681 research outputs found
Morphological Classification of galaxies by Artificial Neural Networks
We explore a method for automatic morphological classification of galaxies by an Artificial Neural Network algorithm. The method is illustrated using 13 galaxy parameters measured by machine (ESO-LV), and classified into five types (E, S0, Sa + Sb, Sc + Sd and Irr). A simple Backpropagation algorithm allows us to train a network on a subset of the catalogue according to human classification, and then to predict, using the measured parameters, the classification for the rest of the catalogue. We show that the neural network behaves in our problem as a Bayesian classifier, i.e. it assigns the a posteriori probability for each of the five classes considered. The network highest probability choice agrees with the catalogue classification for 64 percent of the galaxies. If either the first or the second highest probability choice of the network is considered, the success rate is 90 per cent. The technique allows uniform and more objective classification of very large extragalactic data sets
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
Models of Individual Blue Stragglers
This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined.Comment: Chapter 12, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Stellar Collisions and Ultracompact X-ray Binary Formation
(abridged) We report the results of SPH calculations of parabolic collisions
between a subgiant or slightly evolved red-giant star and a neutron star (NS).
Such collisions are likely to form ultracompact X-ray binaries (UCXBs) observed
today in old globular clusters. In particular, we compute collisions of a 1.4
Msun NS with realistically modelled parent stars of initial masses 0.8 and 0.9
Msun, each at three different evolutionary stages (corresponding to three
different radii R). The distance of closest approach for the initial orbit
varies from 0.04 R (nearly head-on) to 1.3 R (grazing). These collisions lead
to the formation of a tight binary, composed of the NS and the subgiant or
red-giant core, embedded in an extremely diffuse common envelope (CE) typically
of mass ~0.1 to 0.3 Msun. Our calculations follow the binary for many hundreds
of orbits, ensuring that the orbital parameters we determine at the end of the
calculations are close to final. Some of the fluid initially in the envelope of
the (sub)giant, from 0.003 to 0.023 Msun in the cases we considered, is left
bound to the NS. The eccentricities of the resulting binaries range from about
0.2 for our most grazing collision to about 0.9 for the nearly head-on cases.
In almost all the cases we consider, gravitational radiation alone will cause
sufficiently fast orbital decay to form a UCXB within a Hubble time, and often
on a much shorter timescale. Our hydrodynamics code implements the recent SPH
equations of motion derived with a variational approach by Springel & Hernquist
and by Monaghan. Numerical noise is reduced by enforcing an analytic constraint
equation that relates the smoothing lengths and densities of SPH particles. We
present tests of these new methods to help demonstrate their improved accuracy.Comment: 41 pages, 17 figures, accepted by Ap
Revised Relativistic Hydrodynamical Model for Neutron-Star Binaries
We report on numerical results from a revised hydrodynamic simulation of
binary neutron-star orbits near merger. We find that the correction recently
identified by Flanagan significantly reduces but does not eliminate the
neutron-star compression effect. Although results of the revised simulations
show that the compression is reduced for a given total orbital angular
momentum, the inner most stable circular orbit moves to closer separation
distances. At these closer orbits significant compression and even collapse is
still possible prior to merger for a sufficiently soft EOS. The reduced
compression in the corrected simulation is consistent with other recent studies
of rigid irrotational binaries in quasiequilibrium in which the compression
effect is observed to be small. Another significant effect of this correction
is that the derived binary orbital frequencies are now in closer agreement with
post-Newtonian expectations.Comment: Submitted to Phys. Rev.
Gravitational radiation from corotating binary neutron stars of incompressible fluid in the first post-Newtonian approximation of general relativity
We analytically study gravitational radiation from corotating binary neutron
stars composed of incompressible, homogeneous fluid in circular orbits. The
energy and the angular momentum loss rates are derived up to the first
post-Newtonian (1PN) order beyond the quadrupole approximation including
effects of the finite size of each star of binary. It is found that the leading
term of finite size effects in the 1PN order is only smaller than that in the Newtonian order, where means the ratio of the gravitational radius to the mean radius of
each star of binary, and the 1PN term acts to decrease the Newtonian finite
size effect in gravitational radiation.Comment: 26 pages, revtex, 9 figures(eps), accepted for publication in Phys.
Rev.
Truncated post-Newtonian neutron star model
As a preliminary step towards simulating binary neutron star coalescing
problem, we test a post-Newtonian approach by constructing a single neutron
star model. We expand the Tolman-Oppenheimer-Volkov equation of hydrostatic
equilibrium by the power of , where is the speed of light, and
truncate at the various order. We solve the system using the polytropic
equation of state with index and 3, and show how this
approximation converges together with mass-radius relations. Next, we solve the
Hamiltonian constraint equation with these density profiles as trial functions,
and examine the differences in the final metric. We conclude the second
`post-Newtonian' approximation is close enough to describe general relativistic
single star. The result of this report will be useful for further binary
studies.
(Note to readers) This paper was accepted for publication in Physical Review
D. [access code dsj637]. However, since I was strongly suggested that the
contents of this paper should be included as a section in our group's future
paper, I gave up the publication.Comment: 5 pages, RevTeX, 3 eps figs, epsf.sty, accepted for publication in
PRD (Brief Report), but will not appea
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