arXiv subjects

Carlos Abia

Publications and source records attributed to Carlos Abia.

At least 19 recordsLinked to original sources

arXiv2026

Tracing the s-Process: Spectroscopic Insights into Chemical Abundances in O- and C-rich Evolved Stars

Oxygen and carbon-rich AGB stars - and objects directly polluted by them - are excellent laboratories to investigate the nucleosynthesis and mixing processes occurring during the later phases of the of low- and intermediate-mass star evolution. The determination of the abundances of several s-elements is a key tool for constraining theoretical AGB models. This contribution discusses the main results, recent advances, and current problems on this subject.

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arXiv2026

s-process nucleosynthesis in low-mass AGB stars by the $^{13}$C($α$,n)$^{16}$O neutron source

In this review we outline our knowledge on slow neutron captures, concentrating on its main part occurring during the final stages of stellar evolution for low or intermediate-mass stars when they evolve during the Asymptotic Giant Branch, or AGB, stars. We focus our attention on how, in this field, studies passed from a first era of inquiries based on nuclear systematics, to numerical nucleosynthesis computations performed in stellar codes. We then discuss how these last were forced, by observational constraints, to almost abandon, for the synthesis of nuclei between Sr and Pb, the rather naturally activated Ne22 neutron source (operating efficiently at T > 30 keV, and producing a neutron density N_n > 5 10^8 cm^-3). This implied considering the alternative reaction 13C(alpha,n)16O, that can be activated locally after each of the TDU. The mentioned crucial reaction occurs at T< 8 keV, in the time intervals separating two subsequent thermal pulses (TP). The layers where 13C(alpha,n)16O operates are characterized by a radiative equilibrium and their low temperature also yields low values for the neutron density (N_n < 10^7 cm^-3).

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arXiv2023

On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios

Using 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 vs metallicity or abundance ratios vs 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] vs 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 vs secondary or odd elements) and on the lifetimes of their sources (short-lived vs long-lived ones). The latter property is critical in determining the single- or double- branch behavior of an elementary abundance ratio in the Solar neighborhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different timescales 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 behavior in other disc regions.

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arXiv2023

The carbon star mystery: forty years later

In 1981 Icko Iben Jr published a paper entitled 'The carbon star mystery: why do the low mass ones become such, and where have all the high mass ones gone?', where he discussed the discrepancy between the theoretical expectation and its observational counterpart about the luminosity function of AGB carbon stars. After more than 40 years, our understanding of this longstanding problem is greatly improved, also thanks to more refined stellar models and a growing amount of observational constraints. In this paper we review the state of the art of these studies and we briefly illustrate the future perspectives.

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arXiv2022

Sub-stellar engulfment by a main sequence star: where is the lithium?

In this work, we study whether the engulfment of a brown dwarf (BD) by a solar-like main-sequence (MS) star can significantly alter the structure of the star and the Li content on its surface. We perform 3D Smoothed Particle Hydrodynamics simulations of the engulfment of a BD with masses 0.01 and 0.019 Msun, onto an MS star of 1 Msun and solar composition, in three different scenarios: a head-on collision, a grazing collision, and a merger. We study the dynamics of the interaction in detail, and the relevance of the type of interaction and the mass of the BD on the final fate of the sub-stellar object and the host star in terms of mass loss of the system, angular momentum transfer, and changes in the Li abundance in the surface of the host star. We found that most of the BD mass is diluted in the denser region of the MS star. Only in the merger scenario a significant fraction (40%) of the BD material would remain in the outer layers. We find a clear increase in the surface rotational velocity of the host star after the interaction, ranging between 25 km/s (grazing collision) to 50 km/s (merger). We also find a significant mass loss from the system (1e-4 - 1e-3 Msun) due to the engulfment, which in the case of the merger, may form a circumstellar disk-like structure. Assuming that neither the depth of the convective envelope of the host star nor its mass content are modified during the interaction, a small change in the surface Li abundance in the head-on and grazing collisions is found. However, in the merger we find large Li enhancements, by factors 20-30, depending on the BD mass. Some of these features could be detected observationally in the host star provide they remain long enough time.

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arXiv2022

Evidence of Deep Mixing in IRS 7, a Cool Massive Supergiant Member of the Galactic Nuclear Star Cluster

The center of the Milky Way contains stellar populations spanning a range in age and metallicity, with a recent star formation burst producing young and massive stars. Chemical abundances in the most luminous stellar member of the Nuclear Star Cluster (NSC), IRS 7, are presented for $^{19}$F, $^{12}$C, $^{13}$C, $^{14}$N, $^{16}$O, $^{17}$O, and Fe from an LTE analysis based on spherical modeling and radiative transfer with a 25M$_{\odot}$ model atmosphere, whose chemistry was tailored to the derived photospheric abundances. We find IRS 7 to be depleted heavily in both $^{12}$C (~-0.8 dex) and $^{16}$O (~-0.4 dex), while exhibiting an extremely enhanced $^{14}$N abundance (~+1.1 dex), which are isotopic signatures of the deep mixing of CNO-cycled material to the stellar surface. The $^{19}$F abundance is also heavily depleted by ~1 dex relative to the baseline fluorine of the Nuclear Star Cluster, providing evidence that fluorine along with carbon constrain the nature of the deep mixing in this very luminous supergiant. The abundances of the minor isotopes $^{13}$C and $^{17}$O are also derived, with ratios of $^{12}$C/$^{13}$C~5.3 and $^{16}$O/$^{17}$O~525. The derived abundances for IRS 7, in conjunction with previous abundance results for massive stars in the NSC, are compared with rotating and non-rotating models of massive stars and it is found that the IRS 7 abundances overall follow the behavior predicted by stellar models. The depleted fluorine abundance in IRS 7 illustrates, for the first time, the potential of using the $^{19}$F abundance as a mixing probe in luminous red giants.

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arXiv2022

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.

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arXiv2019

Fluorine Abundances in the Galactic Disk

The chemical evolution of fluorine is investigated in a sample of Milky Way red giantstars that span a significant range in metallicity from [Fe/H] $\sim$ -1.3 to 0.0 dex. Fluorine abundances are derived from vibration-rotation lines of HF in high-resolution infraredspectra near $λ$ 2.335 $μ$m. The red giants are members of the thin and thick disk / halo,with two stars being likely members of the outer disk Monoceros overdensity. At lowermetallicities, with [Fe/H]<-0.4 to -0.5, the abundance of F varies as a primary element with respect to the Fe abundance, with a constant subsolar value of [F/Fe] $\sim$ -0.3 to -0.4 dex. At larger metallicities, however, [F/Fe] increases rapidly with [Fe/H] anddisplays a near-secondary behavior with respect to Fe. Comparisons with various models of chemical evolution suggest that in the low-metallicity regime (dominated hereby thick disk stars), a primary evolution of $^{19}$F with Fe, with a subsolar [F/Fe] valuethat roughly matches the observed plateau can be reproduced by a model incorporatingneutrino nucleosynthesis in the aftermath of the core collapse in supernovae of type II (SN II). A primary behavior for [F/Fe] at low metallicity is also observed for a model including rapid rotating low-metallicity massive stars but this overproduces [F/Fe] atlow metallicity. The thick disk red giants in our sample span a large range of galactocentric distance (Rg $\sim$ 6--13.7 kpc), yet display a $\sim$constant value of [F/Fe], indicating a very flat gradient (with a slope of 0.02 $\pm$ 0.03 dex/kpc) of this elemental ratio over asignificant portion of the Galaxy having|Z|>300 pc away from the Galaxy mid-plane.

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arXiv2019

Fluorine Abundances in the Globular Cluster M 4

We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of the globular cluster M 4, as well as six red giants of the globular cluster $ω$ Centauri. The chemical abundances were calculated in LTE via spectral synthesis. The spectra analyzed are high-resolution spectra obtained in the near-infrared region around $λ$2.3$μ$m with the Phoenix spectrograph on the 8.1m Gemini South Telescope, the IGRINS spectrograph on the McDonald Observatory 2.7m Telescope, and the CRIRES spectrograph on the ESO 8.2m Very Large Telescope. The results indicate a significant reduction in the fluorine abundances when compared to previous values from the literature for M 4 and $ω$ Centauri, due to a downward revision in the excitation potentials of the HF(1-0) R9 line used in the analysis. The fluorine abundances obtained for the M 4 red giants are found to be anti-correlated with those of Na, following the typical pattern of abundance variations seen in globular clusters between distinct stellar populations. In M 4, as the Na abundance increases by $\sim$+0.4 dex, the F abundance decreases by $\sim$-0.2 dex. A comparison with abundance predictions from two sets of stellar evolution models finds that the models predict somewhat less F depletion ($\sim$-0.1 dex) for the same increase of +0.4 dex in Na.

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arXiv2016

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.

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arXiv2015

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 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.

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arXiv2011

Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database

By using updated stellar low mass stars models, we can systematically investigate the nucleosynthesis processes occurring in AGB stars, when these objects experience recurrent thermal pulses and third dredge-up episodes. In this paper we present the database dedicated to the nucleosynthesis of AGB stars: the FRUITY (FRANEC Repository of Updated Isotopic Tables & Yields) database. An interactive web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 < M/Msun < 3.0 and metallicities 1e-3 < Z < 2e-2, is discussed here. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parametrization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and [Pb/hs] ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the third dredge-up efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find a good agreement with observations.

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arXiv2009

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.

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arXiv2004

Magnesium abundances in mildly metal-poor stars from different indicators

We present magnesium abundances derived from high resolution spectra using several Mg I and two high excitation Mg II lines for 19 metal-poor stars with [Fe/H] values between -1.1 and +0.2. The main goal is to search for systematic differences in the derived abundances between the two ionisation state lines. Our analysis shows that the one-dimensional LTE and N-LTE study finds a very good agreement between these features. The [Mg/Fe] vs. [Fe/H] relationship derived, despite the small sample of stars, is also in agreement with the classical figure of increasing [Mg/Fe] with decreasing metallicity. We find a significant scatter however, in the [Mg/Fe] ratio at [Fe/H]$\sim -0.6$ which is currently explained as a consequence of the overlap at this metallicity of thick and thin disk stars, which were probably formed from material with different nucleosynthesis histories. We speculate on the possible consequences of the agreement found between Mg I and Mg II lines on the very well known oxygen problem in metal-poor stars. We also study the [O/Mg] ratio in the sample stars using oxygen abundances from the literature and find that the current observations and nucleosynthetic predictions from type II supernovae disagree. We briefly discuss some alternatives to solve this discrepancy.

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arXiv2002

Lithium abundances in the atmospheres of SLR C--giants WZ Cas and WX Cyg from resonance and subordinate Li I lines

Lithium abundances in the atmospheres of the super Li-rich C-giants WZ Cas and WX Cyg are derived by the spectral synthesis technique {\bf using} the Li I resonance line at λ670.8 nm and {\bf three subordinate} lines at λλ812.6, 610.4 and 497.2 nm. The differences between the Li abundances derived from the λ670.8 nm line and the λλ497.2, 812.6 nm lines do not exceed $\pm$ 0.5 dex. The lithium line at \la610.4 nm provides typically lower abundances than the resonance line (by $\approx$ 1 dex). The mean LTE and NLTE Li abundances from three Li I lines (excluding \la610.4 nm) are 4.7, 4.9 for WZ Cas, and 4.6, 4.8 for WX Cyg, respectively.

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arXiv2000

The role of Z=0 AGB stars on the early chemical enrichment

We present theoretical evolutionary models for Z=0 stars in the mass range 4< M/Mo <8 from the pre-main sequence up to the AGB phase. Contrary to previous calculations we found that these stars develop normal thermal pulses and third dredge-up episodes. Special attention is devoted to analyze the chemical enrichment in the envelope due to the above mechanisms. As a consequence, we show that these stars become carbon and nitrogen rich. Using different IMFs proposed in the literature for the Population III stars, we study their contribution to the pre-galactic chemical enrichment. It is found that Z=0 AGB stars could significantly contribute to 7Li, 12C and 14N and produce extreme non-solar 24Mg/25Mg/26Mg ratios. However, the net contribution is very sensitive to the IMF adopted and to the fraction of primordial matter which goes into stars.

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arXiv2000

The Chemical Composition of Carbon Stars II: The J-type stars

Abundances of Li, heavy elements and carbon isotope ratios have been measured in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly normal. Tc is not present in most of them, although upper limits have been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type carbon stars. The Rb abundances, obtained from the resonance 7800 Å Rb I line, are surprisingly low, probably due to strong non-LTE effects in the formation of this line in cool carbon-rich stars. Lithium and $^{13}$C are found to be enhanced in all the stars. These results together with the nitrogen abundances and oxygen isotope ratios measured by Lambert et al. (1986) and Harris et al. (1987) are used to discuss the origin of J-stars. The luminosity and variability class of the stars studied would indicate that they are low mass (M$\leq 2-3$ M$_\odot$), less evolved objects than the normal carbon stars, although the presence of some luminous (M$_{bol}<-5.5$) J-stars in our galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence of at least two types of J-stars, with different formation scenarios depending upon the initial mass of the parent star. Standard evolutionary AGB models are difficult to reconcile with all the observed chemical characteristics. In fact, they suggest the existence of an extra-mixing mechanism which transports material from the convective envelope down to hotter regions where some nuclear burning occurs. This mechanism would act preferably on the early-AGB phase in low-mass stars. Mixing at the He-core flash and the binary system hypothesis are also discussed as alternatives to the above scenario.

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arXiv1999

The chemical composition of the rare J-type carbon stars

Abundances of lithium, heavy elements and carbon isotope ratios have been derived in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly solar. Tc is not present in most of them. The Rb abundances, obtained from the resonance 7800 Å RbI line, are surprisingly low, probably due to stroong non-LTE effects. Lithium and $^{13}$C are found to be enhanced in all the stars. These results are used to discuss the origin of J-stars.

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