Core He-burning Phase Research Articles

Convective overshooting and production of s-nuclei in massive stars during their core He-burning phase

With the "post-processing" technique we explore the role of the convective overshooting on the production of s-nuclei in stellar models of different initial mass and metallicity ($15 \leq M_{ZAMS}/M_{\odot} \leq 25$; $10^{-4} \leq Z \leq 0.02$), considering a range of values for the parameter $f$, which determines the overall efficiency of convective overshooting.We find enhancements in the production of s-nuclei until a factor $\sim 6$ (measured as the average overproduction factor of the 6 s-only nuclear species with $60\lesssim A\lesssim90$) in all our models of different initial mass and metallicity with $f$ in the range $0.01{-}0.035$ (i.e. models with overshooting) compared to the production obtained with "no-overshooting" models (i.e. models with the same initial mass and metallicity, but $f=10^{-5}$). Moreover the results indicate that the link between the overshooting parameter $f$ and the s-process efficiency is essentially monotonic in all our models of different initial mass and metallicity. Also evident is the higher s-process efficiency when we progressively increase for a given f value both the mass of the models from 15 M$_\odot$ to 25 M$_\odot$ and the Z value from 10$^{-4}$ to 0.02. We also briefly discuss the possible consequences of these results for some open questions linked to the s-process weak component efficiency, as well as a "rule of thumb" to evaluate the impact of the convective overshooting on the yields of a generation of stars.

The impact of an updated14N(p,γ)15O reaction rate on advanced evolutionary stages of low-mass stellar models

We have investigated the impact of the $^{14}N(p,\gamma)^{15}O$ reaction rate recently redetermined by the LUNA experiment, on the shell H-burning and core He-burning phases of low-mass, metal poor stellar models. The new reaction rate has small but noticeable effects, the largest one being a $\sim$7-8\% reduction of the red giant branch lifetimes. To different degrees, the lifetimes and luminosities of horizontal branch models, the mass of the stellar models evolving within the RR Lyrae instability strip, the luminosity of the red giant branch luminosity function bump, the theoretical calibrations of the R-parameter and tip of the red giant branch luminosity are also affected. Predictions for the tip of the red giant branch luminosity, in particular, are in very good agreement with the currently available empirical constraints.

Asteroseismology and interferometry of the red giant star ϵ Ophiuchi

The GIII red giant star <i>ϵ<i/> Oph has been found to exhibit several modes of oscillation by the MOST mission. We interpret the observed frequencies of oscillation in terms of theoretical radial <i>p<i/>-mode frequencies of stellar models. Evolutionary models of this star, in both shell H-burning and core He-burning phases of evolution, are constructed using as constraints a combination of measurements from classical ground-based observations (for luminosity, temperature, and chemical composition) and seismic observations from MOST. Radial frequencies of models in either evolutionary phase can reproduce the observed frequency spectrum of <i>ϵ<i/> Oph almost equally well. The best-fit models indicate a mass in the range of 1.85 <i>±<i/> 0.05 with radius of 10.55 <i>±<i/> 0.15. We also obtain an independent estimate of the radius of <i>ϵ<i/> Oph with highly accurate interferometric observations in the infrared <i>K'<i/> band, using the CHARA/FLUOR instrument. The measured limb-darkened disk angular diameter of <i>ϵ<i/> Oph is 2.961 <i>±<i/> 0.007 mas. Together with the <i>Hipparcos<i/> parallax, this translates into a photospheric radius of <i>R<i/> = 10.39 <i>±<i/> 0.07 . The radius obtained from the asteroseismic analysis matches the interferometric value quite closely even though the radius was not constrained during the modelling.

Dark matter annihilations in Population III stars

We study the impact of the capture and annihilation of weakly interacting massive particles (WIMPs) on the evolution of Population III stars. With a suitable modification of the Geneva stellar evolution code, we study the evolution of 20 and $200{M}_{\ensuremath{\bigodot}}$ stars in dark matter halos with densities between ${10}^{8}$ and ${10}^{11}\text{ }\text{ }\mathrm{GeV}/{\mathrm{cm}}^{3}$ during the core H-burning phase, and, for selected cases, until the end of the core He-burning phase. We find that for WIMP densities higher than 5.3 ${10}^{10}({\ensuremath{\sigma}}_{p}^{\mathrm{SD}}/{10}^{\ensuremath{-}38}\text{ }\text{ }{\mathrm{cm}}^{2}{)}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{GeV}\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ the core H-burning lifetime of $20{M}_{\ensuremath{\bigodot}}$ and $200{M}_{\ensuremath{\bigodot}}$ stars exceeds the age of the Universe, and stars are sustained only by WIMP annihilations. We determine the observational properties of these ``frozen'' objects and show that they can be searched for in the local Universe thanks to their anomalous mass-radius relation, which should allow unambiguous discrimination from normal stars.

Vilnius Photometry of Red Giant Stars in the Open Cluster NGC 7789

Photoelectric Vilnius seven-color photometry is presented and analyzed for a sample of 24 red giant branch and clump stars in the open cluster NGC 7789. For each star we have determined photometric spectral type, absolute magnitude, interstellar reddening, effective temperature, metallicity, and surface gravity. From averages over the stars in the sample we find the mean reddening to the cluster EY−V = 0.21± 0.02 (s.d.), or EB−V = 0.25, and the apparent distance modulus (m−M) V = 12.21± 0.10 (s.d.), which yield a distance of 1840 pc. The mean overall metallicity is found to be [Fe/H] = −0.18± 0.09 (s.d.). The clump stars, on average, appear to be slightly more metal-rich than the other red giants, which is most probably caused by evolutionary changes of carbon and nitrogen molecular bands falling in the photometric passbands. A difference in mass between the two groups of stars has also been detected, which suggests that the clump stars might have undergone extra mass loss before reaching their core He-burning phase of evolution.

Evolutionary Properties of Intermediate-Mass Stars

AbstractWe briefly review the main problems related to the computation of the evolution of intermediate-mass stars: the treatment of turbulent convection and the occurrence of blue loops during the core He-burning phase. It is shown that, in order to obtain more accurate and reliable stellar models for this class of stars, one has to consider all possible theoretical and observational constraints. These include observations of low-mass stars to constrain the treatment of envelope convection, and the analysis of the pulsational behaviour of Cepheid stars.

Understanding AGB Carbon Star Nucleosynthesis from Observations

AbstractRecent advances in the knowledge of the evolutionary status of asymptotic giant branch (AGB) stars and of the nucleosynthesis processes occurring in them are discussed, and used to interpret abundance determinations for s-process elements, lithium and CNO isotopes in several types of AGB stars. We focus our attention mainly on carbon-rich AGB stars. By combining these different constraints we conclude that most carbon stars in the solar neighborhood are of low mass (M≤3 M⊙), their abundances being a consequence of the operation of thermal pulses and the third dredge-up. However, the observed abundances in carbon stars of the R and J types cannot be explained by this standard scenario. These stars may not be on the AGB, but possibly in the core-He burning phases; their envelopes may have been polluted with nuclear ashes of the core-He flash, followed by CNO re-processing enhancing 13C. Observational evidence suggesting the operation of non-standard mixing mechanisms during the AGB phase is also discussed.

The Chemical Composition of White Dwarfs as a Test of Convective Efficiency during Core Helium Burning

Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution. Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen (Metcalfe and coworkers). We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing. By combining the uncertainty of the convection theory with the error on the relevant reaction rate, we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate 12C(α,γ)16O reaction rate are able to account for a high central oxygen abundance. The rate of the 12C(α,γ)16O used in these models agrees with the one recently obtained in laboratory experiments by Kunz and coworkers. On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the 12C(α,γ)16O reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so-called breathing pulses) that are usually encountered in modeling late stage of core He-burning phase.

S-process nucleosynthesis in massive stars and the weak component. I - Evolution and neutron captures in a 25 solar mass star

view Abstract Citations (99) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS S-Process Nucleosynthesis in Massive Stars and the Weak Component. I. Evolution and Neutron Captures in a 25 Msun Star Raiteri, C. M. ; Busso, M. ; Gallino, R. ; Picchio, G. ; Pulone, L. Abstract Evolutionary calculations of a typical massive star are run in order to analyze the n-capture process occurring during the hydrostatic core He-burning phase, studying the effects of variations in the C-12(alpha, gamma)O-16 and Ne-22(alpha, n)Mg-25 rates within their present uncertainties and in other nuclear inputs. The nucleosynthetic mechanism is characterized by a low mean neutron density, which never exceeds 10 to the 6th n/cu cm. A good production of the s-isotopes in the atomic mass range 70-90 is found, the actual neutron exposure depending on some critical parameters. Important contributions are also found for several light n-rich species and for the heavy rare isotopes Gd-152 and Ta-180. How s-processing in a typical massive star compares with predictions by classical analysis is addressed. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169622 Bibcode: 1991ApJ...367..228R Keywords: Neutrons; Nuclear Astrophysics; Nuclear Capture; Nuclear Fusion; Stellar Evolution; Stellar Mass; Asymptotic Giant Branch Stars; Reaction Kinetics; Solar System; Astrophysics; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; STARS: ABUNDANCES; STARS: EVOLUTION; STARS: MASSIVE full text sources ADS |

Convective shells and the core He-burning phase of massive stars

In this paper we briefly discuss the effect of complete homogenization in the intermediate unstable layers of massive stars on the effective temperature of the core He-burning models. To this end, a 20M ⊙ star of Population I chemical composition (X=0.700,Z=0.020) has been allowed to evolve from the Main Sequence into the core He-exhaustion stage without taking into account semiconvective mixing. The results show that the models are systematically bluer than those computed with the same physical parameters but with the inclusion of semiconvection.