3,429 research outputs found
Constraining the mass of the planet(s) sculpting a disk cavity. The intriguing case of 2MASS J16042165-2130284
The large cavities observed in the dust and gas distributions of transition
disks may be explained by planet-disk interactions. At ~145 pc, 2MASS
J16042165-2130284 (J1604) is a 5-12 Myr old transitional disk with different
gap sizes in the mm- and m-sized dust distributions (outer edges at ~79
and at ~63 au, respectively). Its CO emission shows a ~30 au cavity.
This radial structure suggests that giant planets are sculpting this disk. We
aim to constrain the masses and locations of plausible giant planets around
J1604. We observed J1604 with the Spectro-Polarimetric High-contrast Exoplanet
REsearch (SPHERE) at the Very Large Telescope (VLT), in IRDIFS\_EXT,
pupil-stabilized mode, obtaining YJH- band images with the integral field
spectrograph (IFS) and K1K2-band images with the Infra-Red Dual-beam Imager and
Spectrograph (IRDIS). The dataset was processed exploiting the angular
differential imaging (ADI) technique with high-contrast algorithms. Our
observations reach a contrast of ~12 mag from 0.15" to 0.80"
(~22 to 115 au), but no planet candidate is detected. The disk is directly
imaged in scattered light at all bands from Y to K, and it shows a red color.
This indicates that the dust particles in the disk surface are mainly
m-sized grains. We confirm the sharp dip/decrement in
scattered light in agreement with polarized light observations. Comparing our
images with a radiative transfer model we argue that the southern side of the
disk is most likely the nearest. This work represents the deepest search yet
for companions around J1604. We reach a mass sensitivity of from ~22 to ~115 au according to a hot start scenario. We propose
that a brown dwarf orbiting inside of ~15 au and additional Jovian planets at
larger radii could account for the observed properties of J1604 while
explaining our lack of detection.Comment: 10 pages, 7 Figures. Accepted for publication in A&A . Abridged
abstrac
Respiration, Oxidative Phosphorylation, And Uncoupling Protein In Candida Albicans.
The respiration, membrane potential (Deltapsi), and oxidative phosphorylation of mitochondria in situ were determined in spheroplasts obtained from Candida albicans control strain ATCC 90028 by lyticase treatment. Mitochondria in situ were able to phosphorylate externally added ADP (200 microM) in the presence of 0.05% BSA. Mitochondria in situ generated and sustained stable mitochondrial Deltapsi respiring on 5 mM NAD-linked substrates, 5 mM succinate, or 100 microM N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride plus 1 mM ascorbate. Rotenone (4 microM) inhibited respiration by 30% and 2 micro M antimycin A or myxothiazole and 1 mM cyanide inhibited it by 85%. Cyanide-insensitive respiration was partially blocked by 2 mM benzohydroxamic acid, suggesting the presence of an alternative oxidase. Candida albicans mitochondria in situ presented a carboxyatractyloside-insensitive increase of Deltapsi induced by 5 mM ATP and 0.5% BSA, and Deltapsi decrease induced by 10 microM linoleic acid, both suggesting the existence of an uncoupling protein. The presence of this protein was subsequently confirmed by immunodetection and respiration experiments with isolated mitochondria. In conclusion, Candida albicans ATCC 90028 possesses an alternative electron transfer chain and alternative oxidase, both absent in animal cells. These pathways can be exceptional targets for the design of new chemotherapeutic agents. Blockage of these respiratory pathways together with inhibition of the uncoupling protein (another potential target for drug design) could lead to increased production of reactive oxygen species, dysfunction of Candida mitochondria, and possibly to oxidative cell death.371455-6
Monoenergetic proton beams accelerated by a radiation pressure driven shock
High energy ion beams (> MeV) generated by intense laser pulses promise to be
viable alternatives to conventional ion beam sources due to their unique
properties such as high charge, low emittance, compactness and ease of beam
delivery. Typically the acceleration is due to the rapid expansion of a laser
heated solid foil, but this usually leads to ion beams with large energy
spread. Until now, control of the energy spread has only been achieved at the
expense of reduced charge and increased complexity. Radiation pressure
acceleration (RPA) provides an alternative route to producing laser-driven
monoenergetic ion beams. In this paper, we show the interaction of an intense
infrared laser with a gaseous hydrogen target can produce proton spectra of
small energy spread (~ 4%), and low background. The scaling of proton energy
with the ratio of intensity over density (I/n) indicates that the acceleration
is due to the shock generated by radiation-pressure driven hole-boring of the
critical surface. These are the first high contrast mononenergetic beams that
have been theorised from RPA, and makes them highly desirable for numerous ion
beam applications
The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau
Variations of eclipse arrival times have recently been detected in several
post common envelope binaries consisting of a white dwarf and a main sequence
companion star. The generally favoured explanation for these timing variations
is the gravitational pull of one or more circumbinary substellar objects
periodically moving the center of mass of the host binary. Using the new
extreme-AO instrument SPHERE, we image the prototype eclipsing post-common
envelope binary V471 Tau in search of the brown dwarf that is believed to be
responsible for variations in its eclipse arrival times. We report that an
unprecedented contrast of 12.1 magnitudes in the H band at a separation of 260
mas was achieved, but resulted in a non-detection. This implies that there is
no brown dwarf present in the system unless it is three magnitudes fainter than
predicted by evolutionary track models, and provides damaging evidence against
the circumbinary interpretation of eclipse timing variations. In the case of
V471 Tau, a more consistent explanation is offered with the Applegate
mechanism, in which these variations are prescribed to changes in the
quadrupole moment within the main-sequence sta
DZ Cha: a bona fide photoevaporating disc
DZ Cha is a weak-lined T Tauri star (WTTS) surrounded by a bright
protoplanetary disc with evidence of inner disc clearing. Its narrow \Ha line
and infrared spectral energy distribution suggest that DZ Cha may be a
photoevaporating disc. We aim to analyse the DZ Cha star + disc system to
identify the mechanism driving the evolution of this object. We have analysed
three epochs of high resolution optical spectroscopy, photometry from the UV up
to the sub-mm regime, infrared spectroscopy, and J-band imaging polarimetry
observations of DZ Cha. Combining our analysis with previous studies we find no
signatures of accretion in the \Ha line profile in nine epochs covering a
time baseline of years. The optical spectra are dominated by
chromospheric emission lines, but they also show emission from the forbidden
lines [SII] 4068 and [OI] 6300 that indicate a disc outflow. The
polarized images reveal a dust depleted cavity of au in radius and two
spiral-like features, and we derive a disc dust mass limit of
M_\mathrm{dust}
80 \MJup) companions are detected down to 0\farcs07 ( au,
projected). The negligible accretion rate, small cavity, and forbidden line
emission strongly suggests that DZ Cha is currently at the initial stages of
disc clearing by photoevaporation. At this point the inner disc has drained and
the inner wall of the truncated outer disc is directly exposed to the stellar
radiation. We argue that other mechanisms like planet formation or binarity
cannot explain the observed properties of DZ Cha. The scarcity of objects like
this one is in line with the dispersal timescale ( yr) predicted
by this theory. DZ Cha is therefore an ideal target to study the initial stages
of photoevaporation.Comment: A&A in press, language corrections include
Longitudinal Ion Acceleration from High-Intensity Laser Interactions with Underdense Plasma
Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser
interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed.
The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was
directional along the laser propagation path, with the highest energy ions
being collimated to a cone of less than 10 degrees. 2D particle-in-cell
simulations have been used to investigate the acceleration mechanism. The time
varying magnetic field associated with the fast electron current provides a
contribution to the accelerating electric field as well as providing a
collimating field for the ions. A strong correlation between the plasma density
and the ion acceleration was found. A short plasma scale-length at the vacuum
interface was observed to be beneficial for the maximum ion energies, but the
collimation appears to be improved with longer scale-lengths due to enhanced
magnetic fields in the ramp acceleration region.Comment: 18 pages, 6 figure
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