413 research outputs found
The dependence of convective core overshooting on stellar mass: reality check, and additional evidence
Overshooting from the convective cores of stars more massive than about 1.2
M(Sun) has a profound impact on their subsequent evolution. And yet, the
formulation of the overshooting mechanism in current stellar evolution models
has a free parameter (f[ov] in the diffusive approximation) that remains poorly
constrained by observations, affecting the determination of astrophysically
important quantities such as stellar ages. In an earlier series of papers we
assembled a sample of 37 well-measured detached eclipsing binaries to calibrate
the dependence of f[ov] on stellar mass, showing that it increases sharply up
to a mass of roughly 2 M(Sun), and remains constant thereafter out to at least
4.4 M(Sun). Recent claims have challenged the utility of eclipsing binaries for
this purpose, on the basis that the uncertainties in f[ov] from the model fits
are typically too large to be useful, casting doubt on a dependence of
overshooting on mass. Here we reexamine those claims and show them to be too
pessimistic, mainly because they did not account for all available constraints
--- both observational and theoretical --- in assessing the true uncertainties.
We also take the opportunity to add semi-empirical f[ov] determinations for 13
additional binaries to our previous sample, and to update the values for 9
others. All are consistent with, and strengthen our previous conclusions,
supporting a dependence of f[ov] on mass that is now based on estimates for a
total of 50 binary systems (100 stars).Comment: 14 pages in emulateapj format, including figures and tables. Accepted
for publication in The Astrophysical Journal. One duplicate object has been
removed, and the tables and one figure have been update
The internal structure of neutron stars and white dwarfs, and the Jacobi virial equation. II
In a previous paper we have shown that the function \Gamma(M,
EOS)=\alpha\beta_{GR}/\Lambda^{0.9}(R) is constant (~ 0.4) for pre
main-sequence stars (PMS), white dwarfs (WD) and for some neutron star (NS)
models, where \alpha_{GR} and \beta_{GR} are the form-factors of the
gravitational potential energy and of the moment of inertia. To investigate the
structural evolution of another type of celestial bodies, we use the MESA code
to extend these calculations to gaseous planets. We show that this function is
conserved for all models during the whole planetary evolution and is
independent of the planet mass. We also analyse the cases for which this
function is not conserved during some stellar evolutionary phases. For the PMS
to the WD cooling sequences, we have found a connection between the strong
variations of \Gamma(M, EOS) during the intermediary evolutionary phases and
the specific nuclear power. A threshold for the specific nuclear power was
determined. Below this limit this function is invariant (~ 0.4) for these
models, i.e., at the initial and final stages (PMS and WD). Concerning NS, we
study the influence of the equation of state (EOS) on this function and refine
the exponent of the auxiliary function \Lambda(R) to be ~ 0.8. It is shown that
the function \Gamma(M, EOS) is also invariant (~ 0.4) and is independent of the
EOS and of the stellar mass. Therefore, we confirm that regardless of the final
products of the stellar evolution, NS or WD, they recover the initial value of
\Gamma(M, EOS) ~ 0.4 acquired at the PMS. Finally, we have introduced a
macroscopic stability "criterion" for neutron star models based on the
properties of the relativistic product \alpha\beta_{GR}.Comment: 6 pages, 5 figures, v3: editorial changes, identical to published
versio
Limb and gravity-darkening coefficients for the TESS satellite at several metallicities, surface gravities, and microturbulent velocities
We present new gravity and limb-darkening coefficients for a wide range of
effective temperatures, gravities, metallicities, and microturbulent
velocities. These coefficients can be used in many different fields of stellar
physics as synthetic light curves of eclipsing binaries and planetary transits,
stellar diameters, line profiles in rotating stars, and others. The
limb-darkening coefficients were computed specifically for the photometric
system of the space mission TESS and were performed by adopting the
least-square method. In addition, the linear and bi-parametric coefficients, by
adopting the flux conservation method, are also available. On the other hand,
to take into account the effects of tidal and rotational distortions, we
computed the passband gravity-darkening coefficients using a
general differential equation in which we consider the effects of convection
and of the partial derivative . To generate the limb-darkening coefficients we adopt
two stellar atmosphere models:
ATLAS (plane-parallel) and PHOENIX (spherical, quasi-spherical, and
-method).
The specific intensity distribution was fitted using five approaches: linear,
quadratic, square root, logarithmic, and a more general one with four terms.
These grids cover together 19 metallicities ranging from 10 up to
10 solar abundances, 0 log g 6.0 and 1500 K T 50000 K. The calculations of the gravity-darkening coefficients
were performed for all plane-parallel ATLAS models
A new method to compute limb-darkening coefficients for stellar atmosphere models with spherical symmetry: the space missions TESS, Kepler, Corot, and MOST
One of the biggest problems we can encounter while dealing with the
limb-darkening coefficients for stellar atmospheric models with spherical
symmetry is the difficulty of adjusting both the limb and the central parts
simultaneously. In particular, the regions near the drop-offs are not well
reproduced for most models, depending on Teff, log g, or wavelength. Even if
the law with four terms is used, these disagreements still persist. Here we
introduce a new method that considerably improves the description of both the
limb and the central parts and that will allow users to test models of stellar
atmospheres with spherical symmetry more accurately in environments such as
exoplanetary transits, eclipsing binaries, etc.
The method introduced here is simple. Instead of considering all the
points in the adjustment, as is traditional, we consider only the points until
the drop-off () of each model. From this point, we impose a
condition . All calculations were performed by adopting
the least-squares method.
The resulting coefficients using this new method reproduce the intensity
distribution of the PHOENIX spherical models (COND and DRIFT) quite well for
the photometric systems of the space missions {{\sc TESS}}, {{\sc KEPLER}},
{{\sc COROT}}, and {{\sc MOST}}. The calculations cover the following ranges of
local gravity and effective temperatures:
2.5 log g 6.0 and 1500 K T
12000 K. The new spherical coefficients can easily be adapted to the
most commonly used light curve synthesis codes
Capella (alpha Aurigae) revisited: New binary orbit, physical properties, and evolutionary state
Knowledge of the chemical composition and absolute masses of Capella are key
to understanding the evolutionary state of this benchmark binary system
comprising two giant stars. Previous efforts, including our own 2009 study,
have largely failed to reach an acceptable agreement between the observations
and current stellar evolution models, preventing us from assessing the status
of the primary. Here we report a revision of the physical properties of the
components incorporating recently published high-precision radial velocity
measurements, and a new detailed chemical analysis providing abundances for
more than 20 elements in both stars. We obtain highly precise (to about 0.3%)
masses of 2.5687 +/- 0.0074 and 2.4828 +/- 0.0067 solar masses, radii of 11.98
+/- 0.57 and 8.83 +/- 0.33 solar radii, effective temperatures of 4970 +/- 50 K
and 5730 +/- 60 K, and independently measured luminosities based on the orbital
parallax (78.7 +/- 4.2 and 72.7 +/- 3.6 solar luminosities). We find an
excellent match to stellar evolution models at the measured composition of
[Fe/H] = -0.04 +/- 0.06. Three different sets of models place the primary star
firmly at the end of the core helium-burning phase (clump), while the secondary
is known to be evolving rapidly across the Hertzprung gap. The measured lithium
abundance, the C/N ratio, and the 12C/13C isotopic carbon abundance ratio,
which change rapidly in the giant phase, are broadly in agreement with
expectations from models. Predictions from tidal theory for the spin rates,
spin-orbit alignment, and other properties do not fare as well, requiring a
40-fold increase in the efficiency of the dissipation mechanisms in order to
match the observations.Comment: 15 pages in emulateapj format, including figures and tables, accepted
for publication in The Astrophysical Journa
Las constituciones de los hospitales y los cuidados enfermeros en la España de los Austrias (siglos xvi-xvii)
El siglo XVI conoce en España un
importante cambio de mentalidad en
lo referente a materia de salud. Las
consecuencias de ello fueron la reforma
de los hospitales y la aplicación de nuevas
formas de curar y cuidar a los enfermos.
Las constituciones elaboradas para estos
nuevos hospitales reflejan, en cierta
medida, estos cambios y asumen las
propuestas que la ciencia médica promueve
desde la literatura especializada. En este
trabajo se estudia la profunda imbricación
existente entre las nuevas constituciones
de hospitales castellanos de fines del siglo
XVI y todo el XVII y los cuidados en
ellos prestados por un personal enfermero
cada vez más formado y especializado.
Por otra parte, el hospital se erige en un
centro diseñado también para tratar el
desequilibrio espiritual que padecen los
enfermos y marginales.The 16th century in Spain knows an
important change of mentality in relation
to health. The consequences of this were the
reform of hospitals and the implementation
of new ways of treatment and of taking
care of sick people. The constitutions
drawn up for these new hospitals reflect,
to some extent, these changes and assume
the proposals promoted by medical science
from specialized literature. This paper
explores the deep existing interweaving
between the new constitutions of Castilian
hospitals at the end of the 16th and 17th
century and the care they provided by
a nursing staff, more and more trained
and specialized. On the other hand, the
hospital stands in a facility designed to
treat the spiritual imbalance that the sick
or the poor tramp in them suffered as a
result of their social marginalization
Implications of mitochondrial dynamics on neurodegeneration and on hypothalamic dysfunction
Mitochondrial dynamics is a term that encompasses the movement of mitochondria along the cytoskeleton, regulation of their architecture, and connectivity mediated by tethering and fusion/fission. The importance of these events in cell physiology and pathology has been partially unraveled with the identification of the genes responsible for the catalysis of mitochondrial fusion and fission. Mutations in two mitochondrial fusion genes (MFN2 and OPA1) cause neurodegenerative diseases, namely Charcot-Marie Tooth type 2A and autosomal dominant optic atrophy. Alterations in mitochondrial dynamics may be involved in the pathophysiology of prevalent neurodegenerative conditions. Moreover, impairment of the activity of mitochondrial fusion proteins dysregulates the function of hypothalamic neurons, leading to alterations in food intake and in energy homeostasis. Here we review selected findings in the field of mitochondrial dynamics and their relevance for neurodegeneration and hypothalamic dysfunction
Using binaries containing giants to constrain theories of stellar and tidal evolution
Investigations of stellar and tidal evolution of binary stars with giant components are rare. In this paper, we will investigate such features in three binary systems for which at least one component is a giant star. As some of these giants seem to be in the blue loop, it is an excellent opportunity to investigate the sensitivity of core overshooting on their location in the HR Diagram. We expect that these characteristics shall serve as an incentive to observers to investigate such kinds of binaries, increasing the accuracy of measurements and the number of systems to test the evolutionary models.
Methods. Prior to performing the study of the circularization and synchronization levels, an analysis of the capability of our stellar evolutionary models to reproduce the observed masses, radii and effective temperatures is carried out. Next, the differential equations of tidal evolution are integrated and the corresponding critical times are compared with the inferred age of the system and with the observed eccentricity and rotational velocities (when available).
Results. We have found good agreement between our stellar models and the astrophysical properties of eta And, V2291 Oph and SZ Cen by adopting a moderate core overshooting amount (alpha(ov) = 0.20). Three mechanisms were used to try to explain the observed levels of circularization and synchronization: the hydrodynamical mechanism, turbulent dissipation and radiative damping. In the cases of eta And and SZ Cen, for which the rotational velocities are available, by assuming solid body rotation for both stars of each system we have found that the theoretical ratio between the rotational velocities V-rotA/V-rotB at the inferred ages are in good agreement with the observational ratios.The Spanish MEC (AYA2006-06375)Peer reviewe
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