149 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
Multi-Wavelength Monitoring of the Changing-Look AGN NGC 2617 during State Changes
Optical and near-infrared photometry, optical spectroscopy, and soft X-ray and UV monitoring of the changing-look active galactic nucleus NGC 2617 show that it continues to have the appearance of a type-1 Seyfert galaxy. An optical light curve for 2010-2017 indicates that the change of type probably occurred between 2010 October and 2012 February and was not related to the brightening in 2013. In 2016 and 2017 NGC 2617 brightened again to a level of activity close to that in 2013 April. However, in 2017 from the end of the March to end of July 2017 it was in very low level and starting to change back to a Seyfert 1.8. We find variations in all passbands and in both the intensities and profiles of the broad Balmer lines. A new displaced emission peak has appeared in Hβ. X-ray variations are well correlated with UV-optical variability and possibly lead by ̃2-3 d. The K band lags the J band by about 21.5 ± 2.5 d and lags the combined B + J bands by ̃25 d. J lags B by about 3 d. This could be because J-band variability arises predominantly from the outer part of the accretion disc, while K-band variability is dominated by thermal re-emission by dust. We propose that spectral-type changes are a result of increasing central luminosity causing sublimation of the innermost dust in the hollow bi-conical outflow. We briefly discuss various other possible reasons that might explain the dramatic changes in NGC 2617.Fil: Oknyansky, V. L.. Sternberg Astronomical Institute; RusiaFil: Gaskell, C. M.. Department of Astronomy and Astrophysics. University of California. Santa Cruz; Estados UnidosFil: Mikailov, K. M.. Shamakhy Astrophysical Observatory, National Academy of Sciences.
Pirkuli; AzerbaiyánFil: Lipunov, V. M.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University ; RusiaFil: Shatsky, N. I.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Tsygankov, S. S.. Tuorla Observatory, Department of Physics and Astronomy. University of Turku.; FinlandiaFil: Gorbovskoy, E. S.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Tatarnikov, A. M.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Metlov, V. G.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Malanchev, K. L.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Brotherton, M.B.. University of Wyoming; Estados UnidosFil: Kasper, D.. University of Wyoming; Estados UnidosFil: Du, P.. Institute of High Energy Physics. Chinese Academy of Sciences; ChinaFil: Chen, X.. School of Space Science and Physics. Shandong University; ChinaFil: Burlak, M. A.. Sternberg Astronomical Institute. M.V.Lomonosov Moscow State University; RusiaFil: Buckley, D. A. H.. The South African Astronomical Observatory; SudáfricaFil: Rebolo, R.. Instituto de Astrofisica de Canarias; EspañaFil: Serra-Ricart, M.. Instituto de Astrofisica de Canarias; EspañaFil: Podestá, R.. Universidad Nacional de San Juan; ArgentinaFil: Levato, O. H.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentin
First results of site testing program at Mt. Shatdzhatmaz in 2007 - 2009
We present the first results of the site testing performed at
Mt.~Shatdzhatmaz at Northern Caucasus, where the new Sternberg astronomical
institute 2.5-m telescope will be installed. An automatic site monitor
instrumentation and functionality are described together with the methods of
measurement of the basic astroclimate and weather parameters. The clear night
sky time derived on the basis of 2006 -- 2009 data amounts to 1340 hours per
year. Principle attention is given to the measurement of the optical turbulence
altitude distribution which is the most important characteristic affecting
optical telescopes performance. For the period from November 2007 to October
2009 more than 85\,000 turbulence profiles were collected using the combined
MASS/DIMM instrument. The statistical properties of turbulent atmosphere above
the summit are derived and the median values for seeing ~arcsec
and free-atmosphere seeing ~arcsec are determined.
Together with the estimations of isoplanatic angle ~arcsec and
time constant \tau_0 = 2.58 \mbox{ ms}, these are the first representative
results obtained for Russian sites which are necessary for development of
modern astronomical observation techniques like adaptive optics.Comment: Accepted for publication in MNRAS, 17 pages, 15 figure
ProteinDBS v2.0: a web server for global and local protein structure search
ProteinDBS v2.0 is a web server designed for efficient and accurate comparisons and searches of structurally similar proteins from a large-scale database. It provides two comparison methods, global-to-global and local-to-local, to facilitate the searches of protein structures or substructures. ProteinDBS v2.0 applies advanced feature extraction algorithms and scalable indexing techniques to achieve a high-running speed while preserving reasonably high precision of structural comparison. The experimental results show that our system is able to return results of global comparisons in seconds from a complete Protein Data Bank (PDB) database of 152 959 protein chains and that it takes much less time to complete local comparisons from a non-redundant database of 3276 proteins than other accurate comparison methods. ProteinDBS v2.0 supports query by PDB protein ID and by new structures uploaded by users. To our knowledge, this is the only search engine that can simultaneously support global and local comparisons. ProteinDBS v2.0 is a useful tool to investigate functional or evolutional relationships among proteins. Moreover, the common substructures identified by local comparison can be potentially used to assist the human curation process in discovering new domains or folds from the ever-growing protein structure databases. The system is hosted at http://ProteinDBS.rnet.missouri.edu
Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars
There is no straightforward explanation for intrinsic X-ray emission from
intermediate-mass main-sequence stars. Therefore the observed emission is often
interpreted in terms of (hypothesized) late-type magnetically active companion
stars. We use Chandra imaging observations to spatially resolve in X-rays a
sample of main-sequence B-type stars with recently discovered companions at
arcsecond separation. We find that all spatially resolved companions are X-ray
emitters, but seven out of eleven intermediate-mass stars are also X-ray
sources. If this emission is interpreted in terms of additional sub-arcsecond
or spectroscopic companions, this implies a high multiplicity of B-type stars.
Firm results on B star multiplicity pending, the alternative, that B stars
produce intrinsic X-rays, can not be discarded. The appropriate scenario in
this vein is might be a magnetically confined wind, as suggested for the X-ray
emission of the magnetic Ap star IQ Aur. However, the only Ap star in the
Chandra sample is not detected in X-rays, and therefore does not support this
picture.Comment: 12 pages; accepted for publication in Astronomy & Astrophysic
The Primordial Binary Population - I: A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey with the aim
to detect close companions to Hipparcos members in the three subgroups of the
nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL)
and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type
stars in the Ks band, and a subset also in the J and H band. We find 151
stellar components other than the target stars. A brightness criterion is used
to separate these components into 77 background stars and 74 candidate physical
companion stars. Out of these 74 candidate companions, 41 have not been
reported before (14 in US; 13 in UCL; 14 in LCC). Companion star masses range
from 0.1 to 3 Msun. The mass ratio distribution follows f(q) = q^-0.33, which
excludes random pairing. No close (rho < 3.75'') companion stars or background
stars are found in the magnitude range 12 < Ks < 14. The lack of stars with
these properties cannot be explained by low-number statistics, and may imply a
lower limit on the companion mass of ~ 0.1 Msun. Close stellar components with
Ks > 14 are observed. If these components are very low-mass companion stars, a
gap in the companion mass distribution might be present. The small number of
close low-mass companion stars could support the embryo-ejection formation
scenario for brown dwarfs. Our findings are compared with and complementary to
visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an
overall companion star fraction of 0.52 in this association. This paper is the
first step toward our goal to derive the primordial binary population in Sco
OB2.Comment: 27 pages, to accepted by A&
Exploring the consequences of pairing algorithms for binary stars
Knowledge of the binary population in stellar groupings provides important
information about the outcome of the star forming process in different
environments (see, e.g., Blaauw 1991, and references therein). Binarity is also
a key ingredient in stellar population studies, and is a prerequisite to
calibrate the binary evolution channels. In this paper we present an overview
of several commonly used methods to pair individual stars into binary systems,
which we refer to as pairing functions. These pairing functions are frequently
used by observers and computational astronomers, either for their mathematical
convenience, or because they roughly describe the expected outcome of the star
forming process. We discuss the consequences of each pairing function for the
interpretation of observations and numerical simulations. The binary fraction
and mass ratio distribution generally depend strongly on the selection of the
range in primary spectral type in a sample. The mass ratio distribution and
binary fraction derived from a binarity survey among a mass-limited sample of
targets is thus not representative for the population as a whole. Neither
theory nor observations indicate that random pairing of binary components from
the mass distribution, the simplest pairing function, is realistic. It is more
likely that companion stars are formed in a disk around a star, or that a
pre-binary core fragments into two binary components. The results of our
analysis are important for (i) the interpretation of the observed mass ratio
distribution and binary fraction for a sample of stars, (ii) a range of
possible initial condition algorithms for star cluster simulations, and (iii)
how to discriminate between the different star formation scenarios.Comment: 43 pages, 18 figures, accepted for publication in A&
- …