27 research outputs found
The angular size of the Cepheid L Car: a comparison of the interferometric and surface brightness techniques
Recent interferometric observations of the brightest and angularly largest
classical Cepheid, L Carinae, with ESO's VLT Interferometer (VLTI) have
resolved with high precision the variation of its angular diameter with phase.
We compare the measured angular diameter curve to the one we derive by an
application of the Baade-Wesselink type infrared surface brightness technique,
and find a near-perfect agreement between the two curves. The mean angular
diameters of L Car from the two techniques agree very well within their total
error bars (1.5 %), as do the derived distances (4 %). This result is an
indication that the calibration of the surface brightness relations used in the
distance determination of far away Cepheids is not affected by large biases.Comment: 7 pages, 3 figures, accepted for publication in ApJ
Squared visibility estimator. Calibrating biases to reach very high dynamic range
In the near infrared where detectors are limited by read-out noise, most
interferometers have been operated in wide band in order to benefit from larger
photon rates. We analyze in this paper the biases caused by instrumental and
turbulent effects to estimators for both narrow and wide band cases.
Visibilities are estimated from samples of the interferogram using two
different estimators, which is the classical sum of the squared
modulus of Fourier components and a new estimator for which complex
Fourier components are summed prior to taking the square. We present an
approach for systematically evaluating the performance and limits of each
estimator, and to optimizing observing parameters for each. We include the
effects of spectral bandwidth, chromatic dispersion, scan length, and
differential piston. We also establish the expression of the Signal-to-Noise
Ratio of the two estimators with respect to detector and photon noise. The
estimator is insensitive to dispersion and is always more sensitive
than the estimator. However, the latter allows to reach better
accuracies when detection is differential piston noise limited. Biases and
noise directly impact the dynamic range of reconstructed images. Very high
dynamic ranges are required for direct exoplanet detection by interferometric
techniques thus requiring estimators to be bias-free or biases to be accurately
calibrated. We discuss which estimator and which conditions are optimum for
astronomical applications especially when high accuracy visibilities are
required. We show that there is no theoretical limit to measuring visibilities
with accuracies as good as which is important in the prospect of
detecting faint exoplanets with interferometers.Comment: 23 pages, 6 figures, accepted for publication in Ap
First Results from the CHARA Array VII: Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star
We have obtained high-precision interferometric measurements of Vega with the
CHARA Array and FLUOR beam combiner in the K' band at projected baselines
between 103m and 273m. The measured visibility amplitudes beyond the first lobe
are significantly weaker than expected for a slowly rotating star characterized
by a single effective temperature and surface gravity. Our measurements, when
compared to synthetic visibilities and synthetic spectrophotometry from a
Roche-von Zeipel gravity-darkened model atmosphere, provide strong evidence for
the model of Vega as a rapidly rotating star viewed very nearly pole-on. Our
best fitting model indicates that Vega is rotating at ~91% of its angular
break-up rate with an equatorial velocity of 275 km/s. Together with the
measured vsin(i), this velocity yields an inclination for the rotation axis of
5 degrees. For this model the pole-to-equator effective temperature difference
is 2250 K, a value much larger than previously derived from spectral line
analyses. The derived equatorial T_eff of 7900 K indicates Vega's equatorial
atmosphere may be convective and provides a possible explanation for the
discrepancy. The model has a luminosity of ~37 Lsun, a value 35% lower than
Vega's apparent luminosity based on its bolometric flux and parallax, assuming
a slowly rotating star. The model luminosity is consistent with the mean
absolute magnitude of A0V stars. Our model predicts the spectral energy
distribution of Vega as viewed from its equatorial plane; a model which may be
employed in radiative models for the surrounding debris disk.Comment: 16 pages, 9 figures, accepted by Ap
High-resolution imaging of dust shells using Keck aperture masking and the IOTA Interferometer
We present first results of an experiment to combine data from Keck aperture
masking and the Infrared-Optical Telescope Array (IOTA) to image the
circumstellar environments of evolved stars with ~20 milliarcsecond resolution.
The unique combination of excellent Fourier coverage at short baselines and
high-quality long-baseline fringe data allows us to determine the location and
clumpiness of the inner-most hot dust in the envelopes, and to measure the
diameters of the underlying stars themselves. We find evidence for large-scale
inhomogeneities in some dust shells and also significant deviations from
uniform brightness for the photospheres of the most evolved M-stars. Deviations
from spherically-symmetric mass loss in the red supergiant NML Cyg could be
related to recent evidence for dynamically-important magnetic fields and/or
stellar rotation. We point out that dust shell asymmetries, like those observed
here, can qualitatively explain the difficulty recent workers have had in
simultaneously fitting the broad-band spectral energy distributions and
high-resolution spatial information, without invoking unusual dust properties
or multiple distinct shells (from hypothetical ``superwinds''). This paper is
the first to combine optical interferometry data from multiple facilities for
imaging, and we discuss the challenges and potential for the future of this
method, given current calibration and software limitations.Comment: To appear in ApJ (61 pages: 4 tables, 23 figures). Image resolution
degrade
Direct discovery of the inner exoplanet in the HD 206893 system : Evidence for deuterium burning in a planetary-mass companion
Aims.
HD 206893 is a nearby debris disk star that hosts a previously identified brown dwarf companion with an orbital separation of ∼10 au. Long-term precise radial velocity (RV) monitoring, as well as anomalies in the system proper motion, has suggested the presence of an additional, inner companion in the system.
Methods.
Using information from ongoing precision RV measurements with the HARPS spectrograph, as well as Gaia host star astrometry, we have undertaken a multi-epoch search for the purported additional planet using the VLTI/GRAVITY instrument.
Results.
We report a high-significance detection over three epochs of the companion HD 206893c, which shows clear evidence for Keplerian orbital motion. Our astrometry with ∼50−100 μarcsec precision afforded by GRAVITY allows us to derive a dynamical mass of 12.7MJup and an orbital separation of 3.53 au for HD 206893c. Our fits to the orbits of both companions in the system use both Gaia astrometry and RVs to also provide a precise dynamical estimate of the previously uncertain mass of the B component, and therefore allow us to derive an age of 155 ± 15 Myr for the system. We find that theoretical atmospheric and evolutionary models that incorporate deuterium burning for HD 206893c, parameterized by cloudy atmosphere models as well as a “hybrid sequence” (encompassing a transition from cloudy to cloud-free), provide a good simultaneous fit to the luminosity of both HD 206893B and c. Thus, accounting for both deuterium burning and clouds is crucial to understanding the luminosity evolution of HD 206893c.
Conclusions.
In addition to using long-term RV information, this effort is an early example of a direct imaging discovery of a bona fide exoplanet that was guided in part by Gaia astrometry. Utilizing Gaia astrometry is expected to be one of the primary techniques going forward for identifying and characterizing additional directly imaged planets. In addition, HD 206893c is an example of an object narrowly straddling the deuterium-burning limit but unambiguously undergoing deuterium burning. Additional discoveries like this may therefore help clarify the discrimination between a brown dwarf and an extrasolar planet. Lastly, this discovery is another example of the power of optical interferometry to directly detect and characterize extrasolar planets where they form, at ice-line orbital separations of 2−4 au
DARWIN - A Mission to Detect, and Search for Life on, Extrasolar Planets
The discovery of extra-solar planets is one of the greatest achievements of
modern astronomy. The detection of planets with a wide range of masses
demonstrates that extra-solar planets of low mass exist. In this paper we
describe a mission, called Darwin, whose primary goal is the search for, and
characterization of, terrestrial extrasolar planets and the search for life.
Accomplishing the mission objectives will require collaborative science across
disciplines including astrophysics, planetary sciences, chemistry and
microbiology. Darwin is designed to detect and perform spectroscopic analysis
of rocky planets similar to the Earth at mid-infrared wavelengths (6 - 20
micron), where an advantageous contrast ratio between star and planet occurs.
The baseline mission lasts 5 years and consists of approximately 200 individual
target stars. Among these, 25 to 50 planetary systems can be studied
spectroscopically, searching for gases such as CO2, H2O, CH4 and O3. Many of
the key technologies required for the construction of Darwin have already been
demonstrated and the remainder are estimated to be mature in the near future.
Darwin is a mission that will ignite intense interest in both the research
community and the wider public
VizieR Online Data Catalog: JHKs photometry around α Cen (Kervella+, 2006)
VizieR Online Data Catalog. 2007. vol. 345We thus observed the environment of alpha Cen B using the Nasmyth Adaptive Optics System (NAOS) of the Very Large Telescope (VLT), coupled to the CONICA infrared camera. The combination of these two devices is abbreviated as NACO. The first series of observations were obtained between February 18 and April 10, 2004. We repeated the same observations one year later in order to identify the proper-motion companions, using the Ks filter only because all the sources identified in the J and H bands were also detected in Ks. One image of the southern field was obtained in July 2004, but due to operational constraints, the remaining observations were conducted in February-March 2005. (1 data file)
VizieR Online Data Catalog: JHKs photometry around α Cen (Kervella+, 2006)
VizieR Online Data Catalog. 2007. vol. 345We thus observed the environment of alpha Cen B using the Nasmyth Adaptive Optics System (NAOS) of the Very Large Telescope (VLT), coupled to the CONICA infrared camera. The combination of these two devices is abbreviated as NACO. The first series of observations were obtained between February 18 and April 10, 2004. We repeated the same observations one year later in order to identify the proper-motion companions, using the Ks filter only because all the sources identified in the J and H bands were also detected in Ks. One image of the southern field was obtained in July 2004, but due to operational constraints, the remaining observations were conducted in February-March 2005. (1 data file)