812 research outputs found
The solar abundance problem and eMSTOs in clusters
We study the impact of accretion from protoplanetary discs on stellar
evolution of AFG-type stars. We use a simplified disc model computed using the
Two-Pop-Py code that contains the growth and drift of dust particles in the
protoplanetary disc. It is used to model the accretion scenarios for a range of
physical conditions of protoplanetary discs. Two limiting cases are combined
with the evolution of stellar convective envelopes computed using the Garstec
stellar evolution code. We find that the accretion of metal-poor (gas) or
metal-rich (dust) material has a significant impact on the chemical composition
of the stellar convective envelope. As a consequence, the evolutionary track of
the star diverts from the standard scenario predicted by canonical stellar
evolution models, which assume a constant and homogeneous chemical composition
after the assembly of the star has finished. In the case of the Sun, we find a
modest impact on the solar chemical composition. Accretion of metal-poor
material indeed reduces the overall metallicity of the solar atmosphere, and it
is consistent, within the uncertainty, with the solar Z reported by Caffau et
al. (2011), but our model is not consistent with the measurement by Asplund et
al. (2009). Another effect is the change of the position of the star in the
colour-magnitude diagram. We compare our predictions to a set of open clusters
from the Gaia DR2 and show that it is possible to produce a scatter close to
the turn-off of young clusters that could contribute to explain the observed
scatter in CMDs. Detailed measurements of metallicities and abundances in the
nearby open clusters will provide a stringent observational test of our
proposed scenario.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in A&
Fermi Surface Reconstruction in CeRhCoIn
The evolution of the Fermi surface of CeRhCoIn was studied as
a function of Co concentration via measurements of the de Haas-van Alphen
effect. By measuring the angular dependence of quantum oscillation frequencies,
we identify a Fermi surface sheet with -electron character which undergoes
an abrupt change in topology as is varied. Surprisingly, this
reconstruction does not occur at the quantum critical concentration ,
where antiferromagnetism is suppressed to T=0. Instead we establish that this
sudden change occurs well below , at the concentration x ~ 0.4 where long
range magnetic order alters its character and superconductivity appears. Across
all concentrations, the cyclotron effective mass of this sheet does not
diverge, suggesting that critical behavior is not exhibited equally on all
parts of the Fermi surface.Comment: 4 pages, 4 figure
ARES+MOOG - a practical overview of an EW method to derive stellar parameters
The goal of this document is to describe the important practical aspects in
the use of an Equivalent Width (EW) method for the derivation of spectroscopic
stellar parameters. A general description of the fundamental steps composing
any EW method is given, together with possible differences that may be found in
different methods used in the literature. Then ARES+MOOG is then used as an
example where each step of the method is described in detail. A special focus
is given for the specific steps of this method, namely the use of a
differential analysis to define the atomic data for the adopted line list, the
automatic EW determinations, and the way to find the best parameters at the end
of the procedure. Finally, a practical tutorial is given, where we focus on
simple exercises useful to illustrate and explain the dependence of the
abundances with the assumed stellar parameters. The interdependences are
described and a clear procedure is given to find the "final" stellar
parameters.Comment: 15 pages, 4 figures, accepted for publication as a chapter in
"Determination of Atmospheric Parameters of B, A, F and G Type Stars",
Springer (2014), eds. E. Niemczura, B. Smalley, W. Pyc
Analysis of stellar spectra with 3D and NLTE models
Models of radiation transport in stellar atmospheres are the hinge of modern
astrophysics. Our knowledge of stars, stellar populations, and galaxies is only
as good as the theoretical models, which are used for the interpretation of
their observed spectra, photometric magnitudes, and spectral energy
distributions. I describe recent advances in the field of stellar atmosphere
modelling for late-type stars. Various aspects of radiation transport with 1D
hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly
reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
Damping of the de Haas-van Alphen oscillations in the superconducting state of MgB_2
The de Haas-van Alphen (dHvA) signal arising from orbits on the Fermi
surface sheet of the two-gap superconductor MgB has been observed in the
vortex state below . An extra attenuation of the dHvA signal, beyond
those effects described in the conventional Lifshitz-Kosevich expression, is
seen due to the opening of the superconducting gap. Our data show that the
band gap is still present up to . The data are compared to
current theories of dHvA oscillations in the superconducting state which allow
us to extract estimates for the evolution of the band gap with magnetic
field. Contrary to results for other materials, we find that the most recent
theories dramatically underestimate the damping in MgB.Comment: 10 pages with figures. Submitted to Phys. Rev. B. PDF version with
higher quality figures can be found at
http://www.phy.bris.ac.uk/research/cond_matt/PdfPubs/mgb2RSdhva.pd
Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles
Measurements of quantum oscillations in the cuprate superconductors afford a
new opportunity to assess the extent to which the electronic properties of
these materials yield to a description rooted in Fermi liquid theory. However,
such an analysis is hampered by the small number of oscillatory periods
observed. Here we employ a genetic algorithm to globally model the field,
angular, and temperature dependence of the quantum oscillations observed in the
resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set
to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key
feature of the data is the first identification of the effect of Zeeman
splitting, which separates spin-up and spin-down contributions, indicating that
the quasiparticles in the cuprates behave as nearly free spins, constraining
the source of the Fermi surface reconstruction to something other than a
conventional spin density wave with moments parallel to the CuO2 planes.Comment: 8 pages, 4 figure
Empirically Derived Integrated Stellar Yields of Fe-Peak Elements
We present here the initial results of a new study of massive star yields of
Fe-peak elements. We have compiled from the literature a database of carefully
determined solar neighborhood stellar abundances of seven iron-peak elements,
Ti, V, Cr, Mn, Fe, Co, and Ni and then plotted [X/Fe] versus [Fe/H] to study
the trends as functions of metallicity. Chemical evolution models were then
employed to force a fit to the observed trends by adjusting the input massive
star metallicity-sensitive yields of Kobayashi et al. Our results suggest that
yields of Ti, V, and Co are generally larger as well as anticorrelated with
metallicity, in contrast to the Kobayashi et al. predictions. We also find the
yields of Cr and Mn to be generally smaller and directly correlated with
metallicity compared to the theoretical results. Our results for Ni are
consistent with theory, although our model suggests that all Ni yields should
be scaled up slightly. The outcome of this exercise is the computation of a set
of integrated yields, i.e., stellar yields weighted by a slightly flattened
time-independent Salpeter initial mass function and integrated over stellar
mass, for each of the above elements at several metallicity points spanned by
the broad range of observations. These results are designed to be used as
empirical constraints on future iron-peak yield predictions by stellar
evolution modelers. Special attention is paid to the interesting behavior of
[Cr/Co] with metallicity -- these two elements have opposite slopes -- as well
as the indirect correlation of [Ti/Fe] with [Fe/H]. These particular trends, as
well as those exhibited by the inferred integrated yields of all iron-peak
elements with metallicity, are discussed in terms of both supernova
nucleosynthesis and atomic physics.Comment: 27 pages, 6 figures; Accepted for Publication in the Astrophysical
Journa
NLTE analysis of the methylidyne radical (CH) molecular lines in metal-poor stellar atmospheres
An analysis of the CH molecule in non-local thermodynamic equilibrium (NLTE)
is performed for the physical conditions of cool stellar atmospheres typical of
red giants (log g = 2.0, Teff = 4500 K) and the Sun. The aim of the present
work is to explore whether the G-band of the CH molecule, which is commonly
used in abundance diagnostics of Carbon-Enhanced Metal-Poor (CEMP) stars, is
sensitive to NLTE effects. LTE and NLTE theoretical spectra are computed with
the MULTI code. We use one-dimensional (1D) LTE hydrostatic MARCS model
atmospheres with parameters representing eleven red giant stars with
metallicities ranging from [Fe/H] = -4.0 to [Fe/H] = 0.0 and carbon-to-iron
ratios [C/Fe] = 0.0, +0.7, +1.5, and +3.0. The CH molecule model is represented
by 1981 energy levels, 18377 radiative bound-bound transitions, and 932
photo-dissociation reactions. The rates due to transitions caused by collisions
with free electrons and hydrogen atoms are computed using classical recipes.
Our calculations suggest that NLTE effects in the statistical equilibrium of
the CH molecule are significant and cannot be neglected for precision
spectroscopic analysis of C abundances. The NLTE effects in the G-band increase
with decreasing metallicity. We show that the C abundances are always
under-estimated if LTE is assumed. The NLTE corrections to C abundance inferred
from the CH feature range from +0.04 dex for the Sun to +0.21 dex for a red
giant with metallicity [Fe/H] = -4.0. Departures from the LTE assumption in the
CH molecule are non-negligible and NLTE effects have to be taken into account
in the diagnostic spectroscopy based on the CH lines. We show here that the
NLTE effects in the optical CH lines are non-negligible for the Sun and red
giant stars, but further calculations are warranted to investigate the effects
in other regimes of stellar parameters.Comment: Accepted for publication in Astronomy & Astrophysic
Manganese in dwarf spheroidal galaxies
We provide manganese abundances (corrected for the effect of the hyperfine
structure) for a large number of stars in the dwarf spheroidal galaxies
Sculptor and Fornax, and for a smaller number in the Carina and Sextans dSph
galaxies. Abundances had already been determined for a number of other elements
in these galaxies, including alpha and iron-peak ones, which allowed us to
build [Mn/Fe] and [Mn/alpha] versus [Fe/H] diagrams. The Mn abundances imply
sub-solar [Mn/Fe] ratios for the stars in all four galaxies examined. In
Sculptor, [Mn/Fe] stays roughly constant between [Fe/H]\sim -1.8 and -1.4 and
decreases at higher iron abundance. In Fornax, [Mn/Fe] does not vary in any
significant way with [Fe/H]. The relation between [Mn/alpha] and [Fe/H] for the
dSph galaxies is clearly systematically offset from that for the Milky Way,
which reflects the different star formation histories of the respective
galaxies. The [Mn/alpha] behavior can be interpreted as a result of the
metal-dependent Mn yields of type II and type Ia supernovae. We also computed
chemical evolution models for star formation histories matching those
determined empirically for Sculptor, Fornax, and Carina, and for the Mn yields
of SNe Ia, which were assumed to be either constant or variable with
metallicity. The observed [Mn/Fe] versus [Fe/H] relation in Sculptor, Fornax,
and Carina can be reproduced only by the chemical evolution models that include
a metallicity-dependent Mn yield from the SNe Ia.Comment: 19 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
- …