1,072 research outputs found
Gaia astrometry for stars with too few observations - a Bayesian approach
Gaia's astrometric solution aims to determine at least five parameters for
each star, together with appropriate estimates of their uncertainties and
correlations. This requires at least five distinct observations per star. In
the early data reductions the number of observations may be insufficient for a
five-parameter solution, and even after the full mission many stars will remain
under-observed, including faint stars at the detection limit and transient
objects. In such cases it is reasonable to determine only the two position
parameters. Their formal uncertainties would however grossly underestimate the
actual errors, due to the neglected parallax and proper motion. We aim to
develop a recipe to calculate sensible formal uncertainties that can be used in
all cases of under-observed stars. Prior information about the typical ranges
of stellar parallaxes and proper motions is incorporated in the astrometric
solution by means of Bayes' rule. Numerical simulations based on the Gaia
Universe Model Snapshot (GUMS) are used to investigate how the prior influences
the actual errors and formal uncertainties when different amounts of Gaia
observations are available. We develop a criterion for the optimum choice of
priors, apply it to a wide range of cases, and derive a global approximation of
the optimum prior as a function of magnitude and galactic coordinates. The
feasibility of the Bayesian approach is demonstrated through global astrometric
solutions of simulated Gaia observations. With an appropriate prior it is
possible to derive sensible positions with realistic error estimates for any
number of available observations. Even though this recipe works also for
well-observed stars it should not be used where a good five-parameter
astrometric solution can be obtained without a prior. Parallaxes and proper
motions from a solution using priors are always biased and should not be used.Comment: Revised version, accepted 21st of August 2015 for publication in A&
The First Dynamical Mass Measurement in the HR 8799 System
HR 8799 hosts four directly imaged giant planets, but none has a mass
measured from first principles. We present the first dynamical mass measurement
in this planetary system, finding that the innermost planet HR~8799~e has a
mass of . This mass results from combining
the well-characterized orbits of all four planets with a new astrometric
acceleration detection (5) from the Gaia EDR3 version of the
Hipparcos-Gaia Catalog of Accelerations. We find with 95\% confidence that
HR~8799~e is below , the deuterium-fusing mass limit. We
derive a hot-start cooling age of \,Myr for HR~8799~e that
agrees well with its hypothesized membership in the Columba association but is
also consistent with an alternative suggested membership in the
~Pictoris moving group. We exclude the presence of any additional
5- planets interior to HR~8799~e with semi-major axes
between 3-16\,au. We provide proper motion anomalies and a matrix
equation to solve for the mass of any of the planets of HR~8799 using only mass
ratios between the planets.Comment: Accepted to ApJ Letter
Potential control under thin aqueous layers using a Kelvin Probe
Kelvin Probes can be modified to control as well as monitor potential. The design and operation of two different Kelvin Probe Potentiostats (KPPs) are described in this paper. One approach uses a permanent magnet and double coil to oscillate the needle at a fixed frequency, an AC backing potential, and software analysis and control schemes. This technique can also control the distance between the tip and sample, thereby tracking the topography of the sample. Both KPPs were used to make measurements on Type 304L stainless steel under thin layers of electrolyte. Cathodic polarization curves exhibited a limiting current density associated with oxygen reduction. The limiting current density varied with solution layer thickness over a finite range of thickness. Anodic polarization curves on 304L in a thin layer of chloride solution resulted in pitting corrosion. The breakdown potential did not vary with solution layer thickness. However, the thin layer was observed to increase in volume remarkably during pit growth owing to the absorption of water from the high humidity environment into the layer with ionic strength increased by the pit dissolution. The open circuit potential (OCP) and solution layer thickness were monitored during drying out of a thin electrolyte layer. Pitting corrosion initiated, as indicated by a sharp drop in the OCP, as the solution thinned and increased in concentration.This work was supported in part by the Office of Science and Technology and International (OST&I), Office of Civilian Radioactive Waste Management (OCRWM), US Department of Energy (DOE). The work is carried out as part of the DOE Multi-University Corrosion Cooperative under Cooperative Agreement DE-FC28-04RW12252
htof::A New Open-source Tool for Analyzing Hipparcos, Gaia, and Future Astrometric Missions
We present htof, an open-source tool for interpreting and fitting the
intermediate astrometric data (IAD) from both the 1997 and 2007 reductions of
Hipparcos, the scanning-law of Gaia, and future missions such as the Nancy
Grace Roman Space Telescope (NGRST). htof solves for the astrometric parameters
of any system for any arbitrary combination of absolute astrometric missions.
In preparation for later Gaia data releases, htof supports arbitrarily
high-order astrometric solutions (e.g. five-, seven-, nine-parameter fits).
Using htof, we find that the IAD of 6617 sources in Hipparcos 2007 might have
been affected by a data corruption issue. htof integrates an ad-hoc correction
that reconciles the IAD of these sources with their published catalog
solutions. We developed htof to study masses and orbital parameters of
sub-stellar companions, and we outline its implementation in one orbit fitting
code (orvara, https://github.com/t-brandt/orvara). We use htof to predict a
range of hypothetical additional planets in the ~Pic system, which could
be detected by coupling NGRST astrometry with Gaia and Hipparcos. htof is pip
installable and available at https://github.com/gmbrandt/htof .Comment: Accepted to AJ. References updated in version 2. The Hipparcos 2007
Re-reduction Java Tool Intermediate Astrometric Data are available at , via
the "zip file" link at https://www.cosmos.esa.int/web/hipparcos/hipparcos-2 :
"...human readable version of the IAD of the Java tool in a zip file
[warning: ~350 MB]...
A Comparative Analysis of the Morphology and Evolution of Permanent Sperm Depletion in Spiders
Once thought to be energetically cheap and easy to produce, empirical work has shown that sperm is a costly and limited resource for males. In some spider species, there is behavioral evidence that sperm are permanently depleted after a single mating. This extreme degree of mating investment appears to co-occur with other reproductive strategies common to spiders, e.g. genital mutilation and sexual cannibalism. Here we corroborate that sperm depletion in the golden orb-web spider Nephila clavipes is permanent by uncovering its mechanistic basis using light and electron microscopy. In addition, we use a phylogeny-based statistical analysis to test the evolutionary relationships between permanent sperm depletion (PSD) and other reproductive strategies in spiders. Male testes do not produce sperm during adulthood, which is unusual in spiders. Instead, spermatogenesis is nearly synchronous and ends before the maturation molt. Testis size decreases as males approach their maturation molt and reaches its lowest point after sperm is transferred into the male copulatory organs (pedipalps). As a consequence, the amount of sperm available to males for mating is limited to the sperm contained in the pedipalps, and once it is used, males lose their ability to fertilize eggs. Our data suggest that PSD has evolved independently at least three times within web-building spiders and is significantly correlated with the evolution of other mating strategies that limit males to monogamy, including genital mutilation and sexual cannibalism. We conclude that PSD may be an energy-saving adaptation in species where males are limited to monogamy. This could be particularly important in web-building spiders where extreme sexual size dimorphism results in large, sedentary females and small, searching males who rarely feed as adults and are vulnerable to starvation. Future work will explore possible energetic benefits and the evolutionary lability of PSD relative to other mate-limiting reproductive behaviors
orvara::An Efficient Code to Fit Orbits using Radial Velocity, Absolute, and/or Relative Astrometry
We present an open-source Python package, Orbits from Radial Velocity,
Absolute, and/or Relative Astrometry (orvara), to fit Keplerian orbits to any
combination of radial velocity, relative astrometry, and absolute astrometry
data from the Hipparcos-Gaia Catalog of Accelerations. By combining these three
data types, one can measure precise masses and sometimes orbital parameters
even when the observations cover a small fraction of an orbit. orvara achieves
its computational performance with an eccentric anomaly solver five to ten
times faster than commonly used approaches, low-level memory management to
avoid python overheads, and by analytically marginalizing out parallax,
barycenter proper motion, and the instrument-specific radial velocity zero
points. Through its integration with the Hipparcos and Gaia intermediate
astrometry package htof, orvara can properly account for the epoch astrometry
measurements of Hipparcos and the measurement times and scan angles of
individual Gaia epochs. We configure orvara with modifiable .ini configuration
files tailored to any specific stellar or planetary system. We demonstrate
orvara with a case study application to a recently discovered white dwarf/main
sequence (WD/MS) system, HD 159062. By adding absolute astrometry to literature
RV and relative astrometry data, our comprehensive MCMC analysis improves the
precision of HD 159062B's mass by more than an order of magnitude to
. We also derive a low eccentricity and
large semimajor axis, establishing HD 159062AB as a system that did not
experience Roche lobe overflow.Comment: 24 pages, 5 figures, 5 tables. AJ accepted with minor changes. orvara
is available at https://github.com/t-brandt/orvar
Precise Masses and Orbits for Nine Radial Velocity Exoplanets
Radial velocity (RV) surveys have discovered hundreds of exoplanetary systems
but suffer from a fundamental degeneracy between planet mass and orbital
inclination . In this paper we break this degeneracy by combining RVs with
complementary absolute astrometry taken from the Gaia EDR3 version of the
cross-calibrated Hipparcos-Gaia Catalog of Accelerations (HGCA). We use the
Markov Chain Monte Carlo orbit code to simultaneously fit
literature RVs and absolute astrometry from the HGCA. We constrain the orbits,
masses, and inclinations of nine single and massive RV companions orbiting
nearby G and K stars. We confirm the planetary nature of six companions: HD
29021 b (), HD 81040 b
(), HD 87883 b (), HD 98649 b (), HD 106252 b
(), and HD 171238 b
(). We place one companion, HD 196067 b
() on the planet-brown dwarf boundary, and two
companions in the low mass brown dwarf regime: HD 106515 Ab
(), and HD 221420 b
(). The brown dwarf HD 221420 b, with a
semi-major axis of AU, a period of
years, and an eccentricity of
represents a promising target for high-contrast imaging. The RV orbits of HD
87883 b, HD 98649 b, HD 171238 b, and HD 196067 b are not fully constrained yet
because of insufficient RV data. We find two possible inclinations for each of
these orbits due to difficulty in separating prograde from retrograde orbits,
but we expect this will change decisively with future Gaia data releases
Regularity of Ornstein-Uhlenbeck processes driven by a L{\'e}vy white noise
The paper is concerned with spatial and time regularity of solutions to
linear stochastic evolution equation perturbed by L\'evy white noise "obtained
by subordination of a Gaussian white noise". Sufficient conditions for spatial
continuity are derived. It is also shown that solutions do not have in general
\cadlag modifications. General results are applied to equations with fractional
Laplacian. Applications to Burgers stochastic equations are considered as well.Comment: This is an updated version of the same paper. In fact, it has already
been publishe
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