High Effective Temperature Research Articles

The ultraviolet spectrum and interstellar environment of HD 50896*

By combining the results from 52 IUE spectra of the WN5 star HD 50896 we have constructed a photometrically precise, flux-calibrated, mean high-resolution (⁠|$R\approx10^4$|⁠) data set covering the wavelength range 1150–3274Å. We give measurements of fluxes and widths for all detected emission lines and P Cygni profiles; approximately 25 per cent of the flux emitted in the IUE spectral region comes from line emission. Examination of the interstellar lines shows at least five components contributing to the line-of-sight: H I and H II systems at low velocity, a feature at |$\sim-30\text {kms}^{-1}$| (measured with respect to the low-velocity blend), and absorptions at about −75 and −130km s−1. We use a profile-fitting analysis to estimate column densities for all available ions in each velocity system. Depletions found in the low-velocity H I gas are typical of those found in other sightlines, while remaining systems show evidence of at least some grain destruction; the −75km s−1 system, in particular, shows an almost total return of material from grains to the gas phase. We interpret the gas-phase abundances in terms of recent models. By examining high-resolution spectra of a further 15 early-type stars seen close to 50896 in the plane of the sky (and mostly members of the loose cluster Cr 121) we are able to discuss the geometry and physical nature of the various absorption systems observed. We attribute the strong lines of highly ionized species seen towards 50896 to an H II region local to that star, and calculate simple models to demonstrate that the observed columns can be reproduced. The results are consistent with the view that at least some WR stars have far-UV radiation fields characterized by high effective temperatures (⁠|$\sim65\times 10^3\text K$|⁠). The −30km s−1 system is interpreted as arising in the wind-blown bubble S308 associated with 50896, while the higher velocity systems (seen towards many of the field stars in addition to 50896) are discussed in terms of the multiple shock structure expected from a single old supernova remnant. We argue that this SNR is at ∼0.8 kpc, and is therefore unlikely to be associated with a mooted neutron star companion to 50896, for which we derive a distance of 2–3 kpc.

The evolutionary behaviour of population III intermediate mass stars

Assuming the Big-Bang nucleosynthesis was responsible for the formation of helium, the evolution of first-generation intermediate-mass stars of 5, 7, and 9M⊙ with no metals have been studied from the threshold of stability through the stage of helium exhaustion in the cores of the stars. Hydrogen Main-Sequence positions are marked at effective temperatures higher than those of normal stars. The evolutionary tracks during the hydrogen burning phase start to be similar to those of normal stars when the CN-cycle reactions, which are controlled by the triple-alpha reactions, become operative for hydrogen depletion. Helium Main Sequence of Population III stars of intermediate mass occurs at the high effective temperature region of the H-R diagram and stars stay as blue stars until the end of the core helium exhaustion phase. The total time elapsed is in the range of 3×107 and 108yr. The stars with the initial masses of 5, 7, and 9M⊙ developed a moderately electron degenerate complete hydrogen-exhausted region with masses of 0.77, 1.06, and 1.42M⊙, respectively, in which the most abundant element is carbon.

Predicted continuum spectra of type II supernovae - LTE results

The continuum spectral energy distribution of the flux emerging from type II supernovae is calculated from quasi-static radiative transfer through a power-law density gradient, assuming radiative equilibrium and LTE. It is found that the Balmer jump disappears at high effective temperatures and low densities, while the spectrum resembles that of a dilute blackbody but is flatter with a sharper cutoff at the short-wavelength end. A significant UV excess is found in all models calculated. The calculation should be considered exploratory because of significant effects which are anticipated to arise from departure from LTE.

Very nonadiabatic radial pulsations in luminous helium stars

view Abstract Citations (62) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Very nonadiabatic radial pulsations in luminous helium stars. Saio, H. ; Wheeler, J. C. ; Cox, J. P. Abstract The present investigation of highly nonadiabatic radial pulsations for luminous He star models having the chemical composition Y, X(C) = 0.9, 0.1, notes that radial pulsations are more nonadiabatic in models with higher L/M values and higher effective temperatures. Three types of 'strange modes' appear among the very nonadiabatic pulsations in which the periods of ordinary pulsation modes deviate from the corresponding adiabatic values considerably. The properties of these modes can be explained by taking into account the importance of thermal waves in highly nonadiabatic pulsations. Numerical experiments are currently used in which nonadiabaticity is modified artificially, together with analytic arguments based on a simple plane parallel model. Publication: The Astrophysical Journal Pub Date: June 1984 DOI: 10.1086/162102 Bibcode: 1984ApJ...281..318S Keywords: B Stars; Nonadiabatic Conditions; Stellar Luminosity; Stellar Models; Stellar Oscillations; Carbon Stars; Differential Equations; Eigenvalues; Pulses; Astrophysics full text sources ADS | data products SIMBAD (3)

Concerning the Temperatures of Central Stars of Planetary Nebulae

Recently Pottasch (1981, Astron. Astrophys. 94, L13) published extremely high effective temperatures of some central stars of planetary nebulae (> 200 000 K). Our study of planetary nebulae based on photoelectric photometry does not confirm his results. A histogram of Tz(HI) and Tz(HeII) shows smooth distribution of Tz with the maximum of about 48 000°K (HI) and 90 000°K (HeII), respectively; the effective temperature of none of the 62 planetary nuclei exceeds 120 000°K.

A New Explanation of the High Effective Temperatures in Pulsar Radioemissions

A new nonlinear process, which can explain the high effective temperatures of pulsar radio emissions, is proposed. It is due to the excitation of low frequency Alfvén waves by waves resonantly interacting with the electrons and positrons in the pulsar ultrarelativistic plasma.

Red variable stars. I - UBVRI photometry and photometric properties

view Abstract Citations (21) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Red variable stars I. UBVRI photometry and photometric properties. Celis S., L. Abstract In all phases of the light cycle of red variables stars, their near-infrared colors define a narrow sequence in the two color (V - R, R - I) diagram. This does not occur with other cool variable stars of types Ce, Se, and Ne. The position of a red variable on this sequence is very sensitive to the Me spectral type. The colors are molecular absorption indices, fundamentally of TiO, rather than indices of the color temperature. In general, the photometric magnitudes for a given Me star result from surfaces of different stellar radius and depend on the presence or absence of absorbing bands in the observed spectral continuum. This result is discussed in terms of amplitudes, radius, visual absolute magnitudes, and effective temperature, all of them free of or affected by TiO. Finally, these results give more information on which to build a model whose radial structure would have a relatively high stellar effective temperature and several cool covers of TiO molecules. Publication: The Astronomical Journal Pub Date: December 1982 DOI: 10.1086/113268 Bibcode: 1982AJ.....87.1791C Keywords: Light Curve; M Stars; Stellar Spectrophotometry; Ubv Spectra; Variable Stars; Infrared Astronomy; Stellar Atmospheres; Stellar Temperature; Titanium Oxides; Astrophysics full text sources ADS | data products SIMBAD (49) GCPD (1)

Ionization of low-energy atoms ejected from ion-bombarded solid surfaces

A microscopic semiphenomenological theory of ionization is described and compared with newly available experimental data. The theory assumes that the substrate electrons around the point of ejection are excited by the collision process and have a very high effective temperature. Formally, the approach is similar to that developed by Overbosch et al. to interpret the Na scattering data.

Energy conservation in tropical air conditioning: Environmental variables

The level of thermal cooling in tropical air conditioning is generally kept rather low for the local acclimatized populations. Also it is shown that the effective temperature (ET) comfort index strongly depends upon humidity and should be utilized in regulating the automated air conditioning systems. These considerations together with a site selection on the basis of ET surveys may lead to considerable energy savings. The use of higher ET as basis of tropical cooling would also lead to a physiologically more-comfortable thermal environment.

Mössbauer effect evidence of high internal pressure at iron atoms implanted in diamond

Mössbauer spectra of 57Fe implanted in diamond contain a single-line component which considerably increases in intensity after thermal annealing of the sample. This line indicates an unusually large s-electron density at the iron nuclei and a very high effective Debye temperature of iron atoms. It is attributed to a very large compression of Fe atoms in the diamond matrix. Atoms associated with defect regions are assumed to give rise to a quadrupole-split doublet. The systematics of hyperfine interactions for 57Fe in matrices of α-C, Si, Ge and α-Sn indicate strong atomic volume effects.

Mass loss from metal-poor stars

1. IntroductionIt is essential to consider the effect of mass loss to understand the distribution of supergiant stars in the HR diagram. This research concerns the evolution of massive stars with X=0.700 and Z=0.001 during the phases up to central Heexhaustion with the inclusion of mass loss. Such low value of Z has been chosen in order to allow a comparison with the supergiant stars of SMC. The rate of mass loss is formulated as in Chiosi, Nasi and Sreenivasan (1978). More specifically, in the range of high effective temperatures, we adopt the mass-loss rate relationship for radiation pressure driven wind of Castor, Abbott and Klein (1975), whereas in the range of low effective temperatures we assume the mass loss rate to be driven by the acoustic flux mechanism of Fusi Pecci and Renzini (1975).

A connection between macroturbulence and nonradial oscillations in late-type stars

view Abstract Citations (7) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A connection between macroturbulence and nonradial oscillations in late-type stars Smith, M. A. Abstract The behavior of the mean amplitude of nonradial oscillations in late-type stellar atmospheres is considered. It is pointed out that the nonradial oscillation theory of Christy and Ando predicts larger velocity amplitudes in stars with high effective temperature and low log g. When comparing G dwarfs and K giants, the effective temperature and log g effects approximately cancel, but the lower gravities of K supergiants cause a large jump in nonradial amplitudes at bolometric magnitude of about -1. The resulting step-function dependence with luminosity closely resembles the very peculiar behavior of macroturbulence previously observed by Smith and Dominy. It is concluded that a large component of macroturbulence in the atmospheres of late-type stars is probably intimately tied to, if not directly caused by 5-minute-like nonradial oscillations. Publication: The Astrophysical Journal Pub Date: December 1980 DOI: 10.1086/183414 Bibcode: 1980ApJ...242L.115S Keywords: Atmospheric Turbulence; Late Stars; Stellar Atmospheres; Stellar Oscillations; Amplitudes; Atmospheric Temperature; G Stars; Giant Stars; K Stars; Prediction Analysis Techniques; Astrophysics full text sources ADS |

Massive Stars: Evolution with Mass Loss. III. Low Metal Stars, and WR's in SMC

In this paper we investigate the effect of mass loss by stellar wind on the evolution from the main sequence to the He exhaustion stage of stars in the range of mass 20 to 100 M⊙ with low metal abundance. To this purpose we compute the set of evolutionary sequences with initial mass 20, 30, 40, 60, 80, and 100 M⊙ and chemical composition parameters X=0.700, Z=0.001. The models are first computed at constant mass to provide the fundamental network to which we compare the mass losing models. This choice of the initial chemical composition is aimed to represent the case of luminous supergiants in the Small Magellanic Cloud. The rate of mass loss follows the formulation of Castor, Abbot and Klein (1975), but it incorporates the effect of different metal content in the regions of high effective temperatures in the HR diagram. The rate of mass loss for models at low effective temperature is given by the acoustic flux mechanism according to the formulation of Chiosi, Nasi and Sreenivasan (1978). The results are compared with those of Chiosi, Nasi and Sreenivasan (1978), and those of Chiosi, Nasi and Bertelli (1979) for normal metal abundance supergiant stars of the Galaxy. On the basis of the present results we suggest an interpretative scenario for the existence of single WR stars in galaxies of different chemical composition. These results allow us to interpret also the different percentages of WR's among subclasses that are observed in the Galaxy, LMC, and SMC (Conti and Vanbeveren, 1979).

The Relativistic Boltzmann Equation and the Equations of Magnetohydrodynamics with Radiative Reaction

The relativistic collisionless Boltzmann equation with radiative reaction is derived from the conservation of the number of particles in the reduced six-dimensional phase space. It is then written in a covariant form. By taking invariant moments of the Boltzmann equation, we obtain the system of equations for relativistic magnetohydrodynamics taking account of radiative reaction. 21 and others have demonstrated that, for ultrarelativistic par­ ticles, the radiative reaction force can, in fact, be much greater than the Lorentz force. We have also shown explicitly that radiative reaction, although negligible at low temperatures, becomes dominant in the region of extremely high effective temperatures in our studies of relativistic plasma oscillations. 31 This suggests the necessity of including the force of radiative reaction in many astrophysical problems when the effective temperatures are extremely high. In the present work, we shall derive the relativistic Boltzmann equation and the equations of magnetohydro­ dynamics with radiative reaction taken into account. The formulation used here is similar to that of Goto, 41 and that of our previous work on relativistic classical scalar :fluid. 51 61

The Location of the ZZ Ceti Stars on the Hertzsprung-Russell Diagram

AbstractBy means of a simple analysis, it is shown that the combination of long periods and relatively high effective temperatures in ZZ Ceti variables may be made consistent with each other if the driving region for instability occurs at a temperature of (1–2) × 105K. Since this temperature does not coincide with the zone of He+ ionization, the driving mechanism is then not the same as for the Cepheid variables and the linear extension of the Cepheid strip down to the ZZ Ceti white dwarfs may be regarded as a coincidence. A Stelllngwerf “bump” type mechanism may be more appropriate for the latter. However, it is also shown that the short period modes computed by Dziembowski and found to be unstable, are consistent with the Cepheid mechanism. A special plea is made for further opacity calculations at temperatures of ~105 K and densities within a few orders of magnitude of unity — a ρ-T region of considerable interest for the envelopes of ZZ Ceti variables.

Stability of Accreting White Dwarfs in Close Binary Systems

AbstractWe present results of thermal and vibrational stability analysis for 1 M⊙ white dwarf models corresponding to various accretion rates ≳ 10−11 M⊙/y. Accretion is assumed to be spherically symmetric and stationary. Thermal instability due to nuclear burning of hydrogen (at lower accretion rates) and helium (at higher rates) was found. At medium rates two growing thermal modes are simultaneously present. Vibrational instability was found for all models except those corresponding to highest accretion rates. The excitation rates for some nonradial g-modes are at least 3 orders of magnitude higher than those for radial pulsations. These rates are also higher than the excitation rates for thermal modes in certain range of accretion rates corresponding to high luminosities and effective temperatures. Among objects in which these instabilities may be important are symbiotic stars and nuclei of planetary nebulae.

Some modifications of the theory of colloidal plasmas

Two effects leading to significant modifications of the theory of colloidal plasmas are discussed. The first concerns the fact that depending on the sign of the electric potential on a droplet in a colloidal plasma, the electrons or the ions impact on the droplet with a much higher effective temperature than their background plasma temperature. The analytical expression for this temperature is derived. It is shown that the electron-droplet collision rate in a wet alkali-metal vapour plasma may be substantially modified by the effect of the attractive potential between a droplet and an electron because of the image of the electron in the droplet. This effect is found to be important when the droplet sizes are in the submicron range.

On the Total Energy Output of the Nova Outburst

It is shown that the postmaximum decline in optical light during the outburst of a normal classical nova is caused by a shift in the wavelength of maximum energy to the ultraviolet. The constant-luminosity phase which was directly observed in Nova FH Ser 1970 may, therefore, be common to all novae, regardless of speed class. This implies that all speed classes of novae have similar energy budgets and that it is the total energy involved in the outburst which produces the variation in the observed characteristics of the novae. Recent thermonuclear runaway models for the outburst also show an extended period of high luminosity and high effective temperature following the initial outburst.

Test of the Li+–He interaction potential

The mobility of Li+ ions in He has been measured at a gas temperature of 300 °K over a wide range of E/N, where E is the electric field intensity and N is the gas number density. A new theory of ion mobility is used to calculate from these data the standard Ω integral ?(1,1) of kinetic theory for Li+ in He over a range of effective gas temperature extending from 300 to 28 700 °K. The ab initio quantum mechanical interaction potential for the Li+–He system computed by Catlow et al. is also used to calculate ?(1,1) as a function of Teff. A comparison of the two sets of data serves as a test of the Catlow potential. The test is extended to still higher effective temperatures (up to 60 000 °K) by comparison with values of ?(1,1) computed from the short-range repulsive potential derived from Li+ beam scattering experiments in helium. The Catlow potential reproduces the general features of the dependence of the ’’experimental’’ Ω integral on Teff over the entire range of the test. However, significant discrepancies exist; they could be resolved by making substantial alterations in the Catlow potential in the region of short ion–atom separation distance and smaller adjustments in the region of the potential minimum.

Effects of darkening processes on surfaces of airless bodies

We find the lunar darkening process could be due neither to simple addition of impact-melted glass nor to addition of devitrified glass to crushed lunar rock. There is evidence that lunar soil grains have thin, very light-absorbing coatings that mask absorption bands, seen in the reflection spectra of freshly crushed lunar rock, in the same manner as they are masked in the spectra of lunar soils. We believe the processes that produce these coatings are (1) deposition of atoms sputtered from lunar soil grains by solar wind particles and (2) deposition of vapor species vaporized from lunar soil grains by micrometeorite impacts. Coatings produced in laboratory simulations of these processes owe their strong light-absorbing properties in large part to the presence of abundant metallic Fe grains smaller than 100 Å in diameter. Another process, which depends on implantation of solar wind protons in lunar soil grains and their later mobilization during micrometeorite impacts to produce metallic Fe in the impact glass, also seems reasonable but has not yet been demonstrated experimentally. As a result of impact vaporization the Moon would preferentially lose minor amounts of light elements, principally monatomic oxygen, and this would result in oxygen depletion in the vapor condensate. This type of fraction would be more extreme on airless bodies with lower escape velocities. Sputtering occurs at higher effective temperatures and this would cause loss of all common rock-forming elements in approximately equal amounts. There would be some bias in this process toward retention of very heavy trace elements— a characteristic that has been observed in the lunar soil. This bias would be less important for smaller airless bodies. We describe an apparent new type of fractionation that occurs during deposition of sputtered atoms. This fractionation favors retention of higher mass atoms over lower mass atoms, and appears to be a linear function of mass. This may explain observed isotopic fractionations in lunar soil, in which the heavier isotope always appears to be enriched relative to the lighter one. This “first bounce fractionation” process should operate on all airless bodies. Na and K apparently do not conform to this fractionation process and have a much greater tendency to escape. This may help explain the presence of high Na concentrations around Io.