88 research outputs found
Parent Stars of Extrasolar Planets. XI. Trends with Condensation Temperature Revisited
We report the results of abundance analyses of new samples of stars with
planets and stars without detected planets. We employ these data to compare
abundance-condensation temperature trends in both samples. We find that stars
with planets have more negative trends. In addition, the more metal-rich stars
with planets display the most negative trends. These results confirm and extend
the findings of Ramirez et al. (2009) and Melendez et al. (2009), who
restricted their studies to solar analogs. We also show that the differences
between the solar photospheric and CI meteoritic abundances correlate with
condensation temperature.Comment: 7 pages, 11 figures; to be published in MNRA
Three-dimensional non-LTE radiative transfer computation of the Ca 8542 infrared line from a radiation-MHD simulation
Interpretation of imagery of the solar chromosphere in the widely used
\CaIIIR infrared line is hampered by its complex, three-dimensional and non-LTE
formation. Forward modelling is required to aid understanding. We use a 3D
non-LTE radiative transfer code to compute synthetic \CaIIIR images from a
radiation-MHD simulation of the solar atmosphere spanning from the convection
zone to the corona. We compare the simulation with observations obtained with
the CRISP filter at the Swedish 1--m Solar Telescope. We find that the
simulation reproduces dark patches in the blue line wing caused by Doppler
shifts, brightenings in the line core caused by upward-propagating shocks and
thin dark elongated structures in the line core that form the interface between
upward and downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the sun exhibits both more
vigorous large-scale dynamics as well as small scale motions that are not
resolved within the simulation, presumably owing to a lack of spatial
resolution.Comment: accepted as ApJ lette
Carbon monoxide in the solar atmosphere I. Numerical method and two-dimensional models
The radiation hydrodynamic code CO5BOLD has been supplemented with the
time-dependent treatment of chemical reaction networks. Advection of particle
densities due to the hydrodynamic flow field is also included. The radiative
transfer is treated frequency-independently, i.e. grey, so far. The upgraded
code has been applied to two-dimensional simulations of carbon monoxide (CO) in
the non-magnetic solar photosphere and low chromosphere. For this purpose a
reaction network has been constructed, taking into account the reactions which
are most important for the formation and dissociation of CO under the physical
conditions of the solar atmosphere. The network has been strongly reduced to 27
reactions, involving the chemical species H, H2, C, O, CO, CH, OH, and a
representative metal. The resulting CO number density is highest in the cool
regions of the reversed granulation pattern at mid-photospheric heights and
decreases strongly above. There, the CO abundance stays close to a value of 8.3
on the usual logarithmic abundance scale with [H]=12 but is reduced in hot
shock waves which are a ubiquitous phenomenon of the model atmosphere. For
comparison, the corresponding equilibrium densities have been calculated, based
on the reaction network but also under assumption of instantaneous chemical
equilibrium by applying the Rybicki & Hummer (RH) code by Uitenbroek (2001).
Owing to the short chemical timescales, the assumption holds for a large
fraction of the atmosphere, in particular the photosphere. In contrast, the CO
number density deviates strongly from the corresponding equilibrium value in
the vicinity of chromospheric shock waves. Simulations with altered reaction
network clearly show that the formation channel via hydroxide (OH) is the most
important one under the conditions of the solar atmosphere.Comment: 15 pages, 6 figures, final version will contain online materia
Star formation history in the SMC: the case of NGC602
Deep HST/ACS photometry of the young cluster NGC 602, located in the remote
low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main
sequence stars as well as young stars on the main sequence. The resolved
stellar content thus provides a basis for studying the star formation history
into recent times and constraining several stellar population properties, such
as the present day mass function, the initial mass function and the binary
fraction. To better characterize the pre-main sequence population, we present a
new set of model stellar evolutionary tracks for this evolutionary phase with
metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a
stellar population synthesis code, which takes into account a full range of
stellar evolution phases to derive our best estimate for the star formation
history in the region by comparing observed and synthetic color-magnitude
diagrams. The derived present day mass function for NGC 602 is consistent with
that resulting from the synthetic diagrams. The star formation rate in the
region has increased with time on a scale of tens of Myr, reaching in the last 2.5 Myr, comparable to what is
found in Galactic OB associations. Star formation is most complete in the main
cluster but continues at moderate levels in the gas-rich periphery of the
nebula.Comment: 24 pages. Accepted for publication in A
HD 65949: Rosetta Stone or Red Herring
HD 65949 is a late B star with exceptionally strong Hg II at 3984[A], but it
is not a typical HgMn star. The Re II spectrum is of extraordinary strength.
Abundances, or upper limits are derived here for 58 elements based on a model
with Teff = 13100K, and log(g) = 4.0. Even-Z elements through nickel show minor
deviations from solar abundances. Anomalies among the odd-Z elements through
copper are mostly small. Beyond the iron peak, a huge scatter is found. The
abundance pattern of the heaviest elements resembles the N=126 r-process peak
of solar material, though not in detail. We find a significant correlation of
the abundance excesses with second ionization potentials for elements with Z >
30. This indicates the relevance of photospheric or near-photospheric
processes. We explore a model with mass accretion of exotic material followed
by the more commonly accepted differentiation by diffusion. That model leads to
a number of predictions which challenge future work.
Likely primary and secondary masses are near 3.3 and 1.6 M(solar), with a
separation of ca. 0.25 AU. New atomic structure calculations are presented in
two appendices.Comment: Accepted by MNRAS: 16 pages, 5 figure
Low-mass pre--main-sequence stars in the Magellanic Clouds
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar
stars form in very large numbers. Most attractive places for catching low-mass
star formation in the act are young stellar clusters and associations, still
(half-)embedded in star-forming regions. The low-mass stars in such regions are
still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature
of these objects and the contamination of their samples by the evolved
populations of the Galactic disk impose demanding observational techniques for
the detection of complete numbers of PMS stars in the Milky Way. The Magellanic
Clouds, the companion galaxies to our own, demonstrate an exceptional star
formation activity. The low extinction and stellar field contamination in
star-forming regions of these galaxies imply a more efficient detection of
low-mass PMS stars than in the Milky Way, but their distance from us make the
application of special detection techniques unfeasible. Nonetheless, imaging
with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS
stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of
such objects are identified as the low-mass stellar content of their
star-forming regions, changing completely our picture of young stellar systems
outside the Milky Way, and extending the extragalactic stellar IMF below the
persisting threshold of a few solar masses. This review presents the recent
developments in the investigation of PMS stars in the Magellanic Clouds, with
special focus on the limitations by single-epoch photometry that can only be
circumvented by the detailed study of the observable behavior of these stars in
the color-magnitude diagram. The achieved characterization of the low-mass PMS
stars in the Magellanic Clouds allowed thus a more comprehensive understanding
of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4
figures. Accepted for publication in Space Science Review
Studies in hypertriglyceridaemia VI: serum lathosterol concentration is raised in hypertriglyceridaemic non‐diabetic males with hyperinsulinaemia
Studies in hypertriglyceridaemia 1. Serum triglyceride distribution and its correlates in randomly selected Swedish middle-aged men
Studies in hypertriglyceridaemia, III: glucose tolerance, insulin sensitivity and indices of adipose tissue lipolysis in randomly selected non-diabetic hypertriglyceridaemic Swedish men
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