25,453 research outputs found
How old are the HII Galaxies?
Using a novel approach we have reanalized the question of whether the extreme
star forming galaxies known as HII galaxies are truly young or rejuvenated old
systems. We first present a method of inversion that applies to any monotonic
function of time describing the evolution of independent events. We show that,
apart from a normalization constant, the ``true'' time dependence can be
recovered from the inversion of its probability density function. We applied
the inversion method to the observed equivalent width of Hbeta (EW(Hbeta))
distribution for objects in the Terlevich and collaborators Spectrophotometric
Catalogue of HII galaxies and found that their global history of star formation
behaves much closer to the expectations of a continuos star formation model
than to an instantaneous one. On the other hand, when the inversion method is
applied to samples within a restricted metallicity range we find that their
history of star formation behaves much closer to what the instantaneous model
predicts.
Our main conclusion is that, globally, the evolution of HII galaxies seems
consistent with a succession of short starbursts separated by quiescent periods
and that, while the emission lines trace the properties of the present burst,
the underlying stellar continuum traces the whole star formation history of the
galaxy. Thus, observables like the EW(Hbeta) that combine an emission line
flux, i.e. a parameter pertaining to the present burst, with the continuum
flux, i.e. a parameter that traces the whole history of star formation, should
not be used alone to characterize the present burst.Comment: Accepted for publication in MNRA
AFLOW-SYM: Platform for the complete, automatic and self-consistent symmetry analysis of crystals
Determination of the symmetry profile of structures is a persistent challenge
in materials science. Results often vary amongst standard packages, hindering
autonomous materials development by requiring continuous user attention and
educated guesses. Here, we present a robust procedure for evaluating the
complete suite of symmetry properties, featuring various representations for
the point-, factor-, space groups, site symmetries, and Wyckoff positions. The
protocol determines a system-specific mapping tolerance that yields symmetry
operations entirely commensurate with fundamental crystallographic principles.
The self consistent tolerance characterizes the effective spatial resolution of
the reported atomic positions. The approach is compared with the most used
programs and is successfully validated against the space group information
provided for over 54,000 entries in the Inorganic Crystal Structure Database.
Subsequently, a complete symmetry analysis is applied to all 1.7 million
entries of the AFLOW data repository. The AFLOW-SYM package has been
implemented in, and made available for, public use through the automated,
framework AFLOW.Comment: 24 pages, 6 figure
Maser Flare Simulations from Oblate and Prolate Clouds
We investigated, through numerical models, the flaring variability that may
arise from the rotation of maser clouds of approximately spheroidal geometry,
ranging from strongly oblate to strongly prolate examples. Inversion solutions
were obtained for each of these examples over a range of saturation levels from
unsaturated to highly saturated. Formal solutions were computed for rotating
clouds with many randomly chosen rotation axes, and corresponding averaged
maser light curves plotted with statistical information. The dependence of
results on the level of saturation and on the degree of deformation from the
spherical case were investigated in terms of a variability index and duty
cycle. It may be possible to distinguish observationally between flares from
oblate and prolate objects. Maser flares from rotation are limited to long
timescales (at least a few years) and modest values of the variability index
(), and can be aperiodic or quasi-periodic. Rotation is therefore
not a good model for HO variability on timescales of weeks to months, or of
truly periodic flares.Comment: 11 pages, 12 figures, accepted for publication in MNRA
The ESO Nearby Abell Cluster Survey. XIII. The orbits of the different types of galaxies in rich clusters
We study the orbits of the various types of galaxies observed in the ESO
Nearby Abell Cluster Survey. Galaxies within and outside substructures are
considered separately. We use the mass profile we determined from the
distribution and kinematics of the early-type galaxies (i.e. ellipticals,
excluding the brightest ones, and S0s) outside substructures; the latter were
assumed to be on isotropic orbits, which is supported by the shape of their
velocity distribution. The projected distribution and kinematics of the
galaxies of other types are used to search for equilibrium solutions in the
gravitational potential derived from the early-type galaxies, using the method
described by Binney and Mamon as implemented by Solanes and Salvador-Sole'. For
the brightest ellipticals we are not able to construct equilibrium solutions.
This is most likely the result of the formation history and the special
location of these galaxies at the centres of their clusters. The data for the
early spirals allow equilibrium solutions and are consistent with isotropic
orbits, although there is an apparent radial anisotropy at about 0.45 r200. For
the late spirals an equilibrium solution with isotropic orbits is rejected by
the data. The orbits are nearly isotropic within about 0.7 r200, but then
become increasingly radial outwards. Finally, the data for the galaxies in
substructures indicate that isotropic solutions are not acceptable, and
tangential orbits are indicated. We briefly discuss the possible implications
of these velocity-anisotropy profiles for current ideas of the evolution and
transformation of galaxies in clusters. (Abridged)Comment: A&A, accepted. 13 pages, 10 figure
The magnetic field topology and chemical abundance distributions of the Ap star HD 32633
Previous observations of the Ap star HD 32633 indicated that its magnetic
field was unusually complex in nature and could not be characterised by a
simple dipolar structure. Here we derive magnetic field maps and chemical
abundance distributions for this star using full Stokes vector (Stokes )
high-resolution observations obtained with the ESPaDOnS and Narval
spectropolarimeters. Our maps, produced using the Invers10 magnetic Doppler
imaging (MDI) code, show that HD 32633 has a strong magnetic field which
features two large regions of opposite polarity but deviates significantly from
a pure dipole field. We use a spherical harmonic expansion to characterise the
magnetic field and find that the harmonic energy is predominately in the
and poloidal modes with a small toroidal component. At the
same time, we demonstrate that the observed Stokes parameter profiles of HD
32633 cannot be fully described by either a dipolar or dipolar plus quadrupolar
field geometry. We compare the magnetic field topology of HD 32633 with other
early-type stars for which MDI analyses have been performed, supporting a trend
of increasing field complexity with stellar mass. We then compare the magnetic
field topology of HD 32633 with derived chemical abundance maps for the
elements Mg, Si, Ti, Cr, Fe, Ni and Nd. We find that the iron-peak elements
show similar distributions, but we are unable to find a clear correlation
between the location of local chemical enhancements or depletions and the
magnetic field structure.Comment: 15 pages, 11 figures, accepted for publication in MNRA
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