125 research outputs found
Numerical two-dimensional calculations of the formation of the solar nebula
Numerical two dimensional calculations of the formation of the solar nebula are presented. The following subject areas are covered: (1) observational constraints of the properties of the initial solar nebula; (2) the physical problem; (3) review if two dimensional calculations of the formation phase; (4) recent models with hydrodynamics and radiative transport; and (5) further evolution of the system
In Situ Formation and Dynamical Evolution of Hot Jupiter Systems
Hot Jupiters, giant extrasolar planets with orbital periods shorter than ~10
days, have long been thought to form at large radial distances, only to
subsequently experience long-range inward migration. Here, we propose that in
contrast with this picture, a substantial fraction of the hot Jupiter
population formed in situ via the core accretion process. We show that under
conditions appropriate to the inner regions of protoplanetary disks, rapid gas
accretion can be initiated by Super-Earth type planets, comprising 10-20 Earth
masses of refractory composition material. An in situ formation scenario leads
to testable consequences, including the expectation that hot Jupiters should
frequently be accompanied by additional low-mass planets with periods shorter
than ~100 days. Our calculations further demonstrate that dynamical
interactions during the early stages of planetary systems' lifetimes should
increase the inclinations of such companions, rendering transits rare.
High-precision radial velocity monitoring provides the best prospect for their
detection.Comment: 19 pages, 10 figures, accepted to Ap
Evolution of Ohmically Heated Hot Jupiters
We present calculations of thermal evolution of Hot Jupiters with various
masses and effective temperatures under Ohmic dissipation. The resulting
evolutionary sequences show a clear tendency towards inflated radii for
effective temperatures that give rise to significant ionization of alkali
metals in the atmosphere, compatible with the trend of the data. The degree of
inflation shows that Ohmic dissipation, along with the likely variability in
heavy element content can account for all of the currently detected radius
anomalies. Furthermore, we find that in absence of a massive core, low-mass hot
Jupiters can over-flow their Roche-lobes and evaporate on Gyr time-scales,
possibly leaving behind small rocky cores.Comment: Accepted to The Astrophysical Journal (2011) 735-2, 9 pages, 8
figures, updated figures 2-
Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anti-Cyclonic Vortex
We study the formation of a giant gas planet by the core--accretion
gas--capture process, with numerical simulations, under the assumption that the
planetary core forms in the center of an anti-cyclonic vortex. The presence of
the vortex concentrates particles of centimeter to meter size from the
surrounding disk, and speeds up the core formation process. Assuming that a
planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement
results in considerably shorter formation times than are found in standard
core--accretion gas--capture simulations. Also, formation of a gas giant is
possible in a disk with mass comparable to that of the minimum mass solar
nebula.Comment: 27 pages, 4 figures, ApJ in pres
In Situ Formation and Dynamical Evolution of Hot Jupiter Systems
Hot Jupiters, giant extrasolar planets with orbital periods shorter than ~10 days, have long been thought to form at large radial distances, only to subsequently experience long-range inward migration. Here, we offer the contrasting view that a substantial fraction of the hot Jupiter population formed in situ via the core-accretion process. We show that under conditions appropriate to the inner regions of protoplanetary disks, rapid gas accretion can be initiated by super-Earth-type planets, comprising 10â20 Earth masses of refractory material. An in situ formation scenario leads to testable consequences, including the expectation that hot Jupiters should frequently be accompanied by additional low-mass planets with periods shorter than ~100 days. Our calculations further demonstrate that dynamical interactions during the early stages of planetary systems' lifetimes should increase the inclinations of such companions, rendering transits rare. High-precision radial velocity monitoring provides the best prospect for their detection
Coupled evolutions of the stellar obliquity, orbital distance, and planet's radius due to the Ohmic dissipation induced in a diamagnetic hot Jupiter around a magnetic T Tauri star
We revisit the calculation of the Ohmic dissipation in a hot Jupiter
presented in Laine et al. (2008) by considering more realistic interior
structures, stellar obliquity, and the resulting orbital evolution. In this
simplified approach, the young hot Jupiter of one Jupiter mass is modelled as a
diamagnetic sphere with a finite resistivity, orbiting across tilted stellar
magnetic dipole fields in vacuum. Since the induced Ohmic dissipation occurs
mostly near the planet's surface, we find that the dissipation is unable to
significantly expand the young hot Jupiter. Nevertheless, the planet inside a
small co-rotation orbital radius can undergo orbital decay by the dissipation
torque and finally overfill its Roche lobe during the T Tauri star phase. The
stellar obliquity can evolve significantly if the magnetic dipole is
parallel/anti-parallel to the stellar spin. Our results are validated by the
general torque-dissipation relation in the presence of the stellar obliquity.
We also run the fiducial model in Laine et al. (2008) and find that the
planet's radius is sustained at a nearly constant value by the Ohmic heating,
rather than being thermally expanded to the Roche radius as suggested by the
authors.Comment: about 40 pages, 10 figures, Accepted for publication in The
Astrophysical Journa
The Internal Structural Adjustment due to Tidal Heating of Short-Period Inflated Giant Planets
Several short-period Jupiter-mass planets have been discovered around nearby
solar-type stars. During the circularization of their orbits, the dissipation
of tidal disturbance by their host stars heats the interior and inflates the
sizes of these planets. Based on a series of internal structure calculations
for giant planets, we examine the physical processes which determine their
luminosity-radius relation. In order for young or intensely heated gas giant
planets to attain quasi-hydrostatic equilibria, with sizes comparable to or
larger than two Jupiter radii, their interiors must have sufficiently high
temperature and low density such that degeneracy effects are relatively weak
compared to those in a mature and compact Jupiter. Consequently, the polytropic
index monotonically increases whereas the central temperature increases and
then decreases with the planets' size. These effects, along with a
temperature-sensitive opacity for the radiative surface layers of giant
planets, cause the power index of the luminosity's dependence on radius to
decrease with increasing radius. For planets larger than twice Jupiter's
radius, this index is sufficiently small that they become unstable to tidal
inflation. We make comparisons between cases of uniform heating and cases in
which the heating is concentrated in various locations within the giant planet.
Based on these results we suggest that accurate measurement of the sizes of
close-in young Jupiters can be used to probe their internal structure under the
influence of tidal heating.Comment: 39 pages, 12 figures, 2 tables. Accepted for publication in Ap
The N2K Consortium. II. A Transiting Hot Saturn Around HD 149026 With a Large Dense Core
Doppler measurements from Subaru and Keck have revealed radial velocity
variations in the V=8.15, G0IV star HD 149026 consistent with a Saturn-Mass
planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory
have detected three complete transit events with depths of 0.003 mag at the
predicted times of conjunction. HD 149026 is now the second brightest star with
a transiting extrasolar planet. The mass of the star, based on interpolation of
stellar evolutionary models, is 1.3 +/- 0.1 solar masses; together with the
Doppler amplitude, K=43.3 m s^-1, we derive a planet mass Msin(i)=0.36 Mjup,
and orbital radius of 0.042 AU. HD 149026 is chromospherically inactive and
metal-rich with spectroscopically derived [Fe/H]=+0.36, Teff=6147 K, log g=4.26
and vsin(i)=6.0 km s^-1. Based on Teff and the stellar luminosity of 2.72 Lsun,
we derive a stellar radius of 1.45 Rsun. Modeling of the three photometric
transits provides an orbital inclination of 85.3 +/- 1.0 degrees and (including
the uncertainty in the stellar radius) a planet radius of 0.725 +/- 0.05 Rjup.
Models for this planet mass and radius suggest the presence of a ~67 Mearth
core composed of elements heavier than hydrogen and helium. This substantial
planet core would be difficult to construct by gravitational instability.Comment: 25 pages, 5 figures, accepted by the Astrophysical Journa
Double Core Evolution X. Through the Envelope Ejection Phase
The evolution of binary systems consisting of an asymptotic giant branch star
of mass equal to 3 M_sun or 5 M_sun, and a main sequence star of mass equal to
0.4 M_sun or 0.6 M_sun with orbital periods > 200 days has been followed from
the onset through the late stages of the common envelope phase. Using a nested
grid technique, the three-dimensional hydrodynamical simulations of an
asymptotic giant branch star with radii approximately 1 A.U. indicate that a
significant fraction of the envelope gas is unbound (about 31% and 23% for
binaries of 3 M_sun and 0.4 M_sun, and 5 M_sun and 0.6 M_sun respectively) by
the ends of the simulations, and that the efficiency of the mass ejection
process is about 40%. While the original volume of the giant is virtually
evacuated in the late stages, most of the envelope gas remains marginally bound
on the grid. At the ends of our simulations, when the orbital decay timescale
exceeds about 5 years, the giant core and companion orbit one another with a
period of about 1 day (2.4 days for a binary involving a more evolved giant),
although this is an upper limit to the final orbital period. For a binary of 5
M_sun and 0.4 M_sun, the common envelope may not be completely ejected.Comment: 34 pages, 16 figures, accepted to Ap
The effectiveness of the Austrian disease management programme for type 2 diabetes: a cluster-randomised controlled trial
<p>Abstract</p> <p>Background</p> <p>Disease management programmes (DMPs) are costly and impose additional work load on general practitioners (GPs). Data on their effectiveness are inconclusive. We therefore conducted a cluster-randomised controlled trial to evaluate the effectiveness of the Austrian DMP for diabetes mellitus type 2 on HbA1c and quality of care for adult patients in primary care.</p> <p>Methods</p> <p>All GPs of Salzburg-province were invited to participate. After cluster-randomisation by district, all patients with diabetes type 2 were recruited consecutively from 7-11/2007. The DMP, consisting mainly of physician and patient education, standardised documentation and agreement on therapeutic goals, was implemented in the intervention group while the control group received usual care. We aimed to show superiority of the intervention regarding metabolic control and process quality. The primary outcome measure was a change in HbA1c after one year. Secondary outcomes were days in the hospital, blood pressure, lipids, body mass index (BMI), enrolment in patient education and regular guideline-adherent examination. Blinding was not possible.</p> <p>Results</p> <p>92 physicians recruited 1489 patients (649 intervention, 840 control). After 401 ± 47 days, 590 intervention-patients and 754 controls had complete data. In the intention to treat analysis (ITT) of all 1489 patients, HbA1c decreased 0.41% in the intervention group and 0.28% in controls. The difference of -0.13% (95% CI -0.24; -0.02) was significant at p = 0.026. Significance was lost in mixed models adjusted for baseline value and cluster-effects (adjusted mean difference -0.03 (95% CI -0.15; 0.09, p = 0.607). Of the secondary outcome measures, BMI and cholesterol were significantly reduced in the intervention group compared to controls in ITT after adjustments (-0.53 kg/mÂČ; 95% CI -1.03;-0.02; p = 0.014 and -0.10 mmol/l; 95% CI -0.21; -0.003; p = 0.043). Additionally, more patients received patient education (49.5% vs. 20.1%, p < 0.0001), eye- (71.0% vs. 51.2%, p < 0.0001), foot examinations (73.8% vs. 45.1%, p < 0.0001), and regular HbA1c checks (44.1% vs. 36.0%, p < 0.01) in the intervention group.</p> <p>Conclusion</p> <p>The Austrian DMP implemented by statutory health insurance improves process quality and enhances weight reduction, but does not significantly improve metabolic control for patients with type 2 diabetes mellitus. Whether the small benefit seen in secondary outcome measures leads to better patient outcomes, remains unclear.</p> <p>Trial Registration</p> <p>Current Controlled trials Ltd., ISRCTN27414162.</p
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