scholarly journals Nucleosynthesis in Red Giant Stars

2002 ◽  
Vol 187 ◽  
pp. 57-69
Author(s):  
Nami Mowlavi
Keyword(s):  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Asymptotic Giant Branch Stars ◽  
Chemical Abundance ◽  
Chemical Abundances ◽  
Giant Stars ◽  
Intermediate Mass Stars ◽  
Standard Models ◽  
Red Giant ◽  
Giant Branch Stars

The production of elements from helium-3 to fluorine in low- and intermediate-mass stars is reviewed and compared to chemical abundances observed at the surface of both red giant branch and asymptotic giant branch stars. It is highlighted that, while the trends predicted by standard models are generally well confirmed, many chemical abundances observed at the surface of red giants require the operation of non-standard mixing in the stellar interior. In addition, chemical abundance predictions from presently available asymptotic giant branch models further suffer from the uncertainties affecting the third dredge-up phenomenon, the source of neutrons and the hot bottom burning process.

scholarly journals Observations of Lithium in red giant stars

2009 ◽  
Vol 5 (S268) ◽  
pp. 301-309
Author(s):  
Verne V. Smith
Keyword(s):  
Stellar Evolution ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Asymptotic Giant Branch Stars ◽  
Multiple Sources ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Red Giant ◽  
First Time ◽  
Giant Branch Stars

AbstractConnections between observations of the lithium abundance in various types of red giants and stellar evolution are discussed here. The emphasis is on three main topics; 1) the depletion of Li as stars ascend the red giant branch for the first time, 2) the synthesis of 7Li in luminous and massive asymptotic giant branch stars via the mechanism of hot-bottom burning, and 3) the possible multiple sources of excess Li abundances found in a tiny fraction of various types of G and K giants.

scholarly journals The chemical evolution of the dwarf spheroidal galaxy Sextans

2020 ◽  
Vol 642 ◽  
pp. A176 ◽  
Author(s):  
R. Theler ◽  
P. Jablonka ◽  
R. Lucchesi ◽  
C. Lardo ◽  
P. North ◽  
...  
Keyword(s):  
Star Formation ◽  
Galaxy Evolution ◽  
Dwarf Galaxy ◽  
Small Mass ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Chemical Abundances ◽  
Low Metallicity ◽  
Red Giant ◽  
Giant Branch Stars

We present our analysis of the FLAMES dataset targeting the central 25′ region of the Sextans dwarf spheroidal galaxy (dSph). This dataset is the third major part of the high-resolution spectroscopic section of the ESO large program 171.B-0588(A) obtained by the Dwarf galaxy Abundances and Radial-velocities Team. Our sample is composed of red giant branch stars down to V ∼ 20.5 mag, the level of the horizontal branch in Sextans, and allows users to address questions related to both stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81 stars, which cover the wide [Fe/H] = −3.2 to −1.5 dex range. The abundances of ten other elements are derived: Mg, Ca, Ti, Sc, Cr, Mn, Co, Ni, Ba, and Eu. Despite its small mass, Sextans is a chemically evolved system, showing evidence of a contribution from core-collapse and Type Ia supernovae as well as low-metallicity asymptotic giant branch stars (AGBs). This new FLAMES sample offers a sufficiently large number of stars with chemical abundances derived with high accuracy to firmly establish the existence of a plateau in [α/Fe] at ∼0.4 dex followed by a decrease above [Fe/H] ∼ −2 dex. These features reveal a close similarity with the Fornax and Sculptor dSphs despite their very different masses and star formation histories, suggesting that these three galaxies had very similar star formation efficiencies in their early formation phases, probably driven by the early accretion of smaller galactic fragments, until the UV-background heating impacted them in different ways. The parallel between the Sculptor and Sextans dSph is also striking when considering Ba and Eu. The same chemical trends can be seen in the metallicity region common to both galaxies, implying similar fractions of SNeIa and low-metallicity AGBs. Finally, as to the iron-peak elements, the decline of [Co/Fe] and [Ni/Fe] above [Fe/H] ∼ −2 implies that the production yields of Ni and Co in SNeIa are lower than that of Fe. The decrease in [Ni/Fe] favours models of SNeIa based on the explosion of double-degenerate sub-Chandrasekhar mass white dwarfs.

scholarly journals Grain Formation in the Winds of Cool Red Giant Stars

2000 ◽  
Vol 177 ◽  
pp. 337-347
Author(s):  
E. Sedlmayr ◽  
J. M. Winters
Keyword(s):  
Dynamical Behavior ◽  
Asymptotic Giant Branch ◽  
Dust Formation ◽  
Asymptotic Giant Branch Stars ◽  
Circumstellar Envelopes ◽  
Giant Stars ◽  
Grain Formation ◽  
Circumstellar Shell ◽  
Red Giant ◽  
Giant Branch Stars

The problem of dust formation in the circumstellar envelopes of Asymptotic Giant Branch stars is reviewed. Special emphasis is put on the consistent modelling of the dust-forming circumstellar shell, where due to a strong coupling the dust formation process governs the dynamical behavior of the object.

scholarly journals Asteroseismology of luminous red giants with Kepler. II. Dependence of mass loss on pulsations and radiation

2020 ◽  
Author(s):  
Jie Yu ◽  
Saskia Hekker ◽  
Timothy R Bedding ◽  
Dennis Stello ◽  
Daniel Huber ◽  
...  
Keyword(s):  
Mass Loss ◽  
Mass Loss Rate ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Asymptotic Giant Branch Stars ◽  
Chemical Enrichment ◽  
Dust Mass ◽  
Red Giant ◽  
The Masses ◽  
Loss Rates

Abstract Mass loss by red giants is an important process to understand the final stages of stellar evolution and the chemical enrichment of the interstellar medium. Mass-loss rates are thought to be controlled by pulsation-enhanced dust-driven outflows. Here we investigate the relationships between mass loss, pulsations, and radiation, using 3213 luminous Kepler red giants and 135000 ASAS–SN semiregulars and Miras. Mass-loss rates are traced by infrared colours using 2MASS and WISE and by observed-to-model WISE fluxes, and are also estimated using dust mass-loss rates from literature assuming a typical gas-to-dust mass ratio of 400. To specify the pulsations, we extract the period and height of the highest peak in the power spectrum of oscillation. Absolute magnitudes are obtained from the 2MASS Ks band and the Gaia DR2 parallaxes. Our results follow. (i) Substantial mass loss sets in at pulsation periods above ∼60 and ∼100 days, corresponding to Asymptotic-Giant-Branch stars at the base of the period-luminosity sequences C′ and C. (ii) The mass-loss rate starts to rapidly increase in semiregulars for which the luminosity is just above the red-giant-branch tip and gradually plateaus to a level similar to that of Miras. (iii) The mass-loss rates in Miras do not depend on luminosity, consistent with pulsation-enhanced dust-driven winds. (iv) The accumulated mass loss on the Red Giant Branch consistent with asteroseismic predictions reduces the masses of red-clump stars by 6.3%, less than the typical uncertainty on their asteroseismic masses. Thus mass loss is currently not a limitation of stellar age estimates for galactic archaeology studies.

scholarly journals Detection and characteristics of magnetic fields in evolved intermediate mass stars

2014 ◽  
Vol 10 (S305) ◽  
pp. 42-46
Author(s):  
Laurence Sabin
Keyword(s):  
Magnetic Fields ◽  
Point Of View ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Stellar Objects ◽  
Intermediate Mass Stars ◽  
Theoretical Predictions ◽  
The One ◽  
Loss Control ◽  
Giant Branch Stars

AbstractThe role of magnetic fields in late type stars, such as Asymptotic Giant Branch stars (AGBs), Post-AGBs and Planetary Nebulae (PNe), is poorly known from an observational point of view. Magnetic fields are however believed to have a non-negligible influence on the dynamics (via mass loss control, outflows shaping) and even on the chemistry (e.g. extra mixing) of these stellar objects. We are therefore presenting two different types of investigation, both based on the use of polarimetry, which aim at filling the gap between the observations on the one hand and the theoretical predictions on the other hand.

scholarly journals High Resolution Radio and IR Observations of AGB Stars

2012 ◽  
Vol 8 (S287) ◽  
pp. 245-249
Author(s):  
W. Cotton ◽  
G. Perrin ◽  
R. Millan-Gabet ◽  
O. Delaa ◽  
B. Mennesson
Keyword(s):  
High Resolution ◽  
Molecular Layer ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Near Ir ◽  
The Subject ◽  
Infrared Interferometry ◽  
Giant Branch Stars

AbstractAsymptotic Giant Branch Stars (AGB) are evolved, mass losing red giants with tenuous molecular envelopes which have been the subject of much recent study using infrared and radio interferometers. In oxygen rich stars, radio SiO masers form in the outer regions of the molecular envelopes and are powerful diagnostics of the extent of these envelopes. Spectroscopically resolved infrared interferometry helps constrain the extent of various species in the molecular layer. We made VLBA 7 mm SiO maser, Keck Interferometer near IR and VLTI/MIDI mid IR high resolution observations of the stars U Ari, W Cnc, RX Tau, RT Aql, S Ser and V Mon. This paper presents evidence that the SiO is depleted from the gas phase and speculate that it is frozen onto Al2O3 grains and that radiation pressure on these grains help drive the outflow.

scholarly journals Active red giants: Close binaries versus single rapid rotators

2020 ◽  
Vol 639 ◽  
pp. A63
Author(s):  
Patrick Gaulme ◽  
Jason Jackiewicz ◽  
Federico Spada ◽  
Drew Chojnowski ◽  
Benoît Mosser ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Close Binaries ◽  
Red Giants ◽  
Intermediate Mass ◽  
Giant Stars ◽  
Rotational Modulation ◽  
Low Mass ◽  
Red Giant Stars ◽  
Red Giant ◽  
Giant Branch Stars

Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, which enables detailed studies of the Milky Way. The objective of this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, which relies on the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We selected a sample of about 4500 relatively bright red giants observed by Kepler, and show that about 370 of them (∼8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 nonoscillators are in this group of binaries. On the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of the stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones are less active. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measured are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets that is not available in the Sun.

scholarly journals The long secondary periods in semi-regular variables

2004 ◽  
Vol 193 ◽  
pp. 322-326
Author(s):  
P.R. Wood ◽  
A.E. Olivier ◽  
S.D. Kawaler
Keyword(s):  
Radial Velocity ◽  
Large Amplitude ◽  
Small Amplitude ◽  
Light Curves ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Red Giant ◽  
Eccentric Motion ◽  
Giant Branch Stars ◽  
Nonradial Pulsation

AbstractAbout 25% of variable asymptotic giant branch stars in the LMC have light curves that are modulated by a long secondary period of length ~ 1–4 yr. Solar-vicinity analogs of these stars show radial velocity curves that are asymmetric and of small amplitude (~3–5km s−1). Possible explanations for the photometric and radial velocity variations include: eccentric motion of an orbiting companion of mass ~0.1 M⊙; radial and nonradial pulsation; rotation of an ellipsoidal-shaped red giant; episodic dust ejection; and star spot cycles. We find that there are severe difficulties with each of these models. Thus, the long secondary periods are the only unexplained type of large-amplitude stellar variability known at this time.

scholarly journals Parameterising the Third Dredge-up in Asymptotic Giant Branch Stars

2002 ◽  
Vol 19 (4) ◽  
pp. 515-526 ◽  
Author(s):  
A. I. Karakas ◽  
J. C. Lattanzio ◽  
O. R. Pols
Keyword(s):  
Mass Loss ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Core Mass ◽  
The Third ◽  
The Core ◽  
Intermediate Mass Stars ◽  
Homogeneous Set ◽  
Efficiency Parameter ◽  
Giant Branch Stars

AbstractWe present new evolutionary sequences for low and intermediate mass stars (1−6M⊙) for three different metallicities, Z = 0.02, 0.008, and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch phase. We have two sequences of models for each mass, one which includes mass loss and one without mass loss. Typically 20 or more pulses have been followed for each model, allowing us to calculate the third dredge-up parameter for each case. Using the results from this large and homogeneous set of models, we present an approximate fit for the core mass at the first thermal pulse, Mc1, as well as for the third dredge-up efficiency parameter, λ, and the core mass at the first dredge-up episode, Mcmin, as a function of metallicity and total mass. We also examine the effect of a reduced envelope mass on the value of λ.

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