605 research outputs found
First light of the VLT planet finder SPHERE. I. Detection and characterization of the sub-stellar companion GJ 758 B
GJ758 B is a brown dwarf companion to a nearby (15.76 pc) solar-type,
metal-rich (M/H = +0.2 dex) main-sequence star (G9V) that was discovered with
Subaru/HiCIAO in 2009. From previous studies, it has drawn attention as being
the coldest (~600K) companion ever directly imaged around a neighboring star.
We present new high-contrast data obtained during the commissioning of the
SPHERE instrument at the VLT. The data was obtained in Y-, J-, H-, and Ks-bands
with the dual-band imaging (DBI) mode of IRDIS, providing a broad coverage of
the full near-infrared (near-IR) range at higher contrast and better spectral
sampling than previously reported. In this new set of high-quality data, we
report the re-detection of the companion, as well as the first detection of a
new candidate closer-in to the star. We use the new 8 photometric points for an
extended comparison of GJ758 B with empirical objects and 4 families of
atmospheric models. From comparison to empirical object, we estimate a T8
spectral type, but none of the comparison object can accurately represent the
observed near-IR fluxes of GJ758 B. From comparison to atmospheric models, we
attribute a Teff = 600K 100K, but we find that no atmospheric model can
adequately fit all the fluxes of GJ758 B. The photometry of the new candidate
companion is broadly consistent with L-type objects, but a second epoch with
improved photometry is necessary to clarify its status. The new astrometry of
GJ758 B shows a significant proper motion since the last epoch. We use this
result to improve the determination of the orbital characteristics using two
fitting approaches, Least-Square Monte Carlo and Markov Chain Monte Carlo.
Finally, we analyze the sensitivity of our data to additional closer-in
companions and reject the possibility of other massive brown dwarf companions
down to 4-5 AU. [abridged]Comment: 20 pages, 15 figures. Accepted for publication in A&
High-contrast spectroscopy of SCR J1845-6357 B
Spectral characterization of sub-stellar companions is essential to
understand their composition and formation processes. However, the large
contrast ratio of the brightness of each object to that of its parent star
limits our ability to extract a clean spectrum, free from any significant
contribution from the star. During the development of the long slit
spectroscopy (LSS) mode of IRDIS, the dual-band imager and spectrograph of
SPHERE, we proposed a data analysis method to estimate and remove the
contributions of the stellar spectrum. This method has never been tested on
real data because of the lack of instrumentation capable of combining adaptive
optics (AO), coronagraphy, and LSS. Nonetheless, a similar attenuation of the
star can be obtained using a particular observing configuration. Test data were
acquired using the AO-assisted spectrograph VLT/NACO. We obtained new J- and
H-band spectra of SCR J1845-6357 B, a T6 companion to a nearby (3.85\pm0.02 pc)
M8 star. This system is a well-suited benchmark as it is relatively wide
(~1.0") with a modest contrast ratio (~4 mag), and a previously published JHK
spectrum is available for reference. We demonstrate that (1) our method is
efficient at estimating and removing the stellar contribution, (2) it allows to
properly recover the spectral shape of the companion, and (3) it is essential
to obtain an unbiased estimation of physical parameters. We also show that the
slit configuration associated with this method allows us to use long exposure
times with high throughput producing high signal-to-noise ratio data. However,
the signal of the companion gets over-subtracted, particularly in our J-band
data, compelling us to use a fake companion spectrum to estimate and compensate
for the loss of flux. Finally, we report a new astrometric measurement of the
position of the companion (sep = 0.817", PA = 227.92 deg).Comment: 11 pages, 8 figures, 4 tables. Accepted for publication in A&
Apodization in high-contrast long-slit spectroscopy. Closer, deeper, fainter, cooler
The spectroscopy of faint planetary-mass companions to nearby stars is one of
the main challenges that new-generation high-contrast spectro-imagers are going
to face. In a previous work we presented a long slit coronagraph (LSC), for
which the presence of a slit in the coronagraphic focal plane induces a complex
distribution of energy in the Lyot pupil-plane that cannot be easily masked
with a binary Lyot stop. To alleviate this concern, we propose to use a pupil
apodization to suppress diffraction, creating an apodized long slit coronagraph
(ALSC). After describing how the apodization is optimized, we demonstrate its
advantages with respect to the CLC in the context of SPHERE/IRDIS long slit
spectroscopy (LSS) mode at low-resolution with a 0.12" slit and 0.18"
coronagraphic mask. We perform different sets of simulations with and without
aberrations, and with and without a slit to demonstrate that the apodization is
a more appropriate concept for LSS, at the expense of a significantly reduced
throughput (37%) compared to the LSC. Then we perform detailed end-to-end
simulations of the LSC and the ALSC that include realistic levels of
aberrations to obtain datasets representing 1h of integration time on stars of
spectral types A0 to M0 located at 10 pc. We insert spectra of planetary
companions at different effective temperatures (Teff) and surface gravities
(log g) into the data at angular separations of 0.3" to 1.5" and with contrast
ratios from 6 to 18 mag. Using the SD method to subtract the speckles, we show
that the ALSC brings a gain in sensitivity of up to 3 mag at 0.3" with respect
to the LSC, which leads to a much better spectral extraction below 0.5". In
terms of Teff, we demonstrate that at small angular separations the limit with
the ALSC is always lower by at least 100K, inducing an increase of sensitivity
of a factor up to 1.8 in objects' masses at young ages. [Abridged]Comment: 15 pages, 17 figures. Accepted for publication in A&
The International Deep Planet Survey I. The frequency of wide-orbit massive planets around A-stars
Breakthrough direct detections of planetary companions orbiting A-type stars
confirm the existence of massive planets at relatively large separations, but
dedicated surveys are required to estimate the frequency of similar planetary
systems. To measure the first estimation of the giant exoplanetary systems
frequency at large orbital separation around A-stars, we have conducted a
deep-imaging survey of young (8-400 Myr), nearby (19-84 pc) A- and F-stars to
search for substellar companions in the 10-300 AU range. The sample of 42 stars
combines all A-stars observed in previous AO planet search surveys reported in
the literature with new AO observations from VLT/NaCo and Gemini/NIRI. It
represents an initial subset of the International Deep Planet Survey (IDPS)
sample of stars covering M- to B-stars. The data were obtained with
diffraction-limited observations in H- and Ks-band combined with angular
differential imaging to suppress the speckle noise of the central stars,
resulting in typical 5-sigma detection limits in magnitude difference of 12 mag
at 1", 14 mag at 2" and 16 mag at 5" which is sufficient to detect massive
planets. A detailed statistical analysis of the survey results is performed
using Monte Carlo simulations. Considering the planet detections, we estimate
the fraction of A-stars having at least one massive planet (3-14 MJup) in the
range 5-320 AU to be inside 5.9-18.8% at 68% confidence, assuming a flat
distribution for the mass of the planets. By comparison, the brown dwarf (15-75
MJup) frequency for the sample is 2.0-8.9% at 68% confidence in the range 5-320
AU. Assuming power law distributions for the mass and semimajor axis of the
planet population, the AO data are consistent with a declining number of
massive planets with increasing orbital radius which is distinct from the
rising slope inferred from radial velocity (RV) surveys around evolved A-stars.Comment: 20 pages, 10 figures, 7 tables. Accepted for publication in A&
High-contrast imaging of Sirius~A with VLT/SPHERE: Looking for giant planets down to one astronomical unit
Sirius has always attracted a lot of scientific interest, especially after
the discovery of a companion white dwarf at the end of the 19th century. Very
early on, the existence of a potential third body was put forward to explain
some of the observed properties of the system. We present new coronagraphic
observations obtained with VLT/SPHERE that explore, for the very first time,
the innermost regions of the system down to 0.2" (0.5 AU) from Sirius A. Our
observations cover the near-infrared from 0.95 to 2.3 m and they offer the
best on-sky contrast ever reached at these angular separations. After detailing
the steps of our SPHERE/IRDIFS data analysis, we present a robust method to
derive detection limits for multi-spectral data from high-contrast imagers and
spectrographs. In terms of raw performance, we report contrasts of 14.3 mag at
0.2", ~16.3 mag in the 0.4-1.0" range and down to 19 mag at 3.7". In physical
units, our observations are sensitive to giant planets down to 11 at
0.5 AU, 6-7 in the 1-2 AU range and ~4 at 10 AU. Despite
the exceptional sensitivity of our observations, we do not report the detection
of additional companions around Sirius A. Using a Monte Carlo orbital analysis,
we show that we can reject, with about 50% probability, the existence of an 8
planet orbiting at 1 AU. In addition to the results presented in the
paper, we provide our SPHERE/IFS data reduction pipeline at
http://people.lam.fr/vigan.arthur/ under the MIT license.Comment: 16 pages, 10 figures, accepted for publication in MNRA
Photometric characterization of exoplanets using angular and spectral differential imaging
The direct detection of exoplanets has been the subject of intensive research
in the recent years. Data obtained with future high-contrast imaging
instruments optimized for giant planets direct detection are strongly limited
by the speckle noise. Specific observing strategies and data analysis methods,
such as angular and spectral differential imaging, are required to attenuate
the noise level and possibly detect the faint planet flux. Even though these
methods are very efficient at suppressing the speckles, the photometry of the
faint planets is dominated by the speckle residuals. The determination of the
effective temperature and surface gravity of the detected planets from
photometric measurements in different bands is then limited by the photometric
error on the planet flux. In this work we investigate this photometric error
and the consequences on the determination of the physical parameters of the
detected planets. We perform detailed end-to-end simulation with the CAOS-based
Software Package for SPHERE to obtain realistic data representing typical
observing sequences in Y, J, H and Ks bands with a high contrast imager. The
simulated data are used to measure the photometric accuracy as a function of
contrast for planets detected with angular and spectral+angular differential
methods. We apply this empirical accuracy to study the characterization
capabilities of a high-contrast differential imager. We show that the expected
photometric performances will allow the detection and characterization of
exoplanets down to the Jupiter mass at angular separations of 1.0" and 0.2"
respectively around high mass and low mass stars with 2 observations in
different filter pairs. We also show that the determination of the planets
physical parameters from photometric measurements in different filter pairs is
essentialy limited by the error on the determination of the surface gravity.Comment: 13 pages, 7 figures, 4 tables. Accepted for publication in MNRA
High-contrast imaging constraints on gas giant planet formation - The Herbig Ae/Be star opportunity
Planet formation studies are often focused on solar-type stars, implicitly
considering our Sun as reference point. This approach overlooks, however, that
Herbig Ae/Be stars are in some sense much better targets to study planet
formation processes empirically, with their disks generally being larger,
brighter and simply easier to observe across a large wavelength range. In
addition, massive gas giant planets have been found on wide orbits around early
type stars, triggering the question if these objects did indeed form there and,
if so, by what process. In the following I briefly review what we currently
know about the occurrence rate of planets around intermediate mass stars,
before discussing recent results from Herbig Ae/Be stars in the context of
planet formation. The main emphasis is put on spatially resolved polarized
light images of potentially planet forming disks and how these images - in
combination with other data - can be used to empirically constrain (parts of)
the planet formation process. Of particular interest are two objects, HD100546
and HD169142, where, in addition to intriguing morphological structures in the
disks, direct observational evidence for (very) young planets has been
reported. I conclude with an outlook, what further progress we can expect in
the very near future with the next generation of high-contrast imagers at 8-m
class telescopes and their synergies with ALMA.Comment: Accepted by Astrophysics and Space Science as invited short review in
special issue about Herbig Ae/Be stars; 12 pages incl. 5 figures, 2 tables
and reference
On-sky multi-wavelength phasing of segmented telescopes with the Zernike phase contrast sensor
Future Extremely Large Telescopes will adopt segmented primary mirrors with
several hundreds of segments. Cophasing of the segments together is essential
to reach high wavefront quality. The phasing sensor must be able to maintain
very high phasing accuracy during the observations, while being able to phase
segments dephased by several micrometers. The Zernike phase contrast sensor has
been demonstrated on-sky at the Very Large Telescope. We present the
multi-wavelength scheme that has been implemented to extend the capture range
from \pmlambda/2 on the wavefront to many micrometers, demonstrating that it is
successful at phasing mirrors with piston errors up to \pm4.0 micron on the
wavefront. We discuss the results at different levels and conclude with a
phasing strategy for a future Extremely Large Telescope.Comment: 17 pages, 8 figures, 2 tables. Accepted for publication in Applied
Optics; he final publised version is available on the OSA website:
http://www.opticsinfobase.org/abstract.cfm?msid=13671
Molecular line mapping of the giant molecular cloud associated with RCW 106 - II. Column density and dynamical state of the clumps
We present a fully sampled C^{18}O (1-0) map towards the southern giant
molecular cloud (GMC) associated with the HII region RCW 106, and use it in
combination with previous ^{13}CO (1-0) mapping to estimate the gas column
density as a function of position and velocity. We find localized regions of
significant ^{13}CO optical depth in the northern part of the cloud, with
several of the high-opacity clouds in this region likely associated with a
limb-brightened shell around the HII region G333.6-0.2. Optical depth
corrections broaden the distribution of column densities in the cloud, yielding
a log-normal distribution as predicted by simulations of turbulence.
Decomposing the ^{13}CO and C^{18}O data cubes into clumps, we find relatively
weak correlations between size and linewidth, and a more sensitive dependence
of luminosity on size than would be predicted by a constant average column
density. The clump mass spectrum has a slope near -1.7, consistent with
previous studies. The most massive clumps appear to have gravitational binding
energies well in excess of virial equilibrium; we discuss possible
explanations, which include magnetic support and neglect of time-varying
surface terms in the virial theorem. Unlike molecular clouds as a whole, the
clumps within the RCW 106 GMC, while elongated, appear to show random
orientations with respect to the Galactic plane.Comment: 17 pages, to appear in MNRA
Spectroscopy across the brown dwarf/planetary mass boundary - I. Near-infrared JHK spectra
With a uniform VLT SINFONI data set of nine targets, we have developed an
empirical grid of J,H,K spectra of the atmospheres of objects estimated to have
very low substellar masses of \sim5-20 MJup and young ages of \sim1-50 Myr.
Most of the targets are companions, objects which are especially valuable for
comparison with atmosphere and evolutionary models, as they present rare cases
in which the age is accurately known from the primary. Based on the sample
youth, all objects are expected to have low surface gravity, and this study
investigates the critical early phases of the evolution of substellar objects.
The spectra are compared with grids of five different theoretical atmosphere
models. This analysis represents the first systematic model comparison with
infrared spectra of young brown dwarfs. The fits to the full JHK spectra of
each object result in a range of best fit effective temperatures of +/-150-300K
whether or not the full model grid or a subset restricted to lower log(g)
values is used. This effective temperature range is significantly larger than
the uncertainty typically assigned when using a single model grid. Fits to a
single wavelength band can vary by up to 1000K using the different models.
Since the overall shape of these spectra is governed more by the temperature
than surface gravity, unconstrained model fits did not find matches with low
surface gravity or a trend in log(g) with age. This suggests that empirical
comparison with spectra of unambiguously young objects targets (such as these
SINFONI data) may be the most reliable method to search for indications of low
surface gravity and youth. For two targets, the SINFONI data are a second epoch
and the data show no variations in morphology over time. The analysis of two
other targets, AB Pic B and CT Cha B, suggests that these objects may have
lower temperatures, and consequently lower masses, than previously estimated.Comment: 15 pages, 13 figure
- …