53 research outputs found
Data reduction methods for single-mode optical interferometry - Application to the VLTI two-telescopes beam combiner VINCI
The interferometric data processing methods that we describe in this paper
use a number of innovative techniques. In particular, the implementation of the
wavelet transform allows us to obtain a good immunity of the fringe processing
to false detections and large amplitude perturbations by the atmospheric piston
effect, through a careful, automated selection of the interferograms. To
demonstrate the data reduction procedure, we describe the processing and
calibration of a sample of stellar data from the VINCI beam combiner. Starting
from the raw data, we derive the angular diameter of the dwarf star Alpha Cen
A. Although these methods have been developed specifically for VINCI, they are
easily applicable to other single-mode beam combiners, and to spectrally
dispersed fringes.Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages, 19
figure
A catalog of bright calibrator stars for 200-meter baseline near-infrared stellar interferometry
We present in this paper a catalog of reference stars suitable for
calibrating infrared interferometric observations. In the K band, visibilities
can be calibrated with a precision of 1% on baselines up to 200 meters for the
whole sky, and up to 300 meters for some part of the sky. This work, extending
to longer baselines a previous catalog compiled by Borde et al. (2002), is
particularly well adapted to hectometric-class interferometers such as the Very
Large Telescope Interferometer (VLTI, Glindemann et al. 2003) or the CHARA
array (ten Brummelaar et al. 2003) when observing well resolved, high surface
brightness objects (K<8). We use the absolute spectro-photometric calibration
method introduced by Cohen et al. (1999) to derive the angular diameters of our
new set of 948 G8--M0 calibrator stars extracted from IRAS, 2MASS and MSX
catalogs. Angular stellar diameters range from 0.6 mas to 1.8 mas (median is
1.1 mas) with a median precision of 1.35%. For both the northern and southern
hemispheres, the closest calibrator star is always less than 10 degree away.Comment: 9 pages, 7 figures, submitted to A&A. The full catalog can be found
in http://calys.obspm.fr/~merand/Files/MerandEtAlCatalogue.tx
Asteroseismology and interferometry of the red giant star epsilon Oph
The GIII red giant star epsilon Oph has been found to exhibit several modes
of oscillation by the MOST mission. We interpret the observed frequencies of
oscillation in terms of theoretical radial p-mode frequencies of stellar
models. Evolutionary models of this star, in both shell H-burning and core
He-burning phases of evolution, are constructed using as constraints a
combination of measurements from classical ground-based observations (for
luminosity, temperature, and chemical composition) and seismic observations
from MOST. Radial frequencies of models in either evolutionary phase can
reproduce the observed frequency spectrum of epsilon Oph almost equally well.
The best-fit models indicate a mass in the range of 1.85 +/- 0.05 Msun with
radius of 10.55 +/- 0.15 Rsun. We also obtain an independent estimate of the
radius of epsilon Oph using high accuracy interferometric observations in the
infrared K' band, using the CHARA/FLUOR instrument. The measured limb darkened
disk angular diameter of epsilon Oph is 2.961 +/- 0.007 mas. Together with the
Hipparcos parallax, this translates into a photospheric radius of 10.39 +/-
0.07 Rsun. The radius obtained from the asteroseismic analysis matches the
interferometric value quite closely even though the radius was not constrained
during the modelling.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
EChOSim: The Exoplanet Characterisation Observatory software simulator
EChOSim is the end-to-end time-domain simulator of the Exoplanet
Characterisation Observatory (EChO) space mission. EChOSim has been developed
to assess the capability EChO has to detect and characterize the atmospheres of
transiting exoplanets, and through this revolutionize the knowledge we have of
the Milky Way and of our place in the Galaxy. Here we discuss the details of
the EChOSim implementation and describe the models used to represent the
instrument and to simulate the detection. Software simulators have assumed a
central role in the design of new instrumentation and in assessing the level of
systematics affecting the measurements of existing experiments. Thanks to its
high modularity, EChOSim can simulate basic aspects of several existing and
proposed spectrometers for exoplanet transits, including instruments on the
Hubble Space Telescope and Spitzer, or ground-based and balloon borne
experiments. A discussion of different uses of EChOSim is given, including
examples of simulations performed to assess the EChO mission
A near-infrared interferometric survey of debris disc stars. II. CHARA/FLUOR observations of six early-type dwarfs
High-precision interferometric observations of six early-type main sequence
stars known to harbour cold debris discs have been obtained in the
near-infrared K band with the FLUOR instrument at the CHARA Array. The measured
squared visibilities are compared to the expected visibility of the stellar
photospheres based on theoretical photospheric models taking into account
rotational distortion, searching for potential visibility reduction at short
baselines due to circumstellar emission. Our observations bring to light the
presence of resolved circumstellar emission around one of the six target stars
(zeta Aql) at the 5 sigma level. The morphology of the emission source cannot
be directly constrained because of the sparse spatial frequency sampling of our
interferometric data. Using complementary adaptive optics observations and
radial velocity measurements, we find that the presence of a low-mass companion
is a likely origin for the excess emission. The potential companion has a
K-band contrast of four magnitudes, a most probable mass of about 0.6 Msun, and
is expected to orbit between about 5.5 AU and 8 AU from its host star assuming
a purely circular orbit. Nevertheless, by adjusting a physical debris disc
model to the observed Spectral Energy Distribution of the zeta Aql system, we
also show that the presence of hot dust within 10 AU from zeta Aql, producing a
total thermal emission equal to 1.69 +- 0.31% of the photospheric flux in the K
band, is another viable explanation for the observed near-infrared excess. Our
re-interpretation of archival near- to far-infrared photometric measurements
shows however that cold dust is not present around zeta Aql at the sensitivity
limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove
zeta Aql from the category of bona fide debris disc stars.Comment: 14 pages, accepted for publication in A&
Are dust shell models well-suited to explain interferometric data of late-type stars in the near-infrared?
Recently available near-infrared interferometric data on late-type stars show
a strong increase of diameter for asymptotic giant branch (AGB) stars between
the K (2.0 - 2.4 \mu m) and L (3.4 - 4.1 \mu m) bands. Aiming at an explanation
of these findings, we chose the objects \alpha Orionis (Betelgeuse), SW
Virginis, and R Leonis, which are of different spectral types and stages of
evolution, and which are surrounded by circumstellar envelopes with different
optical thicknesses. For these stars, we compared observations with spherically
symmetric dust shell models. Photometric and 11 \mu m interferometric data were
also taken into account to further constrain the models. -- [...] -- We
conclude that AGB models comprising a photosphere and dust shell, although
consistent with SED data and also interferometric data in K and at 11 \mu m,
cannot explain the visibility data in L; an additional source of model opacity,
possibly related to a gas component, is needed in L to be consistent with the
visibility data.Comment: 12 pages, 4 figures, to be published in A&A; Latex aa class, uses
packages graphicx, hyperref, natbib, and txfonts; keywords: Techniques:
interferometric -- Radiative transfer -- Circumstellar matter -- Infrared:
stars -- Stars: late-type -- Stars: individual: \alpha Orionis, SW Virginis,
R Leoni
Constraining the Nature of the PDS 70 Protoplanets with VLTI/GRAVITY
We present K-band interferometric observations of the PDS 70 protoplanets along with their host star using VLTI/ GRAVITY. We obtained K-band spectra and 100 μas precision astrometry of both PDS 70 b and c in two epochs, as well as spatially resolving the hot inner disk around the star. Rejecting unstable orbits, we found a nonzero eccentricity for PDS 70 b of 0.17 ± 0.06, a near-circular orbit for PDS 70 c, and an orbital configuration that is consistent with the planets migrating into a 2:1 mean motion resonance. Enforcing dynamical stability, we obtained a 95% upper limit on the mass of PDS 70 b of 10 MJup, while the mass of PDS 70 c was unconstrained. The GRAVITY K-band spectra rules out pure blackbody models for the photospheres of both planets. Instead, the models with the most support from the data are planetary atmospheres that are dusty, but the nature of the dust is unclear. Any circumplanetary dust around these planets is not well constrained by the planets’ 1–5 μm spectral energy distributions (SEDs) and requires longer wavelength data to probe with SED analysis. However with VLTI/GRAVITY, we made the first observations of a circumplanetary environment with sub-astronomical-unit spatial resolution, placing an upper limit of 0.3 au on the size of a bright disk around PDS 70 b
The Mass of β Pictoris C from β Pictoris b Orbital Motion
Aims. We aim to demonstrate that the presence and mass of an exoplanet can now be effectively derived from the astrometry of another exoplanet. Methods. We combined previous astrometry of β Pictoris b with a new set of observations from the GRAVITY interferometer. The orbital motion of β Pictoris b is fit using Markov chain Monte Carlo simulations in Jacobi coordinates. The inner planet, β Pictoris c, was also reobserved at a separation of 96 mas, confirming the previous orbital estimations. Results. From the astrometry of planet b only, we can (i) detect the presence of β Pictoris c and (ii) constrain its mass to 10.04-3.10+4.53 MJup. If one adds the astrometry of β Pictoris c, the mass is narrowed down to 9.15-1.06+1.08 MJup. The inclusion of radial velocity measurements does not affect the orbital parameters significantly, but it does slightly decrease the mass estimate to 8.89-0.75+0.75 MJup. With a semimajor axis of 2.68 ± 0.02 au, a period of 1221 ± 15 days, and an eccentricity of 0.32 ± 0.02, the orbital parameters of β Pictoris c are now constrained as precisely as those of β Pictoris b. The orbital configuration is compatible with a high-order mean-motion resonance (7:1). The impact of the resonance on the planets\u27 dynamics would then be negligible with respect to the secular perturbations, which might have played an important role in the eccentricity excitation of the outer planet
Direct Confirmation of the Radial-Velocity Planet β Pictoris c
Context. Methods used to detect giant exoplanets can be broadly divided into two categories: indirect and direct. Indirect methods are more sensitive to planets with a small orbital period, whereas direct detection is more sensitive to planets orbiting at a large distance from their host star. This dichotomy makes it difficult to combine the two techniques on a single target at once. Aims. Simultaneous measurements made by direct and indirect techniques offer the possibility of determining the mass and luminosity of planets and a method of testing formation models. Here, we aim to show how long-baseline interferometric observations guided by radial-velocity can be used in such a way. Methods. We observed the recently-discovered giant planet β Pictoris c with GRAVITY, mounted on the Very Large Telescope Interferometer. Results. This study constitutes the first direct confirmation of a planet discovered through radial velocity. We find that the planet has a temperature of T = 1250 ± 50 K and a dynamical mass of M = 8.2 ± 0.8 MJup. At 18.5 ± 2.5 Myr, this puts β Pic c close to a \u27hot start\u27 track, which is usually associated with formation via disk instability. Conversely, the planet orbits at a distance of 2.7 au, which is too close for disk instability to occur. The low apparent magnitude (MK = 14.3 ± 0.1) favours a core accretion scenario. Conclusions. We suggest that this apparent contradiction is a sign of hot core accretion, for example, due to the mass of the planetary core or the existence of a high-temperature accretion shock during formation
First direct detection of an exoplanet by optical interferometry; Astrometry and K-band spectroscopy of HR8799 e
To date, infrared interferometry at best achieved contrast ratios of a few
times on bright targets. GRAVITY, with its dual-field mode, is now
capable of high contrast observations, enabling the direct observation of
exoplanets. We demonstrate the technique on HR8799, a young planetary system
composed of four known giant exoplanets. We used the GRAVITY fringe tracker to
lock the fringes on the central star, and integrated off-axis on the HR8799e
planet situated at 390 mas from the star. Data reduction included
post-processing to remove the flux leaking from the central star and to extract
the coherent flux of the planet. The inferred K band spectrum of the planet has
a spectral resolution of 500. We also derive the astrometric position of the
planet relative to the star with a precision on the order of 100as. The
GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital
solutions. A small adjustment of a few degrees to the orbital inclination of HR
8799 e can resolve the tension, implying that the orbits are close to, but not
strictly coplanar. The spectrum, with a signal-to-noise ratio of
per spectral channel, is compatible with a late-type L brown dwarf. Using
Exo-REM synthetic spectra, we derive a temperature of \,K and a
surface gravity of cm/s. This corresponds to a radius
of and a mass of , which is an independent confirmation of mass estimates from evolutionary
models. Our results demonstrate the power of interferometry for the direct
detection and spectroscopic study of exoplanets at close angular separations
from their stars.Comment: published in A&
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