1,117 research outputs found
The Mirroring Hypothesis: Theory, Evidence and Exceptions
The mirroring hypothesis predicts that the organizational patterns of a development project (e.g. communication links, geographic collocation, team and firm co-membership) will correspond to the technical patterns of dependency in the system under development. Scholars in a range of disciplines have argued that mirroring is either necessary or a highly desirable feature of development projects, but evidence pertaining to the hypothesis is widely scattered across fields, research sites, and methodologies. In this paper, we formally define the mirroring hypothesis and review 102 empirical studies spanning three levels of organization: within a single firm, across firms, and in open community-based development projects. The hypothesis was supported in 69% of the cases. Support for the hypothesis was strongest in the within-firm sample, less strong in the across-firm sample, and relatively weak in the open collaborative sample. Based on a detailed analysis of the cases in which the mirroring hypothesis was not supported, we introduce the concept of actionable transparency as a means of achieving coordination without mirroring. We present examples from practice and describe the more complex organizational patterns that emerge when actionable transparency allows designers to 'break the mirror.'Modularity, innovation, product and process development, organization design, design structure, organizational structure, organizational ties
The interplay between radiation pressure and the photoelectric instability in optically thin disks of gas and dust
Previous theoretical works have shown that in optically thin disks, dust
grains are photoelectrically stripped of electrons by starlight, heating nearby
gas and possibly creating a dust clumping instability, the photoelectric
instability (PeI), that significantly alters global disk structure. In the
current work, we use the Pencil Code to perform the first numerical models of
the PeI that include stellar radiation pressure on dust grains in order to
explore the parameter regime in which the instability operates. In models with
gas surface densities greater than ,
we see a variety of dust structures, including sharp concentric rings and
non-axisymmetric arcs and clumps that represent dust surface density
enhancements of factors of depending on the run parameters. The
gas distributions show various structures as well, including clumps and arcs
formed from spiral arms. In models with lower gas surface densities, vortices
and smooth spiral arms form in the gas distribution, but the dust is too weakly
coupled to the gas to be significantly perturbed. In one high gas surface
density model, we include a large, low-order gas viscosity, and, in agreement
with previous radiation pressure-free models, find that it observably smooths
the structures that form in the gas and dust, suggesting that resolved images
of a given disk may be useful for deriving constraints on the effective
viscosity of its gas. Broadly, our models show that radiation pressure does not
preclude the formation of complex structure from the PeI, but the qualitative
manifestation of the PeI depends strongly on the parameters of the system. The
PeI may provide an explanation for unusual disk morphologies such as the moving
blobs of the AU Mic disk, the asymmetric dust distribution of the 49 Ceti disk,
and the rings and arcs found in the disk around HD 141569A.Comment: 13 pages, 13 figures; submitted to Ap
Effect of the Canting of Local Anisotropy Axes on Ground-State Properties of a Ferrimagnetic Chain with Regularly Alternating Ising and Heisenberg Spins
The effect of the canting of local anisotropy axes on the ground-state phase
diagram and magnetization of a ferrimagnetic chain with regularly alternating
Ising and Heisenberg spins is exactly examined in an arbitrarily oriented
magnetic field. It is shown that individual contributions of Ising and
Heisenberg spins to the total magnetization basically depend on the spatial
orientation of the magnetic field and the canting angle between two different
local anisotropy axes of the Ising spins.Comment: 3 pages, 3 figure
On shocks driven by high-mass planets in radiatively inefficient disks. I. Two-dimensional global disk simulations
Recent observations of gaps and non-axisymmetric features in the dust
distributions of transition disks have been interpreted as evidence of embedded
massive protoplanets. However, comparing the predictions of planet-disk
interaction models to the observed features has shown far from perfect
agreement. This may be due to the strong approximations used for the
predictions. For example, spiral arm fitting typically uses results that are
based on low-mass planets in an isothermal gas. In this work, we describe
two-dimensional, global, hydrodynamical simulations of disks with embedded
protoplanets, with and without the assumption of local isothermality, for a
range of planet-to-star mass ratios 1-10 M_jup for a 1 M_sun star. We use the
Pencil Code in polar coordinates for our models. We find that the inner and
outer spiral wakes of massive protoplanets (M>5 M_jup) produce significant
shock heating that can trigger buoyant instabilities. These drive sustained
turbulence throughout the disk when they occur. The strength of this effect
depends strongly on the mass of the planet and the thermal relaxation
timescale; for a 10 M_jup planet embedded in a thin, purely adiabatic disk, the
spirals, gaps, and vortices typically associated with planet-disk interactions
are disrupted. We find that the effect is only weakly dependent on the initial
radial temperature profile. The spirals that form in disks heated by the
effects we have described may fit the spiral structures observed in transition
disks better than the spirals predicted by linear isothermal theory.Comment: 10 pages, 8 figures. ApJ, accepte
Medical Termination of Delayed Miscarriage: Four-Year Experience with an Outpatient Protocol
Purpose: To evaluate the efficacy of an outpatient protocol with vaginal misoprostol to treat delayed miscarriage.
Methods: Retrospective analysis of prospectively collected data on women medically treated for missed abortion with an outpatient protocol. The inclusion criteria were: ultrasound-based diagnosis of missed abortion with less than 10 weeks; no heavy bleeding, infection, inflammatory bowel disease or misoprostol allergy; no more than 2 previous spontaneous abortions; the preference of the patient regarding the medical management. The protocol consisted of: 1) a single dose of 800 µg of misoprostol administered intravaginally at the emergency department, after which the patients were discharged home; 2) clinical and ultrasonographic evaluation 48 hours later – if the intrauterine gestational sac was still present, the application of 800 µg of vaginal misoprostol was repeated, and the patients were discharged home; 3) clinical and ultrasonography evaluation 7 days after the initiation of the protocol – if the intrauterine gestational sac was still present, surgical management was proposed. The protocol was introduced in January 2012. Every woman received oral analgesia and written general recommendations. We also gave them a paper form to be presented and filled out at each evaluation.
Results: Complete miscarriage with misoprostol occurred in 340 women (90.2%). Surgery was performed in 37 (9.8%) patients, representing the global failure rate of the protocol. Miscarriage was completed after the first misoprostol administration in 208 (55.2%) women, with a success rate after the second administration of 78.1% (132/169). The average age of the women with complete resolution using misoprostol was superior to the average age of those who required surgery (33.99 years versus 31.74 years; p = 0.031). Based on the ultrasonographic findings in the first evaluation, the women diagnosed with fetal loss achieved greater success rates compared with those diagnosed with empty sac (p = 0.049).
Conclusions: We conclude this is an effective and safe option in the majority of delayed miscarriage cases during the first trimester, reducing surgical procedures and their consequences
Eccentricity Trap: Trapping of Resonantly Interacting Planets near the Disk Inner Edge
Using orbital integration and analytical arguments, we have found a new
mechanism (an "eccentricity trap") to halt type I migration of planets near the
inner edge of a protoplanetary disk. Because asymmetric eccentricity damping
due to disk-planet interaction on the innermost planet at the disk edge plays a
crucial role in the trap, this mechanism requires continuous eccentricity
excitation and hence works for a resonantly interacting convoy of planets. This
trap is so strong that the edge torque exerted on the innermost planet can
completely halt type I migrations of many outer planets through mutual resonant
perturbations. Consequently, the convoy stays outside the disk edge, as a
whole. We have derived semi-analytical formula for the condition for the
eccentricity trap and predict how many planets are likely to be trapped. We
found that several planets or more should be trapped by this mechanism in
protoplanetary disks that have cavities. It can be responsible for the
formation of non-resonant, multiple, close-in super-Earth systems extending
beyond 0.1AU. Such systems are being revealed by radial velocity observations
to be quite common around solar-type stars.Comment: 24 pages, 7 figures, accepted for publication in Ap
Rossby wave instability does not require sharp resistivity gradients
Context. Rossby wave instability (RWI) at dead zone boundaries may play an important role in planet formation. Viscous hydrodynamics results suggest RWI is excited only when the viscosity changes over a radial distance less than two density scale heights. However in the disks around Solar-mass T Tauri stars, it is not viscosity but magnetic forces that provide the accretion stress beyond about 10 AU, where surface densities are low enough so stellar X-rays and interstellar cosmic rays can penetrate.
Aims. We explore the conditions for RWI in the smooth transition with increasing distance, from resistive and magnetically-dead to conducting and magnetically-active.
Methods. We perform 3D unstratified MHD simulations with the Pencil code, using static resistivity profiles.
Results. We find that in MHD, contrary to viscous models, the RWI is triggered even with a gradual change in resistivity extending from 10 to 40 AU (i.e., spanning 15 scale heights for aspect ratio 0.1). This is because magneto-rotational turbulence sets in abruptly when the resistivity reaches a threshold level. At higher resistivities the longest unstable wavelength is quenched, resulting in a sharp decline of the Maxwell stress towards the star. The sharp gradient in the magnetic forces leads to a localized density bump, that is in turn Rossby wave unstable.
Conclusions. Even weak gradients in the resistivity can lead to sharp transitions in the Maxwell stress. As a result the RWI is more easily activated in the outer disk than previously thought. Rossby vortices at the outer dead zone boundary thus could underlie the dust asymmetries seen in the outer reaches of transition disks
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