367 research outputs found
Detecting the Signatures of Uranus and Neptune
With more than 15 years since the the first radial velocity discovery of a
planet orbiting a Sun-like star, the time baseline for radial velocity surveys
is now extending out beyond the orbit of Jupiter analogs. The sensitivity to
exoplanet orbital periods beyond that of Saturn orbital radii however is still
beyond our reach such that very few clues regarding the prevalence of ice
giants orbiting solar analogs are available to us. Here we simulate the radial
velocity, transit, and photometric phase amplitude signatures of the solar
system giant planets, in particular Uranus and Neptune, and assess their
detectability. We scale these results for application to monitoring low-mass
stars and compare the relative detection prospects with other potential
methods, such as astrometry and imaging. These results quantitatively show how
many of the existing techniques are suitable for the detection of ice giants
beyond the snow line for late-type stars and the challenges that lie ahead for
the detection true Uranus/Neptune analogues around solar-type stars.Comment: 20 pages, 4 figures, accepted for publication in Icaru
Determination of Ras-GTP and Ras-GDP in patients with acute myelogenous leukemia (AML), myeloproliferative syndrome (MPS), juvenile myelomonocytic leukemia (JMML), acute lymphocytic leukemia (ALL), and malignant lymphoma: assessment of mutational and indirect activation
The 21-kD protein Ras of the low-molecular-weight GTP-binding (LMWG) family plays an important role in transduction of extracellular signals. Ras functions as a ‘molecular switch’ in transduction of signals from the membrane receptors of many growth factors, cytokines, and other second messengers to the cell nucleus. Numerous studies have shown that in multiple malignant tumors and hematopoietic malignancies, faulty signal transduction via the Ras pathway plays a key role in tumorigenesis. In this work, a non-radioactive assay was used to quantify Ras activity in hematologic malignancies. Ras activation was measured in six different cell lines and 24 patient samples, and sequence analysis of N- and K-ras was performed. The 24 patient samples comprised of seven acute myelogenous leukemia (AML) samples, five acute lymphocytic leukemia (ALL) samples, four myeloproliferative disease (MPD) samples, four lymphoma samples, four juvenile myelomonocytic leukemia (JMML) samples, and WBC from a healthy donor. The purpose of this study was to compare Ras activity determined by percentage of Ras-GTP with the mutational status of the Ras gene in the hematopoietic cells of the patients. Mutation analysis revealed ras mutations in two of the seven AML samples, one in codon 12 and one in codon 61; ras mutations were also found in two of the four JMML samples, and in one of the four lymphoma samples (codon 12). We found a mean Ras activation of 23.1% in cell lines with known constitutively activating ras mutations, which was significantly different from cell lines with ras wildtype sequence (Ras activation of 4.8%). Two of the five activating ras mutations in the patient samples correlated with increased Ras activation. In the other three samples, Ras was probably activated through “upstream” or “downstream” mechanisms
Double-blind test program for astrometric planet detection with Gaia
We use detailed simulations of the Gaia observations of synthetic planetary
systems and develop and utilize independent software codes in double-blind mode
to analyze the data, including statistical tools for planet detection and
different algorithms for single and multiple Keplerian orbit fitting that use
no a priori knowledge of the true orbital parameters of the systems. 1) Planets
with astrometric signatures times the single-measurement error
and period yr can be detected reliably, with a very
small number of false positives. 2) At twice the detection limit, uncertainties
in orbital parameters and masses are typically . 3) Over 70% of
two-planet systems with well-separated periods in the range
yr, , and eccentricity are
correctly identified. 4) Favorable orbital configurations have orbital elements
measured to better than 10% accuracy of the time, and the value of the
mutual inclination angle determined with uncertainties \leq 10^{\degr}. 5)
Finally, uncertainties obtained from the fitting procedures are a good estimate
of the actual errors. Extrapolating from the present-day statistical properties
of the exoplanet sample, the results imply that a Gaia with = 8
as, in its unbiased and complete magnitude-limited census of planetary
systems, will measure several thousand giant planets out to 3-4 AUs from stars
within 200 pc, and will characterize hundreds of multiple-planet systems,
including meaningful coplanarity tests. Finally, we put Gaia into context,
identifying several areas of planetary-system science in which Gaia can be
expected to have a relevant impact, when combined with data coming from other
ongoing and future planet search programs.Comment: 32 pages, 24 figures, 6 tables. Accepted for pubolication in A&
On the convergence of the critical cooling timescale for the fragmentation of self-gravitating discs
We carry out simulations of gravitationally unstable discs using a Smoothed
Particle Hydrodynamics (SPH) code and a grid-based hydrodynamics code, FARGO,
to understand the previous non-convergent results reported by Meru & Bate
(2011a). We obtain evidence that convergence with increasing resolution occurs
with both SPH and FARGO and in both cases we find that the critical cooling
timescale is larger than previously thought. We show that SPH has a first-order
convergence rate while FARGO converges with a second-order rate. We show that
the convergence of the critical cooling timescale for fragmentation depends
largely on the numerical viscosity employed in both SPH and FARGO. With SPH,
particle velocity dispersion may also play a role. We show that reducing the
dissipation from the numerical viscosity leads to larger values of the critical
cooling time at a given resolution. For SPH, we find that the effect of the
dissipation due to the numerical viscosity is somewhat larger than had
previously been appreciated. In particular, we show that using a quadratic term
in the SPH artificial viscosity (beta_{SPH}) that is too low appears to lead to
excess dissipation in gravitationally unstable discs, which may affect any
results that sensitively depend on the thermodynamics, such as disc
fragmentation. We show that the two codes converge to values of the critical
cooling timescale, beta_{crit} > 20 (for a ratio of specific heats of
gamma=5/3), and perhaps even as large as beta_{crit} \approx 30. These are
approximately 3-5 times larger than has been found by most previous studies.
This is equivalent to a maximum gravitational stress that a disc can withstand
without fragmenting of alpha_{GI,crit} \approx 0.013-0.02, which is much
smaller than the values typically used in the literature. It is therefore
easier for self-gravitating discs to fragment than has been concluded from most
past studies.Comment: Accepted for publication by MNRAS. 26 pages, 17 figure
Identifying Near Earth Object Families
The study of asteroid families has provided tremendous insight into the
forces that sculpted the main belt and continue to drive the collisional and
dynamical evolution of asteroids. The identification of asteroid families
within the NEO population could provide a similar boon to studies of their
formation and interiors. In this study we examine the purported identification
of NEO families by Drummond (2000) and conclude that it is unlikely that they
are anything more than random fluctuations in the distribution of NEO
osculating orbital elements. We arrive at this conclusion after examining the
expected formation rate of NEO families, the identification of NEO groups in
synthetic populations that contain no genetically related NEOs, the orbital
evolution of the largest association identified by Drummond (2000), and the
decoherence of synthetic NEO families intended to reproduce the observed
members of the same association. These studies allowed us to identify a new
criterion that can be used to select real NEO families for further study in
future analyses, based on the ratio of the number of pairs and the size of
strings to the number of objects in an identified association.Comment: Accepted for publication in Icarus. 19 pages including 11 figure
Hyades dynamics from N-body simulations: Accuracy of astrometric radial velocities from Hipparcos
The internal velocity structure in the Hyades cluster as seen by Hipparcos is
compared with realistic N-body simulations using the NBODY6 code, which
includes binary interaction, stellar evolution and the Galactic tidal field.
The model allows to estimate reliably the accuracy of astrometric radial
velocities in the Hyades as derived by Lindegren et al. (2000) and Madsen et
al. (2002) from Hipparcos data, by applying the same estimation procedure on
the simulated data. The simulations indicate that the current cluster velocity
dispersion decreases from 0.35 km/s at the cluster centre to a minimum of 0.20
km/s at 8 pc radius (2-3 core radii), from where it slightly increases
outwards. A clear negative correlation between dispersion and stellar mass is
seen in the central part of the cluster but is almost absent beyond a radius of
3 pc. It follows that the (internal) standard error of the astrometric radial
velocities relative to the cluster centroid may be as small as 0.2 km/s for a
suitable selection of stars, while a total (external) standard error of 0.6
km/s is found when the uncertainty of the bulk motion of the cluster is
included. Attempts to see structure in the velocity dispersion using
observational data from Hipparcos and Tycho-2 are inconclusive.Comment: 12 pages, accepted by A&
A Comparison of the Interiors of Jupiter and Saturn
Interior models of Jupiter and Saturn are calculated and compared in the
framework of the three-layer assumption, which rely on the perception that both
planets consist of three globally homogeneous regions: a dense core, a metallic
hydrogen envelope, and a molecular hydrogen envelope. Within this framework,
constraints on the core mass and abundance of heavy elements (i.e. elements
other than hydrogen and helium) are given by accounting for uncertainties on
the measured gravitational moments, surface temperature, surface helium
abundance, and on the inferred protosolar helium abundance, equations of state,
temperature profile and solid/differential interior rotation.Comment: 25 pages, 6 tables, 10 figures Planetary and Space Science, in pres
Holocene Atlantic climate variations deduced from carbonate peri-platform sediments (leeward margin, Great Bahama Bank)
A marine sediment core from the leeward margin of Great Bahama Bank (GBB) was subjected to a multiproxy study. The aragonite dominated core MD992201 comprises the past 7230 years in a decadal time resolution and shows sedimentation rates of up to 13.8 m/kyr. Aragonite mass accumulation rates, age differences between planktonic foraminifera and aragonite sediments, and temperature distribution are used to deduce changes in aragonite production rates and paleocurrent strengths. Aragonite precipitation rates on GBB are controlled by exchange of carbonate ions and CO2 loss due to temperature-salinity conditions and biological activity, and these are dependent on the current strength. Paleocurrent strengths on GBB show high current velocities during the periods 6000–5100 years BP, 3500–2700 years BP, and 1600–700 years BP; lower current speeds existed during the time intervals 5100–3500 years BP, 2700–1600 years BP, and 700–100 years BP. Bahamian surface currents are directly linked to the North Atlantic atmospheric circulation, and thus periods with high (low) current speeds are proposed to be phases of strong (weak) atmospheric circulation
The PTF Orion Project: a Possible Planet Transiting a T-Tauri Star
We report observations of a possible young transiting planet orbiting a
previously known weak-lined T-Tauri star in the 7-10 Myr old Orion-OB1a/25-Ori
region. The candidate was found as part of the Palomar Transient Factory (PTF)
Orion project. It has a photometric transit period of 0.448413 +- 0.000040
days, and appears in both 2009 and 2010 PTF data. Follow-up low-precision
radial velocity (RV) observations and adaptive optics imaging suggest that the
star is not an eclipsing binary, and that it is unlikely that a background
source is blended with the target and mimicking the observed transit. RV
observations with the Hobby-Eberly and Keck telescopes yield an RV that has the
same period as the photometric event, but is offset in phase from the transit
center by approximately -0.22 periods. The amplitude (half range) of the RV
variations is 2.4 km/s and is comparable with the expected RV amplitude that
stellar spots could induce. The RV curve is likely dominated by stellar spot
modulation and provides an upper limit to the projected companion mass of M_p
sin i_orb < 4.8 +- 1.2 M_Jup; when combined with the orbital inclination, i
orb, of the candidate planet from modeling of the transit light curve, we find
an upper limit on the mass of the planetary candidate of M_p < 5.5 +- 1.4
M_Jup. This limit implies that the planet is orbiting close to, if not inside,
its Roche limiting orbital radius, so that it may be undergoing active mass
loss and evaporation.Comment: Corrected typos, minor clarifications; minor updates/corrections to
affiliations and bibliography. 35 pages, 10 figures, 3 tables. Accepted to
Ap
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