Showing papers in "Astrophysical Journal Supplement Series in 1976"

Absolute calibration of millimeter-wavelength spectral lines

Abstract: A detailed analysis of the chopper-wheel method of calibrating the intensity of a millimeter-wavelength spectral line is presented. Special techniques were used to construct a receiver which eliminates most of the usual calibration difficulties. The zenith atmospheric extinction between 3.5 mm and 2.6 mm wavelength was measured, and the intensities of six spectral lines in this range were absolutely calibrated with an estimated uncertainty (1 sigma) of 7 percent. The effects of the antenna power pattern on the corrected antenna temperature T*/sub A/ are calculated for several simple models of the source brightness distribution. (auth)

Structure of the solar chromosphere. II - The underlying photosphere and temperature-minimum region

Abstract: The paper presents a non-LTE empirical model of the quiet solar photosphere and the temperature-minimum region. The continuous spectrum computed from this model is in good overall agreement with available disk-center observations throughout the wavelength range from 0.125 to 500 microns. It is found that (1) absolute-intensity measurements are needed in the range between 1 and 2 microns to establish the structure of the deepest observable layers; (2) absolute-intensity or flux measurements are needed in the range between 20 and 200 microns to determine whether the minimum solar temperature occurring between the photosphere and the chromosphere is as low as indicated by present observations or much higher, as recent theoretical predictions indicate; (3) studies of the far-ultraviolet spectrum based on the assumption of LTE can be substantially in error; and (4) line opacity seems to account for the 'missing opacity' in the ultraviolet.

The structure of supernova shock waves.

Abstract: The structure of strong shock waves is calculated over the range of shock energies (1 to 100 MeV nucleon/sup -1/) and initial number densities (10/sup 15/--10/sup 22/ cm/sup -3/) believed likely to occur in the red-giant-like envelopes of stars undergoing Type II supernova explosions. The general equations governing the structure of such shocks are developed on the basis of a plasma composed of ions, electrons, positrons, and photons, making use of diffusion theory to evaluate the dissipative and transfer terms. The present treatment differs from previous calculations in that the effects of radiation transport on the energy and momentum balance in the shock are taken into account, as well as the relativistic contributions to radiative emission rates due to nondipole electron-ion bremsstrahlung, electron-electron bremsstrahlung, and radiative Compton scattering. An implicit treatment of inverse Compton scattering is also developed in terms of the creation and diffusion of effective photons.Severalmodels of strong shock structure are formulated and solved on the basis of these equations and physical processes. (AIP)

Radiative opacity tables for 40 stellar mixtures.

Abstract: Using improved methods, radiative opacities for 40 mixtures of elements are given for use in calculations of stellar structure, stellar evolution, and stellar pulsation. The major improvements over previous Los Alamos data are increased iron abundance in the composition, better allowance for the continuum depression for bound electrons, and corrections in some bound-electron energy levels. These opacities have already been widely used, and represent a relatively homogeneous set of data for stellar structures. Further improvements to include more bound-bound (line) transitions by a smearing technique and to include molecular absorptions are becoming available, and in a few years these tables, as well as all previous tables, will be outdated. At high densities the conduction of energy will dominate radiation flow, and this effect must be added separately. (AIP)

Copernicus ultraviolet observations of mass-loss effects in O and B stars.

Abstract: Far-UV spectra of 47 O, B, and A stars obtained with the Copernicus satellite are examined for P Cygni profiles. For all 40 stars with displaced absorption lines, values are given for the velocities of the short-wavelength edge, the line center, and the emission peak (if present). Parts of the spectra of 42 stars are reproduced, evidence for mass motions in ground-based spectra is discussed, and the best available data are summarized on the wavelengths and oscillator strengths of most lines likely to show mass-loss effects in either visual or UV spectra. The main conclusions are that: (1) the far-UV transitions, especially resonance lines, show that mass flow is present over a much wider group of stars than revealed by visible data on subordinate lines; (2) most of the line shifts imply mass motion away from the stars; (3) mass flow occurs in all but one star brighter than a bolometric magnitude of -6.0; and (4) the observed terminal velocities generally exhibit no significant correlation with temperature, luminosity, gravity, rotational velocity, or line strength.

Observations of southern emission-line stars

Abstract: A catalog of 1929 stars showing H-alpha emission on photographic plates is presented which covers the entire southern sky south of declination -25 deg to a red limiting magnitude of about 11.0. The catalog provides previous designations of known emission-line stars equatorial (1900) and galactic coordinates, visual and photographic magnitudes, H-alpha emission parameters, spectral types, and notes on unusual spectral features. The objects listed include 16 M stars, 25 S stars, 37 carbon stars, 20 symbiotic stars, 40 confirmed or suspected T Tauri stars, 16 novae, 14 planetary nebulae, 11 P Cygni stars, 9 Bep stars, 87 confirmed or suspected Wolf-Rayet stars, and 26 'peculiar' stars. Two new T associations are discovered, one in Lupus and one in Chamaeleon. Objects with variations in continuum or H-alpha intensity are noted, and the distribution by spectral type is analyzed. It is found that the sky distribution of these emission-line stars shows significant concentrations in the region of the small Sagittarius cloud and in the Carina region.

The emission-line spectrum above the limb of the quiet sun - 1175-1940 A

Abstract: Spectra of a quiet solar region obtained at positions within and above the solar white-light limb in the wavelength region from 1175 to 1940 A are discussed. The spectra were obtained by the slit spectrograph (SO82-B) on Skylab. The spectral resolution is 0.06 A, and the projected slit area on the sun was 2 x 60 arcsec (1450 x 43500 km). Relative line intensities are presented for lines formed in the temperature region of the solar atmosphere from about 8000 to 220,000 K. Representative line profiles of both optically thin and optically thick lines are shown as a function of height above the limb. Random mass-motion velocities are deduced from the optically thin lines, and the relative intensities and profiles of the lines are discussed in terms of current theoretical models. A wavelength list with identifications is given for the spectrum obtained at +4 arcsec above the white-light limb.

Photometric studies of spiral structure. I. The disks and arms of six Sb I and Sc I galaxies

Abstract: This paper presents detailed surface photometry of the galaxies NGC 3031, 4254, 4321, 5194, 5364, and 5457. Profiles showing the variation of surface brightness with azimuthal angle in each galaxy are given and used to separate the light into disk and arm components. The disk level at each radius is defined by the two main minima of the azimuthal light distribution, and the light above this level is assigned to the arms.The disks are exponential with an extrapolated center surface brightness of (B/sub 0/)/sub D/=21.67 +- 0.35 mag arcsec/sup -2/, in excellent agreement with Freeman. The disk colors are very uniform, with radial gradients mostly below the detection limit ( +- 0.010 mag kpc/sup -1/ in B-V), and lie in the narrow range of colors between old galactic clusters (B-Vapprox. =+0.7) and giant elliptical (B-Vapprox. =+0.9).The arms are significantly bluer than the disks and have a highly composite spectrum, as judged from the color indices. The ratio of arm intensity to disk intensity increases with radius, which explains the well-known outward bluing of spiral galaxies. There are indications that the ratio of total arm intensity to total disk intensity, which is a coarse measure of the strength of star formation,more » correlates with van der Kruit's compression strength of the gas. (AIP)« less

The nature of the objects of Joy: A study of the T Tauri phenomenon

Abstract: Proper understanding of the envelope emission processes enables us to correct the observed spectral energy distributions for interstellar extinction and thus obtain reliable bolometric luminosities. These luminosities, together with effective temperatures from the photospheric spectral types, indicate that the T Tauri stars in these two clouds are relatively low-mass stars evolving toward the main sequence along quasi-static equilibrium tracks. Comparison with theoretical evolutionary tracks yields a mass range of 0.5approximately-less-thanMapproximately-less-than3M/sub sun/ and ages generally less than 10/sup 6/ years. Evidence is presented to indicate that the mass-loss rate and envelope emission decrease with time during the T Tauri phase. (AIP)

Radiation from a hot, thin plasma from 1 to 250 A

Abstract: A calculation of emission spectrum of a hot, low-density plasma in the region 1--250 A is presented. The mechanisms considered are electron collision-induced line emission, bremsstrahlung, and radiative recombination; and the temperature range studied is 10/sup 5/--10/sup 7/ K. 795 lines are included. The elemental abundances of the ions of He, C, N, O, Ne, Mg, Si, S, Ca, Fe, and Ni were taken to be as in the solar corona. The line emission of Fe ions produces a maximum in the curve of an emission power between 1 and 250 A versus temperature around 10/sup 6/ K. The emission rate around 10/sup 6/ K is larger than the results calculated by Cox and Tucker and Tucker and Koren. (auth)

Mass ejection from the O4f star Zeta Puppis

Abstract: Profiles of P Cygni lines and displaced absorption lines observed in high-resolution Copernicus UV scans of Zeta Pup from 907 to 1738 A are compared with profile calculations to derive an optical-depth scale of the lines as a function of height or velocity in the expanding envelope. The degrees of ionization of the elements C, N, O, Si, and S under equilibrium conditions at electron temperatures of 20,000 to 1 million K, radiation temperatures of 20,000 to 50,000 K, and electron number densities of 100 million to 1 trillion per cu cm are computed with a dilution factor of 0.1, taking into account collisional ionization by electrons, radiative ionization from the ground state, radiative recombination, and dielectric recombination. Observed and predicted relative ionization fractions are then compared to determine the electron and radiation temperatures. Crude and refined estimates of the mass-loss rate and velocity law are made, and radiative acceleration in the expanding envelope is analyzed. The derived model for Zeta Pup is found to have a mass-loss rate of about 7.2 millionths of a solar mass per year, a wind velocity that increases slowly outward and reaches 700 km/s about 28 solar masses, and a nearly isothermal envelope with an electron temperature of approximately 200,000 K.

Particles, environments, and possible ecologies in the Jovian atmosphere

Abstract: The eddy diffusion coefficient is estimated as a function of altitude, separately for the Jovian troposphere and mesosphere. Complex organic molecules produced by the Ly alpha photolysis of methane may possibly be the absorbers in the lower mesosphere which account for the low reflectivity of Jupiter in the near ultraviolet. The optical frequency chromophores are localized at or just below the Jovian tropopause. Candidate chromophore molecules must satisfy the condition that they are produced sufficiently rapidly that convective pyrolysis maintains the observed chromophore optical depth. The condition is satisfied if complex organic chromophores are produced with high quantum yield by NH3 photolysis at less than 2,300 A. Jovian photoautotrophs in the upper troposphere satisfy this condition well, even with fast circulation, assuming only biochemical properties of comparable terrestrial organisms. An organism in the form of a thin, gas filled balloon can grow fast enough to replicate if (1) it can survive at the low mesospheric temperatures, or if (2) photosynthesis occurs in the troposphere.

Spectroscopy of 206 QSO candidates and radio galaxies

Abstract: Optical positions of 66 of the objects discussed here are given, with rms accuracy of 0.''5, in the Appendix. (AIP)

Detailed observations of the M17 molecular cloud complex.

Abstract: Detailed molecular line observations of CO, CS, H/sub 2/CO, NH/sub 3/, and some of their isotopes toward the giant molecular cloud complex near M17 are presented and discussed. Extensive /sup 12/CO and /sup 13/CO observations are made of two separation clouds characterized by velocities of 20 and 23 km s/sup -1/. The cloud at 20 km s/sup -1/ is found to be approximately 15' x 30' (10 x 20 pc) in extent and to contain a mass in excess of 3 x 10/sup 4/ M/sub sun/. Within the 20 km s/sup -1/ cloud are three bright spots of /sup 12/CO emission, two of which are shown to be localized high-density fragments a few parsecs in size. Subsequent identification of infrared sources and H/sub 2/O maser sources within their boundaries is interpreted as an indication that star formation has recently taken place. Observations of millimeter-wave CO, CS, HCN, H/sub 2/CO, and SO and centimeter-wave NH/sub 3/ emission and H/sub 2/CO absorption are combined with model cloud calculations manner to determine the physical and chemical conditions in M17 SW, the largest of the CO bright spots. Integrated intensity ratios of /sup 13/CO to C/sup 18/O are found to be significantly the terrestrialmore » value. Line widths of /sup 12/CO and /sup 13/CO are found to increase near M17 SW. It is found that the entire cloud cannot be in uniform spherical collapse of any form. Collapse, however, could be in progress in a small region around M17 SW, but lack of sufficient angular resolution prohibits direct observational tests of this possibility. It is concluded that line widths of approx.3 km s/sup -1/ must be maintained by some mechanism other than systematic collapse or expansion motions. Comparison of the molecular line observations with optical, infrared, and radio continuum data is made. The 20 km s/sup -/1 molecular cloud, the M17 H II region, and the star cluster NGC 6618 are apparently related in a geometrical evolutionary sequence. Cloud evolution seems to have proceeded from the outer edge of a proto-molecular cloud inward.« less

Evolved stars in open clusters.

Abstract: radial-velocity observations and MK classifications have been used to study evolved stars in 25 open clusters. Published data on stars in 72 additional clusters are rediscussed and combined with the observations made in this investigation to yield positions in the Hertzsprung-Russell diagram for 559 evolved stars in 97 clusters. Ages for the parent clusters were estimated from the main-sequence turnoff points, earliest spectral types, est stars in the clusters themselves. The evolved stars were sorted into six age groups ranging from 4 x 10/sup 6/ yr to 4 x 10/sup 8/ yr, and the composit H-R diagram for each age group was then used to study the evolutionary tracks for stars of various masses. The observational results were found to be in reasonably good agreement with recent theoretical computations. The composite color-magnitude diagrams were found to be strikingly different from those of the rich open clusters in the Magellanic Clouds. At a given age the red giants in the Small Magellanic Cloud and the Large Magellanic Cloud clusters are brighter and bluer than their galactic counterparts. It is suggested that these effects may be accounted for by differences in metal abundance. (AIP)

S-PROCESS Studies: Branching and the Time Scale

Abstract: The theory of S-process heavy-element formation is reformulated to allow for competition between beta decay and neutron capture at various nuclei along the path Solutions to the resulting branching network equations are presented (under the assumption of constant temperature and neutron flux) that do not require steady flow for the neutron current Using the exponential exposure distribution rho (tau) =G exp(-tau/tau/sub 0/) and recently calculated temperature-dependent beta-decay rates, comparison of several key branches yields the following average conditions for the solar-system S-process environment: Tapprox =31 x 10/sup 8/ K, n/subn/approx =16 x 10/sup 7/ neutrons cm/sup -3/ For tau/sub 0/=025 n mb/sup -1/ we find that about 48 neutron captures per exposed iron seed are required over a time of the order of a few thousand years for synthesis of the bulk of the solarsystem S-process material, with an average neutron capture time approx10 years (for sigmaapprox500 mb) (AIP)

Radio observations of interstellar CH. I.

Abstract: Observations performed at Onsala Space Observatory of the 9cm CH /sup 2/Pi/sub 1;2/, J=1/2 ground-state hyperfine transitions (F=1-0, upper satellite; F=1-1, main line; F=0-1, lower satellite), in the directions of, and in many cases (marked with /sup asterisk/) in extended areas around: Cas A*, W1*, W3*, Ori A*, W12*, W28 (SNR), W31, W43*, W44 (HII), W49A, W51*, M17, Cyg X* (including, e.g., DR 5, DR 15, DR 21, DR 22), ON 1, ON 2, ON 3 (K3--50), NGC 2068*, NGC 7000*, NGC 7538, L134*, L134 N, L1036, L1082, L1399, L1500*/3C 123, L1630*, Heiles's Clouds 1 and 2*, and the Per OB 2 dust cloud* (including NGC 1333 and IC 348) are reported. The observations were made with a traveling-wave maser preamplifier on the Onsala 25.6 m telescope. Zenith system noise temperature was about 34 K.The CH seems to exhibit weak maser characteristics almost everywhere toward H I and H II regions. Observations on and off 3C 123, which is behind L1500, indicate the three CH transitions are probably inverted in dark dust clouds. Absorption was observed only toward M17. Significant departures from equlibrium line intensity ratios appear toward H I and H II regions, but generally not for dark dustmore » clouds. The lower satellite transition is enhanced and, toward H II regions with strong far-infrared emission, is frequently the strongest line. Alternatives are presented for the interpretation of the observations in Heiles's Cloud 2 (G174.3--13.4), where non-LTE line intensity ratios are found. The CH main line excitation tempratures were determined by observations toward and around Cas A. W12, and 3C 123/L1500, are about -15, -10, and -10 K, respectively. Satellite line transitions are generally more inverted than the main line. The OH 1667 MHz line excitation temperature in the H I clouds toward Cas A is about +4 K. The CH observations, together with pertinent information on other interstellar species and far-infrared data on H II regions, are summarized. (AIP)« less