29 research outputs found
Near-Earth asteroid (3200) Phaethon. Characterization of its orbit, spin state, and thermophysical parameters
The near-Earth asteroid (3200) Phaethon is an intriguing object: its
perihelion is at only 0.14 au and is associated with the Geminid meteor stream.
We aim to use all available disk-integrated optical data to derive a reliable
convex shape model of Phaethon. By interpreting the available space- and
ground-based thermal infrared data and Spitzer spectra using a thermophysical
model, we also aim to further constrain its size, thermal inertia, and visible
geometric albedo. We applied the convex inversion method to the new optical
data obtained by six instruments and to previous observations. The convex shape
model was then used as input for the thermophysical modeling. We also studied
the long-term stability of Phaethon's orbit and spin axis with a numerical
orbital and rotation-state integrator. We present a new convex shape model and
rotational state of Phaethon: a sidereal rotation period of 3.603958(2) h and
ecliptic coordinates of the preferred pole orientation of (319,
39) with a 5 uncertainty. Moreover, we derive its size
(=5.10.2 km), thermal inertia (=600200 J m
s K), geometric visible albedo
(=0.1220.008), and estimate the macroscopic surface
roughness. We also find that the Sun illumination at the perihelion passage
during the past several thousand years is not connected to a specific area on
the surface, which implies non-preferential heating.Comment: Astronomy and Astrophysics. In pres
YORP and Yarkovsky effects in asteroids (1685) Toro, (2100) Ra-Shalom, (3103) Eger, and (161989) Cacus
The rotation states of small asteroids are affected by a net torque arising
from an anisotropic sunlight reflection and thermal radiation from the
asteroids' surfaces. On long timescales, this so-called YORP effect can change
asteroid spin directions and their rotation periods. We analyzed lightcurves of
four selected near-Earth asteroids with the aim of detecting secular changes in
their rotation rates that are caused by YORP. We use the lightcurve inversion
method to model the observed lightcurves and include the change in the rotation
rate as a free parameter of optimization. We
collected more than 70 new lightcurves. For asteroids Toro and Cacus, we used
thermal infrared data from the WISE spacecraft and estimated their size and
thermal inertia. We also used the currently available optical and radar
astrometry of Toro, Ra-Shalom, and Cacus to infer the Yarkovsky effect. We
detected a YORP acceleration of for asteroid Cacus. For
Toro, we have a tentative () detection of YORP from a significant
improvement of the lightcurve fit for a nonzero value of . For asteroid
Eger, we confirmed the previously published YORP detection with more data and
updated the YORP value to . We also updated the shape model of
asteroid Ra-Shalom and put an upper limit for the change of the rotation rate
to . Ra-Shalom has a greater than
Yarkovsky detection with a theoretical value consistent with observations
assuming its size and/or density is slightly larger than the nominally expected
values
Dexketoprofen/tramadol 25Â mg/75Â mg: randomised double-blind trial in moderate-to-severe acute pain after abdominal hysterectomy
Statistical Analysis of TOTPAR over 2, 4, 6 and 8Â h (ANCOVA) (ITT Population) (single-dose phase). (DOCX 16Â kb
Dexketoprofen/tramadol 25 mg/75 mg: randomised double-blind trial in moderate-to-severe acute pain after abdominal hysterectomy
Formation of asteroid pairs by rotational fission
Asteroid pairs sharing similar heliocentric orbits were found recently.
Backward integrations of their orbits indicated that they separated gently with
low relative velocities, but did not provide additional insight into their
formation mechanism. A previously hypothesized rotational fission process4 may
explain their formation - critical predictions are that the mass ratios are
less than about 0.2 and, as the mass ratio approaches this upper limit, the
spin period of the larger body becomes long. Here we report photometric
observations of a sample of asteroid pairs revealing that primaries of pairs
with mass ratios much less than 0.2 rotate rapidly, near their critical fission
frequency. As the mass ratio approaches 0.2, the primary period grows long.
This occurs as the total energy of the system approaches zero requiring the
asteroid pair to extract an increasing fraction of energy from the primary's
spin in order to escape. We do not find asteroid pairs with mass ratios larger
than 0.2. Rotationally fissioned systems beyond this limit have insufficient
energy to disrupt. We conclude that asteroid pairs are formed by the rotational
fission of a parent asteroid into a proto-binary system which subsequently
disrupts under its own internal system dynamics soon after formation.Comment: 12 pages, 2 figures, 1 table + Supplementary Informatio
Radar Observations and the Shape of Near-Earth Asteroid 2008 EV5
We observed the near-Earth asteroid 2008 EV5 with the Arecibo and Goldstone
planetary radars and the Very Long Baseline Array during December 2008. EV5
rotates retrograde and its overall shape is a 400 /pm 50 m oblate spheroid. The
most prominent surface feature is a ridge parallel to the asteroid's equator
that is broken by a concavity 150 m in diameter. Otherwise the asteroid's
surface is notably smooth on decameter scales. EV5's radar and optical albedos
are consistent with either rocky or stony-iron composition. The equatorial
ridge is similar to structure seen on the rubble-pile near-Earth asteroid
(66391) 1999 KW4 and is consistent with YORP spin-up reconfiguring the asteroid
in the past. We interpret the concavity as an impact crater. Shaking during the
impact and later regolith redistribution may have erased smaller features,
explaining the general lack of decameter-scale surface structure.Comment: This paper has been accepted for publication in Icarus:
http://www.sciencedirect.com/science/article/B6WGF-5207B2F-4/2/d87cd2ae4da00c2b277e2dc79a532c4
Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxies and a new section on astrobiology. Some highlights are short (the newly identified class of gamma-ray bursts, and the Deep Impact on Comet 9P/ Tempel 1), some long (the age of the universe, which will be found to have the Earth at its center), and a few metonymic, for instance the term "down-sizing" to describe the evolution of star formation rates with redshift
Relative photometry of transiting exoplanet COROT-Exo-2b
Relative CCD photometry of the extrasolar planet COROT-Exo-2b transiting in front of its parent star was carried out at the Astronomical and Geophysical Observatory of Comenius University at Modra (AGO). Physical and orbital parameters were determined and compared with the previous published data