bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spectroscopic and photometric monitoring of southern post-AGB stars
Authors: Pooley D.J. , Cottrell P.L. , Pollard K.R. , Albrow M.D. 
Published: July 2016

AbstractWe present the results of contemporaneous photometric and spectroscopic monitoring of 20 post-AGB stars from Mt John University Observatory. Photometric measures were carried out using Johnson BV and Cousins RI filters, and the radial velocity measurements were acquired using spectra from an echelle spectrograph. Our programme spanned five years and the stars covered a range of spectral types from B to K in order to investigate the behaviour of post-AGB stars as they evolve away from the AGB. A number of stars proved to be variable in ways incompatible with post-AGB models and are reclassified. Periodicities are presented for a number of stars. Photometrically, HD 70379 was found to be pulsating in two modes with periods of 85 and 97 d. The radial velocities also varied, with the peak amplitude occurring when the photometry was also changing most. AI CMi presented three different types of spectra associated with photometric brightness, with varying strengths of narrow emission lines and molecular bandheads. The Hα profiles in almost all of the stars show evidence of emission which varies on time scales of days to months. The Na D line profiles are generally complex showing between 4 and 7 components due to both circumstellar and interstellar material.
Preliminary photometric study of a pulsating component in the eclipsing binary AB Per
Authors: Kim S.-L. , Kwon S.-G. , Lee D.J. , Lee C.-U. , Jin H. , Lee J.W. 
Published: July 2016

AbstractWe present new photometric results of the Algol-type semidetached eclipsing binary AB Per which was recently discovered to have a Delta Scuti pulsating component.
Nonlinear Radial Pulsations for Hot 20-M⊙ Models
Authors: Austin Benjamin A. , Guzik Joyce A. , Despain Kate M. , Templeton Matthew R. 
Published: July 2016

AbstractWe present the results of calculations that predict a radial pulsation instability region for hot 20-M⊙ stars within a narrow band around Teff = 22,000 K. These models have exhausted core hydrogen and are undergoing shell hydrogen burning, and have luminosities similar to the most massive β Cephei stars, but are slightly cooler and more evolved. The pulsations grow to a radial velocity amplitude of ~20 km s-1with a period of ~0.75 d. The amplitude increases for higher Z and lower Y models.
Mode Identification Using Photometry and Spectroscopy
Authors: De Ridder Joris , Aerts Conny , Dupret Marc-Antoine 
Published: July 2016

AbstractWe tested a stepwise approach to combine photometry and spectroscopy for mode identification where we first used the photometric amplitude ratios to restrict the degree l, then did a spectroscopic mode identification, and finally fitted the photometric amplitudes to restrict the list of candidate modes. For the spectroscopic mode identification, we implemented an efficient multi-mode moment method variant. We conclude that this new variant works well, but that the photometric amplitudes are too model sensitive to do any additional mode discrimination.
A Multi-Site Study of The Extreme Helium Star V2076 Oph
Authors: Wright D.J. , Cottrell P.L. , Lynas-Gray A.E. , Kilkenny D. , Shobbrook R. 
Published: July 2016

AbstractResults from a three-site photometric and spectroscopic campaign of the hottest known extreme helium star V2076 Oph are presented. V2076 Oph shows complex multiple periods ranging from 0.3 to 3 d. However for the two campaigns in 1982 and 2000, no definitive single period can be identified in both datasets. Some of the extracted periods appear to be one-day aliases of others. The two most convincing periods are 0.52 d (22.28 mHz) and 2.75 d (4.21 mHz). The spectral lines identified in V2076 Oph were He I, He II, C II (emission), C III (both emission and absorption), C IV and N III. These lines showed profile variations that could not be linked to the photometric variations. Individual line velocities vary by species and ionisation level. The spectral lines vary in phase with an amplitude of ~25 kms−1 – the exception is the strong C III emission line at 5696 Å which shows different behaviour and has the lowest observed average velocity. This indicates that this emission line originates in the stellar wind rather than the photosphere.
A Possible Pulsation Mechanism for B Stars
Authors: Cox Arthur N. 
Published: July 2016

AbstractNumerous possible mechanisms for the cause of B star radial and nonradial pulsations have been investigated in the last 30 years, but none have been clearly shown that they actually work. I briefly review these ideas, and then focus on a new idea about the driving due to time-dependent convection. I suggest that the outward going photons are absorbed by CNONe ions so that they capture the photon momentum. Previous investigations by many have shown that the acceleration of these ions can be two orders of magnitude larger than the local gravity. Levitation and concentration of these elements can occur between temperatures of 100,000K and 600,000K, the former temperature set by the complete ionization of the competing photon capturing helium, and the latter by the existence of simpler and less absorbing excited electron structures of the CNONe elements. Then I suggest that the concentration of these elements gives such a large local photon opacity that a thin convection zone exists down to maybe 10−5 of the mass of a typical B star. However, there would be no overlap with the outer surface convection zone that exists down to 50,000K in the outer 10−9 of the mass. The long time scale of the convection, or even its periodic turning on and off, gives a source of time-lagged luminosity which can be shown in some cases to drive mechanical pulsations in one or more modes. This envelope pulsation mechanism could apply equally to radial and low degree nonradial B star pulsations. It could also explain why not all B stars pulsate unless this unconventional composition structure occurs, disappears, and maybe reoccurs at some evolution stage.
Solar-Like Oscillations in Late Spectral Class Stars
Authors: Cox Arthur N. , Cahn Julius H. 
Published: July 2016

AbstractThere is interest in the minimum mass that can exhibit solar-like nonradial p-mode oscillations. Inspection of models for the solar case shows that there is approximately equal pulsational driving and damping due to the standard radiative effects. It is widely believed that coupling with surface convective elements occasionally tips the balance by strongly driving modes for a few days with the timescale of the convection at the top of the convection zone. Then both the convection and the radiative damping decrease the mode amplitude at the observed decay rate, (which is close to that calculated by Kidman and Cox (1984)) until the next convective reexcitation. Population I composition stellar models at 0.30, 0.45, and 0.75 solar mass have been constructed to see if the near equality of radiative driving and damping exists in them also. If so, then stars down to late spectral class could possibly display solar-like oscillations. We find that the convection timescale decreases to less than 100 seconds at 0.30 solar mass, but the near equality of radiative driving and damping occurs only down to about 0.5 solar mass. Thus there is the possibility that 100 to 300 second oscillations can be observed through most of the K spectral class, if the magnetic activity often seen in these stars does not overwhelm these small amplitude nonradial pulsations.
The Effects of Time-Dependent Convection on White Dwarf Radial Pulsations
Authors: Cox Arthur N. , Starrfield Sumner G. 
Published: July 2016

AbstractAfter the discovery of pulsations in white dwarfs, predictions were made that these DA and the hotter DB stars should be pulsating in radial modes with periods of a few seconds or less. The mechanisms are the normal kappa and gamma effects that periodically block the flow of radiative luminosity and the blocking effect of the frozen-in convection at the bottom of the convection zone. Blue edges of the instability strips are between 12,000K and 13,000K for the DA and between 32,000K and 33,000K for the DB variables. Extensive observations, however, have shown that these stars pulsate only in the few-hundred-second nonradial modes and not in any few-second radial modes. We have added the time dependent convection model of Cox, Brownlee, and Eilers (1966) to our pulsation analyses to further investigate the white dwarf radial modes. Since the time scale of the convection is usually short compared to the radial pulsation periods, convection is able to carry luminosity rapidly enough to nullify the kappa and gamma effects periodic radiation blocking. We find that most, and maybe all, radial pulsations for 0.6 solar mass carbon-oxygen white dwarfs with thin hydrogen or helium surface layers are stabilized for both these DA and DB classes, now finally in agreement with observations.
Star Formation History of the Large Magellanic Cloud and Asymptotic Giant Branch Evolution Obtained from A Study of Long-Period Variables.
Authors: Hughes Shaun M.G. , Wood P.R. 
Published: July 2016

AbstractA survey for Long-Period Variables has been made of the Bar and southern regions of the Large Magellanic Cloud. This has been combined with the results of Reid, Glass and Catchpole (1988) to present a global survey of the LMC. Periods are used as age indicators, giving the star formation history of the LMC from 1 to 10 Gyr, which indicates a major burst of star formation occurred approximately 4 Gyr ago.
Preliminary Results of A World Wide Photoelectric Campaign on δ Scuti
Authors: McNamara B. , Baggett S. , Pena J. , Thompson K. , Moore G. , Mantegazza L. , Sekiguchi K. , Candy M. 
Published: July 2016

AbstractDuring June 17-27, 1987 observers in Mexico, Chile, Italy, South Africa, Australia and the United States monitored the light output of the prototype variable star, δ Scuti. The goal of the network was to obtain this star’s frequency spectrum. During the campaign, over 17,000 observations were obtained in the Stromgren b and y filters. A preliminary analysis has been performed and the frequencies listed in Table 1 have been tentatively identified.
The Importance of 3:1 Resonances in Stellar Pulsations
Authors: Moskalik Pawel , Buchler J. Robert 
Published: July 2016

AbstractThe 2:1 resonance between the fundamental and the second overtone modes has received a great deal of attention in the context of Cepheids. It was clearly shown that it causes the Hertzsprung bump progression and brings about the very characteristic observed variation of the Fourier phases with period (Buchler & Goupil, 1984, Ap.J.,279, 394; Klapp, Goupil & Buchler, 1985, Ap.J.,296, 514; Buchler & Kovacs, 1986, Ap.J.,303, 749).
Nonlinear RR Lyrae Models and Double Mode Pulsation
Authors: Ostlie Dale A. 
Published: July 2016

AbstractNonlinear models of RR Lyrae variables were calculated in the region of the HR diagram containing double mode pulsation. In all models Teff = 7000 K, the luminosity was 60 times solar, X = 0.70, Y = 0.299, and P1/P0 = 0.746, characteristic of double mode RR Lyrae variables in M15 (Cox, Hodson, and Clancy, 1983). Models of 0.65 solar masses were calculated using standard opacities and models of 0.75 solar masses were calculated with opacities artificially enhanced by a factor of 1.2 between log(T) = 5.2 and log(T) = 5.9 (Andreasen and Petersen, 1988).
Pulsations of Eötvös Spheres
Authors: Pesnell W. Dean 
Published: July 2016

AbstractA derivation of Eötvös spheres and their pulsations is presented. These spheres incorporate a short range force in their gravitational potential. We used a Yukawa formulation to agree with other workers in this field, although a slightly different form would guarantee a gravitational force that is always attractive. Various relationships are obeyed by these obejcts, much like the polytropes to which they are similar. An attempt is made to test whether a short range force can resolve two outstanding problems in astrophysics. These are the Cepheid mass discrepancy and the frequencies of high-order solar p-modes. The disagreement in predicted and observed Cepheid masses cannot be explained by a repulsive force, but the solar p-modes have their asympotoic frequency decreased. Linear fits to sequences of models are presented for the radial eigenfrequencies and asymptotic frequencies for nonradial pulsations.
Tokamak Plasmas: A Paradigm for Coronal Equilibrium and Disequilibrium
Authors: Petrasso Richard D. 
Published: July 2016

AbstractTokamaks operate over a wide parameter space, allowing access to plasma conditions relevant to astrophysical plasmas. For high electron density discharges, for example, the central electron density and temperature are ~ 3 × 1014cm-3 and ~ 1.5 keV, and the central plasma region is in coronal equilibrium. Towards the edge of the plasma, however, many ion species will be far out of coronal equilibrium. A novel feature of the edge region is the seemingly contradictory property that it is, simultaneously, both a strongly recombining and a strongly ionizing plasma. Recent tokamak observations of strongly recombining plasmas also show that the G parameter (the ratio of forbidden plus intercombination to resonance lines) is larger by a factor of 3 than the ratio of statistical weights of the triplet-singlet series. Such observations can be of direct consequence to the interpretation of non-equilibrium astrophysical plasmas.
Merging of Iron Lines in Spectra of X-ray Burst Sources
Authors: Madej J. 
Published: July 2016

AbstractModel atmospheres and synthetic spectra of neutron star of the effective temperature 107 K are presented. All the iron spectral features in the energy range 6-10 keV (uncorrected for redshift) are washed out by instrumental or intrinsic broadening, which leaves only a single line of the equivalent width comparable with the observations.
X-ray Spectral Synthesis in Hydrodynamic Flare Models
Authors: Serio S. , Antonucci E. , Dodero M.A. , Peres G. , Reale F. 
Published: July 2016

AbstractCompact solar flares are triggered by sudden energy release in magnetically confined plasma. This class of flares is well suited to be studied with numerical hydrodynamic models. In particular, one can compare the evolution of observed and synthetic X-ray spectra, computed under various assumptions for the mechanism of impulsive energy deposition, to constrain theoretical models and their parameter space. We discuss recent results on solar flares along this line, non thermal to models of energy depositions by relativistic electron beams. We shall also discuss possible applications of X-ray spectral synthesis to stellar flares.
Evolution of the Coronae in Early-Type Galaxies
Authors: David L.P. , Forman W. , Jones C. 
Published: July 2016

AbstractWe present numerical simulations of the gaseous coronae in elliptical galaxies. These models consist of a modified King profile for the luminous portion of the galaxy and an isothermal dark halo. We include evolving stellar mass loss from planetary nebulae, and type I and II supernovae. Our models show that elliptical galaxies are likely to produce strong galactic winds at early times with x-ray luminosities of 1042 — 1044 ergs s-1 and temperatures of 10 keV. Galaxies can lose approximately 10-30% of their initial luminous mass in the wind which has an oxygen-to-iron ratio twice the solar value. Since elliptical galaxies are a principle component of rich clusters and compact groups this early wind phase affects the metallicity and temperature of the intracluster medium.
Innovative Techniques for X-ray Calorimetry
Authors: Labov S. , Silver E. , Landis D. , Madden N. , Goulding F. , Beeman J. , Haller E. , Rutledge J. , Bernstein G. , Timbie P. 
Published: July 2016

AbstractIn our x-ray calorimetry effort, we have developed several techniques which may be helpful to other groups working in this field. We are studying several different monolithic and composite calorimeter designs. In our readout configuration, the preamplifier circuit employs negative voltage feedback which allows us to accurately measure the temporal profile of the thermal pulse produced by an x-ray absorbed in a micro-calorimeter. Rise times of less than two microseconds have been observed in monolithic devices operating at .3 K. Furthermore, the feedback preamplifier can be configured for either positive or negative electro-thermal feedback. This preamplifier system is followed by an analog pulse shaping amplifier with a frequency response that can be adjusted to yield the maximum signal to noise ratio for a given thermal response of the calorimeter. In addition, we have developed several diagnostic procedures which have been useful in determining the operating and noise characteristics of our devices. These include an infrared light-emitting diode which flashes a discrete amount of energy on to the calorimeter, and a capacitively coupled test input to the preamplifier which allows us to directly determine the total noise in the thermal detection system. Finally, we are developing an adiabatic demagnetization refrigerator with a temperature control system that is designed to stabilize the 0.1 K cold stage to better than 8 μK. This is required for a resistive thermal detector with resolving power of 1000.
P/Halley, the Model Comet, in View of the Imaging Experiment Aboard the Vega Spacecraft
Authors: Szegő K. 
Published: July 2016

Abstract.In this paper, those results of the VEGA imaging experiments are summarized that probably have general validity for any comet. Shape, size, surface structure, jet activity, and rotation pattern are considered in this respect. It is pointed out that imaging data provide indispensable information for an understanding of cometary activity.
Plasma Processes in the Outer Coma
Authors: Galeev A. A. 
Published: July 2016

AbstractSpacecraft encounters with comets Giacobini-Zinner and Halley revealed a great variety of collective plasma phenomena accompanying the interaction of the solar wind with comets. In this review, we discuss the theory and in situ measurements of the Alfvén wave turbulence and the solar wind loading by cometary ions, and the structure of the cometary bow shock.
Cryogenically-cooled, HEMT amplifiers and receivers in 1-50 GHz range: state-of-the-art
Authors: Pospieszalski Marian W. 
Published: July 2016

AbstractA review of the recent developments in the design, construction and performance of cryogenicallycoolable, high-electron-mobility transistor (HEMT, MODFET) amplifiers and their application in compact cryogenic receivers for radio astronomy applications is presented.
IVS - a second generation space VLBI mission
Authors: Pilbratt Göran 
Published: July 2016

AbstractThe International VLBI Satellite is under assessment study by the European Space Agency and the USSR Academy of Sciences. The IVS mission is conceived as a second generation space VLBI mission with the goal of providing higher sensitivity, greater maximum resolution and a wider spectral coverage compared to the precursor missions and, in addition, some single-dish capability.
Some Highlights of Interferometry in early Radio Astronomy
Authors: Sullivan Woodruff T. 
Published: July 2016

AbstractTwo important episodes in the early development of interferometry in radio astronomy are traced in detail. The first is the use of the sea-cliff interferometer at the Radiophysics Laboratory in Sydney, first by Pawsey for solar observations and later by Bolton for radio star surveys. The second is the development of the Michelson interferometer and the phase switch by Ryle in Cambridge. This also was employed for important observations of the sun and radio stars.
Numerical Experiments with Low SNR data in Radio Interferometry
Authors: Wilkinson P.N. , Woodall P. 
Published: July 2016

AbstractWe outline the results of numerical experiments on low SNR interferometer data. We confirm several theoretical analyses of the effect of averaging on visibility amplitude and closure phase data. However the triple product method for estimating the closure phase, which is useful in optical interferometry, does not provide a significant improvement over existing practice in radio interferometry. We introduce a way of using the triple product to estimate visibility amplitudes from arrays dominated by a single large telescope.
VLBI phase-referencing for observations of weak radio sources
Authors: Lestrade J.-F 
Published: July 2016

AbstractPhase-referencing is a technique used in VLBI to extend the signal coherence time from a few minutes to a few hours in order to enhance significantly its sensitivity. With this technique, VLBI observations of milliJansky radio sources can be conducted for high-accuracy differential astrometry as well as imaging. We describe the technique in some details and present, as an example, a submilliarcsecond differential astrometric experiment design to identify the star responsible for the weak radio emission in the binary system Algol.
Progress on Chinese VLBI network project
Authors: Shuhua Ye , Tongshan Wan , Zhihan Qian 
Published: July 2016

SummaryThe Chinese VLBI Network (CVN) project is supported by the Astronomy Committee, Chinese Academy of Sciences. At present, the CVN project consists of three dedicated VLBI stations, two part-time VLBI stations, and a data analysis center. The sites and equipment of the CVN stations are listed in Tables I and II, respectively. The current status of the CVN data analysis center will be described in the last section of this paper.
On the Observed Complexity of Chaotic Stellar Pulsation
Authors: Kolláth Z. 
Published: July 2016

The existence of some variable stars producing very complicated light curves is well known. Theoretical calculations suggest that the irregular behaviour of the pulsation models in the RV Tauri and W Virginis regime is low dimensional is the result of period-doubling or tangent bifurcations (see e.g. Buchler and Kovács 1987, Kovács and Buchler 1988, Tanaka and Takeuti 1988).The light variation of the RV Tauri star R Scuti covering 150 years was analyzed by Kolláth (1990). A striking similarity was found between the reconstructed attractor of the Rössler model and that of the light variation of R Scuti. This result confirms the theoretical prediction of the existence of chaos, but a discrepancy still exists between the theory and observation. We could not find evidence for low dimension (D = 2– 3). The analyses of other stars also show rather erratic behaviour (e.g. Cannizzo and Goodings 1988, Cannizzo et al. 1990). A possible answer for this discrepancy is the treatment of stochastic perturbations by convection (Perdang 1991).
The Effect of Convection in Nonlinear One-Zone Stellar Models
Authors: Saitou M. 
Published: July 2016

AbstractWe study the convective nonlinear one-zone models of the pulsating variable stars. In the small convective case, the solution shows chaotic behavior through the period doubling bifurcation as the temperature is lower although the temperature at which the chaos appears is lower than in the case of no convection. On the other hand, in the strong convective case, the solution only shows period-one limit cycle.