Critical Luminosity Research Articles

The final 105 years of stellar AGB evolution in the presence of a pulsating, dust-induced “superwind”

We have computed mass-loss histories and tip-AGB stellar evolution models in the presence of a dust-induced, carbon-rich “superwind”, in the initial mass-range of 1.1 to about 2.5 solar masses and for nearly solar composition (X=0.28, Y=0.70, Z=0.02). Consistent, actual mass-loss rates are used in each time-step, based on pulsating and “dust-driven” stellar wind models for carbon-rich stars (Fleischer et al. 1992) which include a detailed treatment of dust-formation, radiative transfer and wind acceleration. Our tip-AGB mass-loss rates reach about 4 · 10−5M⊙yr−1 and become an influencial factor of stellar evolution.Heavy outflows of 0.3 to 0.6 M⊙ within only 2 to 3·104 yrs, exactly as required for PN-formation, occur with tip-AGB models of an initial stellar mass Mi ≳ 1.3M⊙. The mass-loss of our “superwind” varies strongly with effective temperature (Ṁ ∝ T−8eff, see Arndt et al. 1997), reflecting the temperature-sensitive micro-physics and chemistry of dust-formation and radiative transfer on a macroscopic scale. Furthermore, a thermal pulse leads to a very short (100 to 200 yrs) interruption of the “superwind” of these models.For Mi ≲ 1.1M⊙, our evolution models fail to reach the (Eddington-like) critical luminosity Lc required by the radiatively driven wind models, while for the (initial) mass-range in-between, with the tip-AGB luminosity LtAGB near Lc, thermal pulses drive bursts of “superwind”, which could explain the outer shells found with some PN's. In particular, a burst with a duration of only 800 yrs and a mass-loss of about 0.03 M⊙, occurs right after the last AGB thermal pulse of a model with Mi ≈ 1.1M⊙. There is excellent agreement with the thin CO shells found by Olofsson et al. (e.g., 1990, 1998) around some Mira stars.

On the fate of processed matter in dwarf galaxies

Two dimensional calculations of the evolution of remnants generated by the strong mechanical energy deposited by stellar clusters in dwarf galaxies (M \si $10^9 - 10^{10}$ \msun) are presented. The evolution is followed for times longer than both the blowout time and the presumed span of energy injection generated by a coeval massive stellar cluster. The remnants are shown to end up wrapping around the central region of the host galaxy, while growing to kpc-scale dimensions. Properties of the remnants such as luminosity, size, swept up mass, and expansion speed are given as a function of time for all calculated cases. The final fate of the swept-up galactic gas and of the matter processed by the central starburst is shown to be highly-dependent on the properties of the low density galactic halo. Superbubbles powered by star clusters, with properties similar to those inferred from the observations, slow down in the presence of an extended halo to expansion speeds smaller than the host galaxy escape velocity. Values of the critical luminosity required for the superbubbles to reach the edge of the galaxies with a speed comparable to the escape speed are derived analytically and numerically. The critical luminosities are larger than those in the detected sources and thus, the superbubbles in amorphous dwarf galaxies must have already undergone blowout and are presently evolving into an extended low density halo. This will inhibit the loss of the swept-up and processed matter from the galaxy.

Effects of Radiation Forces on the Frequency of Gravitomagnetic Precession near Neutron Stars

Gravitomagnetic precession near neutron stars and black holes has received much recent attention, particularly as a possible explanation of 15--60 Hz quasi-periodic brightness oscillations (QPOs) from accreting neutron stars in low-mass X-ray binaries, and of somewhat higher-frequency QPOs from accreting stellar-mass black holes. Previous analyses of this phenomenon have either ignored radiation forces or assumed for simplicity that the radiation field is isotropic, and in particular that there is no variation of the radiation field with angular distance from the rotational equatorial plane of the compact object. However, in most realistic accretion geometries (e.g., those in which the accretion proceeds via a geometrically thin disk) the radiation field depends on latitude. Here we show that in this case radiation forces typically have an important, even dominant, effect on the precession frequency of test particles in orbits that are tilted with respect to the star's rotational equator. Indeed, we find that even for accretion luminosities only a few percent of the Eddington critical luminosity, the precession frequency near a neutron star can be changed by factors of up to $\sim 10$. Radiation forces must therefore be included in analyses of precession frequencies near compact objects, in such varied contexts as low-frequency QPOs, warp modes of disks, and trapped oscillation modes. We discuss specifically the impact of radiation forces on models of low-frequency QPOs involving gravitomagnetic precession, and show that such models are rendered much less plausible by the effects of radiation forces.

The faint young sun climatic paradox: Influence of the continental configuration and of the seasonal cycle on the climatic stability

A quasi-three-dimensional climate model is used to study the early state of the Earth when the solar luminosity was 70% of the present value. Usually, climatic simulations going back to this period lead to a completely frozen planet contrasting with the geologic evidences of sedimentary rock formation and thus of the presence of liquid water at the surface of the continents during the Archean (4.6−2.5 billion years before present). Here, several model simulations are performed for solar luminosities varying between 0.7 and 1 times the present value. Using the present-day continental configuration and taking the seasonal cycle into account, a steady state is found in which glaciation is complete but snow covers only some oceanic coasts, leaving the continents essentially snow-free. As a result, the albedo of the continental area is strongly reduced compared to that of the frozen ocean. Some continental temperatures can almost reach the freezing point of water in summer −1°C in the center of Eurasia). This result can be explained by the behavior of the detailed hydrologic cycle included in the model. During the decrease of the solar luminosity, the jump to a completely frozen Earth occurs when the solar luminosity reaches 0.86 times its present value. The behavior of the climatic system is substantially different with a global ocean configuration. In the absence of land surfaces, the meridional heat transport, explicitly calculated, is less effective and the glaciation of a model latitude zone does not lead to the glaciation of its equatorward neighbor. The climate instability is relatively local and the jump to the completely frozen state is much more progressive than in the case of the modem continental configuration. The role of the seasonal cycle in the paleoclimatic simulation is also studied. Due to the non-linearity of the model, removing the seasonal cycle drives the system to an increase of the annual mean planetary albedo and to a decrease of the relative value (0.82) of the critical solar luminosity at which the jump to the completely frozen solution occurs.

Critical radiation fluxes and luminosities of black holes and relativistic stars

The critial luminosity at which the outward force of radiation balances the inward force of gravity plays an important role in many astrophysical systems. We present expressions for the radiation force on particles with arbitrary cross sections and analyze the radiation field produced by radiating matter, such as a disk, ring, boundary layer, or stellar surface, that rotates slowly around a slowly rotating gravitating mass. We then use these results to investigate the critical radiation flux and, where possible, the critical luminosity of such a system in genral relativity. We demonstrate that if the radiation source is axisymmetric and emission is back-front symmetric with repect to the local direction of motion of the radiating matter, as seen in the comoving frame, then the radial component of the radiation flux and the diagonal components of the radiation stress-energy tensor outside the source are the same, to first order in the rotation rates, as they would be if the radiation source and gravitating mass were not rotating. We argue that the critical radiation flux for matter at rest in the locally nonrotating frame is often satisfactory as an astrophysical benchmark flux and show that if this benchmark is adopted, many of the complications potentially introduced by rotation of the radiation source and the gravitating mass are avoided. We show that if the radiation field in the absence of rotation would be spherically symmetric and the opacity is independent of frequency and direction, one can define a critical luminosity for the system that is independent of frequency and direction, one can define a critical luminosity for the system that is independent of the spectrum and angular size of the radiation source and is unaffected by rotation of the source and mass and orbital motion of the matter, to first order. Finally, we analyze the conditions under which the maximum possible luminosity of a star or black hole powered by steady spherically symmetric radial accretion is the same in general relativity as in the Newtonian limit.

Radiation-driven acceleration in photospheres of nonaccreting magnetic white dwarfs

view Abstract Citations (7) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation-driven Acceleration in Photospheres of Nonaccreting Magnetic White Dwarfs Zheleznyakov, Vladimir V. ; Serber, A. V. Abstract Radiation transfer in a pure hydrogen, fully ionized, isothermal photosphere of an isolated white dwarf with dipole magnetic field is considered, and the radiation pressure force, both in the continuum and in the cyclotron line, is determined with the line saturation effect taken into account. It is shown that the magnetic field can reduce the critical luminosity for white dwarfs. This leads to the possibility of photospheric plasma ejection driven by the radiation in the cyclotron line and the formation of radiation-driven winds from sufficiently hot isolated magnetic white dwarfs. It is shown that cyclotron radiation pressure plays a significant role in the force balance of the photospheres of the magnetic white dwarfs GD 229, GrW +70 deg 8247, and PG 1031+234. The strong unidentified depression in the UV spectrum of GD 229 is attributed to cyclotron scattering by the radiation-driven plasma envelope with density N is greater than or approximately 108/cu cm. Publication: The Astrophysical Journal Supplement Series Pub Date: February 1994 DOI: 10.1086/191903 Bibcode: 1994ApJS...90..783Z Keywords: Radiative Transfer; Stellar Atmospheres; Stellar Magnetic Fields; White Dwarf Stars; Black Body Radiation; Cyclotron Radiation; Magnetic Dipoles; Plasma Density; Radiation Pressure; Stellar Luminosity; Ultraviolet Spectra; Astrophysics; RADIATIVE TRANSFER; STARS: ATMOSPHERES; STARS: MAGNETIC FIELDS; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (3)

Radiation-Driven Acceleration in Photospheres of Nonaccreting Magnetic White Dwarfs

AbstractRadiation transfer in a pure hydrogen, fully ionized, isothermal photosphere of an isolated white dwarf with dipole magnetic field is considered, and the radiation pressure force, both in the continuum and in the cyclotron line, is determined with the line saturation effect taken into account. It is shown that the magnetic field can reduce the critical luminosity for white dwarfs. This leads to the possibility of photospheric plasma ejection driven by the radiation in the cyclotron line and the formation of radiation-driven winds from sufficiently hot isolated magnetic white dwarfs.It is shown that cyclotron radiation pressure plays a significant role in the force balance of the photospheres of the magnetic white dwarfs GD 229, GrW +70° 8247, and PG 1031+234. The strong unidentified depression in the UV spectrum of GD 229 is attributed to cyclotron scattering by the radiation-driven plasma envelope with density N ≳ 108 cm−3 .Subject headings: radiative transfer — stars: atmospheres — stars: magnetic fields — white dwarfs

Effect of radiation forces on disk accretion by weakly magnetic neutron stars

Radiation forces are shown to be more important than general relativistic corrections to Newtonian gravitational forces in determining the motion of particles accreting onto a slowly rotating neutron star if the luminosity of the star is greater than about 1 percent of the Eddington critical luminosity (ECL). This is so even if the radius of the star is less than the radius of the innermost stable orbit. In particular, radiation drag causes matter accreting from a disk to lose angular momentum and spiral inward. At luminosities greater than about 0.2 ECL, a substantial fraction of the accreting matter can transfer most of its angular momentum and gravitational binding energy to the radiation field before reaching the stellar surface. These results have important implications for the X-ray spectra, time variability, and spin evolution of neutron stars with very weak magnetic fields and the prospects for detecting general relativistic effects near such stars.

The faint young sun climatic paradox: A simulation with an interactive seasonal climate-sea ice model

A seasonal one-and-a-half-dimensional (1 1 2 -D) energy-balance climate model including a detailed sea ice calculation and an interactive albedo formulation has been developed and is used to investigate the faint young sun climatic paradox. This model is shown to reproduce the present climate and sea ice observations. In spite of its greater complexity, its behavior is globally similar to simple energy-balance models with highly parameterized ice-albedo feedback used in previous studies of this question. It is found that when the solar luminosity drops by more than about 5% below its present value, the ice albedo feedback causes a global irreversible glaciation. Several sensitivity experiments show that the value of the critical solar constant and associated global surface temperature are only little sensitive to the set of model parameters describing the ice and snow albedo and meridional heat transport. In contrast, the absence or polar location of the continental mass introduce a nearly 10% decrease of the critical luminosity. The minimum level of atmospheric CO 2 needed to prevent a global glaciation through enhanced greenhouse warming is calculated as a function of the solar luminosity. A 30% drop in solar output requires a 2 × 10 4-fold increase in atmospheric CO 2, an unacceptably large value. However, in the absence of continents, a carbon dioxide partial pressure of 2000 times the present level is found to be sufficient to stabilize the climate. The effects of a reduced continental area, paleogeographic changes and higher CO 2 greenhouse effect combine to ensure a larger stability of the non-frozen configuration. Their cumulated and interactive effects may be able to solve the young sun paradox.

The maximum entropy production principle in climate models: Application to the faint young sun paradox

AbstractCurrent theories of stellar evolution imply that the solar luminosity was 25‐30% less than at present during early phases of the Earth's history. the response of the global annual climate to changes of the solar input is examined using the maximum entropy production (MEP) principle and a zonal energy balance model including ice‐albedo feedback. It is shown that this formulation reproduces satisfactorily the present meridional heat flux and latitudinal temperature distributions. It is found that the model predicts that the efficiency of the meridional heat transport decreases for low values of the solar constant. Consequently, if the solar irradiance is lowered, a progressive development of the polar caps and a decrease of the global surface temperature are obtained without major irreversibility in the system. A comparison with a similar model including diffusive heat transport formalism shows that the value of the critical solar luminosity causing a global glaciation is decreased when the meridional heat fluxes are determined by the MEP principle.

Origin of 'normal-branch' quasiperiodic oscillations in low-mass X-ray binary systems

QUASIPERIODIC intensity oscillations with frequencies in the range 6–10 Hz have recently been discovered in luminous low-mass X-ray binary systems1,2. These so-called 'normal-branch' oscillations (NBOs) seem to be a nearly universal feature of those systems with luminosities near the Eddington critical luminosity LE (refs 3,4), at which the outward force of radiation balances gravity. Here we report the results of numerical simulations of the approximately radial accretion flow expected near the neutron-star component, which show that the flow becomes overstable (that is, the flow oscillates, with increasing magnitude) when the luminosity rises to within a few per cent of LE. If the radial inflow begins ∼300 km from the neutron star (as expected), the frequency of the oscillations is ∼5–10 Hz, comparable to the observed frequencies of NBOs. The time variation in optical depth is substantial, whereas the variation in luminosity is relatively small. The predicted amplitudes and phases of the resulting intensity oscillations agree with observations5 of the NBOs. We suggest that the 10–20 Hz oscillations in intensity observed in Scorpius X-l when it is on the flaring branch3,4 are caused by photohydrodynamic modes that are excited by the oscillations in the radial flow and grow when the luminosity exceeds LE.

Hot accretion disks with electron-positron pairs

view Abstract Citations (75) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hot Accretion Disks with Electron-Positron Pairs White, Timothy R. ; Lightman, Alan P. Abstract The hot thermal accretion disks of the 1970s are studied and consideration is given to the effects of electron-positron pairs, which were originally neglected. It is found that disks cooled by internally produced photons have a critical accretion rate above which equilibrium is not possible in a radial annulus centered around r = 10 GM/c-squared, where M is the mass of the central object. This confirms and extends previous work by Kusunose and Takahara. Above the critical rate, pairs are created more rapidly than they can be destroyed. Below the critical rate, there are two solutions to the disk structure, one with a high pair density and one with a low pair density. Depending on the strength of the viscosity, the critical accretion rate corresponds to a critical luminosity of about 3-10 percent of the Eddington limit. Publication: The Astrophysical Journal Pub Date: May 1989 DOI: 10.1086/167455 Bibcode: 1989ApJ...340.1024W Keywords: Accretion Disks; Bremsstrahlung; Compton Effect; Electron-Positron Pairs; Nuclear Astrophysics; Luminosity; Pair Production; X Ray Sources; Astrophysics; ELEMENTARY PARTICLES; STARS: ACCRETION full text sources ADS |

Why stars become red giants

It is shown that a radiative envelope in which the Kramers opacity law holds cannot transport a luminosity larger than a critical value, and it is argued that the transition to red giant structure is triggered by the star's luminosity exceeding the critical value. If the Kramers law is used for all temperatures and densities, the radius of the star diverges as the critical luminosity is approached. In real stars the radiative envelope expands as the luminosity increases until the star intersects the Hayashi track. Once on the Hayashi track, luminosities in excess of the critical luminosity can be accommodated by forcing most of the mass of the envelope into the convection zone. 17 references.

Boundary instability of the magnetized photosphere of a neutron star

The cyclotron resonance of electron scattering cross-section is shown to decrease the value of critical luminosity of neutron star below the classical Eddington limit corresponding to Thomson scattering. If the radiative flux prevails over critical one, instability of photospheric boundary in forms of ejection or MHD-pulsations might be evolved, giving rise to generation of hard gamma-photons and relativistic particles.

The cyanogen inhomogeneity of NGC 362

view Abstract Citations (19) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The cyanogen inhomogeneity of NGC 362. Smith, G. H. Abstract The cyanogen distribution among the subgiants in NGC 362 has been investigated by using a panoramic photometry technique employing narrowband filters designed to measure the strength of the lambda 3883 CN band. This distribution is found to be bimodal, with a heavy preference towards CN-rich stars. The cyanogen variations are found among giants two magnitudes fainter than the Sweigart and Mengel critical luminosity at which CN surface abundance variations are predicted to first appear. Publication: The Astronomical Journal Pub Date: March 1983 DOI: 10.1086/113325 Bibcode: 1983AJ.....88..410S Keywords: Abundance; Cyanogen; Globular Clusters; Stellar Spectrophotometry; Subgiant Stars; Hertzsprung-Russell Diagram; Horizontal Branch Stars; Stellar Luminosity; Stellar Magnitude; Astronomy full text sources ADS | data products SIMBAD (1)

Magnetic field strongly reduces critical luminosity of neutron stars and degenerate dwarfs

Magnetic field strongly reduces critical luminosity of neutron stars and degenerate dwarfs

The Interpretation of Light Curves λλ2460 : 35000Å of the Eclipsing Wolf-Rayet Binary V444 Cyg

The structure of the extended atmosphere and the physical characteristics of the WN5 star V444 Cyg have been determined from the analysis of light curves of this system in the range λλ2460 − 35000. The radius of the core of the WN5 star at τ = 1 is ro = 2.9R⊙; the temperature at the surface of the core is equal to 90000oK, the electron density at its surface is 0.9 · 1013 cm−3. The bolometric luminosity of the WN5 star Lb = 2.1039 erg/s is close to the critical Eddington luminosity 2.6 · 1039 erg/s. The electron temperature decreases monotonically with height through the extended atmosphere from the value Te = 80000° − 90000°K at r = 2.9R⊙ to the value Te = 20000° − 30000°K at r = 8R⊙. The outflow velocity at the surface of the core (ro = 2.9R⊙, τ = 1) is vo = 400 km/s; the outflow in the region r ≥ 12R⊙ becomes stationary with the constant velocity v = 1000 − 1200 km/s. The total interstellar absorption for the system V444 Cyg is Av = 2m.66. The results obtained confirm strictly the model of Wolf-Rayet phenomenon proposed by Beals (1944). Chromospheric-coronal effects in the extended atmosphere are absent until r = 20R⊙. Characteristics of the WR star are consistent with the predictions of the evolution theory for massive close binary systems with mass exchange. The WR star is in a late evolutionary stage after the main sequence stage, being a helium remnant that has been formed as a result of the mass exchange.

Envelope ejection to form planetary nebulae

view Abstract Citations (17) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Envelope ejection to form planetary nebulae. Stry, P. E. Abstract Previous calculations, in which the luminosity is about 0.5 times the critical luminosity, are extended to higher luminosities to determine whether partial, small-scale ejections of mass continue to be the general result. The 0.2 solar-mass envelope of a 1.2 solar-mass star was perturbed by raising the interior boundary condition luminosity 10 percent to approximately the critical luminosity. After a brief period of oscillation the entire envelope was ejected. The numerical methods include time-dependent convection (not necessarily efficient) and a simplified version of a spherically extended atmosphere. Publication: The Astrophysical Journal Pub Date: March 1975 DOI: 10.1086/153435 Bibcode: 1975ApJ...196..559S Keywords: Planetary Nebulae; Stellar Envelopes; Stellar Mass Ejection; Supergiant Stars; Astronomical Models; Late Stars; Stellar Atmospheres; Stellar Luminosity; Stellar Structure; Astrophysics full text sources ADS |

A survey of H alf emission in early-type high-luminosity stars.

view Abstract Citations (120) References (83) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A survey of H-alpha emission in early-type high-luminosity stars. Rosendhal, J. D. Abstract High-dispersion Ha spectrograms have been obtained of 62 early-type high-luminosity stars in order to search for evidence for mass loss in stars in the upper left portion of the H-R diagram. in addition to a study of the strength and shape of Ha, detailed radial-velocity measurements have been made to look for possible velocity differences among the strong lines in the red region of the spectrum. The results of this investigation indicate that the lower limits to the absolute visual magnitudes at which Ha begins to show conspicuous emission are Mv -5.8 in the B( stars and Mv -6.8 in the B8-A3 stars. There is evidence from the radial-velocity studies for differential expansion and hence mass loss for stars approximately 0.5 mag less luminous than these limiting luminosities. Among the early B stars it appears that there is a critical luminosity of Mbol -8.2 at which strongly accelerated large-scale flows may develop. The apparent existence of an abrupt change in the velocity-gradient-luminosity relationship between the early and late B stars suggests that the mechanism driving the flows changes at about 13,000 K. Study of the line strengths indicates that the net strength of Ha is correlated with both luminosity and temperature, with there being no discontinuity in the relationship as the line goes from a net absorption to a net emission feature. Evidence is presented to support the hypothesis that all early-type supergiants which show emission are intrinsic spectrum variables. it is also argued that the presence of reversals near the center of the Ha absorption cores in several 0 stars and lower-luminosity early B supergiants may provide evidence for the existence of chromospheres in these stars. Subject headings: early-type stars - emission-line stars - luminous stars - mass loss Publication: The Astrophysical Journal Pub Date: December 1973 DOI: 10.1086/152555 Bibcode: 1973ApJ...186..909R full text sources ADS | data products SIMBAD (61)

The Luminosity Function of Galactic X-Ray Sources - a Cutoff and a "standard Candle"?

We analyze the 2-10 keV luminosity distribution of 36 X-ray sources in the Local Group having known or estimated distances and find that there exists a luminosity cutoff of approximately 10 ergs 1 in agreement with the theoretical (Eddington) limit for the luminosity of an approximately 1 M0 star. Furthermore, among the complete sample of high-luminosity sources, there appears to be a statistically significant group of X-ray "standard candles" at (within < 0.8 mag) the critical luminosity. This finding (which is in agreement with the self-consistent mass flow accretion models) presents the possibility that X-ray sources may be used as extragalactic distance indicators in the next generation of X-ray astronomy experiments. Subject headings: luminosities - X-ray sources