9,076 research outputs found
Generating metal-polluting debris in white dwarf planetary systems from small-impact crater ejecta
Metal pollution in white dwarf photospheres originates from the accretion of some combination of planets, moons, asteroids, comets, boulders, pebbles and dust. When large bodies reside in dynamically stagnant locations β unable themselves to pollute nor even closely approach the white dwarf β then smaller reservoirs of impact debris may become a complementary or the primary source of metal pollutants. Here, we take a first step towards exploring this possibility by computing limits on the recoil mass that escapes the gravitational pull of the target object following a single impact onto an atmosphere-less surface. By considering vertical impacts only with the full-chain analytical prescription from Kurosawa & Takada (2019), we provide lower bounds for the ejected mass for basalt, granite, iron and water-rich target objects across the radii range 100 β 3 km. Our use of the full-chain prescription as opposed to physical experiments or hydrocode simulations allows us to quickly sample a wide range of parameter space appropriate to white dwarf planetary systems. Our numerical results could be used in future studies to constrain freshly-generated small debris reservoirs around white dwarfs given a particular planetary system architecture, bombardment history, and impact geometries
Spectral variability of classical T Tauri stars accreting in an unstable regime
Classical T Tauri stars (CTTSs) are variable in different time-scales. One
type of variability is possibly connected with the accretion of matter through
the Rayleigh-Taylor instability that occurs at the interface between an
accretion disc and a stellar magnetosphere. In this regime, matter accretes in
several temporarily formed accretion streams or `tongues' which appear in
random locations, and produce stochastic photometric and line variability. We
use the results of global three-dimensional magnetohydrodynamic simulations of
matter flows in both stable and unstable accretion regimes to calculate
time-dependent hydrogen line profiles and study their variability behaviours.
In the stable regime, some hydrogen lines (e.g. H-beta, H-gamma, H-delta,
Pa-beta and Br-gamma) show a redshifted absorption component only during a
fraction of a stellar rotation period, and its occurrence is periodic. However,
in the unstable regime, the redshifted absorption component is present rather
persistently during a whole stellar rotation cycle, and its strength varies
non-periodically. In the stable regime, an ordered accretion funnel stream
passes across the line of sight to an observer only once per stellar rotation
period while in the unstable regime, several accreting streams/tongues, which
are formed randomly, pass across the line of sight to an observer. The latter
results in the quasi-stationarity appearance of the redshifted absorption
despite the strongly unstable nature of the accretion. In the unstable regime,
multiple hot spots form on the surface of the star, producing the stochastic
light curve with several peaks per rotation period. This study suggests a CTTS
that exhibits a stochastic light curve and a stochastic line variability, with
a rather persistent redshifted absorption component, may be accreting in the
unstable accretion regime.Comment: 20 pages, 11 figures, 1 table, accepted for publication in MNRA
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