922 research outputs found
Infrared and visual lunar occultations measurements of stellar diameters and new binary stars detections at the Calar Alto 1.5m telescope
We present a program of routine lunar occultations, at optical and near-IR
wavelengths, recently started at the 1.5m Spanish telescope at the Calar Alto
Observatory. Both a CCD and an infrared array detector are used. The program is
aimed mainly at the detection and investigation of binary systems, although
results in other areas of stellar research are also anticipated.
Occultations are reported for a total of 40 stars. Among these, SAO 164567,
SAO 78258 and AG+24 788 have been discovered to be binaries, with projected
separations as small as 0.006". Furthermore, binarity is suspected in the case
of SAO 78119 and SAO 79251. Additionally, the angular diameter of the late-type
giant 30 Psc and of the infrared star V349 Gem have been accurately measured,
this latter for the first time.
We finally evaluate the instrumentation performance in terms of limiting
magnitude and angular resolution, and discuss applications to larger
telescopes.Comment: 15 pages, 3 figures, LaTeX, uses aa.cls. Accepted for publication in
A&
Aldebaran's angular diameter: how well do we know it?
The bright, well-known K5 giant Aldebaran, alpha Tau, is probably the star
with the largest number of direct angular diameter determinations, achieved
over a long time by several authors using various techniques. In spite of this
wealth of data, or perhaps as a direct result of it, there is not a very good
agreement on a single angular diameter value. This is particularly unsettling
if one considers that Aldebaran is also used as a primary calibrator for some
angular resolution methods, notably for optical and infrared long baseline
interferometry. Directly connected to Aldebaran's angular diameter and its
uncertainties is its effective temperature, which also has been used for
several empirical calibrations. Among the proposed explanations for the
elusiveness of an accurate determination of the angular diameter of Aldebaran
are the possibility of temporal variations as well as a possible dependence of
the angular diameter on the wavelength. We present here a few, very accurate
new determinations obtained by means of lunar occultations and long baseline
interferometry. We derive an average value of 19.96+-0.03 milliarcseconds for
the uniform disk diameter. The corresponding limb-darkened value is 20.58+-0.03
milliarcseconds, or 44.2+-0.9 R(sun). We discuss this result, in connection
with previous determinations and with possible problems that may affect such
measurements.Comment: 8 pages, 4 figures, accepted for publication in A&
New high-sensitivity, milliarcsecond resolution results from routine observations of lunar occultations at the ESO VLT
(Abridged) Lunar occultations (LO) are a very efficient and powerful
technique, that achieves the best combination of high angular resolution and
sensitivity possible today at near-infrared wavelengths. Given that the events
are fixed in time, that the sources are occulted randomly, and that the
telescope use is minimal, the technique is very well suited for service mode
observations. We have established a program of routine LO observations at the
VLT observatory, especially designed to take advantage of short breaks
available in-between other programs. We have used the ISAAC instrument in burst
mode, capable of producing continuous read-outs at millisecond rates on a
suitable subwindow. Given the random nature of the source selection, our aim
has been primarily the investigation of a large number of stellar sources at
the highest angular resolution in order to detect new binaries. Serendipitous
results such as resolved sources and detection of circumstellar components were
also anticipated. We have recorded the signal from background stars for a few
seconds, around the predicted time of occultation by the Moon's dark limb. At
millisecond time resolution, a characteristic diffraction pattern can be
observed. Patterns for two or more sources superimpose linearly, and this
property is used for the detection of binary stars. The detailed analysis of
the diffraction fringes can be used to measure specific properties such as the
stellar angular size and the presence of extended light sources such as a
circumstellar shell. We present a list of 191 stars for which LO data could be
recorded and analyzed. Results include the detection of 16 binary and 2 triple
stars, all but one of which were previously unknown. The projected angular
separations are as small as 4 milliarcseconds and magnitude differences as high
as ?K=5.8 mag...Comment: 10 pages, 3 figures, to be published in A&
Detection of a sub-arcsecond dust shell around the Wolf-Rayet star WR 112
A lunar occultation event of the Wolf-Rayet star WR 112 (type WC9) has been
observed simultaneously from two independent telescopes at lambda = 2.2microns,
allowing us to investigate this source with an angular resolution of approx
0.003 arc-seconds. We have detected a circumstellar dust envelope whose
brightness distribution can be approximately fitted by a gaussian with a FWHM
of approx 0.06 arc-seconds (approx 10^15 cm). We present and discuss the
reconstructed brightness profile, which shows an asymmetry in the radial dust
distribution. The derived dust grain temperature at the inner dust zone of
approx 1150 K is consistent with available model calculations. There is no
signature of the central star from our observations, providing a direct
confirmation that the circumstellar shell emission dominates over the
photospheric emission at 2.2microns as predicted by fits to the spectral energy
distribution. Further lunar occultation observations at different position
angles are essential to reconstruct the 2--D image of the dust shell around WR
112. The current series of lunar occultations of WR 112 will continue to the
end of 1999 and will be visible for all equatorial and southern latitude
observatories.Comment: 5 pages, 1 figur
A new wavelet-based approach for the automated treatment of large sets of lunar occultation data
The introduction of infrared arrays for lunar occultations (LO) work and the
improvement of predictions based on new deep IR catalogues have resulted in a
large increase in the number of observable occultations.
We provide the means for an automated reduction of large sets of LO data.
This frees the user from the tedious task of estimating first-guess parameters
for the fit of each LO lightcurve. At the end of the process, ready-made plots
and statistics enable the user to identify sources which appear to be resolved
or binary and to initiate their detailed interactive analysis.
The pipeline is tailored to array data, including the extraction of the
lightcurves from FITS cubes. Because of its robustness and efficiency, the
wavelet transform has been chosen to compute the initial guess of the
parameters of the lightcurve fit.
We illustrate and discuss our automatic reduction pipeline by analyzing a
large volume of novel occultation data recorded at Calar Alto Observatory. The
automated pipeline package is available from the authors.Comment: Accepted to Astronomy & Astrophysics 9 pages, 5 figure
Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic Center
For the first time, the lunar occultation technique has been employed on a
very large telescope in the near-IR with the aim of achieving systematically
milliarcsecond resolution on stellar sources.
We have demonstrated the burst mode of the ISAAC instrument, using a fast
read-out on a small area of the detector to record many tens of seconds of data
at a time on fields of few squared arcsec. We have used the opportunity to
record a large number of LO events during a passage of the Moon close to the
Galactic Center in March 2006. We have developed a data pipeline for the
treatment of LO data, including the automated estimation of the main data
analysis parameters using a wavelet-based method, and the preliminary fitting
and plotting of all light curves.
We recorded 51 LO events over about four hours. Of these, 30 resulted of
sufficient quality to enable a detailed fitting. We detected two binaries with
subarcsec projected separation and three stars with a marginally resolved
angular diameter of about 2 mas. Two more SiO masers, were found to be resolved
and in one case we could recover the brightness profile of the extended
emission, which is well consistent with an optically thin shell. The remaining
unresolved stars were used to characterize the performance of the method.
The LO technique at a very large telescope is a powerful and efficient method
to achieve angular resolution, sensitivity, and dynamic range that are among
the best possible today with any technique. The selection of targets is
naturally limited and LOs are fixed-time events, however each observation
requires only a few minutes including overheads. As such, LOs are ideally
suited to fill small gaps of idle time between standard observations.Comment: A&A in pres
Evidence of asymmetries in the Aldebaran photosphere from multiwavelength lunar occultations
We have recorded three lunar occultations of Aldebaran (α Tau) at different telescopes and using various band-passes, from the ultraviolet to the far red. The data have been analysed using both model-dependent and model-independent methods. The derived uniform-disc angular diameter values have been converted to limb-darkened values using model atmosphere relations and are found in broad agreement among themselves and with previous literature values. The limb-darkened diameter is about 20.3 mas on average. However, we have found indications that the photospheric brightness profile of Aldebaran may have not been symmetric, a finding already reported by other authors for this and for similar late-type stars. At the sampling scale of our brightness profile, between 1 and 2 mas, the uniform and limb-darkened disc models may not be a good description for Aldebaran. The asymmetries appear to differ with wavelength and over the 137-d time span of our measurements. Surface spots appear as a likely explanation for the differences between observations and the models
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