636 research outputs found
Negative mass corrections in a dissipative stochastic environment
We study the dynamics of a macroscopic object interacting with a dissipative stochastic environment using an adiabatic perturbation theory. The perturbation theory reproduces known expressions for the friction coefficient and, surprisingly, gives an additional negative mass correction. The effect of the negative mass correction is illustrated by studying a harmonic oscillator interacting with a dissipative stochastic environment. While it is well known that the friction coefficient causes a reduction of the oscillation frequency, we show that the negative mass correction can lead to its enhancement. By studying an exactly solvable model of a magnet coupled to a spin environment evolving under standard non-conserving dynamics we show that the effect is present even beyond the validity of the adiabatic perturbation theory.We are grateful to M Kolodrubetz for the careful reading of the manuscript and helpful comments. This work was partially supported by BSF 2010318 (YK and AP), NSF DMR-1506340 (LD and AP), AFOSR FA9550-10-1-0110 (LD and AP), ARO W911NF1410540 (LD and AP) and ISF grant (YK). LD acknowledges the office of Naval Research. YK is grateful to the BU visitors program. (2010318 - BSF; DMR-1506340 - NSF; FA9550-10-1-0110 - AFOSR; W911NF1410540 - ARO; ISF grant)Accepted manuscrip
Accretion Disks Around Young Objects. III. Grain Growth
We present detailed models of irradiated T Tauri disks including dust grain
growth with power-law size distributions. The models assume complete mixing
between dust and gas and solve for the vertical disk structure
self-consistentlyincluding the heating effects of stellar irradiation as well
as local viscous heating. For a given total dust mass, grain growth is found to
decrease the vertical height of the surface where the optical depth to the
stellar radiation becomes unit and thus the local irradiation heating, while
increasing the disk emission at mm and sub-mm wavelengths. The resulting disk
models are less geometrically thick than our previous models assuming
interstellar medium dust, and agree better with observed spectral energy
distributions and images of edge-on disks, like HK Tau/c and HH 30. The
implications of models with grain growth for determining disk masses from
long-wavelength emission are considered.Comment: 29 pages, including 11 figures and 1 table, APJ accepte
Treatment of a rapidly expanding thoracoabdominal aortic aneurysm after endovascular repair of descending thoracic aortic aneurysm in an old patient.
Background: Aortic pathology progression and/or procedure related complications following endovascular repair should always be considered mostly in older patients. We herein describe a hybrid procedure for treatment of rapidly expanding thoracoabdominal aneurysm following endovascular treatment of a descending thoracic aortic aneurysm in an older patient. Case presentation: A 82-year-old man at 18 months after endovascular surgery for a contained rupture of descending thoracic aortic aneurysm revealed a type IV thoracoabdominal aneurysm with significant increase of the aortic diameters at superior mesenteric and renal artery levels. A hybrid approach consisting of preventive visceral vessel revascularization and endovascular repair of entire abdominal aorta was performed. Under general anaesthesia and by xyphopubic laparotomy, the infrarenal aneurysmatic aorta and common iliac arteries were replaced by a bifurcated woven prosthetic graf. From each of the prosthetic branches two reverse 14x7 mm bifurcated PTFE prosthetic grafts were anastomized to both renal arteries and to the celiac axis and superior mesenteric artery, respectively. Vessel ischemia was restricted to the time required for anastomosis. Three 10 cm Gore endovascular stent-grafts for a total length of 15 cm, were used. The overlapping of the stent-grafts was carried out from the bottom upwards, starting from the aorto-iliac prosthetic body up to the healthy segment of thoracic aorta, 40 mm from the previous stent-grafts. The patient was discharged on the 9th postoperative day. Conclusion: This technique offers the advantage of a less invasive treatment, reducing the risk of paraplegia, visceral ischaemia and pulmonary complications, mostly in older patients
Resolving Molecular Line Emission from Protoplanetary Disks: Observational Prospects for Disks Irradiated by Infalling Envelopes
Molecular line observations that could resolve protoplanetary disks of ~100
AU both spatially and kinematically would be a useful tool to unambiguously
identify these disks and to determine their kinematical and physical
characteristics. In this work we model the expected line emission from a
protoplanetary disk irradiated by an infalling envelope, addressing the
question of its detectability with subarcsecond resolution. We adopt a
previously determined disk model structure that gives a continuum spectral
energy distribution and a mm intensity spatial distribution that are consistent
with observational constraints of HL Tau. An analysis of the capability of
presently working and projected interferometers at mm and submm wavelengths
shows that molecular transitions of moderate opacity at these wavelengths
(e.g., C17O lines) are good candidates for detecting disk lines at subarcsecond
resolution in the near future. We suggest that, in general, disks of typical
Class I sources will be detectable.Comment: 41 pages, 16 figures. To be published in The Astrophysical Journa
The effect of the regular solution model in the condensation of protoplanetary dust
We utilize a chemical equilibrium code in order to study the condensation
process which occurs in protoplanetary discs during the formation of the first
solids. The model specifically focuses on the thermodynamic behaviour on the
solid species assuming the regular solution model. For each solution, we
establish the relationship between the activity of the species, the composition
and the temperature using experimental data from the literature. We then apply
the Gibbs free energy minimization method and study the resulting condensation
sequence for a range of temperatures and pressures within a protoplanetary
disc. Our results using the regular solution model show that grains condense
over a large temperature range and therefore throughout a large portion of the
disc. In the high temperature region (T > 1400 K) Ca-Al compounds dominate and
the formation of corundum is sensitive to the pressure. The mid-temperature
region is dominated by Fe(s) and silicates such as Mg2SiO4 and MgSiO3 . The
chemistry of forsterite and enstatite are strictly related, and our simulations
show a sequence of forsterite-enstatite-forsterite with decreasing temperature.
In the low temperature regions (T < 600 K) a range of iron compounds and
sulfides form. We also run simulations using the ideal solution model and see
clear differences in the resulting condensation sequences with changing
solution model In particular, we find that the turning point in which
forsterite replaces enstatite in the low temperature region is sensitive to the
solution model. Our results show that the ideal solution model is often a poor
approximation to experimental data at most temperatures important in
protoplanetary discs. We find some important differences in the resulting
condensation sequences when using the regular solution model, and suggest that
this model should provide a more realistic condensation sequence.Comment: MNRAS: Accepted 2011 February 16. Received 2011 February 14; in
original form 2010 July 2
Optical and Radio monitoring of S5 1803+74
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of 30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.Comment: 25 pages, 12 figure
Optical and infrared observations of the supernova SN 1999el
Optical and near-infrared light curves of the Type IIn supernova 1999el in
NGC 6951 are presented. A period of 220 days (416 days in the near-infrared) is
covered from the first observation obtained a few days before maximum light.
Spectroscopic observations are also discussed. Using as a distance calibrator
the Type Ia SN 2000E, which occurred some months later in the same galaxy, and
fitting a blackbody law to the photometric data we obtain a maximum bolometric
luminosity for SN 1999el of erg s. In general, the
photometric properties of SN 1999el are very similar to those of SN 1998S, a
bright and well studied Type IIn SN, showing a fast decline in all observed
bands similar to those of Type II-L SNe. The differences with SN 1998S are
analyzed and ascribed to the differences in a pre-existing circumstellar
envelope in which dust was already present at the moment of the SN outburst. We
infer that light echoes may play a possibly significant role in affecting the
observed properties of the light curves, although improved theoretical models
are needed to account for the data. We conclude that mass loss in the
progenitor RG stars is episodic and occurs in an asymmetric way. This implies
that collapsing massive stars appear as normal Type II SN if this occurs far
from major mass loss episodes, whereas they appear as Type IIn SNe if a large
mass loss episode is in progress.Comment: 30 pages, 8 figures, figure 1 available as jpeg file, ApJ in pres
Diseño y Construcción de un Convertidor Boost de Pequeña Potencia, con PropósitosEducativos y de Investigación
This paper presents the design and construction of a DC-DC boost converter for a nominal power of 200W, with an input voltage of 100V and an output voltage of 200V, on the basis of theoretical analysis with the aim of design and determine all the components associated with the mentioned converter. In this research we use the voltage feedback control with a PI controller tuned by pole placement, and a PWM modulator for the IGBT switching. The converter performance and the control system faced with step changes in the input voltage and the load resistance is verified through computer simulation and then verified by experimental tests using a microcontroller PIC16F877.Este articulo presenta el diseño y construcciĂłn de un convertidor DC-DC boost o elevador de tensiĂłn, para una potencia nominal de 200 W, con una tensiĂłn de entrada de 100 V y una tensiĂłn de salida de 200 V, basĂĄndose previamente en un anĂĄlisis teĂłrico del mismo para asĂ diseñar y determinar todos los componentes asociados a dicho convertidor. En este artĂculo se utiliza el control por retroalimentaciĂłn de voltaje con un controlador PI, sintonizado mediante la asignaciĂłn de polos, ademĂĄs de un modulador PWM (Pulse Width Modulation) para la conmutaciĂłn del IGBT (Insulated Gate Bipolar Transistor). El desempeño del convertidor y del sistema de control ante cambios de tipo escalĂłn en el voltaje de entrada y resistencia carga, es verificado a travĂ©s de simulaciĂłn computational y luego comprobado mediante pruebas experimentales utilizando un microcontrolador PIC16F877
Spitzer Space Telescope study of disks in the young Orionis cluster
We report new Spitzer Space Telescope observations from the IRAC and MIPS
instruments of the young (~ 3 Myr) sigma Orionis cluster. We identify 336 stars
as members of the cluster using optical and near-infrared color magnitude
diagrams. Using the spectral energy distribution (SED) slopes in the IRAC
spectral range, we place objects in several classes: non-excess stars, stars
with optically thick disks(like classical T Tauri stars), class I
(protostellar) candidates, and stars with ``evolved disks''; the last exhibit
smaller IRAC excesses than optically thick disk systems. In general, this
classification agrees with the location expected in IRAC-MIPS color-color
diagrams for these objects. We find that the evolved disk systems are mostly a
combination of objects with optically thick but non-flared disks, suggesting
grain growth and/or settling, and transition disks, systems in which the inner
disk is partially or fully cleared of small dust. In all, we identify 7
transition disk candidates and 3 possible debris disk systems. As in other
young stellar populations, the fraction of disks depends on the stellar mass,
ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in
the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not
decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC
infrared excesses found in stellar clusters and associations with and without
central high mass stars are similar, suggesting that external photoevaporation
is not very important in many clusters. Finally, we find no correlation between
the X-ray luminosity and the disk infrared excess, suggesting that the X-rays
are not strongly affected by disk accretion.Comment: 44pages, 17 figures. Sent to Ap
The Truncated Disk of CoKu Tau/4
We present a model of a dusty disk with an inner hole which accounts for the
Spitzer Space Telescope Infrared Spectrograph observations of the low-mass
pre-main sequence star CoKu Tau/4. We have modeled the mid-IR spectrum (between
8 and 25 mic) as arising from the inner wall of a disk. Our model disk has an
evacuated inner zone of radius ~ 10 AU, with a dusty inner ``wall'', of
half-height ~ 2 AU, that is illuminated at normal incidence by the central
star. The radiative equilibrium temperature decreases from the inner disk edge
outward through the optically-thick disk; this temperature gradient is
responsible for the emission of the silicate bands at 10 and 20 mic. The
observed spectrum is consistent with being produced by Fe-Mg amorphous glassy
olivine and/or pyroxene, with no evidence of a crystalline component. The
mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older
star TW Hya, where it has been suggested that the significant clearing of its
inner disk is due to planet formation. However, no inner disk remains in CoKu
Tau/4, consistent with the star being a weak-emission (non-accreting) T Tauri
star. The relative youth of CoKu Tau/4 (~ 1 Myr) may indicate much more rapid
planet formation than typically assumed.Comment: 32 pages, 9 figures, accepted in Ap
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