bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Frequency Solar Oscillations
Authors: Jefferies Stuart M.
Published: June 2019

This paper reviews the properties of the solar oscillation power spectrum at frequencies near and above the acoustic cut-off frequency for the atmosphere. This region of the spectrum contains over 50% of the total peak structure observed and is a source of information not only on the outer layers of the sun and the origin of solar oscillations, but also on the physics of the solar core and chromosphere.
On the Chromospheric Behaviour of Photospheric Mn 539.47 nm Spectral Line
Authors: Vince I. , Erkapić S.
Published: June 2019

The measured equivalent width of the Mn 539.47 nm line irradiance shows a cycle variability of about 2% (Livingston, 1992). In this paper we analyse the unusually large variation of this line.
Techniques of mm/sub-mm Interferometry in Star Formation
Authors: Sargent Anneila I.
Published: June 2019

Since the earliest stages of star formation occur deep in clouds of gas and dust, they are hidden from optical view. Nevertheless, infrared and millimeter wavelength observations over the last twenty years have enabled increasingly detailed studies of the processes involved in stellar birth. High resolution, aperture synthesis imaging of the millimeter-wave radiation from dust and molecular gas in star-forming clouds has proven particularly effective. On the other hand, there can be pitfalls to be avoided in the use of mm/sub-mm interferometry techniques. Here, we consider what has been learned from currently-operating mm interferometers and the potential of the next generation of arrays.
The Turbulent Star Formation Model - Outline and Tests
Authors: Vázquez-Semadeni Enrique
Published: June 2019

We summarize the current status of the turbulent model of star formation in turbulent molecular clouds. In this model, clouds, clumps and cores form a hierarchy of nested density fluctuations caused by the turbulence, and either collapse or re-expand. Cores that collapse can be either internally sub- or super-sonic. The former cannot further fragment, and can possibly be associated with the formation of a single or a few stars. The latter, instead, can undergo turbulent fragmentation during their collapse, and probably give rise to a cluster of bound objects. The star formation efficiency is low because only a small fraction of the density fluctuations proceed to collapse. Those that do not may constitute a class of “failed” cores that can be associated with the observed starless cores. “Synthetic” observations of cores in numerical simulations of non-magnetic turbulence show that a large fraction of them have subsonic internal velocity dispersions, can be fitted by Bonnor-Ebert column density profiles, and exhibit “coherence” (an apparent independence of linewidth with column density near the projected core centers), in agreement with observed properties of molecular cloud cores.
Chemical Signatures of the Evolutionary State of Cores
Authors: Aikawa Yuri
Published: June 2019

Recent observations with high angular resolution revealed chemical differentiation in several prestellar cores; while N2H+ emission peaks at the core center, CO, CS and CCS emission peaks are offset from the center. Molecular abundances also vary among cores; some cores have high CCS abundance and low N2H+ abundance, but others have less CCS and more N2H+. Numerical calculations of a chemical-reaction network in contracting cores show that these differentiations and variations can be diagnostics of physical evolution of cores, because molecular abundances and distributions are determined by the balance between the dynamical and chemical time scales. In prestellar cores, low temperatures and high densities cause adsorption of molecules onto grains. Depletion time scale varies among species; early-phase species deplete first because of destruction via gas-phase reactions in addition to the adsorption. N2H+ is the last to deplete because of the low adsorption energy of its parent molecule N2 and depletion of main reactants such as CO. Molecular D/H ratio is another probe of core evolution, since it increases as the adsorption proceeds.
Theory of Formation of Massive Stars via Accretion
Authors: Yorke Harold W.
Published: June 2019

The collapse of massive molecular clumps can produce high mass stars, but the evolution is not simply a scaled-up version of low mass star formation. Outflows and radiative effects strongly hinder the formation of massive stars via accretion. A necessary condition for accretion growth of a hydrostatic object up to high masses M ≳ 20M⊙ (rather than coalescence of optically thick objects) is the formation of, and accretion through, a circumstellar disk. Once the central object has accreted approximately 10 M⊙ it has already evolved to core hydrogen-burning; the resultant main sequence star continues to accrete material as it begins to photoevaporate its circumstellar disk (and any nearby disks) on a timescale of ∼105 yr, similar to the accretion timescale. Until the disk(s) is (are) completely photoevaporated, this configuration is observable as an ultra-compact Hii region (UCHii). The final mass of the central star (and any nearby neighboring systems) is determined by the interplay between radiation acceleration, UV photoevaporation, stellar winds and outflows, and the accretion through the disk.

Several aspects of this evolutionary sequence have been simulated numerically, resulting in a “proof of concept”. This scenario places strong constraints on the accretion rate necessary to produce high mass stars and offers an opportunity to test the accretion hypothesis.
Models of Accretion Disks Around Young Stars
Authors: D'Alessio Paola , Calvet Nuria , Hartmann Lee , Muzerolle James , Sitko Michael
Published: June 2019

We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magnitude, to illustrate the effects of accretion on the overall disk structure and emission.
Probing AU-scale Structure using Spectro-astrometry
Authors: Takami Michihiro , Bailey Jeremy , Chrysostomou Antonio , Tamura Motohide , Terada Hiroshi
Published: June 2019

The circumstellar environment within 10 AU of young stars are of particular interest for star and planet formation. Unfortunately, present imaging facilities such as the Hubble Space Telescope or adaptive optics on 10-m telescopes cannot resolve this region. We have proved that “spectro-astrometry” is a powerful technique for discovering pre-main-sequence binaries, determining kinematics of outflows and providing evidence for gaps in circumstellar disks — all down to AU scales. In this paper, we summarise our progress to date.
Detecting Planets within Disks
Authors: Wolf S.
Published: June 2019

Possible ways to detect extra-solar planets in circumstellar disks using high-angular resolution measurements (imaging) are discussed. Since dust is expected to outshine the planet by scattering of stellar radiation and thermal emission, the most promising approach is to look for characteristic signatures in the circumstellar disk caused by the interaction with the planet.
The galactic constants–an interim report
Authors: Kerr F. J.
Published: June 2019

At the IAU General Assembly in Patras in August 1982, Commission 33 set up a Working Group on the Galactic Constants. The Working Group is charged with developing a critical review of the values of the main galactic constants, for publication before the General Assembly in 1985. It has not been specifically charged to come up with a proposal for a revised set of values, although it can do so if it wishes.
How well do we know the rotation curve of our Galaxy?
Authors: Pişmiş Paris
Published: June 2019

The existence of variations from a smooth curve, in the form of waves, in the rotation curves of galaxies was pointed out earlier, and an interpretation was proposed based on the argument that the waves were the manifestation of the coexistence of different populations in a galaxy (see for example PişLmiş 1965, 1974). Observations in the past few years have shown that “undulations” in the rotation curve of spiral galaxies are rather common phenomena; maxima and minima occur roughly at arm and interarm regions, respectively. The velocity fields of the majority of the 23 galaxies compiled by Bosma (1978) exhibit well-defined waves. In particular the velocity field in the 21-cm HI line of M81 by Visser shows clearly the correlation of the waves with the spiral structure.
On the nature of OH/IR stars
Authors: Engels Dieter
Published: June 2019

OH/IR stars are the infrared counterparts of galactic OH maser sources which show a characteristic double-peaked emission-line profile. Their strong radio emission can be detected at large distances, making them excellent tracers of distribution and kinematics of evolved stars in the Milky Way. The OH maser profile is typical for line emission from an expanding circumstellar shell. The circumstellar shells of OH/IR stars absorb the optical emission of the central star nearly completely and reemit the energy in the infrared. Having luminosities ~ 105 L⊙ and energy distributions peaking around 10μm, they may make a major contribution to the interstellar radiation field beyond 5μm. With mass loss rates of 10-5 to 10-4 M⊙/yr they lose several solar masses in a few hundred thousand years. OH/IR stars are therefore important objects for recycling stellar matter into the interstellar medium.

Progress has been made in understanding the nature of OH/IR stars. They are Mira-like large-amplitude variables with periods up to 5 years long. It is proposed that they are stars of intermediate mass (2–10 M⊙) on the asymptotic giant branch (AGB). They have not only larger masses than Mira variables proper, but also longer periods of pulsation and larger mass loss rates. As a result optically thick circumstellar dust shells are formed, which prevent the detection of these more massive Mira-like variables at optical wavelengths. Radial pulsation (Mira variability) is thus thought to occur for all intermediate-mass stars in the course of their evolution on the AGB. In view of their high mass-loss rates, these stars may be key objects in the study of the formation of planetary nebulae.
Stellar chemical-abundance gradient in the direction of the South Galactic Pole - preliminary results
Authors: Trefzger Ch. F. , Pel J. W. , Blaauw A.
Published: June 2019

Using the Walraven VBLUW photometric system, we have studied the metal content of 89 F and G stars in the Galactic South Pole field SA141. Our sample is based on the Basel survey of RGU photometry in Selected Areas, and it contains all stars in SA141 with VJ <14m.5 and (G-R)<1m.15 (spectral types earlier than about G7). The observations were made with the VBLUW photometer and the 90-cm Dutch Telescope at ESO, La Silla.

For unreddened intermediate-type stars the VBLUW photometry enables us to separate the effects of temperature, gravity, and metallicity (cf. Lub and Pel, 1977). Since reddening is negligible in SA141, we can therefore determine these three parameters for each program star once the photometric indices are calibrated in terms of Teff, log g, and [Fe/H]. The latter calibration was made in a semi-empirical way, using VBLUW observations of stars with spectroscopic analyses in combination with theoretical colors based on the model spectra by Kurucz (1979). We used the Hyades main-sequence as a zeropoint, adopting [Fe/H] = +0.15 for this cluster.

The results in the (V-B)-(B-L) diagram are shown in Fig. 1. This diagram is very sensitive to metallicity, but almost gravity-independent. Fig. 1 indicates that most program stars have metallicities in the range −1 ≦ [Fe/H] ≦ 0. The distribution of the program stars in the gravity-sensitive (V-B)-(L-U) diagram is very narrow, log g =4.2 ± 0.3, which means that these stars are probably all dwarfs, with only very few possible subgiants.

The absolute-magnitude calibration was derived by using the data of Cayrel de Strobel et al. (1980) and of Cayrel de Strobel and Bentolila (1983). From their [Fe/H]-catalogue we took all stars with known distances, and within the parameter range of our program stars, to construct an empirical MV - Teff relation. This relation was used to derive distances for the stars in SA141.

These distances are plotted against [Fe/H] in Fig. 2. The diagram clearly shows the correlation between distance and metallicity, suggesting for this particular sample a gradient of −0.6 in [Fe/H] over the first 500 pc. Similar values were found in the Basel RGU program (cf. Trefzger, 1981) and by Blaauw and Garmany (1975).
Star counts, local density and KZ force
Authors: Strömgren Bengt
Published: June 2019

The approach by Bahcall and Soneira to the determination of galactic parameters through the use of star counts is referred to, and tests of the Bahcall-Soneira Galaxy model based on additional observational data are discussed.

The determination of the local mass density by Hill, Hilditch and Barnes through studies of A and F stars in the region of the North Galactic Pole is briefly discussed, as is a recent investigation of the problem by Bahcall.

In the determination of the galactic force Kz and the local mass density from the density distribution ν(z) and the distribution f(W) of velocities at right angles to the galactic plane for a group of tracer stars, it is important to secure homogeneity of the tracer group. This has led Hill, Hilditch and Barnes in a continuation of their investigation to use photoelectric uvby photometry to segregate homogeneous groups of F stars. A similar approach is followed by Danish astronomers, whose work is briefly described.
Studies of 0-F5 stars at the Galactic poles
Authors: Hilditch R. W. , Mcfadzean A.D. , Hill Graham , Barnes J. V.
Published: June 2019

We report progress on a spectroscopic and photometric programme devoted to the study of the dynamics of O-F5 stars within 15° of the North and South Galactic Poles. The aims of the programme are to test dynamical and chemical evolution models of the Galaxy by establishing velocity dispersions as a function of z-distance for stars of different population groups. We are also able to investigate the interstellar reddening at the poles and the kinematic properties of apparently normal early-type stars found more than 1 kpc from the galactic plane.
HI at the outer edge of the Galaxy and its implications for Galactic rotation
Authors: Jackson Peter D.
Published: June 2019

In their analysis of global properties of the Galaxy based on the HI distribution outside of the solar circle, Knapp, Tremaine, and Gunn (1978 - hereafter referred to as KTG) have examined the extreme HI velocities for the outer edge of the Galaxy. They fit these values to a model based on an exponential drop-off of HI surface density with galactocentric radius, R, and a flat rotation curve. Adopting a value of 4 kpc for the scale length, L, of this drop-off, KTG fit the extreme velocities with various values of θo (the circular rotation speed of the Local Standard of Rest), to obtain a best fit of θo − 220 km/s. While KTG obtained sensitive observations with the NRAO 43-m radio telescope for the galactic-longitude range 1 = 80° to 225°, they had to rely on the HI survey by Kerr, Harten, and Ball (1976 - hereafter referred to as KHB), obtained with the Australian 64-m radio telescope at Parkes, for the balance of the Southern Milky Way.
CO survey of the Southern Milky Way
Authors: Cohen R. S. , Thaddeus P. , Bronfman L.
Published: June 2019

When the Columbia Southern Millimeter-Wave Telescope on Cerro Tololo, Chile, began operation in January 1983, one of the main goals was to make the first extensive out-of-plane survey of 2.6-mm CO emission in the Southern Milky Way. Because this telescope, a 1.2-meter Cassegrain, is a close copy of the Columbia telescope in New York – with nearly identical antenna, feed, and calibration – it will be very easy to join our northern and southern surveys to make the first homogeneous radio survey of the entire Galaxy. The Chile telescope does have one important improvement over the New York system: a liquid-nitrogen-cooled receiver with a single-sideband noise temperature of 385 K. Details of the telescope are given in Cohen (1983).
Distribution of CO in the Southern Milky Way and large-scale structure in the Galaxy
Authors: Mccutcheon W.H. , Robinson B. J. , Manchester R. N. , Whiteoak J. B.
Published: June 2019

The southern galactic-plane region, in the ranges 294° ≤ 1 ≤ 358°, −0°.075 ≤ b ≤ 0°.075, has been surveyed in the J = 1–0 line of 12CO with a sampling interval of 3′ arc. Observations were made with the 4-metre telescope at the CSIRO Division of Radiophysics in 1980 and 1981. Details of equipment and observing procedure are given in Robinson et al. (1982, 1983); see also McCutcheon et al. (1983).
Star formation in the Orion Arm
Authors: Blaauw A.
Published: June 2019

We assemble principal constituents of the morphology of the Orion Arm within 1500 pc as a starting point for the study of progression of star formation.
A survey of molecular clouds associated by young open star clusters
Authors: Leisawitz D. , Bash F.
Published: June 2019

A major study of the molecular gas surrounding young star clusters is underway. We are using the Columbia University 1.2–m millimeter-wave telescope to observe emission from the J=1→0 rotation transition of 12CO in the vicinities of 128 open star clusters. The survey region around each cluster is at least 10 cluster diameters in size, typically ≳ 5 square degrees. Sensitivity is sufficient to detect lines as weak as 1 K over a range in velocity ± 83 km/s centered on the cluster velocity and with a velocity resolution of 0.65 km/s. Clusters in this sample have well-determined distances ranging from 1 to 5 kpc, and ages ≳ 100 million years (Myr).
A deep survey for high-velocity clouds
Authors: Hulsbosch Aad N.M.
Published: June 2019

The deep Dwingeloo survey for high-velocity clouds is now nearing completion. This survey consists of more than 28 000 positions in a 1° grid for δ>−18°. The velocity coverage is −1000 to + 1000 km/s and the detection limit (4σ) in Tb about 0.05 K, corresponding to ∫ Tadv = 1.0 K km/s or 10 Jy km/s or 2.2 r2M⊙ (r the distance in kpc) if we assume an average profile width of 25 km/s.
Two-fluid gravitational instabilities in a galactic disk
Authors: Jog Chanda J.
Published: June 2019

We formulate and solve the hydrodynamic equations describing an azimuthally symmetric galactic disk as a two-fluid system. The stars and the gas are treated as two different isothermal fluids of different velocity dispersions (CS ≫ Cg), which interact gravitationally with each other. The disk is supported by rotation and random motion. The formulation of the equations closely follows the one-fluid treatment by Toomre (1964). We solve the linearized perturbation equations by the method of modes, and study the stability of the galactic disk against the growth of axisymmetric two-fluid gravitational instabilities.
Collective phenomena in a multi-component gravitating system
Authors: Fridman A. M. , Morozov A. G. , Palous J. , Piskunov A. , Polyachenko V. L.
Published: June 2019

The paper aims at a demonstration of the principal differences between the oscillation spectra of multi-component systems and a one-component medium. The character of the mutual motions of components appears then to be of importance. Three cases are considered: (1) motionless components in the inertial frame of reference; (2) inertial subsystems moving at constant relative velocities; (3) a rotating n-component system. The oscillation spectra in these three cases have qualitative differences between each other and when compared with those of resting or rotating one-component systems.
Oscillations of rotating gas disks : p-modes, g-modes, and r-modes
Authors: Iye Masanori
Published: June 2019

Short-wavelength adiabatic oscillations of an infinitesimally thin, rotating and self-gravitating gas disk in dynamical equilibrium are studied. The problem is formulated in terms of the theory of oscillations of gas spheres (i.e. stars) in order to help galactic and stellar seismologists towards their mutual understanding of their rather similar subjects.
Interstellar nitrogen isotope ratios
Authors: Güsten R. , Ungerechts H.
Published: June 2019

We report first results of a systematic study of the galactic interstellar nitrogen isotope ratios, based upon measurements of the (J,K) = (1,1) and (2,2) lines of the 14NH3 and 15NH3 molecules. We find significant deviations of the [14N]/[15N3] abundance ratio from the solar-system value (~270), with the most extreme enrichment in the galactic-center region, where [14N]/[15N] ~ 1500. This value is consistent with a nitrogen nuclear-synthesis scheme in which 14N is the main product of normal secondary CNO-burning, whereas 15N is depleted by the same process.
Part II.—Sipylos and Cybele
Authors: Ramsay W. M.
Published: June 2019

“Und jenen Sängern zu Smyrna, Phokäa, Kyme, Neonteichos, Larissa lag immer der majestätische Sipylos vor Augen mit seinen Felsenhöhen und Abgründen, mit seinen Quellen und kleinen Seen, mit der Erinnerung und Mahnung grosser Erdrevolutionen und Zerstörung reichen irdischen Segens und menschlichen Glückes. So ist denn hier das Bild eines Himmels auf Erden, eines zum Himmel strebenden Menschenglückes, aber auch das Bild eines überkühnen Hochmuthes und göttlichen Strafgerichtes vor allen lokal befestigt worden.” These words of Stark (Niobe, p. 409) well describe the peculiar fascination that the splendid mountain still exercises on one that lives under its shadow, and the ever-growing interest with which one returns to its past history. The least satisfactory part of Stark's excellent work is precisely that which treats of Niobe in Sipylos (98–109 and 403–46). It suffersfrom the lack of trustworthy information about the district. Even after he had himself had the opportunity of seeing for a few hours the socalled “Niobe,” and had recognised in it ein Gebilde alter heimischer, in den phrygischen Bildwerken der Göttermutter vielfach sich später aussprechenden Kunst und eines tiefen Naturgefühls, his rationalising treatment of the myth (Nach d. Gr. Orient, pp. 231–54) is very unsatisfactory. We are to believe that a powerful empire under a king Tantalus existed here, that his capital was destroyed by an earthquake, and his empire ruined by an Assyrian invasion (Stark's account seems to waver between the two and finally to adopt both explanations), that his son went down to the seaport of the empire and sailed away to find an empire and a bride ready for him in Greece. Though a poetic and fervid imagination, stimulated by the charm of the wonderful Sipylos, has made Stark's account a seductive picture, yet it requires only a statement of the theory in its bare outlines to show how uncritical it is.
V / Disposal of the Dead
Authors: Wheat Joe Ben
Published: June 2019

Somewhat more than 221 burials have been recovered from Mogollon sites which can be assigned to the periods covered by this study. The precise number cannot be given, nor is it important other than as an index to the validity of our reconstruction of the burial customs represented.

Burials of Mogollon 1 have been found at Cave Creek, Chiricahua:3:25, San Simon, SU, Bluff, Crooked Ridge Village, and Tularosa Cave. At Cave Creek two burials, flexed on the side, occurred in pits cut through the floor of an abandoned pithouse; a third, that of an infant, came from a shallow pit outside the house. Two additional burials, an infant and an adult, were placed in shallow depressions in the silt at Chiricahua:3:25. None of these burials, all Penasco Phase, contained grave furniture. Numerical data are not available on Penasco Phase burials at San Simon Village, but according to Sayles (1945:62-64) the body was commonly flexed on the side and placed in a pit sometimes cut through the house floor, but usually outside the house.
XIV / Mogollon - Anasazi Relationships
Authors: Wheat Joe Ben
Published: June 2019

One of the chief problems confronting Southwestern archeologists today is the definition of relationships between the Anasazl and the Mogollon, for regardless of the position taken as to their taxonomic status, the fact that differences did exist in some degree demands investigation. Such investigation is by no means easy, because there are many similarities between the two groups. To understand these, it is necessary to control the relative chronology; and for this our evidence is poorest for the period most critical in the relationships, that is, prior to A. D. 600. Nevertheless, recent advances in our knowledge make such an attempt necessary.
2. Information Storage and Integration Applied to Low-Contrast Astronomical Images
Authors: Morton G. A.
Published: June 2019

In observational astronomy one is continually faced with the problem of interpreting faint images which are close to the limit of recognizability. The problem of threshold recognition of images at low light levels may be subdivided into two cases: (1) the limit of detectability of an image against a black background, and (2) the minimum perceptible change of illumination which can be detected at small but finite light levels. Until such time as the much-discussed space platform is launched into its orbit around the Earth, the astronomer must be content to view astronomical objects through a thick layer of relatively luminous air. Therefore, the second type of problem, namely that of a low-contrast image, is the main consideration.
Predictions on the Number of Variable Stars for the GAIA Space Mission and for Surveys such as the Ground-Based International Liquid Mirror Telescope
Authors: Eyer Laurent , Cuypers Jan
Published: June 2019

Future space and ground-based survey programmes will produce an impressive amount of photometric data. The GAIA space mission will map the complete sky down to V = 20m, and produce time series for about 1 billion stars. Survey instruments such as the International Liquid Mirror Telescope will observe slices of the sky down to V = 23m. In both experiments, the opportunity exists to discover a huge number of variable stars. Predictions of the expected total number of variable stars and the number of variables in specific subgroups are given.
MONS: Measuring Oscillations in Nearby Stars
Authors: Kjeldsen H. , Bedding T. R. , Christensen-Dalsgaard J.
Published: June 2019

MONS is a proposed space mission to study stellar oscillations. We briefly describe the main features of the mission – more details are available at the MONS Home Page (http://www.obs.aau.dk/MONS)
The MOA Project Data Reduction Pipeline and Database
Authors: Reid M. L. , Sullivan D. J. , Dodd R. J.
Published: June 2019

The MOA Project is a collaboration of Japanese and New Zealand (NZ) astronomers and physicists undertaking a survey of stellar variability in the direction of the Galactic bulge and Magellanic Clouds. The primary motivation of the project is detection and monitoring of gravitational microlensing events although a large amount of data is also collected on stellar variability. Surveying the number of stars required to detect gravitational microlensing events is an operation of some magnitude and requires sophisticated data processing software.
The Fourier Decomposition of the Light Curves of High Amplitude δ Set Stars
Authors: Poretti Ennio
Published: June 2019

A significant improvement of the relationships between observed and physical properties of high amplitude δ Scuti stars (HADS), SX Phe stars, and RRc stars can be obtained by the systematic application of Fourier decomposition to their light curves.
Nonlinear Time-Series Analysis of Pulsation of Post-AGB Stars by Genetic Algorithm/Neural Network Hybrid Systems
Authors: Aikawa Toshiki
Published: June 2019

Some pulsating post-AGB stars have been observed with an Automatic Photometry Telescope (APT) and a considerable amount of precise photometric data has been accumulated for these stars. The datasets, however, are still sparse, and this is a problem for applying nonlinear time series: for instance, modeling of attractors by the artificial neural networks (NN) to the datasets. We propose the optimization of data interpolations with the genetic algorithm (GA) and the hybrid system combined with NN. We apply this system to the Mackey–Glass equation, and attempt an analysis of the photometric data of post-AGB variables.
Pulsating Variable Stars in Local Group Dwarf Galaxies
Authors: Mateo Mario
Published: June 2019

Pulsating variables stars have had a long and prominent history in helping to map out the local Universe. Current observational techniques now make it possible to identify examples of all non-degenerate radial pulsating variables throughout much of the Local Group. I review our current knowledge of such variables in the dwarf galaxies of the Local Group. This summary tries to highlight some recent results that help underscore the use of these stars as important tools to study the physical nature of their parent galaxies as well as their continued use as important distance indicators of nearby galaxies.
The Cepheid P–L Relation and Metallicity
Authors: Laney C. D.
Published: June 2019

Baade–Wesselink radii derived using VJHK photometry are used to test the metallicity dependence of the Cepheid P–L relation using three collections of metallicity estimates from the literature. The hypothesis that Cepheids at the LMC metallicity ([Fe/H] ~ −0.2) could be several tenths of a magnitude fainter than at solar metallicity is not supported by the evidence.
The Status of the Distance Scale
Authors: Tammann G. A. , Parodi B. R. , Reindl B.
Published: June 2019

Cepheids are the pillar of the extragalactic distance scale, but their reach in distance is not sufficient to calibrateH0Yet HST has provided Cepheid distances to eight galaxies which have produced SNe la. The latter are used as nearly perfect standard candles to carry the distance scale to 30 000 km s−1, givingH0= 58 ± 5. This determination finds support in other methods, but its range in distance and its weight is unparalleled.
The Pulsation Mode of Polaris
Authors: Moskalik P. , Ogloza W.
Published: June 2019

Polaris (α UMi) is the only classical Cepheid withHipparcosparallax determined to better than 10% (Feast & Catchpole 1997). The correct identification of its pulsation mode is therefore crucial for calibrating the Cepheid period–luminosity relation. We show that Polaris is an overtone pulsator. Our analysis confirms the finding of Feast & Catchpole, but is entirely independent of the distance estimation.
Cepheid Radial Velocities and Phase Lag
Authors: Sachkov M. E.
Published: June 2019

Our systematic measurements (since 1987) of radial velocities of northern Cepheids with a correlation spectrometer allowed us to make a detailed study of shapes of VR curves. This is a preliminary analysis of the phase lag between Cepheid radial velocity and light curves.
Double Mode Variables in M3
Authors: Benkő J. M. , Jurcsik J.
Published: June 2019

Using recent and earlier observations of M3 obtained at Konkoly Observatory, and all the other available photometry, the five known double mode RR Lyrae (RRd) stars have been analyzed. Long term amplitude and period changes of both modes have been followed.
New Data on Period Changes of the RR Lyrae Stars in the Globular Cluster M53
Authors: Kopacki Grzegorz
Published: June 2019

We present new data on the period changes of seven RRab stars in the globular cluster M53. The most interesting result of this work is the strong evidence for cyclic period changes for more than half of the studied RR Lyrae variables.
A Photometric Survey for Variable Stars in Twelve Metal-Rich Globular Clusters
Authors: Layden Andrew C. , Bowes Benjamin T. , Ritter Laura A. , Welch Douglas L. , Webb Tracy M. A.
Published: June 2019

We describe a photometric survey for variable stars in twelve metal-rich globular clusters, and provide results for the first two clusters.
On the Existence of Luminosity Differences between HB Field and Cluster Stars
Authors: Carretta E. , Gratton R. G. , Clementini G.
Published: June 2019

The discrepancy between the long distance scale as derived, e.g., from Hipparcos-based distances to globular clusters via main sequence fitting to local subdwarfs, and the short distance scale as derived, e.g., from the absolute magnitude of field RR Lyrae stars via statistical parallaxes and the Baade–Wesselink method, could be accounted for if an intrinsic difference in luminosity of about 0.1−0.2 mag were found to exist between horizontal branch (HB) stars populating the sparse general field and the dense globular clusters.
Theoretical Models for Classical Cepheids: Mean Magnitudes and Colors and the Evaluation of Distance, Reddening and Metallicity
Authors: Marconi Marcella , Caputo Filippina , Ripepi Vincenzo
Published: June 2019

We present a detailed analysis of theoretical period–luminosity (PL) and period–luminosity–color (PLC) relations of classical Cepheids, as derived from nonlinear, nonlocal, time-dependent convective pulsating models with different masses and chemical compositions. The predicted PL and PLC relations turn out to depend on the pulsator metallicity and, to a minor extent, on the adopted averages (magnitude-weighted or intensity-weighted) over the light cycle in different bands. We show that the determination of the reddening and true distance modulus of a galaxy from the observed PL and PLC relations is constrained by the metallicity sensitivity of these relations. Moreover, the metallicity sensitivity itself provides us with a powerful tool for deriving self-consistent evaluations of distance, reddening and metallicity of the host galaxy, once Cepheid measurements in at least three photometric bands are available. To illustrate this point, we apply our relations to the BVK data for Cepheids in the Magellanic Clouds, assuming to be unaware of their metallicities. The three filter method is finally adopted to evaluate the metallicity of galactic Cepheids (BVI data) as a function of their position.
Hydrodynamical Modeling of Double-Mode Pulsation
Authors: Kolláth Zoltán
Published: June 2019

The hydrodynamical modeling of steady double-mode (DM) pulsations has been a long-standing quest. Recent improvements in turbulent-convective pulsation codes provide a natural way to calculate nonlinear DM oscillations. The eddy viscosity plays a crucial role in these models. We present models both for RRd and beat Cepheid variables and for different chemical compositions. Simple nonresonant amplitude equations capture the DM phenomenon, giving an overview of modal selection on the HR diagram.
Forced Nonradial Oscillations in Early-Type Rotating Stars
Authors: Witte M. G. , Savonije G. J.
Published: June 2019

A method of calculating nonradial oscillations in rotating stars is presented. Using this method, we are able to calculate the spectrum of g-, f- and p-mode eigenfunctions of a star for different stellar rotation speeds, and also the spectrum of rotational r modes. Stability of the modes as a function of stellar rotation speed can be investigated. By regarding the response of a star which undergoes periodic deformations due to the gravitational force of an orbiting companion as a forced nonradial oscillation, the problem of determining the eigenfrequencies of the star becomes one of finding resonances with the forcing potential. Expanding the potential of the orbiting (point mass) companion in terms of the usual spherical functions, the response of the star to each tidal term , with l and m fixed, can be calculated separately. By varying the forcing frequency σ we are then able to calculate the stellar spectrum. To calculate the response of the star we numerically solve the fully non-adiabatic, but linearised hydrodynamical equations for the star, in which the Coriolis forces due to stellar rotation are fully taken into account. To this end we utilise an implicit 2D finite difference scheme which solves the equations on an (r, ϑ) grid. A calculated solution describes the steady state in which the power σT due to the external driving force is in equilibrium with the internal damping. For results and more references see Witte & Savonije (1999).
The Effect of the Solar Cycle on the Resonant Coupling of g Modes
Authors: Jordinson C. , Gough D. O.
Published: June 2019

We consider the effect of the solar cycle on g-mode parametric resonance, and hence on the limiting amplitude of the overstable solar g1(ℓ = 1) mode. We find no change in the expected limiting amplitude from that found by Dziembowski (1983), who ignored cycle variations.
Instabilities in Very Massive Stars
Authors: Jørgen Knut , Ødegaard Røed
Published: June 2019

Detailed dynamical models and nucleosynthesis yields of very massive stars from early pre-MS through very late stages have been computed. The recent reduction of mass loss rates for the WR stages can have important consequences for both the evolution, surface composition and stability. Depending on mass and metallicity, in the present models instabilities occur during the accretion phase (pre-ZAMS), LBV stage and very late stages (WC).
Amplitude Variability of Delta Scuti Stars: 4 CVn
Authors: Breger Michel
Published: June 2019

The amplitude variability of δ Scuti stars is demonstrated by the behavior of the evolved δ Scuti star 4 CVn. The data on 4 CVn covering over 30 yr shows the amplitudes of almost all modes to be extremely variable with a time scales of ten years or longer. The most rapid decrease of amplitude in V is shown by the 7.38 d−1 mode of 4 CVn, which collapsed from 15 mmag in 1974 to 4 mmag in 1976, and 1 mmag in 1977. Between 1974–6 and 1977–8 phase jumps of almost half a cycle occurred. While a simple beating model of two close frequencies can be ruled out from the photometric data, the change is compatible with the growth of a new mode between 1976 and 1977.
Photometric Variability of Helium Stars in the Z-bump Instability Strip
Authors: Cuadrado R. Aznar , Rodríguez P. Montañés , Jeffery C. S. , Pollacco D.
Published: June 2019

Z-bump opacities are known to excite radial pulsations in helium stars with 102≲L/M≲ 104andTeff∼ 25 000K. Well-studied cases are V652 Her and BX Cir. The absence of variations in the metalpoor star HD 144941 demonstrates the importance of Z-bump opacities to drive these pulsations. Several other helium-rich stars lie close to the theoretical blue-edge of the Z-bump instability strip. One of these, LS IV+6°2, is a known radial velocity variable. We report on recent (1999 June) observations of two helium stars close to the Z-bump instability boundary. Both appear to be variable.
Suppressing g Modes in Shell Hydrogen-Burning δ Scuti Stars
Authors: Bradley P. A. , Guzik J. A.
Published: June 2019

For shell hydrogen-burning δ Scuti stars, many more unstable modes are predicted than are observed. For example, for 4 CVn and δ Scuti itself, only 18 and 6 modes are observed, respectively, whereas several hundred ℓ = 0, 1 and 2 rotationally-split modes are predicted. The predicted modes have a mixed p- and g-mode character, with many g-type nodes present in the H-exhausted core and shell H-burning region, where the Brunt-Väisälä frequency is large.

Here we explore whether the predicted frequency spectrum can be made to agree better with the observed spectrum if the g-mode character of the pulsation modes is partially suppressed. Additional motivation for this approach is provided by the observation by Breger et al. (1999) that 4 modes in 4 CVn identified as ℓ = 1 have nearly equal frequency spacings of ∼14 μHz, reminiscent of consecutive radial orders from a pure p-mode spectrum. We consider 2.1- and 2.3-M⊙ evolutionary models that match the observed Teff, L, and log g, plus an identified radial mode frequency of 4 CVn and δ Scuti. For these models, we experimented with setting the Brunt-Väisälä frequency to zero from the model center to the edge of or beyond H-depleted core, so that the g-type portion of the waves becomes evanescent in this region.
Studies of Non-adiabatic Effects on Radial Pulsations in the Atmospheres of Rapidly Oscillating Ap Stars
Authors: Medupe R. , Kurtz D. W. , Christensen-Dalsgaard J.
Published: June 2019

A code for solving non-adiabatic radial pulsation equations with consistent treatment of radiative transfer is used to study the oscillations in the atmospheres of the rapidly oscillating Ap stars. We find that our results are consistent with the presence of an observable node in the temperature eigenfunction.
Echelle-Diagrams for roAp Stars
Authors: Mkrtichian David E. , Hatzes Artie P.
Published: June 2019

We present the echelle-diagrams for p-mode spectra of roAp stars HD 60435, HR 1217 and γ Equ and discuss some their peculiarities.
First Results of the 17th DSN Campaign: Photometry of XX Pyx
Authors: Arentoft T. , Handle G. , Shobbrook R. R. , Wood M. A. , Crause L. , Crake P. , Podmore F. , Habanyama A. , Oswalt T. , Birch P. V. , Lowe G. , Sterken C. , Meintjes P. , Brink J. , Claver C. F. , Medupe R. , Guzik J. A. , Beach T. E. , Martinez P. , Leibowitz E. M. , Ibbetson P. A. , Smith T. , Ashoka B. N. , Raj N. E. , Kurtz D. W. , Balona L. A. , Costa J. E. S. , Breger M.
Published: June 2019

We present the first results from multi-site observations of the δ Scuti star XX Pyx (CD–24°7599). The observations were carried out as the 17th run of the Delta Scuti Network. We collected 583 hr of B, V time-series photometry, resulting in a detection level (4σ) in the amplitude spectrum of 0.5 mmag. We detect 6 new pulsation frequencies, bringing the total number of frequencies known in this star up 19.
Frequency Oscillations in the Delta Scuti Star V534 Tauri: Preliminary Results of the STEPHI IX Campaign
Authors: Machado L. Fox , Li Z. P. , Michel E. , Alvarez M. , Hernández M. M. , Chevreton M. , Zhou A. , Barban C. , Dolez N. , Belmonte J. A. , Fernández A. , Fremy J. , Pau S. , Servan B. , Hernández F. Pérez , Liu Y. Y. , Fu J. N. , Ringot O.
Published: June 2019

The δ Scuti stars are pulsating variables located in the lower part of the Cepheid instability strip with spectral types from A2 to F0 on the main sequence, and from A3 to F5 at luminosity class III. These variables show short periods (< 0.3day) and luminosity amplitudes ranging from a few thousandths of a magnitude to several tenths. Over the last few years, significant progress has been made in the detection of pulsating modes in the framework of the multisite campaigns, e.g. STACC (Frandsen et al. 1996), DSN (Breger et al. 1998), STEPHI (Michel et al. 2000). For the 1998 STEPHI IX photometry campaign, the δ Scuti star V534 Tau of the Pleiades cluster (see Table 1) was monitored during a three week, three continent run. Preliminary results are reported here.
Pulsating Stars and Other Variables in h and χ Persei
Authors: Krzesiński J. , Pigulski A.
Published: June 2019

We present the current status of the ongoing search for B-type pulsators in the double cluster h and χ Persei.
On the Amplitude Interferometer Program at the University of Maryland
Authors: Currie Douglas G.
Published: June 2019

A coordinated program of instrumental development and astronomical observation in Amplitude Interferometry has been conducted at the University of Maryland for the past ten years. The objective of this overall program will be presented. The method of operation and the fringe detection technique of the University of Maryland Amplitude Interferometer will be described and reviewed from both the theoretical and the instrumental point of view. The relationship between the detection technique of Amplitude Interferometry and the various parameters of the atmospheric model will be described and the measured Modulation Transfer Function of the telescope/atmosphere will be illustrated and compared with the MTF obtained in other techniques. Samples of the data obtained during the Amplitude Interferometry observing program of the past six years will be presented. This has operated to sixth magnitude, and has reproducibility resolved objects to 0.011 arc seconds with precision of better than 0.002 arc seconds. The reproducibility and stability of repeated measurements of the stellar diameters will be presented. The various uses of this data in astronomy will be briefly described.

The future application of Amplitude Interferometry will be described, particularly a system to obtain high sensitivity and very high resolution with the same detection system and same signal processing as the current instrument. However, for the immediate future, the two instrumental questions are being addressed separately in the form of the Multi-Aperture Amplitude Interferometer, which operates on a conventional telescope and has an increased data rate (by a factor of 8,000) and the Very Long Baseline Amplitude Interferometer with the increased resolution. These instruments and their current status will be described as well as their relationship to the earlier program and their eventual use in a Multi-Aperture Very Long Baseline Amplitude Interferometer system. Finally the possible applications of these techniques Interferometry to a satellite system will be briefly mentioned.
The Prototype Very Long Baseline Amplitude Interferometer
Authors: Liewer Kurt M.
Published: June 2019

An Amplitude Interferometer based on the design of an earlier interferometer but with two independent telescopes has been built at Maryland, The design and the current status of the instrument will be discussed.

This interferometer has been built with automatic guiding in each arm and is intended for use with baselines of 50 meters but can be used with longer baselines at restricted declinations.

The visibility detection will be exactly the same as that used in the cassegrain model of the interferometer which has operated successfully for several years on the 100 and 200 inch Hale telescopes. The visibility will be measured simultaneously in three different paths with slightly different delays which will allow for automatic location and tracking of the white light fringe envelope.
The Monteporzio Two Meter Amplitude Interferometer
Authors: Tango William J.
Published: June 2019

The two meter baseline interferometer of the Royal Observatory, Edinburgh, will be described. This instrument was designed specifically for the observation of binary stars which have angular separations intermediate between the classical visual binaries and spectroscopic systems. Observations on bright stars suggest that it should be possible to measure the fringe visibility with an accuracy of a few percent.
Angular Dependence of Atmospheric Turbulence Effect in Speckle Interferometry
Authors: Fried David L.
Published: June 2019

The concepts of speckle interferometry as developed by Labeyrie, and of speckle imagery as formulated by Knox and Thompson are analyzed for dependence on field-of-view size. The preliminary analysis, assuming isoplanatism rederives the results of Korff, and derives the result previously inferred by Knox and Thompson from computer simulation, that allowable spatial frequency separation for difference of phase shift determination must be less than r0/λ When the assumption of isoplanatism is dropped, results are obtained for the expected object power spectrum in speckle interferometry and for the expected bispectrum in speckle imagery, showing the dependence on angular spread for an object consisting of a pair of point sources. An angle, ϑ, is defined (in terms of an integral over the strength of turbulence distribution along the propagation path), which bounds the range within which there are no significant anisoplanatism effects. It is noted that the effect of anisoplanatism is not to attenuate the information bearing signal but rather to impose incorrect information on the signal. Thus anisoplanatism can result in incorrect conclusions with no indication that there is a problem.