scholarly journals First discovery of trans-iron elements in a DAO-type white dwarf (BD−22°3467)

2019 ◽  
Vol 492 (1) ◽  
pp. 528-548
Author(s):  
L Löbling ◽  
M A Maney ◽  
T Rauch ◽  
P Quinet ◽  
S Gamrath ◽  
...  
Keyword(s):  
White Dwarf ◽  
Convection Zone ◽  
Ultraviolet Spectrum ◽  
Asymptotic Giant Branch ◽  
Thermal Pulse ◽  
Low Mass ◽  
Process Elements ◽  
First Time ◽  
Abundance Determination

ABSTRACT We have identified 484 lines of the trans-iron elements (TIEs) Zn, Ga, Ge, Se, Br, Kr, Sr, Zr, Mo, In, Te, I, Xe, and Ba, for the first time in the ultraviolet spectrum of a DAO-type white dwarf (WD), namely BD−22°3467, surrounded by the ionized nebula Abell 35. Our TIE abundance determination shows extremely high overabundances of up to 5 dex – a similar effect is already known from hot, H-deficient (DO-type) WDs. In contrast to these where a pulse-driven convection zone has enriched the photosphere with TIEs during a final thermal pulse and radiative levitation has established the extreme TIE overabundances, here the extreme TIE overabundances are exclusively driven by radiative levitation on the initial stellar metallicity. The very low mass ($0.533^{+0.040}_{-0.025}\, \mathrm{M}_\odot$) of BD−22°3467 implies that a third dredge-up with enrichment of s-process elements in the photosphere did not occur in the asymptotic giant branch (AGB) precursor.

scholarly journals Evolutionary Tracks for Central Stars of Planetary Nebulae

1989 ◽  
Vol 131 ◽  
pp. 463-472 ◽  
Author(s):  
Detlef Schönberner
Keyword(s):  
Mass Loss ◽  
White Dwarf ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Helium Burning ◽  
Hydrogen Burning ◽  
Stellar Envelope ◽  
Thermal Pulse ◽  
Pulse Cycle ◽  
Central Stars

Our understanding of the evolution of Central Stars of Planetary Nebulae (CPN) has made considerable progress during the last years. This was possible since consistent computations through the asymptotic giant branch (AGB), with thermal pulses and (in some cases) mass loss taken into account, became available (Schönberner, 1979, 1983; Kovetz and Harpaz, 1981; Harpaz and Kovetz, 1981; Iben, 1982, 1984; Wood and Faulkner, 1986). It turned out that the evolution depends very sensitively on the inital conditions on the AGB. More precisely, the evolution of an AGB remnant is a function of the phase of the thermal-pulse cycle during which this remnant was created on the tip of the AGB by the planetary-nebula (PN) formation process (Iben, 1984, 1987). This was first shown by Schönberner (1979), and then fully explored by Iben (1984). In short, two major modes of PAGB evolution to the white dwarf stage are possible, according to the two main phases of a thermally pulsing AGB star: the hydrogen-burning or helium-burning mode. If, for instance, the PN formation, i.e. the removal of the stellar envelope by mass loss, happens during a luminosity peak that follows a thermal pulse of the helium-burning shell, the remnant leaves the AGB while still burning helium as the main energy supplier (Härm and Schwarzschild, 1975). On the other hand, PN formation may also occur during the quiescent hydrogen-burning phase on the AGB, and the remnant continues then to burn mainly hydrogen on its way to becoming a white dwarf.

scholarly journals Unusual neutron-capture nucleosynthesis in a carbon-rich Galactic bulge star

2019 ◽  
Vol 622 ◽  
pp. A159 ◽  
Author(s):  
Andreas Koch ◽  
Moritz Reichert ◽  
Camilla Juul Hansen ◽  
Melanie Hampel ◽  
Richard J. Stancliffe ◽  
...  
Keyword(s):  
Neutron Capture ◽  
Galactic Halo ◽  
Asymptotic Giant Branch ◽  
Galactic Bulge ◽  
Element Abundances ◽  
Intermediate Neutron ◽  
Low Mass ◽  
Process Component ◽  
Process Elements ◽  
The Impact

Metal-poor stars in the Galactic halo often show strong enhancements in carbon and/or neutron-capture elements. However, the Galactic bulge is notable for its paucity of these carbon-enhanced metal-poor (CEMP) and/or CH-stars, with only two such objects known to date. This begs the question whether the processes that produced their abundance distribution were governed by a comparable nucleosynthesis in similar stellar sites as for their more numerous counterparts in the halo. Recently, two contenders of these classes of stars were discovered in the bulge, at [Fe/H] = −1.5 and −2.5 dex, both of which show enhancements in [C/Fe] of 0.4 and 1.4 dex (respectively), [Ba/Fe] in excess of 1.3 dex, and also elevated nitrogen. The more metal-poor of the stars can be well matched by standard s-process nucleosynthesis in low-mass asymptotic giant branch (AGB) polluters. The other star shows an abnormally high [Rb/Fe] ratio. Here, we further investigate the origin of the abundance peculiarities in the Rb-rich star by new, detailed measurements of heavy element abundances and by comparing the chemical element ratios of 36 species to several models of neutron-capture nucleosynthesis. The i-process with intermediate neutron densities between those of the slow (s-) and rapid (r)-neutron-capture processes has been previously found to provide good matches of CEMP stars with enhancements in both r- and s-process elements (class CEMP-r/s), rather than invoking a superposition of yields from the respective individual processes. However, the peculiar bulge star is incompatible with a pure i-process from a single ingestion event. Instead, it can, statistically, be better reproduced by more convoluted models accounting for two proton ingestion events, or by an i-process component in combination with s-process nucleosynthesis in low-to-intermediate mass (2–3 M⊙) AGB stars, indicating multiple polluters. Finally, we discuss the impact of mixing during stellar evolution on the observed abundance peculiarities.

(Pre)-white dwarf stars as measuring tools for yields of AGB nucleosynthesis

2019 ◽  
Vol 15 (S357) ◽  
pp. 158-161
Author(s):  
Lisa Löbling
Keyword(s):  
White Dwarf ◽  
Neutron Capture ◽  
Convection Zone ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Heavy Elements ◽  
Agb Stars ◽  
Subsequent Evolution ◽  
Dwarf Stars ◽  
White Dwarf Stars

AbstractIn the helium-rich intershell region of asymptotic giant branch (AGB) stars, slow neutron-capture nucleosynthesis produces heavy elements beyond iron. If the stars experience a final-flash of the He-burning shell, a pulse-driven convection zone establishes, the stars become hydrogen-deficient and exhibit former intershell material at their surfaces. In their subsequent evolution towards the white-dwarf cooling sequence, but still at constant luminosity, a strong stellar wind prevents diffusion to wipe out the information about AGB yields. We present and interpret the analysis results of hydrogen-rich and -deficient post-AGB stars, discuss difficulties in their analysis and review the implications on the understanding of post-AGB evolution.

scholarly journals The Mystery of CEMPs+r Stars and the Dual Core-Flash Neutron Superburst

2009 ◽  
Vol 26 (3) ◽  
pp. 322-326 ◽  
Author(s):  
M. Lugaro ◽  
S. W. Campbell ◽  
S. E. de Mink
Keyword(s):  
Large Fraction ◽  
Convective Zone ◽  
Asymptotic Giant Branch ◽  
Observational Constraints ◽  
Low Mass Stars ◽  
Low Metallicity ◽  
Low Mass ◽  
Process Component ◽  
Process Elements ◽  
Dual Core

AbstractCarbon-enhanced metal-poor (CEMPs+r) stars show large enhancements of elements produced both by the slow and the rapid neutron capture processes (the s and r process, respectively) and represent a relatively large fraction, 30% to 50%, of the CEMP population. Many scenarios have been proposed to explain this peculiar chemical composition and most of them involve a binary companion producing the s-process elements during its Asymptotic Giant Branch (AGB) phase. The problem is that none of the proposed explanations appears to be able to account for all observational constraints, hence, alternatives are needed to be put forward and investigated. In this spirit, we propose a new scenario for the formation of CEMPs+r stars based on S. W. Campbell's finding that during the ‘dual core flash’ in low-mass stars of extremely low metallicity, when protons are ingested in the He-flash convective zone, a ‘neutron superburst’ is produced. Further calculations are needed to verify if this neutron superburst could make the r-process component observed in CEMPs+r, as well as their Fe abundances. The s-process component would then be produced during the following AGB phase.

scholarly journals H- and He-burning Central Stars and the Evolution to White Dwarfs

2003 ◽  
Vol 209 ◽  
pp. 101-108
Author(s):  
T. Blöcker
Keyword(s):  
Mass Loss ◽  
White Dwarf ◽  
Evolutionary History ◽  
Central Star ◽  
Asymptotic Giant Branch ◽  
Stellar Structure ◽  
Transitional Stage ◽  
Thermal Pulse ◽  
Convective Overshoot ◽  
Central Stars

The structure and evolution of central stars of planetary nebulae (CSPNe) is reviewed. CSPNe represent the rapid transitional stage between the Asymptotic Giant Branch (AGB) and the white-dwarf domain. It is shown that the whole evolution off the AGB through the central-star regime depends on the evolutionary history. The detailed evolution into a white dwarf is controlled by the internal stellar structure which, in turn, is determined by the duration of the preceding AGB evolution and therefore by the AGB mass-loss history. The evolution of hydrogen-deficient central stars has been a matter of debate since many years. Convective overshoot appears to be a key ingredient to model these objects. Various thermal-pulse scenarios with inclusion of overshoot are discussed, leading to surface abundances in general agreement with those observed for Wolf-Rayet central stars.

scholarly journals Understanding AGB Carbon Star Nucleosynthesis from Observations

2003 ◽  
Vol 20 (4) ◽  
pp. 314-323 ◽  
Author(s):  
C. Abia ◽  
I. Domínguez ◽  
R. Gallino ◽  
M. Busso ◽  
O. Straniero ◽  
...  
Keyword(s):  
Carbon Star ◽  
Solar Neighborhood ◽  
Asymptotic Giant Branch ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Carbon Stars ◽  
The Core ◽  
Standard Scenario ◽  
Low Mass ◽  
Process Elements

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.

scholarly journals Peculiar Red Giants — What Kind of White Dwarfs do They Become?

1989 ◽  
Vol 106 ◽  
pp. 205-221 ◽  
Author(s):  
Icko Iben
Keyword(s):  
Mass Loss ◽  
White Dwarf ◽  
Buffer Zone ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Additional Mass ◽  
Hydrogen Burning ◽  
Thermal Pulse ◽  
Pulse Cycle ◽  
Model Star

AbstractAfter a brief commentary on the place of “peculiar red giants” in the overall scheme of stellar evolution, an outline is given of the various possibilities for post asymptotic giant branch (AGB) evolution. The behavior of a post-AGB model star is crucially dependent on where in a thermal pulse cycle the mass of the hydrogen-rich envelope is reduced to such an extent that departure from the AGB must follow on a thermal time scale. If departure from the AGB occurs while the model is still burning hydrogen, post-AGB behavior depends on the mass of the helium buffer zone (- zone containing predominantly helium which has been processed through the hydrogen-burning shell following the last thermal pulse on the AGB). If departure occurs at an arbitrary time during the hydrogen-burning phase, then: (1) in - 25% of all cases, the post-AGB model will experience a final helium shell flash, and, in consequence of additional mass loss, may become a non-DA white dwarf; (2) in - 60% of all cases, the model will cease burning hydrogen when the mass in its hydrogen-rich envelope is reduced to ∼ 10-4Mʘ and will evolve into a DA white dwarf; and (3) in - 15% of all cases, the model will experience a final hydrogen shell flash, but the outcome with regard to spectroscopic type is unclear. If departure from the AGB occurs while the model is burning helium, the result is either the same as in option (3) just described, or mass loss during the post-AGB helium-burning phase may turn the star into a non-DA white dwarf.

scholarly journals On the formation of hydrogen-deficient low-mass white dwarfs

2020 ◽  
Vol 638 ◽  
pp. A30
Author(s):  
Tiara Battich ◽  
Leandro G. Althaus ◽  
Alejandro H. Córsico
Keyword(s):  
White Dwarfs ◽  
Evolutionary History ◽  
Main Sequence ◽  
Asymptotic Giant Branch ◽  
Instability Strip ◽  
Giant Star ◽  
Thermal Pulse ◽  
Low Mass ◽  
Effective Temperatures ◽  
Red Giant

Context. Two of the possible channels for the formation of low-mass (M⋆ ≲ 0.5 M⊙) hydrogen-deficient white dwarfs are the occurrence of a very-late thermal pulse after the asymptotic giant-branch phase or a late helium-flash onset in an almost stripped core of a red giant star. Aims. We aim to asses the potential of asteroseismology to distinguish between the hot flasher and the very-late thermal pulse scenarios for the formation of low-mass hydrogen-deficient white dwarfs. Methods. We computed the evolution of low-mass hydrogen-deficient white dwarfs from the zero-age main sequence in the context of the two evolutionary scenarios. We explore the pulsation properties of the resulting models for effective temperatures characterizing the instability strip of pulsating helium-rich white dwarfs. Results. We find that there are significant differences in the periods and in the period spacings associated with low radial-order (k ≲ 10) gravity modes for white-dwarf models evolving within the instability strip of the hydrogen-deficient white dwarfs. Conclusions. The measurement of the period spacings for pulsation modes with periods shorter than ∼500 s may be used to distinguish between the two scenarios. Moreover, period-to-period asteroseismic fits of low-mass pulsating hydrogen-deficient white dwarfs can help to determine their evolutionary history.

scholarly journals Carbon-enhanced metal-poor stars as probes of early Galactic nucleosynthesis

2009 ◽  
Vol 5 (S265) ◽  
pp. 117-117
Author(s):  
O. R. Pols ◽  
R. G. Izzard ◽  
E. Glebbeek ◽  
R. J. Stancliffe
Keyword(s):  
Binary Systems ◽  
Galactic Halo ◽  
Large Fraction ◽  
Mass Function ◽  
Initial Mass Function ◽  
Asymptotic Giant Branch ◽  
Mass Star ◽  
Low Mass ◽  
Process Elements ◽  
Early Galaxy

A large fraction, between 10 and 25%, of very metal-poor stars in the Galactic halo are carbon-rich objects, with enhancements of carbon relative to iron exceeding a factor 10. The majority of these carbon-enhanced metal-poor (CEMP) stars show enhancements of heavy s-process elements and have been found to be spectroscopic binary systems. Many of their properties are well explained by the binary mass transfer scenario, in which a former asymptotic giant branch (AGB) companion star has polluted the low-mass star with its nucleosynthesis products. The same scenario predicts the existence of nitrogen-rich metal-poor (NEMP) stars, with [N/C] > 0.5, from AGB companions more massive than about 3 solar masses. In contrast to CEMP stars, however, such NEMP stars are very rare. Recent studies suggest that the high frequency of CEMP stars requires a modified initial mass function (IMF) in the early Galaxy, weighted towards intermediate-mass stars. Such models also implicitly predict a large number of NEMP stars which is not seen.

scholarly journals The intermediate neutron capture process

2021 ◽  
Vol 648 ◽  
pp. A119
Author(s):  
A. Choplin ◽  
L. Siess ◽  
S. Goriely
Keyword(s):  
Convection Zone ◽  
Isotopic Ratios ◽  
Elemental Distribution ◽  
Low Mass Stars ◽  
Capture Process ◽  
Thermal Pulse ◽  
Intermediate Neutron ◽  
Low Metallicity ◽  
Low Mass ◽  
Temporal And Spatial

Context. Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. Aims. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. Methods. We computed a 1 M⊙ model at [Fe/H] = −2.5 with the stellar evolution code STAREVOL, using a nuclear network of 1091 species (at maximum) coupled to the transport processes. The impact of the temporal and spatial resolutions on the resulting abundances was assessed. We also identified key elements and isotopic ratios that are specific to i-process nucleosynthesis and carried out a detailed comparison between our model and a sample of r/s-stars. Results. At the beginning of the AGB phase, during the third thermal pulse, the helium driven convection zone is able to penetrate the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of ∼4.3 × 1014 cm−3 at the origin of the synthesis of i-process elements. The nuclear energy released by proton burning in the helium-burning convective shell strongly affects the internal structure: the thermal pulse splits and after approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in the opacity, which triggers a strong mass loss and prevents any further thermal pulse. Our numerical tests indicate that the i-process elemental distribution is not strongly affected by the temporal and spatial resolution used to compute the stellar models, but typical uncertainties of ±0.3 dex on individual abundances are found. We show that specific isotopic ratios of Ba, Nd, Sm, and Eu can represent good tracers of i-process nucleosynthesis. Finally, an extended comparison with 14 selected r/s-stars show that the observed composition patterns can be well reproduced by our i-process AGB model. Conclusions. A rich i-process nucleosynthesis can take place during the early AGB phase of low-metallicity low-mass stars and explain the elemental distribution of most of the r/s-stars, but cannot account for the high level of enrichment of the giant stars in a scenario involving pollution by a former AGB companion.

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