656 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
Hubble and Spitzer Observations of an Edge-on Circumstellar Disk around a Brown Dwarf
We present observations of a circumstellar disk that is inclined close to
edge-on around a young brown dwarf in the Taurus star-forming region. Using
data obtained with SpeX at the NASA Infrared Telescope Facility, we find that
the slope of the 0.8-2.5 um spectrum of the brown dwarf 2MASS J04381486+2611399
cannot be reproduced with a photosphere reddened by normal extinction. Instead,
the slope is consistent with scattered light, indicating that circumstellar
material is occulting the brown dwarf. By combining the SpeX data with mid-IR
photometry and spectroscopy from the Spitzer Space Telescope and previously
published millimeter data from Scholz and coworkers, we construct the spectral
energy distribution for 2MASS J04381486+2611399 and model it in terms of a
young brown dwarf surrounded by an irradiated accretion disk. The presence of
both silicate absorption at 10 um and silicate emission at 11 um constrains the
inclination of the disk to be ~70 deg, i.e. ~20 deg from edge-on. Additional
evidence of the high inclination of this disk is provided by our detection of
asymmetric bipolar extended emission surrounding 2MASS J04381486+2611399 in
high-resolution optical images obtained with the Hubble Space Telescope.
According to our modeling for the SED and images of this system, the disk
contains a large inner hole that is indicative of a transition disk (R_in~58
R_star~0.275 AU) and is somewhat larger than expected from embryo ejection
models (R_out=20-40 AU vs. R_out<10-20 AU).Comment: The Astrophysical Journal, in pres
IRAC Observations of Taurus Pre-Main Sequence Stars
We present infrared photometry obtained with the IRAC camera on the Spitzer
Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in
the Taurus star-forming region. We find a clear separation in some IRAC
color-color diagrams between objects with and without disks. A few
``transition'' objects are noted, which correspond to systems in which the
inner disk has been evacuated of small dust. Separating pure disk systems from
objects with remnant protostellar envelopes is more difficult at IRAC
wavelengths, especially for objects with infall at low rates and large angular
momenta. Our results generally confirm the IRAC color classification scheme
used in previous papers by Allen et al. and Megeath et al. to distinguish
between protostars, T Tauri stars with disks, and young stars without (inner)
disks. The observed IRAC colors are in good agreement with recent improved disk
models, and in general accord with models for protostellar envelopes derived
from analyzing a larger wavelength region. We also comment on a few Taurus
objects of special interest. Our results should be useful for interpreting IRAC
results in other, less well-studied star-forming regions.Comment: 29 pages 10 figures, to appear in Ap
Performance of Dairy Goats to Alfalfa Silage Based Diets Supplemented with Different Sources of Carbohydrates
Lactating Saanen dairy goats fed alfalfa silage (AS) based diets in four 4x4 Latin Square designed experiment were studied for the effects of supplementation of three different type of carbohydrates (wheat grain, (W); sorghum grain, (SG) and dry citrus pulp, (DCP)) on milk yield, composition and chewing activities. Sixteen does (45± 10 DIM and 2.016 kg ± 0.48 4% FCM) housed indoors in individual pens in a four 4x4 experiment were fed four diets 1) AS (33.9%DM, 19.9%CP, 44.01%NDF); 2) ASW (52.4%DM, 19.8%CP, 33.7%NDF); 3) ASSG (50.9%DM, 17.9%CP, 37%NDF), 4) ASDCP (52.5%DM, 16.12%CP, 39.1%NDF) with forage-to-concentrate ratios of 100:0 or 65:35, 67:33 and 64:36 respectively. Intake of AS DM (2.78%BW) was different (P\u3c 0.05) from the other treatments (average 3.53 ±0.07 %BW). Chewing efficiency (min/g NDF per kg BW 0.75) decrease (P\u3c 0.05) as a result of AS substitution or concentrate supplementation without effect (P\u3e 0.05) of carbohydrate type or dietary level of NDF. Milk, 4%FCM and fat-protein-corrected milk yield was affected (P\u3c 0.05) by concentrate supplementation. Either milk protein content (g/l) or yield (g/day) were not affected by treatments. Body weight changes appeared related to concentrate supplementation. Supplementation increase total DM intake, decrease forage DM intake and chewing efficiency and increase producing performance without changing milk composition
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
Involvement of phosphodiesterase-cGMP-PKG pathway in intracellular Ca2+ oscillations in pituitary GH3 cells
AbstractThe present study investigates the potential role of the Ca2+-calmodulin-dependent type I phosphodiesterase (PDE)-cGMP-protein kinase G (PKG) pathway in spontaneous [Ca2+]i oscillations in GH3 cells using fura-2 single cell videoimaging. Vinpocetine (2.5â50 ÎŒM), a selective inhibitor of type I PDE, induced a concentration-dependent inhibition of spontaneous [Ca2+]i oscillations in these pituitary cells, and at the same time produced an increase of the intracellular cGMP content. The cell permeable cGMP analog N2,2âČ-O-dibutyryl-cGMP (dB-cGMP) (1 mM) caused a progressive reduction of the frequency and the amplitude of spontaneous [Ca2+]i oscillations when added to the medium. KT5823 (400 nM), a selective inhibitor of cGMP-dependent protein kinase (PKG), produced an increase of baseline [Ca2+]i and the disappearance of spontaneous [Ca2+]i oscillations. When KT5823 was added before vinpocetine, the PKG inhibitor counteracted the [Ca2+]i lowering effect of the cGMP catabolism inhibitor. Finally, the removal of extracellular Ca2+ or the blockade of L-type voltage-sensitive calcium channels (VSCC) by nimodipine produced a decrease of cytosolic cGMP levels. Collectively, the results of the present study suggest that spontaneous [Ca2+]i oscillations in GH3 cells may be regulated by the activity of type I PDE-cGMP-PKG pathway
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
A New Probe of the Planet-Forming Region in T Tauri Disks
We present new observations of the FUV (1100-2200 Angstrom) radiation field
and the near- to mid-IR (3--13.5 micron) spectral energy distribution (SED) of
a sample of T Tauri stars selected on the basis of bright molecular disks (GM
Aur, DM Tau, LkCa15). In each source we find evidence for Ly alpha induced H2
fluorescence and an additional source of FUV continuum emission below 1700
Angstroms. Comparison of the FUV spectra to a model of H2 excitation suggests
that the strong continuum emission is due to electron impact excitation of H2.
The ultimate source of this excitation is likely X-ray irradiation which
creates hot photo-electrons mixed in the molecular layer. Analysis of the SED
of each object finds the presence of inner disk gaps with sizes of a few AU in
each of these young (~1 Myr) stellar systems. We propose that the presence of
strong H2 continuum emission and inner disk clearing are related by the
increased penetration power of high energy photons in gas rich regions with low
grain opacity.Comment: 5 pages, 3 figures, accepted by ApJ Letter
Confirmation of a recent bipolar ejection in the very young hierarchical multiple system IRAS 16293-2422
We present and analyze two new high-resolution (approx 0.3 arcsec),
high-sensitivity (approx 50 uJy beam-1) Very Large Array 3.6 cm observations of
IRAS 16293-2422 obtained in 2007 August and 2008 December. The components
A2alpha and A2beta recently detected in this system are still present, and have
moved roughly symmetrically away from source A2 at a projected velocity of
30-80 km s-1. This confirms that A2alpha and A2beta were formed as a
consequence of a very recent bipolar ejection from A2. Powerful bipolar
ejections have long been known to occur in low-mass young stars, but this is
-to our knowledge-- the first time that such a dramatic one is observed from
its very beginning. Under the reasonable assumption that the flux detected at
radio wavelengths is optically thin free-free emission, one can estimate the
mass of each ejecta to be of the order of 10^-8 Msun. If the ejecta were
created as a consequence of an episode of enhanced mass loss accompanied by an
increase in accretion onto the protostar, then the total luminosity of IRAS
16293-2422 ought to have increased by 10-60% over the course of at least
several months. Between A2alpha and A2beta, component A2 has reappeared, and
the relative position angle between A2 and A1 is found to have increased
significantly since 2003-2005. This strongly suggests that A1 is a protostar
rather than a shock feature, and that the A1/A2 pair is a tight binary system.
Including component B, IRAS 16293-2422 therefore appears to be a very young
hierarchical multiple system.Comment: Accepted for publication in The Astrophysical Journa
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
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