scholarly journals Li and CNO isotopes from magnetically induced extra-mixing in evolved stars

2009 ◽  
Vol 5 (S268) ◽  
pp. 425-426
Author(s):  
Sara Palmerini ◽  
Maurizio Busso ◽  
Roald Guandalini ◽  
Enrico Maiorca
Keyword(s):  
Red Giants ◽  
Agb Stars ◽  
Low Mass Stars ◽  
Physical Mechanisms ◽  
Convective Mixing ◽  
Evolved Stars ◽  
Intermediate Mass Stars ◽  
Wide Range ◽  
Low Mass ◽  
Fast Transport

AbstractEvolved low mass stars (LMS) contribute not only to the synthesis of s-process nuclei, but also to modifications in the isotopic mix of light elements (Li and CNO especially), induced by proton captures. In particular, RGB and AGB stars show a wide range of Li abundances. This spread is currently attributed to deep phenomena of non-convective mixing. These processes can, in principle, either produce or destroy Li, depending on their velocity. This is due to the fact that Li production requires preserving the unstable 7Be, which has a half-life of only 53 days. Physical mechanisms devised so far to explain the existence of deep mixing in low mass stars generally fail in accounting for fast transport and in avoiding 7Be destruction; on the contrary, this is easily obtained in Intermediate Mass Stars, where Hot Bottom Burning can occur. However, as Li-rich low-mass red giants do exist, we propose here a scenario where both production and destruction of Li are possible in LMS, thanks to the buoyancy of magnetized parcels of processed matter, traveling from the H shell to the envelope at different speeds (depending on their size). Consequences of this transport for CNO nuclei are also discussed.

scholarly journals Low Mass Stars or Intermediate Mass Stars? The Stellar Origin of Presolar Oxide Grains Revealed by Their Isotopic Composition

2021 ◽  
Vol 7 ◽  
Author(s):  
S. Palmerini ◽  
S. Cristallo ◽  
M. Busso ◽  
M. La Cognata ◽  
M. L. Sergi ◽  
...  
Keyword(s):  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Low Mass Stars ◽  
Stellar Nucleosynthesis ◽  
Intermediate Mass Stars ◽  
Low Mass ◽  
Oxygen Isotopic ◽  
Red Giant ◽  
Group 2 ◽  
Group 1

Among presolar grains, oxide ones are made of oxygen, aluminum, and a small fraction of magnesium, produced by the 26Al decay. The largest part of presolar oxide grains belong to the so-called group 1 and 2, which have been suggested to form in Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, respectively. However, standard stellar nucleosynthesis models cannot account for the 17O/16O, 18O/16O, and 26Al/27Al values recorded in those grains. Hence, for more than 20 years, the occurrence of mixing phenomena coupled with stellar nucleosynthesis have been suggested to account for this peculiar isotopic mix. Nowadays, models of massive AGB stars experiencing Hot Bottom Burning or low mass AGB stars where Cool Bottom Process, or another kind of extra-mixing, is at play, nicely fit the oxygen isotopic mix of group 2 oxide grains. The largest values of the 26Al/27Al ratio seem somewhat more difficult to account for.

scholarly journals s-Process Enrichment in Low-Mass Agb Stars

1989 ◽  
Vol 106 ◽  
pp. 176-195 ◽  
Author(s):  
R. Gallino
Keyword(s):  
Solar System ◽  
Massive Stars ◽  
Agb Stars ◽  
Intermediate Mass ◽  
Low Mass Stars ◽  
Temporal Behaviour ◽  
Physical Conditions ◽  
Intermediate Mass Stars ◽  
Low Mass ◽  
Main Component

AbstractAfter a brief description of the developments of the theory of s-process nucleosynthesis, the difficulties recently encountered in envisaging reliable astrophysical conditions for obtaining a solar-system distribution of s-isotopes are discussed. In particular, while the reaction 22Ne(α, n)25Mg may account for the nucleosynthesis of the weak s-component in massive stars, it fails to reproduce the main s-component in intermediate mass stars. The efficiency of the alternative reaction 13C(α, n)160 occurring in low mass stars during recurring thermal instabilities of the He shell is then analyzed. It is shown that, contrary to previous expectations, the 13C source well reproduces the main component, provided that realistic physical conditions are assumed for the temporal behaviour of the pulse and the effect of the light n-absorbers (especially 12C) is properly taken into account. The results satisfactorily compare with the constraints of the classical s-analysis. Key observational evidences also appear to be in agreement with this scenario.

scholarly journals Red giant stars: from mixed modes to angular momentum

2019 ◽  
Vol 82 ◽  
pp. 189-211
Author(s):  
K. Belkacem
Keyword(s):  
Angular Momentum ◽  
Current Knowledge ◽  
Red Giants ◽  
Low Mass Stars ◽  
Giant Stars ◽  
Physical Mechanisms ◽  
Star Evolution ◽  
Low Mass ◽  
Mixed Modes ◽  
Red Giant

Solar-like oscillations are ubiquitous to low-mass stars from the main-sequence to the red-giant branch as demonstrated by the space-borne missions CoRoT and Kepler. Understanding the physical mechanisms governing their amplitudes as well as their behavior along with the star evolution is a prerequisite for interpreting the wealth of seismic data and for inferring stellar internal structure. In this paper, I discuss our current knowledge of mode amplitudes with particular emphasis on non-radial modes in red giants (hereafter mixed modes). Then, I will show how these modes permit to unveil the rotation of the inner-most layers of low-mass stars and how they put stringent constraints on the redistribution of angular momentum.

scholarly journals Nucleosynthesis of Light-Element Isotopes in Evolved Stars Experiencing Extended Mixing

2009 ◽  
Vol 26 (3) ◽  
pp. 161-167 ◽  
Author(s):  
S. Palmerini ◽  
M. Busso ◽  
E. Maiorca ◽  
R. Guandalini
Keyword(s):  
Mass Transport ◽  
Isotopic Composition ◽  
Light Element ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Agb Stars ◽  
Intermediate Mass ◽  
Evolved Stars ◽  
Magnetic Buoyancy ◽  
Slow Mixing

AbstractWe present computations of nucleosynthesis in red giants and Asymptotic Giant Branch (AGB) stars of Population I experiencing extended mixing. The assumed physical cause for mass transport is the buoyancy of magnetized structures, according to recent suggestions. The peculiar property of such a mechanism is to allow for both fast and slow mixing phenomena, as required for reproducing the spread in Li abundances displayed by red giants and as discussed in an accompanying paper. We explore here the effects of this kind of mass transport on CNO and intermediate-mass nuclei and compare the results with the available evidence from evolved red giants and from the isotopic composition of presolar grains of AGB origin. It is found that a good general accord exists between predictions and measurements; in this framework we also show which type of observational data best constrains the various parameters. We conclude that magnetic buoyancy, allowing for mixing at rather different speeds, can be an interesting scenario to explore for explaining together the abundances of CNO nuclei and of Li.

Magnetic fields in PNe and other evolved low-mass and intermediate-mass stars

2016 ◽  
Vol 12 (S323) ◽  
pp. 136-140
Author(s):  
Laurence Sabin ◽  
Qizhou Zhang ◽  
Gregg A. Wade ◽  
Agnès Lèbre ◽  
Roberto Vázquez
Keyword(s):  
Magnetic Fields ◽  
Planetary Nebulae ◽  
Agb Stars ◽  
Intermediate Mass ◽  
Efficient Mechanism ◽  
Intermediate Mass Stars ◽  
Low Mass

AbstractMagnetic fields are likely to be an efficient mechanism which can affect evolved intermediate mass stars (i.e. post-AGB stars and planetary nebulae) in different ways such as via the shaping of their envelope. However, observational probes for the presence of those fields are still scarce. I will present a summary of the works, including those from our group, on the detection and measurement of magnetic fields in various evolved objects.

scholarly journals Period spacings in red giants

2018 ◽  
Vol 618 ◽  
pp. A109 ◽  
Author(s):  
B. Mosser ◽  
C. Gehan ◽  
K. Belkacem ◽  
R. Samadi ◽  
E. Michel ◽  
...  
Keyword(s):  
Asymptotic Expansion ◽  
Stellar Evolution ◽  
Mixed Mode ◽  
Frequency Resolution ◽  
Red Giants ◽  
Large Set ◽  
Low Mass Stars ◽  
Mode Pattern ◽  
Low Mass ◽  
Mixed Modes

Context. Oscillation modes with a mixed character, as observed in evolved low-mass stars, are highly sensitive to the physical properties of the innermost regions. Measuring their properties is therefore extremely important to probe the core, but requires some care, due to the complexity of the mixed-mode pattern. Aims. The aim of this work is to provide a consistent description of the mixed-mode pattern of low-mass stars, based on the asymptotic expansion. We also study the variation of the gravity offset εg with stellar evolution. Methods. We revisit previous works about mixed modes in red giants and empirically test how period spacings, rotational splittings, mixed-mode widths, and heights can be estimated in a consistent view, based on the properties of the mode inertia ratios. Results. From the asymptotic fit of the mixed-mode pattern of a large set of red giants at various evolutionary stages, we derive unbiased and precise asymptotic parameters. As the asymptotic expansion of gravity modes is verified with a precision close to the frequency resolution for stars on the red giant branch (10−4 in relative values), we can derive accurate values of the asymptotic parameters. We decipher the complex pattern in a rapidly rotating star, and explain how asymmetrical splittings can be inferred. We also revisit the stellar inclinations in two open clusters, NGC 6819 and NGC 6791: our results show that the stellar inclinations in these clusters do not have privileged orientation in the sky. The variation of the asymptotic gravity offset with stellar evolution is investigated in detail. We also derive generic properties that explain under which conditions mixed modes can be observed.

scholarly journals A Relative Age Test of Pre-Main Sequence Evolutionary Models Based on the Young Quadruple System GG Tauri

2001 ◽  
Vol 200 ◽  
pp. 464-467
Author(s):  
Russel J. White
Keyword(s):  
Spectral Type ◽  
Temperature Scale ◽  
Main Sequence ◽  
Evolutionary Models ◽  
Low Mass Stars ◽  
Relative Age ◽  
Wide Range ◽  
Quadruple System ◽  
Low Mass

The components of the young hierarchical quadruple GG Tau, which span a wide range in spectral type (K7 – M7.5), are used to test both evolutionary models and the temperature scale for very young, low mass stars under the assumption of coeval formation. Of the evolutionary models tested which extend into the substellar regime, those of Baraffe et al. yield the most consistent ages when combined with a temperature scale intermediate between that of dwarfs and giants. The Palla & Stahler and Siess et al. models are also capable of yielding a coeval age down to their lowest mass (0.1 M⊙). These latter two models, which extend to much higher masses than the Baraffe et al. models, agree reasonably well with the Baraffe et al. models at 1.0 M⊙ and thus could be combined to construct a set evolutionary models that extends from Substellar to several solar masses.

scholarly journals A new method to identify subclasses among AGB stars using Gaia and 2MASS photometry

2018 ◽  
Vol 616 ◽  
pp. L13 ◽  
Author(s):  
T. Lebzelter ◽  
N. Mowlavi ◽  
P. Marigo ◽  
G. Pastorelli ◽  
M. Trabucchi ◽  
...  
Keyword(s):  
Stellar Population ◽  
Chemical Properties ◽  
Large Magellanic Cloud ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Agb Stars ◽  
Population Synthesis ◽  
Stellar Population Synthesis ◽  
Low Mass ◽  
Spectral Ranges

Aims. We explore the wealth of high-quality photometric data provided by data release 2 (DR2) of the Gaia mission for long-period variables (LPVs) in the Large Magellanic Cloud (LMC). Our goal is to identify stars of various types and masses along the asymptotic giant branch. Methods. For this endeavour, we developed a new multi-band approach combining Wesenheit functions WRP,BP−RP and WKs,J−Ks in the Gaia BP, RP, and 2MASS J, Ks spectral ranges, respectively, and use a new diagram, (WRP,BP−RP − WKs,J−Ks) versus Ks, to distinguish between different kinds of stars in our sample of LPVs. We used stellar population synthesis models to validate our approach. Results. We demonstrate the ability of the new diagram to discriminate between O- and C-rich objects, and to identify low-mass, intermediate-mass, and massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution and population synthesis models guide the interpretation of the results, highlighting the diagnostic power of the new tool to discriminate between stellar initial masses, chemical properties, and evolutionary stages.

scholarly journals Spitzer observations of molecules and dust in evolved stars in nearby galaxies

2009 ◽  
Vol 5 (H15) ◽  
pp. 557-557
Author(s):  
Mikako Matsuura
Keyword(s):  
High Sensitivity ◽  
Gas Production ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Chemical Enrichment ◽  
Evolved Stars ◽  
Wide Range ◽  
Nearby Galaxies ◽  
The Impact ◽  
High Metallicity

Molecules and dust are formed in and around the asymptotic giant branch (AGB) stars and supernovae (SNe), and are ejected into the interstellar medium (ISM) through the stellar wind. The dust and gas contain elements newly synthesised in stars, thus, dying stars play an important role in the chemical enrichment of the ISM of galaxies. However, quantitative analysis of molecules and dust in these stars had been difficult beyond our Galaxy. The high sensitivity instruments on-board the Spitzer Space Telescope (SST; Werner et al. 2004) have enabled us to study dust and molecules in these stars in nearby galaxies. Nearby galaxies have a wide range in metallicity, thus the impact of the metallicity on dust and gas production can be studied. This study will be useful for chemical evolution of galaxies from low to high metallicity.

scholarly journals A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776

2021 ◽  
Vol 645 ◽  
pp. A41
Author(s):  
R. Luque ◽  
L. M. Serrano ◽  
K. Molaverdikhani ◽  
M. C. Nixon ◽  
J. H. Livingston ◽  
...  
Keyword(s):  
Outer Planet ◽  
Low Mass Stars ◽  
Formation Pathway ◽  
Transiting Planets ◽  
Additional Signal ◽  
Wide Range ◽  
Mass Fractions ◽  
Thermally Driven ◽  
Low Mass ◽  
Photometric Monitoring

We report the discovery and characterization of two transiting planets around the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54 ± 0.03 M ⊙) detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining the TESS photometry with HARPS radial velocities, as well as ground-based follow-up transit observations from the MEarth and LCOGT telescopes, for the inner planet, TOI-776 b, we measured a period of P b = 8.25 d, a radius of R b = 1.85 ± 0.13 R ⊕, and a mass of M b = 4.0 ± 0.9 M ⊕; and for the outer planet, TOI-776 c, a period of P c = 15.66 d, a radius of R c = 2.02 ± 0.14 R ⊕, and a mass of M c = 5.3 ± 1.8 M ⊕. The Doppler data shows one additional signal, with a period of ~34 d, associated with the rotational period of the star. The analysis of fifteen years of ground-based photometric monitoring data and the inspection of different spectral line indicators confirm this assumption. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets have retained a significant atmosphere, with slightly different envelope mass fractions. Thanks to their location near the radius gap for M dwarfs, we can start to explore the mechanism(s) responsible for the radius valley emergence around low-mass stars as compared to solar-like stars. While a larger sample of well-characterized planets in this parameter space is still needed to draw firm conclusions, we tentatively estimate that the stellar mass below which thermally-driven mass loss is no longer the main formation pathway for sculpting the radius valley is between 0.63 and 0.54 M ⊙. Due to the brightness of the star, the TOI-776 system is also an excellent target for the James Webb Space Telescope, providing a remarkable laboratory in which to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.

Sign in / Sign up

Export Citation Format

Share Document