720 research outputs found
Differential interferometry of QSO broad line regions I: improving the reverberation mapping model fits and black hole mass estimates
Reverberation mapping estimates the size and kinematics of broad line regions
(BLR) in Quasars and type I AGNs. It yields size-luminosity relation, to make
QSOs standard cosmological candles, and mass-luminosity relation to study the
evolution of black holes and galaxies. The accuracy of these relations is
limited by the unknown geometry of the BLR clouds distribution and velocities.
We analyze the independent BLR structure constraints given by super-resolving
differential interferometry. We developed a three-dimensional BLR model to
compute all differential interferometry and reverberation mapping signals. We
extrapolate realistic noises from our successful observations of the QSO 3C273
with AMBER on the VLTI. These signals and noises quantify the differential
interferometry capacity to discriminate and measure BLR parameters including
angular size, thickness, spatial distribution of clouds, local-to-global and
radial-to-rotation velocity ratios, and finally central black hole mass and BLR
distance. A Markov Chain Monte Carlo model-fit, of data simulated for various
VLTI instruments, gives mass accuracies between 0.06 and 0.13 dex, to be
compared to 0.44 dex for reverberation mapping mass-luminosity fits. We
evaluate the number of QSOs accessible to measures with current (AMBER),
upcoming (GRAVITY) and possible (OASIS with new generation fringe trackers)
VLTI instruments. With available technology, the VLTI could resolve more than
60 BLRs, with a luminosity range larger than four decades, sufficient for a
good calibration of RM mass-luminosity laws, from an analysis of the variation
of BLR parameters with luminosity.Comment: 19 pages, 14 figures, accepted by MNRAS on December 5, 201
First astrophysical results from AMBER/VLTI
The AMBER instrument installed at the Very Large Telescope (VLT) combines
three beams from as many telescopes to produce spectrally dispersed fringes
from milli-arcsecond angular scale in the near infrared. Two years after
installation, first scientific observations have been carried out during the
Science Demonstration Time and the Guaranteed Time mostly on bright sources due
to some VLTI limitations. In this paper, we review these first astrophysical
results and we show which types of completely new information is brought by
AMBER. The first astrophysical results have been mainly focusing on stellar
wind structure, kinematics, and its interaction with dust usually concentrated
in a disk. Because AMBER has dramatically increased the number of measures per
baseline, this instrument brings strong constraints on morphology and models
despite a relatively poor (u, v) coverage for each object.Comment: SPIE 6268 proceeding
Fresnel diffraction in an interferometer: application to MATISSE
While doing optical study in an instrument similar to the interferometers
dedicated to the Very Large Telescope (VLT), we have to take care of the pupil
and focus conjugations. Modules with artificial sources are designed to
simulate the stellar beams, in terms of collimation and pupil location. They
constitute alignment and calibration tools. In this paper, we present such a
module in which the pupil mask is not located in a collimated beam thus
introducing Fresnel diffraction. We study the instrumental contrast taking into
account the spatial coherence of the source, and the pupil diffraction. The
considered example is MATISSE, but this study can apply to any other instrument
concerned with Fresnel diffraction.Comment: 8 pages- to appear in Proceedings of SPIE Astronomical Telescopes and
Instrumentation 201
Parasitic Interference in Long Baseline Optical Interferometry: Requirements for Hot Jupiter-like Planet Detection
International audienceThe observable quantities in optical interferometry, which are the modulus and the phase of the complex visibility, may be corrupted by parasitic fringes superimposed on the genuine fringe pattern. These fringes are due to an interference phenomenon occurring from stray light effects inside an interferometric instrument. We developed an analytical approach to better understand this phenomenon when stray light causes cross talk between beams. We deduced that the parasitic interference significantly affects the interferometric phase and thus the associated observables including the differential phase and the closure phase. The amount of parasitic flux coupled to the piston between beams appears to be very influential in this degradation. For instance, considering a point-like source and a piston ranging from λ/500 to λ/5 in the L band (λ = 3.5 μm), a parasitic flux of about 1% of the total flux produces a parasitic phase reaching at most one-third of the intrinsic phase. The piston, which can have different origins (instrumental stability, atmospheric perturbations, etc.), thus amplifies the effect of parasitic interference. According to the specifications of piston correction in space or at ground level (respectively λ/500 ≈ 2 nm and λ/30 ≈ 100 nm), the detection of hot Jupiter-like planets, one of the most challenging aims for current ground-based interferometers, limits parasitic radiation to about 5% of the incident intensity. This was evaluated by considering different types of hot Jupiter synthetic spectra. Otherwise, if no fringe tracking is used, the detection of a typical hot Jupiter-like system with a solar-like star would admit a maximum level of parasitic intensity of 0.01% for piston errors equal to λ/15. If the fringe tracking specifications are not precisely observed, it thus appears that the allowed level of parasitic intensity dramatically decreases and may prevent the detection. In parallel, the calibration of the parasitic phase by a reference star, at this accuracy level, seems very difficult. Moreover, since parasitic phase is an object-dependent quantity, the use of a hypothetical phase abacus, directly giving the parasitic phase from a given parasitic flux level, is also impossible. Some instrumental solutions, implemented at the instrument design stage for limiting or preventing this parasitic interference, appear to be crucial and are presented in this paper
First spectro-interferometric survey of Be stars I. Observations and constraints on the disks geometry and kinematics
Context. Classical Be stars are hot non-supergiant stars surrounded by a
gaseous circumstellar disk that is responsible for the observed infrared-excess
and emission lines. The phenomena involved in the disk formation still remain
highly debated. Aims. To progress in the understanding of the physical process
or processes responsible for the mass ejections and test the hypothesis that
they depend on the stellar parameters, we initiated a survey on the
circumstellar environment of the brightest Be stars. Methods. To achieve this
goal, we used spectro-interferometry, the only technique that combines high
spectral (R=12000) and high spatial (=4\,mas) resolutions.
Observations were carried out at the Paranal observatory with the VLTI/AMBER
instrument. We concentrated our observations on the Br emission line to
be able to study the kinematics within the circumstellar disk. Our sample is
composed of eight bright classical Be stars : Col, CMa,
Car, p Car, Cen, Cen, Ara, and \textit{o} Aqr.
Results. We managed to determine the disk extension in the line and the nearby
continuum for most targets. We also constrained the disk kinematics, showing
that it is dominated by rotation with a rotation law close to the Keplerian
one. Our survey also suggests that these stars are rotating at a mean velocity
of V/V\,=\,0.82\,\,0.08. This corresponds to a rotational rate
of \,=\,0.95\,\,0.02 Conclusions. We did not detect
any correlation between the stellar parameters and the structure of the
circumstellar environment. Moreover, it seems that a simple model of a
geometrically thin Keplerian disk can explain most of our spectrally resolved
K-band data. Nevertheless, some small departures from this model have been
detected for at least two objects (i.e, CMa and Col).
Finally, our Be stars sample suggests that rotation is the main physical
process driving the mass-ejection. Nevertheless, smaller effects from other
mechanisms have to be taken into account to fully explain how the residual
gravity is compensated.Comment: Astronomy and Astrophysics (2011) Accepte
Study of the atmospheric refraction in a single mode instrument - Application to AMBER/VLTI
International audienceThis paper presents a study of the atmospheric refraction and its effect on the light coupling efficiency in an instrument using single-mode optical fibers. We show the analytical approach which allowed us to assess the need to correct the refraction in J- and H-bands while observing with an 8-m Unit Telescope. We then developed numerical simulations to go further in calculations. The hypotheses on the instrumental characteristics are those of AMBER (Astronomical Multi BEam combineR), the near infrared focal beam combiner of the Very Large Telescope Interferometric mode (VLTI), but most of the conclusions can be generalized to other single-mode instruments. We used the software package caos (Code for Adaptive Optics Systems) to take into account the atmospheric turbulence effect after correction by the ESO system MACAO (Multi-Application Curvature Adaptive Optics). The opto-mechanical study and design of the system correcting the atmospheric refraction on AMBER is then detailed. We showed that the atmospheric refraction becomes predominant over the atmospheric turbulence for some zenith angles z and spectral conditions: for z larger than 30° in J-band for example. The study of the optical system showed that it allows to achieve the required instrumental performance in terms of throughput in J- and H-bands. First observations in J-band of a bright star, alpha Cir star, at more than 30° from zenith clearly showed the gain to control the atmospheric refraction in a single mode instrument, and validated the operating law
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