27,380 research outputs found
Inner mean-motion resonances with eccentric planets: A possible origin for exozodiacal dust clouds
High levels of dust have been detected in the immediate vicinity of many
stars, both young and old. A promising scenario to explain the presence of this
short-lived dust is that these analogues to the Zodiacal cloud (or exozodis)
are refilled in situ through cometary activity and sublimation. As the
reservoir of comets is not expected to be replenished, the presence of these
exozodis in old systems has yet to be adequately explained.
It was recently suggested that mean-motion resonances (MMR) with exterior
planets on moderately eccentric () orbits could
scatter planetesimals on to cometary orbits with delays of the order of several
100 Myr. Theoretically, this mechanism is also expected to sustain continuous
production of active comets once it has started, potentially over
Gyr-timescales.
We aim here to investigate the ability of this mechanism to generate
scattering on to cometary orbits compatible with the production of an exozodi
on long timescales. We combine analytical predictions and complementary
numerical N-body simulations to study its characteristics.
We show, using order of magnitude estimates, that via this mechanism, low
mass discs comparable to the Kuiper Belt could sustain comet scattering at
rates compatible with the presence of the exozodis which are detected around
Solar-type stars, and on Gyr timescales. We also find that the levels of dust
detected around Vega could be sustained via our proposed mechanism if an
eccentric Jupiter-like planet were present exterior to the system's cold debris
disc.Comment: 15 pages, 12 figures; Accepted for publication in MNRA
Analytical theory for the initial mass function: III time dependence and star formation rate
The present paper extends our previous theory of the stellar initial mass
function (IMF) by including the time-dependence, and by including the impact of
magnetic field. The predicted mass spectra are similar to the time independent
ones with slightly shallower slopes at large masses and peak locations shifted
toward smaller masses by a factor of a few. Assuming that star-forming clumps
follow Larson type relations, we obtain core mass functions in good agreement
with the observationally derived IMF, in particular when taking into account
the thermodynamics of the gas. The time-dependent theory directly yields an
analytical expression for the star formation rate (SFR) at cloud scales. The
SFR values agree well with the observational determinations of various Galactic
molecular clouds. Furthermore, we show that the SFR does not simply depend
linearly on density, as sometimes claimed in the literature, but depends also
strongly on the clump mass/size, which yields the observed scatter. We stress,
however, that {\it any} SFR theory depends, explicitly or implicitly, on very
uncertain assumptions like clump boundaries or the mass of the most massive
stars that can form in a given clump, making the final determinations uncertain
by a factor of a few. Finally, we derive a fully time-dependent model for the
IMF by considering a clump, or a distribution of clumps accreting at a constant
rate and thus whose physical properties evolve with time. In spite of its
simplicity, this model reproduces reasonably well various features observed in
numerical simulations of converging flows. Based on this general theory, we
present a paradigm for star formation and the IMF.Comment: accepted for publication in Ap
Effects of polymer polydispersity on the phase behaviour of colloid-polymer mixtures
We study the equilibrium behaviour of a mixture of monodisperse hard sphere
colloids and polydisperse non-adsorbing polymers at their -point, using
the Asakura-Oosawa model treated within the free-volume approximation. Our
focus is the experimentally relevant scenario where the distribution of polymer
chain lengths across the system is fixed. Phase diagrams are calculated using
the moment free energy method, and we show that the mean polymer size at which gas-liquid phase separation first occurs decreases with increasing
polymer polydispersity . Correspondingly, at fixed mean polymer size,
polydispersity favours gas-liquid coexistence but delays the onset of
fluid-solid separation. On the other hand, we find that systems with different
but the same {\em mass-averaged} polymer chain length have nearly
polydispersity-independent phase diagrams. We conclude with a comparison to
previous calculations for a semi-grandcanonical scenario, where the polymer
chemical potentials are imposed, which predicted that fluid-solid coexistence
was over gas-liquid in some areas of the phase diagram. Our results show that
this somewhat counter-intuitive result arose because the actual polymer size
distribution in the system is shifted to smaller sizes relative to the polymer
reservoir distribution.Comment: Changes in v2: sketch in Figure 1 corrected, other figures improved;
added references to experimental work and discussion of mapping from polymer
chain length to effective radiu
- …