525 research outputs found
Wind loads on heliostats and photovoltaic trackers
New findings on wind loads help to reduce solar energy cos
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Cluster Origin of Triple Star HD 188753 and its Planet
The recent discovery by M. Konacki of a ``hot Jupiter'' in the hierarchical
triple star system HD 188753 challenges established theories of giant-planet
formation. If the orbital geometry of the triple has not changed since the
birth of the planet, then a disk around the planetary host star would probably
have been too compact and too hot for a Jovian planet to form by the
core-accretion model or gravitational collapse. This paradox is resolved if the
star was initially either single or had a much more distant companion. It is
suggested here that a close multi-star dynamical encounter transformed this
initial state into the observed triple, an idea that follows naturally if HD
188753 formed in a moderately dense stellar system--perhaps an open
cluster--that has since dissolved. Three distinct types of encounters are
investigated. The most robust scenario involves an initially single planetary
host star that changes places with the outlying member of a pre-existing
hierarchical triple.Comment: Accepted by ApJL; minor changes from origina
Recommended from our members
Mechanisms underlying temperature extremes in Iberia: a Lagrangian perspective
The mechanisms underlying the occurrence of temperature extremes in Iberia are analysed considering a Lagrangian perspective of the atmospheric flow, using 6-hourly ERA-Interim reanalysis data for the years 1979â2012. Daily 2-m minimum temperatures below the 1st percentile and 2-m maximum temperatures above the 99th percentile at each grid point over Iberia are selected separately for winter and summer. Four categories of extremes are analysed using 10-d backward trajectories initialized at the extreme temperature grid points close to the surface: winter cold (WCE) and warm extremes (WWE), and summer cold (SCE) and warm extremes (SWE). Air masses leading to temperature extremes are first transported from the North Atlantic towards Europe for all categories. While there is a clear relation to large-scale circulation patterns in winter, the Iberian thermal low is important in summer. Along the trajectories, air mass characteristics are significantly modified through adiabatic warming (air parcel descent), upper-air radiative cooling and near-surface warming (surface heat fluxes and radiation). High residence times over continental areas, such as over northern-central Europe for WCE and, to a lesser extent, over Iberia for SWE, significantly enhance these air mass modifications. Near-surface diabatic warming is particularly striking for SWE. WCE and SWE are responsible for the most extreme conditions in a given year. For WWE and SCE, strong temperature advection associated with important meridional air mass transports are the main driving mechanisms, accompanied by comparatively minor changes in the air mass properties. These results permit a better understanding of mechanisms leading to temperature extremes in Iberia
Improvement in the decadal prediction skill of the North Atlantic extratropical winter circulation through increased model resolution
In this study the latest version of the MiKlip decadal hindcast system is analyzed, and the effect of an increased horizontal and vertical resolution on the prediction skill of the extratropical winter circulation is assessed. Four different metrics â the storm track, blocking, cyclone and windstorm frequencies â are analyzed in the North Atlantic and European region. The model bias and the deterministic decadal hindcast skill are evaluated in ensembles of five members in a lower-resolution version (LR, atm: T63L47, ocean: 1.5â L40) and a higher-resolution version (HR, atm: T127L95, ocean: 0.4â L40) of the MiKlip system based on the Max Planck Institute Earth System model (MPI-ESM). The skill is assessed for the lead winters 2â5 in terms of the anomaly correlation of the quantities' winter averages using initializations between 1978 and 2012. The deterministic predictions are considered skillful if the anomaly correlation is positive and statistically significant. While the LR version shows common shortcomings of lower-resolution climate models, e.g., a storm track that is too zonal and southward displaced as well as a negative bias of blocking frequencies over the eastern North Atlantic and Europe, the HR version counteracts these biases. Cyclones, i.e., their frequencies and characteristics like strength and lifetime, are particularly better represented in HR. As a result, a chain of significantly improved decadal prediction skill between all four metrics is found with the increase in the spatial resolution. While the skill of the storm track is significantly improved primarily over the main source region of synoptic activity â the North Atlantic Current â the other extratropical quantities experience a significant improvement primarily downstream thereof, i.e., in regions where the synoptic systems typically intensify. Thus, the skill of the cyclone frequencies is significantly improved over the central North Atlantic and northern Europe, the skill of the blocking frequencies is significantly improved over the Mediterranean, Scandinavia and eastern Europe, and the skill of the windstorms is significantly improved over Newfoundland and central Europe. Not only is the skill improved with the increase in resolution, but the HR system itself also exhibits significant skill over large areas of the North Atlantic and European sector for all four circulation metrics. These results are particularly promising regarding the high socioeconomic impact of European winter windstorms and blocking situations
Clearing the Dust from Globular Clusters
Recent Spitzer observations of the globular cluster M15 detected dust
associated with its intracluster medium. Surprisingly, these observations imply
that the dust must be very short-lived compared to the time since the last
Galactic plane crossing of the cluster.Here we propose a simple mechanism to
explain this short lifetime. We argue that the kinetic energy of the material
ejected during a stellar collision may be sufficient to remove the gas and dust
entirely from a cluster, or to remove the gas as a wind, in addition to
partially destroying the dust. By calculating the rate of stellar collisions
using an N-body model for the cluster, we find remarkable agreement between the
average time between collisions and the inferred dust lifetime in this cluster,
suggesting a possible close relation between the two phenomena. Furthermore, we
also obtain the birthrate of blue stragglers formed through collisions in M15.
By comparing with the observed number of blue stragglers, we derive an upper
limit for their average lifetime which turns out to be consistent with recent
model calculations, thereby lending further support to our model.Comment: 4 pages, 1 figure, to appear in ApJ Letter
No evidence of a hot Jupiter around HD 188753 A
The discovery of a short-period giant planet (a hot Jupiter) around the
primary component of the triple star system HD 188753 has often been considered
as an important observational evidence and as a serious challenge to
planet-formation theories. Following this discovery, we monitored HD 188753
during one year to better characterize the planetary orbit and the feasibility
of planet searches in close binaries and multiple star systems. We obtained
Doppler measurements of HD 188753 with the ELODIE spectrograph at the
Observatoire de Haute-Provence. We then extracted radial velocities for the two
brightest components of the system using our multi-order, two-dimensional
correlation algorithm, TODCOR. Our observations and analysis do not confirm the
existence of the short-period giant planet previously reported around HD 188753
A. Monte Carlo simulations show that we had both the precision and the temporal
sampling required to detect a planetary signal like the one quoted. From our
failure to detect the presumed planet around HD 188753 A and from the available
data on HD 188753, we conclude that there is currently no convincing evidence
of a close-in giant planet around HD 188753 A.Comment: 8 pages, 3 figures, accepted for publication in A&A. Corrected typos
and minor mistake
Models of Ultraluminous X-Ray Sources with Intermediate-Mass Black Holes
We have computed models for ultraluminous X-ray sources ("ULXs") consisting
of a black-hole accretor of intermediate mass ("IMBH"; e.g., ~1000 Msun) and a
captured donor star. For each of four different sets of initial donor masses
and orbital separations, we computed 30,000 binary evolution models using a
full Henyey stellar evolution code. To our knowledge this is the first time
that a population of X-ray binaries this large has been carried out with other
than approximation methods, and it serves to demonstrate the feasibility of
this approach to large-scale population studies of mass-transfer binaries. In
the present study, we find that in order to have a plausible efficiency for
producing active ULX systems with IMBHs having luminosities > 10^{40} ergs/sec,
there are two basic requirements for the capture of companion/donor stars.
First, the donor stars should be massive, i.e., > 8 Msun. Second, the initial
orbital separations, after circularization, should be close, i.e., < 6-30 times
the radius of the donor star when on the main sequence. Even under these
optimistic conditions, we show that the production rate of IMBH-ULX systems may
fall short of the observed values by factors of 10-100.Comment: 5 pages, 2 figures, submitted to Ap
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