131 research outputs found
Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets
The population of close-in super-Earths, with gas mass fractions of up to 10%
represents a challenge for planet formation theory: how did they avoid runaway
gas accretion and collapsing to hot Jupiters despite their core masses being in
the critical range of ? Previous
three-dimensional (3D) hydrodynamical simulations indicate that atmospheres of
low-mass planets cannot be considered isolated from the protoplanetary disc,
contrary to what is assumed in 1D-evolutionary calculations. This finding is
referred to as the recycling hypothesis. In this Paper we investigate the
recycling hypothesis for super-Earth planets, accounting for realistic 3D
radiation hydrodynamics. Also, we conduct a direct comparison in terms of the
evolution of the entropy between 1D and 3D geometries. We clearly see that 3D
atmospheres maintain higher entropy: although gas in the atmosphere loses
entropy through radiative cooling, the advection of high entropy gas from the
disc into the Bondi/Hill sphere slows down Kelvin-Helmholtz contraction,
potentially arresting envelope growth at a sub-critical gas mass fraction.
Recycling, therefore, operates vigorously, in line with results by previous
studies. However, we also identify an "inner core" -- in size 25% of
the Bondi radius -- where streamlines are more circular and entropies are much
lower than in the outer atmosphere. Future studies at higher resolutions are
needed to assess whether this region can become hydrodynamically-isolated on
long time-scales.Comment: 16 pages, 12 figures, accepted for publication at MNRA
Hydrodynamics of Embedded Planets' First Atmospheres. II. A Rapid Recycling of Atmospheric Gas
Following Paper I we investigate the properties of atmospheres that form
around small protoplanets embedded in a protoplanetary disc by conducting
hydrodynamical simulations. These are now extended to three dimensions,
employing a spherical grid centred on the planet. Compression of gas is shown
to reduce rotational motions. Contrasting the 2D case, no clear boundary
demarcates bound atmospheric gas from disc material; instead, we find an open
system where gas enters the Bondi sphere at high latitudes and leaves through
the midplane regions, or, vice versa, when the disc gas rotates sub-Keplerian.
The simulations do not converge to a time-independent solution; instead, the
atmosphere is characterized by a time-varying velocity field. Of particular
interest is the timescale to replenish the atmosphere by nebular gas,
. It is shown that the replenishment rate,
, can be understood in terms of a modified
Bondi accretion rate,
, where
is set by the Keplerian shear or the magnitude of the
sub-Keplerian motion of the gas, whichever is larger. In the inner disk, the
atmosphere of embedded protoplanets replenishes on a timescale that is shorter
than the Kelvin-Helmholtz contraction (or cooling) timescale. As a result,
atmospheric gas can no longer contract and the growth of these atmospheres
terminates. Future work must confirm whether these findings continue to apply
when the (thermodynamical) idealizations employed in this study are relaxed.
But if shown to be broadly applicable, replenishment of atmospheric gas
provides a natural explanation for the preponderance of gas-rich but
rock-dominant planets like super-Earths and mini-Neptunes.Comment: 15 pages, 13 figures, submitted to MNRAS. Comments welcom
How the Internet, the Sharing Economy, and Reputational Feedback Mechanisms Solve the “Lemons Problem”
This paper argues that the sharing economy—through the use of the Internet and real time reputational feedback mechanisms—is providing a solution to the lemons problem that many regulators have spent decades attempting to overcome. Section I provides an overview of the sharing economy and traces its rapid growth. Section II revisits the lemons theory as well as the various regulatory solutions proposed to deal with the problem of asymmetric information. Section III discusses the relationship between reputation and trust and analyzes how reputational incentives affect commercial interactions. Section IV discusses how information asymmetries were addressed in the pre-Internet era. It also discusses how the evolution of both the Internet and information systems (especially the reputational feedback mechanisms of the sharing economy) addresses the lemons problem. Section V explains how these new realities affect public policy and concludes that asymmetric information is not a legitimate rationale for policy intervention in light of technological changes. We also argue that continued use of this rationale to regulate in the name of consumer protection might, in fact, make consumers worse off. This has ramifications for the current debate over regulation of the sharing economy
Atmospheric Recyling of Volatiles by Pebble-Accreting Planets
Planets, embedded in their natal discs, harbour hot envelopes. When pebbles
are accreted by these planets, the contained volatile components may sublimate,
enriching the envelope and potentially changing its thermodynamical properties.
However, the envelopes of embedded planets actively exchange material with the
disc, which would limit the buildup of a vapour-rich atmosphere. To properly
investigate these processes, we have developed a new phase change module to
treat the sublimation process with hydrodynamical simultions. Combined with the
recently developed multi-dust fluid approach, we conduct 2D self-consistent
hydrodynamic simulations to study how pebble sublimation influences the water
content of super-Earths and sub-Neptunes. We find the extent and the amount of
vapour that a planet is able to hold on to is determined by the relative size
of the sublimation front and the atmosphere. When the sublimation front lies
far inside the atmosphere, vapour tends to be locked deep in the atmosphere and
keeps accumulating through a positive feedback mechanism. On the other hand,
when the sublimation front exceeds the (bound) atmosphere, the ice component of
incoming pebbles can be fully recycled and the vapour content reaches a low,
steady value. Low disc temperature, small planet mass and high pebble flux
(omitting accretion heating by pebbles) render the planet atmosphere
vapour-rich while the reverse changes render it vapour-poor. The phase change
module introduced here can in future studies also be employed to model the
chemical composition of the gas in the vicinity of accreting planets and around
snowlines.Comment: 21 pages, 16 figures. Accepted for publication in MNRAS on June 5th
202
Towards integrated youth care : a systematic review of facilitators and barriers for professionals
To overcome fragmentation in support for children and their families with multiple and enduring problems across life domains, professionals increasingly try to organize integrated care. However, it is unclear what facilitators and barriers professionals experience when providing this integrated care. Our systematic review, including 55 studies from a broad variety of settings in Youth Care, showed that integrated care on a professional level is a multi-component entity consisting of several facilitators and barriers. Findings were clustered in seven general themes: ‘Child’s environment’, ‘Preconditions’, ‘Care process’, ‘Expertise’, ‘Interprofessional collaboration’, ‘Information exchange’, and ‘Professional identity’. The identified facilitators and barriers were generally consistent across studies, indicating broad applicability across settings and professional disciplines. This review clearly shows that when Youth Care professionals address a broad spectrum of problems, a variety of facilitators and barriers should be considered. Registration PROSPERO, registration number CRD42018084527
Behavioural problems of adolescents in secure residential youth care:Gender differences and risk factors
Adolescents in secure residential care mostly suffer from serious behavioural problems, often accompanied by trauma and adverse family circumstances. This paper presents findings of a comparison of behavioural problems and risk factors of 255 boys and girls (aged 12 to 18 years) in secure residential care in the Netherlands and their association with behavioural problems. A cross-sectional design and standardized questionnaires were used to measure behavioural problems and individual and familial risk factors. By using independent-sample t tests, the severity of these factors in boys and girls was compared, and by using structural equation modelling (SEM), associations between these factors and behavioural problems were investigated. The findings of the study show that post-traumatic stress disorder (PTSD) symptoms, maladaptive emotion regulation, impaired perceived competence and internalizing behavioural problems were more severe in girls than in boys. Boys experienced more severe externalizing behavioural problems and more family problems than girls. Maladaptive emotion regulation, PTSD symptoms, perceived competence and parenting problems were related to behavioural problems. The results indicate that treatment for girls should address PTSD symptoms, perceived competence and maladaptive emotion regulation and that extra attention for family problems in the treatment of boys is warranted
The outcome of non-residential youth care compared to residential youth care: A multilevel meta-analysis
Objective: This multilevel meta-analysis compared the outcomes of Treatment Foster Care Oregon for Adolescents (TFCO-A) and home-based treatment programs (HBT) with residential youth care for children and youth aged 0 to 23 years. Methods: A total of 145 effect sizes for different types of behavioral problems were derived from 24 controlled studies (n = 16,943 participants). A three-level random-effects meta-analysis was conducted. Results: We found a small statistically significant overall effect (d = 0.21), 95% CI [0.090-0.338], which indicated that non-residential youth care was slightly more effective than residential youth care. However, moderator analysis revealed that TFCO-A yielded a larger effect size (d = 0.36) than HBT (d = 0.08). Conclusions: Our findings suggest that youth treated in treatment foster care have better outcomes than youth in residential care, which is not true for children who are treated at home. Therefore, in case of out-of-home placement treatment foster care should be the first option. Given that residential care has no additional value for youth who are treated a
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