5,987 research outputs found
Photon deflection and precession of the periastron in terms of spatial gravitational fields
We show that a Maxwell-like system of equations for spatial gravitational
fields and (latter being the analogy of a magnetic field),
modified to include an extra term for the field in the expression for
force, leads to the correct values for the photon deflection angle and for the
precession of the periastron
Observation of the Decay B^-→D_s^((*)+)K^-ℓ^-ν̅ _ℓ
We report the observation of the decay B^- → D_s^((*)+)K^-ℓ^-ν̅ _ℓ based on 342 fb^(-1) of data collected at the Υ(4S) resonance with the BABAR detector at the PEP-II e^+e^- storage rings at SLAC. A simultaneous fit to three D_s^+ decay chains is performed to extract the signal yield from measurements of the squared missing mass in the B meson decay. We observe the decay B^- → D_s^((*)+)K^-ℓ^-ν̅ _ℓ with a significance greater than 5 standard deviations (including systematic uncertainties) and measure its branching fraction to be B(B^- → D_s^((*)+)K^-ℓ^-ν̅ _ℓ)=[6.13_(-1.03)^(+1.04)(stat)±0.43(syst)±0.51(B(D_s))]×10^(-4), where the last error reflects the limited knowledge of the D_s branching fractions
The ongoing pursuit of R Coronae Borealis stars: the ASAS-3 survey strikes again
CONTEXT: R Coronae Borealis stars( RCBs) are rare, hydrogen-deficient, carbon rich super giant variable stars that are likely the evolved merger products of pairs of CO and He white dwarfs. Only 55 RCB stars have been found in our galaxy and their distribution on the sky is weighted heavily by microlensing survey field positions. A less biased wide-area survey would enable us to test competing evolutionary scenarios, understand the population or populations that produce RCBs, and constrain their formation rate. AIMS: The ASAS-3 survey monitored the sky south of declination +28deg between 2000 and 2010 to a limiting magnitude of V = 14. We searched ASAS-3 for RCB variables using several different methods to ensure that the probability of RCB detection was as high as possible and to reduce selection biases based on luminosity, temperature, dust production activity and shell brightness. METHODS: Candidates whose light curves were visually inspected were pre-selected based on their infrared (IR) excesses due to warm dust in their circumstellar shells using the WISE and/or 2MASS catalogues. Criteria on light curve variability were also applied when necessary to minimise the number of objects. Initially, we searched for RCB stars among the ASAS-3 ACVS1.1 variable star catalogue, then among the entire ASAS-3 south source catalogue, and finally directly interrogated the light curve database for objects that were not catalogued in either of those. We then acquired spectra of 104 stars to determine their real nature using the SSO/WiFeS spectrograph. RESULTS: We report 21 newly discovered RCB stars and 2 new DY Per stars. Two previously suspected RCB candidates were also spectroscopically confirmed. Our methods allowed us to extend our detection efficiency to fainter magnitudes that would not have been easily accessible to discovery techniques based onlight curve variability. The overall detection efficiencyis about 90% for RCBs with maximum light brighter than V ∼13. CONCLUSIONS: With these new discoveries, 76 RCBs are now known in our Galaxy and 22 in the Magellanic Clouds. This growing sample is of great value to constrain the peculiar and disparate atmosphere composition of RCBs. Most importantly, we show that the spatial distribution and apparent magnitudes of Galactic RCB stars is consistent with RCBs being part of the Galactic bulge population.Department of HE and Training approved lis
On the Reported Death of the MACHO Era
We present radial velocity measurements of four wide halo binary candidates
from the sample in Chaname & Gould (2004; CG04) which, to date, is the only
sample containing a large number of such candidates. The four candidates that
we have observed have projected separations >0.1 pc, and include the two widest
binaries from the sample, with separations of 0.45 and 1.1 pc. We confirm that
three of the four CG04 candidates are genuine, including the one with the
largest separation. The fourth candidate, however, is spurious at the 5-sigma
level. In the light of these measurements we re-examine the implications for
MACHO models of the Galactic halo. Our analysis casts doubt on what MACHO
constraints can be drawn from the existing sample of wide halo binaries.Comment: 6 Pages, 4 Figures, Accepted for MNRAS Letter
Gravitomagnetism in teleparallel gravity
The assumption that matter charges and currents could generate fields, which
are called, by analogy with electromagnetism, gravitoeletric and
gravitomagnetic fields, dates from the origins of General Relativity (GR). On
the other hand, the Teleparallel Equivalent of GR (TEGR), as a gauge theory,
seems to be the ideal scenario to define these fields, based on the gauge field
strength components. The purpose of the present work is to investigate the
nature of the gravitational electric and magnetic fields in the context of the
TEGR, where the tetrad formalism behind it seems to be more appropriated to
deal with phenomena related to observers.
As our main results, we have obtained, for the first time, the exact
expressions for the gravito-electromagnetic fields for the Schwarzschild
solution that in the linear approximation become the usual expected ones. To
improve our understanding about these fields, we have also studied the geometry
produced by a spherical rotating shell in slow motion and weak field regime.
Again, the expressions obtained are in complete agreement with those of
electromagnetism.Comment: 25 pages. Submitted to International Journal of Modern Physics D.
Version 2: some new discussions, references adde
A Spectral Classification System for Hydrogen-deficient Carbon Stars
Stellar spectral classification, and especially the Yerkes system, has been
highly useful in the study of stars. While there is a currently accepted
classification system for carbon stars, the subset of Hydrogen-deficient Carbon
(HdC) stars has not been well described by such a system, due in part to their
rarity and their variability. Here we present a new system for the
classification of HdCs based on their spectra, which is made wholly on their
observable appearance. We use a combination of dimensionality reduction and
clustering algorithms with human classification to create such a system. We
classify over half of the known sample of HdC stars using this, and roughly
calibrate the temperatures of each class using their colors. Additionally, we
express trends in the occurrence of certain spectral peculiarities such as the
presence of Hydrogen and Lithium lines. We also present three previously
unpublished spectra, and report the discovery of three new Galactic dustless
HdC (dLHdC) stars and additionally discuss one especially unique star that
appears to border between the hottest HdCs and the coolest Extreme Helium (EHe)
stars.Comment: 30 pages, 18 figures, submitted to MNRA
First detection of dust clouds around R CrB variable stars
From VLT/NACO diffraction-limited images of RY~Sgr, we report the first
direct detection of heterogeneities in the circumstellar envelope of a R
Coronae Borealis variable star. Several bright and very large dust clouds are
seen in various directions at several hundred stellar radii from RY Sgr,
revealing high activity for the ejection of stellar material by R CrB
variables. These observations do support the current interpretation that
optically thick dust clouds are formed around the surface of this type of
variable stars and, when passing between the star and the observer, produce the
huge and sudden declines characterizing these objects in visible light. This is
the first direct confirmation of a scenario proposed about 70 years ago.Comment: Published in Astronomy and Astrophysics (A&A 428,L13-L16 (2004)
A single low-energy, iron-poor supernova as the source of metals in the star SMSS J 031300.36-670839.3
The element abundance ratios of four low-mass stars with extremely low
metallicities indicate that the gas out of which the stars formed was enriched
in each case by at most a few, and potentially only one low-energy, supernova.
Such supernovae yield large quantities of light elements such as carbon but
very little iron. The dominance of low-energy supernovae is surprising, because
it has been expected that the first stars were extremely massive, and that they
disintegrated in pair-instability explosions that would rapidly enrich galaxies
in iron. What has remained unclear is the yield of iron from the first
supernovae, because hitherto no star is unambiguously interpreted as
encapsulating the yield of a single supernova. Here we report the optical
spectrum of SMSS J031300.36- 670839.3, which shows no evidence of iron (with an
upper limit of 10^-7.1 times solar abundance). Based on a comparison of its
abundance pattern with those of models, we conclude that the star was seeded
with material from a single supernova with an original mass of ~60 Mo (and that
the supernova left behind a black hole). Taken together with the previously
mentioned low-metallicity stars, we conclude that low-energy supernovae were
common in the early Universe, and that such supernovae yield light element
enrichment with insignificant iron. Reduced stellar feedback both chemically
and mechanically from low-energy supernovae would have enabled first-generation
stars to form over an extended period. We speculate that such stars may perhaps
have had an important role in the epoch of cosmic reionization and the chemical
evolution of early galaxies.Comment: 28 pages, 6 figures, Natur
The EROS2 search for microlensing events towards the spiral arms: the complete seven season results
The EROS-2 project has been designed to search for microlensing events
towards any dense stellar field. The densest parts of the Galactic spiral arms
have been monitored to maximize the microlensing signal expected from the stars
of the Galactic disk and bulge. 12.9 million stars have been monitored during 7
seasons towards 4 directions in the Galactic plane, away from the Galactic
center. A total of 27 microlensing event candidates have been found. Estimates
of the optical depths from the 22 best events are provided. A first order
interpretation shows that simple Galactic models with a standard disk and an
elongated bulge are in agreement with our observations. We find that the
average microlensing optical depth towards the complete EROS-cataloged stars of
the spiral arms is , a number that is
stable when the selection criteria are moderately varied. As the EROS catalog
is almost complete up to , the optical depth estimated for the
sub-sample of bright target stars with () is easier to interpret. The set of microlensing events
that we have observed is consistent with a simple Galactic model. A more
precise interpretation would require either a better knowledge of the distance
distribution of the target stars, or a simulation based on a Galactic model.
For this purpose, we define and discuss the concept of optical depth for a
given catalog or for a limiting magnitude.Comment: 22 pages submitted to Astronomy & Astrophysic
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