arXiv subjects

D. Marshall

Publications and source records attributed to D. Marshall.

3 recordsLinked to original sources

arXiv2019

The thermal, mechanical, structural, and dielectric properties of cometary nuclei after Rosetta

The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasizing the progress made during the Rosetta mission. Comets have a low density of 480 $\pm$ 220 kg m-3 and a low permittivity of 1.9 - 2.0, consistent with a high porosity of 70 - 80 %, are weak with a very low global tensile strength $<$100 Pa, and have a low bulk thermal inertia of 0 - 60 J K-1 m-2 s-1/2 that allowed them to preserve highly volatiles species (e.g. CO, CO2, CH4, N2) into their interior since their formation. As revealed by 67P/Churyumov-Gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. The broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. The properties of the top layer (the first meter) are not representative of that of the bulk nucleus. More globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67P small lobe.

astro-ph.EP↗

arXiv2019

3D analysis of spatial resolution of MIRO/Rosetta measurements at 67P/CG

The MIRO instrument's remote sensing capability is integral in constraining water density, temperature and velocity fields in the coma of 67P/Churyumov-Gersimenko. Our aim is to quantify the contribution to the water density from all facets inside and outside the field-of-view (FOV) of MIRO, in both, nadir and limb geometries. This information is crucial for understanding the MIRO derived coma production rates and their relation to the nucleus characteristics, and inherent spatial resolution of the data. This study relies on a detailed 3D nucleus shape model, illumination conditions and the pointing information of the viewing geometry. With these parameters, we can evaluate the relative contribution of water density originating from facets directly inside the MIRO beam as well outside of the beam as a function of distance along the MIRO line-of-sight. We also calculate the ratio of in-beam versus out-of-beam number density. We demonstrate that despite the rather small MIRO field-of-view there is only a small fraction of molecules that originate from facets within the MIRO beam. This is true for nadir, but also translated into the limb observing geometry. The MIRO instrument cannot discriminate active from non-active regions directly from observations. This study also suggests that the beam averaged solar incidence angle, local time and mean normal vectors are not necessary related to molecules within the MIRO beam. These results also illustrate why the 1D spherical Haser model can be applied with relative success for analyzing the MIRO data (and generally any Rosetta measurements). The future possibilities of constraining gas activity distribution on the surface should use 3D codes extracting information from the MIRO spectral line shapes which contain additional information. The presented results are in fact applicable to all relevant instruments onboard Rosetta.

astro-ph.EP↗

arXiv2010

Mapping the column density and dust temperature structure of IRDCs with Herschel

Infrared dark clouds (IRDCs) are cold and dense reservoirs of gas potentially available to form stars. Many of these clouds are likely to be pristine structures representing the initial conditions for star formation. The study presented here aims to construct and analyze accurate column density and dust temperature maps of IRDCs by using the first Herschel data from the Hi-GAL galactic plane survey. These fundamental quantities, are essential for understanding processes such as fragmentation in the early stages of the formation of stars in molecular clouds. We have developed a simple pixel-by-pixel SED fitting method, which accounts for the background emission. By fitting a grey-body function at each position, we recover the spatial variations in both the dust column density and temperature within the IRDCs. This method is applied to a sample of 22 IRDCs exhibiting a range of angular sizes and peak column densities. Our analysis shows that the dust temperature decreases significantly within IRDCs, from background temperatures of 20-30 K to minimum temperatures of 8-15 K within the clouds, showing that dense molecular clouds are not isothermal. Temperature gradients have most likely an important impact on the fragmentation of IRDCs. Local temperature minima are strongly correlated with column density peaks, which in a few cases reach NH2 = 1 x 10^{23} cm^{-2}, identifying these clouds as candidate massive prestellar cores. Applying this technique to the full Hi-GAL data set will provide important constraints on the fragmentation and thermal properties of IRDCs, and help identify hundreds of massive prestellar core candidates.

astro-ph.GA↗