223 research outputs found
VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre
The dust enshrouded star IRS~3 in the central light year of our galaxy was
partially resolved in a recent VLTI experiment. The presented observation is
the first step in investigating both IRS~3 in particular and the stellar
population of the Galactic Centre in general with the VLTI at highest angular
resolution. We will outline which scientific issues can be addressed by a
complete MIDI dataset on IRS~3 in the mid infrared.Comment: 4 pages, 3 figures, published in: The ESO Messenge
Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band
Our objective is to prove that integrated optics (IO) is not only a good
concept for astronomical interferometry but also a working technique with high
performance. We used the commissioning data obtained with the dedicated K-band
integrated optics two-telescope beam combiner which now replaces the fiber
coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of
this IO device and compare its properties to other single mode beam combiner
like the previously used MONA fiber coupler. The IO combiner provides a high
optical throughput, a contrast of 89% with a night-to-night stability of a few
percent. Even if a dispersive phase is present, we show that it does not bias
the measured Fourier visibility estimate. An upper limit of 0.005 for the
cross-talk between linear polarization states has been measured. We take
advantage of the intrinsic contrast stability to test a new astronomical
prodecure for calibrating diameters of simple stars by simultaneously fitting
the instrumental contrast and the apparent stellar diameters. This method
reaches an accuracy with diameter errors of the order of previous ones but
without the need of an already known calibrator. These results are an important
step of integrated optics and paves the road to incoming imaging interferometer
projects
Ground-Based Coronagraphy with High Order Adaptive Optics
We summarize the theory of coronagraphic optics, and identify a dimensionless
fine-tuning parameter, F, which we use to describe the Lyot stop size in the
natural units of the coronagraphic optical train and the observing wavelength.
We then present simulations of coronagraphs matched to adaptive optics (AO)
systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8m telescopes under
various atmospheric conditions, and identify useful parameter ranges for AO
coronagraphy on these telescopes. Our simulations employ a tapered, high-pass
filter in spatial frequency space to mimic the action of adaptive wavefront
correction. We test the validity of this representation of AO correction by
comparing our simulations with recent K-band data from the 241-channel Palomar
Hale AO system and its dedicated PHARO science camera in coronagraphic mode.Comment: To appear in ApJ, May 2001 (28 pages, 10 figs
VLTI/MIDI observations of 7 classical Be stars
We measured the mid-infrared extension of the gaseous disk surrounding seven
Be stars in order to constrain the geometry of their circumstellar environments
and to try to infer physical parameters characterizing these disks. We used the
VLTI/MIDI instrument with baselines up to 130 m to obtain an angular resolution
of about 15 mas in the N band and compared our results with previous K band
measurements obtained with the VLTI/AMBER instrument and/or the CHARA
interferometer. We obtained one calibrated visibility measurement for each of
the four stars, p Car, zeta Tau, kappa CMa, and alpha Col, two for delta Cen
and beta CMi, and three for alpha Ara. Almost all targets remain unresolved
even with the largest VLTI baseline of 130m, evidence that their circumstellar
disk extension is less than 10 mas. The only exception is alpha Ara, which is
clearly resolved and well-fitted by an elliptical envelope with a major axis
a=5.8+-0.8mas and an axis ratio a/b=2.4+-1 at 8 microns. This extension is
similar to the size and flattening measured with the VLTI/AMBER instrument in
the K band at 2 microns. The size of the circumstellar envelopes for these
classical Be stars does not seem to vary strongly on the observed wavelength
between 8 and 12microns. Moreover, the size and shape of Alpha Ara's disk is
almost identical at 2, 8, and 12microns
The environment of the fast rotating star Achernar - Thermal infrared interferometry with VLTI/MIDI and SIMECA modeling
Context: As is the case of several other Be stars, Achernar is surrounded by
an envelope, recently detected by near-IR interferometry.
Aims: We search for the signature of circumstellar emission at distances of a
few stellar radii from Achernar, in the thermal IR domain.
Methods: We obtained interferometric observations on three VLTI baselines in
the N band (8-13 mic), using the MIDI instrument.
Results: From the measured visibilities, we derive the angular extension and
flux contribution of the N band circumstellar emission in the polar direction
of Achernar. The interferometrically resolved polar envelope contributes 13.4
+/- 2.5 % of the photospheric flux in the N band, with a full width at half
maximum of 9.9 +/- 2.3 mas (~ 6 Rstar). This flux contribution is in good
agreement with the photometric IR excess of 10-20% measured by fitting the
spectral energy distribution. Due to our limited azimuth coverage, we can only
establish an upper limit of 5-10% for the equatorial envelope. We compare the
observed properties of the envelope with an existing model of this star
computed with the SIMECA code.
Conclusions: The observed extended emission in the thermal IR along the polar
direction of Achernar is well reproduced by the existing SIMECA model. Already
detected at 2.2mic, this polar envelope is most probably an observational
signature of the fast wind ejected by the hot polar caps of the star.Comment: A&A Letter, in pres
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
First VLTI infrared spectro-interferometry on GCIRS 7 - Characterizing the prime reference source for Galactic center observations at highest angular resolution
Investigating the environment of the massive black hole SgrA* at the center
of the Galaxy requires the highest angular resolution available to avoid source
confusion and to study the physical properties of the individual objects.
GCIRS7 has been used as wavefront and astrometric reference. Our studies
investigate, for the first time, its properties at 2&10um using VLTI/AMBER and
MIDI. We aim at analyzing the suitability of IRS7 as an IF-phase-reference for
the upcoming generation of dual-field facilities at optical interferometers. We
observed with (R~30) and 50m (proj.) baseline, resulting in 9 and 45mas
resolution for NIR and MIR, resp. The first K-band fringe detection of a GC
star suggests that IRS7 could be marginally resolved at 2um, which would imply
that the photosphere of the supergiant is enshrouded by a molecular and dusty
envelope. At 10um, IRS7 is strongly resolved with a visibility of approximately
0.2. The MIR is dominated by moderately warm (200 K), extended dust, mostly
distributed outside of a radius of about 120 AU (15 mas) around the star. A
deep 9.8-silicate absorption in excess of the usual extinction law with respect
to the NIR extinction has been found. This confirms recent findings of a
relatively enhanced, interstellar 9.8-silicate absorption with respect to the
NIR extinction towards another star in the central arcsec, suggesting an
unusual dust composition in that region. Our VLTI observations show that
interferometric NIR phase-referencing experiments with mas resolution using
IRS7 as phase-reference appear to be feasible, but more such studies are
required to definitely characterize the close environment around this star. We
demonstrate that interferometry is required to resolve the innermost
environment of stars at the Galactic center.Comment: 6 pages, 2 figures, accepted for publication in A&
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