1,086 research outputs found
Intrinsic spectral blueshifts in rapidly rotating stars?
Spectroscopic radial velocities for several nearby open clusters suggest that
spectra of (especially earlier-type) rapidly rotating stars are systematically
blueshifted by 3 km/s or more, relative to the spectra of slowly rotating ones.
Comparisons with astrometrically determined radial motions in the Hyades
suggests this to be an absolute blueshift, relative to wavelengths naively
expected from stellar radial motion and gravitational redshift. Analogous
trends are seen also in most other clusters studied (Pleiades, Coma Berenices,
Praesepe, Alpha Persei, IC 2391, NGC 6475, IC 4665, NGC 1976 and NGC 2516).
Possible mechanisms are discussed, including photospheric convection, stellar
pulsation, meridional circulation, and shock-wave propagation, as well as
effects caused by template mismatch in determining wavelength displacements.
For early-type stars, a plausible mechanism is shock-wave propagation upward
through the photospheric line-forming regions. Such wavelength shifts thus
permit studies of certain types of stellar atmospheric dynamics and -
irrespective of their cause - may influence deduced open-cluster membership
(when selected from common velocity) and deduced cluster dynamics (some types
of stars might show fortuitous velocity patterns).Comment: Accepted by Astronomy & Astrophysics; 6 pages, 3 figure
Abundance analysis of prime B-type targets for asteroseismology II. B6--B9.5 stars in the field of view of the CoRoT
The CoRoT satellite is collecting precise time-resolved photometry for tens
of asteroseismology targets. To ensure a correct interpretation of the CoRoT
data, the atmospheric parameters, chemical compositions, and rotational
velocities of the stars must be determined. The main goal of the ground-based
seismology support program for the CoRoT mission was to obtain photometric and
spectroscopic data for stars in the fields monitored by the satellite. These
ground-based observations were collected in the GAUDI archive. High-resolution
spectra of more than 200 B-type stars are available in this database, and about
45% of them is analysed here. To derive the effective temperature of the stars,
we used photometric indices. Surface gravities were obtained by comparing
observed and theoretical Balmer line profiles. To determine the chemical
abundances and rotational velocities, we used a spectrum synthesis method,
which consisted of comparing the observed spectrum with theoretical ones based
on the assumption of LTE. Atmospheric parameters, chemical abundances, and
rotational velocities were determined for 89 late-B stars. The dominant species
in their spectra are iron-peak elements. The average Fe abundance is
7.24+/-0.45 dex. The average rotational velocity is 126 km/sec, but there are
13 and 20 stars with low and moderate Vsin i values, respectively. The analysis
of this sample of 89 late B-type stars reveals many chemically peculiar (CP)
stars. Some of them were previously known, but at least 9 new CP candidates,
among which at least two HgMn stars, are identified in our study. These CP
stars as a group exhibit Vsin i values lower than the stars with normal surface
chemical composition.Comment: 21 pages, 13 figures, accepted to Astronomy and Astrophysic
Individualisation of time-motion analysis : a method comparison and case report series
© Georg Thieme Verlag KG. This study compared the intensity distribution of time-motion analysis data, when speed zones were categorized by different methods. 12 U18 players undertook a routine battery of laboratory- and field-based assessments to determine their running speed corresponding to the respiratory compensation threshold (RCT), maximal aerobic speed (MAS), maximal oxygen consumption (vVO 2max ) and maximal sprint speed (MSS). Players match-demands were tracked using 5 Hz GPS units in 22 fixtures (50 eligible match observations). The percentage of total distance covered running at high-speed (%HSR), very-high speed (%VHSR) and sprinting were determined using the following speed thresholds: 1) arbitrary; 2) individualised (IND) using RCT, vVO 2max and MSS; 3) individualised via MAS per se; 4) individualised via MSS per se; and 5) individualised using MAS and MSS as measures of locomotor capacities (LOCO). Using MSS in isolation resulted in 61 % and 39 % of player's % HSR and % VHSR, respectively, being incorrectly interpreted, when compared to the IND technique. Estimating the RCT from fractional values of MAS resulted in erroneous interpretations of % HSR in 50 % of cases. The present results suggest that practitioners and researchers should avoid using singular fitness characteristics to individualise the intensity distribution of time-motion analysis data. A combination of players' anaerobic threshold, MAS, and MSS characteristics are recommended to individualise player-tracking data
Abundance analysis of Am binaries and search for tidally driven abundance anomalies - III. HD116657, HD138213, HD155375, HD159560, HD196544 and HD204188
We continue here the systematic abundance analysis of a sample of Am binaries
in order to search for possible abundance anomalies driven by tidal interaction
in these binary systems. New CCD observations in two spectral regions
(6400-6500, 6660-6760 AA) of HD116657, HD138213, HD155375, HD159560, HD196544
and HD204188 were obtained. Synthetic spectrum analysis was carried out and
basic stellar properties, effective temperatures, gravities, projected
rotational velocities, masses, ages and abundances of several elements were
determined. We conclude that all six stars are Am stars. These stars were put
into the context of other Am binaries with 10 < Porb < 200 days and their
abundance anomalies discussed in the context of possible tidal effects. There
is clear anti-correlation of the Am peculiarities with v sin i. However, there
seems to be also a correlation with the eccentricity and may be with the
orbital period. The dependence on the temperature, age, mass, and
microturbulence was studied as well. The projected rotational velocities
obtained by us were compared to those of Royer et al. (2002) and Abt & Morrell
(1995).Comment: 11 pages, 3 tables, 12 figures. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Early Results from the Galactic O-Star Spectroscopic Survey: C III Emission Lines in Of Spectra
On the basis of an extensive new spectroscopic survey of Galactic O stars, we
introduce the Ofc category, which consists of normal spectra with C III
\lambda\lambda4647-4650-4652 emission lines of comparable intensity to those of
the Of defining lines N III \lambda\lambda4634-4640-4642. The former feature is
strongly peaked to spectral type O5, at all luminosity classes, but
preferentially in some associations or clusters and not others. The
relationships of this phenomenon to the selective C III \lambda5696 emission
throughout the normal Of domain, and to the peculiar, variable Of?p category,
for which strong C III \lambda\lambda4647-4650-4652 emission is a defining
characteristic, are discussed. Magnetic fields have recently been detected on
two members of the latter category. We also present two new extreme Of?p stars,
NGC 1624-2 and CPD -28^{\circ}2561, bringing the number known in the Galaxy to
five. Modeling of the behavior of these spectral features can be expected to
better define the physical parameters of both normal and peculiar objects, as
well as the atomic physics involved.Comment: 12 pages, 1 table, 2 figures; accepted for ApJL, vol. 71
Understanding Bottom Production
We describe calculations of b b-bar production to next-to-next-to-leading
order (NNLO) and next-to-next-to-leading logarithm (NNLL) near threshold in pp
interactions. Our calculations are in good agreement with the b b-bar total
cross section measured by HERA-B.Comment: contribution to the proceedings of Quark Matter'02, 4 pages, 1 .eps
figure, uses special included style fil
The magnetic Bp star 36 Lyncis, I. Magnetic and photospheric properties
This paper reports the photospheric, magnetic and circumstellar gas
characteristics of the magnetic B8p star 36 Lyncis (HD 79158). Using archival
data and new polarised and unpolarised high-resolution spectra, we redetermine
the basic physical properties, the rotational period and the geometry of the
magnetic field, and the photospheric abundances of various elements.}{Based on
magnetic and spectroscopic measurements, we infer an improved rotational period
of d. We determine a current epoch of the longitudinal
magnetic field positive extremum (HJD 2452246.033), and provide constraints on
the geometry of the dipole magnetic field (i\geq 56\degr, G, unconstrained). We redetermine the effective
temperature and surface gravity using the optical and UV energy distributions,
optical photometry and Balmer line profiles ( K,
), and based on the Hipparcos parallax we redetermine the
luminosity, mass, radius and true rotational speed ( \kms). We
measure photospheric abundances for 21 elements using optical and UV spectra,
and constrain the presence of vertical stratification of these elements. We
perform preliminary Doppler Imaging of the surface distribution of Fe, finding
that Fe is distributed in a patchy belt near the rotational equator. Most
remarkably, we confirm strong variations of the H line core which we
interpret as due to occultations of the star by magnetically-confined
circumstellar gas.Comment: Accepted by Astronomy and Astrophysic
QCD Structure of Leptons
The QCD structure of the electron is defined and calculated. The leading
order splitting functions are extracted, showing an important contribution from
- interference. Leading logarithmic QCD evolution equations are
constructed and solved in the asymptotic region where log behaviour of the
parton densities is observed. Corrections to the naive evolution procedure are
demonstrated. Possible applications with clear manifestation of 'resolved'
photon and weak bosons are discussed.Comment: 10 pages, 5 figures, revised versio
Heavy-quark correlations in deep inelastic scattering
We discuss results for heavy quark correlations in next-to-leading order QCD
in deep inelastic electroproduction.Comment: Talk presented by J. Smith. To appear in proceedings of "QCD and QED
in Higher Orders" 1996 Zuethen Workshop on Elementary Particle Theory, April
22-26, 1996. Latex, 5 figures, uses espcrc2.sty (included
Rotational velocities of A-type stars I. Measurement of vsini in the southern hemisphere
Within the scope of a Key Programme determining fundamental parameters of
stars observed by HIPPARCOS, spectra of 525 B8 to F2-type stars brighter than
V=8 have been collected at ESO. Fourier transforms of several line profiles in
the range 4200-4500 A are used to derive vsini from the frequency of the first
zero. Statistical analysis of the sample indicates that measurement error is a
function of vsini and this relative error of the rotational velocity is found
to be about 6% on average. The results obtained are compared with data from the
literature. There is a systematic shift from standard values from Slettebak et
al. (1975), which are 10 to 12% lower than our findings. Comparisons with other
independent vsini values tend to prove that those from Slettebak et al. are
underestimated. This effect is attributed to the presence of binaries in the
standard sample of Slettebak et al., and to the model atmosphere they used.Comment: 17 pages, includes 18 figures, accepted in A&
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