26,013 research outputs found
The rotation rates of massive stars: How slow are the slow ones?
Context: Rotation plays a key role in the life cycles of stars with masses
above 8 Msun. Hence, accurate knowledge of the rotation rates of such massive
stars is critical for understanding their properties and for constraining
models of their evolution. Aims: This paper investigates the reliability of
current methods used to derive projected rotation speeds v sin i from
line-broadening signatures in the photospheric spectra of massive stars,
focusing on stars that are not rapidly rotating. Methods: We use slowly
rotating magnetic O-stars with well-determined rotation periods to test the
Fourier transform (FT) and goodness-of-fit (GOF) methods typically used to
infer projected rotation rates of massive stars. Results: For our two magnetic
test stars with measured rotation periods longer than one year, i.e., with v
sin i < 1 km/s, we derive v sin i ~ 40-50 km/s from both the FT and GOF
methods. These severe overestimates are most likely caused by an insufficient
treatment of the competing broadening mechanisms referred to as microturbulence
and macroturbulence. Conclusions: These findings warn us not to rely
uncritically on results from current standard techniques to derive projected
rotation speeds of massive stars in the presence of significant additional line
broadening, at least when v sin i <~ 50 km/s. This may, for example, be crucial
for i) determining the statistical distribution of observed rotation rates of
massive stars, ii) interpreting the evolutionary status and spin-down histories
of rotationally braked B-supergiants, and iii) explaining the deficiency of
observed O-stars with spectroscopically inferred v sin i ~ 0 km/s. Further
investigations of potential shortcomings of the above techniques are presently
under way.Comment: 4 pages, 4 figures, accepted for publication in A&A Letter
What heavy quanta bounds could be inferred from a Higgs discovery?
The Higgs couplings can receive non-decoupling corrections due to heavy
quanta, and deviations from the SM can be used to test its presence. The
possible Higgs signal recently reported at LEP, with mh=115 GeV, severely
constrains the presence of heavy quanta, such as a heavy fourth family. At
Tevatron, the Higgs production by gluon fusion, followed by the decay h -> WW*,
can also be used to probe the existence of heavy colored particles, including
additional families, chiral sextet and octet quarks. Within the MSSM, we also
find that gluon fusion is a sensitive probe for the squark spectrum.Comment: 12 pages, 3 tables, 1 figure. Accepted in Mod. Phys. Lett. A (2001
The Feeling of Color: A Haptic Feedback Device for the Visually Disabled
Tapson J, Gurari N, Diaz J, et al. The Feeling of Color: A Haptic Feedback Device for the Visually Disabled. Presented at the Biomedical Circuits and Systems Conference (BIOCAS), Baltimore, MD.We describe a sensory augmentation system designed to provide the visually disabled with a sense of color. Our system consists of a glove with short-range optical color sensors mounted on its fingertips, and a torso-worn belt on which tactors (haptic feedback actuators) are mounted. Each fingertip sensor detects the observed objectpsilas color. This information is encoded to the tactor through vibrations in respective locations and varying modulations. Early results suggest that detection of primary colors is possible with near 100% accuracy and moderate latency, with a minimum amount of training
XMM-Newton EPIC and OM observation of Nova Centauri 1986 (V842 Cen)
We report the results from the temporal and spectral analysis of an
XMM-Newton observation of Nova Centauri 1986 (V842 Cen). We detect a period at
3.510.4 h in the EPIC data and at 4.00.8 h in the OM data. The X-ray
spectrum is consistent with the emission from an absorbed thin thermal plasma
with a temperature distribution given by an isobaric cooling flow. The maximum
temperature of the cooling flow model is keV. Such a
high temperature can be reached in a shocked region and, given the periodicity
detected, most likely arises in a magnetically-channelled accretion flow
characteristic of intermediate polars. The pulsed fraction of the 3.51 h
modulation decreases with energy as observed in the X-ray light curves of
magnetic CVs, possibly due either to occultation of the accretion column by the
white dwarf body or phase-dependent to absorption. We do not find the 57 s
white dwarf spin period, with a pulse amplitude of 4 mmag, reported by Woudt et
al. (2009) either in the Optical Monitor (OM) data, which are sensitive to
pulse amplitudes 0.03 magnitudes, or the EPIC data, sensitive to
pulse fractions 14 2%.Comment: 5 pages, 3 figures; MNRAS, accepte
A search for diffuse bands in fullerene planetary nebulae: evidence of diffuse circumstellar bands
Large fullerenes and fullerene-based molecules have been proposed as carriers
of diffuse interstellar bands (DIBs). The recent detection of the most common
fullerenes (C60 and C70) around some planetary nebulae (PNe) now enable us to
study the DIBs towards fullerene-rich space environments. We search DIBs in the
optical spectra towards three fullerene-containing PNe (Tc 1, M 1-20, and IC
418). Special attention is given to DIBs which are found to be unusually
intense towards these fullerene sources. In particular, an unusually strong
4428A absorption feature is a common charateristic of fullerene PNe. Similar to
Tc 1, the strongest optical bands of neutral C60 are not detected towards IC
418. Our high-quality (S/N > 300) spectra for PN Tc 1, together with its large
radial velocity, permit us to search for the presence of diffuse bands of
circumstellar origin, which we refer to as diffuse circumstellar bands (DCBs).
We report the first tentative detection of two DCBs at 4428 and 5780 A in the
fullerene-rich circumstellar environment around the PN Tc 1. Laboratory and
theoretical studies of fullerenes in their multifarious manifestations (carbon
onions, fullerene clusters, or even complex species formed by fullerenes and
other molecules like PAHs or metals) may help solve the mystery of some of the
diffuse band carriers.Comment: Accepted for publication in Astronomy & Astrophysics (16 pages, 10
figures, and 7 Tables); final version (changes regarding PN M 1-20 and
language corrected
Effects of weak anchoring on C1 and C2 chevron structures
We present a theoretical study of the effect of weak anchoring on the transition between C1 and C2 chevron structures in smectic C liquid crystals. We employ a continuum theory which allows for variable cone, azimuthal and layer tilt angles. Equilibrium profiles for the director cone and azimuthal angles in the C1 and C2 states are calculated from the standard Euler-Lagrange minimisation of the total energy of the system. By comparing the total energies of the C1 and C2 states we can determine the globally stable chevron profile and calculate the critical temperature for the C1-C2 transition, which depends on anchoring strength and pretilt angle variations
Interplanetary propulsion using inertial fusion
Inertial fusion can be used to power spacecraft within the solar system and beyond. Such spacecraft have the potential for short-duration manned-mission performance exceeding other technologies. We are conducting a study to assess the systems aspects of inertial fusion as applied to such missions, based on the conceptual engine design of Hyde (1983) we describe the required systems for an entirely new spacecraft design called VISTA that is based on the use of DT fuel. We give preliminary design details for the power conversion and power conditioning systems for manned missions to Mars of total duration of about 100 days. Specific mission performance results will be published elsewhere, after the study has been completed
Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean
In the global ocean, the number of reported hypoxic sites (oxygen \u3c 30% saturation) is on the rise both near the coast and in the open ocean. But unfortunately, most of the papers on hypoxia only present oxygen data from one or two years, so that we often lack a long-term perspective on whether oxygen levels at these locations are decreasing, steady or increasing. Consequently, we cannot rule out the possibility that many of the newly reported hypoxic areas were hypoxic in the past, and that the increasing number of hypoxic areas partly reflects increased research and monitoring efforts. Here we address this shortcoming by computing oxygen concentration trends in the global ocean from published time series and from time series that we calculated using a global oxygen database. Our calculations reveal that median oxygen decline rates are more severe in a 30 km band near the coast than in the open ocean (\u3e 100 km from the coast). Percentages of oxygen time series with negative oxygen trends are also greater in the coastal ocean than in the open ocean. Finally, a significant difference between median published oxygen trends and median trends calculated from raw oxygen data suggests the existence of a publication bias in favor of negative trends in the open ocean
Correlations between reflected and transmitted intensity patterns emerging from opaque disordered media
The propagation of monochromatic light through a scattering medium produces
speckle patterns in reflection and transmission, and the apparent randomness of
these patterns prevents direct imaging through thick turbid media. Yet, since
elastic multiple scattering is fundamentally a linear and deterministic
process, information is not lost but distributed among many degrees of freedom
that can be resolved and manipulated. Here we demonstrate experimentally that
the reflected and transmitted speckle patterns are correlated, even for opaque
media with thickness much larger than the transport mean free path, proving
that information survives the multiple scattering process and can be recovered.
The existence of mutual information between the two sides of a scattering
medium opens up new possibilities for the control of transmitted light without
any feedback from the target side, but using only information gathered from the
reflected speckle.Comment: 6 pages, 4 figure
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