844 research outputs found
On the need of the Light Elements Primary Process (LEPP)
Extant chemical evolution models underestimate the Galactic production of Sr,
Y and Zr as well as the Solar System abundances of s-only isotopes with
90<A<130. To solve this problem, an additional (unknown) process has been
invoked, the so-called LEPP (Light Element Primary Process). In this paper we
investigate possible alternative solutions. Basing on Full Network Stellar
evolutionary calculations, we investigate the effects on the Solar System
s-only distribution induced by the inclusion of some commonly ignored physical
processes (e.g. rotation) or by the variation of the treatment of convective
overshoot, mass-loss and the efficiency of nuclear processes. Our main findings
are: 1) at the epoch of the formation of the Solar System, our reference model
produces super-solar abundances for the whole s-only distribution, even in the
range 90<A<130; 2) within errors, the s-only distribution relative to 150Sm is
flat; 3) the s-process contribution of the less massive AGB stars (M<1.5 M_SUN)
as well as of the more massive ones (M>4.0 M_SUN) are negligible; 4) the
inclusion of rotation implies a downward shift of the whole distribution with
an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only
distribution; 5) different prescriptions on convection or mass-loss produce
nearly rigid shifts of the whole distribution. In summary, a variation of the
standard paradigm of AGB nucleosynthesis would allow to reconcile models
predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot
be definitely ruled out, because of the uncertainties still affecting stellar
and Galactic chemical evolution models.Comment: Accepted for publication on Ap
The puzzle of the CNO isotope ratios in AGB carbon stars
Previous determinations of the oxygen isotopic ratios in AGB carbon stars
were at odds with the existing theoretical predictions. We aim to redetermine
the oxygen ratios in these stars using new spectral analysis tools and further
develop discussions on the carbon and nitrogen isotopic ratios in order to
elucidate this problem. Oxygen isotopic ratios were derived from spectra in the
K-band in a sample of galactic AGB carbon stars of different spectral types and
near solar metallicity. Synthetic spectra calculated in LTE with spherical
carbon-rich atmosphere models and updated molecular line lists were used. The
CNO isotope ratios derived in a homogeneous way, were compared with theoretical
predictions for low-mass (1.5-3 M_o) AGB stars computed with the FUNS code
assuming extra mixing both during the RGB and AGB phases. For most of the stars
the 16O/17O/18O ratios derived are in good agreement with theoretical
predictions confirming that, for AGB stars, are established using the values
reached after the FDU according to the initial stellar mass. This fact, as far
as the oxygen isotopic ratios are concerned, leaves little space for the
operation of any extra mixing mechanism during the AGB phase. Nevertheless, for
a few stars with large 16O/17O/18O, the operation of such a mechanism might be
required, although their observed 12C/13C and 14N/15N ratios would be difficult
to reconcile within this scenario. Furthermore, J-type stars tend to have lower
16O/17O ratios than the normal carbon stars, as already indicated in previous
studies. Excluding these peculiar stars, AGB carbon stars occupy the same
region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram,
showing little spread. This reinforces the idea that these grains were probably
formed in low-mass stars during the previous O-rich phases.Comment: Accepted fo publication in A&
The chemical composition of carbon stars: The R-type stars
The aim of this work is to shed some light on the problem of the formation of
carbon stars of R-type from a detailed study of their chemical composition. We
use high-resolution and high signal-to-noise optical spectra of 23 R-type stars
selected from the Hipparcos catalogue. The chemical analysis is made using
spectral synthesis in LTE and state-of-the-art carbon-rich spherical model
atmospheres. We derive their CNO content (including the carbon isotopic ratio),
average metallicity, lithium, and light (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm)
s-element abundances. The observed properties of the stars (galactic
distribution, kinematics, binarity, photometry and luminosity) are also
discussed. Our analysis shows that late-R stars are carbon stars with identical
chemical and observational characteristics than the normal (N-type) AGB carbon
stars. We confirm the results of the sole previous abundance analysis of
early-R stars by Dominy (1984, ApJS, 55, 27), namely: they are carbon stars
with near solar metallicity showing enhanced nitrogen, low carbon isotopic
ratios and no s-element enhancements. In addition, we have found that early-R
stars have Li abundances larger than expected for post RGB tip giants. We also
find that a significant number (aprox. 40 %) of the early-R stars in our sample
are wrongly classified, being probably classical CH stars and normal K giants.
In consequence, we suggest that the number of true R stars is considerably
lower than previously believed. We briefly discuss the different scenarios
proposed for the formation of early-R stars. The mixing of carbon during an
anomalous He-flash is favoured, although no physical mechanism able to trigger
that mixing has been found yet. The origin of these stars still remains a
mystery.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
The role of gravitational supernovae in the Galactic evolution of the Li, Be and B isotopes
The observed Be and B relationships with metallicity clearly support the idea that both elements have a primary origin and that are produced by the same class of objects. Spallation by particles accelerated during gravitational events (SNII, SNIb/c) seems to be a likely origin. We show, in the context of a model of chemical evolution, that it is possible to solve the Li, Be and B abundance puzzle with the yields recently proposed by Ramaty et al. (1997), provided that SNII are unable to significantly accelerate helium nuclei and that different mechanisms are allowed to act simultaneously
Horizons: nuclear astrophysics in the 2020s and beyond
Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics,
which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress
and status of the field, the new open questions that have emerged, and the
tremendous scientific opportunities that have opened up with major advances
in capabilities across an ever growing number of disciplines and subfields that
need to be integrated.We take a holistic view of the field discussing the unique
challenges and opportunities in nuclear astrophysics in regards to science, diversity,
education, and the interdisciplinarity and breadth of the field. Clearly
nuclear astrophysics is a dynamic field with a bright future that is entering a
new era of discovery opportunities.National Science Foundation (NSF) PHY1430152
OISE-1927130ExtreMeMatter Institute EMMI at the GSI Helmholtzzentrumfur Schwerionenforschung in DarmstadtEuropean Cooperation in Science and Technology (COST) CA1611
On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios
Support from the Centre National d’Etudes Spatiales
(CNES), France. This work has made use of data from the European
Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Support of ESO, OCA, and CNES is acknowledged for the AMBRE project. Some of the calculations have been performed
with the high-performance computing facility SIGAMM, hosted
by OCA. CA acknowledges partial support by project PGC2018-095317-B-C21 financed by the MCIN/AEI FEDER ‘Una manera de
hacer Europa’, and by project PID2021-123110NB-I00 financed by
MCIN/AEI /10.13039/501100011033/FEDER, UEUsing a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age versus metallicity or abundance ratios versus age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] versus metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary versus secondary or odd elements) and on the lifetimes of their sources (short-lived versus long-lived ones). The latter property is critical in determining the single- or double- branch behaviour of an elementary abundance ratio in the Solar neighbourhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different time-scales and help determining the site of e.g. the r-process(es). We define the ‘abundance distance’ between the thin and thick disc sequences as an important element for such a separation. We also show how the inside-out evolution of the Milky Way disc leads rather to a single-branch behaviour in other disc regions.Centre National d’Etudes Spatiales
(CNES)Gaia Data Processing and Analysis Consortium DPACEuropean Space Agency
ESACentre National d’Etudes Spatiales
PGC2018-095317-B-C21 CNESMinisterio de Ciencia e Innovación
MICINNAgencia Estatal de Investigación
PID2021-123110NB-I00 AE
Characterisation of Galactic carbon stars and related stars from Gaia-EDR3
We extend here a previous investigation on the characteristics of Galactic
carbon stars using more accurate EDR3 astrometry measurements. Based on a much
larger statistics, we confirm that N- and SC-type carbon stars share a very
similar luminosity function, while the luminosities of J-type stars (Mbol) are
fainter by half a magnitude on average. R-hot type carbon stars have
luminosities throughout the RGB, which favours the hypothesis of an external
origin for their carbon enhancement. Moreover, the kinematic properties of a
significant fraction of the R-hot stars are compatible with the thick-disc
population, in contrast with that of N- and SC-type stars, which would belong
mostly to the thin disk. We also derive the luminosity function of a large
number of Galactic extrinsic and intrinsic (O-rich) S stars and show that the
luminosities of the latter are typically higher than the predicted onset of the
third dredge-up during the AGB for solar metallicity. This result is consistent
with these stars being genuine thermally pulsing AGB stars. On the other hand,
using the so-called Gaia-2MASS diagram, we show that the overwhelming majority
of the carbon stars identified in the LAMOST survey as AGB stars are probably
R-hot and/or CH-type stars. Finally, we report the identification of 2660 new
carbon stars candidates that we identified through their 2MASS photometry,
their Gaia astrometry, and their location in the Gaia-2MASS diagram.Comment: 14 pages, 16 figures, accepted for publication in A&
- …