arXiv subjects

D. Souami

Publications and source records attributed to D. Souami.

25 records · Page 2Linked to original sources

arXiv2020

A multi-chord stellar occultation by the large trans-Neptunian object (174567) Varda

We present results from the first recorded stellar occultation by the large trans-Neptunian object (174567) Varda that was observed on September 10$^{\rm th}$, 2018. Varda belongs to the high-inclination dynamically excited population, and has a satellite, Ilmarë, which is half the size of Varda. We determine the size and albedo of Varda and constrain its 3D shape and density. Thirteen different sites in the USA monitored the event, five of which detected an occultation by the main body. A best-fitting ellipse to the occultation chords provides the instantaneous limb of the body, from which the geometric albedo is computed. The size and shape of Varda are evaluated, and its bulk density is constrained, using Varda's mass known from previous works. The best-fitting elliptical limb has semi-major (equatorial) axis of $(383 \pm 3)$km and an apparent oblateness $0.066\pm0.047$ corresponding to an apparent area-equivalent radius $R'_{\rm equiv}= (370\pm7)$km and geometric albedo $p_v=0.099\pm 0.002 $ assuming a visual absolute magnitude $H_V=3.81\pm0.01$. Using three possible rotational periods for the body (4.76h, 5.91h, and 7.87h), we derive corresponding MacLaurin solutions. Furthermore, given the low-amplitude ($0.06\pm0.01$) mag of the single-peaked rotational light-curve for the aforementioned periods, we consider the double periods. For the 5.91h period (the most probable) and its double (11.82h), we find bulk densities and true oblateness of $ρ=(1.78\pm0.06)$ g cm$^{-3}$, $ε=0.235\pm0.050$ and $ρ=(1.23\pm0.04)$ g cm$^{-3}$, $ε=0.080\pm0.049$. However, it must be noted that the other solutions cannot be excluded just yet.

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arXiv2020

Potential asteroid discoveries by the ESA Gaia mission: Results from follow-up observations

Since July 2014, the ESA Gaia mission has been surveying the entire sky down to magnitude 20.7 in the visible. In addition to the millions of stars, thousands of Solar System Objects (SSOs) are observed daily. By comparing their positions to those of known objects, a daily processing pipeline filters known objects from potential discoveries. However, owing to Gaia's specific scanning law designed for stars, potential newly discovered moving objects are characterized by very few observations, acquired over a limited time. This aspect was recognized early in the design of the Gaia data processing. A daily processing pipeline dedicated to these candidate discoveries was set up to release calls for observations to a network of ground-based telescopes. Their aim is to acquire follow-up astrometry and to characterize these objects. From the astrometry measured by Gaia, preliminary orbital solutions are determined, allowing to predict the position of these potentially new discovered objects in the sky accounting for the large parallax between Gaia and the Earth (separated by 0.01 au). A specific task within the Gaia Consortium has been responsible for the distribution of requests for follow-up observations of potential Gaia SSO discoveries. Since late 2016, these calls for observations (called alerts) are published daily via a Web interface, freely available to anyone world-wide. Between November 2016 and July 2020, over 1700 alerts have been published, leading to the successful recovery of more than 200 objects. Among those, six have provisional designation assigned with the Gaia observations, the others being previously known objects with poorly characterized orbits, precluding identification at the time of Gaia observations. There is a clear trend for objects with a high inclination to be unidentified, revealing a clear bias in the current census of SSOs against high inclination populations.

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arXiv2020

On the local and global properties of the gravitational spheres of influence

We revisit the concept of sphere of gravitational activity, to which we give both a geometrical and physical meaning. This study aims to refine this concept in a much broader context that could, for instance, be applied to exo-planetary problems (in a Galactic stellar disc-Star-Planets system) to define a first order "border" of a planetary system. The methods used in this paper rely on classical Celestial Mechanics and develop the equations of motion in the framework of the 3-body problem (e.g. Star-Planet-Satellite System). We start with the basic definition of planet's sphere of activity as the region of space in which it is feasible to assume a planet as the central body and the Sun as the perturbing body when computing perturbations of the satellite's motion. We then investigate the geometrical properties and physical meaning of the ratios of Solar accelerations (central and perturbing) and planetary accelerations (central and perturbing), and the boundaries they define. We clearly distinguish throughout the paper between the sphere of activity, the Chebotarev sphere (a particular case of the sphere of activity), Laplace sphere, and the Hill sphere. The last two are often wrongfully thought to be one and the same. Furthermore, taking a closer look and comparing the ratio of the star's accelerations (central/perturbing) to that of the planetary acceleration (central/perturbing) as a function of the planeto-centric distance, we have identified different dynamical regimes which are presented in the semi-analytical analysis.

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arXiv2020

The GBOT Asteroid Survey (First years: Jan. 2015 - May 2018)

The GBOT group is in charge of the Ground Based Optical Tracking of the Gaia satellite. In concrete terms, since the launch of Gaia, our task is to take every night, using ground based medium-class telescopes, short sequences of $10$ or $20$ images of the Gaia satellite close to its meridian transit. For this purpose, we mainly use the VLT Survey Telescope and the Liverpool Telescope. In these images, taken close to the Sun's opposition - since Gaia is in L$_2$ - we observe many asteroids: between $30$ and $100$ asteroids every night, up to magnitude $22$. In order to extract the astrometric positions as well as the magnitudes of these asteroids, we have developed semi-automatic methods, strategies and tools tailored explicitly for this daily task. In only three and a half years of operation, this system has allowed us to send to the Minor Planet Center the position and the photometry for about $20,000$ asteroids, amongst which $9,000$ new objects. Here we describe all the aspects of the GBOT asteroid survey.

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arXiv2014

Dynamical evolution of V-type asteroids in the central main belt

V-type asteroids are associated with basaltic composition, and are supposed to be fragments of crust of differentiated objects. Most V-type asteroids in the main belt are found in the inner main belt, and are either current members of the Vesta dynamical family (Vestoids), or past members that drifted away. However, several V-type photometric candidates have been recently identified in the central and outer main belt. The origin of this large population of V-type objects is not well understood. Since it seems unlikely that Vestoids crossing the 3J:-1A mean-motion resonance with Jupiter could account for the whole population of V-type asteroids in the central and outer main belt, origin from local sources, such as the parent bodies of the Eunomia, and of the Merxia and Agnia asteroid families, has been proposed as an alternative mechanism. In this work we investigated the dynamical evolution of the V-type photometric candidates in the central main belt, under the effect of gravitational and non-gravitational forces. Our results show that dynamical evolution from the parent bodies of the Eunomia and Merxia/Agnia families on timescales of 2 Byr or more could be responsible for the current orbital location of most of the low-inclined V-type asteroids.

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arXiv2013

Dynamical evolution and chronology of the Hygiea asteroid family

The asteroid (10) Hygiea is the fourth largest asteroid of the Main Belt, by volume and mass, and it is the largest member of its own family. Previous works investigated the long-term effects of close encounters with (10) Hygiea of asteroids in the orbital region of the family, and analyzed the taxonomical and dynamical properties of members of this family. In this paper we apply the high-quality SDSS-MOC4 taxonomic scheme of DeMeo and Carry (2013) to members of the Hygiea family core and halo, we obtain an estimate of the minimum time and number of encounter necessary to obtain a $3σ$ (or 99.7%) compatible frequency distribution function of changes in proper $a$ caused by close encounters with (10) Hygiea, we study the behavior of asteroids near secular resonance configurations, in the presence and absence of the Yarkovsky force, and obtain a first estimate of the age of the family based on orbital diffusion by the Yarkovsky and YORP effects with two methods. The Hygiea family is at least 2 Byr old, with an estimated age of $T = 3200^{+380}_{-120}$ Myr and a relatively large initial ejection velocity field, according to the approach of Vokrouhlický et al. (2006a, b). Surprisingly, we found that the family age can be shortened by $\simeq$ 25% if the dynamical mobility caused by close encounters with (10) Hygiea is also accounted for, which opens interesting new research lines for the dynamical evolution of families associated with massive bodies. In our taxonomical analysis of the Hygiea asteroid family, we also identified a new V-type candidate: the asteroid (177904) (2005 SV5). If confirmed, this could be the fourth V-type object ever to be identified in the outer main belt.

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arXiv2013

A multi-domain approach to asteroid families identification

Previous works have identified families halos by an analysis in proper elements domains, or by using Sloan Digital Sky Survey-Moving Object Catalog data, fourth release (SDSS-MOC4) multi-band photometry to infer the asteroid taxonomy, or by a combination of the two methods. The limited number of asteroids for which geometric albedo was known until recently discouraged in the past the extensive use of this additional parameter, which is however of great importance in identifying an asteroid taxonomy. The new availability of geometric albedo data from the Wide-field Infrared Survey Explorer (WISE) mission for about 100,000 asteroids significantly increased the sample of objects for which such information, with some errors, is now known. In this work we proposed a new method to identify families halos in a multi-domain space composed by proper elements, SDSS-MOC4 (a*,i-z) colors, and WISE geometric albedo for the whole main belt (and the Hungaria and Cybele orbital regions). Assuming that most families were created by the breakup of an undifferentiated parent body, they are expected to be homogeneous in colors and albedo. The new method is quite effective in determining objects belonging to a family halo, with low percentages of likely interlopers, and results that are quite consistent in term of taxonomy and geometric albedo of the halo members.

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