569 research outputs found
Detection of a Third Planet in the HD 74156 System Using the Hobby-Eberly Telescope
We report the discovery of a third planetary mass companion to the G0 star HD
74156. High precision radial velocity measurements made with the Hobby-Eberly
Telescope aided the detection of this object. The best fit triple Keplerian
model to all the available velocity data yields an orbital period of 347 days
and minimum mass of 0.4 M_Jup for the new planet. We determine revised orbital
periods of 51.7 and 2477 days, and minimum masses of 1.9 and 8.0 M_Jup
respectively for the previously known planets. Preliminary calculations
indicate that the derived orbits are stable, although all three planets have
significant orbital eccentricities (e = 0.64, 0.43, and 0.25). With our
detection, HD 74156 becomes the eighth normal star known to host three or more
planets. Further study of this system's dynamical characteristics will likely
give important insight to planet formation and evolutionary processes.Comment: 24 pages, 4 tables, 6 figures. Accepted for publication in ApJ. V2
fixed table 4 page overrun. V3 added reference
A BAYESIAN ANALYSIS OF THE AGES OF FOUR OPEN CLUSTERS
In this paper we apply a Bayesian technique to determine the best fit of stellar evolution models to find the main sequence turn off age and other cluster parameters of four intermediate-age open clusters: NGC 2360, NGC 2477, NGC 2660, and NGC 3960. Our algorithm utilizes a Markov chain Monte Carlo technique to fit these various parameters, objectively finding the best fit isochrone for each cluster. The result is a high precision isochrone fit. We compare these results with the those of traditional “by eye” isochrone fitting methods. By applying this Bayesian technique to NGC 2360, NGC 2477, NGC 2660, and NGC 3960 we determine the ages of these clusters to be 1.35 ± 0.05, 1.02 ± 0.02, 1.64 ± 0.04, and 0.860 ± 0.04 Gyr, respectively. The results of this paper continue our effort to determine cluster ages to higher precision than that offered by these traditional methods of isochrone fitting
Type Ia Supernovae, Evolution, and the Cosmological Constant
We explore the possible role of evolution in the analysis of data on SNe Ia
at cosmological distances. First, using a variety of simple sleuthing
techniques, we find evidence that the properties of the high and low redshift
SNe Ia observed so far differ from one another. Next, we examine the effects of
including simple phenomenological models for evolution in the analysis. The
result is that cosmological models and evolution are highly degenerate with one
another, so that the incorporation of even very simple models for evolution
makes it virtually impossible to pin down the values of and
, the density parameters for nonrelativistic matter and for the
cosmological constant, respectively. Moreover, we show that if SNe Ia evolve
with time, but evolution is neglected in analyzing data, then, given enough SNe
Ia, the analysis hones in on values of and which
are incorrect. Using Bayesian methods, we show that the probability that the
cosmological constant is nonzero (rather than zero) is unchanged by the SNe Ia
data when one accounts for the possibility of evolution, provided that we do
not discriminate among open, closed and flat cosmologies a priori. The case for
nonzero cosmological constant is stronger if the Universe is presumed to be
flat, but still depends sensitively on the degree to which the peak
luminosities of SNe Ia evolve as a function of redshift. The estimated value of
, however, is only negligibly affected by accounting for possible
evolution.Comment: 45 pages, 15 figures; accepted for publication in The Astrophysical
Journal. Minor revisions and clarifications made including addition of recent
reference
Nonlinear Marine Structures With Random Excitation
Introduction Some of the classical nonlinear and time-varying equations of engineering mathematics appear in the modeling of the dynamic behavior of offshore structures. The dynamics of free-hanging risers, tension leg platforms and suspended loads can be cast in the form of Mathieu's equation; wave excitation causes the time variation of the spring parameter (references Disturbingly large subharmonic resonances or chaotic motions can result if the nonlinear equations (reference [4]) or the spring in the Mathieu equation varies harmonically (reference [1] ). This paper presents physical and mathematical arguments which indicate that these large responses are caused by a phase lock between the motion of the structure and the external excitation, something which is generally unlikely to last for long if a structure is subject to a random excitation. To test these predictions, two typical systems are simulated and randomness is introduced into the previously regular forcing in three different ways; as additive white noise, as frequency wander and as bandwidth spread. The responses are Fourier analyzed and maximum, minimum, mean and rms values are recorded. Random inputs cause the Poincare points to wander in a "Poincare region"; these are displayed as a function of the randomness parameter. The size of the subharmonic motions decays quickly with increasing values of the randomness parameter and they are generally small for realistically random wave forcing signals. Where the motion of a vessel is a significant input to a dynamic system, the filtering action of the vessel's dynamics driven by the wave action can generate a relatively regular motion; Pate
Bayesian Analysis of Two Stellar Populations in Galactic Globular Clusters II: NGC 5024, NGC 5272, and NGC 6352
We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival
ACS Treasury observations of Galactic Globular Clusters to find and
characterize two stellar populations in NGC 5024 (M53), NGC 5272 (M3), and NGC
6352. For these three clusters, both single and double-population analyses are
used to determine a best fit isochrone(s). We employ a sophisticated Bayesian
analysis technique to simultaneously fit the cluster parameters (age, distance,
absorption, and metallicity) that characterize each cluster. For the
two-population analysis, unique population level helium values are also fit to
each distinct population of the cluster and the relative proportions of the
populations are determined. We find differences in helium ranging from
0.05 to 0.11 for these three clusters. Model grids with solar
-element abundances ([/Fe] =0.0) and enhanced -elements
([/Fe]=0.4) are adopted.Comment: ApJ, 21 pages, 14 figures, 7 table
Children’s evaluations of deviant peers in the context of science and technology: The role of gender group norms and status
This is the final version. Available on open access from Elsevier via the DOI in this recordWomen are drastically under-represented within computer science, which is in
part informed by societal ideas of who can and should belong in the sciences. Less is
known about how children evaluate their peers who challenge gendered expectations
of who can and should take part in computer science. The present study asked
children (n = 213; female n = 110) from middle (Mage = 8.71, n = 108) and late
childhood (Mage = 10.56, n = 105) to evaluate a gender-matched peer who challenged
a group norm related to either computer science (male-gendered domain) or biology
(less male-gendered domain). Male participants most negatively evaluated a peer who
wanted to take part in a biology activity when the rest of the group wanted to do a
programming activity. Further, male participants expected their group to negatively
evaluate this deviant peer in the programming condition. Mediation analysis revealed
that for boys in the computer science condition, perceived group evaluation predicted
individual evaluation. Female participants, in contrast, did not negatively evaluate
someone who challenged a STEM peer group norm. This study demonstrates that
male peer groups may perpetuate the idea that computer science is for men through
negative evaluation of in-group members who challenge those ideas, and in turn
maintain their dominant position as the high-status group. Achieving equity in the
computer science field will require a greater understanding of these peer group norms.Wellcome Trus
High-Frequency network activity, global increase in Neuronal Activity, and Synchrony Expansion Precede Epileptic Seizures In Vitro
How seizures start is a major question in epilepsy research. Preictal EEG changes occur in both human patients and animal models, but their underlying mechanisms and relationship with seizure initiation remain unknown. Here we demonstrate the existence, in the hippocampal CA1 region, of a preictal state characterized by the progressive and global increase in neuronal activity associated with a widespread buildup of low-amplitude high-frequency activity (HFA) (100 Hz) and reduction in system complexity.HFAis generated by the firing of neurons, mainly pyramidal cells, at much lower frequencies. Individual cycles ofHFAare generated by the near-synchronous (within 5 ms) firing of small numbers of pyramidal cells. The presence of HFA in the low-calcium model implicates nonsynaptic synchronization; the presence of very similar HFA in the high-potassium model shows that it does not depend on an absence of synaptic transmission. Immediately before seizure onset, CA1 is in a state of high sensitivity in which weak depolarizing or synchronizing perturbations can trigger seizures. Transition to seizure is haracterized by a rapid expansion and fusion of the neuronal populations responsible for HFA, associated with a progressive slowing of HFA, leading to a single, massive, hypersynchronous cluster generating the high-amplitude low-frequency activity of the seizure
Gaia, White Dwarfs, and the Age of the Galaxy
The Milky Way is composed of four major stellar populations: the thin disk, thick disk, bulge, and halo. At present, we do not know the age of any of these populations to better than one or two billion years. This lack of knowledge keeps us from answering fundamental questions about the Galaxy: When did the thin disk, thick disk, and halo form? Did they form over an extended period, and if so, how long? Was star formation continuous across these populations or instead occur in distinct episodes? The Gaia satellite is providing precise trigonometric parallaxes for a plethora of white dwarfs in each of these populations. We combine these parallaxes (and hence, distances) with photometry and analyze them using a modeling technique that relies on Bayesian statistics. This allows us to derive precise ages for individual white dwarfs and determine the age distribution and star formation history for each of the constituents of our Galaxy. Here we will present current progress in this endeavor, with emphasis on the ages of individual white dwarfs in the Hyades. Measuring the ages of individual white dwarfs in well-studied clusters provides proof of concept for our technique, as well exploration of any systematic offsets caused from timescales from main sequence models, as well as the initial-final mass relation
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