Formes d'astéroïdes et formation de satellites : rôle de la réaccumulation gravitationnelle

International audienceAsteroid shapes and satellites: role of gravitational reaccumulation. Following current evidences, it is widely accepted that many asteroids would be "gravitational aggregates", i.e. bodies lacking internal cohesion. They could mainly be originated during the catastrophic disruption of some parent bodies, through the gravitational reaccumulation of the resulting fragments. The same events produced the dynamical families that we observe. In this work we address the problem of the origin of shapes of gravitational aggregates, that could contain signatures of their origin. We use a N-body code to simulate the collapse of a cloud of fragments, with a variety of initial velocity distributions and total angular momentum. The fragments are treated as inhelastic spheres, that rapidly accumulate to form rotating aggregates. The resulting shapes and rotational properties are compared with theoretical predictions. The results show that only a precise category of shapes (flattened spheroids) are created via this mechanism. This may provide interesting constraints on the evolution of asteroid shapes, in particular for those with one or more satellites

A Direct Measurement of the Total Gas Column Density in Orion KL

The large number of high-J lines of C^(18)O available via the Herschel Space Observatory provide an unprecedented ability to model the total CO column density in hot cores. Using the emission from all the observed lines (up to J = 15-14), we sum the column densities in each individual level to obtain the total column after correcting for the population in the unobserved states. With additional knowledge of source size, V_(LSR), and line width, and both local thermodynamic equilibrium (LTE) and non-LTE modeling, we have determined the total C^(18)O column densities in the Extended Ridge, Outflow/Plateau, Compact Ridge, and Hot Core components of Orion KL to be 1.4 × 10^(16) cm^(–2), 3.5 × 10^(16) cm^(–2), 2.2 × 10^(16) cm^(–2), and 6.2 × 10^(16) cm^(–2), respectively. We also find that the C^(18)O/C^(17)O abundance ratio varies from 1.7 in the Outflow/Plateau, 2.3 in the Extended Ridge, 3.0 in the Hot Core, and to 4.1 in the Compact Ridge. This is in agreement with models in which regions with higher ultraviolet radiation fields selectively dissociate C^(17)O, although care must be taken when interpreting these numbers due to the size of the uncertainties in the C^(18)O/C^(17)O abundance ratio

Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the SMA

We aim at deriving the molecular abundances and temperatures of the hot molecular cores in the high-mass star-forming region NGC 6334I and consequently deriving their physical and astrochemical conditions. In the framework of the Herschel guaranteed time key program CHESS, NGC 6334I is investigated by using HIFI aboard the Herschel Space Observatory. A spectral line survey is carried out in the frequency range 480-1907 GHz, and auxiliary interferometric data from the SMA in the 230 GHz band provide spatial information for disentangling the different physical components contributing to the HIFI spectrum. The spectral lines are identified with the aid of former surveys and spectral line catalogs. The observed spectrum is then compared to a simulated synthetic spectrum with XCLASS, assuming local thermal equilibrium, and best fit parameters are derived using the model optimization package MAGIX. A total of 46 molecules are identified, with 31 isotopologues, resulting in about 4300 emission and absorption lines. High- energy levels of the dominant emitter methanol and vibrationally excited HCN are detected. The number of unidentified lines remains low with 75, or less than 2 percent of the lines detected. The modeling suggests that several spectral features need two or more components to be fitted properly. Other components could be assigned to cold foreground clouds or to outflows, most visible in the SiO emission. A chemical variation between the two embedded hot cores is found, with more N-bearing molecules identified in SMA1 and O-bearing molecules in SMA2. Spectral line surveys give powerful insights into the study of the interstellar medium. Different molecules trace different physical conditions like the inner hot core, the envelope, the outflows or the cold foreground clouds. The derived molecular abundances provide further constraints for astrochemical models.Comment: 30 pages including appendix, 49 figures, accepted for publication in Astronomy and Astrophysic

Water emission from the high-mass star-forming region IRAS 17233-3606. High water abundances at high velocities

We investigate the physical and chemical processes at work during the formation of a massive protostar based on the observation of water in an outflow from a very young object previously detected in H2 and SiO in the IRAS 17233-3606 region. We estimated the abundance of water to understand its chemistry, and to constrain the mass of the emitting outflow. We present new observations of shocked water obtained with the HIFI receiver onboard Herschel. We detected water at high velocities in a range similar to SiO. We self-consistently fitted these observations along with previous SiO data through a state-of-the-art, one-dimensional, stationary C-shock model. We found that a single model can explain the SiO and H2O emission in the red and blue wings of the spectra. Remarkably, one common area, similar to that found for H2 emission, fits both the SiO and H2O emission regions. This shock model subsequently allowed us to assess the shocked water column density, N(H2O)=1.2x10^{18} cm^{-2}, mass, M(H2O)=12.5 M_earth, and its maximum fractional abundance with respect to the total density, x(H2O)=1.4x10^{-4}. The corresponding water abundance in fractional column density units ranges between 2.5x10^{-5} and 1.2x10^{-5}, in agreement with recent results obtained in outflows from low- and high-mass young stellar objects.Comment: accepted for publication as a Letter in Astronomy and Astrophysic

Submillimeter continuum observations of Sagittarius B2 at subarcsecond spatial resolution

We report the first high spatial resolution submillimeter continuum observations of the Sagittarius B2 cloud complex using the Submillimeter Array (SMA). With the subarcsecond resolution provided by the SMA, the two massive star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact sources. In total, twelve submillimeter cores are identified in the Sgr B2(M) region, while only two components are observed in the Sgr B2(N) clump. The gas mass and column density are estimated from the dust continuum emission. We find that most of the cores have gas masses in excess of 100 M$_{\odot}$ and column densities above 10$^{25}$ cm$^{-2}$. The very fragmented appearance of Sgr B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well fitted by a Plummer density distribution. This would lead one to believe that in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms first in an homogeneous core, and the rest of the cluster forms subsequently in the then fragmenting structure.Comment: 4 pages, 2 figures, accepted by A&A letter

FIRI - a Far-Infrared Interferometer

Half of the energy ever emitted by stars and accreting objects comes to us in the FIR waveband and has yet to be properly explored. We propose a powerful Far-InfraRed Interferometer mission, FIRI, to carry out high-resolution imaging spectroscopy in the FIR. This key observational capability is essential to reveal how gas and dust evolve into stars and planets, how the first luminous objects in the Universe ignited, how galaxies formed, and when super-massive black holes grew. FIRI will disentangle the cosmic histories of star formation and accretion onto black holes and will trace the assembly and evolution of quiescent galaxies like our Milky Way. Perhaps most importantly, FIRI will observe all stages of planetary system formation and recognise Earth-like planets that may harbour life, via its ability to image the dust structures in planetary systems. It will thus address directly questions fundamental to our understanding of how the Universe has developed and evolved - the very questions posed by ESA's Cosmic Vision.Comment: Proposal developed by a large team of astronomers from Europe, USA and Canada and submitted to the European Space Agency as part of "Cosmic Vision 2015-2025

Herschel observations of EXtraordinary Sources: Analysis of the full Herschel/HIFI molecular line survey of Sagittarius B2(N)

A sensitive broadband molecular line survey of the Sagittarius B2(N) star-forming region has been obtained with the HIFI instrument on the Herschel Space Observatory, offering the first high-spectral resolution look at this well-studied source in a wavelength region largely inaccessible from the ground (625-157 um). From the roughly 8,000 spectral features in the survey, a total of 72 isotopologues arising from 44 different molecules have been identified, ranging from light hydrides to complex organics, and arising from a variety of environments from cold and diffuse to hot and dense gas. We present an LTE model to the spectral signatures of each molecule, constraining the source sizes for hot core species with complementary SMA interferometric observations, and assuming that molecules with related functional group composition are cospatial. For each molecule, a single model is given to fit all of the emission and absorption features of that species across the entire 480-1910 GHz spectral range, accounting for multiple temperature and velocity components when needed to describe the spectrum. As with other HIFI surveys toward massive star forming regions, methanol is found to contribute more integrated line intensity to the spectrum than any other species. We discuss the molecular abundances derived for the hot core, where the local thermodynamic equilibrium approximation is generally found to describe the spectrum well, in comparison to abundances derived for the same molecules in the Orion KL region from a similar HIFI survey.Comment: Accepted to ApJ. 64 pages, 14 figures. Truncated abstrac