1,672 research outputs found
Braking the Gas in the beta Pictoris Disk
(Abridged) The main sequence star beta Pictoris hosts the best studied
circumstellar disk to date. Nonetheless, a long-standing puzzle has been around
since the detection of metallic gas in the disk: radiation pressure from the
star should blow the gas away, yet the observed motion is consistent with
Keplerian rotation. In this work we search for braking mechanisms that can
resolve this discrepancy. We find that all species affected by radiation force
are heavily ionized and dynamically coupled into a single fluid by Coulomb
collisions, reducing the radiation force on species feeling the strongest
acceleration. For a gas of solar composition, the resulting total radiation
force still exceeds gravity, while a gas of enhanced carbon abundance could be
self-braking. We also explore two other braking agents: collisions with dust
grains and neutral gas. Grains surrounding beta Pic are photoelectrically
charged to a positive electrostatic potential. If a significant fraction of the
grains are carbonaceous (10% in the midplane and larger at higher altitudes),
ions can be slowed down to satisfy the observed velocity constraints. For
neutral gas to brake the coupled ion fluid, we find the minimum required mass
to be 0.03 M_\earth, consistent with observed upper limits of the
hydrogen column density, and substantially reduced relative to previous
estimates. Our results favor a scenario in which metallic gas is generated by
grain evaporation in the disk, perhaps during grain-grain collisions. We
exclude a primordial origin for the gas, but cannot rule out the possibility of
its production by falling evaporating bodies near the star. We discuss the
implications of this work for observations of gas in other debris disks.Comment: 19 pages, 12 figures, emulateapj. Accepted for publication in Ap
New constrains on Gliese 86 B
We present the results of multi epochs imaging observations of the companion
to the planetary host Gliese 86. Associated to radial velocity measurements,
this study aimed at characterizing dynamically the orbital properties and the
mass of this companion (here after Gliese 86 B), but also at investigating the
possible history of this particular system. We used the adaptive optics
instrument NACO at the ESO Very Large Telescope to obtain deep coronographic
imaging in order to determine new photometric and astrometric measurements of
Gliese 86 B. Part of the orbit is resolved. The photometry of Gliese B
indicates colors compatible with a ~70 Jupiter mass brown dwarf or a white
dwarf. Both types of objects allow to fit the available, still limited
astrometric data. Besides, if we attribute the long term radial velocity
residual drift observed for Gliese A to B, then the mass of the latter object
is ~0.5 Msun. We analyse both astrometric and radial velocity data to propose
first orbital parameters for Gliese B. Assuming Gliese B is a ~0.5 Msun white
dwarf, we explore the constraints induced by this hypothesis and refine the
parameters of the system.Comment: 10 pages, 18 figures, accepted in A&
First NACO observations of the Brown Dwarf LHS 2397aB
Observations of the standard late type M8 star LHS 2397aA were obtained at
the ESO-VLT 8m telescope ``Yepun'' using the NAOS/CONICA Adaptive Optics
facility. The observations were taken during the NACO commissioning, and the
infrared standard star LHS 2397aA was observed in the H, and Ks broad band
filters. In both bands the brown dwarf companion LHS2397aB was detected. Using
a program recently developed (Bouy et al., 2003) for the detection of stellar
binaries we calculated the principal astrometric parameters (angular binary
separation and position angle P.A.) and the photometry of LHS 2397aA and LHS
2397aB. Our study largely confirms previous results obtained with the
AO-Hokupa'a facility at Gemini-North (Freed et al., 2003); however a few
discrepancies are observed.Comment: 5 page
Multiple spiral patterns in the transitional disk of HD 100546
Protoplanetary disks around young stars harbor many structures related to
planetary formation. Of particular interest, spiral patterns were discovered
among several of these disks and are expected to be the sign of gravitational
instabilities leading to giant planets formation or gravitational perturbations
caused by already existing planets. In this context, the star HD100546 presents
some specific characteristics with a complex gas and dusty disk including
spirals as well as a possible planet in formation. The objective of this study
is to analyze high contrast and high angular resolution images of this
emblematic system to shed light on critical steps of the planet formation. We
retrieved archival images obtained at Gemini in the near IR (Ks band) with the
instrument NICI and processed the data using advanced high contrast imaging
technique taking advantage of the angular differential imaging. These new
images reveal the spiral pattern previously identified with HST with an
unprecedented resolution, while the large-scale structure of the disk is mostly
erased by the data processing. The single pattern at the southeast in HST
images is now resolved into a multi-armed spiral pattern. Using two models of a
gravitational perturber orbiting in a gaseous disk we attempted to bring
constraints on the characteristics of this perturber assuming each spiral being
independent and we derived qualitative conclusions. The non-detection of the
northeast spiral pattern observed in HST allows to put a lower limit on the
intensity ratio between the two sides of the disk, which if interpreted as
forward scattering yields a larger anisotropic scattering than derived in the
visible. Also, we found that the spirals are likely spatially resolved with a
thickness of about 5-10AU. Finally, we did not detect the candidate forming
planet recently discovered in the Lp band, with a mass upper limit of 16-18 MJ.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 8
figure
Sparse aperture masking at the VLT II. Detection limits for the eight debris disks stars Pic, AU Mic, 49 Cet, Tel, Fomalhaut, g Lup, HD181327 and HR8799
Context. The formation of planetary systems is a common, yet complex
mechanism. Numerous stars have been identified to possess a debris disk, a
proto-planetary disk or a planetary system. The understanding of such formation
process requires the study of debris disks. These targets are substantial and
particularly suitable for optical and infrared observations. Sparse Aperture
masking (SAM) is a high angular resolution technique strongly contributing to
probe the region from 30 to 200 mas around the stars. This area is usually
unreachable with classical imaging, and the technique also remains highly
competitive compared to vortex coronagraphy. Aims. We aim to study debris disks
with aperture masking to probe the close environment of the stars. Our goal is
either to find low mass companions, or to set detection limits. Methods. We
observed eight stars presenting debris disks ( Pictoris, AU
Microscopii, 49 Ceti, Telescopii, Fomalhaut, g Lupi, HD181327 and
HR8799) with SAM technique on the NaCo instrument at the VLT. Results. No close
companions were detected using closure phase information under 0.5 of
separation from the parent stars. We obtained magnitude detection limits that
we converted to Jupiter masses detection limits using theoretical isochrones
from evolutionary models. Conclusions. We derived upper mass limits on the
presence of companions in the area of few times the diffraction limit of the
telescope around each target star.Comment: 7 pages, All magnitude detection limits maps are only available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5
High resolution imaging of young M-type stars of the solar neighborhood: Probing the existence of companions down to the mass of Jupiter
Context. High contrast imaging is a powerful technique to search for gas
giant planets and brown dwarfs orbiting at separation larger than several AU.
Around solar-type stars, giant planets are expected to form by core accretion
or by gravitational instability, but since core accretion is increasingly
difficult as the primary star becomes lighter, gravitational instability would
be the a probable formation scenario for yet-to-be-found distant giant planets
around a low-mass star. A systematic survey for such planets around M dwarfs
would therefore provide a direct test of the efficiency of gravitational
instability. Aims. We search for gas giant planets orbiting around late-type
stars and brown dwarfs of the solar neighborhood. Methods. We obtained deep
high resolution images of 16 targets with the adaptive optic system of VLT-NACO
in the Lp band, using direct imaging and angular differential imaging. This is
currently the largest and deepest survey for Jupiter-mass planets around
Mdwarfs. We developed and used an integrated reduction and analysis pipeline to
reduce the images and derive our 2D detection limits for each target. The
typical contrast achieved is about 9 magnitudes at 0.5" and 11 magnitudes
beyond 1". For each target we also determine the probability of detecting a
planet of a given mass at a given separation in our images. Results. We derived
accurate detection probabilities for planetary companions, taking into account
orbital projection effects, with in average more than 50% probability to detect
a 3MJup companion at 10AU and a 1.5MJup companion at 20AU, bringing strong
constraints on the existence of Jupiter-mass planets around this sample of
young M-dwarfs.Comment: Accepted for publication in A&
Pressure-induced enhancement of superconductivity and superconducting-superconducting transition in CaC
We measured the electrical resistivity, , of superconducting
CaC at ambient and high pressure up to 16 GPa. For 8 GPa, we found
a large increase of with pressure from 11.5 up to 15.1 K. At 8 GPa,
drops and levels off at 5 K above 10 GPa. Correspondingly, the residual
increases by 200 times and the behavior
becomes flat. The recovery of the pristine behavior after depressurization is
suggestive of a phase transition at 8 GPa between two superconducting phases
with good and bad metallic properties, the latter with a lower and more
static disorder
Orbital characterization of the \beta Pictoris b giant planet
In June 2010, we confirmed the existence of a giant planet in the disk of the
young star Beta Pictoris, located between 8 AU and 15 AU from the star. This
young planet offers the rare opportunity to monitor a large fraction of the
orbit using the imaging technique over a reasonably short timescale. Using the
NAOS-CONICA adaptive-optics instrument (NACO) at the Very Large Telescope
(VLT), we obtained repeated follow-up images of the Bpic system in the Ks and
L' filters at four new epochs in 2010 and 2011. Complementing these data with
previous measurements, we conduct a homogeneous analysis, which covers more
than eight yrs, to accurately monitor the Bpic b position relative to the star.
On the basis of the evolution of the planet's relative position with time, we
derive the best-fit orbital solutions for our measurements. More reliable
results are found with a Markov-chain Monte Carlo approach. The solutions favor
a low-eccentricity orbit e < 0.17, with semi-major axis in the range 8--9 AU
corresponding to orbital periods of 17--21 yrs. Our solutions favor a highly
inclined solution with a peak around i=88.5+-1.7 deg, and a longitude of
ascending node tightly constrained at Omega = -147.5+-1.5 deg. These results
indicate that the orbital plane of the planet is likely to be above the
midplane of the main disk, and compatible with the warp component of the disk
being tilted between 3.5 deg and 4.0 deg. This suggests that the planet plays a
key role in the origin of the inner warped-disk morphology of the Bpic disk.
Finally, these orbital parameters are consistent with the hypothesis that the
planet is responsible for the transit-like event observed in November 1981, and
also linked to the cometary activity observed in the Bpic system.Comment: 10 pages, 12 figures, accepted to A&
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