Giant Envelope Research Articles

ON PREPLANETARY NEBULA FORMATION

ABSTRACT We propose a comprehensive model in which we examine the process of gas outflow by an evolved star (post-AGB stars) and the subsequent circumstellar shell formation. First, we investigate the bouyant transport of an isolated toroid-like tube through the Red Giant convective envelope, originating from the innermost region of the star and composed of strongly magnetized gas (section II). When carrying up to a position immediately above the stellar surface, such a tube is no more confined by the gas pressure existing throughout the Red Giant envelope and consequently expands. It is hypothesized that the confluence of such magnetized tubes can produce a disk-like structure of cool matter which surrounds the star (section III). We have also analyzed the blowing process by a fast wind of the early ejected cool matter. The large and small scale structures of the preplanetary nebula shell such as the bipolarity, the filamentary pattern and the multiple-shell appearance are then discussed (section IV). A critical comparison with other concurrent models (Balick et al. 1987, Soker and Livio 1989, Morris 1990) are also summarily presented.

On the interaction between dust and gas in late-type stellar atmospheres and winds

An assumption inherent to most models of dust-driven winds from cool, evolved stars is that the radiative and collisional drag forces acting on an individual dust grain are in balance throughout the flow. We have checked the validity of this supposition of 'complete momentum coupling' by comparing the grain motion obtained from such a model with that derived from solution of the full grain equation of motion. For physical conditions typical of the circumstellar envelopes of oxygen-rich red giants, we find that silicate grains with initial radii smaller than about 5 x 10 exp -6 cm decouple from the ambient gas near the base of the outflow. The implications of these results for models of dust-driven mass loss from late-type giants and supergiants are discussed.

Stellar encounters involving red giants in globular cluster cores

A 3D smoothed particle hydrodynamics code is used to simulate collisions between a 0.8 solar mass red giant and impactors of mass 0.4 and 0.6 solar mass. The red giant is modeled as a massive core surrounded by a gaseous envelope, while the impactor, assumed to be a white dwarf or a main-sequence star, is treated as a single particle. Encounters at various impact parameters and with relative velocities pertinent to global clusters are studied. It is found that, if the impactor passes within twice the radius of the red giant, mass loss occurs from the red giant envelope and both objects are left bound in elliptical orbits. Calculations suggest that if the impactor has an initial distance at the closest approach of approximately 1.6 R(RG) or less, the impactor will spiral into the red giant envelope on subsequent periastron passages. For bound systems with a greater separation, the orbit may circularize with a radius approximately double that of the initial counter. 27 refs.

Lithium abundances in carbon stars

view Abstract Citations (46) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lithium Abundances in Carbon Stars Denn, Grant R. ; Luck, R. E. ; Lambert, David L. Abstract High-dispersion, high SNR ratio spectra near 6707 A have provided Li abundances for 29 cool carbon stars. For the carbon stars, with the exception of the J star WZ Cas, the Li abundance ranges from log epsilon(Li) = -0.6 to -1.7 and is uncorrelated with the C-12/C-13, C/O and other abundance ratios, and the effective temperature. The Li abundances are on average lower than found for M giants. This difference may be due to a systematic underestimate of the carbon stars' effective temperatures by 200-400 K. It is possible that Li may be destroyed at the base of an asymptotic giant branch star's convective envelope. When the base temperature increases substantially, Li may be produced by the B-7 transport mechanism, as in the super Li-rich SC (J-type) star WZ Cas with log epsilon(Li) = 6.5. Publication: The Astrophysical Journal Pub Date: August 1991 DOI: 10.1086/170393 Bibcode: 1991ApJ...377..657D Keywords: Carbon Stars; Lithium; Stellar Composition; Stellar Spectra; Stellar Temperature; Abundance; Asymptotic Giant Branch Stars; Stellar Envelopes; Stellar Interiors; Astrophysics; NUCLEOSYNTHESIS; STARS: ABUNDANCES; STARS: CARBON; STARS: INTERIORS full text sources ADS | data products SIMBAD (29)

Resonant interaction in common envelopes

view Abstract Citations (16) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Resonant Interaction in Common Envelopes Soker, Noam Abstract The resonant interaction between an orbiting low-mass secondary star and the pulsational modes of the primary in a common envelope binary system is studied. When the secondary is very close to the envelope, and especially if it is inside the envelope, i.e., the system is in a common envelope phase, a rich spectrum of both g- and p-modes can be excited. During the common envelope phase high-order and high harmonic p-modes become important. By means of a simple one-zone model, the amplitudes of the forced oscillations are approximately estimated. It is claimed that if several conditions are met, then a brown dwarf secondary can cause nonnegligible axisymmetrical mass loss through its resonant interaction with a red giant primary's envelope. One implication of this for elliptical planetary nebulae is discussed. Publication: The Astrophysical Journal Pub Date: February 1991 DOI: 10.1086/169654 Bibcode: 1991ApJ...367..593S Keywords: Binary Stars; Brown Dwarf Stars; Red Giant Stars; Stellar Envelopes; Stellar Oscillations; Astronomical Models; Astronomical Spectroscopy; Planetary Nebulae; Stellar Models; Astrophysics; STARS: BINARIES; STARS: PULSATION full text sources ADS | data products SIMBAD (4)

Rotation and magnetism in white dwarfs

view Abstract Citations (62) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rotation and Magnetism in White Dwarfs Schmidt, Gary D. ; Norsworthy, Jane E. Abstract New rotational ephemerides for four isolated magnetic white dwarfs are derived from more than a decade of polarimetric monitoring. The stars span the period range 99 min-17.9 days and effectively double the number of white dwarfs with measured rotation periods. In addition, five stars whose polarization is constant on time scales up to at least 10 years may have very long rotation periods (P greater than about 100 yr). The lack of very short period white dwarfs attests to the coupling of angular momentum into the giant envelope or interstellar medium during late stages of evolution, but thus far there is no evidence that this process involves magnetic braking. With few exceptions, the disk-averaged fields of magnetic degenerates are dominated by dipolar patterns (as opposed to higher order multipoles or spots), and cover a broad range of obliquity to the rotation axis. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169559 Bibcode: 1991ApJ...366..270S Keywords: Magnetic Stars; Stellar Rotation; White Dwarf Stars; Polarimetry; Polarized Radiation; Stellar Envelopes; Stellar Magnetic Fields; Stellar Structure; Astrophysics; STARS: MAGNETIC; STARS: ROTATION; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (10)

Another lithium-rich K giant - HD 30238

The K giant HD 30238 is reported to show a strong feature due to Li I 6707 A, corresponding to a lithium abundance of log epsilon(Li) = 1.2 + or - 0.05. This star appears to be another example of a rare class of otherwise normal, low-mass K giants with unusually high lithium abundances. The C-12/C-13 isotope ratio in HD 30238 is approximately 26 + or - 4; this low value suggests that the star has developed a normal K giant convective envelope. The observed abundance of lithium in the spectrum of the star may be inconsistent with this low carbon isotope ratio. The abundance of lithium in low-mass stars on the red giant branch as a function of effective temperature suggests that lithium continues to be depleted from the convective envelope during evolution up the red giant branch. 31 refs.

Giant and supergiant stars with degenerate neutron cores

Two classes of Thorne-Zytkow (TZ) stellar models are investigated: the giant models with low-mass envelopes and gravitational energy generation, and the super giant ones with more massive envelopes and nuclear energy generation. Bisnovatyi-Kogan and Lamzin (1984) argue that there is a flaw in the TZ match of the giant star's envelope onto their cores, that no equilibrium giant models will be possible when this flaw is corrected, and that such giant stars will turn out to die quickly via runaway neutrino losses and catastrophic contraction. The paper shows that Bisnovatyi-Kogan and Lamzin are mistaken: there is no flaw in the match to the core; fully self-consistent equilibrium models do exist; and the structure of those models suggests that they will be stable against neutrino-loss-triggered contraction. By contrast, there are genuine difficulties with the TZ supergiant models because their hot CNO cycle nuclear reactions get hung up waiting for beta-decays and thus have difficulty generating enough energy for hydrostatic support. An unsuccessful attempt to build self-consistent supergiant models based on a nonequilibrium hot CNO cycle is described. 21 refs.

Cooling flows and a giant red envelope galaxy

We show that recent i-band observations of a giant red envelope galaxy (GREG) are consistent with a population of low-mass stars which has been forming from a cooling flow. This is the first time that the bulk of the accretion population in a cooling flow has been identified.

Stability and mixing of composition inversions in stellar envelopes

The stability of composition inversions in stellar envelopes is studied. An expression is derived for how steep an inversion can be before the onset of instability to large-scale motions. It is shown that, even if the gradient of the inversion is sufficiently small to be stable against such large-scale motions, it should generate small-scale turbulent motions. The size of such small-scale motions and the rate at which they are expected to destroy the inverted composition gradient are estimated, and the estimates are applied to a model of a one solar mass star accreting material from a red giant envelope that has been partially processed by CN cycling and p-p reactions. It is shown that the composition inversion in this case can be eliminated in a time that is shorter than the evolutionary time scale of the star, and it is shown how the resultant mixing can affect the observed surface abundances. 20 references.

The Shapes and Shaping of Planetary Nebulae

One of the most striking attributes of planetary nebulae are their complex, yet highly symmetric shapes. The process(es) which shape planetaries are only beginning to be understood. It is proposed that the morphologies of most PNs can be understood within the context of the “interacting winds” wherein a fast but light wind driven by the nucleus rams into an older, slower, and more massive wind, or red giant envelope (“RGE”) ejected earlier.In order to explain the shapes of noncircular PNs, it seems necessary to hypothesize that the slow wind was originally ejected with an enhanced density along an equatorial plane. The morphologies of nearly all PNs can be understood through two basic parameters: the equatorial density contrast in their RGEs, and the degree of interaction between the fast and slow winds.

The Abundance of Oxygen in M92 Giants

Studies of the carbon and nitrogen abundances in metal poor giants have generally supported the models of Sweigart and Mengel (1979) which indicate the action of meridional circulation in CN-processing metal poor red giant envelopes. Carbon et al. (1982) find the trends in the carbon and nitrogen abundances in M 92 stars to be at least qualitatively consistent with CN-cycle processing of the envelope, but stars at all phases of giant branch evolution have unusual, and unexplained, carbon and nitrogen abundances. The oxygen abundance is the missing piece of information which may explain what’s going on. Sweigart and Mengel’s models suggest that ON-processing might also occur in the most metal poor giants. The sample of giants in M 92 for which Carbon et al. have provided carbon and nitrogen abundances is ideal for the determination of oxygen abundances to look for the effects of ON-processing. Once the oxygen abundances are known, the sum C+N+0 can be computed to see if it is constant along the giant branch or varies from star to star.

A wind-blown bubble model for NGC 6543

view Abstract Citations (45) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Wind-Blown Bubble Model for NGC 6543 Balick, Bruce ; Preston, Heather L. Abstract Detailed kinematic observations of the bright inner portion of the planetary nebula NGC 6543 are presented. These data, combined with imaging observations reported by Balick et al. (1987), show that the kinematics of the nebula are just as peculiar as its morphology. Empirically, it is found that a double bipolar model readily incorporates most observational information regarding the complex structure and kinematics of NGC 6543. It is proposed that fast winds from the central star of NGC 6543 interact hydrodynamically with the denser red giant envelope expelled earlier, as suggested by Kwok (1982) and others. Publication: The Astronomical Journal Pub Date: October 1987 DOI: 10.1086/114528 Bibcode: 1987AJ.....94..958B Keywords: Astronomical Models; Gas Dynamics; Planetary Nebulae; Stellar Winds; Binary Stars; Red Giant Stars; Astrophysics; NEBULAE: PLANETARY full text sources ADS | data products SIMBAD (8)

Observational Constraints on Silicon Chemistry in the Circumstellar Envelopes of Red Giants

We review observations to date of silicon-bearing molecules in circumstellar gas and dust around red giants. New constraints on circumstellar chemistry are provided by the discovery of (i) a systematic decrease in the [SiO]/[H2] ratio with mass loss rate, ii) SiO in carbon-rich CIT 6. The presence/absence of SiO maser emission in oxygen-rich/carbon-rich envelopes is linked to differences in grain composion and the grain-condensation radius.

Detection of hydrogen cyanide emission from the peculiar oxygen–rich evolved star IRC + 10420

Chemical reaction models do not predict appreciable amounts of hydrogen cyanide (HCN) in oxygen rich [O]>[C] circumstellar envelopes. Recently, two detections of HCN in oxygen–rich envelopes have been reported1. Here we report the first detection of HCN in the oxygen–rich evolved star IRC + 10420. This star has been classified as an F8 supergiant and is considered by many to be in transition between a red giant star and a planetary nebula. Its circumstellar envelope may be in a more advanced stage of chemical evolution than red giant envelopes.

Mass loss from red giants - A simple evolutionary model for NGC 7027

NGC 7027 is a young planetary nebula with the remnants of a red giant circumstellar envelope surrounding the central, ionized region. By comparing the outer molecular envelope with the inner ionized material, it is argued that the mass loss rate has decreased by at least a factor of 3, and more probably by about a factor of 10, during the past 1000 years. From this result, it is argued that the luminosity of the central star has also decreased substantially during the same time, consistent with models for the rapid evolution of stars just after they evolve off the asymptotic giant branch. In this picture, the distance to NGC 7027 is less than 1300 pc. NGC 7027 was the last (and best) example of a star where apparently the momentum in the outflowing mass /M(dot)v/ is considerably greater than the momentum in the radiation field (L/c). With the above description of this object, the evidence is now strong that quite often the mass lost from late-type giants is ultimately driven to infinity by radiation pressure on grains. If M(dot)v is as large as L/c for asymptotic branch stars, then it is expected that the total amount of mass lost during this stage of evolution is of the same magnitude as the initial mass of the star, and therefore this mass loss can profoundly affect the star's ultimate fate.

CO line emission from the expanding circumstellar envelopes of red giants

view Abstract Citations (4) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CO line emission from the expanding circumstellar envelopes of red giants. Rose, W. K. Abstract This is the first in a series of papers which will describe calculations of molecular line emission and infrared continuum radiation from the expanding circumstellar envelopes of red giants that are undergoing rapid mass loss. This paper will be concerned primarily with CO line emission from the outer regions of these envelopes. The present method of solution requires the simultaneous solution of the radiative transfer equation for the CO emission lines, the equations of statistical equilibrium that determine the populations of the rotational levels of the CO molecule, and an energy equation, which determines the kinetic temperature distribution within the envelope if the level populations and radiation field are known. Expressions are derived for the heating rate caused by collisions between molecules and dust particles of cylindrical and spherical shape that drift through the gas as a result of radiation pressure. Solutions which were calculated for a range of physical input parameters are described and discussed. Finally, the present calculations are compared to observations of IRC + 10216, and refinements in the calculations that will be included in a subsequent paper are discussed. Publication: The Astrophysical Journal Pub Date: October 1984 DOI: 10.1086/162497 Bibcode: 1984ApJ...285..237R Keywords: Carbon Monoxide; Infrared Radiation; Molecular Spectra; Radiative Transfer; Red Giant Stars; Stellar Envelopes; Emission Spectra; Late Stars; Line Spectra; Stellar Mass Ejection; Astrophysics full text sources ADS | data products SIMBAD (1)

Density inversion in a red giant envelope

Discussion sur le phenomene d'inversion de densite dans la zone de recombinaison a partir d'une analyse detaillee d'une enveloppe particuliere d'une geante rouge

Convection in the envelopes of red giants

view Abstract Citations (36) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Convection in the envelopes of red giants Antia, H. M. ; Chitre, S. M. ; Narasimha, D. Abstract The nature of convection in the envelopes of red giants is investigated, and the linear convective modes are computed to demonstrate the consistency of the mixing length theory. The mixing length at a given depth is tentatively identified with an equivalent width of the luminosity profile of the convective eigenmodes. The dominant convective element turns out to be comparable in size to the radius of the star, and this could account for the observed irregular variations in red giants. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162236 Bibcode: 1984ApJ...282..574A Keywords: Convective Flow; Red Giant Stars; Stellar Envelopes; Astrophysics; Late Stars; Mixing Length Flow Theory; Stellar Luminosity; Stellar Models; Astrophysics full text sources ADS |

Evolution of Unstable Red Giant Envelopes

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.