116 research outputs found
A volume-averaged nodal projection method for the Reissner-Mindlin plate model
We introduce a novel meshfree Galerkin method for the solution of
Reissner-Mindlin plate problems that is written in terms of the primitive
variables only (i.e., rotations and transverse displacement) and is devoid of
shear-locking. The proposed approach uses linear maximum-entropy approximations
and is built variationally on a two-field potential energy functional wherein
the shear strain, written in terms of the primitive variables, is computed via
a volume-averaged nodal projection operator that is constructed from the
Kirchhoff constraint of the three-field mixed weak form. The stability of the
method is rendered by adding bubble-like enrichment to the rotation degrees of
freedom. Some benchmark problems are presented to demonstrate the accuracy and
performance of the proposed method for a wide range of plate thicknesses
First observations and magnitude measurement of Starlink's Darksat
Measure the Sloan g' magnitudes of the Starlink's STARLINK-1130 (Darksat) and
1113 LEO communication satellites and determine the effectiveness of the
Darksat darkening treatment at 475.4\,nm. Two observations of the Starlink's
Darksat LEO communication satellite were conducted on 2020/02/08 and 2020/03/06
using a Sloan r' and g' filter respectively. While a second satellite,
STARLINK-1113 was observed on 2020/03/06 using a Sloan g' filter. The initial
observation on 2020/02/08 was a test observation when Darksat was still
manoeuvring to its nominal orbit and orientation. Based on the successful test
observation, the first main observation was conducted on 2020/03/06 along with
an observation of the second Starlink satellite. The calibration, image
processing and analysis of the Darksat Sloan g' image gives an estimated Sloan
g' magnitude of at a range of 976.50\,km. For STARLINK-1113 an
estimated Sloan g' magnitude of at a range of 941.62\,km was
found. When scaled to a range of 550\,km and corrected for the solar and
observer phase angles, a reduction by a factor of two is seen in the reflected
solar flux between Darksat and STARLINK-1113. The data and results presented in
this work, show that the special darkening coating used by Starlink for Darksat
has darkened the Sloan g' magnitude by \,mag, when the range is
equal to a nominal orbital height (550\,km). This result will serve members of
the astronomical community modelling the satellite mega-constellations, to
ascertain their true impact on both the amateur and professional astronomical
communities. Concurrent and further observations are planned to cover the full
optical and NIR spectrum, from an ensemble of instruments, telescopes and
observatories.Comment: Accepted for publication in A&A Letters. 5 pages, 2 figures and 4
table
Spectroscopic Observations of New Oort Cloud Comet 2006 VZ13 and Four Other Comets
Spectral data are presented for comets 2006 VZ13 (LINEAR), 2006 K4 (NEAT),
2006 OF2 (Broughton), 2P/Encke, and 93P/Lovas I, obtained with the Cerro-Tololo
Inter-American Observatory 1.5-m telescope in August 2007. Comet 2006 VZ13 is a
new Oort cloud comet and shows strong lines of CN (3880 angstroms), the Swan
band sequence for C_2 (4740, 5160, and 5630 angstroms), C_3 (4056 angstroms),
and other faint species. Lines are also identified in the spectra of the other
comets. Flux measurements of the CN, C_2 (Delta v = +1,0), and C_3 lines are
recorded for each comet and production rates and ratios are derived. When
considering the comets as a group, there is a correlation of C_2 and C_3
production with CN, but there is no conclusive evidence that the production
rate ratios depend on heliocentric distance. The continuum is also measured,
and the dust production and dust-to-gas ratios are calculated. There is a
general trend, for the group of comets, between the dust-to-gas ratio and
heliocentric distance, but it does not depend on dynamical age or class. Comet
2006 VZ13 is determined to be in the carbon-depleted (or Tempel 1 type) class.Comment: 8 pages, 6 figures, 6 tables; Accepted by MNRA
A search for evidence of irradiation in Centaurus X-4 during quiescence
We present a study of the neutron star X-Ray Transient Cen X-4. Our aim is to
look for any evidence of irradiation of the companion with a detailed analysis
of its radial velocity curve, relative contribution of the donor star and
Doppler tomography of the main emission lines. To improve our study all our
data are compared with a set of simulations that consider different physical
parameters of the system, like the disc aperture angle and the mass ratio. We
conclude that neither the radial velocity curve nor the orbital variation of
the relative donor's contribution to the total flux are affected by
irradiation. On the other hand, we do see emission from the donor star at
H and HeI 5876 which we tentatively attribute to irradiation effects.
In particular, the H emission from the companion is clearly
asymmetric and we suggest is produced by irradiation from the hot-spot.
Finally, from the velocity of the HeI 5876 spot we constrain the disc opening
angle to alpha=7-14 deg.Comment: 4 pages, 5 figures, accepted for publication in A&A as a R
Optical-to-NIR magnitude measurements of the Starlink LEO Darksat satellite and effectiveness of the darkening treatment
Four observations of Starlink's LEO communication satellites, Darksat and
STARLINK-1113, were conducted on two nights with two telescopes. The Chakana
0.6\,m telescope at the Ckoirama observatory (Chile) observed both satellites
on 5\,Mar\,2020 (UTC) and 7\,Mar\,2020 (UTC) using a Sloan r' and Sloan i'
filter, respectively. The ESO VISTA 4.1\,m telescope with the VIRCAM instrument
observed both satellites on 5\,Mar\,2020 (UTC) and 7\,Mar\,2020 (UTC) in the
NIR J-band and Ks-band, respectively. The calibration, image processing, and
analysis of the Darksat images give r\,\,5.6\,mag,
i\,\,5.0\,mag, J\,\,4.2\,mag, and Ks\,\,4.0\,mag
when scaled to a range of 550\,km (airmass ) and corrected for the solar
incidence and observer phase angles. In comparison, the STARLINK-1113 images
give r\,\,4.9\,mag, i\,\,4.4\,mag, J\,\,3.8\,mag,
and Ks\,\,3.6\,mag when corrected for range, solar incidence, and
observer phase angles. The data and results presented in this work show that
the special darkening coating used by Starlink for Darksat has darkened the
Sloan r' magnitude by 50\,\%, Sloan i' magnitude by 42\,\%, NIR J magnitude by
32\,\%, and NIR Ks magnitude by 28\,\%. The results show that both satellites
increase in reflective brightness with increasing wavelength and that the
effectiveness of the darkening treatment is reduced at longer wavelengths. This
shows that the mitigation strategies being developed by Starlink and other LEO
satellite operators need to take into account other wavelengths, not just the
optical. This work highlights the continued importance of obtaining
multi-wavelength observations of many different LEO satellites in order to
characterise their reflective properties and to aid the community in developing
impact simulations and developing mitigation tools.Comment: Accepted for publication in A&A, 10 pages, 10 figures, 3 table
739 observed NEAs and new 2-4m survey statistics within the EURONEAR network
We report follow-up observations of 477 program Near-Earth Asteroids (NEAs)
using nine telescopes of the EURONEAR network having apertures between 0.3 and
4.2 m. Adding these NEAs to our previous results we now count 739 program NEAs
followed-up by the EURONEAR network since 2006. The targets were selected using
EURONEAR planning tools focusing on high priority objects. Analyzing the
resulting orbital improvements suggests astrometric follow-up is most important
days to weeks after discovery, with recovery at a new opposition also valuable.
Additionally we observed 40 survey fields spanning three nights covering 11 sq.
degrees near opposition, using the Wide Field Camera on the 2.5m Isaac Newton
Telescope (INT), resulting in 104 discovered main belt asteroids (MBAs) and
another 626 unknown one-night objects. These fields, plus program NEA fields
from the INT and from the wide field MOSAIC II camera on the Blanco 4m
telescope, generated around 12,000 observations of 2,000 minor planets (mostly
MBAs) observed in 34 square degrees. We identify Near Earth Object (NEO)
candidates among the unknown (single night) objects using three selection
criteria. Testing these criteria on the (known) program NEAs shows the best
selection methods are our epsilon-miu model which checks solar elongation and
sky motion and the MPC's NEO rating tool. Our new data show that on average 0.5
NEO candidates per square degree should be observable in a 2m-class survey (in
agreement with past results), while an average of 2.7 NEO candidates per square
degree should be observable in a 4m-class survey (although our Blanco
statistics were affected by clouds). At opposition just over 100 MBAs (1.6
unknown to every 1 known) per square degree are detectable to R=22 in a 2m
survey based on the INT data, while our two best ecliptic Blanco fields away
from opposition lead to 135 MBAs (2 unknown to every 1 known) to R=23.Comment: Published in Planetary and Space Sciences (Sep 2013
Représentation du réseau d'assainissement urbain et péri-urbain par l'utilisation de Geo-Pumma dans les bassins versants du Mercier (France) et de l'El Guindo (Chili)
Transit timing variations in the WASP-4 planetary system*
Abstract Transits in the planetary system WASP-4 were recently found to occur 80 s earlier than expected in observations from the TESS satellite. We present 22 new times of mid-transit that confirm the existence of transit timing variations, and are well fitted by a quadratic ephemeris with period decay dP/dt = −9.2 ± 1.1 ms yr−1. We rule out instrumental issues, stellar activity and the Applegate mechanism as possible causes. The light-time effect is also not favoured due to the non-detection of changes in the systemic velocity. Orbital decay and apsidal precession are plausible but unproven. WASP-4 b is only the third hot Jupiter known to show transit timing variations to high confidence. We discuss a variety of observations of this and other planetary systems that would be useful in improving our understanding of WASP-4 in particular and orbital decay in general
The stellar occultation by Makemake on 2011 April 23
We have taken advantage of a stellar occultation by the dwarf planet Makemake on 2011 April 23, to determine several of its main physical properties. We present results from a multisite campaign with 8 positive occultation detections from 5 different sites, including data from the 8-m VLT and 3.5-m NTT telescopes in Chile, which have very high temporal resolution. Because the star was significantly fainter than Makemake (setting a record in the magnitude of a star whose occultation has been detected), the occultation resulted in a drop of just ~0.3 mag in the lightcurves. From the lightcurves we have been able to determine the size and shape of the body, its geometric albedo and constraints on its atmosphere
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