101 research outputs found
Nota previa sobre la presencia de Arqueociátidos en los alrededores de Urda (Montes de Toledo orientales)
It is stated the presence of Archaeocyathids concerning the autcrop calcareous formation in the environs of Urda. Eastern Montes of Toledo, this enabk un to assign a lov Cambrian age to these Urda limestone
Los conceptos en la resolución de problemas de física "bien estructurados": aspectos identificativos y aspectos formales
Este trabajo trata de analizar la resolución de problemas cerrados de física general del tipo de los que se proponen habitualmente en los libros de texto. Consideramos que, al resolver uno de esos problemas, se debe construir una representación del mismo y para ello deben realizarse una serie de tareas que se pueden dividir en dos tipos: de identificación y de competencia lógica. Se han analizado los errores en ambos y se ha concluido que una de las principales causas de los errores en la resolución de este tipo de problemas es la falta de significado más que el uso de un significado propio del resolvente. En la fundamentación teórica se han utilizado ideas de procedencias muy diversas: de Neimark, de Van Dijk y Kintsch y de Eco, entre otros.This paper deals with the solution to problems of General Physics of the type normally proposed in textbooks. It is our opinion that when solving one of these problems we must build a representation of the problem itself. In order to do that we must perform a series of tasks that can be split into two types: of identification and of logical competence. We have analysed the mistakes in both. We have concluded that one of the main reasons for the mistakes in solving this type of problems is the lack of meaning, rather than the use of the own meaning attached by the solver. In the theoretical foundation we have used ideas of different origin: Neimark, van Dijk and Eco, among others
Barium and related stars, and their white-dwarf companions. III. The masses of the white dwarfs
Masses are one of the most difficult stellar properties to measure. In the
case of the white-dwarf companions of Barium stars, the situation is worse.
These stars are dim, cool, and difficult to observe via direct methods.
However, Ba stars were polluted by the Asymptotic Giant Branch progenitors of
these WDs with matter rich in heavy elements, and the properties of their WD
companions contain key information about binary interaction processes involving
AGB stars and about the slow-neutron-capture(s)-process of nucleosynthesis. We
aim to determine accurate and assumption-free masses for the WD companions of
as many Ba stars as possible. We want to provide new observational constraints
that can help us learn about the formation and evolution of these
post-interaction binary systems and about the nucleosynthesis processes that
took place in the interiors of their AGB progenitors. We combined archival
radial-velocity data with Hipparcos and Gaia astrometry using the software
package orvara, a code designed to simultaneously fit a single Keplerian model
to any combination of these types of data using a parallel-tempering Markov
chain Monte Carlo method. We adopted Gaussian priors for the Ba star masses and
for the parallaxes, and assumed uninformative priors for the orbital elements
and the WD masses. We determined new orbital inclinations and companion masses
for 60 Ba star systems, including a couple of new orbits and several improved
orbits for the longest-period systems. We also unravelled a triple system that
was not known before and constrained the orbits and the masses of the two
companions. (Continued in the manuscript)Comment: Accepted for publication in Astronomy & Astrophysic
Supernovae Origin for the Low-Latitude-Intermediate-Velocity Arch and the North-Celestial-Pole Loop
Supernova explosions attributed to the unseen companion in several binary
systems identified by the Third Gaia Data Release (Gaia DR3) may be responsible
for a number of well-known and well-studied features in the radio sky,
including the Low-Latitude-Intermediate-Velocity Arch and the
North-Celestial-Pole Loop. Slices from the Longitude-Latitude-Velocity data
cube of the -21-cm galactic neutral hydrogen HI4PI survey (HI4PI
Collaboration et al. 2016) show multiple signatures of an expanding shell. The
source of this expansion, which includes the Low-Latitude-Intermediate-Velocity
Arch on the approaching side, may be the neutron star candidate in the Gaia DR3
1093757200530267520 binary. If we make the simplifying assumptions that the
expansion of the cavity is uniform and spherically symmetric, then the
explosion took place about 700,000 years ago. The momentum is in reasonable
agreement with recent model estimates for a supernova this old. The HI on the
receding side of this cavity is interacting with the gas approaching us on the
near side of a second cavity. The North-Celestial-Pole Loop appears to be
located at the intersection of these two expanding features. The neutron star
candidate in the Gaia DR3 1144019690966028928 binary may be (in part)
responsible for this cavity. Explosions from other candidates may account for
the observed elongation along the line of sight of this second cavity. We can
use the primary star in these binaries to anchor the distances to the
Low-Latitude-Intermediate-Velocity Arch and North-Celestial-Pole Loop, which
are about 167 and about 220 pc, respectively.Comment: Published in The Astrophysical Journa
Hertzsprung-Russell diagram and mass distribution of barium stars
With the availability of parallaxes provided by the Tycho-Gaia Astrometric
Solution, it is possible to construct the Hertzsprung-Russell diagram (HRD) of
barium and related stars with unprecedented accuracy. A direct result from the
derived HRD is that subgiant CH stars occupy the same region as barium dwarfs,
contrary to what their designations imply. By comparing the position of barium
stars in the HRD with STAREVOL evolutionary tracks, it is possible to evaluate
their masses, provided the metallicity is known. We used an average metallicity
[Fe/H] = -0.25 and derived the mass distribution of barium giants. The
distribution peaks around 2.5 Msun with a tail at higher masses up to 4.5 Msun.
This peak is also seen in the mass distribution of a sample of normal K and M
giants used for comparison and is associated with stars located in the red
clump. When we compare these mass distributions, we see a deficit of low-mass
(1 - 2 Msun) barium giants. This is probably because low-mass stars reach large
radii at the tip of the red giant branch, which may have resulted in an early
binary interaction. Among barium giants, the high-mass tail is however
dominated by stars with barium indices of less than unity, based on a visual
inspection of the barium spectral line; that is, these stars have a very
moderate barium line strength. We believe that these stars are not genuine
barium giants, but rather bright giants, or supergiants, where the barium lines
are strengthened because of a positive luminosity effect. Moreover, contrary to
previous claims, we do not see differences between the mass distributions of
mild and strong barium giants.Comment: 14 pages, 17 figure
Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars
Among Carbon-Enhanced Metal-Poor (CEMP) stars, some are found to be enriched
in s-process elements (CEMP-s), in r-process elements (CEMP-r) or in both s-
and r-process elements (CEMP-rs). The origin of the abundance differences
between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that
the i-process, whose site still remains to be identified, could better
reproduce CEMP-rs abundances than the s-process. We analyze high-resolution
spectra of 25 metal-poor stars, observed with the high-resolution HERMES
spectrograph mounted on the Mercator telescope, La Palma, or with the UVES/VLT
and HIRES/KECK spectrographs. We propose a new, robust classification method
for CEMP-s and CEMP-rs stars using eight heavy element abundances. The
abundance profiles of CEMP-s and CEMP-rs stars are derived and there appears to
be an abundance continuum between the two stellar classes. CEMP-rs stars
present most of the characteristics of extrinsic stars such as CEMP-s, CH,
Barium and extrinsic S stars, with an even larger binarity rate among CEMP-rs
stars than among CEMP-s stars. Stellar evolutionary tracks of an enhanced
carbon composition (consistent with our abundance determinations) are necessary
to explain the position of CEMP-s and CEMP-rs stars in the HR diagram using
Gaia DR2 parallaxes; they are found to lie mostly on the RGB. CEMP-rs stars can
be explained as being polluted by a low-mass, low-metallicity TP-AGB companion
experiencing i-process nucleosynthesis after proton ingestion during its first
convective thermal pulses. The global fitting of our i-process models to
CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. As
such, CEMP-rs stars could be renamed as CEMP-sr stars, since they represent a
particular manifestation of the s-process at low-metallicities. For these
objects a call for an exotic i-process site may not necessarily be required
anymore.Comment: 35 pages, 26 figures, Accepted for publication in A&A, Final versio
Barium & related stars and their white-dwarf companions II. Main-sequence and subgiant stars
Barium (Ba) dwarfs and CH subgiants are the less-evolved analogues of Ba and
CH giants. They are F- to G-type main-sequence stars polluted with heavy
elements by a binary companion when the latter was on the Asymptotic Giant
Branch (AGB). This companion is now a white dwarf that in most cases cannot be
directly detected. We present a large systematic study of 60 objects classified
as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from
HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL
and CORALIE, we determine the orbital parameters of 27 systems. We also derive
their masses by comparing their location in the Hertzsprung-Russell diagram
with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at
different evolutionary stages and have similar metallicities, despite their
different names. Additionally, Ba giants appear significantly more massive than
their main-sequence analogues. This is likely due to observational biases
against the detection of hotter main-sequence post-mass-transfer objects.
Combining our spectroscopic orbits with the Hipparcos astrometric data, we
derive the orbital inclinations and the mass of the WD companion for four
systems. Since this cannot be done for all systems in our sample yet (but
should be with upcoming Gaia data releases), we also analyse the mass-function
distribution of our binaries. We can model this distribution with very narrow
mass distributions for the two components and random orbital orientation on the
sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital
evolution of low-mass Ba systems can be affected by a second phase of
interaction along the Red Giant Branch of the Ba star, impacting on the
eccentricities and periods of the giants.Comment: Accepted for publication in A&A on the 5th of April, 201
Does the i-process operate at nearly solar metallicity?
A sample of 895 s-process-rich candidates has been found among the 454180
giant stars surveyed by LAMOST at low spectral resolution (R~1800). In a
previous study, taking advantage of the higher resolution (R~86 000) offered by
the the HERMES-Mercator spectrograph, we performed the re-analysis of 15 among
the brightest stars of this sample. Among these 15 program stars, having
close-to-solar metallicities, 11 showed mild to strong heavy element
overabundances. The nucleosynthesis process(es) at the origin of these
overabundances were however not questioned in our former study. We derive the
abundances in s- and r-process elements of the 15 targets in order to
investigate whether some stars also show an i-process signature, as sometimes
found in their lower metallicity counterparts (namely, the Carbon-Enhanced
Metal-Poor (CEMP)-rs stars). Abundances are derived from the high-resolution
HERMES spectra for Pr, Nd, Sm, and Eu, using the TURBOSPECTRUM radiative
transfer LTE code with MARCS model atmospheres. Using the new classification
scheme proposed in our recent study we find that two stars show overabundances
in both s- and r-process elements well above the level expected from the
Galactic chemical evolution, an analogous situation to the one of CEMP-rs stars
at lower metallicities. We compare the abundances of the most enriched stars
with the nucleosynthetic predictions from the STAREVOL stellar evolutionary
code and find abundances compatible with an i-process occurring in AGB stars.
Despite a larger number of heavy elements to characterize the enrichment
pattern, the limit between CEMP-s and CEMP-rs stars remains fuzzy. It is
however interesting to note that an increasing number of extrinsic stars are
found to have abundances better reproduced by an i-process pattern even at
close-to-solar metallicities.Comment: Accepted for publication in A&A, 9 pages, 9 figures including the two
in appendi
A neutron star candidate in the long-period binary 56 UMa
56 UMa is a wide binary system that contains a chemically peculiar red giant
and a faint companion. Due to its surface chemical abundances, the red giant
was classified as a barium (Ba) star. This implies that the companion has to be
a white dwarf, since Ba stars form when mass is transferred to them from an
s-process rich Asymptotic Giant Branch (AGB) star. However, in the case of 56
UMa, the companion might be too massive to be the progeny of an AGB star that
efficiently produced s-process elements such as barium. In this Letter, we
revisit the orbital parameters of the system and perform a full spectral
analysis with the goal of investigating the Ba-star classification of the giant
and unravelling the nature of its faint companion. We combined radial-velocity
and astrometric data to refine the orbital parameters of the system, including
the orbital inclination and the companion mass. Then, we re-determined the
stellar parameters of the giant and its chemical abundances using
high-resolution HERMES spectra. Finally, we investigated the morphology of the
interstellar gas in the vicinity of the system. The faint component in 56 UMa
has a mass of M, which, together with the mixed s+r
abundance profile of the red giant, confirms that the giant is not a standard
barium star. Additionally, the clear identification of a cavity surrounding 56
UMa could indicate that a supernova explosion occurred about 10 5 years ago in
the system, suggesting that the faint companion might be a neutron star.
However, finding an evolutionary scenario that explains all the observables is
not trivial, so we discuss different possible configurations of the system and
their respective merits.Comment: Accepted for publication in A&A
Multi-technique investigation of the binary fraction among A-F type candidate hybrid variable stars discovered by Kepler
Hundreds of candidate hybrid pulsators of intermediate type A-F were revealed by the recent space missions. Hybrid pulsators allow to study the full stellar interiors, where p- and g-modes are simultaneously excited. The true hybrid stars must be identified since other processes, due to stellar multiplicity or rotation, might explain the presence of (some) low frequencies observed in their periodograms. We measured the radial velocities of 50 candidate Delta Sct - Gamma Dor hybrid stars from the Kepler mission with the Hermes/Ace spectrographs over a span of months to years. We aim to derive the fraction of binary and multiple systems and to provide an independent and homogeneous determination of the atmospheric properties and vsini for all targets. The objective is to identify the physical cause of the low frequencies. We computed 1-D cross-correlation functions (CCFs) in order to find the best parameters in terms of the number of components, spectral type and vsini for each target. Radial velocities were measured from spectrum synthesis and by using a 2-D cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. We report on the analysis of 478 high-resolution Hermes and 41 Ace spectra of A/F-type candidate hybrid pulsators from the Kepler field. We determined their radial velocities, projected rotational velocities, atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven new systems. Three long-period preliminary orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27% in our sample of candidate hybrid stars
- …