Les graffeurs de Montréal : penser la mobilité dans la construction du social

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal

Asteroid (16) Psyche’s primordial shape: A possible Jacobi ellipsoid

Context. Asteroid (16) Psyche is the largest M-type asteroid in the main belt and the target of the NASA Psyche mission. It is also the only asteroid of this size (D >  200 km) known to be metal rich. Although various hypotheses have been proposed to explain the rather unique physical properties of this asteroid, a perfect understanding of its formation and bulk composition is still missing. Aims. We aim to refine the shape and bulk density of (16) Psyche and to perform a thorough analysis of its shape to better constrain possible formation scenarios and the structure of its interior. Methods. We obtained disk-resolved VLT/SPHERE/ZIMPOL images acquired within our ESO large program (ID 199.C-0074), which complement similar data obtained in 2018. Both data sets offer a complete coverage of Psyche’s surface. These images were used to reconstruct the three-dimensional (3D) shape of Psyche with two independent shape modeling algorithms (MPCD and ADAM). A shape analysis was subsequently performed, including a comparison with equilibrium figures and the identification of mass deficit regions. Results. Our 3D shape along with existing mass estimates imply a density of 4.20  ±  0.60 g cm−3, which is so far the highest for a solar system object following the four telluric planets. Furthermore, the shape of Psyche presents small deviations from an ellipsoid, that is, prominently three large depressions along its equator. The flatness and density of Psyche are compatible with a formation at hydrostatic equilibrium as a Jacobi ellipsoid with a shorter rotation period of ∼3h. Later impacts may have slowed down Psyche’s rotation, which is currently ∼4.2 h, while also creating the imaged depressions. Conclusions. Our results open the possibility that Psyche acquired its primordial shape either after a giant impact while its interior was already frozen or while its interior was still molten owing to the decay of the short-lived radionuclide 26Al.Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 199.C-0074 (principal investigator: P. Vernazza). P. Vernazza, A. Drouard, M. Ferrais and B. Carry were supported by CNRS/INSU/PNP. J.H. and J.D. were supported by grant 18-09470S of the Czech Science Foundation and by the Charles University Research Programme no. UNCE/SCI/023. E.J. is F.R.S.-FNRS Senior Research Associate. The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE)

A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea

(10) Hygiea is the fourth largest main belt asteroid and the only known asteroid whose surface composition appears similar to that of the dwarf planet (1) Ceres1,2, suggesting a similar origin for these two objects. Hygiea suffered a giant impact more than 2 Gyr ago3 that is at the origin of one of the largest asteroid families. However, Hygeia has never been observed with sufficiently high resolution to resolve the details of its surface or to constrain its size and shape. Here, we report high-angular-resolution imaging observations of Hygiea with the VLT/SPHERE instrument (~20 mas at 600 nm) that reveal a basin-free nearly spherical shape with a volume-equivalent radius of 217 ± 7 km, implying a density of 1,944 ± 250 kg m−3 to 1σ. In addition, we have determined a new rotation period for Hygiea of ~13.8 h, which is half the currently accepted value. Numerical simulations of the family-forming event show that Hygiea’s spherical shape and family can be explained by a collision with a large projectile (diameter ~75–150 km). By comparing Hygiea’s sphericity with that of other Solar System objects, it appears that Hygiea is nearly as spherical as Ceres, opening up the possibility for this object to be reclassified as a dwarf planet.P.V., A.D. and B.C. were supported by CNRS/INSU/PNP. M.Brož was supported by grant 18-04514J of the Czech Science Foundation. J.H. and J.D. were supported by grant 18-09470S of the Czech Science Foundation and by the Charles University Research Programme no. UNCE/SCI/023. This project has received funding from the European Union’s Horizon 2020 research and innovation programmes under grant agreement nos 730890 and 687378. This material reflects only the authors’ views, and the European Commission is not liable for any use that may be made of the information contained herein. TRAPPIST-North is a project funded by the University of Liège, in collaboration with Cadi Ayyad University of Marrakech (Morocco). TRAPPIST-South is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant FRFC 2.5.594.09.F. E.J. and M.G. are F.R.S.-FNRS Senior Research Associates

Des murs aux trains. Des graffiteurs de Montréal

Basé sur une recherche ethnographique, cet article traite de l’implication de jeunes dans le monde du graffiti, de l’adolescence à l’âge adulte. Plutôt que de simplement considérer cette pratique artistique comme un acte de vandalisme, ce texte prend en compte la maturation possible au sein de ce milieu afin de mieux cerner les motivations des graffiteurs et la cohérence de cette culture.Based on an ethnographic research, this article deals with youth involved in the world of graffiti from teenagehood to adulthood. Rather than considering this artistic practice only as an act of vandalism, this text focuses on the possible maturation inherent to graffiti in order to better understand the motivations of those young artists and the coherence of this culture

(16) Psyche: A Mesosiderite-like Asteroid With No Moons?

International audienceAsteroid (16) Psyche is the target of the NASA Psyche mission. It is considered as one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such association is however challenged by both its near- and mid-infrared spectral properties (e.g. Hardersen et al. Icarus 175, 2005; Takir et al. AJ, 153, 2017; Landsman et al. Icarus, 304, 2018). We observed (16) Psyche with ESO VLT SPHERE/ZIMPOL as part of our large program (ID 199.C-0074, PI Vernazza) between April 24 and June 6 2018. We use the high-angular resolution of these observations to reconstruct the 3D shape model of Psyche. When combined with the most recent estimates of its mass, the volume that we derive led to a bulk density of 3.99 ± 0.26 g.cm<SUP>-3</SUP> for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites (˜7.8 g.cm<SUP>-3</SUP>), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ˜4.25 g.cm<SUP>-3</SUP>). Although our observations only covered the northern hemisphere of Psyche, they reveal the presence of two peculiar units in the front views, namely one low and one high brightness regions nicknamed Panthia and Meroe. Meroe unit is about 7% brighter than the surrounding region, whereas the Panthia unit is 8% fainter. Panthia is a depression with a width of 90 km and a depth of 10 km. We did not detect any moons around Psyche and estimate the minimum radius of a moon to be detected around Psyche to 730 ± 100 m at 150 km (so 0.2% x R<SUB>Hill</SUB>) of the primary and 400 ± 100 m at 2000 km from the primary corresponding to 3% x R<SUB>Hill</SUB>) where most of the satellites of >100-km asteroids have been seen so far (Yang et al. AJL 820:L35, 2016). Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to the initial hypothesis by Davis et al. (1999) that Psyche could be a plausible candidate parent body for mesosiderites (Viikinkoski et al. submitted to A&A, 2018). This material is based upon work partially supported by the National Science Foundation under Grant No 1743015

(16) Psyche: A Mesosiderite-like Asteroid With No Moons?

International audienceAsteroid (16) Psyche is the target of the NASA Psyche mission. It is considered as one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such association is however challenged by both its near- and mid-infrared spectral properties (e.g. Hardersen et al. Icarus 175, 2005; Takir et al. AJ, 153, 2017; Landsman et al. Icarus, 304, 2018). We observed (16) Psyche with ESO VLT SPHERE/ZIMPOL as part of our large program (ID 199.C-0074, PI Vernazza) between April 24 and June 6 2018. We use the high-angular resolution of these observations to reconstruct the 3D shape model of Psyche. When combined with the most recent estimates of its mass, the volume that we derive led to a bulk density of 3.99 ± 0.26 g.cm<SUP>-3</SUP> for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites (˜7.8 g.cm<SUP>-3</SUP>), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ˜4.25 g.cm<SUP>-3</SUP>). Although our observations only covered the northern hemisphere of Psyche, they reveal the presence of two peculiar units in the front views, namely one low and one high brightness regions nicknamed Panthia and Meroe. Meroe unit is about 7% brighter than the surrounding region, whereas the Panthia unit is 8% fainter. Panthia is a depression with a width of 90 km and a depth of 10 km. We did not detect any moons around Psyche and estimate the minimum radius of a moon to be detected around Psyche to 730 ± 100 m at 150 km (so 0.2% x R<SUB>Hill</SUB>) of the primary and 400 ± 100 m at 2000 km from the primary corresponding to 3% x R<SUB>Hill</SUB>) where most of the satellites of >100-km asteroids have been seen so far (Yang et al. AJL 820:L35, 2016). Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to the initial hypothesis by Davis et al. (1999) that Psyche could be a plausible candidate parent body for mesosiderites (Viikinkoski et al. submitted to A&A, 2018). This material is based upon work partially supported by the National Science Foundation under Grant No 1743015

(16) Psyche: A Mesosiderite-like Asteroid With No Moons?

International audienceAsteroid (16) Psyche is the target of the NASA Psyche mission. It is considered as one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such association is however challenged by both its near- and mid-infrared spectral properties (e.g. Hardersen et al. Icarus 175, 2005; Takir et al. AJ, 153, 2017; Landsman et al. Icarus, 304, 2018). We observed (16) Psyche with ESO VLT SPHERE/ZIMPOL as part of our large program (ID 199.C-0074, PI Vernazza) between April 24 and June 6 2018. We use the high-angular resolution of these observations to reconstruct the 3D shape model of Psyche. When combined with the most recent estimates of its mass, the volume that we derive led to a bulk density of 3.99 ± 0.26 g.cm<SUP>-3</SUP> for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites (˜7.8 g.cm<SUP>-3</SUP>), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ˜4.25 g.cm<SUP>-3</SUP>). Although our observations only covered the northern hemisphere of Psyche, they reveal the presence of two peculiar units in the front views, namely one low and one high brightness regions nicknamed Panthia and Meroe. Meroe unit is about 7% brighter than the surrounding region, whereas the Panthia unit is 8% fainter. Panthia is a depression with a width of 90 km and a depth of 10 km. We did not detect any moons around Psyche and estimate the minimum radius of a moon to be detected around Psyche to 730 ± 100 m at 150 km (so 0.2% x R<SUB>Hill</SUB>) of the primary and 400 ± 100 m at 2000 km from the primary corresponding to 3% x R<SUB>Hill</SUB>) where most of the satellites of >100-km asteroids have been seen so far (Yang et al. AJL 820:L35, 2016). Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to the initial hypothesis by Davis et al. (1999) that Psyche could be a plausible candidate parent body for mesosiderites (Viikinkoski et al. submitted to A&A, 2018). This material is based upon work partially supported by the National Science Foundation under Grant No 1743015

(16) Psyche: A Mesosiderite-like Asteroid With No Moons?

International audienceAsteroid (16) Psyche is the target of the NASA Psyche mission. It is considered as one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such association is however challenged by both its near- and mid-infrared spectral properties (e.g. Hardersen et al. Icarus 175, 2005; Takir et al. AJ, 153, 2017; Landsman et al. Icarus, 304, 2018). We observed (16) Psyche with ESO VLT SPHERE/ZIMPOL as part of our large program (ID 199.C-0074, PI Vernazza) between April 24 and June 6 2018. We use the high-angular resolution of these observations to reconstruct the 3D shape model of Psyche. When combined with the most recent estimates of its mass, the volume that we derive led to a bulk density of 3.99 ± 0.26 g.cm<SUP>-3</SUP> for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites (˜7.8 g.cm<SUP>-3</SUP>), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ˜4.25 g.cm<SUP>-3</SUP>). Although our observations only covered the northern hemisphere of Psyche, they reveal the presence of two peculiar units in the front views, namely one low and one high brightness regions nicknamed Panthia and Meroe. Meroe unit is about 7% brighter than the surrounding region, whereas the Panthia unit is 8% fainter. Panthia is a depression with a width of 90 km and a depth of 10 km. We did not detect any moons around Psyche and estimate the minimum radius of a moon to be detected around Psyche to 730 ± 100 m at 150 km (so 0.2% x R<SUB>Hill</SUB>) of the primary and 400 ± 100 m at 2000 km from the primary corresponding to 3% x R<SUB>Hill</SUB>) where most of the satellites of >100-km asteroids have been seen so far (Yang et al. AJL 820:L35, 2016). Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to the initial hypothesis by Davis et al. (1999) that Psyche could be a plausible candidate parent body for mesosiderites (Viikinkoski et al. submitted to A&A, 2018). This material is based upon work partially supported by the National Science Foundation under Grant No 1743015

(16) Psyche: A mesosiderite-like asteroid?

International audienceContext. Asteroid (16) Psyche is the target of the NASA Psyche mission. It is considered one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such an association is however challenged by both its near- and mid-infrared spectral properties and the reported estimates of its density. Aims: Here, we aim to refine the density of (16) Psyche to set further constraints on its bulk composition and determine its potential meteoritic analog. Methods: We observed (16) Psyche with ESO VLT/SPHERE/ZIMPOL as part of our large program (ID 199.C-0074). We used the high angular resolution of these observations to refine Psyche's three-dimensional (3D) shape model and subsequently its density when combined with the most recent mass estimates. In addition, we searched for potential companions around the asteroid. Results: We derived a bulk density of 3.99 ± 0.26 g cm-3 for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites (˜7.8 g cm-3), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ˜4.25 g cm-3). In addition, we found no satellite in our images and set an upper limit on the diameter of any non-detected satellite of 1460 ± 200 m at 150 km from Psyche (0.2% × RHill, the Hill radius) and 800 ± 200 m at 2000 km (3% × RHill). Conclusions: Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to a long-published initial hypothesis that Psyche could be a plausible candidate parent body for mesosiderites. Based on observations made with 1) ESO Telescopes at the La Silla Paranal Observatory under programs 086.C-0785 (PI Carry) and 199.C-0074 (PI Vernazza); and 2) the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.Tables A1 and A2 and reduced images are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/L