693 research outputs found
The Snow Border
Context. The study of the snow line is an important topic in several domains
of astrophysics, and particularly for the evolution of proto-stellar
environments and the formation of planets. Aims. The formation of the first
layer of ice on carbon grains requires low temperatures compared to the
temperature of evaporation (T > 100 K). This asymmetry generates a zone in
which bare and icy dust grains coexist. Methods. We use Monte-Carlo simulations
to describe the formation time scales of ice mantles on bare grains in
protostellar disks and massive protostars environments. Then we analytically
describe these two systems in terms of grain populations subject to infall and
turbulence, and assume steady-state. Results. Our results show that there is an
extended region beyond the snow line where icy and bare grains can coexist, in
both proto-planetary disks and massive protostars. This zone is not negligible
compared to the total size of the objects: on the order of 0.4 AU for
proto-planetary disks and 5400 AU for high-mass protostars. Times to reach the
steady-state are respectively es- timated from 10^2 to 10^5 yr. Conclusions.
The presence of a zone, a so-called snow border, in which bare and icy grains
co- exist can have a major impact on our knowledge of protostellar
environments. From a theoretical point of view, the progression of icy grains
to bare grains as the temperature increases, could be a realistic way to model
hot cores and hot corinos. Also, in this zone, the formation of planetesimals
will require the coagulation of bare and icy grains. Observationally, this zone
allows high abundances of gas phase species at large scales, for massive
protostars particularly, even at low temperatures (down to 50 K).Comment: accepted in A&
Tracing early evolutionary stages of high-mass star formation with molecular lines
Despite its major role in the evolution of the interstellar medium, the
formation of high-mass stars (M > 10 Msol) is still poorly understood. Two
types of massive star cluster precursors, the so-called Massive Dense Cores
(MDCs), have been observed, which differ in their mid-infrared brightness. The
origin of this difference is not established and could be the result of
evolution, density, geometry differences, or a combination of these. We compare
several molecular tracers of physical conditions (hot cores, shocks) observed
in a sample of mid-IR weak emitting MDCs with previous results obtained in a
sample of exclusively mid-IR bright MDCs. The aim is to understand the
differences between these two types of object. We present single-dish
observations of HDO, H2O-18, SO2 and CH3OH lines at lambda = 1.3 - 3.5 mm. We
study line profiles and estimate abundances of these molecules, and use a
partial correlation method to search for trends in the results. The detection
rates of thermal emission lines are found to be very similar between mid-IR
quiet and bright objects. The abundances of H2O, HDO (1E-13 to 1E-9 in the cold
outer envelopes), SO2 and CH3OH differ from source to source but independently
of their mid-IR flux. In contrast, the methanol class I maser emission, a
tracer of outflow shocks, is found to be strongly anti-correlated with the 12
micron source brightnesses. The enhancement of the methanol maser emission in
mid-IR quiet MDCs may indicate a more embedded nature. Since total masses are
similar between the two samples, we suggest that the matter distribution is
spherical around mid-IR quiet sources but flattened around mid-IR bright ones.
In contrast, water emission is associated with objects containing a hot
molecular core, irrespective of their mid-IR brightness. These results indicate
that the mid-IR brightness of MDCs is an indicator of their evolutionary stage.Comment: 15 pages, 6 figures, 11 tables, accepted for publication in A&A the
11/06/201
Evaluatie Wet kosten bestuurlijke voorprocedures, eindrapport Pro Facto & vakgroep Bestuursrecht en Bestuurskunde
Het onderzoeksverslag betreft een in opdracht van het WODC, ministerie van Justitie uitgevoerd evaluatieonderzoek naar het functioneren van de Wet kosten bestuurlijke voorprocedures, die het sinds 12 maart 2002 mogelijk maakt dat de kosten die in de bestuurlijke voorprocedure worden gemaakt (voor bv. rechtshulp, reiskosten, inschakeling deskundigen) op verzoek worden vergoed. Conclusie van het onderzoek is dat de regeling - gelet op z'n doelstellingen - naar tevredenheid functioneert
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