3,757 research outputs found
High resolution spectroscopy of HgMn stars: a time of surprises
We present the results of a high spectral resolution study of a few
spectroscopic binaries with HgMn primary stars. We detect for the first time in
the spectra of HgMn stars that for many elements the line profiles are variable
over the rotation period. The strongest profile variations are found for the
elements Pt, Hg, Sr, Y, Zr, Mn, Ga, He and Nd. The slight variability of He and
Y is also confirmed from the study of high resolution spectra of another HgMn
star, alpha And.Comment: 2 pages, 2 figures, to appear in "Precision Spectroscopy in
Astrophysics
Nonlinear mirror instability
Slow dynamical changes in magnetic-field strength and invariance of the
particles' magnetic moments generate ubiquitous pressure anisotropies in weakly
collisional, magnetized astrophysical plasmas. This renders them unstable to
fast, small-scale mirror and firehose instabilities, which are capable of
exerting feedback on the macroscale dynamics of the system. By way of a new
asymptotic theory of the early nonlinear evolution of the mirror instability in
a plasma subject to slow shearing or compression, we show that the instability
does not saturate quasilinearly at a steady, low-amplitude level. Instead, the
trapping of particles in small-scale mirrors leads to nonlinear secular growth
of magnetic perturbations, . Our theory explains
recent collisionless simulation results, provides a prediction of the mirror
evolution in weakly collisional plasmas and establishes a foundation for a
theory of nonlinear mirror dynamics with trapping, valid up to .Comment: 5 pages, submitte
New Mn II energy levels from STIS-HST spectrum of the HgMn star HD 175640
The NIST database lists several Mn II lines that were observed in the
laboratory but not classified. They cannot be used in spectrum synthesis
because their atomic line data are unknown. These lines are concentrated in the
2380-2700 A interval. We aimed to assign energy levels and log gf values to
these lines. Semi-empirical line data for Mn II computed by Kurucz were used to
synthesize the ultraviolet spectrum of the slow-rotating, HgMn star HD 175640.
The spectrum was compared with the high-resolution spectrum observed with the
HST-STIS equipment. A UVES spectrum covering the 3050-10000 A region was also
examined. We determined a total of 73 new energy levels, 58 from the STIS
spectrum of HD 175640 and another 15 from the UVES spectrum. The new energy
levels give rise to numerous new computed lines. We have identified more than
50% of the unclassified lines listed in the NIST database and have changed the
assignement of another 24 lines. An abundance analysis of the star HD 175640,
based on the comparison of observed and computed ultraviolet spectra in the
1250-3040 A interval, is the by-product of this study on Mn II.Comment: Paper accepted by Astronomy & Astrophysic
Dynamical detection of three triple stellar systems in open clusters
We present a kinematic analysis of three triple stellar systems belonging to
two open clusters: CPD-60{\deg}961 and HD66137 in NGC2516, and HD315031 in
NGC6530. All three systems are hierarchical triples with a close binary bound
to a third body in a wider orbit, whose presence is detected through velocity
variations of the close binary barycentre. Orbital parameters are derived from
radial velocity curves. Absolute parameters for all stars are estimated
assuming cluster membership. Some dynamical and evolutionary aspects of these
systems are discussed, particularly the possible influence of Kozai cycles. The
two systems of NGC2516 have similar orbital configurations with inner periods
of 11.23 d and 8.70 d and outer periods of 9.79 yr and 9.24 yr. We report also
radial velocity measurements of the components of the visual binary
CPD-60{\deg}944 in NGC2516. Including results from previous works, this cluster
would harbor 5 hierarchical triples. The young system HD315031 has an inner
binary with a period of 1.37 d and a very eccentric (e=0.85) outer orbit with a
period of 483 d. Possible dynamical evolutionary scenarios are discussed.
Long-term radial velocity monitoring is highlighted as strategy for the
detection of subsystems with intermediate separations, which are hard to cover
with normal spectroscopic studies or visual techniques.Comment: 9 pages, 5 figures. Accepted by Monthly Notices of the Royal
Astronomical Societ
Lanthanides and other spectral oddities in a Centauri
Context: There is considerable interest in the helium variable a Cen as a
bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce
III and other possible lanthanides in the spectrum the of hottest chemically
peculiar (CP) star in which these elements have been found. A {Kr II line
appears within a broad absorption which we suggest may be due to a high-level
transition in C II. Methods: Wavelengths and equivalent widths are measured on
high-resolution UVES spectra, analyzed, and their phase-variations
investigated. Results: New, robust identifications of Ce III and Kr II are
demonstrated. Nd III is likely present. A broad absorption near 4619[A] is
present at all phases of a Cen, and in some other early B stars. Conclusions:
The presence of lanthanides in a Cen strengthens the view that this star is a
significant link between the cooler CP stars and the hotter helium-peculiar
stars. Broad absorptions in a Cen are not well explained.Comment: Research Note accepted by Astronomy and Astrophysics; 4 pages, 4
Figs. 2 Table
From Small-Scale Dynamo to Isotropic MHD Turbulence
We consider the problem of incompressible, forced, nonhelical, homogeneous,
isotropic MHD turbulence with no mean magnetic field. This problem is
essentially different from the case with externally imposed uniform mean field.
There is no scale-by-scale equipartition between magnetic and kinetic energies
as would be the case for the Alfven-wave turbulence. The isotropic MHD
turbulence is the end state of the turbulent dynamo which generates folded
fields with small-scale direction reversals. We propose that the statistics
seen in numerical simulations of isotropic MHD turbulence could be explained as
a superposition of these folded fields and Alfven-like waves that propagate
along the folds.Comment: kluwer latex, 7 pages, 7 figures; Proceedings of the International
Workshop "Magnetic Fields and Star Formation: Theory vs. Observations",
Madrid, 21-25 April 2003 -- published version (but the e-print is free of
numerous typos introduced by the publisher
C II abundances in early-type stars: solution to a notorious non-LTE problem
We address a long-standing discrepancy between non-LTE analyses of the
prominent C II 4267 and 6578/82 A multiplets in early-type stars. A
comprehensive non-LTE model atom of C II is constructed based on critically
selected atomic data. This model atom is used for an abundance study of six
apparently slow-rotating main-sequence and giant early B-type stars.
High-resolution and high-S/N spectra allow us to derive highly consistent
abundances not only from the classical features but also from up to 18 further
C II lines in the visual - including two so far unreported emission features
equally well reproduced in non-LTE. These results require the stellar
atmospheric parameters to be determined with care. A homogeneous (slightly)
sub-solar present-day carbon abundance from young stars in the solar vicinity
(in associations and in the field) of log C/H +12= 8.29+/-0.03 is indicated.Comment: 8 pages, 5 figure
Stratification and Isotope Separation in CP Stars
We investigate the elemental and isotopic stratification in the atmospheres
of selected chemically peculiar (CP) stars of the upper main sequence.
Reconfiguration of the UVES spectrograph in 2004 has made it possible to
examine all three lines of the Ca II infrared triplet. Much of the material
analyzed was obtained in 2008.
We support the claim of Ryabchikova, Kochukhov & Bagnulo (RKB) that the
calcium isotopes have distinct stratification profiles for the stars 10 Aql, HR
1217, and HD 122970, with the heavy isotope concentrated toward the higher
layers. Better observations are needed to learn the extent to which Ca-40
dominates in the deepest layers of all or most CP stars that show the presence
of Ca-48. There is little evidence for Ca-40 in the spectra of some HgMn stars,
and the infrared triplet in the magnetic star HD 101065 is well fit by pure
Ca-48. In HR 5623 (HD 133792) and HD 217522 it is likely that the heavy isotope
dominates, though models are possible where this is not the case.
While elemental stratification is surely needed in many cases, we point out
the importance of including adjustments in the assumed Teff and log(g) values,
in attempts to model stratification. We recommend emphasis on profiles of the
strongest lines, where the influence of stratification is most evident.
Isotopic mixtures, involving the 4 stable calcium nuclides with masses
between 40 and 48 are plausible, but are not emphasized.Comment: 16 Pages, 20 Figures, 10 Tables. Accepted for publication in Monthly
Notices of the RA
Turbulent transport in tokamak plasmas with rotational shear
Nonlinear gyrokinetic simulations have been conducted to investigate
turbulent transport in tokamak plasmas with rotational shear. At sufficiently
large flow shears, linear instabilities are suppressed, but transiently growing
modes drive subcritical turbulence whose amplitude increases with flow shear.
This leads to a local minimum in the heat flux, indicating an optimal E x B
shear value for plasma confinement. Local maxima in the momentum fluxes are
also observed, allowing for the possibility of bifurcations in the E x B shear.
The sensitive dependence of heat flux on temperature gradient is relaxed for
large flow shear values, with the critical temperature gradient increasing at
lower flow shear values. The turbulent Prandtl number is found to be largely
independent of temperature and flow gradients, with a value close to unity.Comment: 4 pages, 5 figures, submitted to PR
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