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At least 1,423 records · Page 79
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NASA NTRS1982

The X-ray absorption spectrum of 4U1700-37 and its implications for the stellar wind of the companion HD153919

The first high resolution non-dispersive 2-60 KeV X-ray spectra of 4U1700-37 is presented. The continuum is typical of that found from X-ray pulsars; that is a flat power law between 2 and 10 keV and, beyond 10 keV, an exponential decay of characteristic energy varying between 10 and 20 keV. No X-ray pulsations were detected between 160 ms and 6 min with an amplitude greater than approximately 2%. The absorption measured at binary phases approximately 0.72 is comparable to that expected from the stellar wind of the primary. The gravitational capture of material in the wind is found to be more than enough to power the X-ray source. The increase in the average absorption after phi o approximately 0.5 is confirmed. The minimum level of adsorption is a factor of 2 or 3 lower than that reported by previous observers, which may be related to a factor of approximately 10 decline in the average X-ray luminosity over the same interval. Short term approximately 50% variations in adsorption are seen for the first time which appear to be loosely correlated with approximately 10 min flickering activity in the X-ray flux. These most likely originate from inhomogeneities in the stellar wind of the primary.

White, N. E.↗
NASA NTRS1981

Momentum and energy balance in late-type stellar winds

Observations at ultraviolet and X-ray wavelengths indicate that the classical picture of a static stellar atmosphere containing a radiative equilibrium temperature distribution is inapplicable to the majority of late type stars. Mass loss and the presence of atmospheric regions characterized by gas temperatures in excess of the stellar effective temperature appear to be almost ubiquitous throughout the HR diagram. Evidence pertaining to the thermal and dynamical structure of the outer envelopes of cool stars is summarized. These results are compared with the predictions of several theoretical models which were proposed to account for mass loss from latetype stars. Models in which the outflow is thermally radiatively, or wave driven are considered for identification of the physical processes responsible for the observed wind properties. The observed variation of both the wind, thermal and dynamical structure as one proceeds from the supergiant branch toward the main sequence in the cool portion of the HR diagram give consideration to potential mechanisms for heating and cooling the flow from low gravity stars.

Macgregor, K. B.↗
NASA NTRS1982

The resolution of P Cygni's stellar wind

The 6 cm radio emission from P Cygni's stellar wind has been resolved using the Very Large Array. The wind is apparently spherically symmetric, and the radial surface brightness distribution agrees with that predicted by the constant velocity, isothermal wind model. The wind temperature is directly determined from the observations to be within 2000 K of 18,000, which is consistent with an effective temperature of 20,000 K for P Cygni. The technique makes it possible to study the adiabatic cooling of the wind, which should be important for most early-type stars but which cannot be recognized through its effects on the radio spectrum. To avoid systematically underestimating fluxes, it is essential to fit models directly to the (u-v) data when observing stellar winds with the VLA.

White, R. L.↗
NASA NTRS1982

High-energy observations of stellar flares - Comparison with the sun

This paper reviews recent observations of flaring activity on flare stars of the UV Ceti type, concentrating on X-ray and radio data in comparison with possible solar analogs. Although detailed differences exist, it is concluded that similar mechanisms may work for both cases. Extending the analogy, the hard X-ray and gamma ray fluxes expected from typical stellar flares are estimated. In the solar case, these radiations give information about accelerated particles in the flare region. Hard X-ray observations of stellar flares may be possible, eventually, but the predicted gamma-ray fluxes are prohibitively weak.

Hudson, H. S.↗
NASA NTRS1983

The X-ray absorption spectrum of 4U 1700-37 and its implications for the stellar wind of the companion HD 153919

The first high resolution non-dispersive 2-60 keV X-ray spectra at 4U1700-37 is presented. The continuum is typical of that found from X-ray pulsars; that is a flat powr law between 2 and 10 keV and, beyond 10 keV, an exponential decay of characteristic energy varying between 10 and 20 keV. No X-ray pulsations were detected between 160 ms and 6 min with an amplitude greater than approximately 2 percent. The absorption measured at binary phases approximately 0.72 is comparable to that expected from the stellar wind of the primary. The gravitational capture of material in the wind is found to be more than enough to power the X-ray source. The increase in the average absorption after phi of approximately 0.5 is confirmed. The minimum level of adsorption is a factor of 2 or 3 lower than that reported by previous observers, which may be related to a factor of approximately 10 decline in the average X-ray luminosity over the same interval. Short term approximately 50 percent variations in adsorption are seen for the first time which appear to be loosely correlated with approximately 10 min flickering activity in the X-ray flux. These most likely originate from inhomogeneities in the stellar wind of the primary. Previously announced in STAR as N83-14034

White, N. E.↗
NASA NTRS1983

Stellar populations in local group dwarf elliptical galaxies. I - NGC 147

A color-magnitude diagram of NGC 147 to an I magnitude of 23 is presented. The stellar population in the outer parts of this elliptical galaxy resembles that of the globular clusters of the Milky Way. Quantitative comparison of the giant branch with those of globular clusters yields a mean metallicity of -1.2 + or - 0.2, making NGC 147 a part of the general correlation between mass and metallicity seen in ellipticals. The giant branch appears to be broad, which suggests a metallicity dispersion. The absence of asymptotic giant branch stars at luminosities above that of the red giant branch tip sets an upper limit of 10 percent for the fraction of stars in this NGC 147 field that have ages less than 12 Gyr. This result contrasts with the situation in some of the related, but less massive, dwarf spheroidal systems. If the choice is made to assume, rather than determine the stellar content of NGC 147, a distance of 630 + or - 50 kpc is derived, similar to that of M31.

Mould, J. R.↗
NASA NTRS1983

Introductory Comments on Stellar Atmospheric Structure and Its Modeling

A general historical perspective on stellar atmospheric models is presented. Some comments on the priori speculative-theoretical modeling of the star, its atmosphere, and its environment are made. In contrast to this more speculative type of investigation, an empirical-theoretical program is defined. The objectives of the program are to delineate atmospheric structural patterns, properties of the local stellar environment, and some necessary characteristics of subatmospheric structure as inferred from the observations of nonthermal fluxes and phenomena, and thermodynamic self consistency.

Source record↗
NASA NTRS1984

Is stellar differential rotation observable?

Daily measures of the disk integral 2.8 GHz solar flux from the years 1947 through 1982 are analyzed to determine the detectability of stellar differential rotation using the tracer method. Autocorrelation and power spectral analyses of 1 yr data sets yield rotation periods whose scatter about the mean period is too large to permit detection of the expected differential rotation signal. The principal noise source is the random appearance of tracer regions on the solar surface, in time and longitude. Criteria are given for optimizing stellar observations and analyses to detect differential rotation.

Labonte, B. J.↗
NASA NTRS1983

The nature of stellar active regions

Observational estimates of chromospheric radiative loss rates in Ca II H and K in main-sequence F, G, and K dwarfs are used to compare stars that span a range of activity levels with each other and with the sun. For stars with spectral types similar to that of the sun, it is found that chromospheric radiative losses in stars with active chromospheres exceed the flux emitted by plages on the sun at solar maximum by about a factor of 6. If this difference is due primarily to a difference in the fraction of the stellar surface covered by plage-like material, and not to a change in the character of the emission-line regions, then more than half of the stellar surface in stars with large chromospheric radiative loss rates must be covered by active regions. It is also possible to show that active regions in young solar-type stars extend at least to latitudes + or - 25 deg from the rotational equator and that a typical individual region contributes no more than 2 percent - 3 percent of the total H and K emission. In Kappa Cet, observations show that the rotational modulation persisted in both phase and amplitude for at least 100 days (12 rotation cycles).

Wolff, S. C.↗
NASA NTRS1984

Ultraviolet spectral morphology of the O stars - The remarkable luminosity dependence of the Si IV stellar wind effect

An extensive survey of ultraviolet O type spectra, by means of high-resolution data from the International Ultraviolet Explorer archives, shows a strong correlation between both the photospheric and stellar wind features, and the optical spectral types. In particular, the stellar wind effect in the Si IV resonance doublet displays a strong luminosity dependence which has not been completely described before: there is no effect anywhere on the main sequence, but it develops gradually through the intermediate luminosity classes into a full P Cygni profile in the supergiants. This behavior is in sharp contrast to that of the C IV and N V, which have strong P Cygni profiles throughout most of the O type domain, including the main sequence. The Si IV effect simultaneously provides a sensitive classification criterion and poses an outstanding problem for detailed astrophysical interpretation. In addition, a new emission feature at 1574 A, possibly due to N III or to N IV forbidden line, is reported.

Walborn, N. R.↗
NASA NTRS1984

Observation of stellar remnants from recent supernovae

The current observational situation regarding the presence or absence of collapsed stellar objects associated with supernova remnants is reviewed. The theoretical expectations and observational evidence for the various possible classes of stellar remnants are discussed. The results of searches for these objects in the radio, optical, X-ray and gamma-ray regions of the spectrum are outlined with the conclusion that nine bona fide candidates are currently known. The implications of these results for the origin and evolution of compact objects are summarized.

Helfand, D. J.↗
NASA NTRS1984

Recombination stellar wind model for the coronae of early-type stars

The wind structures of early-type stars which have base coronal zones are discussed. The calculations give rise to what is called a recombination stellar wind structure because of the transition from a coronally driven wind to a radiatively driven wind. The coronal X-rays are a dominant factor in determining the wind structure because of their effect on the ionization structure, which in turn determines the heating, cooling, and the attenuating properties of the stellar wind. The decreased continuum opacity allows for an increase in the emergent X-ray flux at energies less than 1 keV. The X-ray spectra are compared with those observed with the IPC instrument on the Einstein Observatory and are found to be in excellent agreement. This shows that contrary to earlier analyses, base coronae are not ruled out by the existing IPC X-ray data. The predicted emergent X-ray luminosities are in agreement with the observed values. However, the incident X-ray luminosities from the base coronal zones can be three to four orders of magnitude larger. This suggests that the X-ray IPC spectra reflect the absorption properties of the wind and not the nature of the X-ray source.

Waldron, W. L.↗
NASA NTRS1985

Corotating Interaction Regions in Stellar Winds: Particle Acceleration and Non-thermal Radio Emission in Hot Stars

A co-rotating interaction region (CIR) forms in a stellar wind when a fast stream from a rotating star overtakes a slow stream. The CIR's were studied in detail in the solar wind over the past decade primarily because they are efficient sources of particle acceleration. Here, CIR's in OB star winds are used to explain two properties of such winds: deposition of non-radiative energy in the wind far from the stellar surfaces and acceleration of non-thermal particles.

Mullan, D. J.↗
NASA NTRS1985

Solar and stellar coronal plasmas

Progress made in describing and interpreting coronal plasma processes and the relationship between the solar corona and its stellar counterparts is reported. Topics covered include: stellar X-ray emission, HEAO 2 X-ray survey of the Pleiades, closed coronal structures, X-ray survey of main-sequence stars with shallow convection zones, implications of the 1400 MHz flare emission, and magnetic field stochasticity.

Golub, L.↗
NASA NTRS1984

Brackett-alpha line profiles of young stellar objects

Profiles of the Br-alpha line of H I at a velocity resolution of 45 km/s are presented for the compact imbedded infrared objects BN, S106/IRS 3, GLS 490, GL 961, GL 989, Mon. R2/IRS 2, and for the visible objects LkH-alpha 101, T Tau, and R Mon. A proportionality obtained between Br-alpha luminosity and bolometric luminosity is shown to extend over three orders of magnitude, supporting the idea that the physical conditions and gas motions in the circumstellar envelopes of stellar objects are closely related over a wide range of luminosities. The Br-alpha line strengths are compared to radio continuum flux densities in the context of stellar wind models. Momentum deposition rates deduced from Br-alpha or radio continuum fluxes are consistent with those available in the radiation fields, which appear capable of driving the ionized gas outflows in the vicinity of the core sources. The results of a comparison of the H-alpha and Br-alpha profiles for T Tau are discussed.

Persson, S. E.↗
NASA NTRS1985

Stellar evolution at high mass with convective core overshooting

The transition from stellar evolution models with no convective core overshooting (CCO) at all to models in which homogeneous mixing due to CCO reaches far beyond the formal convective core boundary is systematically explored. Overshooting is parameterized in terms of the ratio d/H(p), where d is the distance of convective overshoot beyond the formal convective core boundary and H(p) is the local pressure scale height. It is concluded that CCO in very massive main sequence stars produces a great expansion of the stellar envelope if d/H(p) is large but not excessively large. CCO does not entirely suppress convective instability above the overshoot zone in the envelopes of main sequence stars more massive than about 15 solar masses. A general comparison of theoretically constructed isochrones for young stars with observed main sequence turnups indicates that the observed turnups are longer, brighter, and cooler at the tip than those expected on thfe basis of standard evolutionary theory.

Stothers, R. B.↗
NASA NTRS1985

Mass outflows from young stellar objects

The process of mass loss from young stellar objects is examined in a review of recent observational investigations and theoretical models. Consideration is given to the interaction of the stellar wind with surrounding molecular clouds, the high degree of collimation seen in some embedded objects, alignment of outflows from stars embedded in the same cloud complex, initially isotropic and intrinsically anisotropic models of mass outflow, and the kinds of observations needed to determine the actual mechanisms involved.

Strom, S. E.↗
NASA NTRS1985

The initial mass function and global rates of mass, momentum, and energy input to the interstellar medium via stellar winds

Published observational data are compiled and analyzed, using theoretical stellar-evolution models to determine the global rates of mass, momentum, and energy injected into the interstellar medium (ISM) by stellar winds. Expressions derived include psi = 0.00054 x (M to the -1.03) stars formed/sq kpc yr log M (where M is the initial mass function in solar mass units) and mass-loss = (2 x 10 to the -13th) x (L to the 1.25) solar mass/yr (with L in solar luminosity units). It is found that the wind/supernova injection of energy into the ISM and the mass loss from stars of 5 solar mass or more are approximately balanced by the dissipation of energy by cloud-cloud collisions and the formation of stars, respectively.

Van Buren, D.↗