scholarly journals Theory of Evolution of the Central Star

1978 ◽  
Vol 76 ◽  
pp. 195-199
Author(s):  
Giora Shaviv
Keyword(s):  
White Dwarf ◽  
Planetary Nebulae ◽  
Central Star ◽  
Point Of View ◽  
Linear Size ◽  
Theoretical Point ◽  
Fundamental Feature ◽  
Theory Of Evolution ◽  
The Core ◽  
Hr Diagram

The fundamental feature of the evolution of the central star of planetary nebulae (CPN) is the cooling of the core towards a white dwarf. The basic observations were made by O'Dell (1963), Seaton (1966), and Harman and Seaton (1964); namely, they discovered a correlation between the linear size of the nebula and the position of the CNP in the HR diagram. Typical values for the velocity of expansion are 10–30 km/sec and the largest size observed for a nebula is ∼0.6 pc and hence the lifetime is 2–6 × 104 years. From a theoretical point of view the important fact about the Harman-Seaton sequence (HS) is the implication on the evolution of the central star. The star first heats up, L and Te increase, then L decreases. But Te keeps increasing until finally Te decreases also (shown schematically in Figure 2). Thus the HS is an evolutionary sequence and not a locus of stars.

scholarly journals Planetary Nebula Evolution Traced by Distance-Independent Parameters

1993 ◽  
Vol 155 ◽  
pp. 480-480
Author(s):  
C.Y. Zhang ◽  
S. Kwok
Keyword(s):  
Physical Properties ◽  
Mass Distribution ◽  
Planetary Nebula ◽  
Strong Support ◽  
Planetary Nebulae ◽  
Central Star ◽  
Evolution Model ◽  
The Core ◽  
Independent Parameters ◽  
Central Stars

Making use of the results from recent infrared and radio surveys of planetary nebulae, we have selected 431 nebulae to form a sample where a number of distance-independent parameters (e.g., Tb, Td, I60μm and IRE) can be constructed. In addition, we also made use of other distance-independent parameters ne and T∗ where recent measurements are available. We have investigated the relationships among these parameters in the context of a coupled evolution model of the nebula and the central star. We find that most of the observed data in fact lie within the area covered by the model tracks, therefore lending strong support to the correctness of the model. Most interestingly, we find that the evolutionary tracks for nebulae with central stars of different core masses can be separated in a Tb-T∗ plane. This implies that the core masses and ages of the central stars can be determined completely independent of distance assumptions. The core masses and ages have been obtained for 302 central stars with previously determined central-star temperatures. We find that the mass distribution of the central stars strongly peaks at 0.6 M⊙, with 66% of the sample having masses <0.64 MM⊙. The luminosities of the central stars are then derived from their positions in the HR diagram according to their core masses and central star temperatures. If this method of mass (and luminosity) determination turns out to be accurate, we can bypass the extremely unreliable estimates for distances, and will be able to derive other physical properties of planetary nebulae.

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 An imaging and spectroscopic survey of the Abell Planetary Nebulae

1997 ◽  
Vol 180 ◽  
pp. 287-287
Author(s):  
N. A. Walton ◽  
J. R. Walsh ◽  
G. Dudziak
Keyword(s):  
White Dwarf ◽  
Surface Brightness ◽  
Planetary Nebulae ◽  
Central Star ◽  
Large Size ◽  
Low Surface Brightness ◽  
Thermal Pulses ◽  
Central Stars

The Abell catalogue of planetary nebulae (PN) are distinguished by their large size, low surface brightness and generally faint central stars. They are thought to be old PN approaching the White Dwarf cooling track. A number have evidence for late thermal pulses (H-poor ejecta near the central star, e.g. A78) and binary central stars.

Intellectual conformism depends on institutional incentives, not on socialized culture

2006 ◽  
Vol 29 (6) ◽  
pp. 569-570
Author(s):  
Li Bennich-Björkman
Keyword(s):  
Organizational Culture ◽  
Point Of View ◽  
Intellectual Freedom ◽  
Unexpected Finding ◽  
Theoretical Point ◽  
Academic Behavior ◽  
The Core ◽  
Institutional Incentives

The study by Ceci et al. shows that academic behavior associated with the core principles of intellectual freedom is more shaped by institutional incentives than by organizational culture. From an organizational theoretical point of view, this is quite an unexpected finding, not least because we do believe universities to be fairly strong and explicit cultures that should be successful in socialization.

scholarly journals Rate of change of the pulsation periods in the PG 1159 star PG 0122+200

2008 ◽  
Vol 4 (S252) ◽  
pp. 157-162
Author(s):  
G. Vauclair ◽  
J.-N. Fu ◽  
J.-E. Solheim ◽  
S.-L. Kim ◽  
M. Chevreton ◽  
...  
Keyword(s):  
White Dwarf ◽  
Rapid Evolution ◽  
Theoretical Models ◽  
Variable Stars ◽  
Rate Of Change ◽  
Point Of View ◽  
Short Time Scale ◽  
Theoretical Point ◽  
Instability Strip ◽  
Core Cooling

AbstractThe pre-white dwarf pulsators of PG 1159 type, or GW Virginis variable stars, are in a phase of rapid evolution towards the white dwarf cooling sequence. The rate of change of their nonradial g-mode frequencies can be measured on a reasonably short time scale. From a theoretical point of view, it was expected that one could derive the rate of cooling of the stellar core from such measurements. At the cool end of the GW Virginis instability strip, it is predicted that the neutrinos flux dominates the cooling. PG 0122+200 which defines the red edge of the instability strip is in principle a good candidate to check this prediction. It has been followed-up through multisite photometric campaigns for about fifteen years. We report here the first determination of the rate of change of its 7 largest amplitude frequencies. We find that the amplitudes of the frequency variations are one to two orders of magnitude larger than predicted by theoretical models based on the assumption that these variations are uniquely caused by cooling. The time scales of the variations are much shorter than the ones expected from a neutrino dominated core cooling. These results point to the existence of other mechanisms responsible for the frequency variability. We discuss the role of nonlinearities as one possible mechanism.

The real time evolution of post-AGB stars

2019 ◽  
Vol 15 (S357) ◽  
pp. 154-157
Author(s):  
Marcin Hajduk
Keyword(s):  
Real Time ◽  
Time Evolution ◽  
Planetary Nebulae ◽  
Theoretical Models ◽  
Central Star ◽  
Maximum Temperature ◽  
Additional Parameter ◽  
Agb Stars ◽  
The Real ◽  
Hr Diagram

AbstractEvolution of post-AGB stars is extremely fast. They cross the HR diagram vertically on a timescale of hundreds to some ten thousands of years to reach maximum temperature in their lifetime. This is reflected in an increasing excitation of planetary nebulae on a timescale of years and decades. Since evolutionary timescale of post-AGB stars is very sensitive to their mass, observed changes can be used to determine model dependent central star masses. If an additional parameter is determined (e.g. luminosity or dynamic age), the observed evolution of planetary nebulae can be utilized for observational verification of theoretical models.

scholarly journals Distances of the Central Stars and their Position in the HR Diagram

1983 ◽  
Vol 103 ◽  
pp. 391-409 ◽  
Author(s):  
S.R. Pottasch
Keyword(s):  
Galactic Center ◽  
Planetary Nebulae ◽  
Central Star ◽  
Magellanic Clouds ◽  
Constant Mass ◽  
Theoretical Predictions ◽  
Statistical Distance ◽  
Central Stars ◽  
Hr Diagram

Determination of the distances to individual planetary nebulae are discussed. Especially those methods which are independent of assumed nebular properties (mass, absolute flux, etc.) are assembled and discussed. In this way, reasonable approximations to the distance can be obtained for about 50 planetary nebulae. The accuracy of the distances is tested by comparing nebular properties derived from these distances with the properties of nebulae at the galactic center or in the Magellanic clouds. A comparison is also made with the statistical distance determinations; the conclusion is that the assumption of constant mass often leads to an overestimate of the distance, while the assumption of constant Hβ flux leads to distances having individual uncertainties of up to a factor of 3.The central star temperature determination is summarized. Individual central stars are placed on the HR diagram and compared with theoretical predictions. Deductions concerning the evolution which can be made from the observations are discussed.

scholarly journals The Continuum Emission from Planetary Nebulae

1989 ◽  
Vol 131 ◽  
pp. 226-226
Author(s):  
Sueli M. Viegas-Aldrovandi
Keyword(s):  
Planetary Nebula ◽  
Planetary Nebulae ◽  
Central Star ◽  
The Other ◽  
Continuum Emission ◽  
Other Hand ◽  
The Continuum ◽  
Hr Diagram

The study of nebular continuum emission is important for several reasons (Pottasch 1984, Planetary Nebulae, Dordrecht: Reidel). First of all, it can provide information about the temperature and the density of the nebula, when the object is large enough, or when the central star is weak enough, so that the nebular continuum is easily observed without interference from the stellar continuum. On the other hand, for small planetary nebulae, both the central star and the nebula contribute to the observed continuum. In this latter case, in order to obtain the stellar continuum the theoretical nebular emission must be used. Thus, studies of the evolution of planetary nebula nuclei through the HR diagram rely on a good calculation of the theoretical nebular continuum.

Hα and Hβ Imaging of the Planetary Nebula NGC 6302

1990 ◽  
Vol 8 (04) ◽  
pp. 360-363 ◽  
Author(s):  
Michael C. B. Ashley
Keyword(s):  
Planetary Nebula ◽  
Planetary Nebulae ◽  
Central Star ◽  
Infrared Observations ◽  
Dust Extinction ◽  
The Core ◽  
Optical Images ◽  
5 Ghz ◽  
Good Agreement

Abstract NGC 6302 is one of the highest excitation planetary nebulae known. It has an obscured central star with a temperature estimated at 430,000 K. We present here CCD images in Hα and Hβ of NGC 6302, and interpret the differences between the images as being due to extinction caused by dust within the nebula. The dust appears to be concentrated in the core, as expected from infrared observations. There is no evidence of patchy foreground extinction, although there is a slight difference in the average extinction between the eastern and western lobes of the nebula. A comparison between the Hα image and a 5 GHz map gives a dust extinction of ΔAV~3.5 to the central star. The outer contours of the 5 GHz map are not in good agreement with the optical images, and further observations at this frequency would be useful.

scholarly journals Are Pulsations a Useful Probe of the Structure of the Outer Layers of White Dwarfs ?

1989 ◽  
Vol 114 ◽  
pp. 263-268
Author(s):  
P. Brassard ◽  
G. Fontaine ◽  
F. Vesemael ◽  
S.D. Kawaler
Keyword(s):  
Structural Feature ◽  
White Dwarf ◽  
White Dwarfs ◽  
Point Of View ◽  
Fundamental Aspect ◽  
Ionization Mechanisms ◽  
The Core ◽  
Near Resonance ◽  
Oscillation Modes ◽  
The Many

The most fundamental aspect of white dwarf seismology is the determination of the gravity-mode (g-mode) period structures of models of isolated pulsating white dwarfs. These stars show multiperiodic luminosity variations which result from the superposition of excited pulsation modes. Among the many oscillation modes available in the very rich nonradial g-mode spectra of white dwarfs, the observed modes are selectively chosen by a filtering mechanism. Although the period evolution is strongly tied to the core temperature evolution in a white dwarf, the period structure remains largely specified by the mechanical properties of the star. The most basic structural feature of a white dwarf is its highly degenerate interior, which leads to nearly isothermal and nearly isentropic stratifications in the core region containing more than 99% of the mass of the star. In particular, because the density gradient is almost adiabatic throughout the interior of a white dwarf, the Brünt-Väisälä frequency (see below) is very small there and low-order g-modes cannot propagate. As a result, g-modes are essentially envelope modes in white dwarfs, with large amplitudes occuring only in the non-degenerate outer layers. One can thus expect that g-modes in white dwarfs are extremely sensitive to envelope properties such as compositional stratification and partial ionization mechanisms. Compositional stratification is, in fact, the second structural feature of a white dwarf model which has strong effects on the period structure. Indeed, trapped modes result when a resonance or near-resonance occurs between the local g-mode radial wavelength and the thickness of one of the composition layers. This results in a period structure which strongly bears the signature of compositional stratification in the outer layers. Thus, it has been widely accepted that white dwarf pulsations probe primarily the outer layers of these stars. This point of view has been borne out by detailed pulsation calculations carried out by several independant groups (see Winget 1987 and references therein).

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