bluuebunny/crossref_metadata_2025 huggingface dataset:
LINE ASYMMETRY AND EXCITATION MECHANISM OF SOLAR OSCILLATIONS
Authors: Nigam R. , Kosovichev A.G. , Scherrer P.H. , Schou J.
Published: June 2019

In his opening address at the conference Dr. Tim Brown posed the line asymmetry problem between velocity and intensity as a puzzle in helioseismology that has been resisting theoretical explanation for many years. It was the observations of Duvall et al. (1993) that for the first time indicated that the power spectrum of solar acoustic modes show varying amounts of asymmetry. In particular, the velocity and intensity power spectra revealed an opposite sense of asymmetry. Many doubted the correctness of the experiment and thought it to be a puzzling result (Abrams & Kumar, 1996). Many authors have investigated this problem theoretically and have found that there is an inherent asymmetry whenever there is a localized source exciting the solar oscillations (Gabriel, 1995; Roxburgh & Vorontsov, 1995; Abrams & Kumar, 1996; Nigam et al. 1997). This problem has important implications in helioseismology where the eigenfrequencies are generally determined by assuming that the power spectrum was symmetric and can be fitted by a Lorentzian. This leads to systematic errors in the determination of frequencies and, thus, affects the results of inversions (Rhodes et al. 1997). In this paper we offer an explanation for the difference in parity of the two asymmetries and estimate the depth and type of the sources that are responsible for exciting the solar p-modes.
Are Solar P Modes Correlated?
Authors: Foglizzo T. , Garcí R.A. ,   The GOLF Team
Published: June 2019

We have studied the statistical properties of the energy of individual p modes, extracted from 310 days of GOLF data near the solar minimum. The exponential distribution of the energy of each mode is clearly seen. The modes are found to be uncorrelated with a ±0.6% accuracy, thus supporting the hypothesis of stochastic excitation by the solar convection.

The same analysis performed on the same modes just before the solar maximum, using IPHIR data, rejects the hypothesis of no correlation at a 99.3% confidence level. A simple model suggests that 31.3 ± 9.4% of the energy of each mode is coherent among the modes studied in IPHIR data, corresponding to a mean correlation of 10.7 ± 5.9%.
Replacing Colour Blindness with Depth Perception: Rapid spectroscopy and multicolour photometry of roAp stars
Authors: Matthews Jaymie
Published: June 2019

Warning to the unsuspecting reader: The editors of this volume have (bravely) asked me to “preserve the spirit” of my oral presentation. It is difficult to translate the stirring grandeur of a multi-coloured flashing bowtie into mere words and diagrams. Failing to evoke that grandeur, I have instead settled for occasionally capturing the tackiness of my battery-operated talk in Kyoto.
The Single Life of Rapidly Oscillating Ap Stars
Authors: Hubrig S. , Kharchenko N. , Mathys G.
Published: June 2019

Rapidly oscillating Ap (roAp) stars are cool magnetic Ap SrCrEu stars which pulsate in high-overtone (n ≫ l), low-degree (ℓ ≤ 3) p-modes, with periods from 6 to 15 minutes and typical amplitudes of a few millimagnitudes. 28 such stars are currently known. The roAp phenomenon is confined to a well-defined region of the Strömgren photometry parameter space. However, this region also contains other Ap stars, in which no pulsation could be detected, despite sometimes thorough searches. These apparently constant Ap stars (non-oscillating Ap stars, or noAp stars) appear remarkably similar to the roAp stars in many respects (e.g. colour indices, abundances, magnetic fields). Here we present recently found indications for differences between both groups.
On The Seismic Signature of the HeII Ionization Zone in Stellar Envelopes
Authors: Monteiro Mário J.P.F.G. , Thompson Michael J.
Published: June 2019

Sharp variations of the structure of the star create a characteristic signal in its frequencies of oscillation (e.g. [3]). The zone of the second ionization of helium is such a localized feature of the structure whose properties depend mainly on the abundance of helium and the equation of state. Considering that such a signal should easily be detectable provided the frequencies are measured to rather better than 1μHz accuracy (the COROT project should measure oscillation frequencies with an accuracy of 0.1μHz), we present here a tool to study this aspect of stellar structure.
The Procyon Campaign: Observations from Kitt Peak
Authors: Pilachowski C. A. , Barden S. , Hill F. , Harvey J. W. , Keller C. U. , Giampapa M. S.
Published: June 2019

Time series spectra of the F5IV star Procyon (α CMi) were obtained at the Kitt Peak National Observatory during a 35-night observing run in January-February 1997. The observations were obtained as part of an international collaboration to detect and study acoustic p-mode oscillations in solar-type stars. Spectra covered the wavelength range from 4000 to 5300 Å, with a resolving power of approximately 3500 (1.3 Å resolution). The sampling rate was one observation per minute, and the typical S/N ratio per pixel after averaging along columns is in excess of 1000. We obtained 12,888 spectra. A sample spectrum is shown in Figure 1
Delta Scuti Stars: Theory
Authors: Guzik J. A.
Published: June 2019

The purpose of asteroseismology is not only to derive the internal structure of individual stars from their observed oscillation frequencies, but also to test and extend our understanding of the physics of matter under the extremes of temperature, density, and pressure found in stellar interiors. In this review, I hope to point out what we can learn about the Sun by studying δ Scuti stars, as well as what we can learn about stars more massive or evolved than the Sun. I will discuss some of the difficulties in theoretical approaches to asteroseismology for δ Scuti stars, using FG Vir, δ Scuti, and CD −24° 7599 as examples.
Asymptotic Description and the Diagnostic Properties of Low-Degree Stellar P-Modes
Authors: Roxburgh I.W. , Vorontsov S.V.
Published: June 2019

Standard asymptotic descriptions of stellar p-modes use 1/ω as a small parameter; either the degree ℓ is kept constant in the asymptotic expansions (e.g. Tassoul 1980), or the parameter , which specifies the position of the inner turning point (e.g. Vorontsov 1991). At low degree, due to the strong effects of gravity perturbations, these expansions are known to produce rather poor results, even when developed to higher order (Roxburgh and Vorontsov 1994). Here, we employ an alternative asymptotic expansion, capable of providing much better accuracy at low degree-with as a small parameter instead of 1/ω, and test its diagnostic capabilities.
Magnetic Fields in Molecular Clouds
Authors: Bourke Tyler L. , Goodman Alyssa A.
Published: June 2019

Magnetic fields are believed to play an important role in the evolution of molecular clouds, from their large scale structure to dense cores, protostellar envelopes, and protoplanetary disks. How important is unclear, and whether magnetic fields are the dominant force driving star formation at any scale is also unclear. In this review we examine the observational data which address these questions, with particular emphasis on high angular resolution observations. Unfortunately the data do not clarify the situation. It is clear that the fields are important, but to what degree we don't yet know. Observations to date have been limited by the sensitivity of available telescopes and instrumentation. In the future ALMA and the SKA in particular should provide great advances in observational studies of magnetic fields, and we discuss which observations are most desirable when they become available.
The Most Detailed Picture Yet of an Embedded High Mass YSO
Authors: Greenhill L. J. , Reid M. J. , Chandler C. J. , Diamond P. J. , Elitzur M.
Published: June 2019

High-mass star formation is not well understood chiefly because examples are deeply embedded, relatively distant, and crowded with sources of emission. Using VLA and VLBA observations of H2O and SiO maser emission, we have mapped in detail the structure and proper motion of material 20-500 AU from the closest high-mass YSO, radio source I in the Orion KL region. We observe streams of material driven in a rotating, wide angle, bipolar wind from the the surface of an edge-on accretion disk. The example of source I provides strong evidence that high-mass star formation proceeds via accretion.
Massive Star Formation
Authors: Garay Guido
Published: June 2019

The understanding of the formation process of massive stars requires a detailed knowledge of the physical conditions of the cloud environment which is thought to play a critical role in determining the formation mechanism. In recent years there has been a rapid growth of observational and theoretical studies concerning the formation of massive stars. Here I review observational data gathered during the last few years which are providing key evidence concerning the physical processes that take place during the formation of massive stars.
Testing the Paradigm of Low Mass Star Formation
Authors: Hartmann Lee
Published: June 2019

Protostellar core formation is probably much more dynamic, and magnetic fields are probably much less important, than has been previously assumed in the standard model of low-mass star formation. This revised picture has important consequences: it is easier to understand the observed rapidity of star formation in molecular clouds; cores are more likely to have structures favoring high infall rates at early times, helping to explain the differences between Class 0 and Class I protostars; and core structure and asymmetry will strongly favor post-collapse fragmentation into binary and multiple stellar systems.
The Structures and Kinematics of Protoclusters
Authors: Di Francesco James
Published: June 2019

Embedded clusters produce the greatest numbers of stars within our Galaxy but the process by which clusters themselves form is not well understood. The structures and kinematics of the very youngest embedded clusters, i.e., “protoclusters” with ages <106 yr, should provide needed insight. To gauge their utility in this regard, we examine recent observations of a nearby protocluster in Ophiuchus. Near-infrared images of the Oph A core reveal numerous young objects but the small sky coverages or low sensitivities to highly-embedded objects attained so far limit their usefulness in revealing cluster structure. Submillimeter continuum images do reveal the most embedded objects but their relatively low resolutions preclude effective comparisons with near-infrared images. Interferometric millimeter observations of N2H+ 1-0 show a strong spatial coincidence with the submillimeter continuum objects, and can be used to trace radial velocities of this population at least. The Oph A filament shows small radial velocity dispersions, e.g., ∼0.2 km s-1, at odds with predictions from some theories of cluster formation. The upcoming C2D and COMPLETE surveys of nearby star-forming molecular clouds will produce the sensitive, wide-field data needed to understand better the cluster formation process.
VLTI - Early Results
Authors: Leinert Christoph
Published: June 2019

VLTI - the infrared interferometer integrated into the VLT complex on Paranal - is approaching operation as part of the scientific instrumentation of the VLTs, with the first call for proposals out for the observing period starting in spring 2004. Two first generation scientific instruments will become available: AMBER, working in the near-infrared and combining up to three telescope beams is scheduled to see first fringes and have commisioning early in 2004. MIDI, working in the 10 μm band and combining two telescope beams, already is working in its basic observing mode. This contribution concentrates on a brief description of the latter instrument and the preliminary first scientific results obtained with it.
High Resolution Observations of Herbig-Haro Flows
Authors: Reipurth B. , Andrews S. , Bally J. , Beck T. L.
Published: June 2019

Observational techniques with high angular and high spectral resolution have become available in recent years and have had a major impact on the study of Herbig-Haro outflows from very young stars. We discuss new results obtained using WFPC2, ACS and STIS onboard the Hubble Space Telescope, as well as spectroscopic data obtained at the Keck telescope and with the Integral Field Unit at the Gemini telescope. We also discuss centimeter radio continuum maps from the Very Large Array and their importance in testing scenarios for the formation of giant Herbig-Haro flows.
The Future of High Angular Resolution Star and Planet Formation Science in the Optical/Infrared
Authors: Hillenbrand Lynne A.
Published: June 2019

This presentation summarizes how some of the most pressing questions in the field of star and planet formation can be addressed by high angular resolution optical/infrared capabilities, and how many of these capabilities will in fact be available with realization of the space and ground facilities currently being planned for the 2005-2020 time frame.
Studies of the Milky Way 1850–1930: some highlights
Authors: Smith Robert W.
Published: June 2019

“The Copernicus of the sidereal system is not to be expected for many generations”. So wrote R.A. Proctor1 in his Essays in Astronomy in 1872. Indeed things did look bleak at this time for those who hoped for a good understanding of the size and structure of the Galaxy. Why was this so, and why was there to be such an astonishing transformation of this situation between 1918 and 1930? Certainly these twelve years saw the widespread acceptance of no less than six fundamentally new ways of viewing the Galactic System. These profound shifts, occurring in such a short time, form, I would suggest, one of the most exciting chapters in the entire history of astronomy. And in this paper I shall attempt to describe and analyze what these changes were, what led up to them, as well as to examine the events surrounding them.
The stellar disk component: distribution, motions, age and stellar composition
Authors: Lyngå Gösta
Published: June 2019

The distribution of the stellar disk component is studied with particular emphasis on the properties of open clusters. A scale height of 80 pc is found for clusters younger than 109years while the scale height is 200 pc for older clusters. A radial metallicity gradient in the disk is confirmed but there seem to be significant abundance variations apart from this. There are radial gradients in age as well as in linear diameter. Kinematically, there are a number of regions in the Milky Way with systematic radial velocity residuals.
Computer-based catalogue of open-cluster data
Authors: Lyngå Gösta
Published: June 2019

The third edition of the computer-based catalogue of open-cluster data has now been produced and is disseminated through the Centre de Données Stellaires, 11 rue de l'Université, F-67000 Strasbourg, France. It is also available through World Data Center A, NASA, Greenbelt, MD 20771, USA.
The kinematics of nearby stars and large-scale radial motion in the Galaxy
Authors: Clube S.V.M.
Published: June 2019

The concept of star streams originally due to Kapteyn is revived. It is suggested that the existence of Stream II may have been overlooked, yet it is of crucial importance in specifying the true local standard of rest.
The stellar distribution in the Galactic spheroid
Authors: Gilmore Gerard
Published: June 2019

Our Galaxy is the only galaxy in which the 3-dimensional distribution of visible mass, chemical abundances and the stellar velocity field are all directly measurable. A project to determine these properties is currently underway, utilising direct photographic plates from the UK Schmidt telescope and the Las Campanas du Pont reflector, and the COSMOS and APM automated measuring machines. These provide reliable number-magnitude-colour distributions for complete samples of stars to V = 19 in 20 square degrees, and V = 22 in 1 square degree, in each of eight directions. These data may then be directly interpreted to determine the density profile, shape and luminosity function of the Galactic spheroid.
The vertical distribution of Galactic HI: the Arecibo-Green Bank Survey
Authors: Bania T. M. , Lockman F. J.
Published: June 2019

We have measured the vertical distribution of HI in the first quadrant of galactic longitude between tangent-point radii, Rt = Ro sin ℓ, of 2.5 kpc and 9.75 kpc at 250 pc intervals of tangent-point distance, using the 305m Arecibo telescope and the 91m and 43m NRAO telescopes. The survey is complete to at least z = ±500 pc and it was sampled with half-beamwidth spacings; the angular resolutions at 21cm wavelength are 4, 11 and 20 arcmin for these instruments.
A CO (2–1) survey of the Southern Milky Way
Authors: van de Stadt H. , Israel F. P. , de Graauw Th. , De Vries C.P. , Brand J. , Habing H. J. , Wouterloot J.
Published: June 2019

We have used the Estec/Utrecht heterodyne submillimetre receiver together with the 1.4 m ESO CAT at La Silla (Chile) to survey the southern Galaxy (1 = 270 − 355°) in the CO(2–1) transition at 230 GHz (1.3 mm). The beam used had a HPBW size of 5.5 arcmin, overall system efficiency was 0.35, and the system temperature was 1400 – 1750 K (DSB). Our filterbanks had velocity resolutions of 1.3 and 0.325 km s−1 and velocity ranges of 333 and 83 km s−1 respectively.
Radio continuum emission of the Milky Way and nearby galaxies
Authors: Beck Rainer , Reich Wolfgang
Published: June 2019

The radio continuum emission of the Milky Way and nearby galaxies can be decomposed into a central region, a clumpy “thin disk”, concentrated in the spiral arms, and a smooth “thick disk” (or flattened “halo”). The emissivity ratio of the two disks seems to be related to the magnetic field properties: Galaxies with strong radio spiral arms reveal a highly ordered field following the arm direction, while galaxies with diffuse disks contain a less ordered, smoothly distributed field. The degree of uniformity of the field seems to correlate with the total optical luminosity. The average magnetic field in the Milky Way is weak and turbulent compared to most of the nearby galaxies observed so far.
Which kind of spiral structure can fit the observed gradient of vertex deviation?
Authors: Oblak E. , Crézé M.
Published: June 2019

We analyse the distribution of peculiar velocities of stars within 200 pc of the Sun in terms of space variations of the velocity ellipsoid.
Small-scale structure and motions in the interstellar gas
Authors: Dickey John M.
Published: June 2019

The interstellar medium shows turbulence whose velocity and density spectra are surprisingly like a Kolmogoroff law, in spite of the fact that the turbulence is generated mainly by microscopic explosions, rather than a cascade of energy from larger to smaller scales.
A 10-Ghz radio-continuum survey of the Galactic–plane region at the Nobeyama Radio Observatory - A complex region at ℓ = 22° − 25°
Authors: Sofue Y. , Hirabayashi H. , Akabane K. , Inoue M. , Handa T. , Nakai N.
Published: June 2019

Preliminary results of a 10-GHz radio-continuum survey of the galactic-plane region using the 45-m telescope at NRO are presented. An extensive study of a complex region at 22° ≦ ℓ ≦ 25°, |b|≳1° has been made.
Interstellar sodium and Galactic structure. A high-resolution survey
Authors: Maurice E. , Ardeberg A. , Lindgren H.
Published: June 2019

Observation of absorption lines produced by interstellar gas is a straight-forward way to determine column densities and velocities along the line of sight of interstellar clouds. In practice, peculiar motions often mask galactic rotation and/or cause line blending. We have made a study of absorption lines of interstellar sodium covering a substantial part of the Galaxy at extremely high spectral resolution.
Three large molecular complexes in Norma
Authors: Bronfman L. , Cohen R. S. , Thaddeus P. , Alvarez H.
Published: June 2019

A segment of the Milky Way in Norma, from ℓ = 327° to 335°, |b|≦ 1°, has been studied as part of the Columbia CO survey of the fourth galactic quadrant. Description of the entire survey is given by Cohen elsewhere in this volume.
Comparison of CO in the Galaxy and the Magellanic Clouds
Authors: Israel F. P. , De Graauw Th. , van de Stadt H. , De Vries C.P.
Published: June 2019

We have used the Estec/Utrecht heterodyne submillimetre receiver and the ESO 3.6-m telescope at La Silla (Chile) to observe the CO(2–1) transition at 230 GHz (1.3 mm) in the Magellanic Clouds. We used a beam of 2 arcmin HPBW (corresponding to linear resolutions of 32 and 46 pc for LMC and SMC respectively), the system temperature was 2200 K (DSB) and the overall system efficiency was 0.55. In this paper we summarize the results, which are published in more detail elsewhere (cf. Israel et al., 1982, 1983; Israel, 1984).
A new study of the neutral hydrogen in Gould's Belt
Authors: Sodroski T. J. , Kerr F. J. , Sinha R. P.
Published: June 2019

The structure and kinematics of the neutral hydrogen associated with Gould's Belt have been studied using data of high velocity resolution and large latitude extent covering ℓ = 10o − 350°. The data comprise the Berkeley Survey of Neutral Hydrogen (Weaver and Williams 1973, 1974), and an unpublished survey by Kerr, Bowers, Kerr, and Jackson (1983) using the 60-foot Parkes telescope. The latter is a fully-sampled survey of the region ℓ = 240° to 350°, b = −10° to +10°.
Recombination-line observations of the Galactic Centre
Authors: van Gorkom J.H. , Schwarz U. J. , Bregman J. D.
Published: June 2019

Aperture-synthesis observations of the H76α and H110α recombination lines are presented for the inner (3pc) region of the Galactic Nucleus. The large line width measured with single dishes (Pauls et al., 1974) is caused by well-ordered large-scale motions of the ionized gas. The velocities of the NeII clumps (Lacy et al., 1980) fit well into our smooth velocity field and smooth intensity distribution. We suggest therefore that the cloud picture of the NeII gas is (at least partly) invalid.
Are spiral nuclei “active” in the radio continuum?
Authors: Ho Paul T. P. , Turner Jean L.
Published: June 2019

Using the VLA, we obtained matched-array continuum observations at 6 and 2 cm. An angular resolution of ~1″ and an rms sensitivity of ~0.05 mJy were achieved for a sample of 17 nearby spiral galaxies. Spectral-index maps derived for the nuclear regions reveal a mixture of thermal and nonthermal activity. The use of high angular resolution and high frequencies was the key to the success in detecting thermal activity. Conversion from thermal fluxes to total number of ionizing photons suggests that star formation is very active in some of these cores (inner 500 pc), with a rate typically 10 times greater than in our own nuclear region. A number of the nuclear regions appear to be dominated by extended nonthermal emission with a steep spectrum. Among these, some are closely associated with thermal emission and hence are consistent with supernova activity. However there are sources exhibiting aligned structures, suggesting possible connections with a central active nucleus. In any case, at our achieved sensitivity level 16 out of 17 galaxies were detected in the radio continuum.
A survey for high-velocity clouds in the inner Galaxy
Authors: Mirabel I. F. , Morras R.
Published: June 2019

The results from a search for high-velocity hydrogen around the direction to the galactic center are presented. About 2000 positions were surveyed with the 43-m radiotelescope of NRAO with a rms of 0.03 K on a velocity interval of −1000 to +1000 km s−1.
The Milky Way: a halo, a corona, or both?
Authors: De Boer Klaas S.
Published: June 2019

The detection in absorption lines of gas clouds outside the galactic plane at high velocities by Münch and Zirin (1961), high velocities then defined as velocities differing by more than 20 km/s from the LSR, showed that the space outside the Milky-Way disk contains not just stars. Of course, from a continuity argument it had been all along clear that some transition zone had to exist between the dense (relatively speaking) gas of the Milky-Way plane and the vast (almost) emptiness of intergalactic space. The presence of these clouds requires a mechanism to prevent their evaporation, and Spitzer (1956) proposed that dilute hot gas had to exist outside the Milky-Way disk reaching, in his hydrostatic-equilibrium model, temperatures of a few million K at several tens of kpc. These high temperatures led him to name these gases the Galactic Corona. Observational confirmation of the abundance of these cool clouds came from the measurements of 21-cm HI emission, but no one-to-one correspondence with clouds detected in the visual did appear (Habing 1969). For the majority of the high-velocity (HV) clouds (Hulsbosch 1978) no distances are known, and all of those are believed to exist as a gaseous halo with the halo stars. Thus our Milky Way appears to have outside the disk: a halo, a gaseous halo, and a corona.
A coordinated radio and optical survey of M31
Authors: Walterbos R.A.M. , Kennicutt R. C.
Published: June 2019

At Leiden we are obtaining coordinated radio, optical and infrared observations of the Andromeda galaxy, M31. Its proximity offers us a unique opportunity to study both the large-scale and small-scale structure of a galaxy which is similar in many respects to the Milky Way. The WSRT has been used to obtain high-resolution (24″ × 36″) maps of M31 in the HI line and 21-and 49-cm radio-continuum emission. Recently the radio data have been complemented with optical surface photometry in UBVR and Hα, using the Burrell Schmidt telescope at Kitt Peak and the Palomar Schmidt. Results from the HI and IRAS infrared observations are presented elsewhere. Here we present some preliminary results from the radio-continuum and optical surveys.
A collision between the Large and Small Magellanic Clouds 2×108 years ago
Authors: Fujimoto M. , Murai T.
Published: June 2019

A number of orbits are obtained for the Large and Small Magellanic Clouds (LMC and SMC) revolving around a model Galaxy with a massive halo. It is suggested that the SMC approached the LMC as close as 3 to 7 kpc about 200 million years ago, if these clouds have been in a binary state for the past 1010 years, and the Magellanic Stream (MS) is due to the gravitational interaction among the triple system of the Galaxy, LMC, and SMC.
Heating of stellar disks by massive gas clouds
Authors: Lacey Cedric G.
Published: June 2019

I have analytically calculated the evolution of the three components of velocity dispersion of the stars in a galactic disk when these are scattered by massive gas clouds, in a generalization of Spitzer & Schwarzschild's (1953) calculation. The principal assumptions made are: (i) The stellar orbits obey the epicyclic approximation. (ii) The gas clouds are massive, long-lived and on circular orbits. (iii) The typical star-cloud encounter time is short compared to the orbital time. (iv) The evolution due to encounters is treated as a diffusion process.
Two-dimensional calculations of tightly wound spiral shocks
Authors: Nelson A. H. , Johns T. , Tosa M.
Published: June 2019

Recently it has been possible to perform computer calculations with sufficiently high resolution to obtain the response of the gas in galaxies to a tightly wound spiral potential due to a stellar density wave. Previously for pitch angles ~ 10° the problem had to be simplified using the tightly wound approximation, with gradients parallel to the arms neglected compared to perpendicular gradients, yielding a quasi-1-dimensional calculation. This paper reports the early results of a 2-dimensional calculation, and compares them with the 1-dimensional results.
The stabilizing effects of haloes and spiral structure
Authors: Robe H.
Published: June 2019

The equilibrium and the stability of homogeneous gaseous and uniformly rotating MacLaurin spheroids imbedded in a rigid, homogeneous and spherical stellar halo are considered. Such systems may be useful as crude models of disk-halo galaxies. Explicitly, we have determined the modes of oscillations stemming from the fundamental radial mode of the homogeneous compressible sphere and its non radial “f” (or Kelvin) modes belonging to the second (ℓ=2) and third (ℓ=3) spherical harmonics, together with the first “pressure” and gravity modes p1 and g1 belonging to the first spherical harmonics (ℓ=1). It has been assumed that the oscillations are adiabatic with a ratio of specific heats γ=5/3, and several sequences of spheroids corresponding to various values of the halo mass have been examined.
New N-body experiments on the spiral structure of galaxies
Authors: Thielheim K. O. , Wolff H.
Published: June 2019

N-body experiments with stellar disks like those performed by Hohl (1971) and Sellwood (1981) show regular, global, two-armed spiral structures associated with bar formation. The question is whether the spiral is caused by the bar, and if this is true, how the generating mechanism works.
Velocity dispersion and the stability of galactic disks
Authors: Kormendy John
Published: June 2019

This paper estimates the effect of the stellar velocity dispersion on local and global instabilities in galaxy disks. Measurements of rotation velocities and velocity dispersions are illustrated for the disks of NGC 488 (Illingworth and Kormendy, in preparation), NGC 936 and NGC 1553. These are used to derive the Toomre stability parameter Q, i.e. the ratio of the observed dispersion to that needed for marginal stability against local modes. In both NGC 488 and 1553, Q = 2.5 − 4, depending on the assumed mass-to-light ratio. These values larger than 2 imply that the stellar disks are stable, although the gas in NGC 488 may remain unstable. This is consistent with the observation that both galaxies largely lack coherent spiral structure: NGC 1553 is an SO and NGC 488 a flocculent spiral. The SBO galaxy NGC 936 is more extreme: Q = 5 (estimated error is a factor of two). This is consistent with Sellwood's suggestion that a bar heats the disk until it is too hot to support spiral structure. In contrast to the above galaxies, Toomre has shown that our Galaxy has Q = 1.6 ± 0.5 near the Sun. It is therefore responsive enough to make spiral structure, as observed.

We also explore the stellar kinematics of the disk of NGC 1553 as a function of radius. The exceptionally strong lens component in this galaxy is very hot. At half of the radius of the lens the velocity dispersion appears to be large enough to have an effect even on global stability.
On the 3-Kpc arm: waves excited at the resonance by an oval distortion in the central region
Authors: Yuan Chi
Published: June 2019

A relatively minor oval distortion in the central region of the Galaxy, turning at a representative angular pattern speed, can excite outgoing waves at the outer Lindblad resonance of that pattern speed. Associated with the density crest of these waves is fast-expanding gas flow. The physical basis of this phenomenon can be understood through a linear analysis. However, to explain the observed expanding velocity in the “3-kpc arm”, the non-linear theory must be used. In our calculations an oval distortion turning at 118 km s-1 kpc-1 with a perturbation of 10% of the mean gravitational field at the outer Lindblad resonance (located at 3 kpc in the present case) can generate an outgoing velocity of 53 km s-1 at the first density crest of the wave (located at 3.6 kpc).
Spiral structure in galaxies: large-scale stochastic self-organization of interstellar matter and young stars
Authors: Feitzinger J. V.
Published: June 2019

Galaxies are dissipative systems, and the spatial and time structure of the interstellar medium and young stars is governed by reaction-diffusion equations. The coherent galactic oscillations of star formation self-organized in spiral waves, previously detected by numerical simulations (Seiden, Schulman, Feitzinger, 1982) can be analytically described by the concept of a limit cycle. Analytical work on self-propagating stochastic star formation is also done by Kaufman (1979), Shore (1981, 1982) and Cowie and Rybicki (1982).
The formation and early evolution of the Milky Way
Authors: Fall S. Michael
Published: June 2019

In broad outline, the traditional picture for the formation of the Milky Way can be summarized as follows. The proto-galaxy consisted of a slowly rotating cloud of metal-free gas that cooled by bremsstrahlung and recombination radiation. As the internal pressure of the gas decreased, it collapsed in stages with smaller dimensions, faster rotation velocities and flatter shapes until it reached centrifugal support in a fundamental plane. At the same time, the gas was progressively depleted by the formation of stars and enriched with heavy elements by the ejecta from previous generations. The result is a general correlation between the kinematic properties, chemical compositions and relative ages of the stellar populations within the Galaxy. This picture was formulated at the Vatican symposium by Oort (1958) and others and was elaborated by Eggen, Lynden-Bell & Sandage (1962), Sandage, Freeman & Stokes (1970), Gott & Thuan (1976), Larson (1976) and others. Much of the recent work on galaxy formation has been an attempt to extend these ideas to a more comprehensive picture that includes large quantities of dark matter. The purpose of this article is to review several topics concerning the collapse phase in the evolution of the Milky Way.
IV.—How were the Primitive Weight Standards fixed?
Authors: Ridgeway William
Published: June 2019

In previous articles I have shown that the oldest Greek unit of weight, the talent of gold in the Homeric Poems, was identical with the cow or value of a cow, that the same identity existed between the cow in Italy and the gold unit (itself the same as the Homeric Talanton) which lies at the base of the Roman system, and that the like relation existed between cow and gold unit in Sicily. I had further advanced the suggestion that we ought to seek for the origin of the weight standard or standards from which probably all systems in the Old World, modern as well as ancient (save the modern French), have been derived, arguing that as the cow or ox was the most widely diffused common unit of barter, it was natural that when metals came into use as a medium of exchange, the metallic unit would naturally represent the value of the older unit of barter. Ordinary law of supply and demand would fix more or less accurately the amount of gold which one man would be willing to give, and another man be willing to accept for an ox. One point however I did not make clear, and that was how it came to pass that primitive men were able to fix with what practically was a high degree of accuracy the amount of gold which represented the value of an ox. It is, I think, this difficulty which is supposed to surround the process of fixing accurately the metallic unit thus derived which has induced metrologists to make up their minds that weight units could not have been arrived at empirically, and in consequence of this to seek their origin in the scientific astronomy of Babylonia.
II. The Mantinean Basis
Authors: Gardner Percy
Published: June 2019

As Overbeck in the fourth edition of his Geschichte der Plastik has recanted his doubts as to the period and authenticity of the sculptures of the Basis of Mantineia, it is fair to regard them as undoubted works of about B.C. 370, and as coming at all events from the workshop of the master whose statues stood above them. Thus the discovery of this basis must be regarded as a very fortunate addition to our sources of knowledge, both of the art of Praxiteles, and of the types of the Muses in the fourth century. Fig. 3 gives these slabs in what I hold to be the true arrangement.
IV / Village Pattern and Architecture
Authors: Wheat Joe Ben
Published: June 2019

Having briefly considered the location, environment, and layout of the various Mogollon villages, we may now analyze them to determine whether or not they follow a pattern.

In their choice of village sites, it is clear that, with few exceptions, the Mogollon chose places of some elevation and isolation. Twelve of 2I4. villages were located on a mesa, a bluff, or a ridge; 10 others occupied terraces well above valley bottoms; and one was situated on the outwash fan of nearby mountains. Only one was located in a valley bottom. Furthermore, Danson. (1952) found in his survey that the early people built their villages on mesas or isolated ridges well back from the main stream of travel. Without exception, sites were located in mountainous country or near to mountains. Whether by preference or for some other reason, it was in the mountain country that the Mogollon people had their ecological niche. Many sites lie in secluded valleys on tributary streams; but except, perhaps, in the northernmost sites, the availability of agricultural land seems to have been one of the deciding factors in selecting a village site.
Material Culture
Authors: Jennings Jesse D.
Published: June 2019

Before outlining this section concerned with material culture I should perhaps try to be explicit about the philosophy which guides me in archeological matters. It is my conviction that archeological work does not transcend antiquarianism (a legitimate and justifiable occupation) unless the specimens and the observational data are regarded as reflecting some cultural situation. For me a continuing circular preoccupation with artifacts is sterile. The only excuse for excavation, it seems, is that ever-present hope that by means of archeological search we can catch a meaningful glimpse of, or a series of moments in, a once dynamic living culture. In this interest, of course, I am not alone.

It seems to me we are interested in archeological specimens because we are in reality, and at some distance, and often obscurely, working with a way of life, for which the observed relationships and the artifacts are the meager documentation. One freely admits that the archeological record is very incomplete; it is, moreover, heavily weighted towards the material things.
Variable Star Research by the PLANET Collaboration
Authors: Albrow Michael D. , Pollard K. R. , Beaulieu J.-P. , Caldwell J. A. R. , Menzies J. , Vermaak P. , DePoy D. L. , Gaudi B. S. , Gould A. , Pogge R. W. , Dominik M. , Naber R. M. , Sackett P. D. , Greenhill J. , Hill K. , Kane S. , Watson R. , Martin R. , Williams A. , Sahu K. C. ,   The PLANET collaboration
Published: June 2019

We review the current status and future prospects of the PLANET collaboration, an international team of astronomers performing high-precision photometric monitoring of microlensing events. Our photometric precision and sampling is characterised and the suitability of the database for variable star studies is discussed. Preliminary results on K-giant stability are presented.
Prospects of Variable Star Research by Future Space Missions
Authors: Eyer Laurent
Published: June 2019

ESA and NASA are studying projects having a tremendous return on variable star research. Other national space agencies are also studying or developing projects of smaller costs but with impressive returns. The projects range from global Galactic surveys like the ESA mission GAIA which will give photometric time series for about 1 billion stars, to detailed pulsation-mode studies like the CNES mission COROT which could reach a photometric precision lower than 1 ppm. The presentation will emphasize the future astrometric, asteroseismological and planet detection missions.
COROT – A Unique Database for Low Amplitude Variability Between 1 Minute and 150 Days
Authors: Michel E. , Baglin A. , Barge P. , Catala C. , Auvergne M. , Weiss W. W. , Appourchaux T. , Garrido R. ,   the COROT Team
Published: June 2019

COROT is a high precision wide field photometry experiment from space, funded in the framework of the CNES “Petites Missions” program (the PI is A. Baglin). It will observe approximately 30000 objects with mV between 4.5 and 15.5 over long observational periods (up to 150 d), with a time sampling between 1 s and 16 min, a precision of the order of 10−4 per measurement. The scientific objectives are stellar seismology and the search for telluric planets. The instrument and the core program have already been presented in several places. We here focus on a description of the characteristics of the data to be obtained with COROT. The large amount of high quality data collected by COROT will constitute rich material for several research programs beyond the core program as already defined (http://www.astrsp-mrs.fr/www/corot.html). A call for proposal of additional programs will be made during Northern Spring 2000.
Photometric Properties of the Hubble Space Telescope Fine Guidance Sensors
Authors: Zwintz K. , Kuschnig R. , Weiss W. W. , Witeschnik A.
Published: June 2019

To reach lower noise levels with the Hubble Space Telescope Fine Guidance Sensors (FGS) the photometric properties as well as the systematic effects affecting the data have been investigated.
Multiperiodic and Aperiodic Pulsations: Comparative Study of Algorithms vs. Variability Types
Authors: Andronov Ivan L.
Published: June 2019

A synopsis of mathematical models describing different types of variability and their application to some classes of stars is given.
Nonlinear Pulsations of Luminous Blue Variables
Authors: Dorfl Ernst A. , Feuchtinger Michael U. , Gautschy Alfred
Published: June 2019

We present the results of various nonlinear radial pulsation calculations of LBVs with different luminosity to mass ratios. Depending on the stellar parameters, most of these objects undergo strongly non-adiabatic pulsations which can also modify their stellar atmospheres. In some cases part of the kinetic energy of the motions is transferred by shock waves into the atmospheric layers leading to an overall decrease or increase of the stellar radius. This results in different mean density accompanied by a change of the pulsational behavior which can be compared to the results of a linear stability analysis. We also find that regular pulsations can occur around a different equilibrium than the initial configuration which has been determined by stellar evolution calculations. In cases of even higher luminosity to mass ratios the linear stability analysis predicts several unstable modes and the nonlinear computations can exhibit rather irregular light curves. Due to the large non-adiabaticity of the pulsation the corresponding velocity curves still remain more regular, but show increasing amplitudes until the energy stored is released. The pulsations are far from equilibrium conditions and may also lead to a pulsationally driven mass loss for certain stellar parameters.
Photometry of R Coronae Borealis Stars during the Recovery Phases of Their Declines
Authors: Skuljan Ljiljana , Cottrell Peter L.
Published: June 2019

R Coronae Borealis (RCB) stars are a rare class of variable stars, well-known by their declines, but still posing two fundamental questions: what is the origin and what is the nature of their unpredictable minima? Possible contributions to the answering of these questions could use large-scale surveys, by discovering new RCB stars. In addition, photometric databases from these surveys and other long-term photometric programs of targeted stars can be used to compile information about these decline phases.

The photometric observations (UBVRI) of nine cool RCB stars have been collected at Mt John University Observatory, New Zealand, for the last ten years. Analysis of the magnitude–color diagrams for the recovery phase of a number of declines of these stars has been made in order to determine the extinction properties of the dust. The photometric magnitudes and colors show significant changes during the RCB declines. The asymptotic approach to the normal brightness can be seen on the magnitude–color diagrams from the recovery phase. This is observed for all RCB declines, and without exception is independent of their decline amplitude. According to the most acceptable theory, the distribution of the dust matter in the cloud which causes the light fading can be considered uniform as the stars recover to the maximum brightness. The slopes of the magnitude–color and color–color diagrams from the recovery phase of 26 observed RCB declines have been used in order to define the normalized extinction quantity Aλ/AV. Although the extinction curve in the UBVRI region is not a good indicator of the type and size of the particles, our data show that the material causing the RCB declines has extinction properties similar to that for the interstellar medium. The reddening parameter (RBV = AV/E(B – V)) for the RCB stars in our sample is in the range 2.5 to 4.6, similar to the interstellar dust. Observations have been compared with the theoretical extinction curves for different sorts of grains. The extinction properties in the U region follow the trend of the UV data from other RCB observations as well as laboratory data for amorphous carbon.
A Spectroscopic Study of RV Tauri Stars in the LMC
Authors: Pollard Karen R. , Evans Tom Lloyd
Published: June 2019

Preliminary results from a low-resolution spectroscopic survey of RV Tauri stars and Type II Cepheids in the LMC are presented.
Changes of the Physical State in Semiregular Variables
Authors: Kiss L. L. , Szabó Gy. , Szatmáry K. , Mattei J. A.
Published: June 2019

We present a progress report on a detailed analysis of long-term (70–90 years) visual observations for seven semiregular variables. Fundamental changes of the physical state (amplitude and/or frequency modulations, mode change and switching) are studied with conventional Fourier- and wavelet analyses. Besides two examples of repetitive mode changes we report a simple geometric model of a rotationally induced amplitude modulation in RY Ursae Majoris.
Unusual Secondary Variations in the Mira Star T Cep
Authors: Marsakova Vladislava I. , Andronov Ivan L.
Published: June 2019

Cyclic period variations (P ≈ 19400d) in T Cep were investigated. As the result of cross-correlation analysis, relations between individual cycle parameters were found. Relations between period–amplitude, hump–amplitude, hump–minimum magnitude, and others have been obtained.
Baade–Wesselink Analysis of Magellanic Cloud Cepheids
Authors: Storm Jesper , Carney Bruce W. , Gieren Wolfgang P. , Fouqué Pascal , Fry Anne M.
Published: June 2019

We apply the Barnes–Evans variant of the Baade–Wesselink method to Cepheids in the LMC and SMC in an attempt to determine the distance directly to individual stars in these galaxies and to determine the metallicity effect on the Cepheid period–luminosity relation. We now haveK-band light curves for a sample of SMC stars as well as for many Cepheids in young clusters in the LMC. Using theFV, (V–K) calibration of Fouqué & Gieren (1997) we find preliminary evidence for a metallicity effect which makes metal poor Cepheids brighter. This is at odds with earlier results based on optical photometry and the reason is not entirely understood yet.
A Synoptic Variability Survey of M3
Authors: Szentgyorgyi Andrew , Stanek Krzysztof Z. , Sasselov Dimitar , Kaluzny Janusz , Schwarzenberg-Czerny Aleksander
Published: June 2019

We present preliminary results from a synoptic photometric variability survey of the galactic globular cluster M3. These data consist of between 9 and 13 nights of data, the sampling allows a minimum detectable period of 10 minutes and is complete to MV ~ 21.
Metallicity Effects in Evolutionary Cepheid Models
Authors: Baraffe Isabelle , Alibert Yann
Published: June 2019

Based on consistent stellar evolution and pulsation calculations for Cepheids, we analyse the effect of metallicity on period–luminosity relationships. We discuss the main uncertainties of the results inherent to stellar evolution calculations and linear stability analysis.
Numerical Simulations of the Cepheid Population in the Hipparcos Catalogue
Authors: Groenewegen M. A. T.
Published: June 2019

The population of Cepheids in the Hipparcos catalogue is retrieved. Some observational properties of the sample are modelled using a. Monte Carlo code. This code is then used to verify that the bias introduced by the method of “reduced parallax” in deriving the zero point of the P–L relation is of order 0.01 mag, confirming earlier results. This bias is likely due to Malmquist bias.