bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fundamental problems and basic tests of stellar evolution theory — The case of carbon stars
Authors: Iben Icko 
Published: June 2019

Carbon stars are thought to be in the asymptotic giant branch (AGB) phase of evolution, alternately burning hydrogen and helium in shells above an electron-degenerate carbon-oxygen (CO) core. The excess of carbon relative to oxygen at the surfaces of these stars is thought to be due to convective dredge-up which occurs following a thermal pulse. During a thermal pulse, carbon and neutron-rich isotopes are made in a convective helium-burning zone. In model stars of large CO core mass, the source of neutrons for producing the neutron-rich isotopes is the 22Ne(α, n)25Mg reaction and the isotopes are produced in the solar system s-process distribution. In models of small core mass, the 13C(α, n) 16O reaction is thought to be responsible for the release of neutrons, and the resultant distribution of neutron-rich isotopes is expected to vary considerably from one star to the next, with the distribution in isolated instances possibly resembling the solar system distribution of r-process isotopes. After the dredge-up phase following each pulse, the 13C is made by the reactions 12C(p,γ) 13N(β+ v) 13C in a zone of large 12C abundance and small 1H abundance that has been established by semiconvective mixing during the dredge-up phase. There is qualitative accord between the properties of carbon stars in the Magellanic Clouds and properties of model stars, but considerably more theoretical work is required before a quantitative match is achieved.The observed paucity of AGB stars more luminous than MBOL ∼ −6 is interpreted to mean that the AGB lifetime of a star more luminous than this is at least a factor of ten smaller than the AGB lifetime of stars less luminous than this, or, at most 105 yr. Since, with current estimates of the 22Ne(α, n)25Mg reaction rate R22, only AGB model stars more luminous than MBOL ∼ −6 can produce s-process isotopes in the solar system distribution, it is inferred that either (1) the current estimates of R22 are too small by one to two orders of magnitude, allowing less luminous AGB stars to contribute, (2) the solar system distribution is not equivalent to the average Galactic distribution, being rather the consequence of a unique injection into the protosolar nebula of matter from a massive intermediate-mass AGB star, or (3) the estimates of the temperatures in the convective shell that are given by extant models are too low by, sav, 10 or 15 percent.The absence of carbon stars more luminous than MBOL ∼ −6 is suggested to be due primarily to the fact that ∼ 106 yr of AGB evolution is necessary to produce surface C/O > 1, rather than to be due to the burning of dredged-up carbon into nitrogen at the base of the convective envelope during the interpulse quiescent hydrogen-burning phase. Thus, the positive correlation between the nitrogen and helium abundances in planetary nebulae is perhaps primarily a consequence of the second dredge-up episode rather than a consequence of processes occurring during the thermally pulsing phase.
Constraints on stellar evolution by observation of white dwarfs
Authors: Weidemann V. 
Published: June 2019

The recent observation of white dwarfs in the open cluster NGC 2516 and the determination of their surface gravity and effective temperatures (Reimers and Koester, 1982) has enabled the establishment of the initial-final mass relation for low and intermediate mass stars which was published a few months ago and is presented here (Fig. 1 of Weidemann and Koester, 1983a). The most important conclusions drawn are:
1.The limiting mass for white dwarf progenitors is 8–9 M⊙ rather than 5–6 M⊙, with supernova production beyond;2.The rather flat run of the initial-final mass relation in the main range of star production, 1–5 M⊙, explains the observed narrow mass distribution of white dwarfs and central stars of planetary nebulae around 0.6 M⊙;3.High mass white dwarfs exist, as shown in the rich, young cluster NGC 2516, but are extremely rare in general.
Periodic Orbits and Gas Flow in Barred Spiral Galaxies
Authors: Sanders R. H. , Teuben P. J. , van Albada G. D. 
Published: June 2019

One purpose for studying the gas flow in barred spiral galaxies is to use the observed distribution and kinematics of the gas as a tracer of the underlying gravitational field. By comparing model hydrodynamical calculations with observations of actual systems, one would like to define three basic properties of barred galaxies:
1)The bar strength. How significant is the deviation from axial symmetry in the region of the bar, measured by some parameter such as qt, maximum aximuthal force in terms of the mean radial force (Sanders and Tubbs, 1980).2)The mean radial distribution of matter. Clearly in a system with large deviations from circular motion, the “rotation curve” gives no direct information on the radial mass distribution.3)The angular velocity of the bar. Where is the co-rotation radius (or Lagrange points) with respect to the bar axes? Are other principal resonances present?
The W28 Supernova and the Surrounding Interstellar Medium
Authors: Zealey W. J. , MacGillivray H. T. , Malin D. , Hartl H. 
Published: June 2019

A large shell, 1.5 degrees in diameter, outlined by nebulosity and dust clouds has been found centred on the W28 supernova remnant. Obscuration has been studied over a 2 degree square area centred on W28. The W28 SNR is coincident with a region of reduced obscuration. The densest parts of the obscuration AB≥ 4m is coincident with the molecular clouds associated with M20 and the OH maser to the south of M20.The observed absorption is interpreted as being the result of the expansion of the W28 SNR into an older pre-existing SNR or stellar wind bubble.
Evolutionary Effect in Quasars as a Consequence of Galaxy Formation Process
Authors: Shklovsky I. S. 
Published: June 2019

Many years ago M. Schmidt established the presence of a strong evolutionary effect in quasistellar objects (QSOs). Though this important problem is not yet fully resolved, there can be no doubt that in earlier epochs of the Universe evolution of the spatial density of QSOs (in the comoving coordinates) was much higher than it is now. Thus, for instance, the spatial density of the optically selected QSO is:
according to Schmidt (1977). Here τ = z/(l+z), whence, for instance, if z = 3, ρ ∼ 1000 ρ0, where ρ0 is the local density.
Order and Chaos in the Solar Cycle
Authors: Ruzmaikin A.A. 
Published: June 2019

Solar activity varying with an 11-year cycle is chaotic at large time scales. The evidence comes from an analysis of observations of the sunspot number and of radioactive carbon. Thereby an estimate of the dimension of the solar attractor can be obtained.The origin of the sunspots can be associated with the interactions of the regular, large-scale, chaotic, and intermittent magnetic fields.
Distances of the Central Stars and their Position in the HR Diagram
Authors: Pottasch S.R. 
Published: June 2019

Determination of the distances to individual planetary nebulae are discussed. Especially those methods which are independent of assumed nebular properties (mass, absolute flux, etc.) are assembled and discussed. In this way, reasonable approximations to the distance can be obtained for about 50 planetary nebulae. The accuracy of the distances is tested by comparing nebular properties derived from these distances with the properties of nebulae at the galactic center or in the Magellanic clouds. A comparison is also made with the statistical distance determinations; the conclusion is that the assumption of constant mass often leads to an overestimate of the distance, while the assumption of constant Hβ flux leads to distances having individual uncertainties of up to a factor of 3.The central star temperature determination is summarized. Individual central stars are placed on the HR diagram and compared with theoretical predictions. Deductions concerning the evolution which can be made from the observations are discussed.
High frequency variations in Earth rotation
Authors: Carter W. E. , Robertson D. S. , Fallon F. W. 
Published: June 2019

The combined POLARIS-IRIS Earth orientation time series now span nearly a full cycle of the Chandler-annual beat period, beginning in late 1980. Since April 1985 there is also a nearly continuous coverage of UT1 at daily intervals. We have fit a simple model, consisting of circular 14-month and annual components and a linear drift to the polar motion series, then computed the “along-track” and “cross-track” residuals. Both sets of residuals display structure with amplitudes of tens of milliseconds of arc on time scales of months, but Fourier analysis reveals no significant peaks at shorter periods, including the 40-60 day period found in the UT1 time series.During September, 1986, we introduced a new “quick-look” UT1 time series. The values are typically available within 7 days. The accuracy, which depends strongly on the accuracy of the X and Y pole coordinates used in the computations, ranged from 0.3 to 0.7 milliseconds during the first two weeks, but improved to about 0.1 milliseconds during the latter two weeks of the month. We plan to continue the quick-look UT1 series as a standard product of the IRIS Earth orientation monitoring service.
Detached Main-Sequence Binaries: A Laboratory for the Study of Stellar Structure
Authors: Gimenez A. 
Published: June 2019

Our knowledge of the evolutionary processes taking place in interacting binary stars is based primarily in our understanding of the stellar structure and evolution of single stars. The determination of absolute dimensions in close doble-lined eclipsing binaries, with well-detached components, is the best empirical approach available into stellar structure.Important physical parameters and processes can be studied in some detail within the framework of detached main-sequence binaries, e.g., initial chemical composition, the mixing-length ratio, the relevance of convective overshooting, interactions between matter and radiation in the stellar interior, the concentration of mass towards the center of the stars or the response of stellar envelopes to tidal influences by an external gravitational potential.A combination of observational results from photometry and spectroscopy are used to obtain relevant stellar parameters: absolute dimensions, ages, chemical composition, apsidal motion rate, degree of synchronization and circularization, etc., and a comparison with theoretical models allow us to derive some information about the above mentioned physical mechanisms.
The Masive Binary SV Centauri
Authors: Linnell Albert P. 
Published: June 2019

Current light synthesis models of SV Centauri require a highly overcontact system with an 8000K temperature discontinuity between components. Current evolutionary models support the overcontact status, and describe the system as in the final stage of rapid mass transfer prior to mass reversal, calculated to occur within 2500 years. Light synthesis simulation provides a poor fit to the observations, and requires an ad hoc assumption to fit the substantial O'Connell effect.IUE spectra have been reported to indicate the presence of a 200 000K source of small dimensions in the system. New spectrum synthesis results demonstrate there is no hot spot, but there is extra radiation longwards of 200nm that is unaccounted for by the overcontact model.Postulated presence of an accretion disk around the mass gainer prospectively overcomes the observational anomalies. Formation of the accretion disk apparently violates the Lubow-Shu criterion. A reasonable scenario nonetheless supports accretion disk formation. Conflict with the theoretical models can be resolved on the postulate that the spherical accretion assumed in those models render them inapplicable to this system.
A Magnetic Field Strength vs. Temperature Relation in Sunspots
Authors: Kopp Greg , Rabin Douglas 
Published: June 2019

The near infrared presents several new and powerful advantages in the diagnostics of sunspot atmospheres: (1) increased magnetic sensitivity in Zeeman-split lines, (2) increased sensitivity of umbral brightness to temperature, and (3) reduced scattered light and seeing disturbances due to atmospheric turbulence. This has revealed a strong and consistent relationship between sunspot brightness and magnetic field strength.We have made spatial/spectral observations of sunspots in the highly sensitive (g = 3) Fe I line at λ = 1.5649 μm to compare field strengths with continuum intensities. We find a characteristic but nonlinear relationship between magnetic field strength, B, and brightness temperature, Tb, in sunspots. In umbrae there is an approximately linear relation between B2 and Tb.
A Redshift Survey in the Direction of a Void near the Pisces-Cetus Supercluster
Authors: Brodie Jean P. , Willick Jeffrey , Bowyer Stuart , Burns Jack O. 
Published: June 2019

The work described here has two main goals:
(1) To test the usefulness of Abell clusters for locating voidsIt is well known that Abell clusters are not uniformly distributed in three-dimensional space (e.g. Bahcall and Soneira, 1983). Recently, Batuski and Bums (1985a, b) have suggested that Abell clusters tend to form filamentary structures between which there appear to be large regions (50–200 Mpc across) devoid of bright galaxies. The technique of using Abell clusters as tracers of voids is controversial because the correlation length of Abell clusters is so much larger than the galaxy-galaxy correlation length. However, compared to an all-sky magnitude-limited survey, the technique, if valid, potentially reduces the amount of telescope time required to identify significant voids and map them in redshift space. To investigate this possibility we have undertaken a redshift survey of galaxies in the direction of a nearby and relatively compact candidate void surrounded by a ring of Abell clusters.(2) To identify a void suitable for more extensive study.Ultimately we would like to know not only the degree to which voids are empty, but also to what extent those galaxies that may be found within a void differ from those in the surrounding shell. Some theorists (e.g. Dekel and Silk, 1986 and references therein) suggest that as a result of biased galaxy formation, galaxies within a void might be expected to be preferentially younger, fainter, more irregular, more metal-poor and more gas-rich than shell galaxies. The region we selected for study is one which forms part of the Haynes and Giovanelli (1986 and references therein) 21 cm redshift survey. It was chosen, in part, because it is of interest to compare optical results with radio data to determine whether or not the two techniques reveal the same structure.
Is Coma a Young Aggregate of Old Subclusters?
Authors: Mellier Y. , Mathez G. , Mazure A. 
Published: June 2019

Evolution of gravitational systems is currently believed to follow a 3-stage scheme:
Stage 1: Subclusters form first ouring violent relaxationStage 2: Dynamical friction mass segregationStage 3: contraction of core, expansion of halo
Are Voids Present in the Lyman-Alpha Forest?
Authors: Crotts Arlin P. S. 
Published: June 2019

SummaryIf voids like those seen in the low z galaxy distribution existed in the H I distribution at z ≈ 2, then high quality QSO spectra, with many Ly-α forest lines per unit z, could be used to discern the voids from the usual random fluctuations in observed number density of lines (≡ n). Several such spectra have been obtained, and these show evidence for gaps in the Ly-α distribution on scales of 20 to 50 h−1Mpc (comoving coordinates, with h = H0/66.7 km s−1Mpc−1, assuming q0 = 0.1). These results are summarized in the table below. All QSO spectra with a line-of-sight n of Ly-α lines n > 80 per unit z and total number of lines N > 40 known to the author are included (except that of PKS 2000-330 [c.f. Carswell and Rees 1987], which is broken into five separate segments of Ly-α forest by gaps in the data and a broad absorption line). Excluded are portions of these spectra where n falls more than 25% below the mean due to instrumental bias. For each of these the distribution of gaps between nearest-neighbor Ly-α redshifts is computed as a function of gap size. If the distribution of redshifts were Poisson, the distribution of gaps should be a decreasing exponential function of gap size. For the two best spectra, large deviations from an exponential are found in the range of 20 to 50 h−1Mpc (in the other four cases, it should be noted that a large number of gaps of such sizes are still expected from Poisson fluctuations). The probability that such deviations are statistically consistent with an exponential distribution is shown in the fifth column of the table.
High-Dispersion Spectroscopy of the Brightest Emission-Line Stars of the Magellanic Clouds
Authors: Stahl O. , Wolf B. , de Groot M. , Leitherer C. 
Published: June 2019

We present an atlas of high dispersion spectra of 24 of the brightest peculiar emission-line stars of the Magellanic Clouds.Our spectra cover the wavelength range from 3600 to 4900 Å. They have been obtained from 1970 to 1984 with the coudé spectrograph of the ESO 1.52 m telescope at La Silla, Chile. The spectral resolution is 0.4 Å for most of the spectra and 0.2 Å for the very brightest stars. Up to 11 spectra are available for one star. In addition, we have done UBVRIJHK(LM) photometry at several epochs of all stars of our sample.
Radial velocities with CORAVEL: results on stellar variability and duplicity
Authors: Burki G. , Mayor M. 
Published: June 2019

The complete radial velocity curve has been determined with CORAVEL for many pulsating stars of various classes: cepheid stars in the Galaxy and in the Magellanic Clouds, RR Lyrae, δ Scuti and SX Phoenicis stars. These measurements allow the determination of the radius variation and of the surface acceleration of these stars. In addition, the mean stellar radius of many of these stars has been determined by applying the Baade-Wesselink method.Systematic surveys of definite groups of binary or multiple stars are in progress with CORAVEL in order to determine the distribution functions of the orbital parameters. The eccentricity distributions for the binaries in the open clusters Pleiades, Praesepe, Coma Ber and Hyades are presented and their dependence on the physical processes (star formation mechanisms, mass exchange, tidal circularization, dynamical evolution) is briefly discussed.
On the infrared excess of Alpha Lyrae
Authors: Anandarao B.G. , Vaidya D.B. 
Published: June 2019

SummaryRecent measurements in far-infrared (12–200 μm) by IRAS (Aumann et al, 1984) and KAO (Harper et al, 1984) have shown evidence of infrared excess in the AO star Alpha Lyrae. This excess in the infrared radiation was interpreted as due to circumstellar dust constituted by grains much larger (≳10 μm) than those of interstellar origin. However, there appears to be a difficulty in interpreting these observations in terms of a single or a combination of several temperatures of dust. We have formulated a simple model to explain the far-infrared data in 12–200 μm region available with IRAS and KAO observations in terms of multiple dust shells of different temperatures. We have shown that dust grains of size ~10 μm can explain the excess emission in the entire region 12–200 μm with temperatures of 130 ± 15 K and 40 ± 5 K. The far-infrared fluxes seem to vary as λ−β with μ = 3.0–3.8, in quite contrast with small sized grains. This work lends further support to the interpretation of far-infrared excess in ∝ Lyrae as emission from circumstellar shell with large (∼10 μm) size grains. Our calculations show, however, that grains of size larger than 10 μm cannot explain the fluxes.
What do binaries teach us about mass-loss from late-type stars?
Authors: Reimers Dieter 
Published: June 2019

It is shown that the binary technique - a B star companion is used as a light source which probes the wind of the red giant primary - has yielded accurate mass-loss rates and wind velocities for 8 G to M (super) giants and (in some cases) estimates of wind temperature.Eclipsing binary systems have in addition revealed that G and K supergiants possess extended chromospheres which could be detected outwards to ∼ 1 R* stellar radius) above the photospheres. Electron temperatures Te and hydrogen ionization ne/nH seem to increase with height up to at least 0.5 R* (ne/nH= 10−2, Te = 104 K at 0.5 R*), and the winds start to be accelerated at heights above ∼ 0.5 R*.Mass-loss rates appear to increase steeper than linearly with L/g · R. It is shown that the observed mass-loss rates are consistent with stellar evolution constraints for both Pop. II and Pop I stars.
Optical Imagery of NGC 6302
Authors: Bohigas Joaquín 
Published: June 2019

Images with narrow band filters centred at the most significant emission lines, such as Hβ, Hα, [SII] 6717 and [SII] 6731, were taken to explore the physical conditions of NGC 6302. Observations were secured on May 1991 with the 2.0m telescope of the Observatorio Astronómico Nacional at San Pedro Mártir, B.C., Mexico, the f/7.5 secondary, and a 384 × 576 CCD chip. The most significant results are:
1.Excitation mechanism. The image of the Hα/[SII] lines ratio reveals that photoionization and shocks are equally important throughout most of the object. Photoionization is dominant in the central region and a few isolated patches. Elongated structures pointing towards the central region can be identified with a shock impinging the wall of a bipolar cavity.2.Extinction. The image of the Hα/Hβ lines ratio shows that extinction changes over all distance scales. C(Hα) is nearly equal to 1.7 in the central region. It falls off very rapidly away from it, and at a distance of 10 arcsec it is approximately equal to 1.3. In the mean, Hα/Hβ is 1.25 times larger in the western lobe than in the eastern.3.Excitation. Different lines ratios, such as [SII] 6724/[SIII] 9069, [OIII] 5007/[OI] 6300 and Hel 5876/Hβ, indicate that excitation is larger in the eastern lobe. For instance, Hel 5876/Hβ ≃ 0.15 in the eastern lobe and ≃ 0.13 in the western region.4.Electron density was calculated from the [SII] 6717 and [SII] 6731 images. The density structure is very complex. Within the central region the highest density is found at both sides of the dark lane. No clear correlation exists between density and shock excited regions. Wave-like patterns can be recognized in the density image. These can be attributed to instabilities, sporadic ejections events and/or multiple shocks.