017
This issue of Spatium features an article by Professor Karsten Danzmann of the Max Planck Institute for Gravitation Physics and the Institute for Gravitation Physics of the Leibniz Universität, Hannover. It is devoted to gravitational waves, their sources and the marvellous technologies required for observing them. Over and above, this issue renders homage to Albert Einstein and the scientists of his time, who laid the cornerstone for our understanding of gravitational waves.
Published: 02 April 2017
Spontaneous collapse models are phenomological theories formulated to address major difficulties in macroscopic quantum mechanics. We place significant bounds on the parameters of the leading collapse models, the continuous spontaneous localization (CSL) model, and the Diosi-Penrose (DP) model, by using LISA Pathfinder's measurement, at a record accuracy, of the relative acceleration noise between two free-falling macroscopic test masses. In particular, we bound the CSL collapse rate to be at most (2.96±0.12)×10-8  s–1. This competitive bound explores a new frequency regime, 0.7 to 20 mHz, and overlaps with the lower bound 10-8±2  s-1 proposed by Adler in order for the CSL collapse noise to be substantial enough to explain the phenomenology of quantum measurement. Moreover, we bound the regularization cutoff scale used in the DP model to prevent divergences to be at least 40.1±0.5  fm, which is larger than the size of any nucleus. Thus, we rule out the DP model if the cutoff is the size of a fundamental particle.
Published: 29 April 2017
We report on electrostatic measurements made on board the European Space Agency mission LISA Pathfinder. Detailed measurements of the charge-induced electrostatic forces exerted on free-falling test masses (TMs) inside the capacitive gravitational reference sensor are the first made in a relevant environment for a space-based gravitational wave detector. Employing a combination of charge control and electric-field compensation, we show that the level of charge-induced acceleration noise on a single TM can be maintained at a level close to 1.0  fm s-2 Hz-1/2 across the 0.1–100 mHz frequency band that is crucial to an observatory such as the Laser Interferometer Space Antenna (LISA). Using dedicated measurements that detect these effects in the differential acceleration between the two test masses, we resolve the stochastic nature of the TM charge buildup due to interplanetary cosmic rays and the TM charge-to-force coupling through stray electric fields in the sensor. All our measurements are in good agreement with predictions based on a relatively simple electrostatic model of the LISA Pathfinder instrument.
Published: 27 April 2017
We combine Gaia DR1, PS1, SDSS and 2MASS astrometry to measure proper motions for 350 million sources across three-fourths of the sky down to a magnitude of mr~20. Using positions of galaxies from PS1, we build a common reference frame for the multi-epoch PS1, single-epoch SDSS and 2MASS data, and calibrate the data in small angular patches to this frame. As the Gaia DR1 excludes resolved galaxy images, we choose a different approach to calibrate Gaia DR1 positions to this reference frame: we exploit the fact that the proper motions of stars in these patches are linear. By simultaneously fitting the positions of stars at different epochs -- Gaia DR1, PS1, SDSS, and 2MASS -- we construct an extensive catalog of proper motions, dubbed GPS1. GPS1 has a characteristic systematic error of less than 0.3 mas yr-1, and a typical precision of 1.5-2.0 mas yr-1. The proper motions have been validated using galaxies, open clusters, distant giant stars and QSOs. In comparison with other published faint proper motion catalogs, GPS1's systematic error (<0.3 mas yr-1) is $\sim$ 10 times better than that of PPMXL and UCAC4 (>2.0 mas yr-1). Similarly, its precision (~1.5 mas yr-1) is an improvement by $\sim$ 4 times relative to PPMXL and UCAC4 (~6.0 mas yr-1). For QSOs, the precision of GPS1 is found to be worse (~2.0-3.0 mas yr-1), possibly due to their particular differential chromatic refraction (DCR). The catalog will be released on-line and available via the VizieR Service.
Published: 18 March 2017
New astrometric reductions of the US Naval Observatory CCD Astrograph Catalog (UCAC) all-sky observations were performed from first principles using the TGAS stars in the 8–11 mag range as the reference star catalog. Significant improvements in the astrometric solutions were obtained, and the UCAC5 catalog of mean positions at a mean epoch near 2001 was generated. By combining UCAC5 with Gaia DR1 data, new proper motions were obtained for over 107 million stars on the Gaia coordinate system, with typical accuracies of 1–2 mas yr−1 (R = 11–15 mag) and about 5 mas yr−1 at 16th mag. Proper motions of most TGAS stars are improved over their Gaia data and the precision level of TGAS proper motions is extended to many millions more, fainter stars. External comparisons were made using stellar cluster fields and extragalactic sources. The TGAS data allow us to derive the limiting precision of the UCAC x, y data, which is significantly better than1/100 pixel.
Published: 20 March 2017
Context. Recently, the first installment of data from the ESA Gaia astrometric satellite mission (Gaia DR1) was released, containing positions of more than 1 billion stars with unprecedented precision. This release contains the proper motions and parallaxes, however, for only a subset of 2 million objects. The second release will include those quantities for most objects. Aims. In order to provide a dataset that bridges the time gap between the Gaia DR1 and Gaia DR2 releases and partly remedies the lack of proper motions in the former, Hot Stuff for One Year (HSOY) was created as a hybrid catalogue between Gaia and ground-based astrometry. This catalogue features proper motions (but no parallaxes) for a large percentage of the DR1 objects. While not attempting to compete with future Gaia releases in terms of data quality or number of objects, the aim of HSOY is to provide improved proper motions partly based on Gaia data and to allow studies to be carried out now or as pilot studies for later projects requiring higher precision data. Methods. The HSOY catalogue was compiled using the positions taken from Gaia DR1 combined with the input data from the PPMXL catalogue, employing the same weighted least-squares technique that was used to assemble the PPMXL catalogue itself. Results. This effort resulted in a four-parameter astrometric catalogue containing 583 million stars with Gaia DR1 quality positions and proper motions with precisions from far less than 1 mas/yr to 5 mas/yr, depending on object brightness and location on the sky.
Published: 02 March 2017

Reference: ESA-SCI(2017)1

In February 2014, PLATO (PLAnetary Transits and Oscillation of stars) was selected as the M3 mission in the Cosmic Vision 2015-2025 programme. This report (also known as the Red Book) provides a high-level summary of the large number of scientific and technical documents produced as outcome of the definition study for the PLATO mission.

Published: 05 April 2017

Outbursts occur commonly on comets with different frequencies and scales. Despite multiple observations suggesting various triggering processes, the driving mechanism of such outbursts is still poorly understood. Landslides have been invoked to explain some outbursts on comet 103P/Hartley 2, although the process required a pre-existing dust layer on the verge of failure. The Rosetta mission observed several outbursts from its target comet 67P/Churyumov–Gerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs. However, none of the aforementioned works included definitive evidence that landslides occur on comets. Amongst the many features observed by Rosetta on the nucleus of the comet, one peculiar fracture, 70 m long and 1 m wide, was identified on images obtained in September 2014 at the edge of a cliff named Aswan. On 10 July 2015, the Rosetta Navigation Camera captured a large plume of dust that could be traced back to an area encompassing the Aswan escarpment. Five days later, the OSIRIS camera observed a fresh, sharp and bright edge on the Aswan cliff. Here we report the first unambiguous link between an outburst and a cliff collapse on a comet. We establish a new dust-plume formation mechanism that does not necessarily require the breakup of pressurized crust or the presence of supervolatile material, as suggested by previous studies. Moreover, the collapse revealed the fresh icy interior of the comet, which is characterized by an albedo >0.4, and provided the opportunity to study how the crumbling wall settled down to form a new talus.

The evolution of the collapse of the Aswan cliff, observed by the OSIRIS Narrow Angle Camera (NAC) and the Rosetta Navigation camera (NavCam), is shown in Fig. 1.

[Remainder of abstract truncated due to character limitations]

Published: 21 March 2017
The Rosetta spacecraft spent ~2 years orbiting comet 67P/Churyumov-Gerasimenko, most of it at distances that allowed surface characterization and monitoring at submeter scales. From December 2014 to June 2016, numerous localized changes were observed, which we attribute to cometary-specific weathering, erosion, and transient events driven by exposure to sunlight and other processes. While the localized changes suggest compositional or physical heterogeneity, their scale has not resulted in substantial alterations to the comet's landscape. This suggests that most of the major landforms were created early in the comet's current orbital configuration. They may even date from earlier if the comet had a larger volatile inventory, particularly of CO or CO2 ices, or contained amorphous ice, which could have triggered activity at greater distances from the Sun.
Published: 21 March 2017
18-Jan-2021 17:35 UT

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