| 1. |
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| 2. |
- Brown, A. G. A., et al.
(författare)
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Gaia Data Release 1 Summary of the astrometric, photometric, and survey properties
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
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Tidskriftsartikel (refereegranskat)abstract
- Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues - a realisation of the Tycho-Gaia Astrometric Solution (TGAS) - and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of similar to 3000 Cepheid and RR Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr(-1) for the proper motions. A systematic component of similar to 0.3 mas should be added to the parallax uncertainties. For the subset of similar to 94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr(-1). For the secondary astrometric data set, the typical uncertainty of the positions is similar to 10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to similar to 0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data.
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| 3. |
- Prusti, T., et al.
(författare)
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The Gaia mission
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
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Tidskriftsartikel (refereegranskat)abstract
- Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page.
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| 4. |
- van Leeuwen, F., et al.
(författare)
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Gaia Data Release 1 : Open cluster astrometry: Performance, limitations, and future prospects
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 601
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Tidskriftsartikel (refereegranskat)abstract
- Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information. Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters. Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed. Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier Hipparcos-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters. Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the Hipparcos data, with clearly increased luminosities for older A and F dwarfs.
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| 5. |
- Clementini, G., et al.
(författare)
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Testing parallaxes with local Cepheids and RR Lyrae stars
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
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Tidskriftsartikel (refereegranskat)abstract
- 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.Methods. Classical Cepheids were carefully selected in order to discard known or suspected binary systems. The final sample comprises 102 fundamental mode pulsators with periods ranging from 1.68 to 51.66 days (of which 33 with sigma(omega)/omega < 0 : 5). The Type II Cepheids include a total of 26 W Virginis and BL Herculis stars spanning the period range from 1.16 to 30.00 days (of which only 7 with sigma(omega)/omega 0 : 5). The RR Lyrae stars include 200 sources with pulsation period ranging from 0.27 to 0.80 days (of which 112 with sigma(omega)/omega < 0 : 5). The new relations were computed using multi- band (V; I; J; K-s) photometry and spectroscopic metal abundances available in the literature, and by applying three alternative approaches: (i) linear least-squares fitting of the absolute magnitudes inferred from direct transformation of the TGAS parallaxes; (ii) adopting astrometry-based luminosities; and (iii) using a Bayesian fitting approach. The last two methods work in parallax space where parallaxes are used directly, thus maintaining symmetrical errors and allowing negative parallaxes to be used. The TGAS-based PL; PW; PLZ, and MV [Fe/H] relations are discussed by comparing the distance to the Large Magellanic Cloud provided by different types of pulsating stars and alternative fitting methods.Results. Good agreement is found from direct comparison of the parallaxes of RR Lyrae stars for which both TGAS and HST measurements are available. Similarly, very good agreement is found between the TGAS values and the parallaxes inferred from the absolute magnitudes of Cepheids and RR Lyrae stars analysed with the Baade-Wesselink method. TGAS values also compare favourably with the parallaxes inferred by theoretical model fitting of the multi-band light curves for two of the three classical Cepheids and one RR Lyrae star, which were analysed with this technique in our samples. The K-band PL relations show the significant improvement of the TGAS parallaxes for Cepheids and RR Lyrae stars with respect to the HIPPARCOS measurements. This is particularly true for the RR Lyrae stars for which improvement in quality and statistics is impressive.Conclusions. TGAS parallaxes bring a significant added value to the previous HIPPARCOS estimates. The relations presented in this paper represent the first Gaia-calibrated relations and form a work-in-progress milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia Data Release 2 (DR2) in 2018.
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| 6. |
- Pajola, M., et al.
(författare)
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The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse
- 2017
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Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 1:5
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Tidskriftsartikel (refereegranskat)abstract
- Outbursts occur commonly on comets(1) with different frequencies and scales(2,3). Despite multiple observations suggesting various triggering processes(4,5), the driving mechanism of such outbursts is still poorly understood. Landslides have been invoked(6) 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/ChuryumovGerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs(7,8). 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(9). 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(7). 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(7). 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.
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| 7. |
- Gicquel, A., et al.
(författare)
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Modelling of the outburst on 2015 July 29 observed with OSIRIS cameras in the Southern hemisphere of comet 67P/Churyumov-Gerasimenko
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S178-S185
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Tidskriftsartikel (refereegranskat)abstract
- Images of the nucleus and the coma (gas and dust) of comet 67P/Churyumov-Gerasimenko have been acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) cameras since 2014 March using both the wide-angle camera and the narrow-angle camera (NAC). We use images from the NAC camera to study a bright outburst observed in the Southern hemisphere on 2015 July 29. The high spatial resolution of the NAC is needed to localize the source point of the outburst on the surface of the nucleus. The heliocentric distance is 1.25 au and the spacecraft-comet distance is 186 km. Aiming to better understand the physics that led to the outgassing, we used the Direct Simulation Monte Carlo method to study the gas flow close to the nucleus and the dust trajectories. The goal is to understand the mechanisms producing the outburst. We reproduce the opening angle of the outburst in the model and constrain the outgassing ratio between the outburst source and the local region. The outburst is in fact a combination of both gas and dust, in which the active surface is approximately 10 times more active than the average rate found in the surrounding areas. We need a number of dust particles 7.83 x 10(11) to 6.90 x 10(15) (radius 1.97-185 mu m), which correspond to a mass of dust (220-21) x 10(3) kg.
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| 8. |
- Ip, W. -H, et al.
(författare)
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Physical properties and dynamical relation of the circular depressions on comet 67P/Churyumov-Gerasimenko
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 591
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to characterize the circular depressions of comet 67P/Churyumov-Gerasimenko and investigate whether such surface morphology of a comet nucleus is related to the cumulative sublimation effect since becoming a Jupiter family comet (JFC). Methods. The images from the Rosetta/OSIRIS science camera experiment are used to construct size frequency distributions of the circular depression structures on comet 67P and they are compared with those of the JFCs 81P/Wild 2, 9P/Tempel 1, and 103P/Hartley 2. The orbital evolutionary histories of these comets over the past 100 000 yr are analyzed statistically and compared with each other. Results. The global distribution of the circular depressions over the surface of 67P is charted and classified. Descriptions are given to the characteristics and cumulative size frequency distribution of the identified features. Orbital statistics of the JFCs visited by spacecraft are derived. Conclusions. The size frequency distribution of the circular depressions is found to have a similar power law distribution to those of 9P/Tempel 1 and 81P/Wild 2. This might imply that they could have been generated by the same process. Orbital integration calculation shows that the surface erosion histories of 81P/Wild 2, and 9P/Tempel 1 could be shorter than those of 67P, 103 P/Hartley 2 and 19P/Borrelly. From this point of view, the circular depressions could be dated back to the pre-JFC phase or the transneptunian phase of these comets. The north-south asymmetry in the distribution of the circular depressions could be associated with the heterogeneous structure of the nucleus of comet 67P and/or the solar insolation history.
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| 9. |
- Attree, N., et al.
(författare)
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Tensile strength of 67P/Churyumov-Gerasimenko nucleus material from overhangs
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 611
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Tidskriftsartikel (refereegranskat)abstract
- We directly measured twenty overhanging cliffs on the surface of comet 67P/Churyumov-Gerasimenko extracted from the latest shape model and estimated the minimum tensile strengths needed to support them against collapse under the comet's gravity. We find extremely low strengths of around 1 Pa or less (1 to 5 Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features and the implied previous collapse of another, suggests that they are prone to failure and that the true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of the 67P nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties either with size over the similar to 10-100 m range studied here or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small body (tens of km).
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| 10. |
- Fabricius, C., et al.
(författare)
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Gaia Data Release 1 : Pre-processing and source list creation
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
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Tidskriftsartikel (refereegranskat)abstract
- Context. The first data release from the Gaia mission contains accurate positions and magnitudes for more than a billion sources, and proper motions and parallaxes for the majority of the 2.5 million Hipparcos and Tycho-2 stars. Aims. We describe three essential elements of the initial data treatment leading to this catalogue: the image analysis, the construction of a source list, and the near real-time monitoring of the payload health. We also discuss some weak points that set limitations for the attainable precision at the present stage of the mission. Methods. Image parameters for point sources are derived from one-dimensional scans, using a maximum likelihood method, under the assumption of a line spread function constant in time, and a complete modelling of bias and background. These conditions are, however, not completely fulfilled. The Gaia source list is built starting from a large ground-based catalogue, but even so a significant number of new entries have been added, and a large number have been removed. The autonomous onboard star image detection will pick up many spurious images, especially around bright sources, and such unwanted detections must be identified. Another key step of the source list creation consists in arranging the more than 1010 individual detections in spatially isolated groups that can be analysed individually. Results. Complete software systems have been built for the Gaia initial data treatment, that manage approximately 50 million focal plane transits daily, giving transit times and fluxes for 500 million individual CCD images to the astrometric and photometric processing chains. The software also carries out a successful and detailed daily monitoring of Gaia health.
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| 11. |
- Hu, X., et al.
(författare)
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Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 604
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Tidskriftsartikel (refereegranskat)abstract
- Context: Dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67P /Churyumov-Gerasimenko (67P). The evolution of the dust deposits was captured by the OSIRIS camera system onboard the Rosetta spacecraft having escorted the comet for over two years. The observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology.Aims: We aim to present OSIRIS observations of surface changes over the dust deposits before and after perihelion. The distribution of changes and a timeline of their occurrence are provided. We perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. We further discuss how the results of our investigation are related to other findings from the Rosetta mission.Methods: Surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. Thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. Modeling results were used for the interpretation of the observed surface changes.Results: Surface changes discussed here were concentrated at mid-latitudes, between about 20 degrees N and 40 degrees N, marking a global transition from the dust-covered to rugged terrains. The changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. The occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. The erosion at locations of most changes was at least about 0 : 5 m, but most likely did not exceed several meters. The erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night.Conclusions: The erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67P. Based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. Local inhomogeneity in water abundance at spatial scales below tens of meters is likely. We suspect that dust ejected from the deposits may not have escaped the comet in bulk. That is, at least half of the ejected mass was afloat in the inner-coma or /and redeposited over other areas of the nucleus.
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| 12. |
- Lucchetti, A., et al.
(författare)
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Geomorphological and spectrophotometric analysis of Seth's circular niches on comet 67P/Churyumov-Gerasimenko using OSIRIS images
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S238-S251
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Tidskriftsartikel (refereegranskat)abstract
- We provide a detailed geomorphological and spectrophotometric analysis of the circular niches located on the Seth region of 67P using OSIRIS images. The features can be related to landslide events that occurred on 67P and shaped its surface, as the recent Aswan cliff collapse detected in the same region. We therefore provide an analysis of the area pre -and post-perihelion suggesting that no specific changes have been observed. To assess this, after performing a geomorphological map of the area that allows us to identify different terrain units, we computed the boulders cumulative size frequency distribution (SFD) of the niches, before and after the perihelion passage. The niches SFDs are characterized by a similar trend with two different power-law indices within the same deposit: lower power-law value (between -2.3 and -2.7) for boulders smaller than 5 m and steeper power-law value (between -4.7 and -5.0) for boulders larger than 5 m. These trends suggest that smaller boulders have evolved and progressively have been depleted (lower power-law index), while bigger boulders are more representative of the event that generated the deposit and are less degraded. Then, we perform the spectrophotometric analysis of this region comparing pre-and post-perihelion results. We found colour changes within the area, in particular brighter patches related to the presence of exposed water ice mixed to the refractory materials have been detected in the post-perihelion images.
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| 13. |
- Pommerol, A., et al.
(författare)
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OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 583
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Tidskriftsartikel (refereegranskat)abstract
- Since OSIRIS started acquiring high-resolution observations of the surface of the nucleus of comet 67P/Churyumov-Gerasimenko, over one hundred meter-sized bright spots have been identified in numerous types of geomorphologic regions, but mostly located in areas receiving low insolation. The bright spots are either clustered, in debris fields close to decameter-high cliffs, or isolated without structural relation to the surrounding terrain. They can be up to ten times brighter than the average surface of the comet at visible wavelengths and display a significantly bluer spectrum. They do not exhibit significant changes over a period of a few weeks. All these observations are consistent with exposure of water ice at the surface of boulders produced by dislocation of the weakly consolidated layers that cover large areas of the nucleus. Laboratory experiments show that under simulated comet surface conditions, analog samples acquire a vertical stratification with an uppermost porous mantle of refractory dust overlaying a layer of hard ice formed by recondensation or sintering under the insulating dust mantle. The evolution of the visible spectrophotometric properties of samples during sublimation is consistent with the contrasts of brightness and color seen at the surface of the nucleus. Clustered bright spots are formed by the collapse of overhangs that is triggered by mass wasting of deeper layers. Isolated spots might be the result of the emission of boulders at low velocity that are redepositioned in other regions.
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| 14. |
- Tubiana, C., et al.
(författare)
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Scientific assessment of the quality of OSIRIS images
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 583
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Tidskriftsartikel (refereegranskat)abstract
- Context. OSIRIS, the scientific imaging system onboard the ESA Rosetta spacecraft, has been imaging the nucleus of comet 67P/Churyumov-Gerasimenko and its dust and gas environment since March 2014. The images serve different scientific goals, from morphology and composition studies of the nucleus surface, to the motion and trajectories of dust grains, the general structure of the dust coma, the morphology and intensity of jets, gas distribution, mass loss, and dust and gas production rates. Aims. We present the calibration of the raw images taken by OSIRIS and address the accuracy that we can expect in our scientific results based on the accuracy of the calibration steps that we have performed. Methods. We describe the pipeline that has been developed to automatically calibrate the OSIRIS images. Through a series of steps, radiometrically calibrated and distortion corrected images are produced and can be used for scientific studies. Calibration campaigns were run on the ground before launch and throughout the years in flight to determine the parameters that are used to calibrate the images and to verify their evolution with time. We describe how these parameters were determined and we address their accuracy. Results. We provide a guideline to the level of trust that can be put into the various studies performed with OSIRIS images, based on the accuracy of the image calibration.
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| 15. |
- Agarwal, Jessica, et al.
(författare)
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Acceleration of individual, decimetre-sized aggregates in the lower coma of comet 67P/Churyumov-Gerasimenko
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462, s. S78-S88
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Tidskriftsartikel (refereegranskat)abstract
- We present observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67P/Churyumov-Gerasimenko. The images were obtained with the narrow angle camera of the Optical, Spectroscopic, and Infrared Remote Imaging System on board the Rosetta spacecraft in 2016 January when the comet was at 2 au from the Sun outbound from perihelion. We measure the acceleration of individual aggregates through a 2 h image series. Approximately 50 per cent of the aggregates are accelerated away from the nucleus, and 50 per cent towards it, and likewise towards either horizontal direction. The accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. At least 10 per cent of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the Northern hemisphere. The rocket force plays a crucial role in pushing these aggregates back towards the surface. Our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2 km) of a comet, where gas drag is still significant.
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| 16. |
- Deshapriya, J. D. P., et al.
(författare)
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Exposed bright features on the comet 67P/Churyumov-Gerasimenko : distribution and evolution
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 613
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Tidskriftsartikel (refereegranskat)abstract
- Context. Since its arrival at the comet 67P/Churyumov-Gerasimenko in August 2014, the Rosetta spacecraft followed the comet as it went past the perihelion and beyond until September 2016. During this time there were many scientific instruments operating on board Rosetta to study the comet and its evolution in unprecedented detail. In this context, our study focusses on the distribution and evolution of exposed bright features that have been observed by OSIRIS, which is the scientific imaging instrument aboard Rosetta. Aims. We envisage investigating various morphologies of exposed bright features and the mechanisms that triggered their appearance. Methods. We co-registered multi-filter observations of OSIRIS images that are available in reflectance. The Lommel-Seeliger disk function was used to correct for the illumination conditions and the resulting colour cubes were used to perform spectrophotometric analyses on regions of interest. Results. We present a catalogue of 57 exposed bright features observed on the nucleus of the comet, all of which are attributed to the presence of H2O ice on the comet. Furthermore, we categorise these patches under four different morphologies and present geometric albedos for each category. Conclusions. Although the nucleus of 67P/Churyumov-Gerasimenko appears to be dark in general, there are localised H2O ice sources on the comet. Cometary activity escalates towards the perihelion passage and reveals such volatile ices. We propose that isolated H2O ice patches found in smooth terrains in regions, such as Imhotep, Bes, and Hapi, result from frost as an aftermath of the cessation of the diurnal water cycle on the comet as it recedes from perihelion. Upon the comet's return to perihelion, such patches are revealed when sublimation-driven erosion removes the thin dust layers that got deposited earlier. More powerful activity sources such as cometary outbursts are capable of revealing much fresher, less contaminated H2O ice that is preserved with consolidated cometary material, as observed on exposed patches resting on boulders. This is corroborated by our albedo calculations that attribute higher albedos for bright features with formations related to outbursts.
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| 17. |
- Deshapriya, J. D. P., et al.
(författare)
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Spectrophotometry of the Khonsu region on the comet 67P/Churyumov-Gerasimenko using OSIRIS instrument images
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462, s. S274-S286
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Tidskriftsartikel (refereegranskat)abstract
- Our work focuses on the spectrophotometric analysis of selected terrain and bright patches in the Khonsu region on the comet 67P/Churyumov-Gerasimenko. Despite the variety of geological features, their spectrophotometric properties appear to indicate a similar composition. It is noticeable that the smooth areas in Khonsu possess similar spectrophotometric behaviour to some other regions of the comet. We observed bright patches on Khonsu with an estimation of >40 per cent of normal albedo and suggest that they are associated with H2O ice. One of the studied bright patches has been observed to exist on the surface for more than 5 months without a major decay of its size, implying the existence of potential sub-surface icy layers. Its location may be correlated with a cometary outburst during the perihelion passage of the comet in 2015 August, and we interpret it to have triggered the surface modifications necessary to unearth the stratified icy layers beneath the surface. A boulder analysis on Khonsu leads to a power-law index of -3.1 + 0.2/-0.3 suggesting a boulder formation, shaped by varying geological processes for different morphological units.
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| 18. |
- Feller, C., et al.
(författare)
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Decimetre-scaled spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462, s. S287-S303
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of the photometric and spectrophotometric properties of the 67P/Churyumov-Gerasimenko nucleus derived with the Optical, Spectroscopic and Infrared Remote Imaging System instrument during the closest fly-by over the comet, which took place on 2015 February 14 at a distance of similar to 6 km from the surface. Several images covering the 0 degrees-33 degrees. phase angle range were acquired, and the spatial resolution achieved was 11 cm pixel(-1). The flown-by region is located on the big lobe of the comet, near the borders of the Ash, Apis and Imhotep regions. Our analysis shows that this region features local heterogeneities at the decimetre scale. We observed difference of reflectance up to 40 per cent between bright spots and sombre regions, and spectral slope variations up to 50 per cent. The spectral reddening effect observed globally on the comet surface by Fornasier et al. (2015) is also observed locally on this region, but with a less steep behaviour. We note that numerous metre-sized boulders, which exhibit a smaller opposition effect, also appear spectrally redder than their surroundings. In this region, we found no evidence linking observed bright spots to exposed water-ice-rich material. We fitted our data set using the Hapke 2008 photometric model. The region overflown is globally as dark as the whole nucleus (geometric albedo of 6.8 per cent) and it has a high porosity value in the uppermost layers (86 per cent). These results of the photometric analysis at a decimetre scale indicate that the photometric properties of the flown-by region are similar to those previously found for the whole nucleus.
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| 19. |
- Groussin, O., et al.
(författare)
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Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 583
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths). Methods. We computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface (Imhotep, Ash, Seth, Hathor, and Agilkia), using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, and Philae's footprint) and mechanical considerations. Results. The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0-20 degrees) are covered by a fine material and contain a few large (>10 m) and isolated boulders; ii) intermediate-slope terrains (20-45 degrees) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from <1m to 10m for the majority of them; and iii) high-slope terrains (45-90 degrees) are cliffs that expose a consolidated material and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3-15 Pa (upper limit of 150 Pa), 4-30 Pa for the shear strength of fine surface materials and boulders, and 30-150 Pa for the compressive strength of overhangs (upper limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.
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| 20. |
- Guettler, C., et al.
(författare)
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Characterization of dust aggregates in the vicinity of the Rosetta spacecraft
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S312-S320
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Tidskriftsartikel (refereegranskat)abstract
- In a Rosetta/OSIRIS imaging activity in 2015 June, we have observed the dynamic motion of particles close to the spacecraft. Due to the focal setting of the OSIRIS wide angle camera, these particles were blurred, which can be used to measure their distances to the spacecraft. We detected 109 dust aggregates over a 130 min long sequence, and find that their sizes are around a millimetre and their distances cluster between 2 and 40 m from the spacecraft. Their number densities are about a factor 10 higher than expected for the overall coma and highly fluctuating. Their velocities are small compared to the spacecraft orbital motion and directed away from the spacecraft, towards the comet. From this we conclude that they have interacted with the spacecraft and assess three possible scenarios. In the likeliest of the three scenarios, centimetre-sized aggregates collide with the spacecraft and we would observe the fragments. Ablation of a dust layer on the spacecraft's z panel (remote instrument viewing direction) when rotated towards the Sun is a reasonable alternative. We could also measure an acceleration for a subset of 18 aggregates, which is directed away from the Sun and can be explain by a rocket effect, which requires a minimum ice fraction of the order of 0.1 per cent.
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| 21. |
- Hu, X., et al.
(författare)
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Thermal modelling of water activity on comet 67P/Churyumov-Gerasimenko with global dust mantle and plural dust-to-ice ratio
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S295-S311
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Tidskriftsartikel (refereegranskat)abstract
- We perform a thermo-physical analysis on water activity of comet 67P/Churyumov-Gerasimenko (67P). The sublimation of water is assumed to occur from beneath a global, desiccated dust mantle over the irregular-shaped nucleus. The concept of two thermal models, the recipe of model formulation and the strategy of application to comet 67P are described. For an accurate and efficient evaluation of energy input by insolation and self-heating over the nucleus, a Landscape data base is devised based on polyhedral shape models of the nucleus. We apply the thermal models to investigate the impact of certain parameters of nucleus properties on water production. It is found that the measured water production of 67P can be overall attributed to sublimation of water ice with a mass abundance of a few to 10 per cent beneath a uniform dust mantle of several millimetres to one centimetre in thickness. Insofar as 67P is concerned, we argue against the necessity to invoke assumptions on localized water activity, or on the distinction of active/dormant surface areas.
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| 22. |
- Ott, T., et al.
(författare)
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Dust mass distribution around comet 67P/Churyumov-Gerasimenko determined via parallax measurements using Rosetta's OSIRIS cameras
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469, s. S276-S284
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Tidskriftsartikel (refereegranskat)abstract
- The OSIRIS (optical, spectroscopic and infrared remote imaging system) instrument on board the ESA Rosetta spacecraft collected data of 67P/Churyumov-Gerasimenko for over 2 yr. OSIRIS consists of two cameras, a Narrow Angle Camera and a Wide Angle Camera. For specific imaging sequences related to the observation of dust aggregates in 67P's coma, the two cameras were operating simultaneously. The two cameras are mounted 0.7 m apart from each other, as a result this baseline yields a parallax shift of the apparent particle trails on the analysed images directly proportional to their distance. Thanks to such shifts, the distance between observed dust aggregates and the spacecraft was determined. This method works for particles closer than 6000 m to the spacecraft and requires very few assumptions. We found over 250 particles in a suitable distance range with sizes of some centimetres, masses in the range of 10(-6)-10(2) kg and a mean velocity of about 2.4 m s(-1) relative to the nucleus. Furthermore, the spectral slope was analysed showing a decrease in the median spectral slope of the particles with time. The further a particle is from the spacecraft the fainter is its signal. For this reason, this was counterbalanced by a debiasing. Moreover, the dust mass-loss rate of the nucleus could be computed as well as the Af rho of the comet around perihelion. The summed-up dust mass-loss rate for the mass bins 10(-4)-10(2) kg is almost 8300 kg s(-1).
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| 23. |
- Pajola, M., et al.
(författare)
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The Agilkia boulders/pebbles size-frequency distributions : OSIRIS and ROLIS joint observations of 67P surface
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462, s. S242-S252
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Tidskriftsartikel (refereegranskat)abstract
- By using the images acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) and ROLIS (ROsetta Lander Imaging System) cameras, we derive the size-frequency distribution (SFD) of cometary pebbles and boulders covering the size range 0.05-30.0 m on the Agilkia landing site. The global SFD measured on OSIRIS images, reflects the different properties of the multiple morphological units present on Agilkia, combined with selection effects related to lifting, transport and redeposition. Contrarily, the different ROLIS SFD derived on the smooth and rough units may be related to their different regolith thickness present on Agilkia. In the thicker, smoother layer, ROLIS mainly measures the SFD of the airfall population which almost completely obliterates the signature of underlying boulders up to a size of the order of 1 m. This is well matched by the power-law index derived analysing coma particles identified by the grain analyser Grain Impact Analyser and Dust Accumulator. This result confirms the important blanketing dynamism of Agilkia. The steeper SFD observed in rough terrains from 0.4 to 2 m could point out intrinsic differences between northern and southern dust size distributions, or it may suggest that the underlying boulders 'peek through' the thinner airfall layer in the rough terrain, thereby producing the observed excess in the decimetre size range. Eventually, the OSIRIS SFD performed on the Philae landing unit may be due to water sublimation from a static population of boulders, affecting smaller boulders before the bigger ones, thus shallowing the original SFD.
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| 24. |
- Vincent, J. -B, et al.
(författare)
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Constraints on cometary surface evolution derived from a statistical analysis of 67P's topography
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469, s. S329-S338
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Tidskriftsartikel (refereegranskat)abstract
- We present a statistical analysis of the distribution of large-scale topographic features on comet 67P/Churyumov-Gerasimenko. We observe that the cumulative cliff height distribution across the surface follows a power law with a slope equal to -1.69 +/- 0.02. When this distribution is studied independently for each region, we find a good correlation between the slope of the power law and the orbital erosion rate of the surface. For instance, the Northern hemisphere topography is dominated by structures on the 100 m scale, while the Southern hemisphere topography, illuminated at perihelion, is dominated by 10 m scale terrain features. Our study suggests that the current size of a cliff is controlled not only by material cohesion but also by the dominant erosional process in each region. This observation can be generalized to other comets, where we argue that primitive nuclei are characterized by the presence of large cliffs with a cumulative height-power index equal to or above -1.5, while older, eroded cometary surfaces have a power index equal to or below -2.3. In effect, our model shows that a measure of the topography provides a quantitative assessment of a comet's erosional history, that is, its evolutionary age.
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| 25. |
- Bodewits, D., et al.
(författare)
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Changes in the physical environment of the inner coma of 67p/churyumov-gerasimenko with decreasing heliocentric distance
- 2016
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Ingår i: Astronomical Journal. - : IOP PUBLISHING LTD. - 0004-6256 .- 1538-3881. ; 152:5
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Tidskriftsartikel (refereegranskat)abstract
- The Wide Angle Camera of the OSIRIS instrument on board the Rosetta spacecraft is equipped with several narrow-band filters that are centered on the emission lines and bands of various fragment species. These are used to determine the evolution of the production and spatial distribution of the gas in the inner coma of comet 67P with time and heliocentric distance, here between 2.6 and 1.3 au pre-perihelion. Our observations indicate that the emission observed in the OH, OI, CN, NH, and NH2 filters is mostly produced by dissociative electron impact excitation of different parent species. We conclude that CO2 rather than H2O is a significant source of the [OI] 630 nm emission. A strong plume-like feature observed in the CN and OI filters is present throughout our observations. This plume is not present in OH emission and indicates a local enhancement of the CO2/H2O ratio by as much as a factor of 3. We observed a sudden decrease in intensity levels after 2015 March, which we attribute to decreased electron temperatures in the first few kilometers above the surface of the nucleus.
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