3,216 research outputs found
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&
Fully Analyzing an Algebraic Polya Urn Model
This paper introduces and analyzes a particular class of Polya urns: balls
are of two colors, can only be added (the urns are said to be additive) and at
every step the same constant number of balls is added, thus only the color
compositions varies (the urns are said to be balanced). These properties make
this class of urns ideally suited for analysis from an "analytic combinatorics"
point-of-view, following in the footsteps of Flajolet-Dumas-Puyhaubert, 2006.
Through an algebraic generating function to which we apply a multiple
coalescing saddle-point method, we are able to give precise asymptotic results
for the probability distribution of the composition of the urn, as well as
local limit law and large deviation bounds.Comment: LATIN 2012, Arequipa : Peru (2012
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&
Deep search for companions to probable young brown dwarfs
We have obtained high contrast images of four nearby, faint, and very low
mass objects 2MASSJ04351455-1414468, SDSSJ044337.61+000205.1,
2MASSJ06085283-2753583 and 2MASSJ06524851-5741376 (here after 2MASS0435-14,
SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as
probable isolated young brown dwarfs. Our goal was to search for binary
companions down to the planetary mass regime. We used the NAOS-CONICA adaptive
optics instrument (NACO) and its unique capability to sense the wavefront in
the near-infrared to acquire sharp images of the four systems in Ks, with a
field of view of 28"*28". Additional J and L' imaging and follow-up
observations at a second epoch were obtained for 2MASS0652-57. With a typical
contrast DKs= 4.0-7.0 mag, our observations are sensitive down to the planetary
mass regime considering a minimum age of 10 to 120 Myr for these systems. No
additional point sources are detected in the environment of 2MASS0435-14,
SDSS0443+00 and 2MASS0608-27 between 0.1-12" (i.e about 2 to 250 AU at 20 pc).
2MASS0652-57 is resolved as a \sim230 mas binary. Follow-up observations reject
a background contaminate, resolve the orbital motion of the pair, and confirm
with high confidence that the system is physically bound. The J, Ks and L'
photometry suggest a q\sim0.7-0.8 mass ratio binary with a probable semi-major
axis of 5-6 AU. Among the four systems, 2MASS0652-57 is probably the less
constrained in terms of age determination. Further analysis would be necessary
to confirm its youth. It would then be interesting to determine its orbital and
physical properties to derive the system's dynamical mass and to test
evolutionary model predictions.Comment: Research note, 5 pages, 2 tables and 3 figures, accepted to A&
A companion candidate in the gap of the T Cha transitional disk
T Cha is a young star surrounded by a cold disk. The presence of a gap within
its disk, inferred from fitting to the spectral energy distribution, has
suggested on-going planetary formation. We observed T Cha in L' and K_s with
NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture
masking. We detected a source in the L' data at a separation of 62+-7 mas,
position angle of 78+-1 degrees, and a contrast of delta L' = 5.1+-0.2 mag. The
object is not detected in the Ks band data, which show a 3-sigma contrast limit
of 5.2 mag at the position of the detected L' source. For a distance of 108 pc,
the detected companion candidate is located at 6.7 AU from the primary, well
within the disk gap. If T Cha and the companion candidate are bound, the
comparison of the L' and Ks photometry with evolutionary tracks shows that the
photometry is inconsistent with any unextincted photosphere at the age and
distance of T Cha. The detected object shows a very red Ks-L' color for which a
possible explanation would be a significant amount of dust around it. This
would imply that the companion candidate is young, which would strengthen the
case for a physical companion, and moreover that the object would be in the
substellar regime, according to the Ks upper limit. Another exciting
possibility would be that this companion is a recently formed planet within the
disk. Additional observations are mandatory to confirm that the object is bound
and to properly characterize it.Comment: 4 pages, 4 figures; accepted for publication by A&
Search for cool giant exoplanets around young and nearby stars - VLT/NaCo near-infrared phase-coronagraphic and differential imaging
[Abridged] Context. Spectral differential imaging (SDI) is part of the
observing strategy of current and future high-contrast imaging instruments. It
aims to reduce the stellar speckles that prevent the detection of cool planets
by using in/out methane-band images. It attenuates the signature of off-axis
companions to the star, such as angular differential imaging (ADI). However,
this attenuation depends on the spectral properties of the low-mass companions
we are searching for. The implications of this particularity on estimating the
detection limits have been poorly explored so far. Aims. We perform an imaging
survey to search for cool (Teff<1000-1300 K) giant planets at separations as
close as 5-10 AU. We also aim to assess the sensitivity limits in SDI data
taking the photometric bias into account. This will lead to a better view of
the SDI performance. Methods. We observed a selected sample of 16 stars (age <
200 Myr, d < 25 pc) with the phase-mask coronagraph, SDI, and ADI modes of
VLT/NaCo. Results. We do not detect any companions. As for the sensitivity
limits, we argue that the SDI residual noise cannot be converted into mass
limits because it represents a differential flux, unlike the case of
single-band images. This results in degeneracies for the mass limits, which may
be removed with the use of single-band constraints. We instead employ a method
of directly determining the mass limits. The survey is sensitive to cool giant
planets beyond 10 AU for 65% and 30 AU for 100% of the sample. Conclusions. For
close-in separations, the optimal regime for SDI corresponds to SDI flux ratios
>2. According to the BT-Settl model, this translates into Teff<800 K. The
methods described here can be applied to the data interpretation of SPHERE. We
expect better performance with the dual-band imager IRDIS, thanks to more
suitable filter characteristics and better image quality.Comment: 19 pages, 16 figures, accepted for publication in A&A, version
including language editin
beta Pic b position relative to the Debris Disk
Context. We detected in 2009 a giant, close-by planet orbiting {\beta} Pic, a
young star surrounded with a disk, extensively studied for more than 20 years.
We showed that if located on an inclined orbit, the planet could explain
several peculiarities of {\beta} Pictoris system. However, the available data
did not permit to measure the inclination of {\beta} Pic b with respect to the
disk, and in particular to establish in which component of the disk - the main,
extended disk or the inner inclined component/disk-, the planet was located.
Comparison between the observed planet position and the disk orientation
measured on previous imaging data was not an option because of potential biases
in the measurements. Aims. Our aim is to measure precisely the planet location
with respect to the dust disk using a single high resolution image, and
correcting for systematics or errors that degrades the precision of the disk
and planet relative position measurements. Methods. We gathered new NaCo data
at Ks band, with a set-up optimized to derive simultaneously the orientation(s)
of the disk(s) and that of the planet. Results. We show that the projected
position of {\beta} Pic b is above the midplane of the main disk. With the
current data and knowledge on the system, this implies that {\beta} Pic b
cannot be located in the main disk. The data rather suggest the planet being
located in the inclined component.Comment: 13 pages, 6 figures, to appear in Astronomy and Astrophysic
Characterization of the long-period companions of the exoplanet host stars: HD196885, HD1237 and HD27442
We present the results of near-infrared, follow-up imaging and spectroscopic
observations at VLT, aimed at characterizing the long-period companions of the
exoplanet host stars HD196885, HD1237 and HD27442. The three companions were
previously discovered in the course of our CFHT and VLT coronographic imaging
survey dedicated to the search for faint companions of exoplanet host stars. We
used the NACO near-infrared adaptive optics instrument to obtain astrometric
follow-up observations of HD196885 A and B. The long-slit spectroscopic mode of
NACO and the integral field spectrograph SINFONI were used to carry out a
low-resolution spectral characterization of the three companions HD196885 B,
HD1237 B and HD27442 B between 1.4 and 2.5 microns. We can now confirm that the
companion HD196885 B is comoving with its primary exoplanet host star, as
previously shown for HD1237 B and HD27442 B. We find that both companions
HD196885 B and HD1237 B are low-mass stars of spectral type M1V and M4V
respectively. HD196885 AB is one of the closer (~23 AU) resolved binaries known
to host an exoplanet. This system is then ideal for carrying out a combined
radial velocity and astrometric investigation of the possible impact of the
binary companion on the planetary system formation and evolution. Finally, we
confirm via spectroscopy that HD27442 B is a white dwarf companion, the third
one to be discovered orbiting an exoplanet host star, following HD147513 and
Gliese 86. The detection of the broad Bracket gamma line of hydrogen indicates
a white dwarf atmosphere dominated by hydrogen.Comment: 6 pages, 3 figures and 3 table
Search for cool extrasolar giant planets combining coronagraphy, spectral and angular differential imaging
Spectral differential imaging (SDI) is part of the observing strategy of
current and on-going high-contrast imaging instruments on ground-based
telescopes. Although it improves the star light rejection, SDI attenuates the
signature of off-axis companions to the star, just like angular differential
imaging (ADI). However, the attenuation due to SDI has the peculiarity of being
dependent on the spectral properties of the companions. To date, no study has
investigated these effects. Our team is addressing this problem based on data
from a direct imaging survey of 16 stars combining the phase-mask coronagraph,
the SDI and the ADI modes of VLT/NaCo. The objective of the survey is to search
for cool (Teff<1000-1300 K) giant planets at separations of 5-10 AU orbiting
young, nearby stars (<200 Myr, <25 pc). The data analysis did not yield any
detections. As for the estimation of the sensitivity limits of SDI-processed
images, we show that it requires a different analysis than that used in
ADI-based surveys. Based on a method using the flux predictions of evolutionary
models and avoiding the estimation of contrast, we determine directly the mass
sensitivity limits of the survey for the ADI processing alone and with the
combination of SDI and ADI. We show that SDI does not systematically improve
the sensitivity due to the spectral properties and self-subtraction of point
sources.Comment: 5 pages, 2 figure
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