2,622 research outputs found
How to Find More Supernovae with Less Work: Object Classification Techniques for Difference Imaging
We present the results of applying new object classification techniques to
difference images in the context of the Nearby Supernova Factory supernova
search. Most current supernova searches subtract reference images from new
images, identify objects in these difference images, and apply simple threshold
cuts on parameters such as statistical significance, shape, and motion to
reject objects such as cosmic rays, asteroids, and subtraction artifacts.
Although most static objects subtract cleanly, even a very low false positive
detection rate can lead to hundreds of non-supernova candidates which must be
vetted by human inspection before triggering additional followup. In comparison
to simple threshold cuts, more sophisticated methods such as Boosted Decision
Trees, Random Forests, and Support Vector Machines provide dramatically better
object discrimination. At the Nearby Supernova Factory, we reduced the number
of non-supernova candidates by a factor of 10 while increasing our supernova
identification efficiency. Methods such as these will be crucial for
maintaining a reasonable false positive rate in the automated transient alert
pipelines of upcoming projects such as PanSTARRS and LSST.Comment: 25 pages; 6 figures; submitted to Ap
Tidal dissipation in rotating giant planets
[Abridged] Tides may play an important role in determining the observed
distributions of mass, orbital period, and eccentricity of the extrasolar
planets. In addition, tidal interactions between giant planets in the solar
system and their moons are thought to be responsible for the orbital migration
of the satellites, leading to their capture into resonant configurations. We
treat the underlying fluid dynamical problem with the aim of determining the
efficiency of tidal dissipation in gaseous giant planets. In cases of interest,
the tidal forcing frequencies are comparable to the spin frequency of the
planet but small compared to its dynamical frequency. We therefore study the
linearized response of a slowly and possibly differentially rotating planet to
low-frequency tidal forcing. Convective regions of the planet support inertial
waves, while any radiative regions support generalized Hough waves. We present
illustrative numerical calculations of the tidal dissipation rate and argue
that inertial waves provide a natural avenue for efficient tidal dissipation in
most cases of interest. The resulting value of Q depends in a highly erratic
way on the forcing frequency, but we provide evidence that the relevant
frequency-averaged dissipation rate may be asymptotically independent of the
viscosity in the limit of small Ekman number. In short-period extrasolar
planets, if the stellar irradiation of the planet leads to the formation of a
radiative outer layer that supports generalized Hough modes, the tidal
dissipation rate can be enhanced through the excitation and damping of these
waves. These dissipative mechanisms offer a promising explanation of the
historical evolution and current state of the Galilean satellites as well as
the observed circularization of the orbits of short-period extrasolar planets.Comment: 74 pages, 12 figures, submitted to The Astrophysical Journa
Direct Synthesis of Propene Using Supported Bifunctional Nickel Catalysts: Preparation and Potential
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Core-Collapse Simulations of Rotating Stars
We present the results from a series of two-dimensional core-collapse
simulations using a rotating progenitor star. We find that the convection in
these simulations is less vigorous because a) rotation weakens the core bounce
which seeds the neutrino-driven convection and b) the angular momentum profile
in the rotating core stabilizes against convection. The limited convection
leads to explosions which occur later and are weaker than the explosions
produced from the collapse of non-rotating cores. However, because the
convection is constrained to the polar regions, when the explosion occurs, it
is stronger along the polar axis. This asymmetric explosion can explain the
polarization measurements of core-collapse supernovae. These asymmetries also
provide a natural mechanism to mix the products of nucleosynthesis out into the
helium and hydrogen layers of the star. We also discuss the role the collapse
of these rotating stars play on the generation of magnetic fields and neutron
star kicks. Given a range of progenitor rotation periods, we predict a range of
supernova energies for the same progenitor mass. The critical mass for black
hole formation also depends upon the rotation speed of the progenitor.Comment: 16 pages text + 13 figures, submitted to Ap
The Binarity of Eta Carinae and its Similarity to Related Astrophysical Objects
I examine some aspects of the interaction between the massive star Eta
Carinae and its companion, in particular during the eclipse-like event, known
as the spectroscopic event or the shell event. The spectroscopic event is
thought to occur when near periastron passages the stellar companion induces
much higher mass loss rate from the primary star, and/or enters into a much
denser environment around the primary star. I find that enhanced mass loss rate
during periastron passages, if it occurs, might explain the high eccentricity
of the system. However, there is not yet a good model to explain the presumed
enhanced mass loss rate during periastron passages. In the region where the
winds from the two stars collide, a dense slow flow is formed, such that large
dust grains may be formed. Unlike the case during the 19th century Great
Eruption, the companion does not accrete mass during most of its orbital
motion. However, near periastron passages short accretion episodes may occur,
which may lead to pulsed ejection of two jets by the companion. The companion
may ionize a non-negligible region in its surrounding, resembling the situation
in symbiotic systems. I discuss the relation of some of these processes to
other astrophysical objects, by that incorporating Eta Car to a large class of
astrophysical bipolar nebulae.Comment: Updated version. ApJ, in pres
A hands-on introduction to querying evolutionary relationships across multiple data sources using SPARQL [version 1; peer review: 1 approved, 2 approved with reservations]
The increasing use of Semantic Web technologies in the life sciences, in particular the use of the Resource Description Framework (RDF) and the RDF query language SPARQL, opens the path for novel integrative analyses, combining information from multiple sources. However, analyzing evolutionary data in RDF is not trivial, due to the steep learning curve required to understand both the data models adopted by different RDF data sources, as well as the SPARQL query language. In this article, we provide a hands-on introduction to querying evolutionary data across multiple sources that publish orthology information in RDF, namely: The Orthologous MAtrix (OMA), the European Bioinformatics Institute (EBI) RDF platform, the Database of Orthologous Groups (OrthoDB) and the Microbial Genome Database (MBGD). We present four protocols in increasing order of complexity. In these protocols, we demonstrate through SPARQL queries how to retrieve pairwise orthologs, homologous groups, and hierarchical orthologous groups. Finally, we show how orthology information in different sources can be compared, through the use of federated SPARQL queries
On the tidal interaction of a solar-type star with an orbiting companion: Excitation of g mode oscillation and orbital evolution
We calculate the dynamical tides raised on a non-rotating solar-type star by
a close stellar or planetary companion. Dissipation arising from a turbulent
viscosity operating in the convection zone and radiative damping in the
radiative core are considered. We compute the torque exerted on the star by a
companion in circular orbit, and determine the potentially observable magnitude
of the tidally induced velocity at the stellar photosphere. These calculations
are compared with the results obtained by assuming that a very small frequency
limit can be taken in order to calculate the tidal response (equilibrium tide).
For a standard solar model, the latter is found to give a relatively poor
approximation at the periods of interest of several days, even when the system
is far from resonance with a normal mode. It is shown that although the
companion may go through a succession of resonances as it spirals in under the
action of the tides, for a fixed spectrum of normal modes its migration is
controlled essentially by the non-resonant interaction. We find that the
turbulent viscosity that is required to provide the observed circularization
rates of main sequence solar-type binaries is about fifty times larger than
that simply estimated from mixing length theory for non-rotating stars. We
discuss the means by which this enhanced viscosity might be realized. These
calculations are applied to 51 Pegasi. We show that the perturbed velocity
induced by the tides at the stellar surface is too small to be observed.Comment: 36 pages including 6 PostScript figures, LaTex -- To be published in
ApJ -- Also available at http://www.ucolick.org/~ct/home.htm
On a mechanism for enhancing magnetic activity in tidally interacting binaries
We suggest a mechanism for enhancing magnetic activity in tidally interacting
binaries. We suppose that the deviation of the primary star from spherical
symmetry due to the tidal influence of the companion leads to stellar pulsation
in its fundamental mode. It is shown that stellar radial pulsation amplifies
torsional Alfv{\'e}n waves in a dipole-like magnetic field, buried in the
interior, according to the recently proposed swing wave-wave interaction
(Zaqarashvili 2001). Then amplified Alfv{\'e}n waves lead to the onset of
large-scale torsional oscillations, and magnetic flux tubes arising towards the
surface owing to magnetic buoyancy diffuse into the atmosphere producing
enhanced chromospheric and coronal emission.Comment: Accepted in Ap
Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
We investigate the secular evolution of the orbital semi-major axis and
eccentricity due to mass transfer in eccentric binaries, assuming conservation
of total system mass and orbital angular momentum. Assuming a delta function
mass transfer rate centered at periastron, we find rates of secular change of
the orbital semi-major axis and eccentricity which are linearly proportional to
the magnitude of the mass transfer rate at periastron. The rates can be
positive as well as negative, so that the semi-major axis and eccentricity can
increase as well as decrease in time. Adopting a delta-function mass-transfer
rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution
timescales ranging from a few Myr to a Hubble time or more, depending on the
binary mass ratio and orbital eccentricity. Comparison with orbital evolution
timescales due to dissipative tides furthermore shows that tides cannot, in all
cases, circularize the orbit rapidly enough to justify the often adopted
assumption of instantaneous circularization at the onset of mass transfer. The
formalism presented can be incorporated in binary evolution and population
synthesis codes to create a self-consistent treatment of mass transfer in
eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa
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