555 research outputs found
Combined MASS-DIMM instrument for atmospheric turbulence studies
Several site-testing programs and observatories currently use combined
MASS-DIMM instruments for monitoring parameters of optical turbulence. The
instrument is described here. After a short recall of the measured quantities
and operational principles, the optics and electronics of MASS-DIMM,
interfacing to telescopes and detectors, and operation are covered in some
detail. Particular attention is given to the correct measurement and control of
instrumental parameters to ensure valid and well-calibrated data, to the data
quality and filtering. Examples of MASS-DIMM data are given, followed by the
list of present and future applications.Comment: Accepted by MNRAS, 11 pages, 8 figure
Accurate seeing measurements with MASS and DIMM
Astronomical seeing is quantified by a single parameter, turbulence integral,
in the framework of the Kolmogorov turbulence model. This parameter can be
routinely measured by a Differential Image Motion Monitor, DIMM. A new
instrument, Multi-Aperture Scintillation Sensor (MASS), permits to measure the
seeing in the free atmosphere above ~0.5km and, together with a DIMM, to
estimate the ground-layer seeing. The absolute accuracy of both methods is
studied here using analytical theory, numerical simulation, and experiments. A
modification of the MASS data processing to compensate for partially saturated
scintillation is developed. We find that the DIMM can be severely biased by
optical aberrations (e.g. defocus) and propagation. Seeing measurements with
DIMM and MASS can reach absolute accuracy of ~10% when their biases are
carefully controlled. Pushing this limit to 1% appears unrealistic because the
seeing itself is just a model-dependent parameter of a non-stationary random
process.Comment: 13 pages, 14 figures. Accepted for publication in MNRA
Atmospheric image blur with finite outer scale or partial adaptive correction
Seeing-limited resolution in large telescopes working over wide wavelength
range depends substantially on the turbulence outer scale and cannot be
adequately described by one "seeing" value. We attempt to clarify frequent
confusions on this matter. We study the effects of finite turbulence outer
scale and partial adaptive corrections by means of analytical calculations and
numerical simulations. If a von Karman turbulence model is adopted, a simple
approximate formula captures the dependence of atmospheric long-exposure
resolution on the outer scale over the entire practically interesting range of
telescope diameters and wavelengths. In the infrared (IR), the difference with
the standard Kolmogorov seeing formula can exceed a factor of two. We find that
low-order adaptive turbulence correction produces residual wave-fronts with
effectively small outer scale, so even very low compensation order leads to a
substantial improvement in resolution over seeing, compared to the standard
theory. Seeing-limited resolution of large telescopes, especially in the IR, is
currently under-estimated by not accounting for the outer scale. On the other
hand, adaptive-optics systems designed for diffraction-limited imaging in the
IR can improve the resolution in the visible by as much as two times.Comment: A&A accepte
Comparison of the scintillation noise above different observatories measured with MASS instruments
Scintillation noise is a major limitation of ground base photometric
precision. An extensive dataset of stellar scintillation collected at 11
astronomical sites world-wide with MASS instruments was used to estimate the
scintillation noise of large telescopes in the case of fast photometry and
traditional long-exposure regime. Statistical distributions of the
corresponding parameters are given. The scintillation noise is mostly
determined by turbulence and wind in the upper atmosphere and comparable at all
sites, with slightly smaller values at Mauna Kea and largest noise at Tolonchar
in Chile. We show that the classical Young's formula under-estimates the
scintillation noise.The temporal variations of the scintillation noise are also
similar at all sites, showing short-term variability at time scales of 1 -- 2
hours and slower variations, including marked seasonal trends (stronger
scintillation and less clear sky during local winter). Some correlation was
found between nearby observatories.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
figure
Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries
(hierarchy 2+2) is studied in comparison with triple stars, with respective
sample sizes of 81 and 724. Seven representative quadruple systems are
discussed in greater detail. The properties of multiple stars do not correspond
to the products of dynamical decay of small clusters, hence the N-body dynamics
is not the dominant process of their formation. On the other hand,
rotationally-driven (cascade) fragmentation possibly followed by migration of
inner and/or outer orbits to shorter periods is a promising scenario to explain
the origin of triple and quadruple stars. Our main results are: (i) Quadruple
systems of Epsilon Lyr type with similar masses and inner periods are common.
(ii) The distributions of the inner periods in triple and quadruple stars are
similar and bimodal. The inner mass ratios do not correlate with the inner
periods. (iii) The statistics of outer periods and mass ratios in triples and
quadruples are different. The median outer mass ratio in triples is 0.39
independently of the outer period, which has a smooth distribution. In
contrast, the outer periods of 25% quadruples concentrate in the narrow range
from 10yr to 100yr, the outer mass ratios of these tight quadruples are above
0.6 and their two inner periods are similar to each other. (iv) The outer and
inner mass ratios in triple and quadruple stars are not mutually correlated.
(v) The inner and outer orbital angular momenta and periods in triple and
quadruple systems with inner periods above 30d show some correlation, the ratio
of outer-to-inner periods is mostly comprised between 5 and 10^4. In the
systems with small period ratios the directions of the orbital spins are
correlated, while in the systems with large ratios they are not.Comment: Accepted by MNRAS, 14 pages, 12 figures. Two electronic tables at
http://www.ctio.noao.edu/ftp/pub/tokovinin/quadruples
Spectroscopic orbits of nearby stars
We observed stars with variable radial velocities to determine their
spectroscopic orbits. Velocities of 132 targets taken over a time span reaching
30 years are presented. They were measured with the correlation radial velocity
spectrometers (1913 velocities) and the new VUES echelle spectrograph (632
velocities), with typical accuracy of 0.5 and 0.2 km/s, respectively. We
derived spectroscopic orbits of 57 stars (including 53 first-time orbits),
mostly nearby dwarfs of spectral types K and M. Their periods range from 2.2
days to 14 years, some of those are Hipparcos astrometric binaries. Comments on
individual objects are provided. Many stars belong to hierarchical systems
containing three or more components, including 20 new hierarchies resulting
from this project. The preliminary orbit of the young star HIP~47110B has a
large eccentricity e=0.47 despite short period of 4.4 d; it could be still
circularizing. Our results enrich the data on nearby stars and contribute to a
better definition of the multiplicity statistics.Comment: Accepted by Astronomy & Astrophysics; 17 pages, 5 figures, 3 table
High-resolution imaging at the SOAR telescope
Bright single and binary stars were observed at the 4.1-m telescope with a
fast electron-multiplication camera in the regime of partial turbulence
correction by the visible-light adaptive optics system. We compare the angular
resolution achieved by simple averaging of AO-corrected images (long-exposure),
selection and re-centering (shift-and-add or "lucky" imaging) and speckle
interferometry. The effect of partial AO correction, vibrations, and image
post-processing on the attained resolution is shown. Potential usefulness of
these techniques is evaluated for reaching the diffraction limit in
ground-based optical imaging. Measurements of 75 binary stars obtained during
these tests are given and objects of special interest are discussed. We report
tentative resolution of the astrometric companion to Zeta Aqr B. A concept of
advanced high-resolution camera is outlined.Comment: Accepted for publication in PASP. 14 pages, 9 figures, 2 tabl
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