2,118 research outputs found
Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers
Kepler's first major discoveries are two hot objects orbiting stars in its
field. These may be the cores of stars that have each been eroded or disrupted
by a companion star. The companion, which is the star monitored today, is
likely to have gained mass from its now-defunct partner, and can be considered
to be a blue straggler. KOI-81 is almost certainly the product of stable mass
transfer; KOI-74 may be as well, or it may be the first clear example of a blue
straggler created throughthree-body interactions.
We show that mass transfer binaries are common enough that Kepler should
discover ~1000 white dwarfs orbiting main sequence stars. Most, like KOI-74 and
KOI-81, will be discovered through transits, but many will be discovered
through a combination of gravitational lensing and transits, while lensing will
dominate for a subset. In fact, some events caused by white dwarfs will have
the appearance of "anti-transits" --i.e., short-lived enhancements in the
amount of light received from the monitored star. Lensing and other mass
measurements methods provide a way to distinguish white dwarf binaries from
planetary systems. This is important for the success of Kepler's primary
mission, in light of the fact that white dwarf radii are similar to the radii
of terrestrial planets, and that some white dwarfs will have orbital periods
that place them in the habitable zones of their stellar companions. By
identifying transiting and/or lensing white dwarfs, Kepler will conduct
pioneering studies of white dwarfs and of the end states of mass transfer. It
may also identify orbiting neutron stars or black holes. The calculations
inspired by the discovery of KOI-74 and KOI-81 have implications for
ground-based wide-field surveys as well as for future space-based surveys.Comment: 29 pages, 6 figures, 1 table; submitted to The Astrophysical Journa
The evolution of low mass, close binary systems with a neutron star component: a detailed grid
In close binary systems composed of a normal, donor star and an accreting
neutron star, the amount of material received by the accreting component is, so
far, a real intrigue. In the literature there are available models that link
the accretion disk surrounding the neutron star with the amount of material it
receives, but there is no model linking the amount of matter lost by the donor
star to that falling onto the neutron star.
In this paper we explore the evolutionary response of these close binary
systems when we vary the amount of material accreted by the neutron star. We
consider a parameter \beta, which represents the fraction of material lost by
the normal star that can be accreted by the neutron star. \beta is considered
as constant throughout evolution. We have computed the evolution of a set of
models considering initial donor star masses (in solar units) between 0.5 and
3.50, initial orbital periods (in days) between 0.175 and 12, initial masses of
neutron stars (in solar units) of 0.80, 1.00, 1.20 and 1.40 and several values
of beta. We assumed solar abundances. These systems evolve to ultracompact or
to open binary systems, many of which form low mass helium white dwarfs. We
present a grid of calculations and analyze how these results are affected upon
changes in the value of \beta. We find a weak dependence of the final donor
star mass with respect to \beta. In most cases this is also true for the final
orbital period. The most sensitive quantity is the final mass of the accreting
neutron star.
As we do not know the initial mass and rotation rate of the neutron star of
any system, we find that performing evolutionary studies is not helpful for
determining \beta.Comment: 18 pages, 6 figures, 6 tables. Accepted for publication in MNRA
Evidence for the disintegration of KIC 12557548 b
Context. The Kepler object KIC 12557548 b is peculiar. It exhibits
transit-like features every 15.7 hours that vary in depth between 0.2% and
1.2%. Rappaport et al. (2012) explain the observations in terms of a
disintegrating, rocky planet that has a trailing cloud of dust created and
constantly replenished by thermal surface erosion. The variability of the
transit depth is then a consequence of changes in the cloud optical depth.
Aims. We aim to validate the disintegrating-planet scenario by modeling the
detailed shape of the observed light curve, and thereby constrain the cloud
particle properties to better understand the nature of this intriguing object.
Methods. We analysed the six publicly-available quarters of raw Kepler data,
phase-folded the light curve and fitted it to a model for the trailing dust
cloud. Constraints on the particle properties were investigated with a
light-scattering code. Results. The light curve exhibits clear signatures of
light scattering and absorption by dust, including a brightening in flux just
before ingress correlated with the transit depth and explained by forward
scattering, and an asymmetry in the transit light curve shape, which is easily
reproduced by an exponentially decaying distribution of optically thin dust,
with a typical grain size of 0.1 micron. Conclusions. Our quantitative analysis
supports the hypothesis that the transit signal of KIC 12557548 b is due to a
variable cloud of dust, most likely originating from a disintegrating object.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations
We present a study of a water-in-oil microemulsion in which surfactant coated
water nanodroplets are dispersed in the isotropic phase of the thermotropic
liquid crystal 5CB. As the temperature is lowered below the isotropic to
nematic phase transition of pure 5CB, the system displays a demixing transition
leading to a coexistence of a droplet rich isotropic phase with a droplet poor
nematic. The transition is anticipated, in the high T side, by increasing
pretransitional fluctuations in 5CB molecular orientation and in the
nanodroplet concentration. The observed phase behavior supports the notion that
the nanosized droplets, while large enough for their statistical behavior to be
probed via light scattering, are also small enough to act as impurities,
disturbing the local orientational ordering of the liquid crystal and thus
experiencing pretransitional attractive interaction mediated by paranematic
fluctuations. The pretransitional behavior, together with the topology of the
phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model
which describes the nanodroplets simply as holes in the liquid crystal.Comment: 64 pages, 16 figures, J. Chem. Phys. in pres
Continuum percolation of wireless ad hoc communication networks
Wireless multi-hop ad hoc communication networks represent an
infrastructure-less and self-organized generalization of todays wireless
cellular networks. Connectivity within such a network is an important issue.
Continuum percolation and technology-driven mutations thereof allow to address
this issue in the static limit and to construct a simple distributed protocol,
guaranteeing strong connectivity almost surely and independently of various
typical uncorrelated and correlated random spatial patterns of participating ad
hoc nodes.Comment: 30 pages, to be published in Physica
The path toward a cloud-aware mobile network protocol stack
We are currently observing the softwarization of communication networks, where network functions are translated from monolithic pieces of equipment to programs running over a shared pool of computational, storage, and communication resources. While it is clear that almost any softwarization improves flexibility (eg, the ability to instantiate more servers to cope with increasing traffic demand), in this paper, we advocate for a complete redesign of the communications protocol stack, instead of a mere translation of hardware functions into software. We discuss 2 drivers for this cloud-aware redesign: (1) relaxing the tight interactions between functions and (2) supporting a graceful degradation of the service when resources become scarce. The potential benefits of this redesign are illustrated with the numerical evaluation of one use case.This work has been partially performed within the 5G-MoNArch project, part of the Phase II of the 5th Generation Public Private Partnership (5G-PPP) program partially funded by the European Commission within the Horizon 2020 Framework Program. It has also been partly funded by the Madrid Regional Government through the TIGRE5-CM program
(S2013/ICE-2919) and by the SpanishMinistry of Economy, Industry and Competitiveness of through the 5GCity project (TEC2016-76795-C6-3-R)
Detached white dwarf main-sequence star binaries
We considered the formation of detached white dwarf main-sequence star (WDMS)
binaries through seven evolutionary channels subdivided according to the
evolutionary process that gives rise to the formation of the white dwarf or its
helium-star progenitor: dynamically stable Roche-lobe overflow (Algol-type
evolution), dynamically unstable Roche-lobe overflow (common-envelope
evolution), or stellar winds (single star evolution). We examine the
sensitivity of the population to changes in the amount of mass lost from the
system during stable Roche-lobe overflow, the common-envelope ejection
efficiency, and the initial mass ratio or initial secondary mass distribution.
In the case of a flat initial mass ratio distribution, the local space density
of WDMS binaries is of the order of 10^{-3}/pc^3. This number decreases to
10^{-4}/pc^3 when the initial mass ratio distribution is approximately
proportional to the inverse of the initial mass ratio. More than 75% of the
WDMS binary population stems from wide systems in which both components evolve
as if they were single stars. The remaining part of the population is dominated
by systems in which the white dwarf is formed in a common-envelope phase. The
birthrate of WDMS binaries forming through a common-envelope phase is about 10
times larger than the birthrate of WDMS binaries forming through a stable
Roche-lobe overflow phase. The ratio of the number of helium white dwarf
systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf
systems derived from large samples of observed WDMS binaries by, e.g., future
planet-search missions such as SuperWASP, COROT, and Kepler may furthermore
constrain the common-envelope ejection efficiency.Comment: 22 pages, accepted for publication in A&
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