148 research outputs found
The birth properties of Galactic millisecond radio pulsars
We model the population characteristics of the sample of millisecond pulsars
within a distance of 1.5kpc.We find that for a braking index n=3, the birth
magnetic field distribution of the neutron stars as they switch on as radio
MSPs can be represented by a Gaussian with mean and
and their birth spin period by a Gaussian with mean
ms and ms. Our study, which takes into consideration
acceleration effects on the observed spin-down rate, shows that most MSPs are
born with periods that are close to the currently observed values and with
average characteristic ages typically larger by a factor 1.5 compared to the
true age. The Galactic birth rate of the MSPs is deduced to be \gsimeq 3.2
\times 10^{-6} yr near the upper end of previous estimates and larger
than the semi-empirical birth rate yr of the LMXBs. The
mean birth spin period deduced by us for the radio MSPs is a factor 2 higher
than the mean spin period observed for the accretion and nuclear powered X-ray
pulsars, although this discrepancy can be resolved if we use a braking index
, the value appropriate to spin down caused by angular momentum losses by
gravitational radiation or magnetic multipolar radiation. We discuss the
arguments for and against the hypothesis that accretion induced collapse may
constitute the main route to the formation of the MSPs, pointing out that on
the AIC scenario the low magnetic fields of the MSPs may simply reflect the
field distribution in isolated magnetic white dwarfs which has recently been
shown to be bi-modal with a dominant component that is likely to peak at fields
below G which would scale to neutron star fields below G.Comment: 8 pages, 2 figures, accepted for publication in the MNRA
Magnetic White Dwarfs
In this paper we review the current status of research on the observational
and theoretical characteristics of isolated and binary magnetic white dwarfs
(MWDs).
Magnetic fields of isolated MWDs are observed to lie in the range 10^3-10^9G.
While the upper limit cutoff appears to be real, the lower limit is more
difficult to investigate. The incidence of magnetism below a few 10^3G still
needs to be established by sensitive spectropolarimetric surveys conducted on
8m class telescopes.
Highly magnetic WDs tend to exhibit a complex and non-dipolar field structure
with some objects showing the presence of higher order multipoles. There is no
evidence that fields of highly magnetic WDs decay over time, which is
consistent with the estimated Ohmic decay times scales of ~10^11 yrs. MWDs, as
a class, also appear to be more massive than their weakly or non-magnetic
counterparts.
MWDs are also found in binary systems where they accrete matter from a
low-mass donor star. These binaries, called magnetic Cataclysmic Variables
(MCVs) and comprise about 20-25\% of all known CVs. Zeeman and cyclotron
spectroscopy of MCVs have revealed the presence of fields in the range \,MG. Complex field geometries have been inferred in the high field MCVs
(the polars) whilst magnetic field strength and structure in the lower field
group (intermediate polars, IPs) are much harder to establish.
The origin of fields in MWDs is still being debated. While the fossil field
hypothesis remains an attractive possibility, field generation within the
common envelope of a binary system has been gaining momentum, since it would
explain the absence of MWDs paired with non-degenerate companions and also the
lack of relatively wide pre-MCVs.Comment: 73 pages, 22 figures, 2 large tables. Invited review chapter on
Magnetic White Dwarfs to appear in Space Science Reviews, Springe
The Most Magnetic Stars
Observations of magnetic A, B and O stars show that the poloidal magnetic
flux per unit mass has an upper bound of 10^-6.5 G cm^2/g. A similar upper
bound is found for magnetic white dwarfs even though the highest magnetic field
strengths at their surfaces are much larger. For magnetic A and B stars there
also appears to be a well defined lower bound below which the incidence of
magnetism declines rapidly. According to recent hypotheses, both groups of
stars may result from merging stars and owe their strong magnetism to fields
generated by a dynamo mechanism as they merge. We postulate a simple dynamo
that generates magnetic field from differential rotation. The growth of
magnetic fields is limited by the requirement that the poloidal field
stabilizes the toroidal and vice versa. While magnetic torques dissipate the
differential rotation, toroidal field is generated from poloidal by an Omega
dynamo. We further suppose that mechanisms that lead to the decay of toroidal
field lead to the generation of poloidal. Both poloidal and toroidal fields
reach a stable configuration which is independent of the size of small initial
seed fields but proportional to the initial differential rotation. We pose the
hypothesis that strongly magnetic stars form from the merging of two stellar
objects. The highest fields are generated when the merge introduces
differential rotation that amounts to critical break up velocity within the
condensed object. Calibration of a simplistic dynamo model with the observed
maximum flux per unit mass for main-sequence stars and white dwarfs indicates
that about 1.5x10^-4 of the decaying toroidal flux must appear as poloidal. The
highest fields in single white dwarfs are generated when two degenerate cores
merge inside a common envelope or when two white dwarfs merge by
gravitational-radiation angular momentum loss.Comment: accepted by MNRAS 8 pages, 3 figure
Enigmas from the Sloan Digital Sky Survey DR7 Kleinman White Dwarf Catalog
We report results from a continuation of our searches for high field magnetic
white dwarfs paired in a detached binary with non degenerate companions. We
made use of the Sloan Digital Sky Survey DR7 catalog of Kleinman et al. (2013)
with 19,712 spectroscopically-identified white dwarfs. These include 1,735
white dwarf plus M dwarf detached pairs (almost 10\% of the Kleinman at al.'s
list). No new pairs were found, although we did recover the polar (AM~Herculis
system) ST\,LMi in a low state of accretion. With the larger sample the
original situation reported ten years ago remains intact now at a much higher
level of statistical significance: in the selected SDSS sample, high field
magnetic white dwarfs are not found in an apparently-detached pairing with an M
dwarf, unless they are a magnetic CV in a low state of accretion. This finding
strengthens the case that the fields in the isolated high field magnetic white
dwarfs are generated by stellar mergers but also raises questions on the nature
of the progenitors of the magnetic CVs.Comment: 12 pages, accepted for publication in the Astrophysical Journa
Algunos aportes para el estudio del léxico castellano desde la baja Edad Media hasta el siglo XVI
Como es sabido, muy poca es la bibliografía útil que aborde el estudio del léxico desde la baja Edad Media hasta el siglo XVI porque no hubo o no se conservaron tratados específicos. Solo se puede acudir a observaciones aisladas, por ejemplo, las reflexiones que nos dejó Juan de Valdés en su Diálogo de la Lengua y las que aparecen de modo no sistemático en las fuentes conocidas. Nuestro propósito ha sido acudir a ellas: el Tesoro de la lengua castellana, de Covarrubias, los Diccionarios de Autoridades, de Terreros, Corominas, Moliner, Martín Alonso, entre otros repertorios, más trabajos individuales y de revistas especializadas, sin olvidar los textos incluidos en ediciones críticas valiosas.A continuación, se proporciona un inventario de voces que se sumarán a otros que he presentado en otros lugares los cuales, junto a los resultados de una investigación en curso con ejemplos no registrados en los repertorios antes mencionados, constituirán mis Notas para un glosario castellano (siglos XIV- XVI)
Galactic Escape Speeds in Mirror and Cold Dark Matter Models
The mirror dark matter (MDM) model of Berezhiani et al. has been shown to
reproduce observed galactic rotational curves for a variety of spiral galaxies,
and has been presented as an alternative to cold dark matter (CDM) models. We
investigate possible additional tests involving the properties of stellar
orbits, which may be used to discriminate between the two models. We
demonstrate that in MDM and CDM models fitted equally well to a galactic
rotational curve, one generally expects predictable differences in escape
speeds from the disc. The recent radial velocity (RAVE) survey of the Milky Way
has pinned down the escape speed from the solar neighbourhood to
km s, placing an additional constraint on dark
matter models. We have constructed an MDM model for the Milky Way based on its
rotational curve, and find an escape speed that is just consistent with the
observed value given the current errors, which lends credence to the viability
of the MDM model. The Gaia-ESO spectroscopic survey is expected to lead to an
even more precise estimate of the escape speed that will further constrain dark
matter models. However, the largest differences in stellar escape speeds
between both models are predicted for dark matter dominated dwarf galaxies such
as DDO 154, and kinematical studies of such galaxies could prove key in
establishing, or abolishing, the validity of the MDM model.Comment: Accepted for publication in the European Physical Journal
High-mass star formation in southern disk galaxies
As part of a major study of the physical processes of star formation and the evolution of galactic discs, the detailed distribution of high-mass star formation within southern late-type spirals and Magellanic-type galaxies is being measured by means of narrow-band imaging in Ha and the continuum, spectroscopic studies of prominent HII regions identified in the Ha images, and by radio mapping in neutral hydrogen and the continuum. The radio mapping will be undertaken with the Southern Hemisphere's first large, multi-frequency synthesis array, the Australia Telescope. Some optical imaging and spectroscopic data has already been acquired; the optical data and some preliminary results are described
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