Showing papers in "Astrophysical Journal Supplement Series in 1981"
TL;DR: In this paper, the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semi-empirical models for six components of the quiet solar chromosphere was investigated.
Abstract: The described investigation is concerned with the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semiempirical models for six components of the quiet solar chromosphere. For a given temperature-height distribution, the solution is obtained of the equations of statistical equilibrium, radiative transfer for lines and continua, and hydrostatic equilibrium to find the ionization and excitation conditions for each atomic constituent. The emergent spectrum is calculated, and a trial and error approach is used to adjust the temperature distribution so that the emergent spectrum is in best agreement with the observed one. The relationship between semiempirical models determined in this way and theoretical models based on radiative equilibrium is discussed by Avrett (1977). Harvard Skylab EUV observations are used to determine models for a number of quiet-sun regions.
2,205Â citations
TL;DR: In this article, the nuclear energy generation and nucleosynthesis that occur in hydrogen-rich compositions at temperatures substantially greater than 10/sup 8/K are examined in detail, and reaction network calculations are presented to illustrate the application of this process to exploding supermassive stars, accreting neutron star, novae, and certain chaotic cosmologies.
Abstract: The nuclear energy generation and nucleosynthesis that occur in hydrogen-rich compositions at temperatures substantially greater than 10/sup 8/ K are examined in detail. At these high temperatures, a new kind of nucleosynthetic process (the ''rp-process'') involving the rapid capture of protons on seed nuclei (or on the products of helium burning in a situation with zero initial metallicity) can lead to the production of heavy elements up to and beyond the iron group with an accompanying energy generation rate greatly modified from that of the ..beta..-limited CNO cycle customarily employed in such calculations. New nuclear reaction rates of interest are tabulated, and reaction network calculations are presented to illustrate the application of this process to exploding supermassive stars, accreting neutron star, novae, and certain chaotic cosmologies. Implications for ..gamma..-line astronomy and X-ray burst models are discussed.
541Â citations
TL;DR: In this paper, an autoregressive (AR) model was used to analyze the optical light curve of the quasar 3C 273, and the best AR model was determined from sampled data and transformed to an MA for interpretation.
Abstract: Random process models phased in the time domain are used to analyze astrophysical time series data produced by random processes. A moving average (MA) model represents the data as a sequence of pulses occurring randomly in time, with random amplitudes. An autoregressive (AR) model represents the correlations in the process in terms of a linear function of past values. The best AR model is determined from sampled data and transformed to an MA for interpretation. The randomness of the pulse amplitudes is maximized by a FORTRAN algorithm which is relatively stable numerically. Results of test cases are given to study the effects of adding noise and of different distributions for the pulse amplitudes. A preliminary analysis of the optical light curve of the quasar 3C 273 is given.
284Â citations
TL;DR: In this paper, the authors derived anelastic equations for convection of a compressible fluid in a deep rotating spherical shell, which can help us understand what role the large density variation present in the solar convection zone plays in the maintenance of the solar differential rotation through angular momentum transports by global scale convection.
Abstract: We derive anelastic equations for convection of a compressible fluid in a deep rotating spherical shell. Our motive is to develop equations the solution of which can help us understand what role the large density variation present in the solar convection zone plays in the maintenance of the solar differential rotation through angular momentum transports by global scale convection. As such, the model equations represent a generalization of a Boussinesq system that has been studied extensively with the solar differential rotation problem in mind.
235Â citations
TL;DR: The rotation measures of 555 extragalactic radio sources are calculated as a result of a large number of new linear polarization measurements carried out by us at several wavelenghts as mentioned in this paper.
Abstract: The rotation measures of 555 extragalactic radio sources are calculated as a result of a large number of new linear polarization measurements carried out by us at several wavelenghts.
193Â citations
188Â citations
178Â citations
TL;DR: In this paper, a sample of 1787 nearby galaxies has been observed in the 21 cm line with the NRAO 91 m and 43 m telescopes and the Bonn 100 m telescope.
Abstract: A sample of 1787 nearby galaxies has been observed in the 21 cm line with the NRAO 91 m and 43 m telescopes and the Bonn 100 m telescope. A total of 1171 galaxies were detected. The radio observations provide an accurate heliocentric velocity, an H I flux, and a line profile width for each detection. Literature optical magnitudes, dimensions, and morphological types are reduced to common systems and tabulated. Intrinsic luminosities, dimensions, H I masses, and total masses are determined, assuming distances derived from redshifts.
169Â citations
141Â citations
135Â citations
TL;DR: In this paper, the physical and chemical properties of nine interstellar dark clouds are investigated and compared with theoretical models of the line emission and absorption from intersteller clouds to derive many of the fundamental physical parameters.
Abstract: An investigation of the physical and chemical properties of nine interstellar dark clouds is reported. Observations of a variety of atomic and molecular species have been obtained and the clouds have been partially mapped in the near infrared. The observations have been compared with theoretical models of the line emission and absorption from intersteller clouds to derive many of the fundamental physical parameters. The temperatures, densities, masses, sizes, and abundances of atomic and molecular species have been determined in the nine dark clouds. Also, the variations of the density and temperature as a function of radius in the clouds have been studied.
TL;DR: In this article, carbon and nitrogen abundances, as well as the strengths of calcium II H and K and the..delta..v = 0 cyanogen band, have been measured in red giant stars in the globular clusters M3 and M13.
Abstract: Carbon and nitrogen abundances, as well as the strengths of calcium II H and K and the ..delta..v = 0 cyanogen band, have been measured in red giant stars in the globular clusters M3 and M13. The data consist of spectrophotometric scans of low resolution (10 A) of 29 giants in M3 and 35 giants in M13 in the wavelength region 3000--5000 A.
TL;DR: In this paper, the coherent oscillations observed in high-speed photometry of dwarf novae during eruption were studied and the authors found that the oscillations are in all cases singly periodic and sinusoidal to the limits of measurement.
Abstract: We report an extensive study of the coherent oscillations observed in high-speed photometry of dwarf novae during eruption. The oscillations are in all cases singly periodic and sinusoidal to the limits of measurement. The detection of oscillations in 14 separate eruptions of AH Her and SY Cnc enables a general study of period variations. The stars trace out characteristic loops (''banana diagrams'') in the period-intensity plane. New detections are also reported for SS Cyg, EM Cyg, and HT Cas.
TL;DR: In this article, the Bahcall-Soneira Galaxy model was used to predict star counts in 17 selected directions on the sky as a function of apparent B, V, R, and I magnitudes and absolute visual magnitudes calculated from the Bah call-soneira galaxy model.
Abstract: Tables are presented of predicted star counts in 17 selected directions on the sky as a function of apparent B, V, R, and I magnitudes and absolute visual magnitudes calculated from the Bahcall-Soneira Galaxy model. It is noted that eight of the fields were selected ab initio for their usefulness in determining from star counts either parameters of galactic structure, the faint end of the disk luminosity function, or the detection of a massive stellar halo. Star counts in specified fields to m sub I = 19 mag are able to reveal the faint end of the disk luminosity function down to the end of the hydrogen-burning main sequence. It is thought that star counts at high galactic latitudes to m sub I = 22 mag should reveal the stellar constituents of a massive halo if they are massive enough to be on the hydrogen-burning main sequence.
TL;DR: In this article, the onset of convection for a compressible fluid in a rotating spherical shell via linear anelastic fluid equations for a depth of 40% of the radius, constant kinematic viscosity and thermometric diffusivity, Taylor numbers up to 10/sup 5/, and density stratifications up to seven e-folds across the zone.
Abstract: We study the onset of convection for a compressible fluid in a rotating spherical shell via linear anelastic fluid equations for a depth of 40% of the radius, constant kinematic viscosity and thermometric diffusivity, Taylor numbers up to 10/sup 5/, and density stratifications up to seven e-folds across the zone. The perturbations are expanded in spherical harmonics, and the radially dependent equations are solved with a Newton-Raphson relaxation method.
TL;DR: In this article, the authors provided detailed numerical tabulations for 20 models of intermediate mass whose evolution extends from the zero-age main sequence (ZAMS) to the base of the asymptotic giant branch.
Abstract: Detailed numerical tabulations are provided for 20 models of intermediate mass whose evolution extends from the zero-age main sequence (ZAMS) to the base of the asymptotic giant branch. The models span the ranges of 0.20< or =Y< or =0.36, 0.001< or =Z< or =0.03, and 3 M/sub sun/< or =M/sub asterisk/< or =11 M/sub sun/. A total of seven different compositions are investigated with two to four different model masses considered for each composition. Quantities tabulated are the model age, effective surface temperature, luminosity, radius, surface gravity, central composition, size of the convective core, the location of the base of the convective envelope, and the center of energy production of the hydrogen- and helium-burning shells. The results show that for a given model mass the evolutionary behavior depends strongly on its initial composition. The information contained in the tables can be applied to the construction of synthetic cluster H-R diagrams, and when combined with evolutionary data of low-mass stars (Mengel et al.), the information can be used in the study of galactic evolution.
TL;DR: In this article, an extensive grid of pure helium stellar model atmosphere calculations for hot, high-gravity stars is presented to help elucidate the nature and characteristics of the progenitors of the DB white dwarfs.
Abstract: We present an extensive grid of pure helium stellar model atmosphere calculations for hot, high-gravity stars. These models are intended to help elucidate the nature and characteristics of the progenitors of the DB white dwarfs. The surface gravity in the model grid ranges from log g = 6.0 (1.0) 9.0, and the effective temperature ranges from T/sub e/ = 25,000 K at log g = 6.0 and 8.0 (T/sub e/ = 40,000 K at log g = 7.0 and 9.0) up to temperatures close to the Eddington limit at each gravity. All our models assume hydrostatic equilibrium, steady-state statistical equilibrium, pure helium composition, and plane-parallel geometry. Most of our models are unblanketed and in LTE, and include convective energy transport. In addition to these, smaller preliminary grids of NLTE models (assuming detailed radiative balance in the helium lines) and of LTE, helium-line--blanketed models (including convective energy transport) were computed to assess the importance of these effects.
TL;DR: In this article, simple closed-form, approximate, analytic expressions for Franck-Condon factors are given for molecular band systems for which only vibrational-frequency, equilibrium internuclear separation and reduced mass values are known.
Abstract: Simple closed-form, approximate, analytic expressions for Franck-Condon factors are given. They provide reliable estimates for Franck-Condon factor arrays for molecular band systems for which only vibrational-frequency, equilibrium internuclear separation and reduced mass values are known, as is often the case for astrophysically interesting molecules such as CeO, CoH, CrH, CrO, CuH, GeH, LaO, NiH, SnH, and ZnH for band systems of which Franck-Condon arrays have been calculated.
TL;DR: In this paper, the emission line spectra of 12 Herbig-Haro nebulae were analyzed with multichannel spectrometer on the Mt. Palomar 5.08 m telescope and with the image intensified dissector scanner on the Kitt Peak 2.13 m telescope.
Abstract: Spectrophotometric data have been obtained for 12 Herbig-Haro nebulae with the multichannel spectrometer on the Mt. Palomar 5.08 m telescope and with the image intensified dissector scanner on the Kitt Peak 2.13 m telescope. Optical emission line fluxes are presented for the following Herbig-Haro objects: H-H 1 (NW), H-H 1 (SE), H-H 2A, H-H 2G, H-H 2H, H-H 3, H-H 7, H-H 11, H-H 24A, H-H 30, H-H 32, and H-H 40. Values for the electron temperature and electron density have been determined for 10 of these condensations. Significant inhomogeneities in the line-forming regions of these H-H objects are indicated by the derived N/sub e/-T/sub e/ diagrams. Empirical two-component density models have been constructed to interpret the emission line spectra of the five brightest condensations. Slightly less satisfactory homogeneous models are presented for the remaining five objects.
TL;DR: A simple analog to compressible convection in a rotating spherical shell is described and solved analytically to enable us to understand the basic physics of the prograde velocities that resulted from the numerical calculations of Paper II as discussed by the authors.
Abstract: A simple analog to compressible convection in a rotating spherical shell is described and solved analytically to enable us to understand the basic physics of the prograde velocities that resulted from the numerical calculations of Paper II. The analog is for an inviscid, adibatically stratified, rotating, equatorial annulus of gas for which a form of potential vorticity is conserved. Linear perturbations in this system take the form of vorticity waves which propagate prograde relative to the rotating reference frame as long as the density decreases outward. Their frequencies depend on the longitudinal wavenumber and density stratification in the same way as the convective modes of Paper II. Simple physical arguments explain these dependences.