Hot Bottom Burning Research Articles

New theoretical yields of intermediate mass stars

We present theoretical yields of H, 4 He, 12 C, 13 C, 14 N, and 16 O for stars with initial masses between 0.8 and 8 M and initial metallicities Z =0 :001, 0.004, 0.008, 0.02, and 0.04. We use the evolutionary tracks of the Geneva group up to the early asymptotic giant branch (AGB) in combination with a synthetic thermal-pulsing AGB evolution model to follow in detail the chemical evo- lution and mass loss up to the end of the AGB including the rst, second, and third dredge-up phases. Most of the relations used are metallicity dependent to make a realis- tic comparison with stars of dierent initial abundances. The eect of Hot Bottom Burning (HBB) is included in an approximate way. The free parameters in our calculations are the mass loss scaling parameter AGB for stars on the AGB (us- ing a Reimers law), the minimum core mass for dredge-up M min c , and the third dredge-up eciency . As derived from previous extensive modeling, AGB =4 , M min c = 0.58 M ,a nd =0 :75 including HBB are in best agree- ment with observations of AGB stars both in the Galactic disk and Magellanic Clouds. The influence of specic model assumptions and adopted parameter values on the resulting AGB yields is examined and compared with earlier theoretical work. We compare the abundances predicted during the nal stages of the AGB with those observed in planetary nebu- lae in the Galactic disk and show that the model with the aforementioned parameters is in good agreement with the observations. The metallicity dependent yields of interme- diate mass stars presented in this paper are well suited for use in galactic chemical evolution models.

The chemical composition of IRAS 05341+0852: a post-AGB F supergiant with 21 μm emission

An abundance analysis of the photosphere of the F-type Post-AGB candidate IRAS 05341-+0852 is presented. It shows that the star is metal-poor ([Fe/H]=–1.0) and carbon-rich (C/0≈2.2). Carbon, nitrogen, oxygen, silicon, and possibly lithium and aluminum are found to be over abundant. Most importantly this star has large overabundance of s-process elements which are as follows: [Y/Fe]= 1.80, [Ba/Fe]= 2.58, [La/Fe]= 2.86, [Ce/Fe]= 2.95, [Pr/Fe]= 2.27, [Nd/Fe]= 1.97, and [Sm/Fe]= 0.86. The overabundances of s-process elements and carbon in IRAS 05341+0852 is direct evidence for the association of s-process enhancements with shell-flashes and dredge-up. These are likely responsible for the increase in C/O. The possible overabundance of Li ([Li/Fe]≤2.5) and Al ([Al/Fe]≤ 1.1) in IRAS 05341+0852 could indicate that there was Hot Bottom Burning (HBB), where the base of the convective envelope is hot enough for nucleosynthesis to occur (Lattanzio 1993). HBB has been suggested as the mechanism responsible for the production of Li in the Li-rich AGB stars discovered by Smith and Lambert (1989). In fact these are bright AGB stars which are oxygen-rich rather than carbon-rich. Recent calculations by Sackmann and Boothroyd (1992) showed that Li-rich and O-rich AGB stars are the result of HBB. In the HBB models temperatures of the order of 0.5–1 × 108 K are encountered at the base of the convective envelope (Blöcker and Schönberner 1991). This is hot enough for the reaction 25Mg(p,γ) 26 Al resulting in the production of Al.

Hubble Space TelescopeObservations of Planetary Nebulae in the Magellanic Clouds. V. Mass Dependence of Dredge‐up and the Chemical History of the Large Magellanic Cloud

A photoionization analysis of Hubble Space Telescope UV and ground-based optical spectrophotometry is given for eight more planetary nebulae (PNs) in the Large Magellanic Cloud (LMC). This allows the central stars to be placed accurately on the H-R diagram and permits the determination of the He, C, N, O, Ne, S, and Ar abundances. In some cases, the gas-phase abundances of Mg and Si may also be determined. We have combined these results with the analysis of two other objects published by us in the first two papers of this series. The observed abundance patterns are qualitatively consistent with the (mass-dependent) operation of the various chemical dredge-up processes as predicted by theory. Dredge-up of C during the thermal pulsing stage appears to be most important, and "hot bottom burning" transforms much of this C to N in the more massive stars. There is no sign of dredge-up of 22Ne. We show that the spread in the α-process element abundances can be understood as being due to differences in core mass of the planetary nebula nucleus, which is related directly to initial mass of the precursor star. This is, therefore, a tracer of the age-metallicity relationship for stars in general, and we derive, for the first time, the chemical history of the LMC based on PNs. We find that the base metallicity of the LMC almost doubled ~2 Gyr ago. This is consistent with studies of field stars and of clusters that show that there was a major burst of star formation at that time.

Theory of AGB Evolution

After completion of their central hydrogen and helium burning stars with initial masses between 0.8 and 8M⊙ enter the Asymptotic Giant Branch (AGB) consisting then of a very compact, hydrogen-exhausted core surrounded by a very dilute, nearly fully convective, hydrogen-rich envelope. This core contains a substantial fraction of the stellar mass (0.5…1M⊙) but the radial position of its edge is only about 10-4 of that of the stellar surface. In its centre it consists of carbon and oxygen surrounded by a thin layer of some 10-2 M⊙ occupied by two nuclear-active shells burning He and H. Correspondingly, this core can be thought of as being a very hot white dwarf which grows in mass by accreting nuclear-processed matter from the envelope. Here, we want to discuss the evolution along the upper AGB which is mainly determined by (i) the recurrent thermal instabilities of the helium burning shell (thermal pulses); (ii) the penetration of the convective envelope into the hydrogen burning shell for more massive objects (hot bottom burning), and (iii) strong and steeply increasing mass losses.

Hot Bottom Burning in Asymptotic Giant Branch Stars and the Turbulent Convection Model

Hot Bottom Burning in Asymptotic Giant Branch Stars and the Turbulent Convection Model

Clues for Lithium Production in Galactic C Stars: The 12C/ 13C Ratio

view Abstract Citations (25) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Clues for Lithium Production in Galactic C Stars: The 12C/13C Ratio Abia, C. ; Isern, J. Abstract High-resolution spectroscopy is presented for 11 Population I Galactic carbon stars (C stars). Carbon isotopic ratios (12C/13C) and lithium abundances are derived from LTE synthetic spectra. It is found that the 13C-rich stars are also Li-rich. We argue that the Li and 13C enrichment in Galactic C stars might be produced by a similar mechanism. Evidence of the operation of hot bottom burning (HBB) is found in the 13C-rich stars. However, theoretical models for asymptotic giant branch stars do not predict the simultaneous existence of HBB and Li production in stars with initial masses M ∼2 Msun, which is the typical initial mass of the stars analyzed here. We briefly discuss the observations in terms of the current models for C stars. Publication: The Astrophysical Journal Pub Date: March 1996 DOI: 10.1086/176982 Bibcode: 1996ApJ...460..443A Keywords: STARS: ABUNDANCES; STARS: CARBON; STARS: AGB AND POST-AGB full text sources ADS | data products SIMBAD (11)

Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances

view Abstract Citations (127) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hot Bottom Burning in Asymptotic Giant Branch Stars and Its Effect on Oxygen Isotopic Abundances Boothroyd, Arnold I. ; Sackmann, I. -Juliana ; Wasserburg, G. J. Abstract A self-consistent calculation of asymptotic giant branch (AGB) evolution was carried out, including nucleosynthesis at the base of the convective envelope (hot bottom burning). Hot bottom burning was found to occur for stars between approximately 4.5 and approximately 7 solar mass, producing envelopes with O-18/O-16 less than or equal to 10-6 and 10-3 approximately less than or equal O-17/O-16 approximately less than or equal to 10-1. The O-17 abundance depends sensitively on the nuclear O-17-destruction rate; this rate is only loosely constrained by the requirement that first and second dredge-up models match O-isotope observations of red giant branch (RGB) stars (Boothroyd, Sackmann, & Wasserburg 1994). In some cases, high mass-loss rates can terminate hot bottom burning before further O-17 enrichment takes place or even before all O-18 is destroyed. These predictions are in accord with the very limited stellar observations of J type carbon stars on the AGB and with some of the circumstellar Al2O3 grains from meteorites. In contrast, precise data from a number of grains and data from most low-mass S and C AGB stars (approximately less than 1.7 solar mass) lie in a region of the O-18/O-16 versus O-17/O-16 diagram that is not accessible by first and second dredge-up or by hot bottom burning. We conclude that for AGB stars, the standard models of stellar evolution are not in accord with these observations. We surmise that an additional mixing mechanism must exist that transports material from the cool bottom of the stellar convective envelope to a depth at which O-18 is destroyed. This 'cool bottom processing' mechanism on the AGB is similar to extra mixing mechanisms proposed to explain the excess C-13 (and depleted C-12) observed in the earlier RGB stage of evolution and the large Li-7 depletion observed in low-mass main-sequence stars. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/187806 Bibcode: 1995ApJ...442L..21B Keywords: Abundance; Asymptotic Giant Branch Stars; Nuclear Fusion; Oxygen Isotopes; Stellar Composition; Stellar Models; Carbon Stars; Interstellar Matter; Stellar Envelopes; Stellar Evolution; Stellar Temperature; Astrophysics; ISM: DUST; EXTINCTION; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; ABUNDANCES; STARS: ABUNDANCES; STARS: AGB AND POST-AGB full text sources ADS |

The Creation of Lithium in Giant Stars

A time-dependent “convective diffusion” algorithm for convective transport in the mixing-length framework has been coupled for the first time with a self-consistent full evolutionary computation, in order to investigate theoretically the creation of superrich lithium stars on the asymptotic giant branch. For intermediate mass stars in the mass range from 4 to 7 M⊙ with both Population I and II compositions, hot bottom burning in the convective envelope was found, with maximum temperatures Tce at the base of the convective envelope ranging from 20 to 100 million K, depending on stellar mass and mass loss rates. For Tce ≥ 40 million K, lithium-rich giants were produced (with log ε(7Li) ≳ 1, i.e., above the normal observed range in giants). For Tce ≥ 50 million K, superrich lithium giants were created, with log ε(7Li) ≳ 3 (i.e., larger than the present cosmic7Li abundance).

Predictions of oxygen isotope ratios in stars and of oxygen-rich interstellar grains in meteorites

We carried out detailed, self-consistent calculations for stars from 1 to 9 M_☉ over a wide range of metallicities, following the evolution and nucleosynthesis from the pre-main sequence to the asymptotic giant branch (AGB), in order to provide a self-consistent grid for evaluating stellar oxygen isotopic variations. These were calculated for first and second dredge-up, and for some masses also for third dredge-up and hot convective envelope burning on the AGB. We demonstrate that ^(16)O/^(17)O in red giant envelopes is primarily a function of the star's mass, while ^(16)O/^(18)O is primarily a function of the initial composition. Uncertainties in the ^(17)O-destruction rate have no effect on the ^(16)O/^(17)O ratio for stars from 1 to 2.5 M_☉, but do affect the ratios for higher masses: the stellar ^(16)O/^(17)O observations are consistent with the Landre et al. (1990) rates using ƒ = 0.2 for ^(17)O(p, y)^(18)F and ^(17)O(p, ɑ)^(14)N, and with the Caughlan & Fowler (1988) rates using ƒ ~ 1. The stellar ^(16)O/^(18)O observations require ƒ ~ 0 in the Caughlan & Fowler ^(18)O(p, ɑ)^(15)N rate. First dredge-up has the largest effect on the oxygen isotope ratios, decreasing ^(16)O/^(17)O significantly from the initial value and increasing ^(16)O/^(18)O slightly. Second and third dredge-up have only minor effects for solar metallicity stars. The absence of very low observed ^(16)O/^(18)O ratios is consistent with a major increase in the ^(18)O(ɑ, y)^(22)Ne rate over the Caughlan & Fowler (1988) value. Hot bottom burning in stars above about 5 M_☉ can cause a huge increase in ^(16)O/^(18)O (to ≳10^6), and possibly a significant decrease in ^(16)O/^(17)O; these are accompanied by a huge increase in ^7Li and a value of ^(12)C/^(13)C ≈ 3. The oxygen isotope ratios in the Al_2O_3 grains (Orgueil grain B, the Murchison 83-5 grain, and the new Bishunpur B39 grain) can be accounted for if they originated in stars that did NOT have the same initial ^(16)O/^(18)O ratio. Thus one cannot assume uniform isotope ratios, even for stars of nearly solar composition. The grains' ^(16)O/^(17)O ratios, together with the ^(26)Mg excesses that indicate grain formation in a ^(26)Al-rich environment, indicate that the Orgueil grain B and Murchison 83-5 grain originated in stars of roughly 1.5 M_☉, during third dredge-up on the AGB. The new Bishunpur B39 grain originated in a star of either 2 or of 4-7 M_☉.

Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. I. The Extreme Type I SMP 83/WS 35

view Abstract Citations (40) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. I. The Extreme Type I SMP 83/WS 35 Dopita, Michael A. ; Ford, Holland C. ; Bohlin, Ralph ; Evans, Ian N. ; Meatheringham, Stephen J. Abstract We have obtained Hubble Space Telescope Planetary Camera images in both the Hα and the [O III] λ5007 emission lines of the planetary nebula SMP 83 alias WS35, alias N66 in the Large Magellanic Cloud. By combining these results with optical and UV spectrophotometry, absolute flux measurements, and dynamical and density information, we have been able to construct a fully self-consistent nebular model. This proves that SMP 83 is an extremely massive type I object having a central star having an effective temperature of 170,000 K and a luminosity of nearly 3 × 104 Lsun. The core mass is estimated in the range 1.0-1.2 Msun, for which the main-sequence mass was greater than ∼6 Msun. The nebular abundances are higher than the average for the LMC and show evidence for hot-bottom burning. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173437 Bibcode: 1993ApJ...418..804D full text sources ADS | data products SIMBAD (4) NED (2) MAST (1) ESA (1) Related Materials (6) Part 2: 1994ApJ...426..150D Part 3: 1996ApJS..105..375V Part 4: 1996ApJ...460..320D Part 5: 1997ApJ...474..188D Part 6: 1998ApJS..114..237V Part 7: 1998ApJ...503..253V

Prevention of High-Luminosity Carbon Stars by Hot Bottom Burning

CNO abundances are presented for theoretical models of asymptotic giant branch stars of 4, 5, and 6 M ○. with Z=0.02 and Z=0.001. Self-consistent stellar evolutionary models were computed, starting on the pre-main sequence and terminating after a series of helium shell flashes on the asymptotic giant branch (AGB). The 5 and 6 M ○. cases encountered hot bottom burning, with temperatures at the base of the convective envelope reaching (60-80)×10 6 K for Z=0.02 and (80-100)×10 6 K for Z=0.001. A luminosity boundary near M C bol ∼−6.4 is predicted; for brighter luminosities, hot bottom burning will prevent carbon star formation (these will be S stars, with 3≤ 12 C/ 13 C≤18, and for a period they will be super lithium-rich)

S-Process Nucleosynthesis on the Asymptotic Giant Branch

AbstractIn this article I review our understanding of s-process nucleosynthesis in Asymptotic Giant Branch (AGB) stars. The discussion begins with the nuclear back~ ground and then shows how the required conditions are found in thermally pulsing AGB stars. The two competing neutron sources (Ne22 and C13) are discussed, as is the observational and theoretical evidence for and against each of them. We conclude by discussing the recent evidence for hot bottom burning, including the observed enhancements of Li7 and Al26 in some AGB stars.

An extremely carbon-poor planetary nebula in the Small Magellanic Cloud

An optical and ultraviolet study is presented of object 28 of Sanduleak et al. (1978) in the SMC. This object is an extreme type of I PN which apparently has undergone all three stages of nuclear dredge-up together with an appreciable amount of convective envelope burning. Optical and UV data are well-represented by a model in which the central star has a temperature of 180,000 K and a radius of 0.09 solar radius. A nebular mass of 0.71 solar mass and a central star mass of 0.65-0.71 solar mass are inferred. The object apparently evolved from a massive progenitor with main-sequence mass greater than five solar masses, which underwent both second and third nuclear dredge-up and very efficient hot bottom burning.

Models of AGB Stars Envelopes and Atmospheres

The possible existence of a hot CN-cycle at the bottom of the AGB convective envelopes (“hot-bottom burning” or HBB) encounters some revealing difficulties with observations (e.g. Reid and mould, 1985). On the other hand, existing models disagree with each other about it.