474 research outputs found
Testing excitation models of rapidly oscillating Ap stars with interferometry
Rapidly oscillating Ap stars are unique objects in the potential they offer
to study the interplay between a number of important physical phenomena, in
particular, pulsations, magnetic fields, diffusion, and convection.
Nevertheless, the simple understanding of how the observed pulsations are
excited in these stars is still in progress. In this work we perform a test to
what is possibly the most widely accepted excitation theory for this class of
stellar pulsators. The test is based on the study of a subset of members of
this class for which stringent data on the fundamental parameters are available
thanks to interferometry. For three out of the four stars considered in this
study, we find that linear, non-adiabatic models with envelope convection
suppressed around the magnetic poles can reproduce well the frequency region
where oscillations are observed. For the fourth star in our sample no agreement
is found, indicating that a new excitation mechanism must be considered. For
the three stars whose observed frequencies can be explained by the excitation
models under discussion, we derive the minimum angular extent of the region
where convection must be suppressed. Finally, we find that the frequency
regions where modes are expected to be excited in these models is very
sensitive to the stellar radius. This opens the interesting possibility of
determining this quantity and related ones, such as the effective temperature
or luminosity, from comparison between model predictions and observations, in
other targets for which these parameters are not well determined.Comment: Accepted for publication in the MNRA
Interferometry of chemically peculiar stars: theoretical predictions vs. modern observing facilities
By means of numerical experiments we explore the application of
interferometry to the detection and characterization of abundance spots in
chemically peculiar (CP) stars using the brightest star eps~Uma as a case
study. We find that the best spectral regions to search for spots and stellar
rotation signatures are in the visual domain. The spots can clearly be detected
already at a first visibility lobe and their signatures can be uniquely
disentangled from that of rotation. The spots and rotation signatures can also
be detected in NIR at low spectral resolution but baselines longer than 180~m
are needed for all potential CP candidates. According to our simulations, an
instrument like VEGA (or its successor e.g., FRIEND) should be able to detect,
in the visual, the effect of spots and spots+rotation, provided that the
instrument is able to measure , and/or closure phase. In
infrared, an instrument like AMBER but with longer baselines than the ones
available so far would be able to measure rotation and spots. Our study
provides necessary details about strategies of spot detection and the
requirements for modern and planned interferometric facilities essential for CP
star research.Comment: Accepted by NMRAS, 18 pages, 11 figures, 2 table
Integrated optics for astronomical interferometry. I. Concept and astronomical applications
We propose a new instrumental concept for long-baseline optical single-mode
interferometry using integrated optics which were developed for
telecommunication. Visible and infrared multi-aperture interferometry requires
many optical functions (spatial filtering, beam combination, photometric
calibration, polarization control) to detect astronomical signals at very high
angular resolution. Since the 80's, integrated optics on planar substrate have
become available for telecommunication applications with multiple optical
functions like power dividing, coupling, multiplexing, etc. We present the
concept of an optical / infrared interferometric instrument based on this new
technology. The main advantage is to provide an interferometric combination
unit on a single optical chip. Integrated optics are compact, provide
stability, low sensitivity to external constrains like temperature, pressure or
mechanical stresses, no optical alignment except for coupling, simplicity and
intrinsic polarization control. The integrated optics devices are inexpensive
compared to devices that have the same functionalities in bulk optics. We think
integrated optics will fundamentally change single-mode interferometry.
Integrated optics devices are in particular well-suited for interferometric
combination of numerous beams to achieve aperture synthesis imaging or for
space-based interferometers where stability and a minimum of optical alignments
are wished.Comment: 11 pages, 8 figures, accpeted by Astronomy and Astrophysics
Supplement Serie
The GRAVITY fringe tracker: correlation between optical path residuals and atmospheric parameters
After the first year of observations with the GRAVITY fringe tracker, we
compute correlations between the optical path residuals and atmospheric and
astronomical parameters. The median residuals of the optical path residuals are
180 nm on the ATs and 270 nm on the UTs. The residuals are uncorrelated with
the target magnitudes for Kmag below 5.5 on ATs (9 on UTs). The correlation
with the coherence time is however extremely clear, with a drop-off in fringe
tracking performance below 3 ms.Comment: submitted to SPIE Astronomical Telescopes & Instrumentation 201
The interferometric baselines and GRAVITY astrometric error budget
GRAVITY is a new generation beam combination instrument for the VLTI. Its
goal is to achieve microarsecond astrometric accuracy between objects separated
by a few arcsec. This accuracy on astrometric measurements is the most
important challenge of the instrument, and careful error budget have been
paramount during the technical design of the instrument. In this poster, we
will focus on baselines induced errors, which is part of a larger error budget.Comment: SPIE Meeting 2014 -- Montrea
Spectroscopic and interferometric signatures of magnetospheric accretion in young stars
Methods. We use the code MCFOST to solve the non-LTE problem of line
formation in non-axisymmetric accreting magnetospheres. We compute the
Br{\gamma} line profile originating from accretion columns for models with
different magnetic obliquities. We also derive monochromatic synthetic images
of the Br{\gamma} line emitting region across the line profile. This spectral
line is a prime diagnostics of magnetospheric accretion in young stars and is
accessible with the long baseline near-infrared interferometer GRAVITY
installed at the ESO Very Large Telescope Interferometer.
Results. We derive Br{\gamma} line profiles as a function of rotational phase
and compute interferometric observables, visibilities and phases, from
synthetic images. The line profile shape is modulated along the rotational
cycle, exhibiting inverse P Cygni profiles at the time the accretion shock
faces the observer. The size of the line's emission region decreases as the
magnetic obliquity increases, which is reflected in a lower line flux. We apply
interferometric models to the synthetic visibilities in order to derive the
size of the line-emitting region. We find the derived interferometric size to
be more compact than the actual size of the magnetosphere, ranging from 50 to
90\% of the truncation radius. Additionally, we show that the rotation of the
non-axisymmetric magnetosphere is recovered from the rotational modulation of
the Br{\gamma}-to-continuum photo-centre shifts, as measured by the
differential phase of interferometric visibilities
High spatial resolution monitoring of the activity of BA supergiant winds
There are currently two optical interferometry recombiners that can provide
spectral resolutions better than 10000, AMBER/VLTI operating in the H-K bands,
and VEGA/CHARA, recently commissioned, operating in the visible. These
instruments are well suited to study the wind activity of the brightest AB
supergiants in our vicinity, in lines such as H or BrGamma. We present
here the first observations of this kind, performed on Rigel (B8Ia) and Deneb
(A2Ia). Rigel was monitored by AMBER in two campaigns, in 2006-2007 and
2009-2010, and observed in 2009 by VEGA; whereas Deneb was monitored in
2008-2009 by VEGA. The extension of the Halpha and BrGamma line forming regions
were accurately measured and compared with CMFGEN models of both stars.
Moreover, clear signs of activity were observed in the differential visibility
and phases. These pioneer observations are still limited, but show the path for
a better understanding of the spatial structure and temporal evolution of
localized ejections using optical interferometry.Comment: Proceedings of conf. IAUS272 - Active OB stars - Paris, July 19-23,
201
Modeling the e-APD SAPHIRA/C-RED ONE camera at low flux level: An attempt to count photons in the near-infrared with the MIRC-X interferometric combiner
This is the final version. Available on open access from EDP Sciences via the DOI in this recordContext. We implement an electron avalanche photodiode (e-APD) in the MIRC-X instrument, upgrade of the 6-telescope nearinfrared imager MIRC, at the CHARA array. This technology should improve the sensitivity of near-infrared interferometry.
Aims. We characterize a near-infrared C-RED ONE camera from First Light Imaging (FLI) using an e-APD from Leonardo (previously SELEX).
Methods. We first used the classical Mean-Variance analysis to measure the system gain and the amplification gain. We then developed
a physical model of the statistical distribution of the camera output signal. This model is based on multiple convolutions of the Poisson
statistic, the intrinsic avalanche gain distribution, and the observed distribution of the background signal. At low flux level, this model
constraints independently the incident illumination level, the total gain, and the excess noise factor of the amplification.
Results. We measure a total transmission of 48 ± 3% including the cold filter and the Quantum Efficiency. We measure a system
gain of 0.49 ADU/e, a readout noise of 10 ADU, and amplification gains as high as 200. These results are consistent between the two
methods and therefore validate our modeling approach. The measured excess noise factor based on the modeling is 1.47 ± 0.03, with
no obvious dependency with flux level or amplification gain.
Conclusions. The presented model allows measuring the characteristics of the e-APD array at low flux level independently of preexisting calibration. With < 0.3 electron equivalent readout noise at kilohertz frame rates, we confirm the revolutionary performances of
the camera with respect to the PICNIC or HAWAII technologies. However, the measured excess noise factor is significantly higher
than the one claimed in the literature (<1.25), and explains why counting multiple photons remains challenging with this camera.European Union Horizon 2020Labex OSUG@2020CNRS/INS
Characterization of integrated optics components for the second generation of VLTI instruments
Two of the three instruments proposed to ESO for the second generation
instrumentation of the VLTI would use integrated optics for beam combination.
Several design are studied, including co-axial and multi-axial recombination.
An extensive quantity of combiners are therefore under test in our
laboratories. We will present the various components, and the method used to
validate and compare the different combiners. Finally, we will discuss the
performances and their implication for both VSI and Gravity VLTI instruments.Comment: SPIE Astronomical Instrumentation 2008 in Marseille, France --
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