47,990 research outputs found
The orbits of subdwarf-B + main-sequence binaries. II. Three eccentric systems; BD+29 3070, BD +34 1543 and Feige 87
The predicted orbital-period distribution of the subdwarf-B (sdB) population
is bi-modal with a peak at short ( 250 days) periods.
Observationally, many short-period sdB systems are known, but the predicted
long period peak is missing as orbits have only been determined for a few
long-period systems. As these predictions are based on poorly understood
binary-interaction processes, it is of prime importance to confront the
predictions with reliable observational data. We therefore initiated a
monitoring program to find and characterize long-period sdB stars. In this
paper we aim to determine the orbital parameters of the three long-period
sdB+MS binaries BD+29 3070, BD+34 1543 and Feige 87, to constrain their
absolute dimensions and the physical parameters of the components.
High-resolution spectroscopic time series were obtained with HERMES at the
Mercator telescope on La Palma, and analyzed to determine the radial velocities
of both the sdB and MS components. Photometry from the literature was used to
construct the spectral-energy distribution (SED) of the binaries. Atmosphere
models were used to fit these SEDs and to determine the surface gravities and
temperatures of both components of all systems. Spectral analysis was used to
check the results of the SEDs. An orbital period of 1283 +- 63 d, a mass ratio
of q = 0.39 +- 0.04 and a significant non-zero eccentricity of e = 0.15 +- 0.01
were found for BD+29 3070. For BD+34 1543 we determined P = 972 +- 2 d, q =
0.57 +- 0.01 and again a clear non-zero eccentricity of e = 0.16 +- 0.01. Last,
for Feige 87 we found P = 936 +- 2 d, q = 0.55 +- 0.01 and e = 0.11 +- 0.01.
BD+29 3070, BD+34 1543 and Feige 87 are long period sdB + MS binaries on
clearly eccentric orbits. These results are in conflict with the predictions of
stable Roche-lobe overflow models.Comment: 15 pages, 6 figures, Accepted by A&
The X-ray Iron Emission from Tycho's Supernova Remnant
We present the results of broadband fits to the X-ray spectrum of Tycho's
supernova remnant obtained by the Solid-State Imaging Spectrometers on the ASCA
Observatory. We use single-temperature, single-ionization-age, nonequilibrium
ionization models to characterize the ejecta and the blast-shocked interstellar
medium. Based on the Fe K emission at 6.5 keV, previous spectral studies have
suggested that the Fe ejecta in this Type Ia remnant are stratified interior to
the other ejecta. The ASCA data provide important constraints from the Fe L
emission near 1 keV as well as the Fe K emission. We find that the simplest
models, with emission from the ejecta and blast wave each at a single
temperature and ionization age, severely underestimate the Fe K flux. We show
that there is little Fe emission associated with the Si and S ejecta shell. The
blast-shocked interstellar medium has abundances roughly 0.3 times the solar
value, while the ejecta, with the exception of Fe, have relative abundances
that are typical of Type Ia supernovae. The addition of another component of Fe
emission, which we associate with ejecta, at a temperature at least two times
higher and an ionization age 100 times lower than the Si ejecta, does
provide a good fit to the spectrum. This model is consistent with X-ray imaging
results. Although fluorescent emission from dust in the remnant may contribute
to the Fe K flux, we conclude that it is unlikely to dominate.Comment: 23 pages, LaTex; 4 postscript figures, 2 postscript tables. To appear
in ApJ, vol 49
The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars
In preparation for the upcoming all-sky data releases of the Gaia mission we
compiled a catalogue of known hot subdwarf stars and candidates drawn from the
literature and yet unpublished databases. The catalogue contains 5613 unique
sources and provides multi-band photometry from the ultraviolet to the far
infrared, ground based proper motions, classifications based on spectroscopy
and colours, published atmospheric parameters, radial velocities and light
curve variability information. Using several different techniques we removed
outliers and misclassified objects. By matching this catalogue with astrometric
and photometric data from the Gaia mission, we will develop selection criteria
to construct a homogeneous, magnitude-limited all-sky catalogue of hot subdwarf
stars based on Gaia data.Comment: 11 pages, A&A accepte
A MERLIN Study of 6 GHz Excited-state OH & 6.7 GHz Methanol Masers in ON1
MERLIN observations of 6.668-GHz methanol and both 6.031- and 6.035-GHz
hydroxyl (OH) emission from the massive star-formation region ON1 are
presented. These are the first methanol observations made in full polarization
using 5 antennas of MERLIN, giving high resolution and sensitivity to extended
emission. Maser features are found to lie at the southern edge of the
ultra-compact HII region, following the known distribution of ground-state OH
masers. The masers cover a region ~1 arcsec in extent, lying perpendicular to
the H13CO+ bipolar outflow. Excited-state OH emission demonstrates consistent
polarization angles across the strongest linearly polarized features which are
parallel to the overall distribution. The linear polarizations vary between
10.0 and 18.5 per cent, with an average polarization angle of -60 deg +/- 28
deg. The strongest 6.668-GHz methanol features provide an upper limit to linear
polarization of ~1 per cent. Zeeman splitting of OH shows magnetic fields
between -1.1 to -5.8 mG, and a tentative methanol magnetic field strength of
-18 mG is measured.Comment: 10 Pages, 5 Figure
Parking and the visual perception of space
Using measured data we demonstrate that there is an amazing correspondence
among the statistical properties of spacings between parked cars and the
distances between birds perching on a power line. We show that this observation
is easily explained by the fact that birds and human use the same mechanism of
distance estimation. We give a simple mathematical model of this phenomenon and
prove its validity using measured data
Large-N phase transition in lattice 2-d principal chiral models
We investigate the large-N critical behavior of 2-d lattice chiral models by
Monte Carlo simulations of U(N) and SU(N) groups at large N. Numerical results
confirm strong coupling analyses, i.e. the existence of a large-N second order
phase transition at a finite .Comment: 12 pages, Revtex, 8 uuencoded postscript figure
Discovery of the Putative Pulsar and Wind Nebula Associated with the TeV Gamma-ray Source HESS J1813-178
We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio
supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We
resolve the X-ray emission from the co-located ASCA source into a point source
surrounded by structured diffuse emission that fills the interior of the radio
shell. The morphology of the diffuse emission strongly resembles that of a
pulsar wind nebula. The spectrum of the compact source is well-characterized by
a power-law with index Gamma approx 1.3, typical of young and energetic
rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray
absorption and an association with the nearby star formation region W33, the
2-10 keV X-ray luminosities of the putative pulsar and nebula are L(PSR) =
3.2E33 ergs/s and L(PWN) = 1.4E34 ergs/s, respectively. Both the flux ratio of
L(PWN)/L(PSR) = 4.3 and the total luminosity of this system predict a pulsar
spin-down power of Edot > 1E37 ergs/s, placing it within the ten most energetic
young pulsars in the Galaxy. A deep search for radio pulsations using the
Parkes telescope sets an upper-limit of approx 0.07 mJy at 1.4 GHz for periods
>~ 50 ms. We discuss the energetics of this source, and consider briefly the
proximity of bright H2 regions to this and several other HESS sources, which
may produce their TeV emission via inverse Compton scattering.Comment: 7 pages, 6 figure, Latex, emulateapj style. To appear in the
Astrophysical Journa
Teachers' classroom feedback: still trying to get it right
This article examines feedback traditionally given by teachers in schools. Such feedback tends to focus on children's acquisition and retrieval of externally prescribed knowledge which is then assessed against mandated tests. It suggests that, from a sociocultural learning perspective, feedback directed towards such objectives may limit children's social development. In this article, I draw on observation and interview data gathered from a group of 27 9- to 10-year olds in a UK primary school. These data illustrate the children's perceived need to conform to, rather than negotiate, the teacher's feedback comments. They highlight the children's sense that the teacher's feedback relates to school learning but not to their own interests. The article also includes alternative examples of feedback which draw on children's own inquiries and which relate to the social contexts within which, and for whom, they act. It concludes by suggesting that instead of looking for the right answer to the question of what makes teachers' feedback effective in our current classrooms, a more productive question might be how a negotiation can be opened up among teachers and learners themselves, about how teachers' feedback could support children's learning most appropriately
Landauer formula without Landauer's assumptions
The Landauer formula for dissipationless conductance lies at the heart of
modern electronic transport, yet it remains without a clear microscopic basis.
We analyze the Landauer formula microscopically, and give a straightforward
quantum kinetic derivation for open systems. Some important experimental
implications follow. These lie beyond the Landauer result as popularly
received.Comment: LaTeX, 7pp, one fi
D-instanton partition functions
Duality arguments are used to determine D-instanton contributions to certain
effective interaction terms of type II supergravity theories in various
dimensions. This leads to exact expressions for the partition functions of the
finite N D-instanton matrix model in d=4 and 6 dimensions that generalize our
previous expression for the case d=10. These results are consistent with the
fact that the Witten index of the T-dual D-particle process should only be
non-vanishing for d=10.Comment: 20 pages, harvmac, typos corrected, reference adde
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