scholarly journals A Shortlived, Deep Convective Envelope for Highly Evolved, Blue Stars?

1978 ◽  
Vol 76 ◽  
pp. 207-208 ◽  
Author(s):  
I.-Juliana Sackmann
Keyword(s):  
Planetary Nebulae ◽  
Helium Burning ◽  
Core Mass ◽  
Convective Envelope ◽  
Vast Number ◽  
Left Behind ◽  
New Type ◽  
Red Giant ◽  
Highly Evolved

An interesting new phenomenon was encountered while evolving a star with a core mass, Mc of 0.8 M0 and with a small envelope mass (0.015 M0) away from the red giant branch towards the nuclei of planetary nebulae, while taking the helium shell flashes into account. It was found that the top of the intershell carbon pocket (the carbon-enriched region in between the hydrogen- and helium-burning shells left behind by the flash) was expanded outwards and cooled immensely; namely, cooled to near 20,000°K! This means that the intershell carbon pocket was lifted out to near the photosphere, right into the shallow outer convective envelope surrounding the hydrogen- and helium-ionization zones! The carbon opacity at these cool temperatures is great. It seems likely that all the layers from the outer regions of the intershell carbon pocket right up to the surface will become convective. This would be a totally new type of deep convective envelope with a vast number of fascinating implications. Careful checks of this new phenomenon are now underway. (Supported in part by the National Aeronautics and Space Administration [NSG 7195].)

scholarly journals Fast Winds in Planetary Nebulae

1983 ◽  
Vol 103 ◽  
pp. 305-316 ◽  
Author(s):  
F. D. Kahn
Keyword(s):  
Planetary Nebula ◽  
Planetary Nebulae ◽  
Central Star ◽  
Taylor Instability ◽  
Considerable Time ◽  
Left Behind ◽  
Hot Gas ◽  
Fast Wind ◽  
Rayleigh Taylor Instability ◽  
Red Giant

A planetary nebula consists mainly of gas ejected slowly by a red giant. Its dynamics is dominated by the hot central star which is left behind later. In particular a fast wind from this star forms a bubble of hot gas which fills the inner part of the nebula and pushes the envelope into a shell. This shell remains only partly ionized for a considerable time. Its non-ionized part is subject to a Rayleigh-Taylor instability, and is expected to break up into fragments which remain behind in the HII part of the nebula.

scholarly journals Evolution and Mass Distribution of Central Stars of Planetary Nebulae

1983 ◽  
Vol 103 ◽  
pp. 359-371 ◽  
Author(s):  
D. Schoenberner ◽  
V. Weidemann
Keyword(s):  
Mass Distribution ◽  
Theoretical Calculations ◽  
Planetary Nebulae ◽  
Birth Rates ◽  
Core Mass ◽  
The Core ◽  
Average Factor ◽  
Red Giant ◽  
Central Stars ◽  
Hr Diagram

Considerable progress has been made in our understanding of the evolution of the central stars of planetary nebulae (NPN) compared to the situation five years ago at the Ithaca Symposium where Shaviv (1978) and Paczynski (1978) reviewed the subject. Shaviv stressed the necessity to start theoretical calculations with realistic initial models but doubted - in view of the loops in the HR diagram made by flashing stars - if the Harman-Seaton sequence could be taken as a single evolutionary sequence. Paczynski pointed out how strongly the theoretical rate of evolution depends on the stellar mass - a result which had appeared in his earlier calculations (1971) - and expected the existence of more flashing NPN's of the FG Sagittae type among the luminous (L > 104 L⊙) central stars, for which the core mass luminosity relation (Mc > 0.7 M⊙) combined with the core mass interpulse time relation predicts fairly short (2.10 yrs) intervals between flashing events. Weidemann, however, at the Symposium and shortly thereafter (1977a) concluded in view of the lower effective temperature derived by Pottasch et al. (1978) and the observed narrow mass distribution of white dwarfs around a 0.6 Mo. combined with the theoretical predicted horizontal tracks from the red giant branch towards the NPN region at a luminosity given by the core mass luminosity relation that the high luminosity part (and also the “upturn”) of the Harman-Seaton sequence does not exist. He also proposed an increase in the distances by an average factor of 1.3 compared to the Seaton/Webster (Seaton, 1968) or Cahn/Kaler (1971) scale in order to bring the observed NPN on the 0.6 M⊙ track in the HR diagram and to lower the NPN birth rates to a value compatible with white dwarf birth rates.

scholarly journals Chemical Enrichment and Central Star Properties

1993 ◽  
Vol 155 ◽  
pp. 572-572
Author(s):  
C.Y. Zhang
Keyword(s):  
Planetary Nebulae ◽  
Central Star ◽  
Agb Stars ◽  
Physical Processes ◽  
Chemical Abundances ◽  
Core Mass ◽  
Chemical Enrichment ◽  
The Core ◽  
Stellar Temperature ◽  
Independent Parameters

We have selected a sample of planetary nebulae, for which the core masses are determined using distance-independent parameters (Zhang and Kwok 1992). The chemical abundances of He, N, O, and C are taken from the literature for them. Relationships of the ratios of He/H, N/O, and C/O with various stellar parameters of planetary nebulae (PN), such as the core mass, the mass of the core plus the ionized nebular gas, the stellar age and temperature, are examined. It is found that the N/O increases with increasing mass, while the C/O first increases and then decreases with the core mass. No strong correlation seems to exist between the He/H and the core mass. A correlation of the N/O and He/H with the stellar temperature exists. The current dredge-up theory for the progenitor AGB stars cannot satisfactorily account for these patterns of chemical enrichment in PN. Furthermore, the correlations of the N/O and He/H with the stellar age and temperature indicate that besides the dredge-ups in the RG and AGB stages, physical processes that happen in the planetary nebula stage may also play a role in forming the observed patterns of chemical enrichment in the planetary nebulae.

scholarly journals Characterizing global expansion evolution of the ionized matter in planetary nebulae

2016 ◽  
Vol 12 (S323) ◽  
pp. 352-353
Author(s):  
J. A. López ◽  
M. G. Richer ◽  
M. Pereyra ◽  
M. T. García-Díaz
Keyword(s):  
Galactic Halo ◽  
Evolutionary Rate ◽  
Planetary Nebulae ◽  
Central Star ◽  
Global Expansion ◽  
Kinematic Evolution ◽  
Wide Range ◽  
Model Predictions ◽  
First Time ◽  
Highly Evolved

AbstractBulk outflow or global expansion velocities are presented for a large number of planetary nebulae (PNe) that span a wide range of evolutionary stages and different stellar populations. The sample comprises 133 PNe from the Galactic bulge, 100 mature and highly evolved PNe from the disk, 11 PNe from the Galactic halo and 15 PNe with very low central star masses and low metallicities, for a total of 259 PNe. These results reveal from a statistical perspective the kinematic evolution of the expansion velocities of PNe in relation to changing characteristics of the central star’s wind and ionizing luminosity and as a function of the evolutionary rate determined by the central (CS) mass. The large number of PNe utilized in this work for each group of PNe under study and the homogeneity of the data provide for the first time a solid benchmark form observations for model predictions, as has been described by López et al. (2016).

scholarly journals Core rotation braking on the red giant branch for various mass ranges

2018 ◽  
Vol 616 ◽  
pp. A24 ◽  
Author(s):  
C Gehan ◽  
B. Mosser ◽  
E. Michel ◽  
R. Samadi ◽  
T. Kallinger
Keyword(s):  
Angular Momentum ◽  
Red Giants ◽  
Regular Pattern ◽  
Convective Envelope ◽  
Large Sample ◽  
The Mean ◽  
Mixed Modes ◽  
Red Giant ◽  
Core Rotation ◽  
Dipole Gravity

Context. Asteroseismology allows us to probe stellar interiors. In the case of red giant stars, conditions in the stellar interior are such as to allow for the existence of mixed modes, consisting in a coupling between gravity waves in the radiative interior and pressure waves in the convective envelope. Mixed modes can thus be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for red giant core rotation. Thus large-scale measurements of red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. Aims. This work aims at identifying the components of the rotational multiplets for dipole mixed modes in a large number of red giant oscillation spectra observed by Kepler. Such identification provides us with a direct measurement of the red giant mean core rotation. Methods. We compute stretched spectra that mimic the regular pattern of pure dipole gravity modes. Mixed modes with the same azimuthal order are expected to be almost equally spaced in stretched period, with a spacing equal to the pure dipole gravity mode period spacing. The departure from this regular pattern allows us to disentangle the various rotational components and therefore to determine the mean core rotation rates of red giants. Results. We automatically identify the rotational multiplet components of 1183 stars on the red giant branch with a success rate of 69% with respect to our initial sample. As no information on the internal rotation can be deduced for stars seen pole-on, we obtain mean core rotation measurements for 875 red giant branch stars. This large sample includes stars with a mass as large as 2.5 M⊙, allowing us to test the dependence of the core slow-down rate on the stellar mass. Conclusions. Disentangling rotational splittings from mixed modes is now possible in an automated way for stars on the red giant branch, even for the most complicated cases, where the rotational splittings exceed half the mixed-mode spacing. This work on a large sample allows us to refine previous measurements of the evolution of the mean core rotation on the red giant branch. Rather than a slight slow-down, our results suggest rotation is constant along the red giant branch, with values independent of the mass.

scholarly journals Low-Mass AGB Stellar Models for 0.003 ≤ Z ≤ 0.02: Basic Formulae for Nucleosynthesis Calculations

2003 ◽  
Vol 20 (4) ◽  
pp. 389-392 ◽  
Author(s):  
O. Straniero ◽  
I. Domínguez ◽  
S. Cristallo ◽  
R. Gallino
Keyword(s):  
Effective Temperature ◽  
Convective Instability ◽  
Convective Zone ◽  
Maximum Temperature ◽  
Core Mass ◽  
Convective Envelope ◽  
Appropriate Treatment ◽  
Low Mass ◽  
Thermal Pulses ◽  
Radiative Opacity

AbstractWe have extended our published set of low-mass AGB stellar modelsto lower metallicities. Different mass-loss rates have been explored. We provide interpolation formulae for the luminosity, effective temperature, core mass, mass of dredge up material and maximum temperature in the convective zone generated by thermal pulses. Finally, we discuss the resultant modification of these quantities when we use an appropriate treatment of the inward propagation of the convective instability, as caused by the steeprise in radiative opacity when the convective envelope penetratesthe H-depleted region.

scholarly journals Evolution of Unstable Red Giant Envelopes

1983 ◽  
Vol 103 ◽  
pp. 281-289
Author(s):  
Y. Tuchman
Keyword(s):  
Planetary Nebulae ◽  
Dynamical Behaviour ◽  
Red Giant

Two main observable phenomena are directly connected with the dynamical behaviour of Red Giant (R.G.) envelopes: The variability of Mira stars and Planetary Nebulae (P.N.) ejection.

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 Asymptotic Giant Branch Stars: Thermal Pulses, Carbon Production, and Dredge Up; Neutron Sources and s-Process Nucleosynthesis

1991 ◽  
Vol 145 ◽  
pp. 257-274
Author(s):  
Icko Iben
Keyword(s):  
Neutron Source ◽  
Lower Boundary ◽  
Source Model ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Core Mass ◽  
Convective Envelope ◽  
Thermal Pulses ◽  
Giant Branch Stars

A brief review is given of the structure of asymptotic giant branch (AGB) stars and of the characteristics of the thermal pulses which these stars experience. Following a pulse, model AGB stars with a large core mass easily dredge up fresh carbon, which is the main product of incomplete helium burning, and s-process isotopes, which are made as a consequence of the activation of the 22Ne neutron source. Model AGB stars of small core mass activate the 13C neutron source and produce s-process isotopes in nearly the solar system distribution. They also dredge up fresh carbon and s-process isotopes, but only if overshoot or some other form of “extra” mixing beyond the lower boundary of the convective envelope is invoked.

Initial Masses

1989 ◽  
Vol 131 ◽  
pp. 493-504
Author(s):  
D.C.V. Mallik
Keyword(s):  
Mass Distribution ◽  
Main Sequence ◽  
Planetary Nebulae ◽  
Initial Mass ◽  
Exact Nature ◽  
Red Giants ◽  
Selection Effects ◽  
Widespread Occurrence ◽  
Red Giant ◽  
Central Stars

Planetary nebulae represent a transitory stage in the life of the majority of stars as they proceed towards the end of their nuclear evolution and descend to the domain of white dwarfs. The immediate precursors of the central stars are probably red giants which populate a part of the HR diagram far removed from the region inhabited by the central stars of well recognised nebulae. The problem of determining the initial masses is complicated by the widespread occurrence of massloss on the red giant branch. The total amount of mass lost by a star must depend upon a number of stellar parameters including the initial mass, but the exact nature of this dependence remains to be discovered and a unique relation between the final masses and initial main sequence masses is not yet available. Thus even though the mass distribution of the nuclei of planetary nebulae (NPN) has been derived in the last few years, it has not been possible to deduce from this an unambiguous initial mass distribution of the progenitors. Further, an observed sample always suffers from selection effects and, in the particular case of NPN mass distribution, this has led to irretrievable loss of information.

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