8,139 research outputs found
The role of accretion disks in the formation of massive stars
We present radiation hydrodynamics simulations of the collapse of massive
pre-stellar cores. We treat frequency dependent radiative feedback from stellar
evolution and accretion luminosity at a numerical resolution down to 1.27 AU.
In the 2D approximation of axially symmetric simulations, it is possible for
the first time to simulate the whole accretion phase of several 10^5 yr for the
forming massive star and to perform a comprehensive scan of the parameter
space. Our simulation series show evidently the necessity to incorporate the
dust sublimation front to preserve the high shielding property of massive
accretion disks. Our disk accretion models show a persistent high anisotropy of
the corresponding thermal radiation field, yielding to the growth of the
highest-mass stars ever formed in multi-dimensional radiation hydrodynamics
simulations. Non-axially symmetric effects are not necessary to sustain
accretion. The radiation pressure launches a stable bipolar outflow, which
grows in angle with time as presumed from observations. For an initial mass of
the pre-stellar host core of 60, 120, 240, and 480 Msol the masses of the final
stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2
Msol respectively.Comment: 4 pages, 2 figures, Computational Star Formation Proceedings IAU
Symposium No. 270, 2010, Ed.: J. Alves, B. Elmegreen, J. Girart & V. Trimbl
Receptor mechanisms and their role in drug interactions:Effects of anaesthetics on G-protein-activated intracellular signalling pathways
Different types of receptor-mediated mechanism play a key role in cellular transmembrane communication. The majority of plasma membrane receptors mediate the effects of neurotransmitters and hormones through activation of GTP-binding proteins (G-proteins). Coupling of the activated receptor to a G-protein initiates (occasionally inhibits) a cascade of enzyme-catalysed reactions leading to the production of one or more second messengers, eventually leading to the physiological response. The most commonly known cascades are the phosphoinositide and the cAMP route. This paper will describe the key concepts of G-protein-mediated signalling of both cascades and introduce the concept of 'cross-talk'. Further, the effects of anaesthetics on the intracellular components of these signalling pathways will be reviewed.</p
Dissecting Massive YSOs with Mid-Infrared Interferometry
The very inner structure of massive YSOs is difficult to trace. With
conventional observational methods we often identify structures still several
hundreds of AU in size. But we also need information about the innermost
regions where the actual mass transfer onto the forming high-mass star occurs.
An innovative way to probe these scales is to utilise mid-infrared
interferometry. Here, we present first results of our MIDI GTO programme at the
VLTI. We observed 10 well-known massive YSOs down to scales of 20 mas. We
clearly resolve these objects which results in low visibilities and sizes in
the order of 30 - 50 mas. Thus, with MIDI we can for the first time quantify
the extent of the thermal emission from the warm circumstellar dust and thus
calibrate existing concepts regarding the compactness of such emission in the
pre-UCHII region phase. Special emphasis will be given to the BN-type object
M8E-IR where our modelling is most advanced and where there is indirect
evidence for a strongly bloated central star.Comment: 8 pages, 6 figures, proceedings contribution for the conference
"Massive Star Formation: Observations confront Theory", held in September
2007 in Heidelberg, Germany; to appear in ASP Conf. Ser. 387, H. Beuther et
al. (eds.
CB17: Inferring the dynamical history of a prestellar core with chemo-dynamical models
We present a detailed theoretical study of the isolated Bok globule CB17
(L1389) based on spectral maps of CS, HCO, CO, CS, and
HCO lines. A phenomenological model of prestellar core evolution, a
time-dependent chemical model, and a radiative transfer simulation for
molecular lines are combined to reconstruct the chemical and kinematical
structure of this core. We developed a general criterion that allows to
quantify the difference between observed and simulated spectral maps. By
minimizing this difference, we find that very high and very low values of the
effective sticking probability are not appropriate for the studied
prestellar core. The most probable value for CB17 is 0.3--0.5. The spatial
distribution of the intensities and self-absorption features of optically thick
lines is indicative of UV irradiation of the core. By fitting simultaneously
optically thin and optically thick transitions, we isolate the model that
reproduces all the available spectral maps to a reasonable accuracy. The line
asymmetry pattern in CB17 is reproduced by a combination of infall, rotation,
and turbulent motions with velocities km s, km
s, and km s, respectively. These parameters corresponds
to energy ratios , , and (the rotation
parameters are determined for ). The chemical age of the core is
about 2 Myrs. In particular, this is indicated by the central depletion of CO,
CS, and HCO. Based on the angular momentum value, we argue that the core is
going to fragment, i.e., to form a binary (multiple) star. (abridged)Comment: ApJ, in pres
Hierarchical fragmentation and collapse signatures in a high-mass starless region
Aims: Understanding the fragmentation and collapse properties of the dense
gas during the onset of high-mass star formation. Methods: We observed the
massive (~800M_sun) starless gas clump IRDC18310-4 with the Plateau de Bure
Interferometer (PdBI) at sub-arcsecond resolution in the 1.07mm continuum
andN2H+(3-2) line emission. Results: Zooming from a single-dish low-resolution
map to previous 3mm PdBI data, and now the new 1.07mm continuum observations,
the sub-structures hierarchically fragment on the increasingly smaller spatial
scales. While the fragment separations may still be roughly consistent with
pure thermal Jeans fragmentation, the derived core masses are almost two orders
of magnitude larger than the typical Jeans mass at the given densities and
temperatures. However, the data can be reconciled with models using
non-homogeneous initial density structures, turbulence and/or magnetic fields.
While most sub-cores remain (far-)infrared dark even at 70mum, we identify weak
70mum emission toward one core with a comparably low luminosity of ~16L_sun,
re-enforcing the general youth of the region. The spectral line data always
exhibit multiple spectral components toward each core with comparably small
line widths for the individual components (in the 0.3 to 1.0km/s regime). Based
on single-dish C18O(2-1) data we estimate a low virial-to-gas-mass ratio
<=0.25. We discuss that the likely origin of these spectral properties may be
the global collapse of the original gas clump that results in multiple spectral
components along each line of sight. Even within this dynamic picture the
individual collapsing gas cores appear to have very low levels of internal
turbulence.Comment: 8 pages, 4 figures, A&A in pres
Detecting Extrasolar Planets with Integral Field Spectroscopy
Observations of extrasolar planets using Integral Field Spectroscopy (IFS),
if coupled with an extreme Adaptive Optics system and analyzed with a
Simultaneous Differential Imaging technique (SDI), are a powerful tool to
detect and characterize extrasolar planets directly; they enhance the signal of
the planet and, at the same time, reduces the impact of stellar light and
consequently important noise sources like speckles. In order to verify the
efficiency of such a technique, we developed a simulation code able to test the
capabilities of this IFS-SDI technique for different kinds of planets and
telescopes, modelling the atmospheric and instrumental noise sources. The first
results obtained by the simulations show that many significant extrasolar
planet detections are indeed possible using the present 8m-class telescopes
within a few hours of exposure time. The procedure adopted to simulate IFS
observations is presented here in detail, explaining in particular how we
obtain estimates of the speckle noise, Adaptive Optics corrections, specific
instrumental features, and how we test the efficiency of the SDI technique to
increase the signal-to-noise ratio of the planet detection. The most important
results achieved by simulations of various objects, from 1 M_J to brown dwarfs
of 30 M_J, for observations with an 8 meter telescope, are then presented and
discussed.Comment: 60 pages, 37 figures, accepted in PASP, 4 Tables adde
Nonlinear propagation of planet-generated tidal waves
The propagation and evolution of planet-generated density waves in
protoplanetary disks is considered. The evolution of waves, leading to the
shock formation and wake dissipation, is followed in the weakly nonlinear
regime. The local approach of Goodman & Rafikov (2001) is extended to include
the effects of surface density and temperature variations in the disk as well
as the disk cylindrical geometry and nonuniform shear. Wave damping due to
shocks is demonstrated to be a nonlocal process spanning a significant fraction
of the disk. Torques induced by the planet could be significant drivers of disk
evolution on timescales of the order 1-10 Myr even in the absence of strong
background viscosity. A global prescription for angular momentum deposition is
developed which could be incorporated into the study of gap formation in a
gaseous disk around the planet.Comment: AASTeX, 26 pages, 4 figures, 1 table, submitted to Ap
The search for planetary mass companions to field brown dwarfs with HST/NICMOS
We present the results of a high-resolution spectral differential imaging
survey of 12 nearby, relatively young field L dwarfs (<1 Gyr) carried out with
HST/NICMOS to search for planetary mass companions at small physical
separations from their host. The survey resolved two brown dwarf binaries: the
L dwarf system Kelu-1AB and the newly discovered L/T transition system 2MASS
J031059+164815AB. For both systems common proper motion has already been
confirmed in follow-up observations which have been published elsewhere. The
derived separations of the binaries are smaller than 6 AU and consistent with
previous brown dwarf binary statistics. Their mass ratios of q > 0.8 confirm
the preference for equal mass systems similar to a large number of other
surveys. Furthermore, we found tentative evidence for a companion to the L4
dwarf 2MASS W033703-175807, straddling the brown dwarf/planetary mass boundary
and revealing an uncommonly low mass ratio system (q ~ 0.2) compared to the
vast majority of previously found brown dwarf binaries. With a derived minimum
mass of 10 - 15 Mjup, a planetary nature of the secondary cannot be ruled out
yet. However, it seems more likely to be a very low mass brown dwarf secondary
at the border of the spectral T/Y transition regime, primarily due to its
similarities to recently found very cool T dwarfs. This would make it one of
the closest resolved brown dwarf binaries (0.087" 0.015", corresponding
to 2.52 0.44 AU at a distance of 29 pc) with the coolest (Teff ~ 600-630
K) and least massive companion to any L or T dwarf.Comment: 33 pages, 8 figures, 2 tables, accepted for publication by Ap
Article 9 and the Characterization and Treatment of Tenant Security Deposits
Each day, thousands of lessees enter into contracts under which they lease either real or personal property. Under the majority of these contracts, the lessee agrees to pay (and does pay) a security deposit to the lessor. The lessor typically agrees to refund the deposit at the conclusion of the lease term if the lessee fully performs its obligations under the lease contract. Is Article 9 relevant to this transaction? Has the lessor taken a security interest in the lessee\u27s property to secure the lessee\u27s obligations under the lease contract?
In Part I, we highlight two opinions representative of the majority of case decisions that have treated lessee security deposits as a debt. In Part II, we briefly explore the extent to which Article 9\u27s existing provisions appropriately address tenant security deposits, focusing particularly upon some potentially troubling differences that may follow if a security deposit is not made with property of the debtor (such as cash or a check) but is instead made with a credit card or similar payment mechanism. Part III argues that instead of attempting to modify Article 9 to more aptly govern tenant security deposits, the Uniform Law Commission should incorporate explicit provisions for the characterization and handling of tenant security deposits into the revised URLTA provisions that acknowledge the security deposit as a form of secured transaction, but that address the expectations of residential landlords and tenants more appropriately than the existing provisions of Article 9. Part III concludes with statutory language proposed for inclusion within the revised URLTA
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